Controllable surface reconstruction of copper foam for electrooxidation of benzyl alcohol integrated with pure hydrogen production

Electrocatalytic water splitting that is coupled with electrocatalytic chemical oxidation is considered as one of the promising methods for efficiently obtaining hydrogen energy and fine chemicals. Herein, we focus on an electrochemical redox activation strategy to rationally manipulate the microstructure and surface valence states of copper foam (CF) and boost the corresponding performance towards electrocatalytic benzyl alcohol oxidation (EBA), accompanied by the efficient hydrogen production. Correspondingly, the Cu(II)‐dominated species are gradually formed on the CF surface with the dissolution and redeposition of copper in the suitable potential range. The new species containing Cu2O, CuO, and Cu(OH)2 during surface reconstruction process of the CF were confirmed by multiple characterization techniques. After 220‐cycled activation (CF‐220), the activated CF achieves an increase of current density for EBA in anode from 9.5 for the original CF to 29.3 mmol/cm2, while the pure hydrogen yield increases threefold than that of the original CF at 1.5 VRHE. The produced new species can endow the CF‐220 with abundant acidity sites, which can enhance the adsorption toward Lewis‐basicity benzyl alcohol, confirmed by NH3‐temperature‐programmed desorption. In situ Raman result further reveals that the as‐produced CuO, Cu(OH)2, and Cu(OH)42− are the main active species toward the EBA process.


| INTRODUCTION
Water molecules can be broken to produce hydrogen as an energy carrier, which in turn can be combusted to supply energy with zero emissions. 1One of the most promising options for achieving truly clean "Green Hydrogen" is to drive the breaking of O−H bonds in water molecules through electricity from renewable energy sources. 2 Nowadays, renewable energy is not only limited to traditional solar or wind energy, some new forms of renewable energy, such as triboelectric nanogenerators, also have great potential for future applications. 35][6] Unfortunately, the overall water splitting is impeded by the sluggish kinetics of the fourelectron transfer formed by the O−O bond during the OER process, 7 and the mixture of hydrogen and oxygen produced at the same time is also prone to safety hazards.To obtain the hydrogen, it is still necessary to impose a potential much higher than 1.23 V (the thermodynamic potential of the OER) and to separate the cathode and anode by using a proton exchange membrane electrolyzers with an expensively fluorinated membrane, 8 which is not conducive to the large-scale production of water splitting.Despite advanced precious metal-free catalysts for OER have been extensively investigated, the input voltage of around 1.6 − 2.0 V is still required for water splitting in practical electrolyzers. 7,9To alleviate the overpotentials and simplify electrolysis equipment, hybrid water electrolysis systems involved in the conversion of chemicals such as benzyl alcohol (BA), 5-hydroxymethylfurfural (HMF), and glycerol have been highly concerned.It is known that the thermodynamic potentials driving the electrooxidation of BA, glycerol, and HMF are 0.48, 0.69, and 0.11 V, respectively, which are superior to the OER. 7,10The BA molecule is one of the simplest aromatic alcohols and its oxidation products is the primary chemical feedstock for the production of various bulk commodities, polymers, and pharmaceuticals. 11,12As such, electrocatalytic benzyl alcohol oxidation (EBA) integrated with pure hydrogen production is one of the promising technologies.How to drive this process well will be full of great challenges, especially, the urgent need for high-efficiency catalysts.
