An asymmetric photothermal platform for coupled seawater splitting and desalination

Domain: neuroscience
Published: 2026-03-27
Editorial outcome: major_revision

Abstract
Photocatalytic seawater splitting provides a sustainable route to green hydrogen, but faces barriers of inefficiency and scalability. Graphitic carbon nitride (g-C3N4, CN) offers saline tolerance, yet its catalytic potential requires atomic-level control of structure and charge dynamics. Herein, we design three single-atom Co configurations on CN, including symmetric Co–N4, vacancy-anchored Co–N3, and asymmetric Co–N4. The asymmetric Co–N4 in hierarchically porous carbon nitride (CoSA-hCN) introduces second-shell carbon vacancies that reshape electronic asymmetry and charge dynamics, establishing a robust pathway for effective salinity-mediated charge transfer and in situ Pt photodeposition for H2 evolution. On a scalable 60 cm2 floating photothermal platform under 1 sun, H2 production (47.7 mmol m−2 h−1) and interfacial seawater evaporation (1.88 kg m−2 h−1) are achieved directly from natural seawater. In this work, we offer design principles for a single-atom catalyst on scalable photothermal platform for coupled seawater splitting and desalination.

Introduction
Recently, seawater splitting to hydrogen has been a research hot spot.1–4 Compared with electrocatalysis,4,5 solar seawater splitting will deliver green H2 at a global scale,1,2 while seawater’s ionic complexity heavily corrodes most catalysts (Supplementary Fig. 1a).6,7 Graphitic carbon nitride (g-C3N4, CN) showed somewhat tolerance to saline environments,8,9 where polar surface and ion-catalyst interactions may enhance charge separation and redox kinetics.10 Nevertheless, its intrinsic catalytic activity remains limited by poor charge transfer and insufficient active sites. Also, the underlying mechanisms governing these processes have been insufficiently explored, and unlocking the full catalytic potential of CN requires precise structural engineering that integrates active-site design with controlled electronic and geometric modulation.11,12 Recent advances highlight CN-supported single-atom metal photocatalysts (SACs) as an ideal platform,13–15 yet their translation to seawater splitting has scarcely been realized, where complex ionic matrices demand in-depth mechanistic understanding and design strategies. In CN-integrated SACs, first-shell vacancy engineering and coordination number modulation of metal centers has been widely studied.16,17 By contrast, second-shell asymmetry, arising from nearby vacancies or ligand field distortions, has been recognized to strongly tune orbital hybridization and charge redistribution in carbon-supported single-atom electrocatalysts,18,19 however, its role in CN-SACs, particularly in shaping photocatalysis and ionic interactions in seawater conditions, has rarely been explored. Advancing understandings of coordination asymmetry at different atomic levels will be essential for guiding next-generation CN-SAC designs for solar H2 production from seawater.

Beyond atomic-level photocatalyst engineering, integration of materials into scalable and robust systems is equally important for advancing practical seawater splitting. Direct seawater utilization can significantly enhance sustainability by reducing freshwater dependence in arid and coastal regions.20 A unified platform that simultaneously produces H2 and freshwater would unlock the full potential of seawater, offering a broadly applicable strategy for sustainable energy and water supply.20,21 Yet, the inherently distinct physicochemical demands of photocatalysis and desalination have led to their separate treatment in previous investigations.22–24 Integration attempts, such as sheet-assisted architectures20 or photothermal evaporative substrates,25 remain largely a proof-of-concept. These systems often suffer from structural complexity, limited scalability, and trade-offs between freshwater and hydrogen yields. These limitations highlight the need for a structurally simple and scalable platform that can simultaneously deliver efficient photocatalysis and desalination.

In this work, we establish a molecular-level coordination-engineering strategy to construct three single-atom Co configurations on CN: symmetric Co–N4 (CoSA-CN), vacancy-anchored Co–N3 (CoSA@hCN), and asymmetric Co–N4 with second-shell carbon vacancies (CoSA-hCN). The asymmetric coordination in CoSA-hCN induces a distinct electronic-asymmetry and facilitates electron transfer from Co to the CN framework for efficient in situ Pt cocatalyst photodeposition. From a catalyst design perspective, this tailored coordination and surface charge strongly promote selective interactions with seawater ions and improve salinity-mediated charge transfer (Supplementary Fig. 1b). Under visible light irradiation, CoSA-hCN exhibits 4- and 3-fold higher H2 evolution rate than CoSA-CN and CoSA@hCN, respectively. Remarkably, CoSA-hCN presents a distinct solar-induced photothermal effect, contributing to its record-high performance. Most importantly, we integrate CoSA-hCN into a floating photothermal platform using a 60 cm2 commercial sponge support (Fig. 1a) to couple efficient H2 generation with seawater desalination (Supplementary Fig. 1b). This work demonstrates that asymmetric single-atom site design can be easily translated into device-level integration and the sponge-supported floating architecture provides a structurally simple and modular basis for real seawater operation. These advances in atomic-level innovation and scalability design establish a general blueprint for future solar energy conversion and clean water technologies.

Results
Catalyst synthesis and characterizations

All CN-SACs were synthesized via pyrolysis of dicyandiamide (DCDA) and cobalt (II) acetylacetonate (Co(acac)2) precursors (Fig. 1b-d). Direct pyrolysis led to immobilization of Co single-atoms (CoSA) within melon polymer chains, producing CoSA-CN with a compact, non-porous morphology (Fig. 1b and Supplementary Fig. 2-4). To construct hierarchical porous SACs with an enlarged surface area (Supplementary Fig. 3-5 and Supplementary Table 1), we adopted a silica template-assisted pyrolysis strategy. Close-packed silica spheres provide a confinement effect to promote single-atom loading.13,14,26 Additionally, silica templating introduces structural modifications such as vacancy formation,27,28 yet its influences on the coordination environment of single-atom centers remain unexplored. Accordingly, a one-step silica template-assisted pyrolysis method was used to induce vacancy formation and modulate the local electronic structure, thereby breaking the coordination symmetry of Co centers and producing CoSA-hCN (Fig. 1c and Supplementary Fig. 6). For comparison, Co atoms were anchored onto pre-formed hCN via a multi-step self-assembly and pyrolysis method to produce CoSA@hCN catalysts (Fig. 1d). All samples show similar Co loadings, as determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Supplementary Fig. 7).

Aberration-corrected high-angle annular dark field scanning transmission electron microscopy (AC HAADF-STEM, Fig. 1e and Supplementary Fig. 8) and energy-dispersive X-ray (EDX, Fig. 1f) mapping images confirm atomically dispersed Co across all catalysts. This aligns with X-ray diffraction (XRD), showing no detectable Co-related crystalline peak (Supplementary Fig. 9). CoSA@hCN and CoSA-hCN show weakened (100) and (002) peaks, suggesting NH3-induced C atom etching and reduced stacking periodicity.28,29 X-ray photoelectron spectroscopy (XPS) reveals a notable decrease in the atomic C/N ratio in CoSA-hCN and CoSA@hCN than CoSA-CN, confirming the formation of C vacancies (Supplementary Fig. 10). Fourier transform infrared (FTIR) spectra show intensified −OH/−NH/−NH2 stretching at 2800-3600 cm−1 (Supplementary Fig. 11), further confirming C atom removal and amine group formation.30 Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy was conducted to pinpoint vacancy locations (Fig. 1g). The declining i1:i2 ratio (characteristic C atoms in N=C–N to N=C–N(NH2) from CoSA-CN to CoSA-hCN and CoSA@hCN) evidences the loss of C atoms in the N=C–Nx motif. Solid-state electron paramagnetic resonance (EPR) spectroscopy shows that CoSA-hCN exhibits the strongest signal at the g-value of 2.004, indicating the most abundant unpaired π-electrons localized at the C vacancy sites (Supplementary Fig. 12). In contrast, CoSA@hCN shows a slightly weaker signal than hCN, implying partial vacancy occupation by Co atoms in CoSA@hCN.31

Synchrotron-based X-ray absorption spectroscopy was used to investigate the structural and electronic properties of catalysts. The C K-edge X-ray absorption near-edge structure (XANES, Supplementary Fig. 13a) spectra show a weakened π*(C−N−C) peak in CoSA-hCN, indicating disruption of the C−N−C coordination,15 likely caused by the disorder in the sp2 in-plane structure by C vacancies. This may induce interlayer C–N interaction and create conductive paths across the layers for improved charge transfer.32 N K-edge XANES spectra reveal attenuated π* resonances for triangular edge π*(C–N–C) and bridging N (π*(N–C)) in CoSA-hCN than hCN33 (Supplementary Fig. 13b). This suggests orbital hybridization and electron donation from Co to N atoms, considering lower electronegativity of Co relative to C and N.32 Meanwhile, Co K-edge XANES (Fig. 1h) reveals a positive shift in CoSA-hCN, indicating increased electropositivity and charge delocalization.34,35 The weaker pre-edge peak and stronger white-line peak in CoSA-hCN suggest the reduced symmetry and rearranged charge polarization.34,36

Fourier-transformed Co K-edge X-ray absorption fine structure (EXAFS) spectra (Fig. 1i) show a main peak at ~1.6 Å at R space for all samples, corresponding to Co–N first-shell coordination. CoSA@hCN exhibits reduced peak intensity, indicating a lower first-shell coordination number than CoSA-CN and CoSA-hCN. EXAFS fitting results reveal that the first-shell coordination numbers of CoSA-CN, CoSA-hCN and CoSA@hCN are 4.3, 4.4, and 3.3. respectively (Supplementary Fig. 14 and Supplementary Table 2). The Co–C second-shell coordination (~2.5 Å, referenced against CoPc) shows diminished intensity in CoSA-hCN relative to CoSA-CN, reflecting C vacancies-induced loss of second-shell C atoms. For CoSA@hCN, the weakened second-shell Co–C signal and broadened first-shell peak suggest that Co atoms are anchored at vacancy sites, where shortened Co–C distances lead to overlapping Co–N/C scattering paths in the first-shell region. The absence of Co–Co (2.2 Å) coordination confirms single-atom feature of all samples. Wavelet transform (WT) analysis further validates exclusive Co–N coordination without Co–Co or Co–O contributions (Fig. 1j). Collectively, the above structural characterizations reveal symmetric Co–N4 configurations for CoSA-CN, Co–N3 at C vacancies for CoSA@hCN and asymmetric Co–N4 with second-shell C vacancy for CoSA-hCN, enabling distinct electronic modulation.

Structure-property correlation

To elucidate the impact of asymmetric coordination on electronic modulation, we performed density functional theory (DFT) calculations. Local structural models of CoSA-CN, CoSA-hCN, and CoSA@hCN were constructed and optimized to explore how coordination symmetry affects electronic structure and charge transport (Supplementary Fig. 15, Supplementary Data 1). The spatial charge density distribution in Fig. 2a and Supplementary Fig. 15 shows that introducing the C vacancy in CoSA-hCN induces charge redistribution, with electron accumulation on the tri-s-triazine framework. Bader charge analysis reveals that the amount of electrons transferred from Co to surrounding N atoms increases from 0.90 (CoSA-CN) to 1.12 (CoSA@hCN) and 1.13 e (CoSA-hCN). The total and projected density of states (DOS) further highlight these differences (Fig. 2b). In CoSA-hCN, the Co-d states show an enrichment and broadening near the Fermi level (EF), as well as significant Co–N hybridization. The increased near-EF Co-d spectral weight provides more accessible states for charge transfer. Interestingly, the projected DOS appears symmetric in CoSA-CN and CoSA@hCN, suggesting weak hybridization,37 while CoSA-hCN displays an irregular and asymmetric line shape, reflecting stronger orbital hybridization, which facilitates charge delocalization and charge transfer. The spin density plot (Supplementary Fig. 16) further reveals localized spin polarization on the Co atom and neighboring N sites. This confirms that the asymmetric coordination environment induces a localized unpaired electron distribution, which may facilitate spin-selective charge transfer and thereby enhance the photocatalytic activity of CoSA-hCN.

We further examined the influences of structural variations on electronic properties and carrier dynamics. Steady-state photoluminescence (PL) spectroscopy in Fig. 2c shows that the emission intensities follow: CN > hCN > CoSA-CN > CoSA@hCN > CoSA-hCN, implying the maximally inhibited intrinsic radiative recombination of photocarriers in CoSA-hCN. A slight red shift of the emission peak from 466 (CoSA-CN) to 468 (CoSA@hCN) and 470 nm (CoSA-hCN) is attributed to the increased trap state and defect density (Supplementary Fig. 17). Time-resolved PL (TRPL) spectroscopy reveals a decreased average fluorescence lifetime (τave) from 5.24 (CoSA-CN) to 5.05 (CoSA@hCN) and 3.83 ns for CoSA-hCN, suggesting accelerated charge transfer in CoSA-hCN (Fig. 2d and Supplementary Table 3). The shortened lifetime indicates an increased contribution of non-radiative recombination pathways. Such non-radiative relaxation is expected to contribute to localized photothermal heating under illumination,38 which may support faster interfacial reaction kinetics. The findings of electronic function align well with the DFT simulations, confirming that the vacancy-induced symmetry breaking could facilitate more charge delocalization and promote carrier dynamics. These calculations are based on idealized models and do not capture the full surface complexity or dynamic ionic environment under operating conditions; therefore, they provide qualitative rather than quantitative insight into structure-property relationships.

Solid-state EPR was performed to track light-induced electron delocalization within the vacancy-engineered framework (Fig. 2e). Upon light irradiation, CoSA-hCN exhibits a pronounced intensity increase, indicating enhanced delocalization of photocarriers across the π-conjugated network. This is attributed to the electronic perturbation introduced by carbon vacancies. In further assessing charge transport capability, electrochemical impedance spectra profiles indicate that CoSA-hCN exhibits the lowest charge transfer resistance among all catalysts (Supplementary Fig. 18). Consistently, CoSA-hCN shows the highest photocurrent density, confirming its great capability for charge separation and migration (Supplementary Fig. 19). The electronic band structures were evaluated using UV-vis diffuse reflectance spectroscopy (DRS, Supplementary Fig. 20) and valence band (VB) XPS to determine valence band maximum differences (Supplementary Fig. 21) as well as ultraviolet photoelectron spectroscopy (UPS) to extract the absolute valence band position (Supplementary Fig. 22). These measurements show that CoSA-hCN possesses the most negative conduction band potential, favoring 2H+/H2 photoreduction (Supplementary Fig. 23).

In photocatalytic H2 evolution systems by CN-SACs, Pt nanoparticles were typically introduced via in situ photodeposition using H2PtCl6.39–43 Their formation is inherently a charge transfer-driven process and highly sensitive to the photocatalyst’s electronic structure. However, the underlying mechanisms governing Pt deposition and its interaction with single-atom sites remain insufficiently understood. Building on the above insights into asymmetric coordination and electronic modulation, we further explored how local coordination features affect cocatalyst integration and interfacial charge transfer dynamics. To probe the dynamics of in situ Pt photodeposition on SACs, we conducted open-circuit potential (OCP) measurements (Fig. 2f). Under light irradiation, all catalysts exhibited gradual photoanodic shifts, indicating the electron extraction. Notably, CoSA-hCN showed a greater potential increase upon H2PtCl6 addition than others, suggesting more efficient electron transfer from the photoexcited matrix to reduce Pt4+ and facilitate Pt nanoparticle formation. The presence of Pt nanoparticles was verified in HAADF-STEM and EDX mappings (Supplementary Fig. 24 and inset in Fig. 2f). ICP-OES analysis indicates comparable Pt loadings across the three catalysts, with CoSA-hCN exhibiting the highest Pt loading at 4.08 wt% (Supplementary Fig. 25a). This indicates more favorable Pt photodeposition on CoSA-hCN. TEM images show that the Pt nanoparticles on CoSA-hCN and CoSA@hCN are more evenly dispersed and exhibit a narrower size distribution, typically below 5 nm, whereas Pt on CoSA-CN presents less uniform dispersion and shows larger aggregates (Supplementary Fig. 25b-d), reflecting the distinct structural and electronic characteristics of the supports.

To probe the oxidation states and interfacial electronic interactions of photodeposited Pt species, we performed XPS analysis of samples before and after deposition. Upon Pt loading, N 1s peaks of CoSA-hCN slightly shifted to higher binding energies (Supplementary Fig. 26), suggesting electron transfer from the substrate to Pt. In contrast, negligible spectral changes were observed in hCN and CoSA-CN; minor in CoSA@hCN (Supplementary Fig. 27-29), indicating relatively weak Pt-substrate interaction and less effective Pt anchoring. The Pt 4f spectra of CoSA-hCN after photodeposition display distinct peaks at 73.0 and 76.3 eV for Pt2+ species from partial reduction of H2PtCl6, along with sub-peaks at 70.8 and 74.6 eV for metallic Pt0 (Fig. 2g). The coexistence of Pt0 and Pt2+ species implies a dual functional role, where Pt0 facilitates proton reduction and Pt2+ prevents the undesirable H2 backward reaction.44 Other samples exhibited weak or indistinct Pt 4f signals under identical photodeposition conditions. These differences highlight the favorable electronic structure of CoSA-hCN for Pt4+ photoreduction and dual-functional Pt generation, ultimately enhancing H2 photoproduction. To further study the electronic interaction, we performed theoretical calculations on the optimized Pt-loaded CoSA-CN, CoSA-hCN and CoSA@hCN models (Supplementary Data 2). DOS profiles in Supplementary Fig. 30 reveal that CoSA-hCN induces a more pronounced redistribution of electronic states near the Fermi level, indicating stronger electronic coupling between Pt and the Co single-atom center. Bader charge analysis (Supplementary Fig. 31) shows that Pt undergoes pronounced electron enrichment upon interacting with the substrates, confirming spontaneous interfacial charge transfer consistent with its role as an efficient electron acceptor for proton reduction. Among all configurations, Pt on Co-hCN exhibits the most negative Bader charge, further confirming the strongest electronic interaction.

The charge transfer on Co sites was also investigated. Post-deposition Co K-edge XANES spectra were re-examined in pure water containing H2PtCl6 under visible light irradiation. All catalysts display a slight edge shift toward higher energy, consistent with electron transfer from Co centers to reduce Pt4+ during Pt nucleation (Fig. 2h and Supplementary Fig. 32). With the addition of TEOA after reaction, CoSA-hCN shows a small shift toward lower energy, reflecting the reductive action of TEOA on the Co sites (Fig. 2h). Fourier-transformed Co K-edge EXAFS analysis indicates the Co active sites still preserve their singly dispersed nature after operation (Supplementary Fig. 33). The enhanced white line intensity suggests reduced local symmetry and increased unoccupied 3d states around the Co centers. TRPL spectroscopy highlights the dominant role of single-atom Co in facilitating charge transport, with minimal contribution from Pt (Supplementary Fig. 34-35). These findings support a directional charge transfer mechanism: photoexcited electrons first transiently stabilize at the redox-flexible Co sites, then relay through surrounding N sites, and ultimately reach Pt cocatalysts to drive proton reduction (Fig. 2i).

Photocatalytic H2 production from seawater

Photocatalytic H2 evolution performances were evaluated under visible light irradiation using TEOA as a sacrificial agent and in situ photodeposited Pt as the cocatalyst (Fig. 3a and Supplementary Fig. 36). Among all catalysts, CoSA-hCN exhibited the highest H2 production rate of 6.4 ± 0.2 mmol g–1 h–1 in artificial seawater (ASTM D1141-98 (2013) standard, Supplementary Table 4), which is 4- and 3-fold higher than that of CoSA-CN and CoSA@hCN, respectively. As discussed above, this enhanced activity primarily arises from the asymmetric Co–N4 coordination and vacancy-induced electronic modulation, which strengthen charge separation and promote efficient in situ Pt photodeposition, while physicochemical improvements such as higher surface area provide additional contributions. The apparent quantum efficiency (AQE) reached 8.96% at 420 nm. Under combined UV and visible light irradiation, the H2 evolution rate of CoSA-hCN increased to 8.2 ± 0.3 mmol g–1 h–1 (Fig. 3b and Supplementary Fig. 36b). When operated without temperature control, photothermal self-heating raised the system temperature to ~65 °C (Supplementary Fig. 37), boosting the rate to 22.5 ± 1.2 mmol g–1 h–1, 2.7-fold increase compared to that at 25 °C. This photothermal enhancement effect, evidenced by the experimentally observed temperature increase of the reaction system, has rarely been reported in CN-SAC and CN-based systems for photocatalytic H2 production from water or seawater. It enables an competitive H2 evolution activity with previously reported CN-SAC photocatalysts (primarily evaluated in water)39–43,45–50, and CN-based photocatalysts in seawater8,51–59 (Supplementary Table 5). The structural stability of CoSA-hCN was verified by cyclic tests and unchanged XRD pattern, FTIR spectrum and C/N K-edge XANES of the post-reaction samples (Supplementary Fig. 38). Notably, CoSA-hCN demonstrates proof-of-concept photocatalytic performance in natural seawater under laboratory conditions, achieving a H2 production rate of 5.9 mmol g–1 h–1 in natural seawater (Fig. 3a). Furthermore, photoelectrochemical corrosion tests (Fig. 3c) revealed that CoSA-hCN exhibits a lower corrosion potential (0.56 V) than CN (0.62 mV), hCN (0.69 V), CoSA-CN (0.60 V), and CoSA@hCN (0.58 V) under simulated sunlight irradiation. This suggests the robust anti-corrosion capability of CoSA-hCN, highlighting its potential for real-life applications in marine environments.

