Sulfophenylated centimeter-sized graphene membrane in a direct methanol fuel cell

Domain: earth_science
Published: 2025-11-27
Editorial outcome: major_revision

Abstract
An ideal proton exchange membrane should only allow protons to pass and remain impermeable to fuels, as required in fuel cell applications. In methanol fuel cells, high proton conductivity enables high power density, whereas methanol crossover between the electrodes degrades the catalyst activity and lowers efficiency. In conventional polymer membranes, however, conductivity and selectivity are often antagonistic: long transport pathways are needed to achieve selectivity, but these introduce additional ionic resistance. Graphene, a two-dimensional material consisting of a single atomic layer of carbon, intrinsically addresses both requirements. Its basal plane is impermeable to water and other molecules, while still exhibiting a measurable degree of proton conductivity. Here, we show that chemical functionalization of monolayer graphene with sulfophenyl groups significantly enhances its proton transport properties. The conductance increases from 6.9±1.1 to 30.9±2.3 S·cm-2, while the energy barrier for proton transport is reduced to 6.9 kJ·mol-1. These findings propose functionalized graphene as an alternative to polymer membranes for electrochemical energy devices.

Introduction
Protons, as one form of hydrons1, can translocate through graphene and other 2D materials such as hexagonal boron nitride (h-BN)2 and 2D mica3 at room temperature, making two-dimensional (2D) materials promising candidates for proton-exchange membrane applications4. Beyond monolayer 2D materials, proton transport has also been demonstrated using 2D laminates (i.e., 2D crystals assembled in a layered structure, made from graphene flakes5, and other 2D nanosheets6,7, and covalent organic frameworks8), making them suitable for membrane-based applications. Remarkably, functionalizing 2D materials and reducing interlayer distances down to sub-nanometer dimensions yielded membranes with high selectivity, as well as opportunities to observe ion transport under highly confined spaces9,10. However, the ion transport distance in these systems is currently, particularly in laminated structures11, beyond ‘one atom’, thus yielding additional parallel resistances. Protons were also shown not to permeate through defect-free regions of graphene12, which demonstrates the need for proton-selective pathways. Open questions focus on the role of defects and strain13, and – we believe with our work – also the role of out-of-plane polar sp3 distortions installed onto the graphene basal plane.

The main challenge for polymeric membranes lies in balancing proton conductivity and selectivity, with methanol crossover posing an additional obstacle due to its high solubility in water and uncharged nature, which facilitates migration from the anode to the cathode. To mitigate this issue, various strategies have been pursued to enhance the power output of direct methanol fuel cells (DMFCs)14. For instance, incorporating layered additives has been shown to improve performance; the power output increased to 35 mW cm−2 with nano-silica additives15 and to 39 mW cm−2 with graphene oxide16. The power output of DMFCs using a Nafion 117 membrane was enhanced to 79 mW cm−2 by impregnating the membrane with Pd17. Additionally, recasting the Nafion membrane with blended additives has been shown to boost DMFC to over 100 mW cm−2 of the maximum power outputs18,19. The power output of DMFCs was enhanced to 25 mW cm−2 by sandwiching single-layer graphene between two layers of Nafion membrane20 and increased to 120 mW cm−2 by incorporating single-layer graphene onto the electrode21.

To open pores enabling transmembrane ion transport in graphene22 and other 2D materials, such as MoS2 23, methods have been established using electron beams, ion bombardment, and plasma exposure24–30. Ion bombardment allows obtaining a high pore density and small pores (>1014 cm−2)25, while as generated porous graphene suffers from a lower control over the resulting pore size distribution and chemistry. Pore and pore-like defects have also been realized via bottom-up approaches, including chemical vapor deposition (CVD) growth of amorphous carbon31,32, using 2D polymers33,34, graphyne35, and graphdiyne36,37. These bottom-up strategies benefit from remarkable scalability in combination with the possibility of controlling the size, shape, and chemistry of the pore, unfortunately, yet with pores that are larger than the diameter of a hydrated proton. The controlled manipulation of electron density and hydrogenation within a double-gated graphene lattice also results in an adjustable proton conductance38. In CVD graphene, cracks and defects are commonly observed at the microscale39, influencing its transport properties. At a smaller scale than larger-sized cracks, theoretical studies indicate that small lattice disorders, such as hydrogenated graphene seven- and higher-membered rings, effectively lower the proton permeation barrier through graphene40–42. Diffusive proton conductance experiments also demonstrate improved proton transport through graphene with eight-membered rings43, vacancy defects25, and other structural modifications such as nitrogen-doped graphene44. Therefore, due to the presence of intrinsic defects, the proton conductance of CVD graphene was reported up to three orders of magnitude higher than that of defect-free graphene 2D crystals45.

Results
Here, we introduce sp3 lattice distortions paired with sulfophenyl groups on the basal plane of single-layer graphene via a diazotization strategy46 using 4-sulfobenzenediazonium (4-SBD) as a reactant (Fig. 1A, Supplementary material, section 2.1). The functionalization converts sp2 carbons from graphene into an active sp3 site carrying a hard charge (from the sulfonate; pK~−2.8 in water) and a resulting out-of-plane dipole. Chemically, a delocalized electron from the graphene lattice reacts with the 4-SBD cation, leading to the covalent grafting of a sulfophenyl radical orthogonally to the graphene lattice with N2 expelled as a leaving group. The resulting aryl radical reacts with a sp2 carbon atom on the graphene basal plane, which is converted to sp3 upon grafting. Meanwhile, side reactions might also occur: 4-SBD can react with an already grafted aryl group (at the ortho position) either via a radical or azo-coupling mechanism, both of which might result in oligomerization47 of the aryl species on the graphene basal plane.

Using circumcoronene models (only the middle pyrene unit is shown in Fig. 1B), we calculated the extend of lattice expansion upon diazotization, including hydrogenated and phenylated circumcoronene (Table S1, see Supplementary material 1.1) to explore their impact on proton transmembrane conductance through graphene membranes. Figure 1B shows that larger functional groups induced greater distortion in the hexagonal ring (up to 6.8% for double sulfophenyl functionalization), resulting in reduced potential energy barriers and increased proton flux. Functionalization, on both sides, with phenyl or sulfophenyl groups increased the proton flux by eight to nine orders of magnitude, while hydrogenation only increased the flux by one order of magnitude. Notably, quantum tunneling was less prominent with larger ring sizes. Additionally, we investigated the shuttling of the proton from the water bulk to the graphene basal plane by performing 1.0 ns of ReaxFF MD plus metadynamics (see Supplementary Material 1.2)48 on a model system with one sulfophenylated sp3 functionality bonded to the graphene basal plane (Fig. S3). First, the proton is captured from the water bulk by the SO3- functionality with a barrier of ~0.15–0.20 eV and then shuttled and released closer to the basal plane (~3 Å) with a lower barrier of ~0.1 eV (Fig. S4). Next, the proton could tunnel the graphene basal plane to the other side of the graphene.40,49 The energy barrier of the proton quantum tunneling for the case of hydrogenated graphene was estimated by Feng et al. to be <1.0 eV.40 The distortion produced by the sulfophenylated functionality when compared to a case involving only hydrogenations shows an enhanced proton flux (Fig. 1B) with a more efficient tunneling.40 Therefore, the proton-shuttling effect of the sulfophenylated functionality coupled to the enhanced proton tunneling induced by the distortion of the graphene basal plane--where the sulfophenylated functionality is located-- fosters an effective proton-transport mechanism.

Next, we prepared SO3--graphene as a proton-selective membrane in a direct methanol fuel cell (DMFC) (Fig. 2A). Centimeter-sized single-crystal CVD graphene films, which were seamlessly stitched into well-aligned domains50, were used as a source of graphene with minimal defects and grain boundaries. Figure 2B shows scanning electron microscopy (SEM) images of >200 μm wide CVD graphene single crystals grown on a copper foil. Multilayer patches were negligibly observed in the samples. However, sparsely distributed defects and cracks introduced during graphene manipulation, particularly during its assembly in the DMFC shown in Fig. 2A, cannot be entirely prevented and may contribute to additional permeation in contrast to the proton conductance that would be measured with perfectly defect-free graphene. We then transferred CVD graphene to a quantifoil TEM grid and incubated graphene with 4-SBD for up to 6 days (1 mg·ml-1 4-SBD in 0.1 M HCl) and characterized SO3--graphene by atomic force microscopy (AFM, Fig. 2C) and high-resolution transmission electron microscopy (HRTEM, Fig. 2D). From the AFM images, the arithmetic average (Ra) and root mean square roughness (RMS) increased by 0.5 nm within the first 24 h of incubation (Fig. 2C). After 24 h, only a minimal change in surface roughness occurred (Fig. 2C and Fig. S18). However, from the error image channels (the bottom panel in Fig. 2C, for a spatially better-resolved image) a clear change in morphology occurred for longer treatment times: surface features on graphene such as folds resulting from the transfer became decreasingly visible while denser agglomerates appeared upon longer 4-SBD treatments. Pristine graphene is therefore assumed to be covered by 4-SBD already after 24 h after which the 4-SBD layer becomes thicker and more homogenous. Oligomerization, which is commonly observed during diazonium treatment46 and here on AFM images (the top panel in Fig. 2C), could likely lead to additional membrane resistance. The spontaneous oligomerization of SBD, giving rise to agglomerates visible in AFM, was however not distinguishable by HRTEM from common hydrocarbon contaminations resulting from the growth of CVD graphene (supplementary material, Fig. S20-S22). We also used HRTEM to check the potential damage induced by the chemical functionalization at the nanometer scale. HRTEM images of graphene after 4-day and 6-day of SBD treatment did not show the apparition of large >1 nm pores (Fig. 2E). Instead, the large defect-free graphene area suggests that individual grafting sites could not be resolved by HRTEM on the monolayer regions (Fig. 2E). Additionally, no apparent expansion of the graphene lattice due to sp3 distortions were observed by HRTEM. To confirm covalent functionalization, we conducted Raman spectroscopy and elemental analysis and the results are discussed in the next section.

Additionally, the intensity ratio of the 2D peak (~2700 cm−1) over the G peak (0.6 < I2D /IG < 0.8) indicates no major changes in the quality and number of layers of graphene, although the value is not 2.0 in the case of polymer (Nafion) coated graphene52 (see Supplementary material Fig. S15, I2D /IG > 2.0 for untreated graphene, on SiO2 /Si, and after 5 days treatment I2D /IG = 0.6).

Additionally to Raman, we used X-ray photoelectron spectroscopy (XPS) to study the sulfur content and hybridization of the carbon atoms on the lattice upon 4-SBD functionalization. Figure 3B shows XPS core level spectra (C1s, N1s, and S2p) of pristine graphene and after 2 days, 4 days, and 6 days of treatment with 4-SDB. The fitting of the C1s spectral leads to four distinct species including the characteristic C sp2 (BE≈284.5 eV) and C sp3 (BE≈285.3 eV) of graphene.

Moreover, semi-quantitative analyses were also conducted to determine the chemical composition of the grafted sites (Table S2). The sp3 C/sp2 C ratio, initially at 0.2 for pristine graphene, indicates impurities from transfer and manipulation as pristine graphene should be free of sp3 hybridization. Therefore, the absolute value of the sp3 C/sp2 C ratio could not reveal the number of sp3 sites introduced by the SBD treatment. However, discussing the sp3 C/sp2 C ratio evolution reveals the state of sp3/sp2 conversion of graphene upon 4-SBD treatment. Relative ratios of -SO3- and -N=N- with respect to the total chemical species from XPS, particularly the S2p, C1s and N1s spectrum, indicate the relative increase of sulfophenyl groups grafted on the graphene surface vs reaction time. For ease of comparison, we plotted sp2 C/sp3 C and -SO3-, -N=N- in Fig. 3C. Incubating graphene with diluted 4-SBD (0.1 mg·ml−1 instead of 1 mg·ml−1), resulted in no more changes in the relative sp3 C/sp2 C (0.86) and S2p content (1.7) after 2-day of SBD treatment (Table S2), validating that the changes in relative component ratios were raised by 4-SBD. On the contrary, the sp3 C/sp2 C ratio, for the treatment with 1 mg·ml−1 of 4-SBD, first increased to 1.0 after 2-day treatment and then dropped to 0.3 after 6-day treatment. Indeed, the oligomerization procedure only introduces more sp2 C in benzene rings instead of more sp3 sites on graphene. Accordingly, the relative ratio of -SO3- increases from 0 for pristine graphene to 5.2 after 4 days of treatment and further increases to 6.4 after 6-day of treatment. The relative ratio of -N=N- shows a similar trend to SO3-, which also supports simultaneous azo coupling of sulfophenyl groups, in agreement with AFM. According to the Raman results, however, the intensity of the D peak plateaued after 4 days of treatment, suggesting that only oligomerization (coupling between 4-SBD and grafted sulfophenyl groups) occurred for treatment times longer than 4 days.

Next, we tested SO3--graphene as a membrane in a DMFC. We measured three independent batches of SO3--graphene after each day of treatment (i.e., after 0, 1, 2, 3, 4, 5, and 6 days; 21 samples in total) and determined maximum power densities, membrane conductance, and methanol crossover. Figure 4A shows the power-current (P-I) and voltage-current (V-I) plots for pristine CVD graphene, Nafion 117, and SO3--graphene at 60 °C. To assess the contribution of the spin-coated Nafion ionomer, we tested the Nafion ionomer/PC membrane in DMFCs. The membrane exhibited high permeability to the methanol/water mixture, resulting in almost no cell current (Fig. S24). This confirms that the graphene layer serves as the primary selective barrier, and the DMFC observations effectively demonstrate the selective proton conductance of graphene.

Pristine graphene showed a maximum power density of 25.8±15.4 mW·cm−2, at 212 mV, which is about half (~45%) compared to Nafion 117. Using pristine graphene, we would expect minimal power output unless defects and cracks form during the membrane electrode assembly (MEA) that incorporates CVD graphene. It is notable that, however, for SO3--graphene after 4 days of reaction time with 4-SBD, the maximum power density increased to 109.8±14.8 mW·cm−2 (i.e., ~2 times higher than Nafion). The power output did not increase further with a prolonged 4-SBD reaction time. Instead, it decreased to 44.9±17.3 mW·cm−2 after a 6-day treatment, again in line with our hypothesis of the oligomerization of the sulfophenyls after 4 days of treatment. Subsequently, we monitored the power density for a range of operating temperatures ranging from room temperature (rt, 20 °C) to 70 °C (Fig. 4B). In general, an increase in temperature leads to an elevated catalytic activity as well as an increased diffusion rate of both protons and methanol across the membrane. As expected, the power density for 4-day treated SO3--graphene increased linearly with the temperature rising from 20 °C to 70 °C (Fig. 4B). In contrast, for pristine graphene and 6-day treated graphene, the maximum power densities did not show a linear increase with temperature, and even decreased for operation temperatures above 50 °C. We attribute this observation to a higher methanol crossover rate at higher temperatures, suggesting the importance of an optimal 4-SBD treatment time of 4 days. In fact, in the pristine sample, proton transport occurs primarily through inherent defects, which have poor proton selectivity, and we presume therefore more methanol crossover. Similarly, for the oligomerized samples, the proton selective sites are likely to be blocked, resulting in a relatively low proton conductance. As a result, inherent defects also facilitate the methanol crossover.