Recently, metallic Cu or copper-containing catalysts have attracted extensive research due to their exceptionally catalytic properties. 13,146][17] Also, copper and its derivatives have been gradually extended to electrocatalytic oxidation in recent years due to their wide availability and variable valence electronic structure. 18ore recently, Wang's group 19 proposed to couple the oxidation of glucose to promote hydrogen production with copper hydroxide as the anodic electrode.The onset potential over copper hydroxide anode is lower than that of nickel, cobalt, or iron anode by 0.6 V since the organic substrate in anode is indirectly oxidized in the oxidation-reduction reaction of the copper catalyst itself.Choi et al. 20 utilized electrodeposition to deposit corncob-like dendrite copper species on copper foil as an anode to catalyze 5-HMF to 2,5-furandicarboxylic acid with a yield of 96.4%.In general, for the electrooxidation of organic compounds, copper or its oxides/hydroxides as an effective active site have a lower onset potential than that of other transition metals, and exhibit high selectivity toward the target product.Several highperformance electrocatalysts had been synthesized in previous theoretical and experimental investigations employing copper foils or copper foam (CF) as catalyst precursors.Unlike cathodic reduction, the complexity of copper anode catalysis is mainly reflected in the dissolution/reprecipitation of copper species at different potentials.Electrochemical redox activation, such as CV and pulsed potential, has been used to improve the catalytic performance of graphite and copper foils in water splitting and CO 2 reduction. 17,21However, the structure evolution and species conversion that occur through these activation strategies are not well understood for CF, which is challengeable for designing high-performance catalysts and understanding the key catalytic players in hybrid water electrolysis.
In this study, a strategy to rationally modify the valence states of the CF surface through electrochemical redox activation is proposed, which ultimately triggers the process of accelerated EBA integrated with pure hydrogen production.We found that the range of activation potential influences the dissolution of copper in alkaline electrolytes.Meanwhile, the valence and morphology of CF depend strongly on the cycles of electrochemical redox activation.With the increase of cycles, the percentage of Cu(II) tends to stabilize, reaching 63.8% after 220 activation cycles.Compared to the original CF, the CF after 220 cycles achieves a threefold improvement in pure H 2 yield, reaching 90.8 mmol/cm 2 in a membrane-free reactor at 1.5 V RHE (RHE = reversible hydrogen electrode) toward the EBA.In addition, multiple characterizations complementarily confirm that a mixed phase containing Cu 2 O, CuO and Cu(OH) 2 is formed on the surface after activation.With the support of in situ Raman spectra and NH 3temperature-programmed desorption (NH 3 -TPD), the accelerated EBA process achieved by electrochemical redox activation of the catalyst is explained by the mechanism of the acidity sites of the catalyst matching with the Lewis basicity of the BA.

| Surface reconstruction of CF
It is known that for the oxidation potential of copper in alkaline systems, the conversion from metallic Cu to Cu 2 O occurs above 0.3 V RHE with a characteristic peak of around 0.6 V RHE , while the high-valence Cu(II) ions appear at above 0.8 V RHE . 22,23Thus, the CF can be activated at the abovementioned potentials, corresponding to the cyclic voltammetry (CV) curves shown in Supporting Information: Figure S1.There is a clear oxidation P1 peak assigned as the formed Cu(I) species and oxidation P2 peak corresponding to Cu(II) species at the initial state.The derivatively possible oxidation reaction is listed in Supporting Information: Table S1. 17,20After 10-cycled activation, it is noted that the electrolyte is changed from colorless to blue, as shown in Supporting Information: Figure S2, which can be explained by the soluble blue Cu(OH) 4  2− species produced by the reaction Equations ( 3) and ( 5) (Supporting Information: Table S1).Further, this dissolved Cu(OH) 4 2− will be reduced to metallic Cu, which covers the cathode, as shown in the inset of Supporting Information: Figure S2.To avoid the rapid dissolution of copper species and precipitate at the cathode, the activation potential is set in the range of 1.3-1.8V RHE .In this case, the dissolution diffusion of Cu species can be suppressed and the corresponding elemental content of Cu species in the electrolyte is only trace in the inductively coupled plasma results compared with that after 10-cycled CV activation at 0.3-1.8V RHE (Supporting Information: Figure S3).The corresponding CV curves (1.3-1.8V RHE ) of CF activation were also documented in Supporting Information: Figure S4, where the OER is dominated.Meanwhile, the overpotential of the OER decreases with the increase of the number of cycles, indicating the enhanced reactivity of the OER.Before 220-cycled CV activation, the required potential decreases gradually with the increased cycle number (Supporting Information: Figure S4A-C).However, when the number of cycles is increased from 220 to 500, the potential changes slightly and remains essentially constant (Supporting Information: -Figure S4D), implying that the activation of CF reaches equilibrium and stabilizes at around 220 cycles.