To explore how interfacial characteristics of catalysts correlate with their behaviors in seawater, we studied the interfacial wettability and surface charge properties. As shown in Fig. 3d, the contact angle of CoSA-hCN decreased from 59.7° in pure water to 47.9° in seawater, indicating enhanced hydrophilicity under saline conditions. Compared with CoSA@hCN (64.6°) and CoSA-CN (56.1°), the enhanced hydrophilicity in CoSA-hCN promotes better contact between the catalyst surface and seawater matrix, facilitating mass transfer and interfacial reactivity (Supplementary Fig. 39). Zeta potential measurements revealed that CoSA-hCN possessed a more negatively charged surface than CoSA@hCN and CoSA-CN (Fig. 3e). This may help suppress local Cl− accumulation at the interface for undesired side reactions. Meanwhile, the negatively charged surface electrostatically attracts seawater cations (e.g., Na+, Mg2+, Ca2+), which may modulate surface polarization and interfacial charge distribution, thereby enhancing electron mobility.1 Simultaneously, H⁺ ions are drawn toward the negatively charged surface, promoting H+-coupled electron transfer for H2 evolution. Theoretically, Gibbs free energy calculations confirm that CoSA-hCN exhibits the great intrinsic activity for proton adsorption and H2 evolution (Supplementary Fig. 40, the atomic coordinates of the computational models have been provided in Supplementary Data 3). Simplified computational models are employed to evaluate intrinsic proton adsorption, while the effects of cocatalysts and ionic species under realistic reaction conditions are not explicitly included.

To clarify the influence of specific seawater ions on H2 production, we evaluated individual salts. As shown in Fig. 3a, NaCl led to the highest H2 evolution rate, outperforming MgCl2 and CaCl2. TRPL spectroscopy was used to investigate the effect of different saline components on charge transfer (Fig. 3f and Supplementary Fig. 41-43). Fig. 3g shows that NaCl exhibits the most pronounced effect in shortening carrier lifetimes, reflecting its dominant role in promoting charge transfer. Given NaCl’s important effect, DFT calculations were performed to evaluate the adsorption behavior of Na+ and Cl− on different catalysts (Supplementary Data 4). As revealed by the adsorption energy (Eads), CoSA-CN and CoSA@hCN possess more negative Eads of Cl− than Na+, indicating higher affinity to Cl− (Fig. 3g and Supplementary Fig. 44). In comparison, CoSA-hCN can significantly enhance the adsorption of Na+ (Eads of −3.21 eV), showing a preference to Na+ than Cl− (−3.19 eV). Relative adsorption energies (Fig. 3h, Supplementary Fig. 45 and Supplementary Data 5) further suggest that the asymmetric atomic structure of CoSA-hCN promotes selective adsorption of seawater cations over Cl⁻, favoring cation interaction (Fig. 3i). This selectivity suppresses chlorine by-product formation, improving anticorrosion ability,60 while the cation adsorption induces strong polarization and facilitates photocarrier transport.1 Furthermore, band center analysis (Supplementary Fig. 46) shows that CoSA-hCN exhibits optimal orbital alignment for efficient reactant sorption and favorable reaction kinetics. Taken together, asymmetric electronic structure engineering of CoSA-hCN enables selective ion interactions and salinity-mediated charge transfer, thereby enhancing photocatalytic H2 production in saline environments.

Scaled-up seawater splitting and desalination under practical conditions

To evaluate device-level feasibility, we developed a pilot-scale photothermal device to simultaneously achieve H2 evolution and seawater desalination (Fig. 4a-b and Supplementary Fig. 47). A floatable sponge photoreactor was constructed using a commercial sponge (L100 × W60 × H50 mm), featuring a white melamine‐formaldehyde (MF) foam at the upper layer and a black synthetic fiber at lower layer (Supplementary Fig. 48). Photocatalysts were immobilized onto the upper layer via a simple dip-coating method (Fig. 4c and Supplementary Fig. 49-50). The melamine foam contains abundant N–H and C–H functional groups that provide favorable sites for anchoring CoSA-hCN during impregnation (Supplementary Fig. 51). A mechanical stability test demonstrated the robust interfacial bonding between the photocatalyst and the melamine support (Supplementary Fig. 52). During operation, capillary action transports seawater from the base to the catalyst-coated surface (Fig. 4b-c), while the open-cell MF provides strong buoyancy to keep the catalytic surface afloat (Fig. 4d and Supplementary Fig. 53). The hydrophobic backing further enhances flotation by trapping air within its porous matrix and distributing flotation across the photoreactor, preventing waterlogging and ensuring full light exposure of the catalyst surface. This structure facilitates a unidirectional seawater flow, improving reactant delivery and promoting H2 evolution. Simultaneously, photothermal heating evaporates seawater at the interface, and the resulting vapor condenses on the overhead cover and is collected as freshwater, enabling integrated H2 and freshwater co-production.

Under simulated sun irradiation (average 103 mW cm–2) and ambient conditions (~25 °C, ~40% humidity), the Pt-deposited CoSA-hCN-embedded sponge exhibited rapid surface heating to 37.5 °C within 10 min and stabilized at ~45 °C (Fig. 4e and Supplementary Fig. 54), significantly higher than the bulk sponge (~33 °C, Supplementary Fig. 55). This photothermal enhancement enabled a higher evaporation rate of 1.75 kg m−2 h−1 from artificial seawater (Fig. 4f), outperforming the bulk sponge (1.05 kg m−2 h−1). The evaporation rate reached 1.88 kg m−2 h−1 in 6 h from natural seawater (Supplementary Fig. 56). To assess the desalination performance, we quantified the amount of freshwater condensed during a 6 h light-irradiation run in the coupled H2 evolution and desalination process (Fig. 4g-h). The freshwater yield at 7.5 kg m−2, with ion concentrations (Na+, Mg2+, Ca2+ and K+) in the condensate reduced below the World Health Organization drinking threshold (Fig. 4i and Supplementary Fig. 57). The system also demonstrates good applicability with natural seawater, yielding 8.3 kg m−2 purified water.

Simultaneous photocatalytic H2 production was realized using Pt-deposited CoSA-hCN-immobilized sponge (Fig. 5a-b), achieving a H2 production rate of 55.3 ± 4.1 mmol m−2 h−1 in artificial seawater under 6 h light irradiation (Fig. 5c-d and Supplementary Fig. 58), and maintaining high efficiency under realistic aqueous solutions (Supplementary Fig. 59). Triethanolamine functioned both as a stabilizer for catalyst immobilization and as a sacrificial electron donor (Fig. 5c). We further validated the generality of the integrated system by replacing catalysts with commercial TiO2 (P25). Under identical conditions, P25 showed a relatively lower H2 production rate (Supplementary Fig. 60), confirming that the enhanced performance in our system arises from the synergistic coupling between CoSA-hCN and the photothermal reactor. The system exhibited stable long-term performance for H2 production and seawater desalination (Fig. 5e and Supplementary Fig. 61-62), with minimal catalyst leaching (Supplementary Fig. 63). The unchanged C and N K-edge profiles indicate that the CoSA-hCN catalysts remain structurally stable after the operation (Supplementary Fig. 64). During cycling tests, catalyst retention at the sponge surface was maintained via gravity-assisted redistribution. This concept can be adapted for large-scale floating systems by incorporating passive tilting or slow rotational movement, allowing the catalyst layer to self-redistribute under operational conditions in open-water environments. Compared to state-of-the-art floatable photocatalytic systems20,21,25,61,62 (Supplementary Table 6), our platform features simple and modular design that allows facile fabrication through a one-step dipping process. It provides a practically viable strategy to integrate efficient hydrogen production and seawater purification.

Methods
Materials

All chemicals used are of an analytical grade without further purification. Ammonium hydroxide solution (NH3·H2O, 28.0 - 30.0% NH3 basis), ammonium hydrogen difluoride (NH4HF2, pure), acetone (≥99.5%), cobalt(II) acetylacetonate (Co(C5H7O2)2, ≥99.0%), dicyandiamide (DCDA, 99%), ethanol (≥99.9%, absolute, analytical level), isopropyl alcohol (≥99.9%), and tetraethyl orthosilicate (TEOS, ≥99.0%) were purchased from Sigma-Aldrich. Ultrapure water (18.2 MΩ cm−1) was used. Artificial seawater was prepared using the ASTM D1141-98(2021) standard, specifically, 24.53 g L–1 NaCl, 5.20 g L–1 MgCl2, 4.09 g L–1 Na2SO4, 1.16 g L–1 CaCl2, 0.69 g L–1 KCl, and 0.20 g L–1 NaHCO3 solution. Natural seawater was collected on the Grange Beach in Adelaide, South Australia. A commercial sponge (Big Job Magic Eraser, Chux) was purchased from a Woolworths supermarket in Adelaide, South Australia.

Fabrication of ordered silica templates

Ordered silica microsphere templates were synthesized using a modified Stöber method. In this process, Solution A was prepared by adding 192 mL of ethanol to 8 mL of TEOS and stirring at 500 rpm at 25 °C. Simultaneously, Solution B was made by combining 14 mL NH3·H2O, 56.6 mL ethanol, and 29.4 mL ultrapure water, also stirred at 500 rpm at 25 °C. After 30 min, Solution B was gradually and slowly added to Solution A while stirring at 750 rpm. The resulting mixture was centrifuged at 18,900 g using a high-speed constant temperature centrifuge at 20 °C. The collected silica was washed with ultrapure water to remove residuals, dried in a preheated oven at 60 °C overnight, and then ground and dispersed in 5 wt% ultrapure water. This solution was placed into vials and subjected to ultrasonication, followed by evaporation in a preheated oven at 110 °C for 24 h, allowing the silica microspheres to self-assemble into silica strips on the vial walls.

Fabrication of pristine carbon nitride (CN)

Pristine CN was synthesized by calcining 0.6 g of DCDA at 520 °C for 2 h (2 °C·min–1), followed by further calcination at 550 °C for an additional 2 h (4 °C·min–1) under an argon (Ar) atmosphere. The resulting CN solid was washed thoroughly with ultrapure water and subsequently dried at 60 °C.

Fabrication of hierarchical porous carbon nitride (hCN)

The hCN was synthesized by calcining DCDA with the help of a silica template. Specifically, 0.6 g DCDA was ground with 1 g silica template and subsequently calcined at 520 °C for 2 h (2 °C min–1), followed by further calcination at 550 °C for an additional 2 h (4 °C min–1) under an Ar atmosphere. The resulting product was etched with 50 mL of a 4 mol L–1 NH4HF2 solution (pH of 4.5 ± 0.3) for 48 h to remove the silica template. The as-obtained hCN solids were then washed several times with ultrapure water and dried overnight at 60 °C.

Fabrication of CoSA-hCN

A mixture of DCDA and Co(C5H7O2)2 was dissolved in a 300 mL solution comprising ethanol and acetone in a ratio of 1:1. The solution was stirred at 400 rpm and heated to 75 °C for 2 days for evaporation. The resulting solid was washed with ultrapure water several times and dried in a preheated oven at 60 °C overnight. The collected solids were ground. Subsequently, 0.6 g of collected solids were further ground with 1 g silica template, and calcined at 520 °C for 2 h (2 °C min–1), followed by 550 °C for 2 h (4 °C min–1) in Ar. The product was etched with 50 mL of 4 M NH4HF2 solution (pH of 4.5 ± 0.3) for 48 h to remove the silica template. Finally, the CoSA-hCN solid was collected, rinsed with ultrapure water, and dried overnight at 60 °C.

Fabrication of CoSA-CN

A mixture of DCDA and Co(C5H7O2)2 was dissolved in a 300 mL solution comprising ethanol and acetone in a 1:1 ratio. The solution was stirred at 400 rpm and heated to 75 °C for 2 days for evaporation. The resulting solid was washed with ultrapure water several times and dried in a preheated oven at 60 °C overnight. The collected solids were ground and calcined at 520 °C for 2 h (2 °C min–1), followed by 550 °C for 2 h (4 °C min–1) in Ar. The CoSA-CN samples were rinsed with ultrapure water and dried overnight at 60 °C.

Fabrication of CoSA@hCN

A mixture of hCN and Co(C5H7O2)2 was dissolved in a 300 mL solution comprising ethanol and acetone in a 1:1 ratio. The solution was stirred at 400 rpm and heated to 75 °C for 2 days for evaporation. The resulting solid was washed with ultrapure water several times and dried in a pre-heated oven at 60 °C overnight. The collected solids were ground and calcined at 550 °C for 2 h (2 °C min–1) in Ar. Finally, the CoSA@hCN solid was collected, rinsed with ultrapure water, and dried overnight at 60 °C.

Electron microscopy

All electron microscopy characterizations were conducted at Adelaide Microscopy, which provides advanced instrumentation for high-resolution structural and compositional analysis at the nanoscale. Scanning electron microscopy (SEM) imaging was carried out using an FEI Quanta 450 microscope to examine surface morphology. Transmission electron microscopy (TEM) observations were performed on an FEI Tecnai G2 Spirit instrument to investigate microstructural features and particle dispersion. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), combined with energy-dispersive X-ray spectroscopy (EDX) for single-atom metal identification, elemental mapping and compositional analysis, was conducted using an FEI Titan Themis 80−200 microscope.

Physicochemical and surface properties

A specific surface area was acquired by N2 sorption measurement on a Micromeritics Tristar 3000 at −196 °C. The surface potential was analyzed using Zetasizer Nano ZS (Malvern Instruments, Germany). The hydrophilicity property was measured using Attension Optical Tensiometers (Nanoscience Instruments). For each measurement, 30 mg of catalyst powder was pressed into a 7-mm-diameter pellet under a pressure of 1.5 tons to obtain a flat and uniform surface suitable for optical analysis.

Spectroscopy

The Co contents of catalysts were measured by ICP-OES (Agilent 5110) after autoclave digestion of the samples. X-ray diffraction (XRD) was performed using a Rigaku MiniFlex 600 X-ray diffractometer. Fourier transform infrared (FTIR) spectroscopy was carried out on a Nicolet 6700 (Thermo Fisher). Soft X-ray absorption spectra at the C and N K-edges and Co L-edge XANES were measured at the Soft X-ray Spectroscopy beamline of the Australian Synchrotron, part of Australian Nuclear Science and Technology Organisation (ANSTO) in Melbourne. Data analysis was performed using the QANT software package provided by the facility.63 Co K-edge XANES and EXAFS spectra were collected in transmission mode at the same synchrotron. Measurements were conducted under ambient conditions. All spectra data were processed using ATHENA mode of the IFEFFIT package for energy calibration, background correction and normalization.64 Surface chemistry of samples was analyzed by X-ray photoelectron spectroscopy (XPS) using an X-ray photoelectron spectrometer (K-alpha, Thermo Fisher Scientific). All the binding energies were referenced to the C 1s peak at 284.8 eV. The VB-XPS and UPS spectra were obtained using a Thermo Fisher Scientific ESCALAB Xi spectrometer.

Electronic structure and functions study

UV-visible diffuse reflectance spectra were acquired using a Cary 100 UV-Vis spectrophotometer (Agilent, USA). The band gap was estimated by Tauc plots derived from the Kubelka-Munk function. The position of the valence band was determined through valence band XPS using an X-ray photoelectron spectrometer (K-alpha, Thermo Fisher Scientific, UK). The steady-state photoluminescence (PL) was obtained using an RF-5301PC spectrofluorophotometer (Shimadzu, Japan). The steady-state PL map (variable excitation and emission wavelengths) and TRPL spectra (excitation of 350 nm, emission of 470 nm) were obtained using an FLS1000 spectrometer (Edinburgh Instrument). A Bruker Magnettech ESR5000 was used to measure the Electron Spin Resonance (EPR) signal of solid samples.

Electrochemical measurements

Electrochemical measurements were carried out using a CHI760E electrochemical workstation (standard three-electrode configuration). Open-circuit potential (OCP) measurements were performed in artificial seawater (pH = 8.33) as the electrolyte. A saturated calomel electrode (SCE) served as the reference electrode, and a Pt wire was used as the counter electrode.

The measured potentials can be converted to the reversible hydrogen electrode (RHE) scale via the following equation:
ERHE = ESCE + E0SCE + 0.0592 × pH.
Where, ERHE is the converted potential versus RHE, E0SCE is 0.242 V at 25 °C, and ESCE is the experimentally measured potential against SCE reference. The working electrode was prepared by depositing the catalyst onto fluorine-doped tin oxide (FTO, 1 × 1.5 cm2) glass. For electrode preparation, 5 mg of catalyst was dispersed in 200 μL isopropanol with 5 μL Nafion solution to form a homogeneous ink. The ink was drop-casted onto a defined circular area (diameter = 5 mm) of the FTO substrate and dried under ambient conditions. Polarization curves were measured using the same three-electrode configuration and electrode preparation method. Electrochemical impedance spectroscopy (EIS) was conducted using a similar setup; however, 0.5 M Na2SO4 (pH = 7) was used as the electrolyte. EIS spectra were recorded at open-circuit potential over a frequency range of 105 to 0.01 Hz with an AC amplitude of 0.005 V. All measurements were conducted without iR compensation. The electrochemical analysis was primarily intended for qualitative comparison of interfacial charge-transfer behavior, and the reported potentials are therefore discussed in a comparative rather than absolute benchmarking context. The electrochemical measurements complement the photocatalytic evaluation and are not intended to represent standalone electrocatalytic performance. The photocurrent density measurement was conducted using a similar setup using 0.5 M Na2SO4 (pH = 7) as the electrolyte. A 300 W xenon-arc lamp (Aulight CEL-HX, Beijing, with 420 cut-off filter) was used as a light source.

Photocatalytic H2 production from seawater

The photocatalytic H2 production tests were carried out using a homemade sealed black poly(tetrafluoroethylene) reactor equipped with a quartz window and a water-cooling jacket (Supplementary Fig. 65). A 300 W xenon-arc lamp was used as a light source (emitted UV and visible light, Aulight CEL-HX, Beijing). Visible light-driven photocatalytic H2 production tests were performed using a 420 cut-off filter. The light intensity was measured by a 340 solar meter (OAI, USA), and determined as 147 mW cm–2 at the light spot. A water circulation system was supplied to keep the temperature at 25 °C for the H2 production tests unless further specified. A solution of 0.4 g L–1 photocatalyst and 10 vol% TEOA (as a sacrificial agent) was prepared. Before irradiation, a 1 mg L–1 H2PtCl6 solution was added and vigorously stirred in the dark for 30 min. Ultrapure nitrogen (N2, 99.9%) gas was purged into the system simultaneously, ensuring anaerobic conditions. The H2 product was analyzed using an online gas chromatograph (Agilent 490 Micro GC) equipped with a thermal conductivity detector (TCD). For, apparent quantum efficiency (AQE) test, the experiment was conducted for 1h under a 300 W xenon-arc lamp using a 420 nm bandpass filter. After adding H2PtCl6 (1 mg L–1) into a pre-mixed suspension of 0.4 g L–1 photocatalyst and 10 vol% TEOA, the mixture was vigorously stirred in the dark for 30 min, followed by irradiation for 30 min photodeposit Pt co-catalysts.