To establish a correlation between areal proton conductance and DMFC performance (i.e., power density; and in the next section also fuel crossover rates), we first investigated how the membrane resistance of SO3--graphene correlates with the operating temperature of the fuel cell. Figure 4C shows the equivalent circuit and Nyquist plots obtained by electrochemical impedance spectroscopy (EIS), in which the intercept of the x-axis indicates the overall resistance of the membrane. Remarkably, the lowest resistance was also achieved after 4 days of treatment with 4-SBD (i.e., R = 0.59 Ω, which is ~30% lower than 6-day treated graphene and 2.5-fold lower than Nafion 117). The higher resistance after 6 days of reaction with 4-SBD also suggests a hindrance of the proton transport pathway due to oligomerization.

As opposed to the conductance normalized to the electrode area, electrodes cover the entire SO3--graphene membrane, while graphene is only free-standing over 16% compared to the electrode area (i.e. only ~16% of the polycarbonate membrane area is open with free-standing graphene). Importantly, pristine CVD graphene shows a proton conductance of 6.9±1.1 S·cm−2, normalized to the exposed CVD graphene area, which originates from defects in the form of pinholes or cracks in CVD graphene after the membrane electrode assembly13,53, at room temperature (20 °C). For SO3--graphene, similarly, the areal conductance observed after 4 days of reaction time is 30.9±2.3 S·cm−2 per free-standing graphene area (25 devices measured; respectively 33.8 S·cm−2 for the one device where the membrane was mounted in the reverse direction with the sulfophenylated groups facing the anode, see the Supplementary material Section 3.2 and Fig. S30).

By analyzing the measured conductance and temperature-dependence, the proton conductances of SO3--graphene membranes exhibit an Arrhenius behavior as described by Eq. (1):

ln(σT) = ln(σ0) - (Ea / kB × T) (1)

where σ is the measured conductance, T is the temperature, Ea is the activation energy, and kB is the Boltzmann’s constant. Samples after 1, 2, 3, and 4 days of 4-SBD treatment show a linear relationship of ln(σT) vs. T-1, with a fitted Ea of 6.9 kJ·mol−1 (0.072 eV) after 4 days of 4-SBD treatment (Fig. 4D, and Table S3). Pristine graphene does not show an Arrhenius behavior. The relatively low Ea after 4 days of 4-SBD treatment suggests that protons cross the membrane via a Grotthus mechanism (≤0.4 eV) where the proton hops through the membrane at a faster rate than dissociating from the hydronium ion54. After 5-day and 6-day, ln(σT) vs. T-1 presents a less linear relationship compared to 4-day of treatment (Fig. S29), which could be caused by the polymerization of 4-SBD, therefore hindering proton transport, leaving larger defects/cracks with a lower proton selectivity compared to methanol crossover.

Next, we evaluated the methanol crossover by testing the DMFCs with a higher methanol concentration and determining fuel efficiency (details see Supplementary material section 3.3). After a switch from 1 M to 5 M methanol, the maximum power density with Nafion 117 dropped by ~53% (Fig. 4E), while remaining similar for sulfophenylated graphene. Additionally, the drop in maximum power density, switching methanol from 1 M to 5 M, was reduced by ~10% after 4 days of reaction time with 4-SBD (Fig. 4F). The fuel efficiency increased after 4-day of 4-SBD treatment, indicating that the introduced proton path is selective for proton over methanol (Fig. S31). For the 4-day SO3--graphene at 60°C, η reaches ~79% at a current density of 457.5 mA cm-2.

To conclude, the functionalization of centimeter-sized CVD graphene with a sulfophenyl radical increases the membrane conductance of CVD graphene in a DMFC from 6.9±1.1 S·cm−2 to 30.9±2.3 S·cm−2 at room temperature (20°C), with a power density of 127 mW·cm−2 at 60°C, which is more than twice the power density measured with Nafion 117. The activation energy barrier of -SO3--graphene is as low as 6.9 kJ·mol−1, indicating a reduced proton transport barrier, in line with the improved proton conductance through graphene after sulfophenylation. However, further investigations are required to understand the factors contributing to enhancing the proton conductance, particularly defects, sp3 distortions, and functional groups at the interface between graphene and the electrolyte. Controlling how functional groups are oriented on both sides of the membrane could play a crucial role in modulating the transport of protons across the membrane. The creation of active sites as intrinsic proton pathways on graphene, as well as controlling the polarization of 2D membranes via these active sites – particularly in more complex two-dimensional polymer architectures55,56 – would be a natural next step of this research, to establish correlations between the chemistry of the graphene and electrolyte interface and the efficiency of proton transport, for example using performance parameters of a fuel cell or using ion transport measurements.

Methods
4-sulfo-benzene-diazonium (4-SBD) synthesis
To prepare 4-SBD, which is also mentioned as 4-sulfobenzenediazonium in this manuscript, in its salt form with tetrafluoroborate as counterion, a suspension of sulfanilic acid (2.00 g, 11.5 mmol, 1.0 eq) in 48% aqueous tetrafluoroboric acid (4 mL, 30.3 mmol, 2.6 eq) was cooled to −5 °C. While stirring, a pre-cooled aqueous saturated solution of sodium nitrite (1.20 g, 17.3 mmol, 1.5 eq) was added dropwise to the reaction mixture. During the addition, it became increasingly difficult to maintain stirring which was solved by adding 4 mL of cooled ultrapure water. After this, the reaction mixture was stirred at −5 °C for 5 h after which the product was isolated by filtration. The collected precipitate was washed with ice-cold diethyl ether, ice-cold diethyl ether/methanol (4:1 v/v), ice-cold ethanol and dried overnight under vacuum to yield 4-SBD as a white solid (2.43 g, 9.94 mmol, 77%). The compound was stored at 4 °C.

Graphene for the Raman characterization
For Nafion/graphene samples, we spin-coated a Nafion solution (~5% D521 from Fuel cell store) on single-crystalline graphene on a copper foil at 2000 rpm for 1 min followed by baking on a hot plate at 60 °C for 30 mins to evaporate the solvent. Then, the Cu film was etched away by floating the copper/graphene/Nafion stack on a 0.5 M aqueous solution of ammonium persulfate (APS, from Sigma-Aldrich) in a glass petri dish. Nafion/graphene was then rinsed by transferring the film onto the water-air meniscus (again floating) in a fresh ultrapure water bath for 20 mins. This procedure was repeated 3 times. For the 4-SBD treatment, Nafion/graphene was floated on the 4-SBD solution (0.1 mg·ml−1) in a Petri dish for the time indicated. Then, the samples were floated on 0.1 M HCl for 40 mins and rinsed 3 times for 20 mins each time in ultrapure water. Finally, the Nafion/graphene stack was transferred to a SiO2 /Si wafer (with 285 nm thermal oxide layer, purchased from Siegert Wafer) before Raman characterization.

X-ray photoelectron spectroscopy (XPS) characterization
Measurements were performed with a Thermo Fisher VG Escalab 250 spectrometer as part of the cluster tool DAISY-SOL. It was equipped with a monochromatic X-ray source (Al Kα = 1486.6 eV) set at 15 mA and 15 kV with a spot size of 650 µm in diameter. The pressure inside the analytical chamber was monitored below 2 × 10–9 mbar. Except where otherwise stated, survey spectra were acquired with a pass energy of 50 eV, a step size of 0.1 eV, and a dwell time of 50 ms per measurement point. The detailed scans were acquired with lower pass energy (10 eV) and lower step size (0.05 eV); up to 50 scans were made to increase the signal-to-noise ratio. Metallic foils (Ag, Au, Cu) were cleaned by Ar sputtering (3 kV for 180 s) and used for the spectrometer calibration (Fermi edge of Ag at 0.0 eV, Au 4f 7/2 = 84.0 eV, Ag 3d 5/2 = 368.3 eV, Cu 2p 3/2 = 932.7 eV). The Fermi level of the cleaned silver was also used to determine an instrumental resolution of 0.35 eV for XPS (pass energy of 10 eV). After XPS spectra acquisition, they were slightly adjusted (one sample with a shift of 0.4 and the other +/− 0.1 eV) using the C 1s sp2 peak at binding energy (BE) of 284.5 eV. For measuring the 4-sulfonate benzen diazonium (4-SBD) powder on foil, a flood gun was required to bring the adventitious carbon state as close as possible to 284.8 eV; despite that, charge correction of the spectra to lower BE (≈1.8 eV) were made afterward.

Atomic force microscopy (AFM) characterization
AFM images were recorded using a JPK NanoWizard Ultra Speed microscope, and the obtained data were processed using the JPK SPM Data Processing software. All experiments were performed using a silicon probe (Olympus, OMCL-AC240R3) with a nominal resonance frequency of 70 kHz. The images were all scanned and recorded (with a resolution of 1024 × 1024 pixels) in intermittent contact mode in the air at room temperature (20 °C). For each incubation time, three individual regions were recorded, and the surface roughness was calculated from the combined three regions. The AFM characterization was conducted with graphene on the TEM grid to allow the characterization of free-standing graphene, which is prepared by the following steps. A droplet of isopropanol was added to the TEM grid (carbon film on top), and a piece of flat graphene/Cu film was gently put on the droplet (graphene side on bottom). Then, the sample was dried at room temperature (20 °C). After the drying out of isopropanol, the sample was flipped over and floated on 0.5 M APS solution to fully etch away Cu film. Then the sample was floated on ultrapure water for 20 mins and this procedure was repeated 3 times.
For the consistency of samples, the samples for HRTEM characterization are prepared in the same manner as samples for AFM characterization. HRTEM images were acquired with the image-side CC/CS-corrected SALVE microscope operated at 80 kV with a resolution of 76 pm (SALVE: Sub-Angstrom Low-Voltage Electron Microscopy). Data acquisition was conducted on a Ceta CMOS camera.

Membrane-electrode assembly fabrication
A single-crystalline CVD graphene membrane was used in a DMFC application, where cracks or defects were minimized during the chemical vapor deposition process. Graphene-based membranes were prepared with a porous polycarbonate (PC) membrane as a support (Nuclepore track-etch membrane with a pore diameter of 3 μm, Whatman), and a proton-conducting ionomer (Nafion dispersion D521, Fuel cell store) was used as a stabilizer to prevent direct contact between the graphene lattice and electrodes.
To prepare the membrane, CVD graphene on Cu (1.2 by 1.2 cm²) was first spin-coated with the Nafion solution (D521) at 2000 rpm for 1 min and then baked at 80 °C for 30 min on a hot plate. The Nafion/graphene/Cu was then placed on a piece of PC membrane, and the Cu was etched by floating the Nafion/graphene/Cu/PC sample on a 0.5 M ammonium persulfate (APS) aqueous solution in a petri dish. The sulfophenylation with SBD was conducted by floating Nafion/Graphene/PC on the 4-SBD solution (1 mg/ml, 0.1 M HCl). The resulting Nafion/graphene/PC composite membrane was rinsed and stored in 0.1 M HCl before being fixed between two PTFE gasket sheets using soft replica rubber (Reprorubber Thin Pour, Flexbar Machine Corp.). A Nafion solution was drop-casted twice onto the backside of the PC support, followed by baking at 80 °C for 30 min each time (to act as a mechanical contact buffer during assembly of the fuel cell and following the compression from the electrodes). The anode, with a Pt/Ru loading rate of 4 mg cm−2, and the cathode, with a Pt loading rate of 4 mg cm−2 (Fuel cell store), were then put on the sides of the MEA before the measurements.

