Chlorine radical-mediated electrochemical propylene epoxidation from seawater

Domain: biology
Published: 2026-03-14
Editorial outcome: major_revision

Abstract
Electrocatalytic chlorine-mediated propylene oxidation utilizing seawater represents a
promising route for sustainable propylene oxide (PO) synthesis. Unfortunately, inefficient
utilization of active chlorine species results in compromised yield and Faradaic efficiency (F.E.).
Herein, we achieve efficient chlorine radical-mediated propylene to PO by incorporating Li into
Co O , attaining a PO F.E. approaching 99.0% and a yield of 20.6 mol/m2/h in simulated seawater
3 4
with long-term stability for over 100 h. Furthermore, a PO yield of 78.9 mol/m2/h is obtained even
under an industrial-level current density of 0.5 A cm-2, demonstrating competitive performance
compared with most reported noble metal catalysts. Mechanistic investigations reveal that Li
doping transforms the adsorption configuration of Cl– on Co O from linear Co-Cl structure to
3 4
more favorable triangular Li-Cl-O configuration, thereby enhancing •Cl generation, and
consequently redirecting propylene oxidation from conventional HClO pathway to a more efficient
•Cl route. This work advances the mechanistic understanding of the mechanism of electrocatalytic
propylene oxidation and offers great opportunities for the sustainable synthesis of commercial
chemicals using waste lithium-ion batteries, seawater, and renewable electricity.

Introduction
Propylene oxide (PO) is a crucial industrial intermediate, serving as a precursor to a variety of
commercial chemicals including polymers, propylene glycol, polyurethane foams, and propylene
carbonate.1 It ranks as the third most significant propylene derivative with an annual global
production exceeding 10 million tonnes.2 Conventional industrial methods for PO production
include chlorohydrin process, hydrogen peroxide-based process (HPPO) and Halcon process (Fig.
1a-c).3 Nevertheless, these methods face challenges such as high costs, generation of
stoichiometric side products and severe pollution.4 Therefore, there is an urgent demand to develop
a more sustainable PO synthetic route. The electrochemical oxidation of propylene driven by
renewable electricity presents a promising alternative for PO production under ambient
conditions.5, 6, 7, 8, 9, 10 While direct electrooxidation via water-derived active oxygen species offers
a viable route but is severely hindered by propylene’s poor solubility and inevitable overoxidation,
resulting in limited Faradaic efficiency (F.E.) (<70%), low current density (<10 mA cm-2) and
insufficient stability (<12 h), rendering the efficiency of direct electrocatalytic oxidation of
propylene far from satisfactory (Fig. 1d).11, 12, 13, 14

The indirect chlorination-mediated route overcomes these limitations by integrating
electrochemical chloroxidation with propylene oxidation, enabling stable chloropropanol
production which subsequently reacts with the base generated from the cathodic hydrogen
evolution reaction to yield PO (Fig. 1e).2 Notably, besides serving as a redox mediator, chloride
provide the additional advantage of allowing direct use of seawater as the electrolyte. Current
studies identify hypochlorous acid (HClO) as the active species in chlorination-mediated olefin
oxidation, generated through the oxidation of Cl– to Cl
2
, followed by Cl
2
disproportionation with
H
2
O.15 Despite achieving high current density and selectivity, the spontaneous irreversible
decomposition of HClO into non-reactive ClO– still hampers the F.E.16 Furthermore, the oxidized
Cl⁻ were partially reduced back to unreactive HCl during the stoichiometric disproportionation
reaction that formed HClO, resulting in inefficient charge utilization and additional energy
losses.17, 18 In addition, the competitive oxygen evolution reaction (OER) further constrains the
F.E., as the standard electrode potential for chlorine evolution reaction (CER) (1.36 V vs. RHE)
exceeds that of OER (1.23 V vs. RHE).19 Due to the limited utilization of active chlorine species
and the significant OER side reaction, achieving chlorine-mediated electrooxidation of propylene
with high efficiency remains a significant challenge.

Chlorine radicals (•Cl), as a potent oxidant with high oxidation potentials (2.47 V) have been an
appealing option for mediating various oxidation reactions.20, 21, 22, 23, 24 As an electrophilic reagent,
•Cl can react with unsaturated organics such as olefins in addition reactions to form C-Cl bonds
by generating a stabilized secondary carbon-centered radical.25, 26, 27, 28, 29, 30 Therefore, the
utilization of •Cl holds great promise for achieving efficient propylene oxidation. During
electrochemical chloroxidation, •Cl is typically produced via the Volmer step.31 Nevertheless, the
marked tendency of •Cl to mutual couple to generate Cl limits its effective utilization. Modulating
2
the adsorption behavior of Cl– on the catalyst surface is expected to reduce the probability of
coupling. For instance, Zhang et al. utilized discontinuous Cl– adsorption sites to inhibit Cl
2
evolution, thereby harnessing •Cl for the electrosynthesis of α,α-dichloroketones.32 Similarly, Liu
et al. enhanced •Cl utilization by increasing the spatial separation between •Cl species, which
mitigated mutual recombination and facilitated the •Cl-mediated epoxide ring-opening process.33
However, due to the lack of direct regulation over the intrinsic adsorption behavior of Cl–, the
utilization efficiency of •Cl remains suboptimal.

Herein, by precisely modulating Cl– adsorption configuration, we realized efficient •Cl-mediated
conversion of propylene to PO on Li-doped Co O , resulting in a production rate of 20.6 mol/m2/h,
3 4
representing 5.2 and 1.4 times higher over pristine Co O and commercial RuIrTi anodes, along
3 4
with a F.E. of 99.0% and long-term stability for over 100 h. Furthermore, a high PO yield of 78.9
mol/m2/h was obtained even under an industrial-level current density of 0.5 A cm-2. Mechanistic
studies revealed that Li doping altered Cl– adsorption configuration on Co O from a linear Co-Cl
3 4
structure to a more favorable triangular Li-Cl-O configuration, which significantly promoted the
generation and utilization of •Cl species. This enhancement in •Cl availability effectively
redirected the propylene oxidation from the conventional HClO route to a more efficient •Cl-
mediated pathway, ultimately resulting in a markedly improved yield and F.E. for propylene
epoxidation on Li-Co O .