Preparation of the sponge-photoreactor

The illustration of the sponge-photoreactor preparation process is shown in Supplementary Fig. 38. A commercial sponge (Big Job Magic Eraser, Chux) was thoroughly cleaned before catalyst immobilization. The sponge was first immersed in 750 mL of ultrapure water until fully saturated, and then manually compressed to expel the trapped liquid. This process was repeated 10 times until the disappearance of foam in the expelled water to remove residual surfactants. The sponge was then subjected to ultrasonication in 750 mL of 10 vol% ethanol solution for 30 min, followed by three additional cycles of immersion and compression to eliminate any remaining contaminants. The cleaned sponge was dried overnight in a preheated oven at 80 °C and cooled to room temperature for further use. The catalyst slurry was prepared via the following process. Photocatalyst (40 mg) was dispersed in 20 mL of ethanol under magnetic stirring for 30 min. Pt cocatalysts (1.0 wt%, using H2PtCl6 as the precursor) were introduced via photodeposition under light irradiation for 15 min. After Pt loading, 4 mL of TEOA was added and stirred for an additional 15 min. TEOA serves as a mild stabilizing agent and a binding facilitator, promoting both the dispersion of the catalyst and its adhesion to the sponge support. The resulting slurry was transferred to a flat container. The dried sponge was placed with the white side down into the slurry and allowed to absorb the suspension for 15 s. It was then dried in an oven at 60 °C for 1 h. This dip-coating and drying process was repeated twice, and then dried in an oven at 60 °C. Upon cooling to room temperature, the sponge-photoreactor was obtained. Then, the photocatalyst-coated layer was edge-sealed with a ring of standard transparent tape to secure the catalyst layer and reduce lateral wicking. The as-prepared sponge-photoreactor can then be directly employed in seawater for solar H2 production and seawater evaporation.

Integrated photocatalytic H2 production and seawater desalination

A custom-built device was designed to simultaneously enable photocatalytic H2 evolution and solar-driven seawater desalination (Supplementary Fig. 47). The device consists of two compartments, a seawater pool and a purified water collection chamber. The upper enclosure was sealed with a slanted quartz glass window, which allowed efficient light transmission while promoting the condensation of water vapor. The tilted design facilitated a directional flow of condensed droplets into the purified water collection chamber. During operation, the sponge-photoreactor was placed in the seawater pool with the white-coated surface facing upward. Then, 500 mL of seawater was added to the seawater pool. The internal environment was maintained under ambient air conditions to simulate real-world operation. The device was sealed to enable gas accumulation. A 300 W xenon-arc lamp equipped with an AM 1.5 filter was used as a simulated solar light source. The average light intensity was measured by 340 solar meter (OAI, USA) at 5 representative points, and determined as 103 mW cm–2. Gas products were analyzed using a gas chromatograph (GC, Agilent 8890) with a thermal conductivity detector (TCD). The purified water was collected from the purified water chamber and quantified gravimetrically. Its metal ion content was determined via atomic absorption spectroscopy (AAS) to assess desalination performance.