Direct methanol fuel cell (DMFC) operation
All electrochemistry measurements were conducted using a potentiostat (PGSTAT204) equipped with an electrochemical impedance spectroscopy model (FRA32M) and operated using the Nova software. DMFC measurements were carried out using an electrolyzer from Dioxide Materials, with a titanium anode plate and a cathode made from 904L stainless steel. The cell were operated using a methanol/water mixture at a flow rate of 30 ml min−1 passing through the anode. To ensure the reliability of all measurements, all cells were started with 5 rounds of CV scanning from 0 to 1.5 V at a speed of 10 mV s−1, and cathode starvation was performed by floating N2 gas instead of O2, followed by 20 min of running the cell at 200 mV. The open circuit voltage was determined by fixing the cell current to 0 and recording it after 5 min with no voltage variation. After the cell was fully stabilized and the current variation (under 200 mV) was less than 10 μA min−1, the polarization curves were obtained by linear sweep from open circuit voltage to short circuit voltage at a 10 mV s−1 scan rate. The resistance was determined by reading the intercept from the Nyquist plot of the electrochemical impedance spectrum, which was measured with frequencies from 1 KHz to 0.1 Hz and 10 mV amplitude under an open circuit. Data were acquired using three independent samples for reproducibility check, and the error bars indicate the maximum and minimum derivation instead of standard deviations to directly show the experimental results.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In the DMFC, the 2D framework of graphene film shows the possibility to realize the proton permeability while stopping\nmethanol crossover, however it faces many challenges. In this manuscript, authors using the chemical modification way to\nfunctionalize the monolayer graphene film with sulfophenylated sp3 dislocations by diazotization. This method maintains the\nintegrity of 2D graphene and makes the graphene only permeate proton. The results are attractive. Follows are comments for\nthis manuscript.\n1. According to the theoretical calculation, the sulfophenylated sp3 dislocations can reduce the energy barrier of cross-plane\ntransportation of proton. However, the reduction in the energy barrier is not the direct evidence for the high proton\nconductivity. The energy barrier for the cross-plane transportation of proton should be provided.\n2. Measurements reveal that the monolayer graphene is highly functionalized, and why these functional groups can’t be\ndirectly seen in the high-resolution TEM images?\n3. The pristine graphene shows a high proton conductivity of 6.9 ±1.1 S·cm-2, and authors thought it comes from the defects\nin the form of pinholes or cracks in CVD graphene. As we know the chemical modification can reduce the mechanical\nstrength once many sp3 carbon atoms are produced on the graphene plane, the functionalized graphene will become more\nvulnerable during transfer and MEA preparation. Thus, authors should confirm that such large enhancement in proton\nconductivity of the SO3-graphene isn’t originated from the defects.\n4. Using the proton-accessible 2D materials in the DMFC is attractive for solving the problem of methanol crossover. Some\nreports using 2D graphdiyne with intrinsic atomic-level pores have close concept with this manuscript, and should not be\nignored in this manuscript. (Angew. Chem. Int. Ed DOI: 10.1002/anie.201910588; Nano Today 202. 39, 101213).", "reviewer_2": "The authors have demonstrated the approach by functionalization of CVD graphene with sulfophenyl groups, which not only\nenhanced the proton transport rate, but also maintained good selectivity to methanol. This work is interesting, the paper itself\nis well-written and almost ready for publication. But I have some concerns:\n1. The calculation in Figure 1 seems to be a strong support to show that this functionalization is much better than\nhydrogenation, especially the proton flux can be 7 to 8 orders of magnetite higher than that of H-Gr. As I understand this is\nalso correlated to the energy barrier change compared to pristine graphene. However, from Ref 36, the energy barrier\ndifference from pristine to H-Gr is already very large (>2 eV), but in Figure 1B, the difference is only 0.15 eV, why there is a\nsuch big difference? This needs to be checked, otherwise will be misleading.\n2. The measured energy barrier for proton transport from Figure 4D for the sample of 4d treatment is only 0.072 eV, which is\nreally very small. This is far below any case that have been reported for proton transport so far. The authors need to check if\nthis fitting is really meaningful as if the conductance change is small, you can fit to any equation.\n3. As now the demonstration of using the functionalized Gr membrane for methanol fuel cell is core for this paper, a clear\ncomparison with other state-of-art membrane systems on this field is necessary to show the merit of this approach.\n4. The results for the reverse assembly (Fig. S28) shows almost similar performance to that of forward one, does this in\nconsistent to the calculations made for Figure 1?\nminor points:\nno D’ peak was not found, no shall be deleted\nNo scale bar information for Fig. S13", "reviewer_3": "Zhang et al. report on Sulfophenylated centimeter-size graphene membrane for a direct methanol fuel cells. The manuscript\nis well written and seems to present detailed characterization for graphene and with an anchor around the main claim of\ncovalent functionalization of single crystalline graphene.\nThe lack of novelty and incomplete support for conclusion publication at this stage is premature (see below for details).\nFirstly, the use of graphene for methanol fuel cells has been reported by several groups.\nDOI: 10.1002/aenm.201601216\nDOI: 10.1016/j.jpowsour.2016.02.030\nDOI 10.1149/MA2018-01/30/1763\nSecondly, some major scientific things to consider are:\n1. The core claim of sulfonation allowing for enhanced proton transport is unsupported. Intrinsic defects exist within the\nindividual CVD domains forming the film. How do the authors rule the presence of defects and the etching of these defects\nwith longer sulfonation times or processing during membrane fabrication? Authors should show direct evidence of\nexpansion of the ring with covalent functionalization e.g. TEM images with increased bond length or distorted rings and\nRaman spectra.\n2. The authors use CVD grown single crystalline graphene - how do they rule out defects in single crystalline graphene over\ncentimeter scale areas? Several reports have shown that single crystalline graphene are not devoid of defects. These\naspects appear to have been overlooked.\n3. Point 1 and 2 are supported by the so called \"pristine\" graphene in this study showing > 3mS/cm2 (value for exfoliated\ngraphene).\n4. How do the authors rule out the formation of crack or tears in graphene as the spin coated Nafion film swells during\nremoval of Cu while etching in acid?\n5. The data for 4 day treatment shows better power density than 6 days. This further indicates defects and not covalent\nfunctionalization and a trade-off.\n6. Referencing is inadequate and several key papers in the field on centimeter scale graphene PEMs are not cited.", "reviewer_4": "The manuscript describes chemical modification by sulfophenylation as a strategy to (selectively) tune proton conductivity\nthrough graphene membranes. Specifically, unless the mechanically exfoliated membrane is properly clamped, the\npossibility that edge leakage from lifted membranes contributes to the ~5000-fold increase in conductance cannot be\nexcluded. Moreover, the modest selectivity between H+ to K+ ions transport across the membrane does not adequately\naddress this concern. I suggest that perhaps a conclusion based on the CVD graphene data, where sulfophenylation\ndemonstrably helps seal defects, is adequate. As it stands, the indirect approach of ruling out leakages for measurements on\nsulfophenylated mechanically exfoliated graphene membranes is not sufficiently convincing. I recommend the manuscript for\npublication as long as this limitation of measurements made on mechanically exfoliated membrane is not dribbled around\nbut clearly acknowledged. In my opinion, the merits of the improvement made by the chemical modification to graphene are\nworth publishing in Nature Communications."}, "author_response": "Response to reviewers\nWe thank all the reviewers for their valuable feedback and comments. In response to the growing\ndemand for broadening the applications of large-area 2D materials, we developed a chemical\nfunctionalization strategy to enhance proton conductance through centimeter-scale CVD graphene while\npreserving its proton selectivity. The performance of our DMFC serves as direct evidence of the\nmembrane’s efficiency, as it is sensitive to proton conductivity—reflected by the internal circuit\nresistance and proton selectivity—indicated by the methanol crossover current. The DMFC exhibits\nsuperior power output resulting from the sulfophenylation of graphene which we attribute to a high\nproton conductivity and selectivity.\nWe characterized the sulfophenylated graphene to confirm the functionalization using state-of-the-art\ntechniques: Raman spectroscopy, high-resolution transmission electron microscopy, atomic force\nmicroscopy, and X-ray photoelectron spectroscopy. Additionally, we conducted theoretical studies that\ndescribe the chemical states of the functionalized graphene, providing insight into the improved proton\nconductance.\nWe understand that the reviewers' primary concerns are whether the increased proton conductance is\nsolely due to the functionalized sites and how these sites contribute to the overall proton conductance.\nTo address this, we have included additional evidences in the manuscript (detailed below). Our findings\nconfirm that the enhanced proton conductance and selectivity are attributed to sulfophenylation,\nsupported by analyses that rule out potential damage from treatment/operations and comparisons with\ndefected graphene.\nWe hope our response will satisfactorily address the reviewers' questions and concerns.\nPoint-by-point response\nReviewer #1 (Remarks to the Author):\nIn the DMFC, the 2D framework of graphene film shows the possibility to realize the proton permeability\nwhile stopping methanol crossover, however it faces many challenges. In this manuscript, authors using\nthe chemical modification way to functionalize the monolayer graphene film with sulfophenylated sp3\ndislocations by diazotization. This method maintains the integrity of 2D graphene and makes the\ngraphene only permeate proton. The results are attractive. Follows are comments for this manuscript.\nWe thank the reviewer for his-her report and we answered the questions below.\n1. According to the theoretical calculation, the sulfophenylated sp3 dislocations can reduce the energy\nbarrier of cross-plane transportation of proton. However, the reduction in the energy barrier is not the\ndirect evidence for the high proton conductivity. The energy barrier for the cross-plane transportation\nof proton should be provided.\nWe agree with the reviewer that the energy reduction does not directly relate to the proton conductivity.\nIn fact, we have not simulated the conductivity, but the proton flux through a 6-fold ring in a graphene\nmodel. The flux was estimated from the WKB approximation, which we used to calculate tunnelling\neffects and estimate the classical vs. quantum components of the flux, as we did before in another work\n(An et al., Adv. Mater. 2020, 2002442).\nIf we understand the request of the reviewer correctly, below, we provide the correlation between the\nenergy barrier and the ring area, which should correspond to the cross-plane for proton transport. We\nfind a linear correlation between the two except for pristine graphene (*). We excluded this point from\nthe fitting because this is the only system that retains the perfect π-conjugation, which is disrupted in\nthe functionalized graphene models. We added the following content to the revised supplementary\nmaterial (Fig. S2, Page 4 in supplementary material).\nFigure R1. Correlation between ring area and the energy barrier for proton transfer through graphene\n(using circumcoronene model structure). Energy barrier calculated at the DFT/PBE0/MDB level of\ntheory; ring area calculated via triangle area summation in a 6-fold ring. Pristine graphene is marked\nwith a star, the functionalized graphene with black circles. For the set of points for functionalized\ngraphene, we find a linear correlation between the ring area and the energy barrier.\n2. Measurements reveal that the monolayer graphene is highly functionalized, and why these functional\ngroups can’t be directly seen in the high-resolution TEM images?\nWe attempted to observe the functionalized sites of graphene using HRTEM, however we could not\nimage individual carbon atoms grafted by SBD (Figure 2E, Figure S10-22). However, as previously\nreported (Nature Chem, 2011, 3, 279–286), identifying individual functionalized sites on a graphene\nlattice with low functionalization levels proved challenging (using Raman, we measured a I /I of ~0.4,\nD G\nindicating a L of ~16 nm corresponding to 1 sp3 out of >3000 sp2 carbon atoms, by L =√𝐴/(𝐼(D)/𝐼(G),\nd d\nwhere A is 102 nm2, Nano Letters, 2012, 12, 3925-3930). We could not observe such sp3 by electron\nmicroscopy, nor using the diffraction mode. Additionally, the potential effects from the transmission\nelectron microscopy beam on the conversion of sp3 to sp2 (Journal of Applied Physics, 2010, 107 (1),\n013709 and the electron-beam induced cleavage (Nanomaterials, 2022, 3 (12)) make the observation on\nisolated grafting site more challenging. Additionally, this type of functionalization typically does not\ndisplay a measurable change in the local crystalline structure nearby the distortion. Even for high\nfunctionalization degrees (J. Mater. Chem., 2012, 22, 2063-2068) a direct observation by HRTEM of\nthe grafted sites was not possible.\n3. The pristine graphene shows a high proton conductivity of 6.9 ±1.1 S·cm-2, and authors thought it\ncomes from the defects in the form of pinholes or cracks in CVD graphene. As we know the chemical\nmodification can reduce the mechanical strength once many sp3 carbon atoms are produced on the\ngraphene plane, the functionalized graphene will become more vulnerable during transfer and MEA\npreparation. Thus, authors should confirm that such large enhancement in proton conductivity of the\nSO3-graphene isn’t originated from the defects.\nThe observed increases in proton conductance can be attributed to the inherent properties of pristine\ngraphene. Given the higher proton selectivity over methanol after treatment and considering that\nmethanol molecules are neutral and subnanometer in size, the defects responsible for this selectivity\nmust also be subnanometer in scale. Unlike mechanically introduced defects, which tend to propagate\nand cause tearing, the sulfophenylation process does not cause mechanical damage. This explains the\nimproved proton conductance while maintaining structural integrity.\nWhile it is true that covalent functionalization can compromise the stability of the graphene lattice and\nsignificantly reduce its mechanical strength, such as chlorination and hydrogenation, this was not the\ncase in our study. Given the modest level of functionalization, we did not observe any degradation in\nthe overall integrity of the graphene lattice, as shown in Figure R2.\nFigure R2. Optical image (A) and I /I (B) mapping of Raman spectra of corresponding area for a\nD G\nNafion-casted graphene floated on 0.1 mg/ml SBD solution for 3 days.\nDuring the chemical treatment, the structural integrity of the graphene lattice was maintained over\nseveral days, as demonstrated by the fact that graphene floated at the water-air interface without cracking\nwith only a Nafion-casted layer. Upon sulfophenylation, Raman spectroscopy showed a gradual increase\nin the D peak, indicating functionalization, which plateaued after 4 days of treatment with no further\nincrease observed during the subsequent 2 days (Fig. 3A). This trend was consistent with our\nobservations on a rigid SiO /Si substrate (Fig. S16). From this, we ruled out the likelihood of significant\n2\nmechanical damage during treatment. Unlike the floated graphene, which exhibited a flat surface, the\ngraphene transferred onto a rigid substrate displayed more wrinkles, with these regions showing a lower\nI /I ratio. Consequently, the SBD treatment primarily affected the monolayer areas and did not\nD G\ninfluence the wrinkles or multilayer islands.