Results
Preparation and characterization of Li-Co O
3 4
Co O samples containing varying amounts of Li doping were synthesized via a simple
3 4
impregnation method followed by flash Joule heating treatment (Fig. S1, Table S1).34, 35 High-
resolution transmission electron microscopy (HR-TEM) revealed that the layered structure of
Co O was retained, with the primary exposed facet was (111) after Li doping (Fig. 2a-b, S2). The
3 4
distribution of Li was further characterized using time-of-flight secondary ion mass spectrometry
(TOF-SIMS).36 As depicted in Fig. 2c-d, regions corresponding to fractured flake structures
exhibited higher Li concentrations, suggesting the potential infiltration of Li into the Co O lattice.
3 4
Depth profiling revealed an inverse trend in the distribution of Li and Co with depth strongly
suggests that Li had substituted part of Co sites within the Co O lattice (Fig. 2e). Inductively
3 4
coupled plasma atomic emission spectroscopy (ICP-AES) determined the Li mass loading to be
5.2 wt%. The X-ray diffraction (XRD) patterns of both Li-Co O and Co O matched well with
3 4 3 4
the standard card of Co O (PDF #97-002-8158) (Fig. S3). Raman spectra revealed redshifts in
3 4
both octahedral (688 cm-1) and tetrahedral (195 cm-1) Co-O vibrations,37 indicative of weakened
Co-O bonds and suggesting Li likely substituted Co sites within Co O (Fig. S4).38 The normalized
3 4
Co K-edge X-ray absorption near edge structure (XANES) spectra of Li-Co O displayed a
3 4
positive shift in the position of the adsorption edge compared with that of Co
3
O
4
(Fig. 2f),
indicating the valence state of Co was elevated due to Li doping. Similar results were observed
from the high-resolution Co 2p X-ray photoelectron spectroscopy (XPS) analysis, in which the
peak area ratio of Co2+/Co3+ decreased from 1.17 to 1.03 after Li introduction (Fig. S5).
The Fourier transformed extended X-ray absorption fine structure (FT-EXAFS) of the Co K-edge for
Co
3
O
4
showed three distinct peaks, assigned to Co-O, octahedral Co-Co
Oh
, and tetrahedral Co-
Co bonds, respectively (Fig. 2g).39 However, the Co-Co signal in Li-Co O significantly
Td Td 3 4
weakened (Fig. 2j-l), confirming Li substituted tetrahedral Co sites within Co O . Based on the
3 4
aforementioned analysis, we constructed a structural model featuring Li substitution at tetrahedral
Co sites (Fig. 2i), with the close agreement between fitted and experimental Co K-edge FT-EXAFS
providing conclusive confirmation of Li occupation in tetrahedral coordination within the Co O
3 4
lattice (Fig. 2h). DFT calculations further confirmed that the tetrahedral Co-substitution
configuration exhibits the lowest energy compared with Li occupying an octahedral site or an
interstitial site, thereby corroborating the preferential substitution of Li at tetrahedral Co sites in
Co O . (Fig. S6). X-ray photoelectron spectroscopy (XPS) O 1s spectrum revealed peaks at 529.9
3 4
eV, 531.2 eV, and 532.3 eV, corresponding to surface lattice oxygen (O ), adsorbed oxygen on
α
surface O (O ), and surface adsorbed H O (O ).The enhanced O signal in XPS denoted the
v β 2 γ β
generation of more O on Li-Co O , in line with electron paramagnetic resonance (EPR) results
v 3 4
(Fig. S7-8).40 Meanwhile, the O of Li-Co O shifted towards lower binding energy, indicating the
α 3 4
weakened Co-O covalency with Li incorporation and electron transfer along the Li-O-Co atomic
unit.41 The shift of peaks toward lower temperatures in the O temperature-programmed desorption
2
(TPD) profile of Li-Co O further confirmed the enhanced lattice oxygen activity (Fig. S9).42 The
3 4
valence of Li was +1 (Fig. S10).

Electrochemical propylene epoxidation performance
We evaluated the electrochemical performance of Li-Co O and Co O with a focus on their CER
3 4 3 4
behaviors. As illustrated by the linear sweep voltammetry (LSV) curves, Li-Co O exhibited better
3 4
water activation ability in chloride-free electrolyte. After replacing the electrolyte with 0.5 mol/L
NaCl, both Co O and Li-Co O showed largely increase in current density, whereas Li-Co O
3 4 3 4 3 4
demonstrated the quickest start with a sharp increase in the anodic current density, with an earlier
onset potential, underscoring the substantial enhancement in Cl– activation induced by Li doping.
Upon introducing propylene into a 0.5 mol/L NaCl, Li-Co O exhibited a significantly larger
3 4
current increase compared with Co O and the RuIrTi anode, demonstrating its enhanced activity
3 4
toward propylene oxidation (Fig. 3a, S11). Additionally, Li-Co O exhibited a larger
3 4
electrochemical surface areas and lower charge transfer resistance (Fig. S12-13, Table S2). Active
chlorine (AC) selectivity and F.E. of Li-Co O and Co O at different applied potentials were
AC 3 4 3 4
measured using iodometric titration,31 and the results showed that Li-Co O consistently exhibited
3 4
higher AC selectivity and F.E. across potentials from 1.8-2.2 V vs. RHE. Notably, Li-Co O
AC 3 4
achieved a F.E. of 99.3% even at a high potential of 2.2 V vs. RHE, compared to 76.4% for
AC
Co O and 74.8% for RuIrTi anode under identical conditions (Fig. S14). This highlighted the
3 4
improved Cl– activation activity and selectivity of Li-Co O . Subsequently, the electrocatalytic
3 4
performance of Li-Co O and Co O for propylene oxidation was evaluated in a H-type cell using
3 4 3 4
simulated neutral seawater (0.5 mol/L NaCl, pH 6.5).43 1H nuclear magnetic resonance (1H NMR)
and gas chromatography-mass spectrometry (GC-MS) analyses revealed that the anodic
electrooxidation products was chloropropanol. The cathodic reaction was characterized by
vigorous hydrogen evolution, rendering the electrolyte alkaline. By simply mixing the post-
reaction anodic and cathodic electrolytes, efficient conversion of chloropropanol to PO was
achieved (Fig. 3c, S15-20). Quantification of the resulting PO via 1H NMR demonstrated that Li-
Co O exhibited a dramatic enhancement in PO yield and F.E. at the potentials from 1.8-2.2 V vs.
3 4
RHE compared to Co O (Fig. 3b). At 2.2 V vs. RHE, Li-Co O achieved a PO yield rate and F.E.
3 4 3 4
of 20.6 mol/m2/h and 99.0%, respectively, representing a 5.2-fold and 1.3-fold increase over Co O
3 4
(4.0 mol/m2/h and 75.2%), and an enhancement of 1.4 times in both metrics compared to the RuIrTi
anode (15.1 mol/m2/h and 70.4%). The remaining F.E. on Co O and RuIrTi anode was attributed
3 4
to the OER side reaction, compounded by the incomplete utilization of intermediate chlorine
species due to disproportionation constraints (Fig. S21). As illustrated in the radar chart (Fig. 3e),
Li-Co O outperforms both pristine Co O and commercial RuIrTi anode across a broad spectrum
3 4 3 4
of key performance indicators including current density, F.E. , PO yield rate, F.E. and catalyst
AC PO
cost,31 demonstrating its well-rounded advantages. Notably, Li-Co O also demonstrated
3 4
competitive performance compared with previously reported catalysts (Table S3).2, 12, 13, 14, 16, 19, 41,
44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58. When natural seawater (collected from Yellow Sea, China,
120°18′ E, 35°82′ N) was used as the electrolyte, Li-Co O maintained a PO yield of 7.2 mol/m2/h
3 4
and F.E. of 74.5% at 2.2 V vs. RHE, demonstrating the practical feasibility of chlorine-mediated
propylene epoxidation in seawater (Fig. 3d, S22). The decline in activity may be attributed to the
gradual passivation of the electrode surface caused by the accumulation of complex alkali metal
cations present in natural seawater during the oxidation process.31, 59 Durability tests of Li-Co O
3 4
showed slight decreases in current density and F.E. over 100 hours of electrolysis in 0.5 mol/L
NaCl, while post-reaction characterizations confirmed that the Li-Co O retained its structural
3 4
stability, suggesting Li-Co O was stable during propylene electrooxidation (Fig. 3g, S23-24).
3 4
Moreover, Li-Co O maintained a PO yield of 78.9 mol/m2/h and F.E. of 84.5% with good stability
3 4
and selectivity even under industrial-level current density of 0.5 A cm-2 (corresponding to
approximately 4.2 V vs. RHE), whereas Co O rapidly deactivated under identical conditions (Fig.
3 4
3f, S25).