Theoretical calculations

The periodic electronic structure calculations were carried out within the framework of spin-polarized density functional theory (DFT) using the Vienna Ab initio Simulation Package (VASP).65,66 The electron-ion interactions were described by the projector augmented wave (PAW) approach.67 The exchange-correlation functional was described within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) formulation. Dispersion interactions of the van der Waals type were described using Grimme’s DFT-D3 semi-empirical correction.68 The plane-wave expansion was truncated at a kinetic energy cutoff of 500 eV. Geometry optimizations were conducted with a Gamma (Γ)-centered 1 × 1 × 1 Monkhorst-Pack k-point grid for Brillouin zone sampling. For surface slab models, a vacuum region of approximately 15 Å was introduced perpendicular to the slab plane to avoid spurious interactions between periodically repeated images. All atomic positions were fully optimized during the relaxation process without any geometric constraint, ensuring unbiased structural evolution toward the minimum-energy configuration. All structures exhibiting magnetic moments were fully relaxed without imposing any constraint. Structural optimization proceeded until the total energy converged to below 1 × 1 × 10-6 eV and the residual forces on each atom were reduced to less than 0.02 eV Å-1. The charge density difference (CDD) was calculated to visualize the redistribution of electronic charge upon the formation of the Co−N coordination environment. The CDD is defined as:
Δρ(r) = ρCoSA + CN (r) − ρCoSA(r) − ρCN(r).
Where, ρCoSA+CN denotes the charge density of the complete Co single-atom structure, and ρCoSA and ρCN represent the charge densities of the isolated Co center and the surrounding N coordination environment, respectively, each computed in the same geometric configuration as in the combined system. This definition highlights the intrinsic charge redistribution between the metal center and its coordinated ligands. The isosurface was visualized at an isovalue of 0.001 e Bohr−3, which provides a clear yet physically meaningful depiction of charge transfer regions. In the plotted isosurfaces, yellow regions denote electron accumulation (Δρ > 0), while cyan regions correspond to electron depletion (Δρ < 0). Thus, the color contrast intuitively reflects the direction and spatial extent of charge transfer between the CN adsorbate and the Co active site.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This study presents a combination of atomic-scale catalyst design and system-level integration for photocatalytic H₂ evolution from seawater. The novelty lies in engineering asymmetric Co–N₄ single-atom sites on carbon nitride with second-shell carbon vacancies to modulate electronic asymmetry and salinity-mediated charge transfer. This work contributed to addressing two long-standing hurdles simultaneously: realizing high catalytic activity in saline environments and coupling hydrogen production with concurrent freshwater generation. The deployment on a scalable floating photothermal platform under sunlight elevates the work from materials discovery to a technology concept with practical relevance, suggesting pathways for modular, off-grid hydrogen supply in coastal or remote settings. To better support the claims and conclusions and improve the quality of this work, the following questions are suggested to be addressed before considering it for publication.\n1. Page 3, lines 87-88, Supplementary Fig. 6: TGA is not accurate enough to identify the Co loadings. As shown in the TGA curve, the adsorbed water may contribute to the mass loss during the heating process. To improve this data, ICP-AES/AAS is suggested to measure the Co loadings.\n2. It was claimed that carbon vacancies were formed in CoSA-hCN. Elemental analysis is necessary to identify the composition of all three catalysts: CoSA-CN, CoSA@hCN, and CoSA-hCN, especially the C/N ratio in these samples.\n3. What is a possible mechanism of forming Co-N4 with carbon vacancies via the silica-templated one-step process?\n4. Page 8, line 199-200, Supplementary Fig. 19: VB-XPS alone gives relative VBM to Femi level (EF). A work function from UPS/secondary-electron cutoff or Kelvin probe is required to place the bands on an absolute (vacuum) scale and convert them to SHE/NHE.\n5. Page 7, Fig. 2g, lines 223-226: the intensities of Pt peaks from different catalysts vary significantly. What is the exact loading amount of Pt on each catalyst? What is the particle size of Pt on these catalysts? As the Pt nanoparticles act as active sites for the H2 evolution reaction, the particle size and loading amount of Pt nanoparticles on photocatalysts play an important role in the H2 evolution rate. ICP-AES/AAS and HRTEM are required to answer these questions.\n6. Page 9, lines 223-228: “It was claimed that the coexistence of Pt0 and Pt2+ species implies a dual functional role…….” Is it possible that the Pt0 in photo-deposited Pt nanoparticles is partially oxidized to Pt2+ when the sample is exposed to air?\n7. Page 9, lines 230-232; “Post-deposition Co K-edge XANES spectra reveal a light shift in the absorption edge of CoSA-hCN toward lower energy, indicating partial reduction of Co centers by photoexcited electrons (Fig. 2h).” Is this phenomenon observed on CoSA-hCN only? How about other catalysts after Pt deposition?\n8. Material synthesis procedure: What is the pH value of 4 mol/L NH4HF2? Is it a strong acid (pH <1) that can etch the Co species from CoSA-hCN?\n9. The catalysts CoSA-CN, CoSA@hCN, and CoSA-hCN with different Co coordination were synthesized via method engineering. which showed remarkably different photocatalytic H2 evolution activity. Material characterizations demonstrated that multiple structural features and properties of these materials were distinct, including surface area, surface hydrophilicity, zeta potential, conductivity, charge transfer, Pt loading (maybe amount and particle size), and band alignment (bandgap and band position). What is the key/major factor contributing to the enhanced H2 evolution rate of CoSA-hCN in pure water and seawater, respectively? For instance, the surface area of CoSA-hCN is three times that of CoSA-CN, and the H2 evolution rate of CoSA-hCN is around 4-fold higher than that of CoSA-CN. Is there a direct relationship between the surface area and the H2 evolution rate?", "reviewer_2": "This work presents a floating photothermal platform designed for the simultaneous production of hydrogen and freshwater from seawater. The approach is innovative, leveraging single-atom cobalt configurations on carbon nitride photocatalysts applied onto foam supports. The rational design of the catalyst, supported by detailed characterizations, clearly demonstrates the originality and significance of the work. The manuscript is well written, and the conclusions are appropriately supported by the presented data.\nHowever, several important aspects related to the practical implementation and long-term stability of the platform need further clarification and discussion:\n1. Stability of the active layer: The long-term stability of the photothermal layer on the melamine foam raises concerns, especially under prolonged operation in real seawater environments. The possibility of catalyst leaching and the downward migration of active materials should be more thoroughly investigated. The authors are encouraged to provide quantitative data or estimates on material loss over time, if available.\n2. Interaction Between Catalyst and Support: It remains unclear whether there is any chemical or physical interaction between the melamine foam and the catalyst layer. A discussion on this point, supported by surface characterization or adhesion tests, would strengthen the understanding of the structural integrity of the composite system.\n3. Gravity-Assisted Re-Accumulation Strategy: The authors mention a gravity-assisted re-accumulation mechanism to mitigate catalyst loss. However, the practical feasibility of this approach under real operational conditions is questionable. Further elaboration is required to assess how effective and scalable this strategy would be in a realistic application scenario, especially in open water environments.\n4. Salt Fouling and Long-Term Operation: Salt accumulation or fouling is a major concern for any system operating with seawater. The manuscript should include a discussion on how the platform design addresses this issue, especially regarding the impact of salt crystallization on both photothermal performance and catalyst activity over extended use.\nOverall, the study is highly promising and contributes meaningfully to the field. Addressing the points above would greatly improve the manuscript and provide a more comprehensive understanding of the system's robustness and scalability for real-world applications.", "reviewer_3": "The manuscript presents an interesting study on molecularly engineered single-atom Co catalysts integrated into a scalable photothermal reactor for seawater splitting and desalination. The authors provide a well-designed system with comprehensive structural characterizations, mechanistic analyses, and impressive device-level demonstrations, highlighting strong H evolution activity and freshwater production directly from natural seawater. The integration of photothermal enhancement with catalyst design is an important strength.\nHowever, the central claim of single-atom catalysis requires more careful justification. The experimental results suggest that the primary photocatalytic activity is largely derived from the in situ photodeposited Pt clusters, while the role of the Co single atoms and the hCN substrate remains less clearly distinguished. Theoretical analysis should be clarified to separate the contributions of Pt clusters, Co single atoms, and the carbon nitride support. Furthermore, some mechanistic interpretations (e.g., band center analysis, charge density differences, and spin localization) require stronger evidence and clearer presentation.\nOverall, while the work is promising and suitable for publication, I recommend major revision before acceptance. Addressing the mechanistic role of individual components, clarifying computational and experimental analyses, and improving figure annotations will significantly enhance the rigor and impact of this manuscript.\n1. Typo: There is a missing closing parenthesis starting with “(characteristic” in Line 112, Page 5:\n“The declining i1:i2 ratio (characteristic C atoms in N=C–N2 to N=C–N(NHx) from CoSA-CN to CoSA-hCN and ~”\n2. Please provide the isovalue of the charge density difference in Fig. 2, and reduce the isovalue to clearly identify the atomic sites of charge accumulation and depletion. Indicate in the caption:\n(a) what the colors in the isosurfaces represent, and\n(b) how the charge density difference was calculated (e.g., the difference between CoSA and CN).\n3. Since DFT often gives poor predictions of spin localization due to spin-delocalization error, please provide the spin density of CoSA-hCN to justify the asymmetric spin density of states shown in Fig. 2b, as well as to offer insights into its relationship with photocatalytic activity.\n4. For all theoretical calculations of adsorption energies (Fig. 3h and i) and the free energy diagram (Supplementary Fig. 30), please specify the active center or adsorption site, or include the optimized structures, to clearly indicate the adsorption configurations.\n5. The band center analysis in Supplementary Fig. 34 does not show a clear correlation with the adsorption strength of the Volmer intermediate (*H, Supplementary Fig. 30) or saline ions (Fig. 3h). According to the band center theory, the upshift or downshift of the metal valence (d-)band center near the Fermi level should affect the adsorption strength of intermediates. However, the provided d-band center, if it corresponds to the active metal center, does not correlate with H adsorption energy, while the p- and s-band centers are too far below the Fermi level to modulate the occupation of bonding and antibonding states.\nFurthermore, the band center analysis in Supplementary Fig. 34 should specify which element or species it refers to, and it is meaningful only if the band center of the active site (e.g., the Pt cluster) is considered. Since Supplementary Fig. 34 shows an undefined band center, the following statements appear contradictory:\n* (below the caption of Supplemetary Fig. 34)\n\"Among all models, CoSA-hCN displays the moderate band center positions, which is the most favorable alignment to support effective adsorption and dissociation of reactant species\"\n* (In the Structure-property correlation section)\n\"In CoSA-hCN, the total and projected Co d-orbital DOSs shift closer to the Fermi level, indicating stronger Co-N orbital interaction and greater charge transfer capability.\"\nIn addition, I recommend analyzing the Bader charge of the Pt cluster, since:\n(a) it is more meaningful for an isolated cluster than its band center, and\n(b) the authors have already mentioned efficient charge transfer in CoSA-hCN.\n6. In Fig. 3h and 3i, how did you define the adsorption energy of \"charged\" ions? Also, to avoid confusion, the y-axis in Fig. 3h should be Eads (eV) with signs consistent with Fig. 3i, rather than using the magnitude of adsorption energy.\n7. In the last sentences of \"Structure-property correlation\" section, the authors suggest a possible mechanism of photoelectron transfer, visualized in Fig. 2i and 5e. Please include the supporting density of states of Pt-cluster-hybridized CoSA-hCN to show the relative positions of projected DOS states of Pt, Co and CN.\n8. On Page 9, the authors report post-deposition Co K-edge XANES and state that the lower energy observed in CoSA-hCN indicates \"partial reduction of Co centers by photoexcited electrons\". Is it sufficient to claim localization of photoexcited electrons at Co centers, considering their limited lifetime?\n9. In the computational details, what does \"The bottom layers of about half of the structure\" mean? Isn't it a single layer structure of hCN? Moreover, the use of “about” is ambiguous and should be clarified.\n10. The caption of Supplementary Fig. 19 can be misleading, as it does not show the (theoretical) electronic band structure but the experimentally estimated band edge alignments.\n11. Although Co plays a crucial role in relaying photoexcited electrons, the main HER catalytic activity is attributed to the Pt cluster on CoSA-hCN. In this case, how can we describe it as a \"single-atom Co photocatalyst\"?", "reviewer_4": "In this work, the authors reported a carbon nitride–supported single-atom cobalt photocatalyst integrated into a floating photothermal device for simultaneous seawater splitting and desalination. The study presents extensive structural characterizations (including XAS and DFT) to support the claim that asymmetric Co–N₄ configurations with second-shell carbon vacancies enhance charge transfer and salinity tolerance. The device-level demonstration of hydrogen and freshwater co-production is ambitious, and the characterization of ion rejection mechanisms is a strength of the work.\nHowever, there are several concerns that limit the impact and clarity of this manuscript. Overall, while the manuscript demonstrates well-characterized materials and an integrated device concept, the mechanistic ambiguity, overstatement of “real-world” relevance, and insufficient benchmarking significantly weaken the impact. These issues should be thoroughly addressed before the manuscript can be considered for publication in Nature Communication.\nMajor Concerns\n1. Although the structural analysis of the catalysts is comprehensive, the manuscript lacks a clear mechanistic link between the observed atomic/electronic modifications and the improvements in hydrogen evolution and photothermal effects. The authors should clarify which structural factors are most important, and how this “molecular engineering single approach” provides a general strategy for improving performance.\n2. The reported heating of the CN-based material to ~65 °C (line 294) is unusual given that CN is not black. In the Methods section (line 515), the reactor is described as being constructed from black PTFE. It remains unclear whether the heating originates from the photothermal effect of the CoSA–CA catalyst itself or from the reactor substrate. This ambiguity raises concerns about the interpretation of photothermal contributions.\n3. The role of photothermal effects is not adequately explained. The authors should provide a mechanistic discussion of how the CoSA–CA structure contributes to photothermal enhancement and how this property translates into improved desalination and HER performance.\n4. The system relies on triethanolamine (TEOA) as a sacrificial electron donor. This is not sustainable and contradicts the claim that the device has been tested under “real-world” conditions. The authors should moderate their claims, as the use of TEOA fundamentally differentiates the system from practical overall water splitting.\n5. When comparing results with the literature, the authors do not distinguish between sacrificial hydrogen production and overall water-splitting systems. Since sacrificial systems typically exhibit higher HER rates, the current comparisons risk overstating the relative performance of the reported catalyst (Fig.5).\nIn addition, a few minor points should be addressed:\n1. It is unclear how the contact angle measurements in Fig. 3 were conducted (directly on powders, or on films drop-cast onto substrates). This should be clarified.\n2. In Supplementary Tables S6 and S7, systems using sacrificial agents should be clearly separated from overall water-splitting systems.\n3. The justification for designing the system specifically for seawater use requires more careful discussion. In practice, desalination contributes less than 5% of the overall energy cost of green hydrogen production, and the broader benefits of seawater utilization should be explained.", "reviewer_5": "This manuscript reports on molecular engineering of single atom catalysts in a scalable photothermal reactor for integrated seawater splitting and desalination. The authors constructed three single-atom Co configurations on CN: symmetric Co–N4 (CoSA-CN), vacancy-anchored Co–N3 (CoSA@hCN), and asymmetric Co–N4 with second-shell carbon vacancies (CoSA-hCN). The asymmetric Co–N4 in hierarchically porous carbon nitride (CoSA-hCN) introduces second-shell carbon vacancies that reshape electronic asymmetry and charge dynamics, establishing a robust pathway for effective salinity-mediated charge transfer and in situ Pt photodeposition for H2 evolution. On a scalable 60 cm2 floating photothermal platform with one sun, simultaneous H2 (47.7 mmol m−2 h−1) and freshwater (1.88 kg m−2 h−1) are produced directly from natural seawater. Given the significance of this work in advancing a generalizable design paradigm for integrated solar hydrogen production and seawater desalination, it merits publication in Nature Communications, a leading multidisciplinary journal, provided that the following comments are addressed thoroughly.\n1. What is the reason for choosing CoSA-hCN as the catalyst? I believe the main novelty of this work lies in the integration of a photothermal reactor for seawater splitting and desalination (ref. 20 in the manuscript), along with floatable systems for photocatalysis (ref. 33 in the Supplementary Information), which synergistically enhance both solar hydrogen production and seawater desalination. The novelty does not primarily reside in the catalyst design itself. Honestly, I believe the choice of photocatalyst is less important in this case than the integration of the two systems. Could the author expand the reactions with other photocatalysts (TiO2, ZnO, CdS….) with proper cocatalyst deposition, and compare the result with CoSA-hCN, to justify their choice of photocatalyst?\n2. While photothermal systems are properly referenced (refs. 20–25), floatable photocatalyst systems are rarely cited in the manuscript. The authors should properly reference recent developments in floatable photocatalyst systems and compare their merits with the state-of-the-art in the main manuscript.\n3. While the authors demonstrated that their system operates stably for 6 * 6-hour cycles, they did not provide in-depth characterization of the spent catalyst. Although they claim that the asymmetric Co–N₄ sites in CoSA-hCN introduce second-shell carbon vacancies that reshape electronic asymmetry and charge dynamics—establishing a robust pathway for effective salinity-mediated charge transfer—it remains questionable whether the initial active site structure is preserved during operation."}, "author_response": "RESPONSE TO REVIEWERS’ COMMENTS\nManuscript ID: NCOMMS-25-70370-T\nTitle: Molecular Engineering Single Atom Catalysts in a Scalable Photothermal Reactor for Integrated\nSeawater Splitting and Desalination\nResponses to Reviewer #1\nReviewer’s general comments: This study presents a combination of atomic-scale catalyst design and\nsystem-level integration for photocatalytic H evolution from seawater. The novelty lies in engineering\n2\nasymmetric Co–N single-atom sites on carbon nitride with second-shell carbon vacancies to modulate\n4\nelectronic asymmetry and salinity-mediated charge transfer. This work contributed to addressing two\nlong-standing hurdles simultaneously: realizing high catalytic activity in saline environments and\ncoupling hydrogen production with concurrent freshwater generation. The deployment on a scalable\nfloating photothermal platform under sunlight elevates the work from materials discovery to a technology\nconcept with practical relevance, suggesting pathways for modular, off-grid hydrogen supply in coastal\nor remote settings. To better support the claims and conclusions and improve the quality of this work,\nthe following questions are suggested to be addressed before considering it for publication.\nResponse to comments: We sincerely thank the reviewer for the constructive and encouraging\ncomments, as well as the positive recommendation. We have carefully evaluated all suggestions and\nrevised the manuscript accordingly. Each comment has been addressed, and the corresponding changes\nare highlighted in the revised manuscript.\nComment 1-1: Page 3, lines 87-88, Supplementary Fig. 6: TGA is not accurate enough to identify the\nCo loadings. As shown in the TGA curve, the adsorbed water may contribute to the mass loss during the\nheating process. To improve this data, ICP-AES/AAS is suggested to measure the Co loadings.\nResponse: We thank the reviewer for the comment on the limitation of TGA in Co loading investigations.\nIn response, we re-measured the Co contents of all samples using the recommended method of\ninductively coupled plasma-optical emission spectroscopy (ICP-OES) for more precise quantification.\nChanges: On page 3, last paragraph, the description of Co loading determination has been revised as\nfollows:\n“For comparison, Co atoms were anchored onto pre-formed hCN via a multi-step self-assembly and\npyrolysis method to produce CoSA@hCN (Fig. 1d). All samples show similar Co loadings, as\ndetermined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Supplementary\nFig. 7).”\nOn page 22, second paragraph, the use of ICP-OES was added to the section of Materials and Methods.\nSpectroscopy\n“The Co contents of catalysts were measured by ICP-OES (Agilent 5110) after autoclave digestion of\nthe samples.”\nSupplementary Information, page 8, the following figure and caption were added:\n1\n2.0\n%)\nwt 1.50 wt%\nn\n(1.5 1.38 wt% 1.41 wt%\no\nati\nntr\ne\nc1.0\nn\no\nC\nht\ng\nei0.5\nW\no\nC\n0.0\nCoSA-CN CoSA-hCN CoSA@hCN\nSupplementary Fig. 7 | Co concentration analysis. Inductively coupled plasma-optical emission\nspectroscopy (ICP-OES) analysis of CoSA-CN, CoSA-hCN, and CoSA@hCN after autoclave digestion.\nEach measurement was performed in triplicate, and the average Co weight concentration of each sample\nwas calculated.\nComment 1-2: It was claimed that carbon vacancies were formed in CoSA-hCN. Elemental analysis is\nnecessary to identify the composition of all three catalysts: CoSA-CN, CoSA@hCN, and CoSA-hCN,\nespecially the C/N ratio in these samples.\nResponse: Thanks for the great suggestion. To further verify the formation of C vacancies, we performed\nXPS analysis to quantify the atomic C/N ratios of all samples. The measurements show a clear decrease\nin the C/N ratio from CN to hCN, confirming carbon atom removal during the synthesis. For single-atom\ncatalysts, CoSA-hCN and CoSA@hCN exhibit a lower C/N ratio than CoSA-CN, further supporting the\nformation of carbon vacancies in these systems.\nChanges: On page 5, second paragraph, the following description of the C/N ratio using XPS was added:\n“X-ray photoelectron spectroscopy (XPS) reveals a notable decrease in the atomic C/N ratio in CoSA-\nCN and CoSA@hCN than CoSA-CN, confirming the formation of C vacancies (Supplementary Fig.\n10).”\nSupplementary Information, page 11, the following figure for C/N ratio was added:\nC 1s\nCoSA@hCN (C/N = 0.865)\n)\nu.