\nFigure R3. Optical image (A) and I /I (B) mapping of Raman spectra of corresponding area for a\nD G\ngraphene on SiO /Si substrate immersed in 0.1 mg/ml SBD solution for 3 days.\nduring this step, it would have manifested as large cracks rather than small pinholes. Such cracks would\nhave been evident from the start-up of the cell, resulting in a significantly reduced open-circuit voltage\nand a very low power output, owing to the methanol leakage (as shown in Figure R4, compromised\nmembranes). In contrast, small pinholes would still permit effective methanol selectivity. Our Arrhenius\nanalysis confirmed that membrane conductance remained linear after 4 days of SBD treatment,\nindicating robust thermal stability. Additionally, we observed a reduced methanol/proton permeation\nrate, further supporting the integrity of the treated membrane.\nFigure R4. DMFC performance of a graphene membrane with cracks.\nAdditionally, we explored the role of active sites and defects by functionalizing graphene using\nnitrogen plasma treatment. This exposure led to an increase in defect density, which initially boosted\nthe DMFC power output (Figure R5). However, despite the defect density being significantly higher\nthan that of sulfophenylated graphene (Figure R6) with an I /I ratio of ~1.2 (0.4~0.6 for sulfophenylated graphene),\nD G\nthe maximum power output achieved was only one-third of that seen with sulfophenylated graphene.\nThis finding further substantiates that the enhanced proton conductance results from sp3 dislocations\nand the presence of ph-SO - functional groups.\nFigure R6. Raman spectrum of defected graphene by nitrogen plasma.\n4. Using the proton-accessible 2D materials in the DMFC is attractive for solving the problem of\nmethanol crossover. Some reports using 2D graphdiyne with intrinsic atomic-level pores have close\nconcept with this manuscript, and should not be ignored in this manuscript. (Angew. Chem. Int. Ed DOI:\n10.1002/anie.201910588; Nano Today 2021. 39, 101213).\nWe thank the reviewer for referring to these publications. We have now incorporated these references\ninto the introduction.\nReviewer #2 (Remarks to the Author):\nThe authors have demonstrated the approach by functionalization of CVD graphene with sulfophenyl\ngroups, which not only enhanced the proton transport rate, but also maintained good selectivity to\nmethanol. This work is interesting, the paper itself is well-written and almost ready for publication. But\nI have some concerns:\nWe thank the reviewer’s positive feedback.\n1. The calculation in Figure 1 seems to be a strong support to show that this functionalization is much\nbetter than hydrogenation, especially the proton flux can be 7 to 8 orders of magnetite higher than that\nof H-Gr. As I understand this is also correlated to the energy barrier change compared to pristine\ngraphene. However, from Ref 36, the energy barrier difference from pristine to H-Gr is already very\nlarge (>2 eV), but in Figure 1B, the difference is only 0.15 eV, why there is a such big difference? This\nneeds to be checked, otherwise will be misleading.\nThe differences between our results and those reported in Ref. 46 can be explained as we functionalize\nonly two C atoms with hydrogen, whereas Ref. 46 reports results for complete 6-fold ring\nfunctionalization. In our previous work (An et al., Adv. Mater. 2020, 2002442), we investigated how\nvarying degrees of hydrogenation (2, 4, or 6 H atoms on a 6-fold ring) affect the energy barrier and\nproton flux. For these cases, we observed that the energy barrier changes from 1.41 eV for pristine\nconditions to 1.23 eV for 2 H, 0.79 eV for 4 H, and 0.70 eV for 6 H. In comparison, Ref. 36 reports\nenergy changes from 3.65 eV for pristine graphene down to 3.24 eV (a decrease of only 0.41 eV) for 2\nH, 3.07 eV for 4 H, and 1.82 eV for 6 H. The discrepancy in absolute values is due to the transition state\nin Ref. 36, which involves a bound proton that stabilizes both the product and reactant, leading to a\nhigher energy barrier. Also, we did not consider water capturing the proton in the form of hydronium.\nHowever, the relative trend remains similar, with 6 H atoms reducing the barrier by roughly half\ncompared to pristine graphene.\nFull hydrogenation in Ref. 46 results in lowering of energy barrier to about 1 eV for chair confirmation\nand to 0.76 eV for boat one.\n2. The measured energy barrier for proton transport from Figure 4D for the sample of 4d treatment is\nonly 0.072 eV, which is really very small. This is far below any case that have been reported for proton\ntransport so far. The authors need to check if this fitting is really meaningful as if the conductance\nchange is small, you can fit to any equation.\nThe activation energy of our 4-day treated samples is approximately 30% to 65% lower than that\nreported for polymeric proton exchange membranes, which typically show values around 0.1 eV for\nhigh-performance PEMs. Activation energies of 0.075 eV (J. Mater. Chem. A, 2020, 8, 1147-1153),\n0.085 eV (J. Mater. Chem. A, 2023, 11, 3446-3453), and 0.17 eV for Nafion HP (J. Solid State\nElectrochem, 2019, 23, 2639–2656) were previously reported.\nWe have therefore verified our calculations. The conductance of our samples increased from 4.9 S cm−2\nto 6.5 S cm−2 when the temperature was raised from 20 to 60 °C. Under this temperature range, the\nconductance of a Nafion 117 membrane increased from 2.4 S cm−2 to 3.1 S cm−2, with reported activation\nenergies ranging between 9 to 14 kJ mol−1 (J. Electrochem. Soc. 2014, 161, F1395). For the linear fitting,\nthe R2 is 0.9935, 0.9986, 0.9887, 0.9724 for 1-4 days sample. The R2 for 0-day sample is 0.7311, as we\nobserved a poor linearity and thermal stability. The following table was added to the supplementary\nmaterial for the clarification of our calculations (Table S3, page 37).\nTable R1. Datasheet of Arrhenius behavior analysis\nT (K) 1000/T 0-day 1-day 2-day 3-day 4-day 5-day\n(K-1) σ (S ln(σ𝑇) σ (S ln(σ𝑇) σ (S ln(σ𝑇) σ (S ln(σ𝑇) σ (S ln(σ𝑇) σ (S ln(σ𝑇)\ncm-2) cm-2) cm-2) cm-2) cm-2) cm-2)\n293.15 3.41 1.13 5.81 1.86 6.30 3.15 6.83 4.08 7.08 4.94 7.28 3.93 6.70\n313.15 3.19 1.39 6.08 2.39 6.62 3.77 7.08 4.91 7.34 5.75 7.50 4.41 6.93\n323.15 3.09 2.30 6.61 2.65 6.75 4.04 7.17 5.22 7.43 6.12 7.59 4.53 7.00\n333.15 3.00 2.19 6.59 2.83 6.85 4.32 7.27 5.50 7.51 6.29 7.65 4.54 7.02\n343.15 2.91 1.81 6.43 3.01 6.94 4.55 7.35 5.68 7.58 6.35 7.69 4.41 7.04\n3. As now the demonstration of using the functionalized Gr membrane for methanol fuel cell is core\nfor this paper, a clear comparison with other state-of-art membrane systems on this field is necessary\nto show the merit of this approach.\nWe appreciate the reviewer’s suggestion and have expanded the introduction to highlight recent\nadvancements in the field of DMFCs, with an emphasis on membrane technologies. This elaboration\nunderscores the progress made in improving membrane performance to enhance the overall efficiency\nand power output of DMFCs. We also removed a sentence from the abstract which may be a speculative\nperspective of the work: ‘Selective to protons, transmembrane areal conductances surpass those of\npolymer membranes, while providing proton selectivity over methanol through such an atomically thin\nlayer.’\nThe main challenge for polymeric membranes lies in balancing proton conductivity and selectivity, with\nmethanol crossover posing an additional obstacle due to its high solubility in water and uncharged\nnature, which facilitates migration from the anode to the cathode. To mitigate this issue, various\nstrategies have been pursued to enhance the power output of direct methanol fuel cells (DMFCs) (J\nPolym Res., 2024 31, 125). For instance, incorporating layered additives has been shown to improve\nperformance; the power output increased to 35 mW cm-2 with nano-silica additives (Electrochemistry\nCommunications, 2004, 6(10), 1069–1074) and to 39 mW cm-2 with graphene oxide (ACS Appl. Eng.\nMater., 2023, 1(3), 947–954). The power output of DMFCs using a Nafion 117 membrane was enhanced\nto 79 mW cm-2 by impregnating the membrane with Pd (Electrochemistry Communications, 2003, 5(7),\n571–574). Additionally, recasting the Nafion membrane with blended additives has been shown to boost\nDMFC power output to over 100 mW cm-2 (Electrochimica Acta, 2018, 292, 855–864; Journal of\nMembrane Science, 2017, 541, 127–136). The power output of DMFCs was enhanced to 25 mW cm-2\nby sandwiching single-layer graphene between two layers of Nafion membrane (Journal of Power\nSources, 2016, 311, 188-194) and increased to 120 mW cm-2 by incorporating single-layer graphene\nonto the electrode (Adv. Energy Mater., 2017, 7 (5), 7).\n4. The results for the reverse assembly (Fig. S28) shows almost similar performance to that of forward\none, does this in consistent to the calculations made for Figure 1?\nWe originally aimed to test whether a directional DFMC performance dependency would stand from\nmounting the sulfophenylated graphene membrane in the reverse direction. As shown in Figure S29, the\nreverse assembly demonstrated a nearly identical performance to the forward assembly, confirming that\nthe proton conductance is non-directional. In the simulations in Figure 1, the proton flux is influenced\nprimarily by the energy barrier associated with proton tunneling across the functionalized sites,\nirrespective of the direction.\nminor points:\nno D’ peak was not found, no shall be deleted\nThe D' peak is not visible in our case due to the relatively low I /I ratio.\nD G\nNo scale bar information for Fig. S13\nThe scale bar is added.\nReviewer #3 (Remarks to the Author):\nZhang et al. report on Sulfophenylated centimeter-size graphene membrane for a direct methanol fuel\ncells. The manuscript is well written and seems to present detailed characterization for graphene and\nwith an anchor around the main claim of covalent functionalization of single crystalline graphene.\nThe lack of novelty and incomplete support for conclusion publication at this stage is premature (see\nbelow for details).\nTo our knowledge, no prior studies have reported similar observations on proton transport through\nchemically functionalized graphene lattices. We also provided comprehensive evidence for the\nfunctionalized membrane, including Raman spectroscopy, HRTEM, AFM, XPS, and full-scale fuel cell\ntests conducted with over 30 samples. The characterizations were conducted by the most state-of-art\ntechniques. We believe our work is new and important.\nFirstly, the use of graphene for methanol fuel cells has been reported by several groups.\nDOI: 10.1002/aenm.201601216\nDOI: 10.1016/j.jpowsour.2016.02.030\nDOI 10.1149/MA2018-01/30/1763\nWe incorporated the top two references into a comprehensive paragraph that introduces recent\nadvancements in membrane technology for DMFCs.\nThe last reference, however, is a conference abstract which we did not added. See our addition below:\nThe main challenge for polymeric membranes lies in balancing proton conductivity and selectivity, with\nmethanol crossover posing an additional obstacle due to its high solubility in water and uncharged\nnature, which facilitates migration from the anode to the cathode. To mitigate this issue, various\nstrategies have been pursued to enhance the power output of direct methanol fuel cells (DMFCs) (J\nPolym Res., 2024 31, 125). For instance, incorporating layered additives has been shown to improve\nperformance; the power output increased to 35 mW cm-2 with nano-silica additives (Electrochemistry\nCommunications, 2004, 6(10), 1069–1074) and to 39 mW cm-2 with graphene oxide (ACS Appl. Eng.\nMater., 2023, 1(3), 947–954). The power output of DMFCs using a Nafion 117 membrane was enhanced\nto 79 mW cm-2 by impregnating the membrane with Pd (Electrochemistry Communications, 2003, 5(7),\n571–574). Additionally, recasting the Nafion membrane with blended additives has been shown to boost\nDMFC power output to over 100 mW cm-2 (Electrochimica Acta, 2018, 292, 855–864; Journal of\nMembrane Science, 2017, 541, 127–136). The power output of DMFCs was enhanced to 25 mW cm-2\nby sandwiching single-layer graphene between two layers of Nafion membrane (Journal of Power\nSources, 2016, 311, 188-194) and increased to 120 mW cm-2 by incorporating single-layer graphene\nonto the electrode (Adv. Energy Mater., 2017, 7 (5), 7).\nSecondly, some major scientific things to consider are:\n1. The core claim of sulfonation allowing for enhanced proton transport is unsupported. Intrinsic defects\nexist within the individual CVD domains forming the film. How do the authors rule the presence of\ndefects and the etching of these defects with longer sulfonation times or processing during membrane\nfabrication?\nIntrinsic defects in CVD graphene are unavoidable, which contrasts with the controlled introduction of\nfunctionalized graphene samples. To better understand these intrinsic properties, we conducted studies\non non-functionalized CVD graphene. We did not observe that intrinsic defects were further etched\nduring the functionalization process, as evidenced by HRTEM (Figure 2E). Raman spectroscopy\n(Figure R2), and AFM (refer to Figures S19). HRTEM observations revealed no discernible alterations\nbeyond the pristine lattice structure. In fact, despite our efforts, we could not directly visualize the sp3\ndefects as intended (refer to our previous response regarding the imaging challenges of distortion).\nFigure R2. Optical image (A) and I /I (B) mapping of Raman spectra of corresponding area for a\nD G\nNafion-casted graphene floated on 0.1 mg/ml SBD solution for 3 days.\nAuthors should show direct evidence of expansion of the ring with covalent functionalization e.g. TEM\nimages with increased bond length or distorted rings and Raman spectra.\nWhat the referee is asking is not possible with current techniques. And samples are not yet compatible\nto carry STM. Efforts have been made to directly observe the distortion after SBD treatment. As\npreviously reported (Nature Chem, 2011, 3, 279–286), identifying individual functionalized sites on a\ngraphene lattice with low functionalization levels proved challenging.\n2. The authors use CVD grown single crystalline graphene - how do they rule out defects in single\ncrystalline graphene over centimeter scale areas? Several reports have shown that single crystalline\ngraphene are not devoid of defects. These aspects appear to have been overlooked.\nWe did not overlook the potential defects from pristine graphene. Instead, all the\nanalysis/characterization involve the untreated graphene as a control to compare the changes in transport\nproperties upon the chemical treatment (Figure 2, observations by HRTEM and AFM with pristine\ngraphene included, Figure 3 Raman and XPS characterizations include the pristine samples, Figure 4\nfuel cell tests with all parameters from pristine graphene discussed).\nWe are also curious about the proton transport through the intrinsic graphene lattice (Nature, 2023, 620,\n782-786) vs. sulfophenylated sites (arXiv:2308.16112)\nObserving and measuring sparsely distributed active sites vs non-functional defects sites is inherently\nchallenging, especially when they are spread over a large surface area. An operando system is required\nto correlate the changes in the lattice and resulting proton conductance. Therefore, we will build an on-\nchip platform allowing us to observe the introduced changes (by electron beam, electrochemical reaction\nand electrical field) and the correlated changes in the chemical states by spectroscopy (Raman and IR),\nideally with a probing system similar to SECCM. But this is beyond the content this manuscript and the\nwork of the team of PD and PhD students authoring this manuscript.\n3. Point 1 and 2 are supported by the so called \"pristine\" graphene in this study showing > 3mS/cm2\n(value for exfoliated graphene).\nWe added in the main text that the 3mS/cm2 were measured for exfoliated flakes as a graphene 2D\ncrystal.\n4. How do the authors rule out the formation of crack or tears in graphene as the spin coated Nafion\nfilm swells during removal of Cu while etching in acid?