To assess the industrial viability of the electrochemical Cl-mediated route, we performed a techno-
economic analysis (TEA) to evaluate both the levelized cost of propylene oxide (PO) production
and the end-of-life net present value (NPV). The analysis revealed that the process became
economically viable when the operating current density exceeded 0.5 A cm-2 with a F.E. of at least
62%, or when a lower current density of 0.1 A cm-2 was maintained with a minimum F.E. of 56%,
assuming an electricity cost of $0.10 per kWh. Under these conditions, the •Cl-mediated
electrochemical epoxidation of propylene over Li-Co O , as demonstrated in this work, offers
3 4
significantly improved profitability (Fig. 3h, S26-28, Supplementary Text S1), underscoring
attractive industrial application potential.11, 41, 60

Identification of reactive chlorine species
Cyclic voltammetry measurements in 0.5 mol/L NaCl without C H revealed the Cl reduction (Cl
3 6 2
reduction or Cl desorption) peak near 1.7 V, which disappeared upon C H introduction and
ads 3 6
reappeared after its removal (Fig. 4a), confirming a Cl-mediated C H oxidation pathway.52, 61
3 6
HClO was generally considered to be the active chlorine species for olefin oxidation.15, 62, 63
Therefore, the HClO concentration in the electrolytes after reaction at different potentials in the
absence of C H were quantified by UV colorimetry (Fig. 4b, S29-30),64 and found the HClO
3 6
concentration in Co O was higher than that in Li-Co O . To probe the role of HClO, control
3 4 3 4
experiments were conducted by introducing C H into HClO standard solutions matching the
3 6
concentrations generated by Co O and Li-Co O at 2.2 V vs. RHE, followed by NaOH treatment
3 4 3 4
to fully convert chloropropanol to PO. The results showed that the PO yield rate was markedly
lower than that from Li-Co O -catalyzed electrooxidation, while Co O exhibited comparable
3 4 3 4
performance to its HClO counterpart (Fig. 4c). Similarly, no SPO formation was observed when
C H was introduced into NaClO solution. These results unambiguously demonstrated that, in
3 6
contrast to Co O , HClO was not the primary active species in the Li-Co O -catalyzed oxidation
3 4 3 4
process.

To further elucidate the specific active species responsible for C H oxidation on Li-Co O , in situ
3 6 3 4
EPR characterization was employed. The EPR spectra of Li-Co O revealed pronounced •Cl and
3 4
•OH signal within the voltage range of 1.8-2.2 V, with the signal intensity increasing with higher
potentials.65, 66 In contrast, no significant •Cl or •OH signals were detected on Co O (Fig. 4d, S31).
3 4
This implied that Li-Co O was more inclined to activate Cl– into highly reactive •Cl and can be
3 4
stabilized rather than rapidly converted to Cl , which was consistent with the result that HClO was
2
instead lower on Li-Co O . Moreover, a significantly stronger •OH signal was detected on Li-
3 4
Co O in chloride-free electrolyte (Fig. 4e), which may be attributable to the enhanced activation
3 4
of H O at O on Li-Co O surface.13 Under the concerted attack of •Cl and •OH, C H was oxidized
2 v 3 4 3 6
to chloropropanol, predominantly following the Markovnikov’s rule, with the Cl moiety
preferentially attaching to the terminal carbon of C H .67 Radical quenching experiments were
3 6
further carried out to confirm the decisive role of •Cl and •OH. When tert-butanol was added as
•Cl and •OH scavenger to the reaction system of Li-Co O , the yield and F.E. both drastically
3 4
decreased across all potential ranges, with reductions reaching as high as 96% and 92% at 2.2 V
vs. RHE. Similarly, upon the addition of nitrobenzene as a •OH scavenger, the PO yield and F.E.
markedly decreased to 1.5 mol·m-2·h-1 and 19.7% at 2.2 V vs. RHE, respectively (Fig. 4f),
demonstrating the critical involvement of •OH in the epoxidation process.68 Furthermore, the
kinetic isotope effect (KIE) value was measured to be 1.38 (Fig. S32), indicating that water
dissociation to produce hydroxyl played a crucial role in the epoxidation pathway.69 These results
provide robust evidence for the coexistence and cooperative mechanism of •Cl and •OH in the
epoxidation process.

DFT calculations were conducted to reveal the processes of chloride and water activation on Li-
Co O and Co O surfaces, demonstrating that Li doping markedly lowered the theoretical
3 4 3 4
overpotential for the CER from 0.46 to 0.07 eV (Fig. 4g). Notably, on pristine Co O , the formation
3 4
of *Cl served as the rate-determining step (RDS) with a high energy barrier of 1.82 eV. Upon Li
incorporation, this barrier significantly decreased to 1.29 eV, and the RDS shifted to Cl formation
2
with an energy barrier of 1.43 eV (Fig. 4h), thereby confirming the enhanced generation and
stabilization of •Cl on Li-Co O surface. Concurrently, Li doping also facilitated water activation
3 4
by reducing the energy barrier for •OH formation from 2.06 eV on Co O to 1.60 eV, in agreement
3 4
with the stronger •OH signal observed for Li-Co O in in situ EPR measurements (Fig. 4i).
3 4

Recognition and understanding of Cl– adsorption and activation
To elucidate the mechanism underlying the enhanced generation of •Cl on Li-Co O , we
3 4
investigated the adsorption behavior of Cl– given its pivotal role in the formation of active chlorine
species. Comparative high-resolution XPS analysis of Cl 2p spectra after reaction revealed an
increase in peak area for Li-Co O versus pristine Co O , demonstrating the creation of additional
3 4 3 4
active adsorption sites with enhanced Cl– chemisorption capability.31 The spectral deconvolution
showed fundamental differences in Cl– coordination, Co O exhibits a singular Co-Cl signal at
3 4
201.0 eV, while Li-Co O displayed two deconvoluted peaks at 201.7 eV (O-Cl) and 200.1 eV (Li-
3 4
Cl) (Fig. 5a).70 Meanwhile, in the post-reaction O 1s spectrum of Li-Co O (Fig. S33), an
3 4
additional signal at 530.5 eV corresponding to O-Cl species was observed.59 Those result
suggested that dual adsorption centers where Li and activated lattice oxygen may synergistically
stabilize chloride species.

DFT calculations were performed to explore the possible adsorption configurations of Cl– on both
Li-Co O and Co O surface. Based on experimental results, two plausible adsorption
3 4 3 4
configurations of Cl– on Co O were constructed, where Cl– coordinated with Co and O sites,
3 4
respectively (Fig. S34). For Li-Co O , four potential adsorption configurations were modeled,
3 4
including Cl– coordinated with Li, Co, and O sites, as well as a Li-Cl-O triangular structure via the
synergistic adsorption of Cl– by Li and lattice oxygen (Fig. S35). The results revealed that Cl–
predominantly adsorb on Co site with an adsorption energy of -1.29 eV on Co O . In contrast, Cl–
3 4
preferentially adopt a Li-Cl-O triangular adsorption mode on Li-Co O , exhibiting a much stronger
3 4
adsorption energy of -1.85 eV (Fig. 5b). To further determine the adsorption configuration of Cl–,
simulated Raman spectra were employed to compare the vibrational features of Cl– adsorption
modes on Li-Co O . When Cl– was co-adsorbed by Li and lattice oxygen to form a triangular Li-
3 4
Cl-O structure, distinct peaks appeared around 311 and 356 cm-1, consistent with the experimental
Raman features observed on Li-Co O after electrolysis in NaCl solution (Fig. 5c).31, 70 Charge
3 4
density difference and Bader charge analysis demonstrated that this triangular configuration
enabled Cl– to receive 0.19 |e| from the adjacent Li and O atoms (Fig. 5g), resulting in increased
electrophilicity of Cl– activation, which was favorable for the reaction activity.59 In contrast, linear
adsorption of Cl– on either Li or Co sites led to lower charge transfers of 0.16 and 0.13 |e|,
respectively, underscoring the enhanced activation capability of the Li-Cl-O motif (Fig. S36).