\nb. CoSA-hCN (C/N = 0.873)\nr\na\n(\ny CoSA-CN (C/N = 0.926)\nsit\nn\ne\nnt\nhCN (C/N = 0.753)\nI\nCN (C/N = 0.981)\n200 250 300 350 400 450 500\nBinding energy (eV)\nSupplementary Fig. 10 | C/N ratio analysis. X-ray photoelectron spectroscopy (XPS) spectra of C 1s\nand N 1s of CN, hCN, CoSA-CN, CoSA@hCN and CoSA-hCN.\nXPS reveals that the atomic C/N ratio decreases from CN (0.981) to hCN (0.753), indicating carbon\nremoval during template processing. Among the single-atom catalysts, CoSA-hCN (0.873) and\nCoSA@hCN (0.865) exhibit lower C/N ratios than CoSA-CN (0.962), confirming the presence of\npronounced carbon vacancies in CoSA-hCN and CoSA@hCN.\nComment 1-3: What is a possible mechanism of forming Co-N with carbon vacancies via the silica-\n4\ntemplated one-step process?\nResponse: We thank the reviewer for the insightful question. We have now included a mechanism for\nthe formation of Co−N sites with adjacent carbon vacancies during the one-step pyrolysis. The\n4\n−Si−OH/−Si−O groups on the surface of SiO could react with C atoms in the heptazine framework\n2\nduring pyrolysis. Specifically, the localized −O/−OH species mildly oxidize C(N) sites to generate\n2 4\nschematic illustration have been added in Supplementary Fig. 6 and detailed in the Supplementary\nInformation.\nChanges: On page 3, last paragraph, the following description was added:\n“Accordingly, a one-step silica template-assisted pyrolysis method was used to induce vacancy formation\nand modulate the local electronic structure, thereby breaking the coordination symmetry of Co centers\nand producing CoSA-hCN (Fig. 1c and Supplementary Fig. 6).”\nSupplementary Information, page 7, the following figure of a possible mechanism for the formation of\nCo−N sites with adjacent carbon vacancies was added:\n4\nC\nduring the silica-templated pyrolysis process.\nDuring the condensation and polymerization of dicyandiamide into carbon nitride under the silica-\ntemplated environment, trace surface −OH/−O species on SiO react with adjacent carbon atoms in the\n2\nheptazine framework at elevated temperature. This mild oxidation generates CO/CO and leaves carbon\n2\nvacancies, which distort the local coordination environment of the embedded Co centers. After template\nremoval, these vacancies stabilize asymmetric Co−N configurations of CoSA-hCN.\nlevel (EF). A work function from UPS/secondary-electron cutoff or Kelvin probe is required to place the\nbands on an absolute (vacuum) scale and convert them to SHE/NHE.\nResponse: We thank the reviewer for this professional and insightful comment. We fully agree that VB-\nXPS provides only the relative VB maximum positions with respect to the Fermi level, and absolute band\npositions require work function measurements. In our work, the VB-XPS results were used to determine\nthe relative VBM differences among different samples (Supplementary Fig. 21). Following the\nreviewer’s suggestion, we used UPS to determine the valence band energy of a representative sample\n(CN, E = 2.01 eV, Supplementary Fig. 22) on an absolute vacuum scale (ref. to Nature Energy 2021,\nVB\n6, 388-397). The E was calculated from the secondary-electron cutoff and Fermi edge using E = hν\nVB VB\n+ E − E , and converted to the NHE scale via E = −E − 4.44 eV. Relative VBM shifts\nFermi cutoff vacuum NHE\n4\nobtained from the VB-XPS were then applied to determine E for the remaining samples. Finally, their\nVB\nCB positions were determined using the optical bandgap values extracted from UV-vis DRS, enabling\nthe construction of a consistent band alignment diagram for all samples, as shown in Supplementary\nFig. 23.\nChanges: On page 9, first paragraph, the following description has been revised:\n“The electronic band structures were evaluated using UV-vis diffuse reflectance spectroscopy (DRS,\nSupplementary Fig. 20), valence band (VB) XPS to determine VBM differences (Supplementary Fig.\n21) and ultraviolet photoelectron spectroscopy (UPS) to extract the absolute valence band position\n(Supplementary Fig. 22). These measurements show that CoSA-hCN possesses the most negative\nconduction band potential, favoring 2H+/H photoreduction (Supplementary Fig. 23).”\n“The VB-XPS and UPS spectra were obtained using a Thermo Fisher Scientific ESCALAB Xi\nspectrometer.”\nSupplementary Information, page 23, the following figure for UPS of CN was added:\nSupplementary Fig. 22 | Ultraviolet photoelectron spectroscopy (UPS) spectrum of CN. a, secondary\nelectron cutoff (Ecutoff); b, Linear extrapolation used to determine the E .\nFermi\nUPS was employed to determine the ionization potential,7 corresponding to the valence band energy (E )\nVB\nof the samples. Using linear extrapolation, the E of the representive CN sample was calculated as −6.45\nVB\neV (vs. vacuum) based on:\nE = hv + E − E\nVB Fermi cutoff\nWhere, hν = 21.22 eV (He I source), E = 4.07 eV, and E = 18.84 eV. The E value was converted\nFermi cutoff VB\nto the normal hydrogen electrode (NHE) scale according to the relationship between the vacuum energy\n(E ) and the NHE potential (E ),\nvacuum NHE\nvacuum NHE\nAccordingly, the valence band maximum of CoSA-hCN was determined to be +2.01 V (vs. NHE). VBM\noffsets from VB-XPS were used to determine E for hCN, CoSA-CN, CoSA-hCN, CoSA@hCN.\nVB\nCombining these values with optical bandgap from UV-vis diffuse reflectance spectra allowed\ndetermination of the conduction band positions.\nSupplementary Information, page 24, the following band structure alignments have been revised:\nSupplementary Fig. 23 | Band structure alignments for CN, hCN, CoSA-CN, CoSA@hCN and CoSA-\nhCN.\nSupplementary Information References:\nZhao, D. et al. Boron-doped nitrogen-deficient carbon nitride-based Z-scheme heterostructures for\nphotocatalytic overall water splitting. Nat. Energy 6, 388–397 (2021).\nComment 1-5: Page 7, Fig. 2g, lines 223-226: the intensities of Pt peaks from different catalysts vary\nsignificantly. What is the exact loading amount of Pt on each catalyst? What is the particle size of Pt on\nthese catalysts? As the Pt nanoparticles act as active sites for the H evolution reaction, the particle size\n2\nand loading amount of Pt nanoparticles on photocatalysts play an important role in the H evolution rate.\nResponse: We thank the reviewer for this important comment on the Pt nanoparticles. To quantify the\nPt loading, we performed ICP-OES of SAC samples after Pt photodeposition (CoSA-CN: 3.86 wt%;\nCoSA-hCN: 4.08 wt%; CoSA@hCN: 3.74 wt%). We also performed the transmission electron\nmicroscopy (TEM) analyses to further confirm the uniform dispersion of Pt nanoparticles on the catalyst\nsurfaces. The Pt nanoparticles are uniformly distributed on both the CoSA-hCN and CoSA@hCN\nframeworks, with sizes predominantly < 5 nm, whereas CoSA-CN shows a broader size distribution with\na noticeable fraction of particles larger than 10 nm. These observations indicate that the hierarchical\nporous structures of CoSA-hCN and CoSA@hCN promote more effective anchoring and controlled Pt\nnucleation during photodeposition.\n“The presence of Pt nanoparticles was verified by HAADF-STEM and EDX mappings (Supplementary\nFig. 24 and inset in Fig. 2f). ICP-OES analysis indicates comparable Pt loadings across the three catalysts,\nwith CoSA-hCN exhibiting the highest Pt loading at 4.08 wt% (Supplementary Fig. 25a). This indicates\nmore favorable Pt photodeposition on CoSA-hCN. TEM images show that the Pt nanoparticles on CoSA-\nhCN and CoSA@hCN are more evenly dispersed and exhibit a narrower size distribution, typically\nbelow 5 nm, whereas Pt on CoSA-CN presents less uniform dispersion and includes larger aggregates\n(Supplementary Fig. 25b-d), reflecting the distinct structural and electronic characteristics of the\nsupports.”\nSupplementary Information, page 26, the following figure for Pt nanoparticle analysis was added:\nSupplementary Fig. 25 | Pt nanoparticle cocatalysts analysis. a, ICP-OES analysis of CoSA-CN,\nCoSA-hCN, and CoSA@hCN after 1 h Pt photodeposition under visible light irradiation. Each\nmeasurement was performed in triplicate, and the average Pt weight concentration of each sample was\ncalculated. TEM image of b, CoSA-CN, c, CoSA-hCN, and d, CoSA@hCN after 1 h Pt photodeposition\nunder visible light irradiation. Pt nanoparticles on CoSA-hCN and CoSA@hCN exhibit a more uniform\n7\nsize distribution (typically < 5 nm) and more homogeneous dispersion than those on CoSA-CN,\nconsistent with the structural differences of the carbon nitride supports.\nComment 1-6: Page 9, lines 223-228: “It was claimed that the coexistence of Pt0 and Pt2+ species implies\na dual functional role…….” Is it possible that the Pt0 in photo-deposited Pt nanoparticles is partially\noxidized to Pt2+ when the sample is exposed to air?\nResponse: We appreciate the reviewer’s very thoughtful comment. While some surface oxidation of Pt0\nupon air exposure is possible in principle, the coexistence of Pt0 and Pt2+ observed here is more\nreasonably explained by intrinsic features of the photodeposition process and the catalyst-support\ninteraction: (i) incomplete reduction of the Pt4+ precursor (H PtCl ) under the photodeposition conditions,\n2 6\nwhich can leave a fraction of Pt in the Pt2+ state; and (ii) strong metal-support interactions between Pt\nand CoSA-hCN support that stabilize the coexistence of Pt0 and Pt2+ states via interfacial charge transfer.\nSimilar dual-state Pt0/Pt2+ systems on carbon nitride have been reported (ChemSusChem 2019, 12, 2029-\n2034), where Pt0 acts as the proton-reduction site while Pt2+ suppresses the undesired H backward\n2\nreaction. Our findings further demonstrate that engineering the Co-N coordination environment provides\nan additional means to modulate Pt nanoparticle formation and stabilize the functional Pt0/Pt2+ hybrid\nstate.\nChanges: On page 10, first paragraph, the description of H PtCl photoreduction has been revised:\n2 6\n“The Pt 4f spectra of CoSA-hCN after photodeposition display distinct peaks at 73.0 and 76.3 eV for\nPt2+ species from partial reduction of H PtCl , along with sub-peaks at 70.8 and 74.6 eV for metallic Pt0\n2 6\n(Fig. 2g). The coexistence of Pt0 and Pt2+ species implies a dual functional role, where Pt0 facilitates\nproton reduction and Pt2+ prevents the undesirable H backward reaction44.”\nXing, W. et al. Anchoring Active Pt2+/Pt0 Hybrid Nanodots on g-C N Nitrogen Vacancies for\n3 4\nPhotocatalytic H Evolution. ChemSusChem 12, 2029–2034 (2019).\nin the absorption edge of CoSA-hCN toward lower energy, indicating partial reduction of Co centers by\nphotoexcited electrons (Fig. 2h).” Is this phenomenon observed on CoSA-hCN only? How about other\ncatalysts after Pt deposition?\nResponse: We thank the reviewer for this important comment. We sincerely apologize for the unclear\ndescription, which may have caused misunderstanding regarding the Co electronic changes during\noperation. We realized that the influence of Pt photodeposition on the Co electronic structure was not\nsufficiently considered in our original manuscript. To clarify this, we conducted additional\nCo K-edge XANES measurements for CoSA-hCN under visible light irradiation in pure water with H PtCl ,\n2 6\nand\nthen probed charge transfer at Co sites during operation in the presence of TEOA. We also conducted\nmeasurements for CoSA-CN and CoSA@hCN with the addition of H PtCl under light irradiation in\n2 6\npure water for comparison. In pure water, all catalysts show a slight edge shift toward higher energy,\nreflecting mild oxidation of Co sites as electrons are transferred for Pt nucleation. This behavior is\ncommon to all samples. When TEOA is introduced, CoSA-hCN instead exhibits a small shift toward\n8\nlower energy, which arises from the reductive action of TEOA. These additional measurements provide\na clearer and more accurate description of the evolution of Co centers during Pt photodeposition and\nphotocatalysis, which has been incorporated into the revised manuscript.\nChanges: On page 10, last paragraph, the discussion of charge transfer on Co sites has been revised:\n“The charge transfer on Co sites was also investigated. Post-deposition Co K-edge XANES spectra were\nre-examined in pure water containing H PtCl under visible light irradiation. All catalysts display a slight\n2 6\nedge shift toward higher energy, consistent with electron transfer from Co centers to reduce Pt4+ during\nPt nucleation (Fig. 2h and Supplementary Fig. 32). With the addition of TEOA after reaction, CoSA-\nhCN shows a small shift toward lower energy, reflecting the reductive action of TEOA on the Co sites\n(Fig. 2h).”\nFig. 2 | Electronic structures and functions investigation. h, Co K-edge XANES of CoSA-hCN before\nand after operation with the addition of H PtCl and TEOA under visible light irradiation at 25 °C.\n2 6\nSupplementary Information, page 33, the figure for the Co K-edge XANES spectra of post-reacted\nsamples was added:\nK-edge XANES spectra of all catalysts after Pt photodeposition. Co K-edge XANES spectra of b, CoSA-\nCN, c, CoSA@hCN, and d, CoSA-hCN before and after Pt photodeposition performed in pure water\nwith the addition of H PtCl under visible light irradiation. All three catalysts exhibit a slight edge shift\n2 6\ntoward higher energy after Pt photodeposition, indicating electron transfer from Co centers to reduce Pt4+.\nComment 1-8: Material synthesis procedure: What is the pH value of 4 mol/L NH HF ? Is it a strong\n4 2\nacid (pH <1) that can etch the Co species from CoSA-hCN?\nResponse: We thank the reviewer for this important question. The pH value of 4 M NH HF solution\n4 2\nwas measured to be approximately 4.5 ± 0.3, indicating that it is weakly acidic rather than strongly acidic.\nThe mild acidity may remove a small fraction of unstable or weakly bound surface Co species; however,\nthe majority of Co single atoms remain coordinated with nitrogen and are stably anchored within the\ncarbon nitride framework. To clarify this point, the measured pH value of the NH HF solution has been\n4 2\nadded into the section of Materials and Methods.\nChanges: On page 20, third paragraph, the pH of 4 mol L–1 NH HF solution was added:\n4 2\n“The resulting product was etched with 50 mL of a 4 mol L–1 NH HF solution (pH of 4.5 ± 0.3) for 48\n4 2\nh to remove the silica template.”\nOn page 21, first paragraph.\n“The product was etched with 50 mL of 4 M NH HF solution (pH of 4.5 ± 0.3) for 48 h to remove the\n4 2\nsilica template.”\nComment 1-9: The catalysts CoSA-CN, CoSA@hCN, and CoSA-hCN with different Co coordination\nwere synthesized via method engineering, which showed remarkably different photocatalytic H\n2\nevolution activity. Material characterizations demonstrated that multiple structural features and\nproperties of these materials were distinct, including surface area, surface hydrophilicity, zeta potential,\nconductivity, charge transfer, Pt loading (maybe amount and particle size), and band alignment\n(bandgap and band position). What is the key/major factor contributing to the enhanced H evolution rate of CoSA-\n2\nhCN in pure water and seawater, respectively? For instance, the surface area of CoSA-hCN is three times\nthat of CoSA-CN, and the H evolution rate of CoSA-hCN is around 4-fold higher than that of CoSA-\nproperties. The dominant contribution comes from the asymmetric Co–N\n4\ncoordination, which induces\nlocal charge polarization, enhances electron delocalization, and promotes fast electron transfer from Co\nsites into the CN framework. This facilitates more efficient charge separation and enables highly effective\nin situ Pt photodeposition, resulting in stronger synergy between the single-atom Co sites and Pt\ncocatalysts during H evolution.\n2\nimproved hydrophilicity, which mainly enhances the exposure of active Co sites and Pt cocatalyst\ndispersion, rather than directly determining the catalytic rate. Notably, hCN without Co loading shows a\nlarger surface area but a much lower H evolution rate, confirming that the improved performance of\nmodulation rather than the surface area or other physicochemical properties alone.\nChanges: On page 11, second paragraph, the following structure-activity relationship has been added:\n“Among all catalysts, CoSA-hCN exhibited the highest H production rate of 6.4 mmol g–1 h–1 in artificial\n2\nseawater (ASTM D1141-98 (2013) standard, Supplementary Table 5), which is 4- and 3-fold higher\nthan that of CoSA-CN (1.6 mmol g–1 h–1) and CoSA@hCN (2.2 mmol g–1 h–1), respectively. As discussed\nabove, this enhanced activity primarily arises from the asymmetric Co–N coordination and vacancy-\n4\ninduced electronic modulation, which strengthen charge separation and promote efficient in situ Pt\nphotodeposition, while physicochemical improvements such as higher surface area provide additional\ncontributions.”\nReviewer’s general comments: This work presents a floating photothermal platform designed for the\nsimultaneous production of hydrogen and freshwater from seawater. The approach is innovative,\nleveraging single-atom cobalt configurations on carbon nitride photocatalysts applied onto foam supports.\nThe rational design of the catalyst, supported by detailed characterizations, clearly demonstrates the\noriginality and significance of the work. The manuscript is well written, and the conclusions are\nappropriately supported by the presented data. However, several important aspects related to the practical\nimplementation and long-term stability of the platform need further clarification and discussion.\nResponse to comments: We sincerely thank the reviewer for the positive and encouraging comments on\nthe originality, design, and clarity of our manuscript. We appreciate the reviewer’s insightful recognition\nof the innovation in developing a floating photothermal platform for simultaneous hydrogen and\nfreshwater production. In response to the reviewer’s valuable suggestions regarding the practical\nimplementation and long-term stability of the system, we have provided additional experimental\nevidence and discussion in the revised manuscript.\nComment 2-1: Stability of the active layer: The long-term stability of the photothermal layer on the\nmelamine foam raises concerns, especially under prolonged operation in real seawater environments.\nThe possibility of catalyst leaching and the downward migration of active materials should be more\nthoroughly investigated. The authors are encouraged to provide quantitative data or estimates on material\nloss over time, if available.\nResponse: We thank the reviewer for this valuable comment regarding the structural stability of the\nphotothermal active layer. To evaluate the robustness of the coating, we recorded photographs of the\ncomposite before and after photocatalytic operation (Supplementary Fig. 61). From the top view, the\nactive layer in yellow remained complete and clearly visible after the reaction. The side view further\nshows that the active layer was still well attached to the melamine foam without any noticeable downward\nmigration or detachment. To quantitatively evaluate material loss, we collected and weighed the residual\nsolids from seawater after operation. Only 4.2 mg of catalyst was recovered, compared with the initial\n~80 mg loaded onto the sponge (Supplementary Fig. 63). These observations confirm that the\nphotothermal coating exhibits excellent structural and interfacial stability during prolonged seawater-\nsplitting operation.\nafter 6-h operation. Slight downward migration of the catalyst was observed during operation due to\nwater permeation, but most catalysts remained concentrated near the surface.\nSupplementary Fig. 63 | Illustration of the stability of the sponge-photoreactor. Photos of the device\nafter the operation. The collected catalyst solid was approximately 4.2 mg from the seawater residues\n(initial ~80 mg catalysts loaded onto the sponge). The sponge-reactor was inverted after each run to\nenable gravity-assisted re-accumulation of the catalysts at the active interface.\nComment 2-2: Interaction Between Catalyst and Support: It remains unclear whether there is any\nchemical or physical interaction between the melamine foam and the catalyst layer. A discussion on this\npoint, supported by surface characterization or adhesion tests, would strengthen the understanding of the\nstructural integrity of the composite system.\nResponse: We appreciate the reviewer’s valuable suggestion regarding the interaction between the\ncatalyst layer and the melamine foam support. The strong adhesion primarily arises from physical\ninterlocking and surface interactions rather than chemical bonding. The melamine foam possesses an\nopen-cell porous network with abundant –N–H and –C–H groups (Supplementary Fig. 51), which\nfacilitates tight anchoring of the catalyst through capillary infiltration during impregnation and mild van\nder Waals/hydrogen-bond interactions with the TEOA-assisted precursor.\nTo experimentally assess the adhesion, we conducted a mechanical stability test (as shown in\nSupplementary Fig. 52) in which the catalyst-loaded sponge was placed upside down and slid across a\nflat paper surface under increasing applied weights. No detectable catalyst shedding or visible abrasion\noccurred, demonstrating that the coating remains firmly attached even under external mechanical stress.\nChanges: On page 15, first paragraph, the discussion of the structural integrity and mechanical stability\nwas added:\n“Photocatalysts were immobilized onto the upper layer via a simple dip-coating method (Fig. 4c and\nSupplementary Fig. 49-50). The melamine foam contains abundant N–H and C–H functional groups\nthat provide favorable sites for anchoring CoSA-hCN during impregnation (Supplementary Fig. 51). A\nmechanical stability test demonstrated the robust interfacial bonding between the photocatalyst and the\nmelamine support (Supplementary Fig. 52).”\nSupplementary Information, pages 52, the surface chemistry of sponge photoreactor was added:\nfoam before dipping photocatalysts and CoSA-hCN-loaded sponge photoreactor.\nSupplementary Information, page 53, the following figure for mechanical stability tests was added:\nSupplementary Fig. 52 | Mechanical adhesion tests of CoSA-hCN-loaded sponge. Photographs\nshowing the mechanical adhesion tests of the CoSA-hCN-loaded sponge. The sample was placed upside\ndown on an A4 paper surface and gently slid under different applied loads (metal weights). a, 132.23 g\nand b, 338.19 g metal weights. No visible detachment or powder shedding was observed, confirming the\ngreat physical adhesion and structural integrity of the catalyst coating on the sponge framework.\nComment 2-3: Gravity-Assisted Re-Accumulation Strategy: The authors mention a gravity-assisted re-\naccumulation mechanism to mitigate catalyst loss. However, the practical feasibility of this approach\nunder real operational conditions is questionable. Further elaboration is required to assess how effective\nand scalable this strategy would be in a realistic application scenario, especially in open water\nenvironments.\nthe gravity-assisted re-accumulation method. In our laboratory-scale setup, after each run, the floating\nreactor was removed from water and gently inverted to allow excess seawater to drain and the catalyst\nlayer to redistribute uniformly under gravity. This simple step effectively prevents local detachment or\naccumulation of catalyst particles, maintaining a uniform and intact active layer throughout repeated\ncycles. In a realistic, larger-scale deployment, this principle can be translated into practical engineering\nsolutions. For example, floating photothermal platforms can incorporate slow, periodic tilting,\nbuoyancy-driven rocking, or low-speed rotational motion induced by natural wave activity. These mechanisms\nwould allow the catalyst layer to redistribute passively without manual intervention. Such design\nelements are widely used in floating environmental and solar devices and can be integrated into modular\nplatforms to maintain long-term coating uniformity and minimize material loss. Thus, while simple at\nthe laboratory scale, the gravity-assisted strategy is directly scalable through passive mechanical design\nfeatures in open-water environments.\nChanges: On page 17, last paragraph, the discussion of using the gravity-assisted re-accumulation\napproach has been added:\n“During cycling tests, catalyst retention at the sponge surface was maintained via gravity-assisted\nredistribution. This concept can be adapted for large-scale floating systems by incorporating passive\ntilting or slow rotational movement, allowing the catalyst layer to self-redistribute under operational\nconditions in open-water environments.”\nComment 2-4: Salt Fouling and Long-Term Operation: Salt accumulation or fouling is a major concern\nfor any system operating with seawater. The manuscript should include a discussion on how the platform\ndesign addresses this issue, especially regarding the impact of salt crystallization on both photothermal\nperformance and catalyst activity over extended use.\nResponse: We thank the reviewer for raising the concern regarding salt fouling. In our floating sponge\nreactor, salt crystallization is largely mitigated by the continuous capillary flow of seawater, which keeps\nthe catalyst layer fully wetted during operation and prevents salt accumulation. As shown in\nSupplementary Fig. 61, no visible salt deposition is observed on the surface under operating (wet)\nconditions. When the reactor is completely dried (Supplementary Fig. 62), a small amount of salt\ncrystallites can indeed appear on the surface and within the porous structure due to natural evaporation.