\nDuring the membrane fabrication process, the graphene was supported by a rigid PC membrane, which\nmitigated any potential impact from size variations in Nafion during DMFC operation. The casted\nNafion layer was approximately tens nanometers thick and exhibited an amorphous structure, unlike\nreinforced membranes such as Nafion 117. This configuration provided sufficient space among the\npolymer chains to allow for hydration and dehydration without significant size changes in the composite\nmembrane. To confirm this stability, Raman spectroscopy and optical observations were conducted on\nthe Nafion-casted graphene. Figure R2 shows the graphene floating on the SBD solution for three days,\nrevealing only homogeneous D peaks without any evidence of cracks, alleviating concerns about\nstructural integrity.\n5. The data for 4 day treatment shows better power density than 6 days. This further indicates defects\nand not covalent functionalization and a trade-off.\nThe referee refers to defects which we do not see in HRTEM. We respectfully disagree with the assertion\nthat the oligomerization of SBD is responsible for the improvement in DMFC power output. Instead,\nthe enhancement in proton conductance is the primary factor driving this improvement. The proton\nconductance increased after SBD treatment, whereas one would expect a decline in conductance if only\nan additional layer had formed atop the graphene while no functionality of the lattice was changed. An\nadditive mechanism could only justify an improvement in power output if the reduction in methanol\ncrossover would be the main contributor.\nOur findings show that diazonium groups graft onto the basal plane within the initial 4-day treatment,\nfollowed by oligomerization in extended treatments. This conclusion is supported by Raman\nspectroscopy and AFM observations (Figure S19). Specifically, after 4 days of treatment, the D peak no\nlonger increased, indicating minimal changes to the lattice with prolonged exposure. However, AFM\nimaging revealed that longer treatments resulted in a polymer-like layer developing on the graphene\nsurface, rather than discrete grafted sites. This explains the observed decrease in power output after 6\ndays of treatment.\nExperimentally, we ensured that the membrane floated on the treatment solution, making it unlikely for\nnon-covalently bound layers to adhere to the graphene surface, especially without external electron to\ninitiate further polymerization (as it would be, for example, by electrografting). Additionally, the treated\ngraphene was floated on water before being assembled with the electrode, further ruling out residual\nsurface-bound layers.\n6. Referencing is inadequate and several key papers in the field on centimeter scale graphene PEMs\nare not cited.\nAs the reviewer noticed, the single-layer graphene as a membrane in fuel cells is relevant to our work,\nshowing the proton/methanol transport through the lattice. All crucial works featuring proton transport\nthrough a (pristine) graphene lattice have been discussed in our manuscript.\nReviewer #4 (Remarks to the Author):\nThe manuscript describes chemical modification by sulfophenylation as a strategy to (selectively) tune\nproton conductivity through graphene membranes. Specifically, unless the mechanically exfoliated\nmembrane is properly clamped, the possibility that edge leakage from lifted membranes contributes to\nthe ~5000-fold increase in conductance cannot be excluded. Moreover, the modest selectivity between\nH+ to K+ ions transport across the membrane does not adequately address this concern. I suggest that\nperhaps a conclusion based on the CVD graphene data, where sulfophenylation demonstrably helps seal\ndefects, is adequate. As it stands, the indirect approach of ruling out leakages for measurements on\nsulfophenylated mechanically exfoliated graphene membranes is not sufficiently convincing. I\nrecommend the manuscript for publication as long as this limitation of measurements made on\nmechanically exfoliated membrane is not dribbled around but clearly acknowledged. In my opinion, the\nmerits of the improvement made by the chemical modification to graphene are\nworth publishing in Nature Communications.\nWe appreciate the valuable feedback provided by the reviewer. At this stage, we cannot entirely exclude\nthe possibility of leakage in our experiments involving exfoliated graphene, as reported in our preprint\non (arXiv:2308.16112). In this preprint manuscript, we investigated proton transport through chemically\nfunctionalized exfoliated graphene. While the presence of leakage cannot be ignored, which makes the\nconfirmation of proton conductance less certain, we submitted the fuel cell part separately (this\nmanuscript under review).\nAcknowledging the recent advancements in understanding proton transport through graphene lattices,\nwe have also highlighted the recent work by our colleagues (Nature, 2024, 630, 619-624; Proceedings\nof the National Academy of Sciences, 2024, 120, e2300481120). Additionally, we believe that for\nexfoliated graphene, a form of clamping is beneficial. Here, clamping is achieved using Nafion in\ncombination with a rigid polymer support."}, {"round": 2, "reviewer_comments": {"reviewer_1": "This paper is well revised according to reviewer's suggestions. Now, it can be acceptable.", "reviewer_2": "The paper presents a promising and attractive approach for controlling ion transport through chemically functionalized\ngraphene, and now I recommend its publication.", "reviewer_3": "Previous comments in italics and new comments in normal text\nZhang et al. report on Sulfophenylated centimeter-size graphene membrane for a direct methanol fuel cells and the reviewer\nappreciates the effort to clarify some of the finer points.\nHowever, this reviewer lack of novelty based on prior work (that have now been cited) and incomplete support for the core\nclaims as alluded to by reviewer 1 – point 3.\n1. The core claim of sulfonation allowing for enhanced proton transport is unsupported.\nNote reviewer #1 also bring up this point #3 The pristine graphene shows a high proton conductivity of 6.9 ±1.1 S·cm-2,\nThe authors responded with optical images and Raman which simply doesn’t have adequate resolution for quantifying all\nkinds of defects. Raman below a certain level of defect density is not adequate.\nThe authors report proton conductance 20 times more than pristine graphene at 3mS cm-2, there is no explanation for this.\n(Nature volume 516, pages227–230 (2014); ACS Nano 2019, 13, 10, 12109–12119; J. Mater. Chem. A, 2022,10, 19797-\n19810) crack and tears in centimeter scale membranes are known to enhance transport.\nCaution should be exercised in attributing all of it to doping without direct evidence, considering point 2 below.\nThe devices have Nafion so they won’t show up in the performance. The nitrogen doping data actually supports the\nreviewer’s claim that increased proton transport is not from small defects but from sporadic cracks and tears in the graphene\nfor the SO2- case.\n2. Intrinsic defects exist within the individual CVD domains forming the film. How do the authors rule the presence of defects\nand the etching of these defects with longer sulfonation times or processing during membrane fabrication? Authors should\nshow direct evidence of expansion of the ring with covalent functionalization e.g. TEM images with increased bond length or\ndistorted rings and Raman spectra.\nSince the authors cannot confirm these aspects with HRTEM or find evidence of sp3 in Raman, they should avoid\nspeculative claims.\nThey should try to find out the real reason for enhanced transport as suggested by 2 independent reviewers instead of trying\nto push through this manuscript in its current state.\n2. The authors use CVD grown single crystalline graphene - how do they rule out defects in single crystalline graphene over\ncentimeter scale areas? Several reports have shown that single crystalline graphene are not devoid of defects. These\naspects appear to have been overlooked.\nThe authors misunderstood the point the reviewer was making. The pristine graphene they refer to is made via CVD! That\nalso have defects that manifest even in single crystalline graphene membranes. Characterizing these intrinsic defects over\ncentimeter scale areas is challenging with HRTEM, AFM, and Raman, and several groups around the world have developed\ndiffusive transport as an alternative solution that provides information in a few hours with reliable statistics. The authors may\nwant to look into those works and try to characterize their materials thoroughly with relevant techniques that will un-ravel the\nscience behind the effects seen here – instead of claiming it comes from doping, SO3- without direct evidence.\n3. Point 1 and 2 are supported by the so called \"pristine\" graphene in this study showing > 3mS/cm2 (value for exfoliated\ngraphene).\nThe authors missed the point that even CVD graphene has shown 3-4 mS cm-2 similar to exfoliated graphene. (Nature\nvolume 516, pages227–230 (2014); ACS Nano 2019, 13, 10, 12109–12119; J. Mater. Chem. A, 2022,10, 19797-19810).\nDoes this mean the CVD single crystalline graphene made here is not as high in quality as even polycrystalline graphene\nused in other studies? Or is the higher transport from cracks and tears?\nIn light of points 1 and 2 the reviewer urges extreme caution from making claims without evidence by the authors.\n4. How do the authors rule out the formation of crack or tears in graphene as the spin coated Nafion film swells during\nremoval of Cu while etching in acid?\nAlthough the authors used a rigid PC membrane and used Raman these don’t allow for probing over macroscopic areas\nwith high resolution.\n5. The data for 4 day treatment shows better power density than 6 days. This further indicates defects and not covalent\nfunctionalization and a trade-off.\nThe authors mention they don’t see defects but no HR TEM is provided as evidence.\nThe reviewer is merely point to defects and variability might be responsible for this since one would not expect a reduction\nafter saturation if this was purely functionalization.\n6. Referencing is inadequate and several key papers in the field on centimeter scale graphene PEMs are not cited.\nThe reviewer recommends citing relevant papers in the field focusing on proton transport using CVD graphene to\nbenchmark and contextualize the current work."}, "author_response": "Response to reviewers\nWe sincerely appreciate the additional comments from Reviewer #3. We understand\nthat his/her primary concerns are on proton permeation through CVD graphene and the\ncorrelation between enhanced proton permeation and chemical functionalization vs. the\npresence of defects and cracks. We can not exclude the presence of defects in CVD\ngraphene. In response, we have revised our manuscript to provide additional\nexplanations on these points. We also confirm that the observed effects originate from\nchemical functionalization, proposing that the chemical functionalization with SBD\ndoes not bring additional defects, such as treatment-induced cracks or pinholes.\nThe method we employed introduces sp³ distortions, -SO - groups, and a polymerized\n3\nlayer simultaneously, making it challenging to isolate any single factor as the direct\ncause of the increased transport of protons, as questioned by the reviewer. A\ncomprehensive analysis of this correlation exceeds the scope of a single manuscript,\ngiven the need to carefully isolate all these contributing factors. However, we are\nconfident that our observations, along with the control experiments we have conducted,\nprovide sufficient evidence to demonstrate that sulfophenylated graphene exhibits\nperformance comparable to Nafion polymer membranes in a direct methanol fuel cell.\nIn the laboratory, we are currently isolating the influence of individual factors,\nincluding sp³ defects, sulfonate groups, and vacancy-type defects, for exfoliated\ngraphene flakes and CVD graphene. These studies require careful statistics before we\ncan conclusively isolate and identify which factors influence proton/ion transport\nthrough chemically functionalized graphene.\nWe believe this revision adequately addresses now the reviewer’s questions and\nconcerns.\nPoint-by-point response\nReviewer #3 (Remarks to the Author):\nPrevious comments in italics and new comments in normal text\nZhang et al. report on Sulfophenylated centimeter-size graphene membrane for a direct\nmethanol fuel cells and the reviewer appreciates the effort to clarify some of the finer\npoints.\nHowever, this reviewer lack of novelty based on prior work (that have now been cited)\nand incomplete support for the core claims as alluded to by reviewer 1 – point 3.\n1. The core claim of sulfonation allowing for enhanced proton transport is\nunsupported.\nThe claim that sulfophenylation enhances the fuel cell performance is supported. That\nthe increased performance of the fuel cell is attributed to the selective transport of\nprotons is not the claim of this manuscript; instead, it is a hypothesis we formulate. The\nverification of this hypothesis requires further investigation, not using a fuel cell but on\nfreestanding graphene. We revised the manuscript where our description of the results\nmay have conveyed that we conclude on the selective transport of protons caused\nthrough sulfophenylation (see our detailed answer below).\nNote reviewer #1 also bring up this point #3 The pristine graphene shows a high proton\nconductivity of 6.9 ±1.1 S·cm-2,\nFor pristine graphene, this conductivity is caused by either defects intrinsically present\nin the CVD graphene or (what we now believe most) the interfacial chemistry between\ngraphene and the fuel cell membrane assembly components.\nThe authors responded with optical images and Raman which simply doesn’t have\nadequate resolution for quantifying all kinds of defects. Raman below a certain level of\ndefect density is not adequate.\nWe agree. We extended the original description of the HRTEM results. See below in\nthe detailed answer.\nThe authors report proton conductance 20 times more than pristine graphene at 3mS\ncm-2, there is no explanation for this. (Nature volume 516, pages227–230 (2014); ACS\nNano 2019, 13, 10, 12109–12119; J. Mater. Chem. A, 2022,10, 19797-19810) crack\nand tears in centimeter scale membranes are known to enhance transport.\nFor dry Nafion a conductance of 3mS/cm2 was indeed reported in 2014. We noted in\nour manuscript that intrinsic defects, along with interfacial contacts in a membrane\nfabricated from CVD graphene, could contribute to higher proton conductance. This\nvalue could be up to three orders of magnitude greater than 3 mS/cm2 (Science, 2021,\n374, eabd7687). Details of our modifications are provided in response to the following\nspecific questions.\nCaution should be exercised in attributing all of it to doping without direct evidence,\nconsidering point 2 below.\nWe believe that we now have revised the manuscript by taking into consideration our\nresults carefully. We formulated hypothesizes to explain our data.\nModification we made following the comments above:\nL39-47: Selective to protons, transmembrane areal conductances surpass those of\npolymer membranes, while The membrane conductance of pristine graphene in direct\nmethanol fuel cells, derived mainly from intrinsic defects, is 6.9 ± 1.1 S·cm-2. After\nchemical functionalization, the proton conductance increases to 30.9 ± 2.3 S·cm-2,\nnormalized to the free-standing graphene area, indicating that covalently\nfunctionalizing graphene with sulfophenyl groups efficiently lowers the membrane\nresistance presumably because the sulfophenylation lowers the energy barrier for\nprotons to transport across graphene. By creating proton-conductive and selective paths\nthrough graphene, we unveil a covalent chemical route to rationalize transmembrane\nproton transport through 2D materials.\nWe modified the introduction by adding more references discussing proton transport\nthrough CVD graphene, especially the defects\nL84-96: Defects in graphene, such as seven- and higher-member rings, together with\nsmall lattice disorders such as sp3 distortions and polar functional groups are promising\napproaches for enhancing proton conductivity, while maintaining the impermeability\nof the basal plane to other substances, which are particularly of interests in energy\ndevices such as fuel cells. In CVD graphene, for example, cracks and defects are\ncommonly observed at the microscale46, influencing proton transport properties. At a\nsmaller scale than larger sized cracks, theoretical studies indicate that small lattice\ndisorders, such as hydrogenated graphene seven- and higher-membered rings,\neffectively lower the proton permeation barrier through graphene47-49. Diffusive proton\nconductance experiments also demonstrate improved proton transport through\ngraphene with eight-membered rings50, vacancy defects33, and other structural\nmodifications such as nitrogen-doped graphene51. Therefore, due to the presence of\nintrinsic defects, the proton conductance of CVD graphene was reported up to three\norders of magnitude higher than that of defect-free graphene 2D crystals52.