Based on the aforementioned results, we proposed the adsorption and activation mechanisms of Cl– activation were proposed. On pristine
Co O , Cl– was adsorbed and activated at Co sites, followed by a surface-mediated reaction with
3 4
solution-phase Cl– to generate Cl . The formed Cl subsequently underwent stoichiometric
2 2
disproportionation in aqueous solution to produce HClO, while the oxidized Cl⁻ were partially
reduced back to unreactive HCl, resulting in inefficient charge utilization. In contrast, on Li-Co O ,
3 4
Cl– was co-adsorbed and activated at Li and lattice oxygen sites through a Li-Cl-O coordination
motif, which facilitated the direct generation of highly reactive •Cl species, thereby enhancing the
overall oxidative efficacy.

The adsorption strength of Cl– was further characterized via H temperature-programmed
2
reduction (H -TPR) analysis (Fig. 5d). Li-Co O displayed distinct Cl– desorption peaks at 437 °C,
2 3 4
indicating stronger Cl binding compared to the lower-temperature peak at 273 °C on Co O .71
3 4
Given that both C H and Cl– are electron-rich, their adsorption are directly modulated by the
3 6
charge state of the catalyst surface. Zeta potential measurements revealed that Li doping reversed
the negative surface charge of Co O to positive (Fig. S37), thereby facilitating the adsorption and
3 4
activation of both C H and Cl–.31, 72 C H -TPD characterization demonstrated significant C H
3 6 3 6 3 6
desorption peaks at 304 °C and 420 °C on Li-Co O , whereas no significant desorption signals
3 4
were observed on Co O (Fig. 5e),7 indicating the substantial enhancement in C H adsorption
3 4 3 6
capacity induced by Li doping. Notably, Cl adsorption on Li-Co O elevated the C H desorption
3 4 3 6
temperature to 473 °C and enlarged the peak area, indicating strong reactivity of the Li-Cl-O
species toward C H . DFT calculations further confirmed those by showing adsorption energies of
3 6
-1.07 eV for C H on Li-Co O and -0.66 eV on Co O . Upon Cl– adsorption on Li-Co O , the
3 6 3 4 3 4 3 4
adsorption energy of C H was further increased to -1.20 eV, confirming the high reactivity of the
3 6
Li-Cl-O motif toward C H activation (Fig. 5f, S38).
3 6

The reaction pathways of electrochemical propylene epoxidation
In-situ electrochemical Raman spectroscopy was employed to track Cl– activation process on Li-
Co O and Co O surfaces. As shown in Fig. 6a, the characteristic Co-Cl stretching vibration at
3 4 3 4
142 and 245 cm-1,73 as identified by the reference spectrum of CoCl (Fig. S39), were observed on
2
Co O , confirming Co sites as the primary Cl– activation centers. In contrast, Li-Co O exhibited
3 4 3 4
simultaneous emergence of Cl-O (799 cm-1) and Li-Cl (724 cm-1) vibrational under applied
potentials,71 unambiguously demonstrating dual activation centers where both Li and lattice
oxygen synergistically served as Cl– adsorption sites (Fig. 6d, S40). Notably, with increasing
applied potential, Li-Co O displayed progressive intensification of vibrational features attributed
3 4
to the Li-Cl-O configuration at 310 and 381 cm-1, while no Co-Cl signals were detected. This
observation indicated that, unlike the Co-Cl coordination mode that dominated on Co O , Cl⁻
3 4
adsorption on Li-Co O preferentially proceeded via a Li-Cl-O tri-coordinated configuration,
3 4
highlighting a mechanistic divergence in chloride activation pathways induced by Li doping. To
obtain a deeper insight into the mechanism behind the improved electrocatalytic performance, we
monitored the reaction process of C H on the surfaces of Li-Co O and Co O . In the in-situ
3 6 3 4 3 4
electrochemical attenuated total reflectance surface-enhanced infrared absorption spectroscopy
(ATR-SEIRAS) spectrum of Co O (Fig. 6b), vibrational signals at 800 and 840 cm-1 were
3 4
observed, corresponding to a C-Cl-C cyclic structure formed via π-bond in C H interaction with
3 6
chloronium ions as indicated by theoretical frequency analysis, thereby confirming that the C H
3 6
oxidation on Co O predominantly followed a HClO-mediated pathway. In contrast, the
3 4
substantially attenuated of those signals observed on Li-Co O suggested that the oxidation of
3 4
C H followed a distinct mechanism in which not proceed predominantly through the HClO
3 6
pathway. Instead, C-Cl bond vibrations at 945 and 1043 cm-1, as supported by theoretical frequency
analysis, suggested a direct electrophilic addition of surface-generated •Cl to C H (Fig. 6e, S41).
3 6

Electrokinetic studies were conducted to further elucidate the oxidation mechanism.58 The current
densities on both Co O and Li-Co O were found to be proportional to Cl– concentration,
3 4 3 4
confirming the critical role of Cl– activation in C H oxidation, while Li-Co O displaying a steeper
3 6 3 4
slope indicative of stronger Cl– activation capability (Fig. 6c). Notably, the current density on
Co O depended solely on chloride oxidation and was insensitive to C H concentration,
3 4 3 6
corroborating a HClO-mediated oxidation mechanism without C H adsorption in bulk
3 6
electrolyte.51 In contrast, Li-Co O showed a current density that initially increased with C H
3 4 3 6
concentration before plateauing (Fig. 6f), indicating that the reaction between C H and •Cl
3 6
followed an Eley-Rideal mechanism wherein activated Cl sites on Li-Co O readily reacted with
3 4
C H at low C H concentrations, resulting in continuous current growth, whereas at higher C H
3 6 3 6 3 6
concentrations, the saturation of C H adsorption on Cl sites limited further current increase.74
3 6
DFT calculations were conducted to investigate the formation pathway of PO via the reaction of
C H with HClO in the bulk electrolyte on Co O , and with •Cl on Li-Co O (Fig. 6g). The results
3 6 3 4 3 4
revealed a substantially lower reaction barrier for C H oxidation on Li-Co O . Attributed to the
3 6 3 4
characteristic Li-Cl-O coordination structure, Li-Co O enabled the activation of Cl– into •Cl with
3 4
a low energy barrier of -0.07 eV. In contrast, Cl– activation on Co O requires overcoming a barrier
3 4
of 0.46 eV. The highly reactive •Cl spontaneously and exothermically bonded with C H , followed
3 6
by subsequent attack from •OH generated via H O activation at O , ultimately leading to
2 v
chloropropanol formation. In contrast, on Co O , the activated Cl– species preferentially evolved
3 4
into Cl , whose reversible hydrolysis with H O generates HClO via a high-energy barrier process.
2 2
Moreover, the reactivity of •Cl toward C H was found to be significantly higher than that of HClO.
3 6
The oxidation of C H by HClO required overcoming an energy barrier of 0.57 eV, in sharp
3 6
contrast to the spontaneous and exothermic reaction (-0.48 eV) between •Cl and C H . These
3 6
findings highlight the critical role of Cl adsorption configuration in governing the formation of
active chlorine species and underscore the higher reactivity of •Cl in C H oxidation.
3 6

Based on the foregoing analysis, distinct C H oxidation mechanisms over Li-Co O and Co O
3 6 3 4 3 4
were elucidated (Fig. 6h-i). For Li-Co O , Cl– were preferentially adsorbed and activated at Li and
3 4
O sites via a Li-Cl-O co-adsorption configuration, thereby enabling the generation of highly
reactive •Cl species. •Cl initiated C H oxidation by forming stabilized carbon radical
3 6
intermediates, which were subsequently attacked by •OH derived from H O activation to yield
2
chloropropanol, ultimately leading to epoxide formation via base-assisted deprotonation.69, 75 In
contrast, Co O activated Cl– at Co sites to generate Cl ,62 which subsequent disproportionation in
3 4 2
aqueous solution to produce HClO, enabling the oxidation of C H through a carbocation
3 6
intermediate pathway to produce chloropropanol, which subsequently hydrolyzing under alkaline
conditions to afford PO.