\nHowever, these residues form only after drying and are not present during actual operation and therefore\ndo not influence the catalytic performance within the tested cycling duration.\nafter 6-h operation. Slight downward migration of the catalyst was observed during operation due to\nwater permeation, but most catalysts remained concentrated near the surface.\nChanges: Supplementary Information, page 63, the following figure for salt crystallization was added:\nSupplementary Fig. 62 | Salt crystallization behavior of the sponge-based photoreactor. Digital\nphotographs of fully dried catalyst-loaded sponges before reaction, after operation in pure water, and in\nseawater. Minor salt crystallites are observed on the surface, lateral sides, and internal cross-sectional\nregions only after complete drying, whereas no visible salt accumulation occurs under wet operating\nconditions.\nTo further examine the salt-fouling behavior, we compared dried sponge samples before reaction and\nafter operation in pure water and seawater. As shown in Supplementary Fig. 62, slight salt crystallization\nappears on the surface, lateral sides, and within the internal porous cross-section only after complete\ndrying, originating from natural evaporation of residual seawater. Importantly, these deposits are absent\n16\nunder wet operating conditions (Supplementary Fig. 61), where continuous capillary water transport\nkeeps the catalyst layer fully hydrated and prevents salt accumulation.\nFinal Comment: Overall, the study is highly promising and contributes meaningfully to the field.\nAddressing the points above would greatly improve the manuscript and provide a more comprehensive\nunderstanding of the system's robustness and scalability for real-world applications.\nResponse: We sincerely thank the reviewer for their positive and encouraging remarks. We are delighted\nthat the study is viewed as both promising and impactful. We have carefully addressed all the comments\nraised, which have helped us significantly improve the clarity, robustness, and completeness of the\nmanuscript. We greatly appreciate the reviewer’s constructive input, which has helped improve the\noverall quality of this work.\nReviewer’s general comments: The manuscript presents an interesting study on molecularly engineered\nsingle-atom Co catalysts integrated into a scalable photothermal reactor for seawater splitting and\ndesalination. The authors provide a well-designed system with comprehensive structural\ncharacterizations, mechanistic analyses, and impressive device-level demonstrations, highlighting strong\nH evolution activity and freshwater production directly from natural seawater. The integration of\n2\nphotothermal enhancement with catalyst design is an important strength.\nHowever, the central claim of single-atom catalysis requires more careful justification. The experimental\nresults suggest that the primary photocatalytic activity is largely derived from the in situ photodeposited\nPt clusters, while the role of the Co single atoms and the hCN substrate remains less clearly distinguished.\nTheoretical analysis should be clarified to separate the contributions of Pt clusters, Co single atoms, and\nthe carbon nitride support. Furthermore, some mechanistic interpretations (e.g., band center analysis,\ncharge density differences, and spin localization) require stronger evidence and clearer presentation.\nOverall, while the work is promising and suitable for publication, I recommend major revision before\nacceptance. Addressing the mechanistic role of individual components, clarifying computational and\nexperimental analyses, and improving figure annotations will significantly enhance the rigor and impact\nof this manuscript.\nResponse: We sincerely thank the reviewer for the constructive and encouraging assessment. We\nespecially appreciate the professional advice and insightful comments from the aspect of theoretical\ncalculation, which have greatly helped us refine the mechanistic interpretation. All comments have been\ncarefully addressed, and the manuscript has undergone thorough revision to enhance clarity and rigor. In\nparticular, we expanded both experimental and computational analyses to clarify the respective roles of\nPt clusters, Co single atoms, and the carbon nitride support. These revisions substantially strengthen the\nmechanistic discussion and enhance the overall robustness of the work.\nComment 3-1: Typo: There is a missing closing parenthesis starting with “(characteristic” in Line 112,\nPage 5: “The declining i1:i2 ratio (characteristic C atoms in N=C–N2 to N=C–N(NHx) from CoSA-CN\nto CoSA-hCN and CoSA@hCN”\nResponse: We thank the reviewer for carefully identifying the typographical errors. The missing closing\nparenthesis in the sentence has been corrected in the revised manuscript.\nChanges: On page 5, second paragraph, the missing closing parenthesis was added:\n“The declining i : i ratio (characteristic C atoms in N=C–N to N=C–N(NH ) from CoSA-CN to CoSA-\n1 2 2 x\nhCN and CoSA@hCN) evidences the loss of C atoms in the N=C–N motif.”\nisovalue to clearly identify the atomic sites of charge accumulation and depletion. Indicate in the caption:\n(a) what the colors in the isosurfaces represent, and\n(b) how the charge density difference was calculated (e.g., the difference between CoSA and CN).\nResponse: We appreciate the reviewer’s suggestion regarding the charge density difference analysis. In\nFig. 2a, the charge density difference is visualized using an isovalue of 0.001 e Bohr–3, which is a widely\n18\nadopted threshold in DFT studies of heterogeneous and single-atom catalysts. This value provides an\noptimal balance between suppressing numerical noise and preserving physically meaningful charge\naccumulation/depletion features, as commonly used in the literature (e.g., Nat. Commun. 2023, 14, 6994;\nNat. Commun. 2022, 13, 2430; J. Am. Chem. Soc. 2018, 140, 12469-12475). To clearly identify the\nspecific atomic sites involved in charge redistribution, we have added atom labels directly onto the charge\ndensity difference maps (Supplementary Fig. 15). In response to comment 3-2a, the yellow and cyan\nisosurfaces represented charge accumulation and depletion, respectively. We have also updated the figure\ncaption accordingly. In response to comment 3-2b, the detailed calculation method of charge density\ndifference has been added into the section of Materials and Methods.\nChanges: On page 8, the clarification of isosurface have been included:\nFig. 2 | Electronic structures and functions investigation.\na, Charge density difference of CoSA-CN, CoSA-hCN, and CoSA@hCN. The isosurface levels are 0.001\ne Bohr–3. The yellow and cyan isosurfaces represent charge accumulation and depletion, respectively.\nOn page 25, first paragraph, detailed calculation method of charge density difference has been added:\n“The charge density difference (CDD) was calculated to visualize the electronic redistribution upon\nformation of the Co−N coordination environment. The CDD is defined as:\nΔρ(r) = ρCoSA + CN (r) − ρCoSA(r) − ρCN(r)\nWhere, ρCoSA+CN denotes the charge density of the combined Co single-atom structure, and ρCoSA\nand ρCN represent the charge densities of the isolated Co atom and the surrounding N coordination\nenvironment, respectively, each computed in the same geometric configuration as in the combined system.\nThis definition highlights the intrinsic charge redistribution between the metal center and its coordinated\nligands. The isosurface was visualized at an isovalue of 0.001 e Bohr−3, which provides a clear yet\nphysically meaningful depiction of charge transfer regions. In the plotted isosurfaces, yellow regions\ndenote electron accumulation (Δρ > 0), while cyan regions correspond to electron depletion (Δρ < 0).\nThus, the color contrast intuitively reflects the direction and spatial extent of charge transfer between the\nCN adsorbate and the Co active site.”\nSupplementary Information, page 16, the atoms in the charge density difference maps have been labeled\naccordingly.\n(DFT) calculations. a, CN; b, CoSA-CN; c, CoSA-hCN; d, CoSA@hCN. Charge density difference maps\nwith labeled atoms are shown for CoSA-CN, CoSA-hCN, and CoSA@hCN.\nComment 3-3: Since DFT often gives poor predictions of spin localization due to spin-delocalization\nerror, please provide the spin density of CoSA-hCN to justify the asymmetric spin density of states shown\nin Fig. 2b, as well as to offer insights into its relationship with photocatalytic activity.\nResponse: We thank the reviewer for this insightful suggestion. To validate the asymmetric spin\ndistribution and support the discussion in Fig. 2b, we have now included the spin-density map of CoSA-\nhCN (Supplementary Fig. 16). The spin density was calculated using spin-polarized DFT under the\nsame computational parameters as in the main text. The isosurface of the spin density was plotted at\n0.001 e Bohr–3, where yellow and cyan regions represent spin-up and spin-down electron localization,\nrespectively. The result clearly shows spin localization around the Co center and adjacent N atoms,\ncorroborating the proposed electronic asymmetry relevant to photocatalytic activity.\nChanges: On page 7, first paragraph, the following discussion on spin density plot has been added:\n“Interestingly, the projected DOS appears symmetric in CoSA-CN and CoSA@hCN, suggesting weak\nhybridization,37 while CoSA-hCN displays an irregular and asymmetric line shape, reflecting stronger\norbital hybridization, which facilitates charge delocalization and charge transfer. The spin density plot\n(Supplementary Fig. 16) further reveals localized spin polarization on the Co atom and neighboring N\nsites. This confirms that the asymmetric coordination environment induces a localized unpaired electron\ndistribution, which may facilitate spin-selective charge transfer and thereby enhance the photocatalytic\nactivity of CoSA-hCN.”\nSupplementary Information, page 16, spin density plot has been added:\nisovalue of 0.001 e Bohr–3. Yellow and cyan regions correspond to spin-up and spin-down electron\ndensities, respectively.\nComment 3-4: For all theoretical calculations of adsorption energies (Fig. 3h and i) and the free energy\ndiagram (Supplementary Fig. 30), please specify the active center or adsorption site, or include the\noptimized structures, to clearly indicate the adsorption configurations.\nResponse: We thank the reviewer for this helpful suggestion. The adsorption configurations (optimized\ntheoretical models) for all theoretical calculations, including adsorption energy and free energy diagrams,\nhave been provided.\nChanges: On page 13: The description of optimized theoretical models has been added to the caption of\nFigure 3:\nh, Calculated adsorption energies of Na* and Cl* of CoSA-CN, CoSA-hCN and CoSA@hCN. The\nadsorption configurations (optimized theoretical models) can be found in Supplementary Fig. 44. i,\nCalculated adsorption energies of Na*, Mg*, K*, Ca* towards Cl* of CoSA-CN and CoSA-hCN. Neutral\nNa, Mg, K, Ca and Cl atoms were used in the DFT calculations. The ionic labels represent cation-like or\nanion-like adsorption behavior, rather than the formal ionic charge of the adsorbate. The adsorption\nconfigurations (optimized theoretical models) can be found in Supplementary Fig. 45.\nSupplementary Information, page 45, theoretical models for Na and Cl adsorption have been added:\nwith adsorbed Na and Cl atoms. Neutral Na and Cl atoms were used in the DFT calculations. In the\ncorresponding discussion in the manuscript, the ionic labels represent cation-like or anion-like adsorption\nbehavior, rather than the formal ionic charge of the adsorbate.\nSupplementary Information, page 46, theoretical models for seawater ions adsorption have been added:\nSupplementary Fig. 45 | Optimized theoretical models for CoSA-CN and CoSA-hCN with adsorbed\nNa, Mg, K, Ca and Cl atoms. Neutral Na, Mg, K, Ca and Cl atoms were used in the DFT calculations.\nIn the corresponding discussion in the manuscript, the ionic labels represent cation-like or anion-like\nadsorption behavior, rather than the formal ionic charge of the adsorbate.\nSupplementary Fig. 40 | Theoretical calculations for the intrinsic H evolution capabilities of CoSA-\nH*\nhydrogen evolution reaction on catalyst surface (Optimized theoretical models were provided). The H\natom adsorbs at the Co single-atom site, and the calculated ΔG reflects the thermodynamic feasibility\nH*\nof the hydrogen evolution reaction on each catalyst surface.\nThe intrinsic H evolution capabilities of different catalysts were evaluated by the Gibbs free energy\n2\ncalculation for the adsorbed hydrogen atom (ΔG , Supplementary Fig. 40). CoSA-hCN shows a\nH*\nsmaller ΔG of 0.32 eV compared to CoSA-CN (0.51 eV) and CoSA@hCN (0.53 eV), indicating more\nH*\nthermodynamically favorable H* adsorption and superior intrinsic activity for proton adsorption and H\n2\nformation.\nComment 3-5: The band center analysis in Supplementary Fig. 34 does not show a clear correlation with\nthe adsorption strength of the Volmer intermediate (*H, Supplementary Fig. 30) or saline ions (Fig. 3h).\nAccording to the band center theory, the upshift or downshift of the metal valence (d-)band center near\nthe Fermi level should affect the adsorption strength of intermediates. However, the provided d-band\ncenter, if it corresponds to the active metal center, does not correlate with H adsorption energy, while the\np- and s-band centers are too far below the Fermi level to modulate the occupation of bonding and\nantibonding states.\nResponse: We appreciate the reviewer’s careful assessment of this point. In our study, the presented s-,\np-, and d-band centers were derived from the total density of states of the entire catalyst system, while\nthe d-band contribution originates exclusively from the single-atom Co sites, which are the active centers\nfor H adsorption. We also thank the reviewer’s observation regarding the correlation between the d-band\ncenter and H adsorption strength. As pointed out in a previous work by G. Pacchioni et al. (ACS Catal.\n2022, 12, 5846-5856), the conventional d-band center model, although broadly effective for extended\nmetal surfaces, does not always apply quantitatively to isolated single-atom catalysts, where discrete\n23\nenergy levels, coordination asymmetry, and charge redistribution govern the adsorption behavior. We\nagree that a more comprehensive correlation analysis combining theoretical descriptors with the intrinsic\ncatalytic behavior from a computational aspect would further deepen the mechanistic understanding of\nsingle-atom catalysts in photocatalytic hydrogen evolution. This aspect will be further explored in our\nfuture investigations.\nChanges: Supplementary Information, page 41, the description and discussion about the DFT-calculated\nGibbs free energy profiles for hydrogen adsorption have been revised:\nSupplementary Fig. 40 | Theoretical calculations for the intrinsic H evolution capabilities of CoSA-\nH*\nhydrogen evolution reaction on catalyst surface (Optimized theoretical models were provided). The H\natom adsorbs at a Co site, and the calculated ΔG reflects the thermodynamic feasibility of the hydrogen\nH*\nevolution reaction on each catalyst surface.\nThe intrinsic H evolution capabilities of different catalysts were evaluated by the Gibbs free energy\n2\ncalculation for the adsorbed hydrogen atom (ΔG , Supplementary Fig. 40). CoSA-hCN shows a\nH*\nsmaller ΔG of 0.32 eV compared to CoSA-CN (0.51 eV) and CoSA@hCN (0.53 eV), indicating more\nH*\nthermodynamically favorable H* adsorption and superior intrinsic activity for proton adsorption and H\n2\nformation.\nFurthermore, the band center analysis in Supplementary Fig. 34 should specify which element or species\nit refers to, and it is meaningful only if the band center of the active site (e.g., the Pt cluster) is considered.\nSince Supplementary Fig. 34 shows an undefined band center, the following statements appear\ncontradictory:\n* (below the caption of Supplementary Fig. 34)\n\"Among all models, CoSA-hCN displays the moderate band center positions, which is the most favorable\nalignment to support effective adsorption and dissociation of reactant species\"\n* (In the Structure-property correlation section)\n\"In CoSA-hCN, the total and projected Co d-orbital DOSs shift closer to the Fermi level, indicating\nstronger Co-N orbital interaction and greater charge transfer capability.\"\nResponse: We thank the reviewer for the constructive comments. In this work, the band-center and\nadsorption analyses were intentionally performed on Pt-free models, because the aim of this theoretical\ncomponent is to probe the intrinsic electronic structure and inherent catalytic properties of the Co single-\natom catalysts. We have clarified in the caption of Supplementary Fig. 46 and the associated text that\nthe band-center calculations were derived from the total density of states of the Pt-free model, while the\nd-band contribution corresponds exclusively to the single-atom Co sites. Co on theoretical models is the\nactive site in the computational study for *H and seawater ion atoms adsorption.\nChanges: On page 6, last paragraph, the following discussion on DOS has been revised:\n“The total and projected density of states (DOS) further highlight these differences (Fig. 2b). In CoSA-\nhCN, the Co-d states show an enrichment and broadening near the Fermi level (E ), as well as significant\nF\nCo–N hybridization. The increased near-E Co-d spectral weight provides more accessible states for\nF\ncharge transfer.”\nChanges: Supplementary Information, page 47, the description and discussion about the band center\nhave been revised:\nSupplementary Fig. 46 | Band center analysis. Calculated band centers of CoSA-CN, CoSA@hCN\nand CoSA-hCN.\nThe band-center positions of CoSA-CN, CoSA@hCN and CoSA-hCN (theoretical models as shown in\nSupplementary Fig. 15) were calculated. The band center positions of the s-, p-, and d-orbitals relative\nto the Fermi level were shown in Supplementary Fig. 46. Among all models, CoSA-hCN exhibits an\nintermediate d-band center but with increased Co-d spectral weight near the Fermi level (Fig. 2b),\nproviding an optimal balance between orbital filling and reactivity. This balanced electronic\nconfiguration supports effective adsorption and dissociation of reactant species, thereby promoting\nfavorable reaction kinetics.\n(a) it is more meaningful for an isolated cluster than its band center, and\n(b) the authors have already mentioned efficient charge transfer in CoSA-hCN.\nResponse: We appreciate the reviewer’s valuable suggestion regarding the Bader charge analysis of the\nPt clusters. To directly address the reviewer’s concern regarding Co-Pt interfacial interactions, we\nconstructed and optimized three models by loading representative Pt on CoSA-CN, CoSA-hCN and\n13\nspecies (Supplementary Fig. 31).\nSupplementary Fig. 30 | Theoretical studies of electronic interaction between Co single-atom sites\nand Pt nanoparticles. Optimized models of Pt -loaded a, CoSA-CN, b, CoSA-hCN and c, CoSA@hCN.\n13\n“Bader charge analysis (Supplementary Fig. 31) shows that Pt undergoes pronounced electron\nenrichment upon interacting with the substrates, confirming spontaneous interfacial charge transfer\nconsistent with its role as an efficient electron acceptor for proton reduction. Among all configurations,\nPt on Co-hCN exhibits the most negative Bader charge (−0.99 e), further confirming the strongest\n13\nelectronic interaction.”\nSupplementary Information, page 32, the figure for Bader charge analysis at Pt\n13\nspecies has been added:\n26\ne) 0.0\ns\n(\ne\nci -0.1\ne\np\ns\nof\nPt 1 3 -0.2 CoSA- C N\ne -0.3\ng\nar\nc h -0.4\ner\nd\na -0.5\nv\ne\nB\n@h C\nN\nR\nel\nati -0.6\nCoSA-h\nC N\nCoSA\nSupplementary Fig. 31 | Bader charge analysis of optimized Pt -loaded CoSA-CN, CoSA-hCN and\navoid confusion, the y-axis in Fig. 3h should be Eads (eV) with signs consistent with Fig. 3i, rather than\nusing the magnitude of adsorption energy.\nResponse: We thank the reviewer for this insightful question. In DFT calculations, charged adsorbates\n(Na+, Mg2+, Cl–, etc.) cannot be directly modelled because introducing net charges leads to artificial\nCoulomb interactions and the need for a compensating background charge, which renders the adsorption\nenergies unreliable. Therefore, we adopted a widely used approach in heterogeneous catalysis modelling,\nespecially in H evolution reaction from seawater (Nano Lett. 2023, 23, 4390–4398; ACS Nano 2023,17,\n2\n18217–18226; ACS Catal. 14, 13768–13780), in which neutral atoms are used as electronic probes to\nevaluate the surface affinity toward cation-like and anion-like species. Specifically, neutral Na, K, Mg,\nand Ca atoms readily donate electrons upon adsorption and thus exhibit cation-like adsorption behavior,\nwhereas neutral Cl atoms strongly attract electrons from the surface and therefore behave in an anion-\nlike manner. The ionic labels in Figs. 3h and i (Na+, Mg2+, Cl–, etc.) are used to denote these cation-like\nor anion-like adsorption trends, rather than the formal charge of the adsorbate. We have explicitly\nclarified this point in the revised figure caption. Following the reviewer’s suggestion, the y-axis label in\nFig. 3h has been corrected to “E (eV)” with signs consistent with Fig. 3i, avoiding confusion in the\nads\ncomparison of adsorption energies.\nChanges: On page 13, the following figure caption has been revised to clarify the adsorbed atoms:\nh, Calculated adsorption energies of Na* and Cl* of CoSA-CN, CoSA-hCN and CoSA@hCN. The\nadsorption configurations (optimized theoretical models) can be found in Supplementary Fig. 44. i,\nCalculated adsorption energies of Na*, Mg*, K*, Ca* towards Cl* of CoSA-CN and CoSA-hCN. Neutral\nNa, Mg, K, Ca and Cl atoms were used in the DFT calculations. The ionic labels represent cation-like or\nanion-like adsorption behavior derived from charge-transfer trends, rather than the formal ionic charge\n27\nof the adsorbate. The adsorption configurations (optimized theoretical models) can be found in\nSupplementary Fig. 45.\nThe y-axis label in Fig. 3h has been corrected to “E (eV)”\nads\nCl− Na+\n) -4\nV\ne\n(\ns -2\nd\na\nE\nSupplementary Information, pages 45-46, the figure for the optimized theoretical models with adsorbed\nseawater ion atoms have been added:\nSupplementary Fig. 44 | Optimized theoretical models for CoSA-CN, CoSA-hCN and CoSA@hCN\nwith adsorbed Na and Cl atoms. Neutral Na and Cl atoms were used in the DFT calculations. In the\ncorresponding discussion in the manuscript, the ionic labels represent cation-like or anion-like adsorption\nbehavior derived from charge-transfer trends, rather than the formal ionic charge of the adsorbate.\nNa, Mg, K, Ca and Cl atoms. Neutral Na, Mg, K, Ca and Cl atoms were used in the DFT calculations.\nIn the corresponding discussion in the manuscript, the ionic labels represent cation-like or anion-like\nadsorption behaviour derived from charge-transfer trends, rather than the formal ionic charge of the\nadsorbate.\nComment 3-7: In the last sentences of \"Structure-property correlation\" section, the authors suggest a\npossible mechanism of photoelectron transfer, visualized in Fig. 2i and 5e. Please include the supporting\ndensity of states of Pt-cluster-hybridized CoSA-hCN to show the relative positions of projected DOS\nstates of Pt, Co and CN.\nResponse: We thank the reviewer for this great suggestion. As mentioned in the response to Comment\n3-5, to further support the proposed electron transfer and electronic interaction, we have additionally\nconstructed and optimized theoretical models by loading Pt on CoSA-CN, CoSA-hCN and\nChanges: On page 10, first paragraph, the following discussion on the total and projected DOS of Pt -\n13\nloaded models has been added:\n“To further study the electronic interaction, we performed theoretical calculation on the optimized Pt -\n13\nloaded CoSA-CN, CoSA-hCN and CoSA@hCN models. DOS profiles in Supplementary Fig. 30 reveal\nthat CoSA-hCN induces a more pronounced redistribution of electronic states near the Fermi level,\nindicating stronger electronic coupling between Pt and the Co single-atom center.”\nand Pt nanoparticles. Optimized models of Pt -loaded a, CoSA-CN, b, CoSA-hCN and c, CoSA@hCN.\nenergy observed in CoSA-hCN indicates \"partial reduction of Co centers by photoexcited electrons\". Is\nit sufficient to claim localization of photoexcited electrons at Co centers, considering their limited\nlifetime?\nResponse: We thank the reviewer for raising this important question. We sincerely apologize for the\nearlier unclear description, which may have caused misunderstanding regarding the Co electronic\nchanges during operation. As clarified in our response to Comment 1-7, we conducted additional\nmeasurements to clarify the change of oxidation state of Co during the reaction. In pure water without\nthe addition of TEOA, all three catalysts (CoSA-CN, CoSA@hCN, and CoSA-hCN) show a slight\npositive edge shift caused by electron transfer from Co sites to reduce Pt4+, resulting in mild oxidation of\nCo. In the presence of TEOA (after reaction), CoSA-hCN instead displays a small negative shift because\nTEOA provides a reducing environment under illumination. We apologize for the earlier insufficient\nconsideration of the role of TEOA in influencing the Co oxidation state. The relevant discussion has now\nbeen revised to avoid confusion.