\nThe devices have Nafion so they won’t show up in the performance. The nitrogen doping\ndata actually supports the reviewer’s claim that increased proton transport is not from\nsmall defects but from sporadic cracks and tears in the graphene for the SO2- case.\nNitrogen doping does not create cracks but nanopores. We ruled out the possibility that\nthe enhanced proton conductance originates from enlarged cracks and tears caused by\nchemical functionalization. This conclusion is supported by tests on stand-alone spin-\ncoated Nafion ionomer and extensive HRTEM observations.\nWe spin-coated a Nafion ionomer layer onto a flat Cu foil without graphene and then\ntransferred it onto a porous PC membrane by etching away the Cu, floating the\nNafion/Cu/PC stack on a Cu etchant. The Nafion/PC membrane was subsequently\nrinsed in ultrapure water and 0.1 M HCl, following the same procedure used for\nfabricating the Nafion/graphene/PC membranes in our study. Our results showed that\nthe Nafion ionomer alone was highly permeable to the methanol/water mixture. In\nDMFC tests, this led to significant methanol crossover, causing near-zero cell current\nand complete activation failure (see Fig. R1). Thus, the spin-coated Nafion layer alone\nhad negligible impact on membrane performance, particularly in sealing leaks. Based\non this, we conclude that any cracks or tears in the graphene would be detectable\nthrough methanol crossover measurements.\nWe added this observation to the following discussion:\nL258-262: To assess the contribution of the spin-coated Nafion ionomer, we tested the\nNafion ionomer/PC membrane in DMFCs. The membrane exhibited high permeability\nto the methanol/water mixture, resulting in almost no cell current (Fig. S24). This\nconfirms that the graphene layer serves as the primary selective barrier, and the DMFC\nobservations effectively demonstrate the selective proton conductance of graphene.\nFigure R1 (S24). DMFC performance of a Nafion spin-coated PC membrane.\nIf the original proton-conducting sites had developed larger cracks or tears, methanol\ncrossover would have increased, but we did not observe such an effect (also for nitrogen\ndoped graphene as shown in our previous response, pasted below for information).\nSecondly, we aimed to investigate the presence of smaller openings, such as pinholes.\nHowever, extensive HRTEM observations of three batches—comprising a total of\neleven independent samples—did not reveal any pinholes or vacancy-type defects after\nchemical functionalization. To maintain a more neutral perspective, we have revised\nthe following discussion accordingly.\nOur comments on N-doped graphene\nAdditionally, we explored the role of active sites and defects by functionalizing graphene using\nnitrogen plasma treatment. This exposure led to an increase in defect density, which initially boosted\nthe DMFC power output (Figure R5). However, despite the defect density being significantly higher\nthan that of sulfophenylated graphene (Figure R6) with an I /I ratio of ~1.2 (0.4~0.6 for\nD G\nsulfophenylated graphene), the maximum power output achieved was only one-third of that seen\nwith sulfophenylated graphene. This finding further substantiates that the enhanced proton\nconductance results from sp3 dislocations and the presence of ph-SO - functional groups.\nFigure R’6. Raman spectrum of defected graphene by nitrogen plasma.\nL164-166: We checked the potential damage induced by the chemical functionalization\nat the nanometer scale. HRTEM images of graphene after 4-day and 6-day of SBD\ntreatment did not show the apparition of large >1 nm pores (Fig. 2E)\nFig. 2E (S20) TEM image with diffraction pattern of pristine CVD graphene and SO -\n3\n-graphene after 4 days and 6 days 4-SBD (batch 1, 1 mg·ml-1 in 0.1 M HCl) treatment.\nFig. 2E (S21) HRTEM images of CVD graphene after 1 to 6 days of 4-SBD (batch\n2, 1 mg·ml-1 in 0.1 M HCl) treatment.\nFig. 2E (S22) HRTEM images of CVD graphene after 2 and 4 days of 4-SBD (batch\n3, 1 mg·ml-1 in 0.1 M HCl) treatment.\nMoreover, during the functionalization with SBD, graphene floats on 0.1M HCl in\nwater. We carried control experiment where graphene was floated on pure water and\nwater containing 0.1M HCl and mounted in the DMFC. We did not measure significant\nimprovement in power density, membrane resistance and open circuit voltages for those\nsamples, ruling out the possibility that the enhanced proton conductance was due to\ngraphene degradation due to water or 0.1M HCl during the treatment.\n)\n2\n-m 60\nc\nW\nm\n(\ny 40\nsit\nn\ne\nd\ne r 20\nw\no\np\nx\na\nM 0\n2 3 4 5 6 7 8 9 10 11 12\nTreatment days\nFig. R5 (S25) Maximum power output of Direct Methanol Fuel Cells (DMFCs)\nwith graphene membranes floated on pure water. Graphene membranes were\nfloated on ultrapure water using the same conditions applied for the SBD treatment.\nThe fuel cell tests were operated at 60 °C, with same flow rate of 1 M methanol solution\nand oxygen as tests conducted with SO --graphene membranes.\n3\n2)\n-m\n60\nc\nW\nm\n(\ny 40\nsit\nn\ne\nd\nr\ne 20\nw\no\np\nx.\na\nM 0\n2 3 4 5 6\nTreatment time (day)\nFig. R6 (S26) Maximum power output of Direct Methanol Fuel Cells (DMFCs)\nwith graphene membranes floated on 0.1 M HCl solution. Graphene membranes\nwere floated on 0.1 M HCl using the same conditions applied for the SBD treatment.\nThe fuel cell experiments were performed at 60 °C, with same flow rate of the 1 M\nmethanol solution and oxygen as experiments conducted with SO --graphene\n3\nmembranes.\n2. Intrinsic defects exist within the individual CVD domains forming the film. How do\nthe authors rule the presence of defects and the etching of these defects with longer\nsulfonation times or processing during membrane fabrication? Authors should show\ndirect evidence of expansion of the ring with covalent functionalization e.g. TEM\nimages with increased bond length or distorted rings and Raman spectra.\nSince the authors cannot confirm these aspects with HRTEM or find evidence of sp3 in\nRaman, they should avoid speculative claims.\nSulfophenylation does not cause the etching of original defects into larger one. We have\nperformed extensive TEM characterization, and we could not observe any holes\nforming upon functionalization. We recognize that once the fuel cell is mounted,\nadditional damage may occur, however, still, sulfophenylated graphene membranes\nyield better fuel cell performances than pristine graphene or Nafion alone. We have\nrevised the discussion to leave room for an open dialogue on the potential influence of\ndefects to the increased fuel cell performances.\nModification in the manuscript:\nCVD graphene, L146-149 : Fig. 2B shows scanning electron microscopy (SEM)\nimages of >200 μm wide CVD graphene single crystals grown on a copper foil with\nminimized grain boundaries and intrinsic defects. Multilayer patches were negligibly\nobserved in the samples. However, the sparsely distributed defects and cracks\nintroduced during membrane manipulation cannot be entirely prevented and may\ncontribute to additional permeation in contrast to the proton conductance measured with\na perfectly defect-free graphene 2D crystal.\nHRTEM, L164-170: We checked the potential damage induced by the chemical\nfunctionalization at the nanometer scale. HRTEM images of graphene after 4-day and\n6-day of SBD treatment did not show the apparition of large >1 nm pores (Fig. 2E).\nInstead, the large defect-free graphene area suggests that individual grafting sites could\nnot be observed on the monolayer regions (Fig. 2E). Additionally, no apparent\nexpansion of the graphene lattice due to sp3 distortion was observed. To confirm\ncovalent functionalization, we conducted Raman spectroscopy and elemental analysis\nand the results are discussed in the next section.\nRaman, L193-206 : The increasing value of the D peak intensity ratio (~1350 cm-1)\nover the G peak (~1580 cm-1) (I /I ) indicates the formation of defects upon 4-SBD\nD G\ntreatment, primarily the introduction of sp3 distortions58. Since the I /I peak remains\nD G\nsimilar after the 4-day treatment, we speculate that the introduced defects are unlikely\nto be vacancies or cracks. Additionally, after mounting in the fuel cell, the presence of\nactive edges that could further enlarge over time would yield to an increase of methanol\ncrossover, which we did not measure. We cannot rule out that sulfophenylation could\nseal grain boundaries, or sp3 distortions, however, we could not observe such an effect\nin HRTEM images. With our hypothesis that the combination of a sp3 distortion and a\ncharged SO - group contributes to an increased proton transport.\nfunctionalization of grain boundaries and defects could enhance proton transport\nselectivity through these regions, rather than decreasing it. This is because\nsulfophenylation may effectively seal and passivate grain boundaries, guiding protons\nthrough them. However, our analysis showed no clear preference for functionalization\nat grain boundaries on the microscale, as indicated by the homogeneous distribution of\nRaman signals across the graphene surface (Fig. S14-S15).\nThey should try to find out the real reason for enhanced transport as suggested by 2\nindependent reviewers instead of trying to push through this manuscript in its current\nstate.\nWe formulate hypothesis that could possibly explain our results wherever this is\npossible. Besides the modifications mentioned above, we refined the conclusion\naccordingly:\nL343-360: To conclude, the functionalization of centimeter-size CVD graphene with\nan out-of-plane polar group and an sp3 distortion by diazotization with a sulfophenyl\nradical increases the membrane conductance of CVD graphene in a working DMFC\nfrom 6.9 ± 1.1 S·cm-2 to 30.9 ±2.3 S·cm-2 at room temperature, with a power density of\n127 mW·cm-2 at 60 °C, which is more than twice the power density measured with\nNafion 117. which yielded highly selective proton conductivity (30.9 ±2.3 S·cm-2) and\nalso enabled application in a direct methanol fuel cell delivering a power density more\nthan twice higher than its NafionTM counterpart according to the electrode area (127\nmW·cm-2 at 60 °C). However, further investigations are required to understand the\nfactors contributing to enhancing the proton conductance, particularly defects, sp³\ndistortions, and functional groups at the interface between graphene and the electrolyte.\nControlling how functional groups are oriented on both side of the membrane could\nplay a crucial role in modulating the transport of protons across the membrane. The\ncreation of active sites as intrinsic proton pathways on graphene, as well as controlling\nthe polarization of 2D membranes via these active sites – particularly in more complex\ntwo-dimensional polymer architectures62,63 – would be a natural next step of this\nresearch, to establish correlations between the chemistry of the graphene and electrolyte\ninterface and the efficiency of proton transport, for example using performance\nparameters of a fuel cell or using ion transport measurements.\n2. The authors use CVD grown single crystalline graphene - how do they rule out\ndefects in single crystalline graphene over centimeter scale areas? Several reports have\nshown that single crystalline graphene are not devoid of defects. These aspects appear\nto have been overlooked.\nThe graphene we use is single crystalline over centimeter scales. Still it shows a\nconductance of 6.9 S/cm2 with wet Nafion in its pristine state. This conductance\nincreases to 30.9 S/cm2 after SBD functionalization. We discussed the possible role of\ndefects in the revised manuscript:\nL84-96: Defects in graphene, such as seven- and higher-member rings, together with\nsmall lattice disorders such as sp3 distortions and polar functional groups are promising\napproaches for enhancing proton conductivity, while maintaining the impermeability\nof the basal plane to other substances, which are particularly of interests in energy\ndevices such as fuel cells. In CVD graphene, for example, cracks and defects are\ncommonly observed at the microscale46, influencing proton transport properties. At a\nsmaller scale than larger sized cracks, theoretical studies indicate that small lattice\ndisorders, such as hydrogenated graphene seven- and higher-membered rings,\neffectively lower the proton permeation barrier through graphene47-49. Diffusive proton\nconductance experiments also demonstrate improved proton transport through\ngraphene with eight-membered rings50, vacancy defects33, and other structural\nmodifications such as nitrogen-doped graphene51. Therefore, due to the presence of\nintrinsic defects, the proton conductance of CVD graphene was reported up to three\norders of magnitude higher than that of defect-free graphene 2D crystals52.\nL264-267: Pristine graphene showed a maximum power density of 25.8 ±15.4 mW·cm-\n2, at 212 mV, which is about half (~ 45%) compared to Nafion 117. These results can\nbe attributed to defects and cracks, resulting from the membrane electrode assembly\n(MEA) including CVD graphene. These results can be attributed to the fact that\ngraphene blocks proton transport compare to Nafion.\nThe authors misunderstood the point the reviewer was making. The pristine graphene\nthey refer to is made via CVD! That also have defects that manifest even in single\ncrystalline graphene membranes. Characterizing these intrinsic defects over centimeter\nscale areas is challenging with HRTEM, AFM, and Raman, and several groups around\nthe world have developed diffusive transport as an alternative solution that provides\ninformation in a few hours with reliable statistics. The authors may want to look into\nthose works and try to characterize their materials thoroughly with relevant techniques\nthat will un-ravel the science behind the effects seen here – instead of claiming it comes\nfrom doping, SO3- without direct evidence.\nIn our manuscript we do not perform ion transport measurements and do not\nsystematically try to make correlations between the active site types and the fuel cell\nperformance. To address the misunderstanding regarding CVD graphene, we have\nrevised the following sections to discuss the presence of cracks in CVD graphene and\nthe role of defects and how we confirm the contribution of chemical functionalization\nto the higher fuel cell performances.\nSingle-crystalline CVD graphene vs. exfoliated graphene:\nL84-96: Defects in graphene, such as seven- and higher-member rings, together with\nsmall lattice disorders such as sp3 distortions and polar functional groups are promising\napproaches for enhancing proton conductivity, while maintaining the impermeability\nof the basal plane to other substances, which are particularly of interests in energy\ndevices such as fuel cells. In CVD graphene, for example, cracks and defects are\ncommonly observed at the microscale46, influencing proton transport properties. At a\nsmaller scale than larger sized cracks, theoretical studies indicate that small lattice\ndisorders, such as hydrogenated graphene seven- and higher-membered rings,\neffectively lower the proton permeation barrier through graphene47-49. Diffusive proton\nconductance experiments also demonstrate improved proton transport through\ngraphene with eight-membered rings50, vacancy defects33, and other structural\nmodifications such as nitrogen-doped graphene51. Therefore, due to the presence of\nintrinsic defects, the proton conductance of CVD graphene was reported up to three\norders of magnitude higher than that of defect-free graphene 2D crystals52.\nL142-149: The centimeter-size single-crystal CVD graphene films, which were\nseamlessly stitched into well-aligned domains57, were used to minimize the defects and\ngrain boundaries. Fig. 2B shows scanning electron microscopy (SEM) images of >200\nμm wide CVD graphene single crystals grown on a copper foil with minimized grain\nboundaries and intrinsic defects. Multilayer patches were negligibly observed in the\nsamples. However, the sparsely distributed defects and cracks introduced during\nmembrane manipulation cannot be entirely prevented and may contribute to additional\npermeation in contrast to the proton conductance observed with a perfectly structured\ngraphene 2D crystal.