Methods
Chemicals and materials
Except noted, all chemicals were purchased and used without further purification. Cobaltous
nitrate (Co(NO ) •6H O) (AR), urea (CH N O) (AR), lithium nitrate (LiNO ) (AR), sodium
3 2 2 4 2 3
chloride (NaCl) (AR), potassium dihydrogen phosphate (KH PO ) (AR), sulfuric acid (H SO )
2 4 2 4
(98%), ethylenediaminetetraacetic acid disodium salt (EDTA-Na ) (AR), hydrochloric acid (HCl)
2
(AR) and dibasic sodium phosphate (Na HPO ) (AR) were purchased from Sinopharm Chemical
2 4
Reagent Co., Ltd. China. 5,5-dimethyl-1-pyrroline N-oxide (DMPO) (AR), Nafion solution (5
wt%), Dimethyl Sulfoxide (DMSO) (AR), Deuterium oxide (D
2
OS) (99.9%) were purchased from
Aladdin Chemical Co., Ltd. China. Deionized (DI) water with a resistivity of 18.2 MΩ•cm was
used. Propylene (99.99%) was purchased by NewradargRas (Wuhan, China). The DSA consisted of
10% Ir, 40% Ru and 50% Ti, and was purchased from Suzhou Shuer Tai Industrial Technology.
Seawater was collected from Bohai Sea, China, 120°81′ E, 40°61′ N.

Preparation of catalysts
Synthesis of Co O : A homogeneous solution was prepared by dissolving 30 mmol
3 4
Co(NO ) •6H O and 60 mmol urea in 100 mL deionized water under continuous magnetic stirring
3 2 2
at room temperature (25° C) for 1 h. The resulting mixture was subsequently transferred into a
Teflon-lined stainless-steel autoclave and subjected to hydrothermal treatment at 120 °C for 20 h.
After natural cooling to ambient temperature, the precipitate was collected via vacuum filtration,
repeatedly washed with deionized water to remove residual impurities, and dried at 90 °C for 12 h
in a convection oven. The dried intermediate was finely ground into a uniform powder, placed in
an alumina crucible, and calcined at 375 °C under a nitrogen atmosphere for 2 h. The final Co O
3 4
product was obtained following flash Joule heating treatment of the calcined powder.
Synthesis of Li-Co O : 1 g of Co O powder (prior to flash Joule heating) was dispersed into 40 mL
3 4 3 4
of 0.33 mol L-1 aqueous LiNO under vigorous stirring. The suspension was maintained at 80 °C
3
until complete evaporation of the solvent, yielding a dried composite. The resultant solid was
thoroughly pulverized and then calcined in an alumina crucible at 375 °C under a nitrogen
atmosphere for 2 h, followed by Joule thermal treatment to obtain the Li-Co O catalyst. A series
3 4
of Li-Co O samples with varying Li doping concentrations were synthesized by adjusting the
3 4
quantity of LiNO precursor during the impregnation step.
3
Joule heating treatment: The as-synthesized samples were subjected to a two-step pulsed Joule
heating process under vacuum. In the first stage, a discharge voltage of 20 V was applied with a
peak current of 30 A, a pulse duration of 100 ms, and a cycle period of 110 ms, repeated for 5 cycles.
In the second stage, the discharge voltage was increased to 30 V with a peak current of 36 A, while
maintaining the same pulse duration and cycle period, also for 5 cycles. Upon completion of the
discharge treatment, the samples were allowed to cool naturally to ambient temperature before
collection.

Characterization methods
The as-prepared catalyst of X-ray diffraction (XRD) patterns were measured using a Bruker D8
ADVANCE diffractometer equipped with Cu Kα radiation source (λ = 1.5406 Å) at 40 kV and 40
mA. The Li and Co contents were determined by inductively coupled plasma optical emission
spectrometry (ICP-OES, Thermo ICAEP PRO). Depth-resolved compositional analysis was
conducted using TOF-SIMS with a Tescan AMBER spectrometer. The high-resolution transmission
electron microscopy (HR-TEM) images obtained using a FEI TF20microscope. Different
elements of energy-dispersive spectroscopy (EDS) were obtained on the same equipment. The
scanning electron microscope (SEM) images were measured by Zeiss Sigma500. Electron
paramagnetic resonance (EPR) spectrum was collected using Bruker Micro 6/1. The liquid products
were quantified by a Bruker 400 MHz NMR spectrometer (Bruker AVANCE III HD). The gas
chromatography-mass spectrometry (GC-MS) data was collected by Trace 1300-ISQ (Thermo
Scientific, U.S.A.). The valences of different elements on the surface of the catalyst were analyzed
by XPS (Thermo Scientific Escalab 250 Xi analyzer), all binding energies were calibrated with C
1s = 284.8 eV. Temperature-programmed desorption (TPD) and temperature-programmed
reduction (TPR) experiments were conducted by Micromeritics AutoChem II 2920. The in-situ
attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of the samples towards
propylene oxidation were collected by a Nicolet iS50 FT-IR spectrometer (Thermo Scientific,
U.S.A.).