\nChanges: On page 10, second paragraph, the discussion of charge transfer on Co sites has been revised:\n“The charge transfer on Co sites was also investigated. Post-deposition Co K-edge XANES spectra were\nre-examined in pure water containing H PtCl under visible light irradiation. All catalysts display a slight\n2 6\nedge shift toward higher energy, consistent with electron transfer from Co centers to reduce Pt4+ during\nPt nucleation (Fig. 2h and Supplementary Fig. 32). With the addition of TEOA after reaction, CoSA-\nhCN shows a small shift toward lower energy, reflecting the reductive action of TEOA on the Co sites\n(Fig. 2h).”\nand after operation with the addition of H PtCl and TEOA under visible light irradiation at 25 °C.\n2 6\nSupplementary Information, page 33, the figure for the Co K-edge XANES spectra of post-reacted\nsamples was added:\nSupplementary Fig. 32 | Co K-edge XANES spectra of post-reacted samples. a, Co K-edge XANES\nspectra of all catalysts after Pt photodeposition. Co K-edge XANES spectra of b, CoSA-CN, c,\nCoSA@hCN, and d, CoSA-hCN before and after Pt photodeposition performed in pure water. All three\ncatalysts exhibit a slight edge shift toward higher energy after Pt photodeposition, indicating electron\ntransfer from Co centers to reduce Pt4+ species.\nComment 3-9: In the computational details, what does \"The bottom layers of about half of the structure\"\nmean? Isn't it a single layer structure of hCN? Moreover, the use of “about” is ambiguous and should be\nclarified.\nwording was imprecise and did not accurately reflect the single-layer carbon nitride models used in this\nwork. This description has been removed, and we have clarified that all atoms in the structure were fully\nrelaxed without any positional constraints during the VASP calculations. We have also carefully re-\nexamined the entire manuscript, particularly the section of Materials and Methods, to ensure consistency\nand to avoid any inaccurate or ambiguous descriptions.\nChanges: Page 24, first paragraph, the Theoretical calculations in Materials and Methods have been\nrevised:\n“All atoms in the structure were fully relaxed without any positional constraints imposed during the\nVASP calculations. All structures with a dynamic magnetic moment were optimized without any\nrestriction until their total energies were converged to < 1×10-6 eV, and the average residual forces were\n< 0.02 eV/Å. The charge density difference (CDD) was calculated to visualize the redistribution of\nelectronic charge upon the formation of the Co−N coordination environment. The CDD is defined as:\nΔρ(r) = ρCoSA + CN (r) − ρCoSA(r) − ρCN(r)\nWhere, ρCoSA+CN denotes the charge density of the complete Co single-atom structure, and ρCoSA\nand ρCN represent the charge densities of the isolated Co center and the surrounding N coordination\nenvironment, respectively, each computed in the same geometric configuration as in the combined system.\nThis definition highlights the intrinsic charge redistribution between the metal center and its coordinated\nligands. The isosurface was visualized at an isovalue of 0.001 e Bohr−3, which provides a clear yet\nphysically meaningful depiction of charge transfer regions. In the plotted isosurfaces, yellow regions\ndenote electron accumulation (Δρ > 0), while cyan regions correspond to electron depletion (Δρ < 0).\nThus, the color contrast intuitively reflects the direction and spatial extent of charge transfer between the\nCN adsorbate and the Co active site.”\nComment 3-10: The caption of Supplementary Fig. 19 can be misleading, as it does not show the\n(theoretical) electronic band structure but the experimentally estimated band edge alignments.\nResponse: We thank the reviewer for pointing out this potential ambiguity. The caption of\nSupplementary Fig. 23 (previous Supplementary Fig. 19) has been revised accordingly to better reflect\nthe content.\nChanges: Supplementary Information, page 24, the caption has been revised:\nfor CN, hCN, CoSA-CN, CoSA@hCN and CoSA-hCN.\nComment 3-11: Although Co plays a crucial role in relaying photoexcited electrons, the main HER\ncatalytic activity is attributed to the Pt cluster on CoSA-hCN. In this case, how can we describe it as a\n\"single-atom Co photocatalyst\"?\nResponse: We thank the reviewer for the thoughtful question. In our manuscript, the term “single-atom\nCo photocatalyst” refers to the photocatalytic system where Co single atoms act as the primary light-\nresponsive and charge separation centers, rather than implying that Co itself is the dominant active site\nfor H evolution. In our system, Co–N sites and carbon nitride are responsible for harvesting photons,\n2\ngenerating charge carriers, and efficiently relaying photoexcited electrons for in situ photodeposition of\nPt. This photodeposited Pt cluster subsequently serves as the cocatalyst. Therefore, describing the\nmaterial as a “single-atom Co photocatalyst” reflects the fact that the photocatalytic functionality, light\nabsorption, charge separation, and electron transfer are governed by the Co single-atom coordination\nenvironment, while the Pt nanoparticles produced in situ are the H evolution-active species. We have\n2\nclarified this distinction in the revised manuscript to avoid misunderstanding.\nReviewer’s general comments: In this work, the authors reported a carbon nitride–supported single-\natom cobalt photocatalyst integrated into a floating photothermal device for simultaneous seawater\nsplitting and desalination. The study presents extensive structural characterizations (including XAS and\nDFT) to support the claim that asymmetric Co–N₄ configurations with second-shell carbon vacancies\nenhance charge transfer and salinity tolerance. The device-level demonstration of hydrogen and\nfreshwater co-production is ambitious, and the characterization of ion rejection mechanisms is a strength\nof the work. However, there are several concerns that limit the impact and clarity of this manuscript.\nOverall, while the manuscript demonstrates well-characterized materials and an integrated device\nconcept, the mechanistic ambiguity, overstatement of “real-world” relevance, and insufficient\nbenchmarking significantly weaken the impact. These issues should be thoroughly addressed before the\nmanuscript can be considered for publication in Nature Communication.\nResponse: We sincerely thank the reviewer for the constructive and insightful comments. We greatly\nappreciate the recognition of our device design and comprehensive characterizations. All comments have\nbeen carefully addressed point by point, with detailed revisions made throughout the manuscript to\nimprove mechanistic interpretation, benchmarking, and discussion of real-world applicability. We\nbelieve these revisions have substantially enhanced the overall clarity and overall impact of the work.\nMajor Concerns:\nComment 4-1: Although the structural analysis of the catalysts is comprehensive, the manuscript lacks\na clear mechanistic link between the observed atomic/electronic modifications and the improvements in\nhydrogen evolution and photothermal effects. The authors should clarify which structural factors are\nmost important, and how this “molecular engineering single approach” provides a general strategy for\nimproving performance.\nResponse: We thank the reviewer for this insightful comment. As also discussed in our response to\nComment 1-9 (Reviewer #1), multiple structural features can collectively influence photocatalytic\nperformance, which is common in catalyst optimization. However, the core objective of this work is to\nestablish a molecular-engineering approach that directly tailors the coordination environment and\nelectronic structure of isolated Co sites on carbon nitride, thereby providing a molecular-level mechanism\nfor the observed performance enhancement. In this framework, the most important structural factor is the\nasymmetric Co−N coordination created by second-shell carbon vacancies. This configuration induces\n4\nlocalized electronic polarization, strengthens Co to CN charge delocalization, and facilitates charge\nseparation and transfer for in situ Pt cocatalyst photodeposition that accelerates proton reduction. Overall,\nthe molecular engineering strategy establishes a general approach: precise manipulation of the active\nsite’s electronic structure governs charge processes, catalytic behavior and light-heat conversion\nbehavior, thereby providing a unified route to performance improvement.\nChanges: On page 2, last paragraph, the molecular-level engineering strategy was highlighted:\n“Herein, we establish a molecular-level coordination-engineering strategy to construct three single-atom\nCo configurations on CN: symmetric Co–N (CoSA-CN), vacancy-anchored Co–N (CoSA@hCN), and\n4 3\nasymmetric Co–N with second-shell carbon vacancies (CoSA-hCN).”\n4\n“Among all catalysts, CoSA-hCN exhibited the highest H production rate of 6.4 mmol g–1 h–1 in artificial\n2\nseawater (ASTM D1141-98 (2013) standard, Supplementary Table 5), which is 4- and 3-fold higher\nthan that of CoSA-CN (1.6 mmol g–1 h–1) and CoSA@hCN (2.2 mmol g–1 h–1), respectively. As discussed\nabove, this enhanced activity primarily arises from the asymmetric Co–N coordination and vacancy-\n4\ninduced electronic modulation, which strengthen charge separation and promote efficient in situ Pt\nphotodeposition, while physicochemical improvements such as higher surface area provide additional\ncontributions.”\nOn page 18, last paragraph, the molecular-level engineering strategy was further highlighted:\n“In summary, this study presents molecular-level coordination engineering of single-atom photocatalysts\ncombined with a scalable photothermal device for efficient solar seawater splitting and desalination.”\nComment 4-2: The reported heating of the CN-based material to ~65 °C (line 294) is unusual given that\nCN is not black. In the Methods section (line 515), the reactor is described as being constructed from\nblack PTFE. It remains unclear whether the heating originates from the photothermal effect of the CoSA-CA\ncatalyst itself or from the reactor substrate. This ambiguity raises concerns about the interpretation\nof photothermal contributions.\nResponse: We thank the reviewer for pointing out this important ambiguity. To clarify the origin of heat\ngeneration, we conducted additional control experiments to decouple the photothermal contribution of\nthe CoSA-hCN catalyst from that of the reactor substrate. In the absence of a catalyst (pure water only),\nthe temperature increased only modestly: from room temperature to 28.5 °C within 10 min and reaching\n44.1 °C after 3 h in the black PTFE reactor. By contrast, when CoSA-hCN was added, the temperature\nrise was significantly higher: 38 °C within 10 min and 63.9 °C after 3 h. We have also compared black\nand white PTFE reactors under identical illumination. The white PTFE reactor showed a similar final\ntemperature (63.2 °C) after 3 h light illumination. This clearly shows that the catalyst itself has an\nintrinsic light-absorption driven photothermal effect beyond that of the PTFE substrate. These results\nhave been included in the revised manuscript to clarify that the observed heating arises primarily from\nthe CoSA-hCN material. In the sealed reactor used for photocatalytic H evolution, heat dissipation is\n2\nmore limited, and the actual operating temperature is therefore expected to be slightly higher than the\nvalues measured in this open setup. While the present work focuses on how moderate photothermal\nheating enhances photocatalytic H evolution, we agree that a more systematic investigation of the\n2\ncatalyst’s intrinsic photothermal properties will be valuable and plan to pursue this in future work.\nChanges: On page 11, second paragraph, the figure for temperature rise has been introduced:\n“When operated without temperature control, photothermal self-heating raised the system temperature\nto ~65 °C (Supplementary Fig. 37), boosting the rate to 22.5 mmol g–1 h–1, 2.7-fold increase compared\nto that at 25 °C.”\nOn page 13, the caption in Fig. 3 has been revised:\n35\nb, Photocatalytic H production evaluation using artificial seawater (3 h) under visible light irradiation\n2\nat 25 °C, combined UV and visible light irradiation at 25 °C, and combined UV and visible light\nirradiation with solar-induced photothermal self-heating to increase the system temperature to ~65 °C\n(Photothermal heating behaviour as shown in Supplementary Fig. 37).\nIn Supplementary Information, page 38, the following figure for the temperature rise under operation\nhas been added.\nSupplementary Fig. 37 | Photothermal heating behavior in control systems. Systems include (a)\nTemperature evolution of CoSA-hCN in a black PTFE reactor, (b) CoSA-hCN in a white PTFE reactor,\nand (c) pure water in a black PTFE reactor under 1-sun illumination. The IR thermal images share a\nunified temperature scale ranging from 15 to 70 °C.\nComment 4-3: The role of photothermal effects is not adequately explained. The authors should provide\na mechanistic discussion of how the CoSA-CA structure contributes to photothermal enhancement and\nhow this property translates into improved desalination and HER performance.\nResponse: We appreciate the reviewer’s thoughtful comment regarding the role of photothermal effects.\nOur initial intention is to clarify how light-induced heating, an intrinsic feature of non-isothermal\nphotocatalysis, acts synergistically to enhance both hydrogen evolution kinetics and water evaporation\nrate within the integrated device. We thank the reviewer for highlighting this aspect, which indeed\nprovides an opportunity to more clearly articulate the mechanistic link between catalyst structure and\nphotothermal behavior.\nConventional semiconductors can generate localized photothermal hot spots through non-radiative\nrelaxation of photoexcited carriers. In CoSA-hCN, the asymmetric Co–N coordination and second-shell\n4\ncarbon vacancies increase the degree of electronic polarization and strengthen Co–N hybridization. This\nenhanced coupling accelerates carrier relaxation, as confirmed by the pronounced reduction in τ\nave\nobserved in TRPL measurements. The faster non-radiative decay provides an additional channel for\n36\nlocalized heat generation, which translates into higher interfacial temperature at catalytic sites and\naccelerates proton reduction kinetics.\nChanges: On page 8, second paragraph, the discussion of the mechanistic role of non-radiative\nrelaxation and its relation to the observed localized hot-spot heating has been revised:\n“Time-resolved PL (TRPL) spectroscopy reveals a decreased average fluorescence lifetime (τ ) from\nave\n5.1 (CoSA-CN) to 4.4 (CoSA@hCN) and 3.0 ns for CoSA-hCN, suggesting accelerated charge transfer\nin CoSA-hCN (Fig. 2d and Supplementary Table 3). The shortened lifetime also signifies an increased\ncontribution of non-radiative recombination channels, which can generate localized photothermal hot\nspots under illumination.38 This localized heating supports faster interfacial reaction kinetics and\ncontributes to the enhanced photocatalytic performance.”\nComment 4-4: The system relies on triethanolamine (TEOA) as a sacrificial electron donor. This is not\nsustainable and contradicts the claim that the device has been tested under “real-world” conditions. The\nauthors should moderate their claims, as the use of TEOA fundamentally differentiates the system from\npractical overall water splitting.\nResponse: We much appreciate the reviewer for this important comment, which has helped us refine and\nmoderate our claims to make them more rigorous and accurate. We agree that the use of TEOA as a\nsacrificial electron donor means that our current study specifically examines the half reaction of H -\n2\nevolution, rather than overall seawater splitting. We have revised the manuscript to clarify that the present\nwork demonstrates the H -evolution half reaction rather than overall water splitting. Ongoing research is\n2\nfocused on extending this molecular engineering concept to overall seawater splitting without the use of\nsacrificial agents.\nChanges: On page 2, last sentence of first paragraph:\n“Advancing understandings of coordination asymmetry at different atomic levels will be essential for\nguiding next-generation CN-SAC designs for solar H production from seawater.”\nFig. 1 | Preparation and structural characterizations of single-atom Co photocatalysts. a, Illustration\nof the single-atom Co photocatalysts on a 60 cm2 floating sponge photoreactor for seawater-to-H\n2\nproduction and seawater desalination.\nOn page 11, second paragraph:\n“This photothermal enhancement effect has rarely been reported for CN-SAC and CN-based systems for\nphotocatalytic H production from water or seawater.”\nFig. 3 | Photocatalytic H production performances and salinity effects. c, Comparison with current\n2\nliterature in carbon nitride-supported single-atom photocatalysts for H production, and carbon nitride-\n2\nintegrated systems in photocatalytic H production from seawater. CN-SACs in photocatalytic H\n2 2\nevolution (PC-HER) include Fe@g-C N ;39 Co-S-C N ;42 Co/P/CN-sc;45 Co1/2DPCN;46 PCNNi\n3 4 3 4\n37\n(without TEOA);47 g-C N –Ni (without TEOA);48 p-CN/Ni-COF;49 Cu1@HCNS;41 Cu /CN;50 Zn-\n3 4 1\nPCN;43 WSA-CN-PUNS.40 CN-based materials for photocatalytic H production from seawater include\n2\ntubular CN;51 CsI-CN;52 CN-NaK;53 CN-m;54 CN-CP;55 VN-HCN;8 C N ;56 ACN;57 CCN4;58\n3 4\nHCCN/CAN.59 Detailed data are provided in Supplementary Table 5, sacrificial agents are involved\nunless otherwise specified.\nOn page 16:\nFig. 4 | Scaled-up H production and seawater desalination. a, Illustration of a pilot-scale\n2\nphotocatalytic device integrating H production and seawater desalination system.\nFig. 5 | Sponge-photoreactor for photocatalytic H production from seawater.\n“In summary, this study presents molecular-level coordination engineering of single-atom photocatalysts\ncombined with a scalable photothermal device for efficient solar seawater-to-H production and seawater\n2\ndesalination. By rationally designing three single-atom Co coordination environments, symmetric Co–\nN , vacancy-anchored Co–N , and asymmetric Co–N with second-shell carbon vacancies, on carbon\n4 3 4\nnitride-supported photocatalysts, we optimized H evolution performance and uncovered the pronounced\n2\nrole of coordination asymmetry.”\nComment 4-5: When comparing results with the literature, the authors do not distinguish between\nsacrificial hydrogen production and overall water-splitting systems. Since sacrificial systems typically\nexhibit higher HER rates, the current comparisons risk overstating the relative performance of the\nreported catalyst (Fig.5).\nResponse: We thank the reviewer for the valuable comment. To ensure a fair evaluation of our catalyst\nunder comparable reaction conditions, the performance comparison in Fig. 3c was carried out primarily\nusing catalysts reported under identical sacrificial-agent conditions (TEOA-containing systems,\nSupplementary Table 5) to maintain consistency. The intention of Fig. 5 is to compare the photothermal\nreactor performance, i.e., the ability of different sponge-based systems to harvest solar energy and drive\nH evolution under identical photothermal conditions. The figure evaluates reactor-level enhancement,\n2\nrather than the intrinsic activity of photocatalysts under mechanistically identical reactions. To evaluate\nthe advantage of our catalyst within this system, we extended the same platform to several benchmark\nphotocatalysts (e.g., TiO P25), under which CoSA-hCN consistently exhibited a higher activity.\nwith and without TEOA.\nFig. 3 | Photocatalytic H production performances and salinity effects. c, Comparison with current\n2\nliterature in carbon nitride-supported single-atom photocatalysts for H production, and carbon nitride-\n2\nintegrated systems in photocatalytic H production from seawater. CN-SACs in photocatalytic H\n2 2\nevolution (PC-HER) include Fe@g-C N ;39 Co-S-C N ;42 Co/P/CN-sc;45 Co1/2DPCN;46 PCNNi\n3 4 3 4\n(without TEOA);47 g-C N –Ni (without TEOA);48 p-CN/Ni-COF;49 Cu1@HCNS;41 Cu /CN;50 Zn-\n3 4 1\n2\ntubular CN;51 CsI-CN;52 CN-NaK;53 CN-m;54 CN-CP;55 VN-HCN;8 C N ;56 ACN;57 CCN4;58\n3 4\nHCCN/CAN.59 Detailed data are provided in Supplementary Table 5, sacrificial agents are involved\nunless otherwise specified.\nIn Supplementary Information, pages 72-73, the comparison table has been revised to clearly distinguish\nthe systems with and without TEOA.\nSupplementary Table 5 | Comparison between different systems: carbon nitride-supported single-atom\ncatalysts for photocatalytic H production from pure water; carbon nitride-based photocatalysts in\n2\nphotocatalytic H production from seawater.\nPhotocatalyst Reactant solution Light Source 2 Ref.\n(μmol g–1 h–1) (%)\n8.96%\nArtificial seawater with 10vol% 300 W Xe lamp, This\nCoSA-hCN 6400.5 at 420\nTEOA and 1wt% Pt λ>420 nm work\nnm\nReal seawater with 10vol% 300 W Xe lamp, This\nCoSA-hCN 5909.4 -\nTEOA and 1wt% Pt λ>420 nm work\nArtificial seawater with 10vol% This\nCoSA-hCN and Vis, without 22465.9 -\nTEOA and 1wt% Pt work\nsystem cooling\nCarbon nitride-supported single-atom catalysts in H2 evolution\nWith sacrificial agents\n6.89%\nPurewater with 10vol% TEOA 500 W Xe lamp,\nFe@g-C N 3390 at 420 8\n3 4 and 3vol% H PtCl λ>420 nm\n2 6 nm\nCo-S-C N Purewater with 3vol% TEOA 910.6 - 9\n3 4 1.5G\nCo/P/CN-sc Purewater with 20vol% TEOA 3730.4 - 10\nλ>420 nm\n1.38%\nPurewater with TEOA and 300 W Xe lamp,\nCo1/2DPCN 1.58 at 420 11\n0.75wt% Pt λ>420 nm\nnm\n0.408\np-CN/Ni-COF Purewater with 10vol% TEOA 2290 at 420 12\nλ>420 nm\nnm\nPurewater with 10vol% TEOA 300 W Xe lamp, 7.1% at\nCu1@HCNS 3261 13\nand 3wt% Pt λ>420 nm 420 nm\nPurewater with 20vol% TEOA 300 W Xe lamp,\nCu /CN 1436.1 - 14\n1 and 1wt% Pt λ>400 nm\nPurewater with 10vol% TEOA 500 W Xe lamp, AM\nZn-PCN 1172.9 - 15\nand 1wt% Pt 1.5G\nWSA-CN- Purewater with TEOA and 300 W Xe lamp, 4.1% at\n3020 16\nPUNS 1wt% Pt λ>420 nm 420 nm\nWithout sacrificial agents\n1.12%\nPurewater without sacrificial 300 W Xe lamp,\nPCNNi 26.6 at 420 17\nagents λ>420 nm\nnm\nPurewater without sacrificial 500 W Xe lamp, AM\ng-C N –Ni 53.3 - 18\n3 4 agents 1.5G\nCarbon nitride in H2 evolution from seawater\nWith sacrificial agents\n39\n3.5% NaCl solution with 10wt% 300 W Xe lamp, 6.9% at\nTubular CN 3391 19\nTEOA and 1wt% Pt λ>450 nm 420 nm\nNatural seawater with 10wt% 300 W Xe lamp, 72% at\nCsI-CN 2101.25 20\nTEOA and 2wt% Pt λ>420 nm 420 nm\n3.5wt% NaCl solution with 50 W White LED 60% at\nCN-NaK 5560 21\n10wt% TEOA and 3wt% Pt light, λ > 420 nm 420 nm\n3wt% NaCl solution with 50 W White LED\nCN-m 3000 - 22\n10wt% TEOA and 3wt% Pt light, λ > 420 nm\nSeawater with 10wt% TEOA 300 W Xe lamp,\nCN-CP 2197 - 23\nand 3wt% Pt λ>420 nm\nArtificial seawater with 50vol% 300 W Xe lamp, AM 7.8%\nVN-HCN 8700 24\nmethanol and 0.01 mmol L–1 Pt 1.5G 405 nm\nArtificial seawater with 10vol% 300 W Xe lamp,\nC N 399 - 25\n3 4 TEOA and 0.5wt% Pt λ>420 nm\nArtificial seawater with TEOA 500 W Xe lamp, AM\nACN 238.7 26\nand 3wt% Pt 1.5G\n0.5 mmol L–1 solution with\n2.4% at\nCCN4 20vol% methanol and 0.5 wt% 300 W Xe lamp 3808.8 27\n420 nm\nPt\n3.5wt% NaCl solution with 3.6% at\nHCCN/CAN 300 W Xe lamp 3147 28\n20wt% TEOA and 1wt% Pt 420nm\nIn addition, a few minor points should be addressed:\nComment 4-6: It is unclear how the contact angle measurements in Fig. 3 were conducted (directly on\npowders, or on films drop-cast onto substrates). This should be clarified.\nResponse: We thank the reviewer for raising this point. The contact angle measurements in Fig. 3 were\nconducted on pressed catalyst pellets rather than loose powders. Specifically, 30 mg of catalyst powder\nwas pressed into 7-mm-diameter pellets under a consistent pressure of 1.5 tons to ensure a flat and\nuniform surface suitable for optical contact angle analysis. This clarification has been added to the\nMaterials and Methods section of the revised manuscript.\nChanges: On page 22, first paragraph, the detailed sample preparation procedure for the contact angle\nmeasurements has been added:\n“The hydrophilicity property was measured using Attension Optical Tensiometers (Nanoscience\nInstruments). For each measurement, 30 mg of catalyst powder was pressed into a 7-mm-diameter pellet\nunder a pressure of 1.5 tons to obtain a flat and uniform surface suitable for optical analysis.”\nComment 4-7: In Supplementary Tables S6 and S7, systems using sacrificial agents should be clearly\nseparated from overall water-splitting systems.\nResponse: We thank the reviewer for the helpful suggestion. As mentioned in the response to Comment\n4-5, in the revised version, Supplementary Table S5 has been updated to clearly detail the use of\nsacrificial agents in each system. The systems listed in Supplementary Table S6 focus on suspended\nphotothermal reactors reported in the literature, in which no sacrificial agent such as TEOA was added.