Resultantly, the optimal and stable structure CF is formed and produced and such reconstruction of the CF surface strongly depends on the number of cycles.
The changes in the CF surface morphology before and after the activation at different cycles were characterized by scanning electron microscope (SEM) and atomic force microscope (AFM).The commercial CF shows a flat surface as shown in the SEM and AFM images before CV activation (Supporting Information: Figure S5).However, as the number of cycles increases to 30 (Figure 1A), the surface of the original CF becomes rough and the corresponding three-dimensional images of the AFM clearly show different degrees of bumps.
Due to the continuous local dissolution/reprecipitation of copper species, the surface is gradually converted to nanowires after 220 cycles and finally to nanosheet arrays after 500 cycles (Figure 1C,D).Meanwhile, the color of CF undergoes a clear macroscopic change during activation, shifting from its intrinsic red-orange color to black (SEM inset), implying the generation of new copper species, such as black copper oxide.In addition, the concentration-dependent characteristics of CF in different concentrations of KOH electrolytes (0.5, 1.5, and 2 mol/L) were carried out and further investigated.As shown in the Supporting Information: Figures S6-S8, the CF surface can be reconstructed in various concentrations of KOH electrolytes.With low concentrations of KOH (0.5 mol/L), the surface of CF begins to appear more dispersed nanosheets after 500-cycled CV activation.With the increase of KOH concentration to 1.5 and 2 mol/L, the CF surface has generated the nanosheets after 220 and 50 cycles, respectively.This result indicates that the higher the concentration of KOH is, the faster the nanosheets are produced on the CF surface.This phenomenon can be explained by the fact that the higher the KOH concentration is, a larger current density there is at the same potential, resulting in a faster surface reconstruction.
In general, the electrochemical redox activationinduced surface reconstruction leads to the formation of new copper species.The surface reconstruction process was further confirmed by the electrochemically active surface area (ECSA), which can be used to detect the available active sites.The CF with different cycles of activation in the non-Faraday potential range was obtained from CV curves at scan rates of 0.02-0.1 V/s.Higher values of the electrochemical double-layer capacitance (C dl ) imply a higher adsorption capacity of the electrode for OH − .As shown in the Supporting Information: Figure S9, after 220 cycles, C dl is 25 mF/cm 2 (Supporting Information: Figure S10), which is sixfold higher than that of the original CF (4 mF/cm 2 ), but only increases to 29 mF/ cm 2 after 500 cycles.This result indicates that electrochemical redox activation generates new active sites and their production occurs mainly within the first 220 cycles.
To further clearly indicate the difference in the performance of CF before and after electrochemical redox activation, the linear sweep voltammetry (LSV, without IR compensation) curves of CF after 220-cycled activation (CF-220) and pristine CF were measured and compared.As shown in Figure 2A, after activation, the potential of 1.57 V RHE is required for CF-220 to reach a current density of 10 mA/cm 2 (red line), while the pristine CF requires a potential of more than 1.6 V RHE to reach 10 mA/cm 2 (purple line).][26][27][28] To more clearly ascertain the generation of new species during electrochemical redox activation of CF, X-ray diffraction (XRD) was first employed to investigate the structural transformation of copper.As shown in Figure 2B, THE strong diffraction peaks located at 43.4°, 50.6°, and 74.4°correspond to the (111), (200), and (220) phases of Cu (PDF#04-0836), respectively.With the increase of the cycles of electrochemical redox activation, new diffraction peaks gradually appear at 36.1°and 39.3°, which can be more clearly seen in Figure 2C, where the intensity of these peaks gradually enhances with increasing number of cycles.These new peaks correspond to the (002) and (111) crystal planes of CuO (PDF#34-1354), respectively, and the increase in intensity indicates a gradual increase in the crystallinity of CuO.