Catalytic performance tests
Preparation of the working electrode. A homogeneous catalyst ink was prepared by dispersing
20 mg of Co O or Li-Co O powder in 3.85 mL isopropanol, followed by the addition of 155 μL
3 4 3 4
of a 5 wt% Nafion solution. The mixture was subjected to ultrasonication for 30 minutes to achieve
complete dispersion. The resulting ink was then uniformly spray-coated onto a 1×1 cm2 carbon
paper. After allowing the solvent to evaporate completely under ambient conditions, the dried
electrodes were utilized for subsequent electrochemical characterizations.
Preparation of electrolyte. Accurately weigh 14.61 g of NaCl using an analytical balance and
dissolve it in 500 mL of deionized water to obtain a 0.5 mol L-1 NaCl solution as the electrolyte.
The solution was stored at room temperature (25° C) and was used within 3 days. The pH of the
solution was measured to be 6.5 ± 0.1 using a pH meter.
Treatment of membranes. The proton exchange membrane Nafion 117 (2.5 × 2.5 cm2, 183 µm).
was pretreated by heating in deionized water at 80 °C for 1 h, followed by treated with 5 wt% H O
2 2
at 80 °C for 1 h, then rinsed repeatedly with deionized water and soaked for 30 min. Subsequently,
it was treated with 5 wt% H SO solution at 80 °C for 1 h, rinsed thoroughly with deionized water,
2 4
and finally soaked in deionized water for further use.
Calibration of the Reference Electrode. The reference electrode was calibrated in 0.5 mol/L
H SO solution. The solution was saturated with H for 30 min before test, and Pt foil worked as
2 4 2
both working electrode and counter electrode, and the Ag/AgCl electrode was set as reference
electrode. The CV test was performed at a scan rate of 2 mV/s, and the onset potential of HER is
the zero potential relative to the RHE.
LSV test. The LSV test was performed in a H-cell. The H-cell contains a catholyte and an anolyte
chamber, which was separated by Nafion 117 membrane. Both sides of the cell contained 30 mL
electrolyte. In the typical three-electrode system, an Ag/AgCl electrode (Gaoss Union, saturated
KCl solution) and Pt foil (Gaoss Union, 99.9%) were used as reference electrode and counter
electrode respectively, and carbon paper with catalyst was used as working electrode (1 × 1 cm2).
All potentials in LSV test were measured against the Ag/AgCl with 80% iR-compensated and
converted to the RHE scale as follow:
E ( V v s . R H E ) = E ( V v s . A g / A g C l ) + 0 . 0 5 9 p H + 0 . 1 9 7 V (1)
Electrochemical epoxidation experiments. For the electrooxidation of propylene, the
electrochemical measurements were carried out in a H-cell using simulated seawater (0.5 mol/L
NaCl, pH = 6.5) as the electrolyte under ambient conditions. The anodic and cathodic chambers
with volumes of 30 mL were separated by Nafion 117 membrane. The area of the working
electrode was 1×1 cm2. Pt foil and Ag/AgCl electrodes were used as the counter electrode and
reference electrode, respectively. The potentials were controlled by CHI 760E (Shanghai Chenhua
Apparatus Corporation, China). High-current stability experiments were performed on a CS300M
electrochemical workstation (Wuhan CorrTest Instruments Corp., Ltd.). Agitation was maintained
throughout all electrochemical tests. All electrochemical epoxidation measurements were not
corrected for iR compensation. Propylene was continuously introduced at a flow rate of 30 SCCM,
all reported activity data were obtained after 0.5 h of electrolysis at each applied potential or
current density. At the end of electrolysis, the anolyte and catholyte were mixed in equal volumes,
enabling the formation of PO via a saponification reaction. The liquid products in the electrolyte
were quantified by 1H NMR analysis. Typically, 0.5 mL of the electrolyte after electrolysis was
mixed with 0.2 mL of D O which containing DMSO as internal standard.76 The NMR spectra were
2
measured on a Bruker 400 MHz NMR spectrometer with water suppression. The F.E. was
calculated as follow:
F . E . = c  V  Q / (N  F) (2)
where c represents the concentration of PO. V represents the volume of the electrolyte. N
represents the number of transferred electrons for products, which is 2 for PO. F is the Faraday
constant, and Q represents quantity of electric charge integrated by i-t curve.47 All activity data
were evaluated by at least three independent measurements.
For the high-current-density and long-term stability tests of electrochemical PO synthesis, the
anolyte and catholyte were continuously circulated through the electrolytic cell using peristaltic
pump. During electrolysis, propylene was continuously fed into the reactor at a flow rate of
30 SCCM. The mixed electrolyte was taken out at regular intervals for quantitative analysis via 1H
NMR.
The calculation method for C was determined by measuring the capacitive current associated
dl
with double-layer charging from the scan-rate dependence of CV in a non-faradaic potential window.
The CV measurements were conducted in a three-electrode cell equipped with an Ag/AgCl
reference electrode and a Pt counter electrode using 0.5 mol/L NaCl as the electrolyte. The CV
measurements were conducted at various scan rates (5, 10, 20, 30, 40 and 50 mV s-1) under Ar
atmosphere. The C was estimated by plotting the Δj (Δj = j - j ) at the middle of the scan range
dl a c
against the scan rates, where j and j were the anodic and cathodic current densities, respectively.
a c
The linear slope was twice that of the C value.
dl
Resistance measurements. The EIS measurements were recorded at OCP over a frequency range
from 100 kHz to 0.1 Hz with an amplitude of 10 mV.
Iodometric titration for AC detection
AC Faraday efficiency (F.E. ) was evaluated by iodometric titration. In a typical procedure, 30
AC
mL of 1 mol/L KI solution, 0.3 mL of 1 mol/L H SO , and 0.3 mL of starch solution were mixed
2 4
thoroughly to form a colorless solution. Subsequently, an appropriate volume of the anolyte
obtained after electrolysis was added, resulting in an immediate color change to blue. The blue
solution was titrated with 0.005 mol/L Na S O solution, the endpoint of the titration was indicated
2 2 3
by the disappearance of the blue starch-iodine complex. The volume of Na S O consumed was
2 2 3
recorded, and the FE was calculated using the following formula:31
AC
F . E . = c(Na S O )  V / (Q  F) (3)
AC 2 2 3 2 2 3
where c(Na S O ) and V(Na S O ) stand for concentration of Na S O solution and the volume of
2 2 3 2 2 3 2 2 3
Na S O used during titration, F is the Faraday constant, and Q represents quantity of electric
2 2 3
charge integrated by i-t curve.
HClO quantification
The concentration of HClO was quantitatively determined using 4-amino-N, N-diethyl aniline
sulfate (DPD) as the chromogenic reagent via UV–vis spectrophotometry, following the standard
method GBT 14424-2008. The anolyte collected after reactions at different applied potentials was
diluted tenfold prior to analysis. Briefly, 1 mL of sample solution was added into 1 mL of phosphate
buffer (pH = 6.5). Then, 1 mL of DPD reagent was added to the above solution and monitored at
550 nm with UV-vis 2500 spectrophotometer (Shimadzu, Japan) immediately. The standard curve
is plotted from absorbance versus the hypochlorite concentration.
O -TPD
2
O temperature-programmed desorption (TPD) experiments were conducted on a chemical
2
adsorption instrument (Micromeritics AutoChem II 2920). The samples (100 mg) of Co O and
3 4
Li-Co O were loaded into U-shaped quartz tubes respectively. The sample was pretreated with
3 4
pure Ar (30 mL/min) at 200 °C for 1 h and cooled to 50 °C to remove surface impurities. Thereafter,
the catalyst was exposed to 20% O (30 mL/min) at 50 °C for 1 hour. Unadsorbed O was then
2 2
purged with Ar (50 mL/min) for another hour. Finally, under a continuous Ar flow (30 mL/min),
the temperature was ramped from 50 °C to 700 °C at a rate of 10 °C/min.
H -TPR
2
H temperature-programmed reduction (T PR) experiments were conducted on a chemical
2
adsorption instrument (Micromeritics AutoChem II 2920) equipped with a mass spectrometer
(HRP-20 EGA) to collected the signals of HCl (m/z = 36) at atmospheric pressure. After reaction
in 0.5 mol/L NaCl solution at 2.2 V vs. RHE, 100 mg of Co O and Li-Co O samples were
3 4 3 4
separately loaded into U-shaped quartz tubes. The sample was pretreated with pure Ar (30 mL/min)
at 200 °C for 1 h and cooled to 50 °C to remove surface impurities, followed by ramping the
temperature at a rate of 10 °C/min from 50 °C to 500 °C in 10 vol.% H (30 mL/min). During the
2
temperature ramping process, surface-adsorbed chlorine species on the catalyst were removed by