\nIn contrast, our system includes a small amount of TEOA, which functions both as a binder/stabilizer to\nimmobilize the catalyst on the sponge and as a sacrificial electron donor for the H -evolution half-\n2\nreaction.\ndistinguish the systems with and without TEOA.\nSupplementary Table 5 | Comparison between different systems: carbon nitride-supported single-atom\ncatalysts for photocatalytic H production from pure water; carbon nitride-based photocatalysts in\n2\nphotocatalytic H production from seawater.\nPhotocatalyst Reactant solution Light Source 2 Ref.\n(μmol g–1 h–1) (%)\n8.96%\nArtificial seawater with 10vol% 300 W Xe lamp, This\nCoSA-hCN 6400.5 at 420\nTEOA and 1wt% Pt λ>420 nm work\nnm\nReal seawater with 10vol% 300 W Xe lamp, This\nCoSA-hCN 5909.4 -\nTEOA and 1wt% Pt λ>420 nm work\nArtificial seawater with 10vol% This\nCoSA-hCN and Vis, without 22465.9 -\nTEOA and 1wt% Pt work\nsystem cooling\nCarbon nitride-supported single-atom catalysts in H2 evolution\nWith sacrificial agents\n6.89%\nPurewater with 10vol% TEOA 500 W Xe lamp,\nFe@g-C N 3390 at 420 8\n3 4 and 3vol% H PtCl λ>420 nm\n2 6 nm\nCo-S-C N Purewater with 3vol% TEOA 910.6 - 9\n3 4 1.5G\nCo/P/CN-sc Purewater with 20vol% TEOA 3730.4 - 10\nλ>420 nm\n1.38%\nPurewater with TEOA and 300 W Xe lamp,\nCo1/2DPCN 1.58 at 420 11\n0.75wt% Pt λ>420 nm\nnm\n0.408\np-CN/Ni-COF Purewater with 10vol% TEOA 2290 at 420 12\nλ>420 nm\nnm\nPurewater with 10vol% TEOA 300 W Xe lamp, 7.1% at\nCu1@HCNS 3261 13\nand 3wt% Pt λ>420 nm 420 nm\nPurewater with 20vol% TEOA 300 W Xe lamp,\nCu /CN 1436.1 - 14\n1 and 1wt% Pt λ>400 nm\nPurewater with 10vol% TEOA 500 W Xe lamp, AM\nZn-PCN 1172.9 - 15\nand 1wt% Pt 1.5G\nWSA-CN- Purewater with TEOA and 300 W Xe lamp, 4.1% at\n3020 16\nPUNS 1wt% Pt λ>420 nm 420 nm\nWithout sacrificial agents\n1.12%\nPurewater without sacrificial 300 W Xe lamp,\nPCNNi 26.6 at 420 17\nagents λ>420 nm\nnm\nPurewater without sacrificial 500 W Xe lamp, AM\ng-C N –Ni 53.3 - 18\n3 4 agents 1.5G\nCarbon nitride in H2 evolution from seawater\nWith sacrificial agents\n41\n3.5% NaCl solution with 10wt% 300 W Xe lamp, 6.9% at\nTubular CN 3391 19\nTEOA and 1wt% Pt λ>450 nm 420 nm\nNatural seawater with 10wt% 300 W Xe lamp, 72% at\nCsI-CN 2101.25 20\nTEOA and 2wt% Pt λ>420 nm 420 nm\n3.5wt% NaCl solution with 50 W White LED 60% at\nCN-NaK 5560 21\n10wt% TEOA and 3wt% Pt light, λ > 420 nm 420 nm\n3wt% NaCl solution with 50 W White LED\nCN-m 3000 - 22\n10wt% TEOA and 3wt% Pt light, λ > 420 nm\nSeawater with 10wt% TEOA 300 W Xe lamp,\nCN-CP 2197 - 23\nand 3wt% Pt λ>420 nm\nArtificial seawater with 50vol% 300 W Xe lamp, AM 7.8%\nVN-HCN 8700 24\nmethanol and 0.01 mmol L–1 Pt 1.5G 405 nm\nArtificial seawater with 10vol% 300 W Xe lamp,\nC N 399 - 25\n3 4 TEOA and 0.5wt% Pt λ>420 nm\nArtificial seawater with TEOA 500 W Xe lamp, AM\nACN 238.7 26\nand 3wt% Pt 1.5G\n0.5 mmol L–1 solution with\n2.4% at\nCCN4 20vol% methanol and 0.5 wt% 300 W Xe lamp 3808.8 27\n420 nm\nPt\n3.5wt% NaCl solution with 3.6% at\nHCCN/CAN 300 W Xe lamp 3147 28\n20wt% TEOA and 1wt% Pt 420nm\nComment 4-8: The justification for designing the system specifically for seawater use requires more\ncareful discussion. In practice, desalination contributes less than 5% of the overall energy cost of green\nhydrogen production, and the broader benefits of seawater utilization should be explained.\nResponse: We thank the reviewer for this insightful comment. We agree that the direct energy cost of\ndesalination contributes only a minor portion of the overall green hydrogen production process. Our\nintention is not to position seawater utilization as an energy-saving strategy. The motivation for designing\na seawater-compatible system lies in the broader practical, environmental, and infrastructural benefits\nthat extend beyond the energy balance. Specifically, a direct use of seawater enables access to an\nabundant, non-competitive water source, which is particularly important for arid, water-scarce, or rapidly\nindustrializing coastal regions. Moreover, seawater-compatible systems have the potential to reduce the\nenvironmental footprint associated with large centralized desalination infrastructure, including brine\ndischarge and land use. We have clarified these points in the revised manuscript to more accurately reflect\nthe practical and environmental motivation for seawater-compatible devices.\nChanges: on page 2, second paragraph, the broader benefits of seawater utilization have been added:\n“Beyond atomic-level photocatalyst engineering, the integration of materials into scalable and robust\nsystems is equally important for advancing practical seawater splitting. Direct seawater utilization can\nsignificantly enhance sustainability by reducing freshwater dependence in arid and coastal regions.20”\nReference:\nPornrungroj, C. et al. Hybrid photothermal–photocatalyst sheets for solar-driven overall water splitting\ncoupled to water purification. Nat. Water 1, 952–960 (2023).\nReviewer’s general comments: This manuscript reports on molecular engineering of single atom\ncatalysts in a scalable photothermal reactor for integrated seawater splitting and desalination. The authors\nconstructed three single-atom Co configurations on CN: symmetric Co–N (CoSA-CN), vacancy-\n4\nanchored Co–N (CoSA@hCN), and asymmetric Co–N with second-shell carbon vacancies (CoSA-\n3 4\nhCN). The asymmetric Co–N in hierarchically porous carbon nitride (CoSA-hCN) introduces second-\n4\nshell carbon vacancies that reshape electronic asymmetry and charge dynamics, establishing a robust\npathway for effective salinity-mediated charge transfer and in situ Pt photodeposition for H evolution.\n2\nand freshwater (1.88 kg m−2 h−1) are produced directly from natural seawater. Given the significance of\nthis work in advancing a generalizable design paradigm for integrated solar hydrogen production and\nseawater desalination, it merits publication in Nature Communications, a leading multidisciplinary\njournal, provided that the following comments are addressed thoroughly.\nResponse: We sincerely thank the reviewer for the positive and encouraging evaluation of our work and\nfor recommending it for publication. The reviewer’s detailed and insightful assessment has been very\nhelpful in improving the overall quality of our work. We have carefully revised the manuscript in\nresponse to all comments, with particular attention to clarifying the mechanistic interpretation, refining\ntheoretical analysis, and strengthening the contextual discussion. The following sections provide detailed\npoint-by-point responses to each comment.\nComment 5-1: What is the reason for choosing CoSA-hCN as the catalyst? I believe the main novelty\nof this work lies in the integration of a photothermal reactor for seawater splitting and desalination (ref.\n20 in the manuscript), along with floatable systems for photocatalysis (ref. 33 in the Supplementary\nInformation), which synergistically enhance both solar hydrogen production and seawater desalination.\nThe novelty does not primarily reside in the catalyst design itself. Honestly, I believe the choice of\nphotocatalyst is less important in this case than the integration of the two systems. Could the author\nexpand the reactions with other photocatalysts (TiO , ZnO, CdS….) with proper cocatalyst deposition,\n2\nand compare the result with CoSA-hCN, to justify their choice of photocatalyst?\nResponse: We sincerely thank the reviewer for recognizing the novelty of integrating the photothermal\nreactor with a floatable photocatalytic platform. We agree that this system-level design is a central\nstrength of the work. At the same time, the choice and molecular-level engineering of the photocatalyst\nis also crucial. As mentioned in the Introduction, ‘Graphitic carbon nitride (g-C N , CN) showed\n3 4\nsomewhat tolerance to saline environments, where polar surface and ion-catalyst interactions may\nenhance charge separation and redox kinetics.’, however, pristine carbon nitride still suffers from limited\nhydrogen evolution activity from seawater. By anchoring atomically dispersed Co sites and further\nmodulating their coordination environment, we enable effective electronic coupling with the Pt cocatalyst,\nthereby improving charge transfer and interfacial reaction kinetics.\nTo clarify this novelty, we have revised the Introduction to better highlight the importance of the catalyst\nselection and design. To further extend the applicability of our integrated system, we also have performed\ncomparative experiments using commercial TiO (P25) under identical experimental conditions.\n2\n“Simultaneous photocatalytic H production was realized using Pt-deposited CoSA-hCN-immobilized\n2\nsponge (Fig. 5a-b), achieving a H production rate of 55.3 mmol m−2 h−1 in artificial seawater under 6 h\n2\nlight irradiation (Fig. 5c-d and Supplementary Fig. 58), and maintaining high efficiency under realistic\naqueous solutions (Supplementary Fig. 59). Triethanolamine functioned both as a stabilizer for catalyst\nimmobilization and as a sacrificial electron donor (Fig. 5c). We further validated the generality of the\nintegrated system by replacing catalysts with commercial TiO (P25). Under identical conditions, P25\n2\nshowed a relatively lower H production rate (Supplementary Fig. 60), confirming that the enhanced\n2\nperformance in our system arises from the synergistic coupling between CoSA-hCN and the\nphotothermal reactor.”\nIn Supplementary Information, page 61, the following figure has been added to present the P25 TiO\n2\ncontrol experiment:\nSupplementary Fig. 60 | Photocatalytic H production using a P25 TiO -loaded sponge-\n2 2\nphotoreactor. a, Photos of P25 TiO -loaded sponge-photoreactor. b, H production performance.\n2 2\nOn page 1, last paragraph, the following discussion in the Introduction section has been revised:\n“Graphitic carbon nitride (g-C N , CN) showed somewhat tolerance to saline environments,8,9 where\n3 4\npolar surface and ion-catalyst interactions may enhance charge separation and redox kinetics.10\nNevertheless, its intrinsic catalytic activity remains limited by poor charge transfer and insufficient active\nsites. Also, the underlying mechanisms governing these processes have been insufficiently explored, and\nunlocking the full catalytic potential of CN requires precise structural engineering that integrates active-\nsite design with controlled electronic and geometric modulation.11,12”\nOn page 2, last paragraph, the following discussion in the Introduction section has been revised:\n“Herein, we rationally construct three single-atom Co configurations on CN: symmetric Co–N (CoSA-\n3 4\n44\nvacancies (CoSA-hCN). The asymmetric coordination in CoSA-hCN induces a distinct electronic-\nasymmetry and facilitates electron transfer from Co to the CN framework for efficient in situ Pt cocatalyst\nphotodeposition. From a catalyst design perspective, this tailored coordination promotes selective\ninteractions with seawater ions and improves salinity-mediated charge transfer (Supplementary Fig.\n1b).”\nComment 5-2: While photothermal systems are properly referenced (refs. 20–25), floatable\nphotocatalyst systems are rarely cited in the manuscript. The authors should properly reference recent\ndevelopments in floatable photocatalyst systems and compare their merits with the state-of-the-art in the\nmain manuscript.\nResponse: We thank the reviewer for the constructive suggestion. We fully agree that a clear comparison\nwith floatable photocatalyst systems is important. We would like to clarify that the cited systems we\ncompared in Fig. 5f and Supplementary Table 6 are floatable architectures. To clearly mark the types\nof floatable supports, we have updated the floatable type (e.g., gel, wood, film, etc.) in Supplementary\nTable 6. We also have revised the corresponding discussion to emphasize comparison between our\nplatform and existing floatable photocatalyst designs.\nChanges: On page 17, second paragraph, the comparison with state-of-the-art floatable systems has\nbeen revised as follows:\n“The system exhibited stable long-term performance for H production and seawater desalination (Fig.\n2\n5e, Supplementary Fig. 61-62), with minimal catalyst leaching (Supplementary Fig. 63). The\nunchanged C and N K-edge profiles indicate that the CoSA-hCN catalysts remain structurally stable after\nthe operation (Supplementary Fig. 64). During cycling tests, catalyst retention at the sponge surface\nwas maintained via gravity-assisted redistribution. This concept can be adapted for large-scale floating\nsystems by incorporating passive tilting or slow rotational movement, allowing the catalyst layer to self-\nredistribute under operational conditions in open-water environments. Compared to state-of-the-art\nfloatable photocatalytic systems20,21,25,61,62 (Fig. 5f and Supplementary Table 6), our platform features\nsimple and modular design that allows facile fabrication through a one-step dipping process. It provides\na practically viable strategy to integrate efficient hydrogen production and seawater purification.”\nIn Supplementary Information, pages 74-75, the comparison table for the state-of-the-art floatable\nsystems has been revised.\nSupplementary Table 6 | Comparison between different solar-driven systems for seawater splitting and\npurification.\nSystem Fabrication simplicity H production Water Long-term Scalability Ref\n2\n& accessibility1 rate (mmol h– purification stability (cm2)\n1 m–2) capability (h)\n(kg m−2 h−1)\nIntegrated seawater splitting and purification systems:\nCoSA-hCN on Facile dipping with 55.3 (artificial Evaporation 24 60 This\ncommercial the help of TEOA seawater); (production): work\nsponge\n(Highly facile) 47.7\n45\n(natural 1.75\nseawater) (artificial\nseawater).\n1.88 (natural\nseawater),\nused in Fig.\n5f for\ncomparison.\nSVG-PC sheet Drop casting on pre- 17.8 Evaporation 154 25 29\n(Floatable film- treated carbon paper (production):\ntype) (Facile) 0.95\nPVA/RGO/STO Aerogel-assisted in 17.9 Evaporation 18 4.3 30\n(Floatable gel- situ crosslinking (production):\ntype) method (Moderate) 1.56\nPTC gel PTC gel with a 3.26 Evaporation 20 7.3 31\n(Floatable gel- hydrophobic (production):\ntype) membrane (Moderate) 1.49\nC-wood/TiO Hydrogel- 9.7 Evaporation 15 9 32\n2\n(Floatable wood- encapsulated porous (production):\ntype) wood supported 1.92\ncatalyst loading\n(Relatively complex)\nPDA- 3D-printing 0.226 Evaporation 100 11.3 33\nBTO/CPSS-FH (Technically (production):\n(Floatable gel- demanding) 2.51\ntype)\nOther similar systems:\nPt/TiO HPU–PPG gelation, 163 - - 1000 34\n2\ncryoaerogel catalyst-loaded\n(Floatable gel- bilayer assembly\ntype)\nWood/CoO Spin-coating on 2207.4 - 48 7.8 35\n(Floatable wood- carbonized wood\ntype)\nMPPH-S Cross-linking with 45.5 5.07 9 - 36\n(Floatable gel- calcination and\ntype) ultrasonication\nPM6:ITIC:L8- Impregnation method 153.6 - 20 16 37\nBO\n(Floatable wood\nfiber foam-type)\nTiO -Au/CuS Spin-coating 230 - 24 1.5 38\n(Floatable gel- freezing and alkali-\ntype) treatment\nP- Calcination and 4.22 0.97 300 - 40\nTiO (B)/Ni P/NF photo-deposition on\n2 2\nNickel foam\n46\n(Ni foam-\nsupport)\nTiO microneedle Polydimethylsiloxane- 775.5 2.04 60 7.5 41\n2\n(Floatable gel- supported fabrication\ntype)\nComment 5-3: While the authors demonstrated that their system operates stably for 6*6-hour cycles,\nthey did not provide in-depth characterization of the spent catalyst. Although they claim that the\nasymmetric Co–N sites in CoSA-hCN introduce second-shell carbon vacancies that reshape electronic\n4\nasymmetry and charge dynamics, establishing a robust pathway for effective salinity-mediated charge\ntransfer, it remains questionable whether the initial active site structure is preserved during operation.\nResponse: We thank the reviewer for this important comment. To further evaluate the structural stability\nof the catalyst under both laboratory-scale and scaled-up operation, we performed soft X-ray absorption\n(SXR) characterization of the post-reaction samples obtained from (i) powder-level photocatalytic H\n2\nevolution and (ii) the sponge-photoreactor. In both cases, C and N K-edge XANES spectra remain\nessentially unchanged, indicating the preservation of the carbon nitride framework. In addition, Co K-\nedge FT-EXAFS confirms that Co sites remain atomically dispersed after operation, with no detectable\nCo−Co coordination. Taken together, these results demonstrate that the single-atom Co centers and the\nsurrounding carbon nitride scaffold are structurally robust across both small-scale and device-level tests.\nChanges: On page 10, last paragraph, the following discussion of the Co K-edge XANES and EXAFS\nanalyses has been added:\n“The charge transfer on Co sites was also investigated. Post-deposition Co K-edge XANES spectra were\nre-examined in pure water containing H PtCl under visible light irradiation. All catalysts display a slight\n2 6\nedge shift toward higher energy, consistent with electron transfer from Co centers to reduce Pt4+ during\nPt nucleation (Fig. 2h and Supplementary Fig. 32). With the addition of TEOA after reaction, CoSA-\nhCN shows a small shift toward lower energy, reflecting the reductive action of TEOA on the Co sites\n(Fig. 2h). Fourier-transformed Co K-edge EXAFS analysis indicates the Co active sites still preserve\ntheir singly dispersed nature after operation (Supplementary Fig. 33).”\nOn page 11, second paragraph, the following discussion of the C/N K-edge XANES of the post-reaction\nsamples has been included:\n“The structural stability of CoSA-hCN was verified by cyclic tests, and unchanged XRD pattern, FTIR\nspectrum and C/N K-edge XANES of the post-reaction samples (Supplementary Fig. 38).”\nOn page 17, second paragraph, the following discussion of the C/N K-edge XANES of the post-operation\nsamples on sponge photoreactor has been included:\n“The system exhibited stable long-term performance for H production and seawater desalination (Fig.\n2\n5e, Supplementary Fig. 61-62), with minimal catalyst leaching (Supplementary Fig. 63). The\nunchanged C and N K-edge profiles indicate that the CoSA-hCN catalysts remain structurally stable after\nthe operation (Supplementary Fig. 64).”\nCoSA-hCN has been added:\nCoSA-hCN\nCoSA-hCN with Pt deposited\nCoSA-hCN after reaction\n-3)\nA\n)\n(ô\nR)\n(c\nT(\nF\nô\n1 2 3 4 5 6\nRadical distance (Å)\nSupplementary Fig. 33 | Fourier-transformed Co K-edge EXAFS of CoSA-hCN. a, Before reaction,\nb, after the addition of H PtCl in water under visible light irradiation at 25 °C, c, after the addition of\n2 6\nH PtCl and TEOA in water under visible light irradiation at 25 °C.\n2 6\nIn Supplementary Information, page 39, the following C/N K-edge EXAFS analyses on post-reaction\nCoSA-hCN has been added:\nphotocatalysts. b, XRD; c, FTIR; d, C and e, N K-edge XANES studies of CoSA-hCN before and after\nphotocatalytic seawater splitting. Performance was evaluated using a sealed black\npolytetrafluoroethylene reactor under visible light irradiation with system cooling to maintain the\ntemperature at 25 °C. Samples after reaction were collected via a filtration process and washed with\nultrapure water.\nIn Supplementary Information, page 65, the following C/N K-edge EXAFS analyses on post-operation\nCoSA-hCN on the sponge-photoreactor has been added:\nK-edge XANES studies of CoSA-hCN before and after operation. The post-operation sample was\ncarefully removed from the sponge, then washed, dried, and subsequently measured using SXR\nspectroscopy.\nManuscript ID: NCOMMS-25-70370A\nTitle: Molecular Engineering Single Atom Catalysts in a Scalable Photothermal Reactor for\nIntegrated Seawater Splitting and Desalination\nReviewer #1\nReviewer’s comments: No further comments.\nResponse: We thank the reviewer for the positive evaluation and note that there is no further\ncomment.\nReviewer #2\nReviewer’s comments: The authors have effectively addressed the reviewer’s concerns, and I\ntherefore recommend proceeding with publication.\nResponse: We thank the reviewer for the positive assessment and recommendation for publication.\nReviewer #3\nReviewer’s comments: The authors have made a substantial effort to address my comments, and\nmany of the technical clarifications and additional analyses are helpful. Overall, the manuscript\nhas been significantly improved. Before publication, however, one minor point should still be\naddressed:\nIn Supplementary Fig. 30, the projected density of states is shown without a clear legend. Please\nprovide a legend indicating the color and orbital/species assignments to improve clarity and\nreadability.\nResponse: We appreciate the reviewer for the positive evaluation and the helpful suggestions. As\nsuggested, a clear legend specifying the color and orbital/species assignments has now been added\nin Supplementary Fig. 30 to improve clarity and readability.\nChanges:\nOn page 31, supplementary information, supplementary Fig. 30 has been revised with a clear\nlegend being added.\nsites and Pt nanoparticles. Optimized models of Pt -loaded a, CoSA-CN, b, CoSA-hCN and c,\nReviewer #4\nReviewer’s general comments: The authors have addressed my previous comments satisfactorily,\nand I appreciate the improved clarity in mechanistic explanation and benchmarking. I am generally\nsupportive of publication. Before final acceptance, I recommend the following minor refinements\nto further align claims with evidence.\nResponse: We thank the reviewer for the positive evaluation and support for publication, and we\nhave carefully implemented the recommended minor refinements to further align the claims with\nthe supporting experimental evidence.\nComment 4-1: In several places, the manuscript still uses strong language that could be interpreted\nas implying near-term deployment, despite the use of a sacrificial agent and simulated illumination.\nFor example, statements such as “demonstrates strong applicability under real-world conditions”\n(e.g., around Fig. 3 discussion) would be better framed as “demonstrates proof-of-concept\nperformance in natural seawater under laboratory conditions.” A consistent adjustment of this type\nthroughout the main text would avoid overstatement while preserving impact.\nResponse: We greatly appreciate the reviewer for this kind suggestion. As suggested, the\nmanuscript has been systematically revised to moderate overly strong language and to consistently\navoid implications of near-term deployment. The statements implying real-world applicability\n2\nhave been rephrased to emphasize proof-of-concept performance under laboratory conditions,\nincluding the use of sacrificial reagents and simulated illumination.\nChanges:\nOn page 1: “Our findings offer design principles relevant to the development of future solar-driven\nseawater hydrogen production and desalination systems.”\nOn page 11: “Notably, CoSA-hCN demonstrates proof-of-concept photocatalytic performance in\nnatural seawater under laboratory conditions, achieving a high H production rate of 5.9 mmol g–\n2\n1 h–1 in natural seawater (Fig. 3a).”\nOn page 15: “To evaluate device-level feasibility, we developed a pilot-scale photothermal device\nto simultaneously achieve H evolution and seawater desalination (Fig. 4a-b and Supplementary\nOn page 17: “To assess the desalination performance, we quantified the amount of freshwater\ncondensed during a 6 h light-irradiation run in the coupled H evolution and desalination process\n2\n(Fig. 4g-h).”\nOn page 19: “These findings provide insights into coordination asymmetry and photothermal\neffects in seawater splitting and offer design principles for developing scalable photothermal\ncatalytic systems for sustainable solar energy conversion and seawater purification.”\nComment 4-2: The discussion linking TRPL behavior, non-radiative relaxation, and photothermal\nheating is reasonable and well explained. To avoid over-interpretation, I suggest ensuring that the\nmanuscript consistently distinguishes between experimentally observed temperature increases and\nmechanistic interpretation of their origin.\nResponse: We thank the reviewer for the constructive suggestion. The manuscript has been\ncarefully revised to consistently distinguish between experimentally observed temperature\nincreases and the mechanistic interpretation of their origin, thereby avoiding over-interpretation.\nChanges:\nOn page 8: “Time-resolved PL (TRPL) spectroscopy reveals a decreased average fluorescence\nlifetime (τ ) from 5.1 (CoSA-CN) to 4.4 (CoSA@hCN) and 3.0 ns for CoSA-hCN, suggesting\nave\naccelerated charge transfer in CoSA-hCN (Fig. 2d and Supplementary Table 3). The shortened\nlifetime indicates an increased contribution of non-radiative recombination pathways. Such non-\nradiative relaxation is expected to contribute to localized photothermal heating under\nillumination,38 which may support faster interfacial reaction kinetics.”\ntemperature increase of the reaction system, has rarely been reported for CN-SAC and CN-based\nsystems in photocatalytic H\n2\nproduction from water or seawater.”\nReviewer #5\nReviewer’s comments: This reviewer is satisfied with the revision.\nResponse: We thank the reviewer for the positive assessment and note that no further comment\nwas raised.\n4"}]

Decision letter