\nBesides the modifications we made to the original text of the manuscript, we also would\nlike to comment on how we included the contributions of cracks in our original data set:\n1. Cracks:\nStability of transferred graphene:\nTo assess the stability of transferred graphene, we floated the sample on water and a 0.1\nM HCl solution for several days. No significant changes were observed, indicating that\npotential cracks would not enlarge simply due to liquid contact (Fig. R5&R6).\n)\n2\n-m 60\nc\nW\nm\n(\ny 40\nsit\nn\ne\nd\ne r 20\nw\no\np\nx\na\nM 0\n2 3 4 5 6 7 8 9 10 11 12\nTreatment days\nFig. R5 (S25) Maximum power output of Direct Methanol Fuel Cells (DMFCs)\nwith graphene membranes floated on pure water. Graphene membranes were\nfloated on ultrapure water using the same conditions applied for the SBD treatment.\nThe fuel cell tests were operated at 60 °C, with same flow rate of 1 M methanol solution\nand oxygen as tests conducted with SO --graphene membranes.\n3\n2)\n-m\n60\nc\nW\nm\n(\ny 40\nsit\nn\ne\nd\nr\ne 20\nw\no\np\nx.\na\nM 0\n2 3 4 5 6\nTreatment time (day)\nFig. R6 (S26) Maximum power output of Direct Methanol Fuel Cells (DMFCs)\nwith graphene membranes floated on 0.1 M HCl solution. Graphene membranes\nwere floated on 0.1 M HCl using the same conditions applied for the SBD treatment.\nThe fuel cell experiments were performed at 60 °C, with same flow rate of the 1 M\nmethanol solution and oxygen as experiments conducted with SO --graphene\n3\nmembranes.\nRaman study of SBD treatment and controlled defect growth by diazotization:\nTo evaluate the impact of the chemical treatment, we conducted Raman spectroscopy\non Nafion/graphene samples floating in the reactive SBD solution over multiple days.\nThe spectral changes remained homogeneous throughout the treatment period, ruling\nout the possibility of treatment-induced cracks at the microscale (Fig. R8). The defect\ndensity increased only up to four days of treatment, aligning with the expected surface\ngrafting process, where only a limited number of sites could be functionalized. If the D\npeak were associated with reactive edges, a much more rapid increase in defect density\nand a significantly larger D peak would be expected.\nFig. R8 (S14) Raman mapping of peak intensity ratios, I /I and I /I , images of\nD G 2D G\nNafion/graphene after floating on SBD solution for 0 to 6 days. The scale bar is 2 μm.\n2. Pinholes\nWe performed three batches of HRTEM measurements on eleven independent samples\nafter different time of SBD treatment. We did not observe the formation of pinholes\n(Fig R2-4).\nFig. R2 (S20) TEM image with diffraction pattern of pristine CVD graphene and SO -\n3\n-graphene after 4 days and 6 days 4-SBD (batch 1, 1 mg·ml-1 in 0.1 M HCl) treatment.\nFig. R3 R10 (S21) HRTEM images of CVD graphene after 1 to 6 days of 4-SBD (batch\n2, 1 mg·ml-1 in 0.1 M HCl) treatment.\nFig. R4 (S22) HRTEM images of CVD graphene after 2 and 4 days of 4-SBD (batch\n3, 1 mg·ml-1 in 0.1 M HCl) treatment.\n3. Point 1 and 2 are supported by the so called \"pristine\" graphene in this study\nshowing > 3mS/cm2 (value for exfoliated graphene).\nThe authors missed the point that even CVD graphene has shown 3-4 mS cm-2 similar\nto exfoliated graphene. (Nature volume 516, pages227–230 (2014); ACS Nano 2019,\n13, 10, 12109–12119; J. Mater. Chem. A, 2022,10, 19797-19810).\nWe pasted our reply this question here again:\nFor dry Nafion a conductance of 3mS/cm2 was indeed reported in 2014. We noted in\nour manuscript that intrinsic defects, along with interfacial contacts in a membrane\nfabricated from CVD graphene, could contribute to higher proton conductance. This\nvalue could be up to three orders of magnitude greater than 3 mS/cm2 (Science, 2021,\n374, eabd7687).\nSingle-crystalline CVD graphene vs. exfoliated graphene:\nL84-96: Defects in graphene, such as seven- and higher-member rings, together with\nsmall lattice disorders such as sp3 distortions and polar functional groups are promising\napproaches for enhancing proton conductivity, while maintaining the impermeability\nof the basal plane to other substances, which are particularly of interests in energy\ndevices such as fuel cells. In CVD graphene, for example, cracks and defects are\ncommonly observed at the microscale46, influencing proton transport properties. At a\nsmaller scale than larger sized cracks, theoretical studies indicate that small lattice\ndisorders, such as hydrogenated graphene seven- and higher-membered rings,\neffectively lower the proton permeation barrier through graphene47-49. Diffusive proton\nconductance experiments also demonstrate improved proton transport through\ngraphene with eight-membered rings50, vacancy defects33, and other structural\nmodifications such as nitrogen-doped graphene51. Therefore, due to the presence of\nintrinsic defects, the proton conductance of CVD graphene was reported up to three\norders of magnitude higher than that of defect-free graphene 2D crystals52.\nDoes this mean the CVD single crystalline graphene made here is not as high in quality\nas even polycrystalline graphene used in other studies? Or is the higher transport from\ncracks and tears?\nIn light of points 1 and 2 the reviewer urges extreme caution from making claims without\nevidence by the authors.\nThe CVD graphene has single crystalline domains of centimeters and was used to\nminimize grain boundaries and inherent defects, as highlighted in the manuscript.\nHowever, when mounted in the fuel cell, we measured a non-infinite membrane\nresistance, suggesting that the MEA contributes to the membrane resistance. Compared\nto polycrystalline graphene, which contains more grain boundaries and multilayer\nregions, single-crystalline graphene offers a more controlled structure with fewer\nintrinsic imperfections. We added to the manuscript:\nL142-149: The centimeter-size single-crystal CVD graphene films, which were\nseamlessly stitched into well-aligned domains57, were used to minimize the defects and\ngrain boundaries. Fig. 2B shows scanning electron microscopy (SEM) images of >200\nμm wide CVD graphene single crystals grown on a copper foil with minimized grain\nboundaries and intrinsic defects. Multilayer patches were negligibly observed in the\nsamples. However, when mounted in the fuel cell, we measured a non-infinite\nmembrane resistance, suggesting that the MEA contributes to the membrane resistance\nwith an original conductance of pristine graphene of 6.9 ± 1.1 S·cm-2.\n4. How do the authors rule out the formation of crack or tears in graphene as the spin\ncoated Nafion film swells during removal of Cu while etching in acid?\nIf large cracks and tears would form, the power density of the fuel cell would reach near\nzero as shown in Figure R1 (Figure S24). But still, we measure a conductance for\npristine graphene which accounts for 6.9 ± 1.1 S·cm-2.\nAlthough the authors used a rigid PC membrane and used Raman these don’t allow for\nprobing over macroscopic areas with high resolution.\nAfter the membrane electrode assembly and fuel cell testing, we can not image the\ngraphene layer. This would be optimal but the graphene is stuck with Nafion and the\ncatalyst layers and can not be taken apart.\n5. The data for 4 day treatment shows better power density than 6 days. This further\nindicates defects and not covalent functionalization and a trade-off.\nThe polymerization of the diazonium compound occurs as a result of surface grafting,\na process confirmed through AFM observations showing that the surface roughness\nincreases with increasing treatment time (see Figure 2C and 2D). The formation of a\nthick polymer layer is likely responsible for the observed decrease in membrane\nperformance. This outlines the reaction mechanism involved in the chemical treatment\nwith both grafting and polymer growth phases.\nThe authors mention they don’t see defects but no HR TEM is provided as evidence.\nWe have discussed our HRTEM studies and images in the main text. The complete set\nof HRTEM results is provided in the supplementary information (Fig. S20-S22). Those\ndo not show defects that appear from the SBD treatment. As such, in fuel cells, we do\nnot expect the SBD treatment itself to create but also can not be compared with\ngraphene after the fuel cell has operated.\nThe reviewer is merely point to defects and variability might be responsible for this\nsince one would not expect a reduction after saturation if this was purely\nfunctionalization.\nThe sulfophenyl radical reacts with a sp2 carbon atom on the graphene basal plane,\nwhich is converted to sp3 upon grafting. Meanwhile, side reactions might also occur: 4-\nSBD can react with an already grafted aryl group (at the ortho position) either via a\nradical or azo-coupling mechanism, both of which might result in oligomerization54 of\nthe aryl species on the graphene basal plane.\nPristine graphene is therefore covered by 4-SBD already after 24 h after which the 4-\nSBD layer becomes thicker and more homogenous. Oligomerization, which is\ncommonly observed during diazonium treatment53 likely leads to the additional\nresistance that protons encounter. The spontaneous oligomerization of SBD, giving rise\nto agglomerates visible in AFM, were therefore not distinguishable by HRTEM from\ncommon hydrocarbon contaminations resulting from the growth of CVD graphene\n(supplementary material, Fig. S20-S22).\n6. Referencing is inadequate and several key papers in the field on centimeter scale\ngraphene PEMs are not cited.\nThe reviewer recommends citing relevant papers in the field focusing on proton\ntransport using CVD graphene to benchmark and contextualize the current work.\nWe revised the discussion of graphene PEMs with the following literatures selected for\nthe discussion:\nThe main challenge for polymeric membranes lies in balancing proton conductivity and\nselectivity, with methanol crossover posing an additional obstacle due to its high\nsolubility in water and uncharged nature, which facilitates migration from the anode to\nthe cathode. To mitigate this issue, various strategies have been pursued to enhance the\npower output of direct methanol fuel cells (DMFCs)22. For instance, incorporating\nlayered additives has been shown to improve performance; the power output increased\nto 35 mW cm-2 with nano-silica additives23 and to 39 mW cm-2 with graphene oxide24.\nThe power output of DMFCs using a Nafion 117 membrane was enhanced to 79 mW\ncm-2 by impregnating the membrane with Pd25. Additionally, recasting the Nafion\nmembrane with blended additives has been shown to boost DMFC to over 100 mW cm-\n2 of the maximum power outputs26,27. The power output of DMFCs was enhanced to 25\nmW cm-2 by sandwiching single-layer graphene between two layers of Nafion\nmembrane28 and increased to 120 mW cm-2 by incorporating single-layer graphene onto\nthe electrode29.\nbased proton exchange membranes for direct methanol fuel cells. Journal of Polymer Research\n31, 125 (2024). https://doi.org/10.1007/s10965-024-03964-y\nmembranes prepared by PECVD for direct methanol fuel cells. Electrochemistry\nCommunications 6, 1069-1074 (2004).\nhttps://doi.org/https://doi.org/10.1016/j.elecom.2004.07.006\nACS Applied Engineering Materials 1, 947-954 (2023).\nhttps://doi.org/10.1021/acsaenm.2c00234\nnanocomposite Nafion membrane for use in high-concentration methanol fuel in DMFC.\nElectrochemistry Communications 5, 571-574 (2003).\nhttps://doi.org/https://doi.org/10.1016/S1388-2481(03)00130-9\nproton selective hybrid membrane for direct methanol fuel cells. Electrochimica Acta 292, 855-\n864 (2018). https://doi.org/https://doi.org/10.1016/j.electacta.2018.09.193\nmembranes for direct methanol fuel cells with reduced methanol crossover. Journal of\nMembrane Science 541, 127-136 (2017).\nhttps://doi.org/https://doi.org/10.1016/j.memsci.2017.06.081\nsandwich membrane for direct methanol fuel cells. Journal of Power Sources 311, 188-194\n(2016). https://doi.org/10.1016/j.jpowsour.2016.02.030\nAdv. Energy Mater. 7, 7 (2017). https://doi.org/10.1002/aenm.201601216\nWhile numerous studies have explored the use of nanoflake additives to enhance PEM\nperformance, many of these involve permeation pathways that include interlayer\ntransport rather than purely transmembrane mechanisms. Regarding single-layer\ngraphene used as PEMs, we discuss now also relevant diffusion experiments with\nparticular emphasis on ionic permeation behavior. See below the added paragraph in\nthe revised introduction:\nL84-96: Defects in graphene, such as seven- and higher-member rings, together with\nsmall lattice disorders such as sp3 distortions and polar functional groups are promising\napproaches for enhancing proton conductivity, while maintaining the impermeability\nof the basal plane to other substances, which are particularly of interests in energy\ndevices such as fuel cells. In CVD graphene, for example, cracks and defects are\ncommonly observed at the microscale46, influencing proton transport properties. At a\nsmaller scale than larger sized cracks, theoretical studies indicate that small lattice\ndisorders, such as hydrogenated graphene seven- and higher-membered rings,\neffectively lower the proton permeation barrier through graphene47-49. Diffusive proton\nconductance experiments also demonstrate improved proton transport through\ngraphene with eight-membered rings50, vacancy defects33, and other structural\nmodifications such as nitrogen-doped graphene51. Therefore, due to the presence of\nintrinsic defects, the proton conductance of CVD graphene was reported up to three\norders of magnitude higher than that of defect-free graphene 2D crystals52.\nselective membrane. ACS Nano 13, 12109-12119 (2019).\nhttps://doi.org/10.1021/acsnano.9b06505\nmicrometer cracks in graphene. Carbon 118, 556-560 (2017).\nhttps://doi.org/https://doi.org/10.1016/j.carbon.2017.03.054\ncrystals. Journal of Physical Chemistry Letters 8, 6009-6014 (2017).\nhttps://doi.org/10.1021/acs.jpclett.7b02820\npermeability and isotope selectivity of single-layer graphene. Adv. Mater. 32, 2002442 (2020).\nhttps://doi.org/10.1002/adma.202002442\nmulti-protonation: A cooperative mechanism for proton permeation. Carbon 144, 724-730\n(2019). https://doi.org/10.1016/j.carbon.2018.12.086\nDefects. ACS Nano 14, 7280-7286 (2020). https://doi.org/10.1021/acsnano.0c02496\nNano Letters 21, 4314-4319 (2021). https://doi.org/10.1021/acs.nanolett.1c00813\natomically thin membranes: Present and future applications. Science 374, eabd7687 (2021).\nhttps://doi.org/10.1126/science.abd7687"}, {"round": 3, "reviewer_comments": {"reviewer_3": "The reviewer appreciates the efforts by Zhang et al. to clarify some aspects including\naddition of HRTEM images and changes to the manuscript to factor in several aspects\nthat were previously overlooked.\nThe author’s response seems to broadly agree with the reviewer’s concerns:\n“The method we employed introduces sp³ distortions, -SO3- groups, and a polymerized\nlayer simultaneously, making it challenging to isolate any single factor as the direct\ncause of the increased transport of protons, as questioned by the reviewer. A\ncomprehensive analysis of this correlation exceeds the scope of a single manuscript,\ngiven the need to carefully isolate all these contributing factors. However, we are\nconfident that our observations, along with the control experiments we have conducted,\nprovide sufficient evidence to demonstrate that sulfophenylated graphene exhibits\nperformance comparable to Nafion polymer membranes in a direct methanol fuel cell.”\n1. Proton conductance increased by ~5x after sulfophenylation and attributed to\npresumably lower energy barrier. But this should be straightforward to measure with\ntemperature to get activation energy barriers. Flushing this out completely will provide\ndetailed insights and position this paper for a high impact journal.\nThanks you for the suggestion. We agree. Element of this analysis, including the results\nin Table S3 (now mentioned in the revised manuscript) together with Figure 4D and\nS28 show, indeed, that the activation energy barrier decreases down to 6.9 kJ·mol-1 after\n4 days of SBD treatment, which is approximately half of the one reported for Nafion\nmembranes under fully humidified conditions (~13.5 kJ·mol-1, Journal of Power\nSources, Volume 196, Issue 18, 15 September 2011, Pages 7363-7371).