hydrogen and released in the form of hydrogen chloride, which was subsequently detected by mass
spectrometry.
C H -TPD
3 6
C H temperature-programmed desorption (TPD) experiments were conducted on a chemical
3 6
adsorption instrument (Micromeritics AutoChem II 2920) equipped with a mass spectrometer
(HRP-20 EGA) to collected the signals of C H (m/z = 41) at atmospheric pressure. The 100 mg
3 6
sample was loaded into U-shaped quartz tubes. The sample was pretreated under a pure argon flow
(30 mL/min) at 200 °C for 1 hour and subsequently cooled to 50 °C to remove surface impurities.
Thereafter, the catalyst was exposed to high-purity C H (30 mL/min) at 50 °C for 1 hour to
3 6
achieve adsorption saturation. Unadsorbed C H was then purged with Ar (50 mL/min) for another
3 6
hour. Finally, under a continuous Ar flow (30 mL/min), the temperature was ramped from 50 °C
to 700 °C at a rate of 10 °C/min, and the desorbed C H were monitored using mass spectrometry.
3 6
XAS
X-ray absorption spectroscopy (XAS) spectra was obtained at the BL14W beamline of the
Shanghai Synchrotron Radiation Facility (SSRF). The spectra of Co K-edge were collected in
transmission mode by Lytle detector at room temperature (25° C). The collected XAS data were
processed using the Athena software package for background deduction, normalization and Fourier
transform. The FT-EXAFS spectra data were obtained by fitting using Artemis software77, 78. For
Wavelet Transform analysis, the χ(k) exported from Athena was imported into the Hama Fortran
code.
In situ ATR-FTIR
In-situ attenuated total reflection Fourier transform infrared (In situ ATR-FTIR) spectra of the
samples towards propylene oxidation were collected by a Nicolet iS50 FT-IR spectrometer
(Thermo Scientific, U.S.A.) in the region of 750-4000 cm-1 with the resolution of 2 cm-1. The
experiments were conducted in 0.5 mol/L NaCl solution with continuous C H flow to the three-
3 6
electrode PTFE cell. The catalyst was loaded onto an Au film-coated Si prism to serve as the
working electrode, with Pt wire as the counter electrode and Ag/AgCl as the reference electrode.
Each spectrum was collected after 180 s of electrolysis.
In situ Raman
In situ Raman spectra was collected by LabRAM Soleil (HORIBA FRANCE SAS) confocal
spectrometer equipped with a 532 nm single longitudinal-mode laser, with catalyst on carbon paper
serving as the working electrode, Pt wire as the counter electrode, and Ag/AgCl as reference
electrode. The testing was conducted in constant potential modes using a CHI 760E
electrochemical workstation. Each spectrum was collected after 180 s of electrolysis. The Raman
spectroscope was manually focused to locate the surface of the electrode using a 50× microscope
objective, and the exposure time was 3 s for each measurement. All Raman spectra were recorded
after calibration using a standard silicon wafer (520.7 cm-1) as the reference.
In-situ EPR
The in-situ electron paramagnetic resonance (in situ EPR) spectra were detected by EPR
spectrometer (Bruker Micro 6/1, Germany) with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and
2,2,6,6-tetramethylpiperidine oxide (TEMPO) as the spin traps. The applied potential started from
1.8 V vs. RHE and was increased in 100 mV intervals. At each potential, the reaction was
maintained for 180 s, after which 100 μL of the anodic electrolyte was accurately withdrawn and
mixed with 0.4 mol/L, 100 μL of DMPO or TEMPO. The mixture was immediately subjected to
EPR analysis. The instrumental parameters were set as follows: center field, 3510 G; sweep width,
200 G; sweep time, 30 s; receiver gain, 30 dB; modulation amplitude, 1.00 G; number of scan, 2;
microwave attenuation, 10 dB; microwave power, 20 mW.
DFT calculations
All the DFT calculations were conducted based on the Vienna Ab-inito Simulation Package (VASP).
The exchange-correlation effects were described by the Perdew-Burke-Ernzerhof (PBE) functional
within the generalized gradient approximation (GGA) method. The core-valence interactions were
accounted by the projected augmented wave (PAW) method. The energy cutoff for plane wave
expansions was set to 400 eV, and the 3×3×1 Monkhorst-Pack grid k-points were selected to sample
the Brillouin zone integration. The structural optimization was completed for energy and force
convergence set at 1.0×10-4 eV and 0.05 eV Å-1, respectively. Atomic coordinates of the optimized
computational structure are provided in Supplementary Data 1.
The Gibbs free energy change (ΔG) of each step is calculated using the following formula:
ΔG = ΔE +ΔZPE −TΔS (4)
where ΔE is the electronic energy difference directly obtained from DFT calculations, ΔZPE is the
zero point energy difference, T is the room temperature (298.15 K) and ΔS is the entropy change.
ZPE could be obtained after frequency calculation by:
ZPE = 1/2 hvi (5)
And the TS values of adsorbed species are calculated according to the vibrational frequencies:
TS =k T ln [1 + hv/k T] + ln [1/(1-e-hv/k T)] + 1 (6)
B B B
It is worth mentioning that the reaction-free energy diagrams were calculated under applied
electrode potential at U=1.36 V, the ΔG correction was considered as 1.36 eV for CER, correlated
ref
to the reversible potentials of the chlorine and hydrogen electrodes.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, Ming Cheng and co-workers report electrochemical propylene oxide synthesis using Li–Co3O4 catalysts. The authors claim that Li doping reconfigures chloride adsorption on the cobalt oxide surface, thereby promoting a chlorine radical–mediated pathway. They support this with spectroscopic analyses (EPR, FT-IR) and computational studies. The catalytic performance is impressive, and the mechanistic proposal is intriguing. I would recommend publication after the following issues are addressed:\ni) A central claim is that propylene oxidation proceeds via chlorine radicals. The in situ EPR study is presented as key evidence, but the Methods section lacks critical details. Please provide a clear description of the spin-trapping protocol (concentration of trapping agents, timing of addition, acquisition parameters). Also, control experiments should be conducted to confirm that spin traps do not interfere with the electrolysis.\nii) The EPR data suggest the involvement of both •Cl and •OH, yet only tert-butanol quenching is reported. Additional radical scavengers or spin trapping (e.g., benzoquinone, BMPO (5-tert-Butoxycarbonyl-5-methyl-1-pyrroline N-oxide), or TEMPO) would provide complementary mechanistic evidence.\niii) The manuscript emphasizes chlorine radicals but also shows stronger •OH signals. The authors attribute this to H2O activation at oxygen vacancies. How do •Cl and •OH interact in the reaction network? Is there any possibility of overoxidation by •OH at higher potentials that could compromise PO selectivity?\niv) A D2O/H2O kinetic isotope effect (KIE) study may help to confirm the hydroxyl involvement in the epoxidation process.\nv) Electrochemical analysis requires clarification. Potentials are presented without iR-compensation and referenced to Ag/AgCl, making comparisons with CER/OER a bit ambiguous. Please provide iR-corrected polarization curves, convert potentials to the RHE scale, and quantify solution resistance and membrane contributions by EIS. Reported onset shifts and performance enhancements should be re-evaluated accordingly, to show that the obtained results are intrinsic to Li doping rather than ohmic artifacts.", "reviewer_2": "In this manuscript, the authors successfully incorporated Li into Co3O4 via flash Joule heating treatment, attained a remarkable propylene oxide (PO) Faradaic efficiency approaching 99.0% and a production rate of 20.6 mol m-2 h-1 in simulated neutral seawater, along with excellent stability exceeding 100 hours. Mechanistic investigations revealed that HClO was the primary active species in the Co3O4-catalyzed oxidation process, while on Li-Co3O4, the more efficient •Cl pathway becomes dominant. This shift in reaction mechanism is attributed to the Li doping transformed the Cl- adsorption configuration from a linear Co-Cl structure to a more favorable triangular Li-Cl-O configuration, thereby enhancing •Cl generation. This work not only demonstrates a promising route for utilizing seawater resources in organic synthesis but also offers new mechanistic insights into chlorine-mediated electrocatalytic propylene oxidation. However, there are some shortcomings in the manuscripts.\n1. In this paper, the author highlights the advantages of the •Cl pathway over the HClO route. However, radicals are generally known for their inherent lack of selectivity. Why the •Cl-mediated process in this system still achieves high selectivity towards propylene oxide? The author should give more discussion.\n2. In Figure 3b, the RuIrTi electrode shows the highest HClO production, yet the Faradaic efficiency for PO is comparable to that of Co3O4. This observation appears inconsistent and needs further explanation.\n3. In Figure 4c, the experimental Raman spectrum lacks the peak at 155 cm-1 that is present in the simulated spectrum. The author should give more discussion.\n4. In line 275, the formation of HClO on Co3O4 is attributed to the evolution of Cl2. However, HClO could also be generated through the direct oxidation of Cl- (Appl. Catal. B-Environ. Energy 2025, 361, 124644). The authors should provide the corresponding evidence.\n5. In the proposed mechanism, the authors suggest that transient and highly reactive •OH attacks transient carbon radical intermediates to form chloropropanol. However, the selectivity and efficiency of coupling between two transient, highly reactive free radicals are theoretically limited (Sci. China Chem. 2025, 68, 3770-3777, J. Am. Chem. Soc. 2018, 140, 2438−2441). The authors should reconsider this pathway or supply additional data to validate the role of •OH.\n6. Figure 3a shows a significant enhancement in the Cl⁻ reduction peak intensity following the addition and removal of cyclopropane, despite no theoretical change in chloride ion concentration. The authors should provide an explanation and discussion for this discrepancy.\n7. All electrode potentials presented in the text and figures should be converted to the reversible hydrogen electrode (RHE) scale to ensure consistent and standardized reporting.", "reviewer_3": "In this manuscript, the author designed a catalyst by doping lithium recovered from spent lithium-ion batteries into Co₃O₄, enabling highly efficient electrocatalytic synthesis of propylene oxide in simulated seawater via enhanced active chlorine generation. This whole work not only offers new insights into the mechanism of propylene epoxidation but also opens a new pathway for environmentally sustainable and low-cost synthesis of industrial chemicals. There are only two minor questions that need to be supplemented:\n1. The Li-Co₃O₄ catalyst exhibited good stability during the 100-hour test. Is there any data indicating that Li⁺ does not leach or migrate under strong anodic polarization conditions? Is it necessary to perform post-reaction characterization (such as ICP or XPS) of the catalyst to confirm the retention of Li and the local structural stability?\n2. In the catalyst preparation process, flash Joule heating was employed to treat Li-doped Co₃O₄. The article merely mentioned the treatment steps without elaborating on the relevant flash Joule heating parameters (such as heating rate, holding time). The author may supplement as necessary.", "reviewer_4": "This study presents a incorporates lithium into Co₃O₄ via flash Joule heating treatment, achieving efficient chlorine radical-mediated conversion of propylene to propylene oxide (PO), with an excellent PO Faradaic efficiency approaching 99.0% and a yield of 20.6 mol/m²/h in simulated neutral seawater, maintaining long-term stability for over 100 hours. This study provides new insights into the mechanism of electrocatalytic propylene oxidation. Overall, this work holds potential for publication in Nature Communications, but several major issues must be addressed to enhance experimental rigor and clarity before I can recommend for its publication.\nMajor revisions:\n1. Through FT-EXAFS analysis, the authors concluded that Li preferentially replaces tetrahedral Co sites (as evidenced by the significant weakening of the Co-Co Td signal) and constructed a corresponding structural model. Did the authors perform theoretical calculations to support this selective replacement tendency? What is the energy difference between Li⁺ entering tetrahedral and octahedral sites? Are there other possible Li occupation sites (such as interstitial sites)?\n2. The TOF-SIMS results (Figures 1c-1d) showed a higher concentration of Li in fractured flake regions. Does this imply that the Li doping process is influenced by material morphology or defect density? Could such non-uniform distribution induce compromise the reproducibility of overall catalytic performance?\n3. Multiple characterization techniques indicated that Li has entered the lattice and induced changes in electronic structure and local coordination environment, however, no noticeable peak shifts were observed in the XRD patterns (Figure S2). Could the authors explain why no detectable lattice expansion or contraction occurs upon Li doping? Is this due to the low doping level, or could it be attributed to the similar ionic radii of Li⁺ and Co²⁺? Please provide ICP or other quantitative analyses to confirm the actual Li doping content.\n4. In the active chlorine (AC) generation (Figure S11) and propylene oxidation tests (Figure 2b), for both Co₃O₄ and RuIrTi catalysts, the lost Faradaic efficiency, could this be attributed to OER or other side reactions? At least, the FE for OER should be measured at one potential to clarify the fate of the missing electrons (FE).\n5. During the stability test in Figure 2h, the reaction potential is extremely high (up to 3.6 V vs. Ag/AgCl or may be > 4 V vs. RHE?), which appears inconsistent with the LSV results in Figure 2a. This observation is quite surprising, as both the catalysts and carbon paper substrate remain stable at such a high potential. Have the authors did any characterizations of the catalyst after stability reaction?\n6. Theoretical calculations suggest that the OER energy barrier is reduced on Li-Co₃O₄ (Figure 3i). However, during the OER test in Figure 2a, the OER performance of Li-Co₃O₄ shows no improvement compared to Co₃O₄, which seems contradictory. Could the authors discuss this discrepancy?\n7. The authors employ several characterization techniques commonly used in thermal catalysis studies (such as H₂ temperature-programmed reduction (H₂-TPR) in Figure 4d and temperature-programmed desorption (TPD) in Figure 4e) to investigate species adsorption. However, the real electrochemical environment involves a solid–liquid interface. So can these gas-phase-based methods or characterizations accurately reflect the adsorption behavior of the catalyst under actual operating conditions?\n8. Why is the formation of Cl* on Li-Co₃O₄ thermodynamically spontaneous (Figure 5g)? Were the theoretical calculations performed under applied electrode potential?\nOther minor comments:\n1. Regarding the O 1s XPS spectra, more detailed assignment of different oxygen species should be provided, rather than simply labeling them with α, β or γ.\n2. The manuscript should include more experimental details. The current version lacks sufficient information, making it confusing and difficult to follow. For example, various reaction solutions, including artificial seawater and real seawater are used in experiments in Figure 2. The exact composition of the artificial seawater should be specified, and it should be clarified whether the electrolytes used in Figure 2g–2h are identical to those in Figure 2a.\n3. The LSV tests with propylene and NaCl should also be provided, similar to Figure 2a.\n4. The assignment of EPR signals for Cl• and OH• radicals should be supported by cited references or spectral simulations in Figure 3d."}, "author_response": "We greatly appreciate reviewer’s positive comments on our study. Per reviewer’s suggestions, additional experiments have been conducted, with new results and corresponding discussions added in the revised manuscript and Supplementary Information, which have significantly improved the quality of this work. [Detailed point-by-point responses follow for all reviewers, including R1-i through R4-minor-4 as provided in the text]."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have adequately addressed my comments, and I believe the manuscript is ready for publication.", "reviewer_2": "The authors provide detailed and satisfactory responses to the comments raised, which makes the manuscript more concise and clear. Therefore, I am in favor of its publication.", "reviewer_3": "The revision has addressed the comments from reviewers properly. I recommend this manuscript to be accepted.", "reviewer_4": "The authors have sufficiently addressed all my previous comments, the manuscript can now be recommended for publication."}, "author_response": ""}]

Decision letter