\nNow our manuscript states:\nBy analyzing the measured conductance and temperature-dependence, the proton\nconductances of SO --graphene membranes exhibit an Arrhenius behavior as described\n3\nby equation (1):\nln(σ𝑇) = ln(σ ) − ln(𝑒)𝐸/𝑘×𝑇-1 (1)\n0 𝑎 𝐵\nwhere σ is the measured conductance, T is the temperature, E is the activation energy\na\nand k is the Boltzmann’s constant. Samples after 1, 2, 3 and 4 days of SBD treatment\nB\nshow a linear relationship of ln(sT) vs. T-1, with a fitted E of 6.9 kJ·mol-1 (0.072 eV)\na\nafter 4 days of SBD treatment (Fig. 4D, and Table S3). Pristine graphene does not show\nan Arrhenius behavior. The relatively low E after 4 days of SBD treatment suggests\na\nthat protons cross the membrane via a Grotthus mechanism (≤0.4 eV) where the proton\nhopping through the membrane at a faster rate than dissociating from the hydronium ion61.\nAfter 5-day and 6-day, ln(sT) vs. T-1 present a less linear relationship compared to 4-\nday of treatment (Fig. S28), which could be caused by the polymerization of SBD,\ntherefore hindering proton transport, leaving larger defects/cracks with a lower proton\nselectivity compared to methanol crossover.\nFigure 4 | DMFC performance with CVD graphene and SO --graphene membranes.\n3\n(A) Power-current density (P-I) and voltage-current density (V-I) curves for Nafion 117,\npristine graphene, 4-day, and 6-day 4-SBD treated SO --graphene. The cell voltage (V)\n3\nand the power density (P) as a function of current density (I) were measured in a 1 M\nmethanol/water solution at 60 °C. (B) Maximum power density as a function of\ntemperature. (C) Equivalent circuit and Nyquist plots of electrochemical impedance\nspectroscopy (EIS) measured at the open circuit voltage of the DMFC with graphene\nand SO --graphene membranes. A NafionTM layer was spin coated on graphene and\n3\nof PC with ca. 3μm diameter holes) filled with NafionTM. (D) ln(σ𝑇) as a function of the\ninverse of temperature for graphene membranes. The energy barriers were calculated\nbased on eq. (1) following an Arrhenius-type analysis. (E) P-I curves of the DMFC fuel\ncell operating at 60 °C with aqueous solutions of methanol (respectively 1 and 5 M) for\na ~183 μm thick Nafion 117 layer and 4-day SO --graphene. (F) Decreases of maximum\n3\npower density upon operating the DMFC in 5 M methanol compared to 1 M methanol.\nThe blue dashed line corresponds to the decrease of the power density of a DMFC with\nonly Nafion 117 when switching from 1 M methanol to 5 M methanol.\nWe also added the activation energy values, to reflect on the correlation between the\nactivation energy and the increased proton conductance upon sulfophenylation by SBD.\nWe revised the abstract:\nL39-44: The membrane conductance of pristine graphene in direct methanol fuel cells,\nderived mainly from intrinsic defects, is 6.9 ± 1.1 S·cm-2. After chemical\nfunctionalization, the proton conductance increases to 30.9 ± 2.3 S·cm-2, normalized to\nthe free-standing graphene area, indicating that covalently functionalizing graphene\nwith sulfophenyl groups efficiently lowers the membrane resistance presumably\nbecause the sulfophenylation lowers the energy barrier to 6.9 kJ·mol-1 for protons to\ntransport across graphene.\nWe also revised the conclusion:\nL341-346: To conclude, the functionalization of centimeter-size CVD graphene with a\nsulfophenyl radical increases the membrane conductance of CVD graphene in a\nworking DMFC from 6.9 ± 1.1 S·cm-2 to 30.9 ±2.3 S·cm-2 at room temperature, with a\npower density of 127 mW·cm-2 at 60 °C, which is more than twice the power density\nmeasured with Nafion 117. The activation energy barrier of -SO --graphene is as low\n3\nas 6.9 kJ·mol-1, indicating a reduced proton transport barrier, in line with the improved\nproton conductance through graphene after sulfophenylation.\n2. How do the authors rule out formation of additional proton permeable defects in the\nCVD graphene lattice upon functionalization but hindering methanol transport? The\nID/IG ratio in Raman clearly increases for upto 4 days of functionalization indicating\ndefect formation in graphene. The polymer from functionalization layer or transfer\ncontamination will preferentially cover defects preventing observation in HRTEM\nimages. Reviewer notes authors do mention HRTEM doesn’t show defects >1nm but\npolymer/contamination covered defects won’t be visible in HRTEM and doesn’t mean\nthey don’t exist. In fact no defect is seen in the HRTEM images presented!\nWe did not observe that the sulfophenylation creates pores larger than a sp3 distortion.\nInstead, AFM suggests the polymerization of SBD after 4 days. And as there are no\npores to start with, the polymerized layer increases the resistance, which results in the\ndecrease in the power output. If pores larger than a sp3 distortion were present before\nSBD functionalization, they would be visible on HRTEM images of pristine graphene.\nWe discuss the grain boundaries below.\n3. In the context of the above, reviewer urges caution and closer analysis of data and\nincluding formation of additional defects as a possibility along with “With our\nhypothesis that the combination of a sp3 distortion and a charged SO3- group\ncontributes to an increased proton transport”\nAuthors already allude to this “This is because sulfophenylation may effectively seal\nand passivate grain boundaries, guiding protons through them.”\nAt this stage of the research, several parameters may influence the proton transport\nacross functionalized graphene. For example, the distance between the -SO - group and\n3\ngraphene, the presence of a sp3 site together with a charged group, the combination of\na charge and a grain boundary. SBD functionalization does not allow us to disentangle\neach of these separate contributions. We added in the second round of revision:\nHowever, further investigations are required to understand the factors contributing to\nenhancing the proton conductance, particularly defects, sp³ distortions, and functional\ngroups at the interface between graphene and the electrolyte. Controlling how\nfunctional groups are oriented on both side of the membrane could play a crucial role\nin modulating the transport of protons across the membrane. The creation of active sites\nas intrinsic proton pathways on graphene, as well as controlling the polarization of 2D\nmembranes via these active sites – particularly in more complex two-dimensional\npolymer architectures62,63 – would be a natural next step of this research, to establish\ncorrelations between the chemistry of the graphene and electrolyte interface and the\nefficiency of proton transport, for example using performance parameters of a fuel cell\nor using ion transport measurements.\nWhat we like to convey by ‘This is because sulfophenylation may effectively seal and\npassivate grain boundaries, guiding protons through them.’ is that adding a charged\ngroup either next to a lattice distortion or to a grain boundary site may increase proton\ntransport. Any defects larger than the benzene ring sealed by a polymer chain upon\npolymerization of SBD (for example, a heptagon in a Stone-Wales grain boundary\ndefect) could be yielding a reduced proton conductance. Additionally, the presence of\ncracks from the membrane electrode assembly can contribute to a baseline conductance\n(as we measured through the cracks), which we expect to be the same for pristine and\nSBD functionalized graphene."}, "author_response": "Response to reviewers\nWe thank Reviewer #3 for the valuable questions which helped to further increase the\nclarity of our manuscript. Please find our detailed responses to each specific question\nbelow. We also amended the manuscript with these revisions (in red).\nReviewer #3 (Remarks to the Author):\nThe reviewer appreciates the efforts by Zhang et al. to clarify some aspects including\naddition of HRTEM images and changes to the manuscript to factor in several aspects\nthat were previously overlooked.\nThe author’s response seems to broadly agree with the reviewer’s concerns:\n“The method we employed introduces sp³ distortions, -SO3- groups, and a polymerized\nlayer simultaneously, making it challenging to isolate any single factor as the direct\ncause of the increased transport of protons, as questioned by the reviewer. A\ncomprehensive analysis of this correlation exceeds the scope of a single manuscript,\ngiven the need to carefully isolate all these contributing factors. However, we are\nconfident that our observations, along with the control experiments we have conducted,\nprovide sufficient evidence to demonstrate that sulfophenylated graphene exhibits\nperformance comparable to Nafion polymer membranes in a direct methanol fuel cell.”\n1. Proton conductance increased by ~5x after sulfophenylation and attributed to\npresumably lower energy barrier. But this should be straightforward to measure with\ntemperature to get activation energy barriers. Flushing this out completely will provide\ndetailed insights and position this paper for a high impact journal.\nThanks you for the suggestion. We agree. Element of this analysis, including the results\nin Table S3 (now mentioned in the revised manuscript) together with Figure 4D and\nS28 show, indeed, that the activation energy barrier decreases down to 6.9 kJ·mol-1 after\n4 days of SBD treatment, which is approximately half of the one reported for Nafion\nmembranes under fully humidified conditions (~13.5 kJ·mol-1, Journal of Power\nSources, Volume 196, Issue 18, 15 September 2011, Pages 7363-7371).\nNow our manuscript states:\nBy analyzing the measured conductance and temperature-dependence, the proton\nconductances of SO --graphene membranes exhibit an Arrhenius behavior as described\n3\nby equation (1):\nln(σ𝑇) = ln(σ ) − ln(𝑒)𝐸/𝑘×𝑇-1 (1)\n0 𝑎 𝐵\nwhere σ is the measured conductance, T is the temperature, E is the activation energy\na\nand k is the Boltzmann’s constant. Samples after 1, 2, 3 and 4 days of SBD treatment\nB\nshow a linear relationship of ln(sT) vs. T-1, with a fitted E of 6.9 kJ·mol-1 (0.072 eV)\na\nafter 4 days of SBD treatment (Fig. 4D, and Table S3). Pristine graphene does not show\nan Arrhenius behavior. The relatively low E after 4 days of SBD treatment suggests\na\nthat protons cross the membrane via a Grotthus mechanism (≤0.4 eV) where the proton\nhopping through the membrane at a faster rate than dissociating from the hydronium ion61.\nAfter 5-day and 6-day, ln(sT) vs. T-1 present a less linear relationship compared to 4-\nday of treatment (Fig. S28), which could be caused by the polymerization of SBD,\ntherefore hindering proton transport, leaving larger defects/cracks with a lower proton\nselectivity compared to methanol crossover.\nFigure 4 | DMFC performance with CVD graphene and SO --graphene membranes.\n3\n(A) Power-current density (P-I) and voltage-current density (V-I) curves for Nafion 117,\npristine graphene, 4-day, and 6-day 4-SBD treated SO --graphene. The cell voltage (V)\n3\nand the power density (P) as a function of current density (I) were measured in a 1 M\nmethanol/water solution at 60 °C. (B) Maximum power density as a function of\ntemperature. (C) Equivalent circuit and Nyquist plots of electrochemical impedance\nspectroscopy (EIS) measured at the open circuit voltage of the DMFC with graphene\nand SO --graphene membranes. A NafionTM layer was spin coated on graphene and\n3\nof PC with ca. 3μm diameter holes) filled with NafionTM. (D) ln(σ𝑇) as a function of the\ninverse of temperature for graphene membranes. The energy barriers were calculated\nbased on eq. (1) following an Arrhenius-type analysis. (E) P-I curves of the DMFC fuel\ncell operating at 60 °C with aqueous solutions of methanol (respectively 1 and 5 M) for\na ~183 μm thick Nafion 117 layer and 4-day SO --graphene. (F) Decreases of maximum\n3\npower density upon operating the DMFC in 5 M methanol compared to 1 M methanol.\nThe blue dashed line corresponds to the decrease of the power density of a DMFC with\nonly Nafion 117 when switching from 1 M methanol to 5 M methanol.\nWe also added the activation energy values, to reflect on the correlation between the\nactivation energy and the increased proton conductance upon sulfophenylation by SBD.\nWe revised the abstract:\nL39-44: The membrane conductance of pristine graphene in direct methanol fuel cells,\nderived mainly from intrinsic defects, is 6.9 ± 1.1 S·cm-2. After chemical\nfunctionalization, the proton conductance increases to 30.9 ± 2.3 S·cm-2, normalized to\nthe free-standing graphene area, indicating that covalently functionalizing graphene\nwith sulfophenyl groups efficiently lowers the membrane resistance presumably\nbecause the sulfophenylation lowers the energy barrier to 6.9 kJ·mol-1 for protons to\ntransport across graphene.\nWe also revised the conclusion:\nL341-346: To conclude, the functionalization of centimeter-size CVD graphene with a\nsulfophenyl radical increases the membrane conductance of CVD graphene in a\nworking DMFC from 6.9 ± 1.1 S·cm-2 to 30.9 ±2.3 S·cm-2 at room temperature, with a\npower density of 127 mW·cm-2 at 60 °C, which is more than twice the power density\nmeasured with Nafion 117. The activation energy barrier of -SO --graphene is as low\n3\nas 6.9 kJ·mol-1, indicating a reduced proton transport barrier, in line with the improved\nproton conductance through graphene after sulfophenylation.\n2. How do the authors rule out formation of additional proton permeable defects in the\nCVD graphene lattice upon functionalization but hindering methanol transport? The\nID/IG ratio in Raman clearly increases for upto 4 days of functionalization indicating\ndefect formation in graphene. The polymer from functionalization layer or transfer\ncontamination will preferentially cover defects preventing observation in HRTEM\nimages. Reviewer notes authors do mention HRTEM doesn’t show defects >1nm but\npolymer/contamination covered defects won’t be visible in HRTEM and doesn’t mean\nthey don’t exist. In fact no defect is seen in the HRTEM images presented!\nWe did not observe that the sulfophenylation creates pores larger than a sp3 distortion.\nInstead, AFM suggests the polymerization of SBD after 4 days. And as there are no\npores to start with, the polymerized layer increases the resistance, which results in the\ndecrease in the power output. If pores larger than a sp3 distortion were present before\nSBD functionalization, they would be visible on HRTEM images of pristine graphene.\nWe discuss the grain boundaries below.\n3. In the context of the above, reviewer urges caution and closer analysis of data and\nincluding formation of additional defects as a possibility along with “With our\nhypothesis that the combination of a sp3 distortion and a charged SO3- group\ncontributes to an increased proton transport”\nAuthors already allude to this “This is because sulfophenylation may effectively seal\nand passivate grain boundaries, guiding protons through them.”\nAt this stage of the research, several parameters may influence the proton transport\nacross functionalized graphene. For example, the distance between the -SO - group and\n3\ngraphene, the presence of a sp3 site together with a charged group, the combination of\na charge and a grain boundary. SBD functionalization does not allow us to disentangle\neach of these separate contributions. We added in the second round of revision:\nHowever, further investigations are required to understand the factors contributing to\nenhancing the proton conductance, particularly defects, sp³ distortions, and functional\ngroups at the interface between graphene and the electrolyte. Controlling how\nfunctional groups are oriented on both side of the membrane could play a crucial role\nin modulating the transport of protons across the membrane. The creation of active sites\nas intrinsic proton pathways on graphene, as well as controlling the polarization of 2D\nmembranes via these active sites – particularly in more complex two-dimensional\npolymer architectures62,63 – would be a natural next step of this research, to establish\ncorrelations between the chemistry of the graphene and electrolyte interface and the\nefficiency of proton transport, for example using performance parameters of a fuel cell\nor using ion transport measurements.\nWhat we like to convey by ‘This is because sulfophenylation may effectively seal and\npassivate grain boundaries, guiding protons through them.’ is that adding a charged\ngroup either next to a lattice distortion or to a grain boundary site may increase proton\ntransport. Any defects larger than the benzene ring sealed by a polymer chain upon\npolymerization of SBD (for example, a heptagon in a Stone-Wales grain boundary\ndefect) could be yielding a reduced proton conductance. Additionally, the presence of\ncracks from the membrane electrode assembly can contribute to a baseline conductance\n(as we measured through the cracks), which we expect to be the same for pristine and\nSBD functionalized graphene."}]

Decision letter