It is well known that the two most critical factors for determining electrochemical activity are the surface morphology of catalysts and surface copper species. 29he XPS analysis was performed to probe the transition of valence states for copper species on the CF surface before and after different cycles of activation.Before activation, in the Cu LMM auger electron spectra (AES) region of the pristine CF (Supporting Information: Figure S11), there are characteristic peaks at 570.2, 568.3, and 567.0 eV, ascribed to Cu(I), Cu(II) and metallic Cu, respectively.These species of Cu(I) and Cu(II) are derived from the natural oxide layer on the surface.After different intensities of activation, the proportion of surface valence states is significantly changed from AES (Figure 2D).More intuitively, the ratios of metallic Cu, Cu(I), and Cu(II) were calculated by deconvolution curves and recorded in Figure 2E.For the original CF, the percentages of metallic Cu, Cu(I), and Cu(II) are  To further distinguish the newly formed copper species, the vibrational modes of original CF and CF-220 were compared by Raman spectroscopy, as shown in Figure 2H.For pristine CF, the native oxide layer has no obvious Raman characteristic peaks, which has been confirmed by previous work. 30,31After 220-cycled electrochemical redox activation, the CF-220 electrode exhibits three new characteristic peaks at 210, 300, and 485 cm −1 , attributed to Cu 2 O, CuO, and Cu(OH) 2 , respectively.
The CF-220 was further examined by transmission electron microscopy (TEM), showing a needle-like structure as shown in Supporting Information: Figure S12, which is consistent with the observations of SEM and AFM images.In addition, high-resolution TEM (HR-TEM) shows lattice stripes with different lattice spacings (Figure 3A) corresponding to different copper species.The three different lattice spacings of CuO correspond to the (200) plane, (002) plane, and (111) plane, which are consistent with the reported lattice spacings.In addition, (111) and (002) of Cu(OH) 2 and (111) planes of Cu 2 O can also be detected.The fast Fourier transform (FFT) of Figure 3B also confirms that the CF-220 surface is composed of polycrystals.The high-angle annular dark field-TEM (Figure 3C) and the corresponding elemental mapping images (Figure 3D,E) show that the oxygen and copper uniformly distributed.It has already been reported that the wettability of the catalyst matches with the reactivity, 32 so it is necessary to compare the surface wettability of the activated CF with that of the original CF by contact angle.It is obviously seen from Supporting Information: Figure S13 that the CF-220 is more hydrophilic, indicating that the activation can facilitate sufficient yet intimate contact between the electrolyte and the electrode.

| EBA
After 220-cycled electrochemical oxidation, the surface of the CF was activated and endowed with Cu(II)-enriched features, which enables the activated CF to be a potential electrocatalyst for EBA.First, the relationship between the concentration of BA and the current density was investigated, as shown in Figure 4A.The polarization curves at different BA concentrations show that the current density increases with boosting BA concentration.This is because the higher the concentration of the substrate, the smaller the diffusion resistance between the solution and the electrode surface, thus favoring the occurrence of electrochemical reactions.The current density increases slightly at the same potential when the concentration of BA increases to greater than 0.05 mol/L, which is caused by the low solubility of BA in water.The delivered current density featured a concentrationdependent behavior.Further, the electrochemical performances of pristine CF and CF-220 were measured in 1 mol/L KOH aqueous solution with or without 0.05 mol/ L BA.As shown in Figure 4B, the pristine CF and CF-220 featured an inert response to OER.In sharp contrast, the required potential for CF-220 to reach a current density of 10 mA/cm 2 is reduced by 150 mV with BA in comparison to the original CF.In addition, the concentration of BA can be fitted to the first-order kinetic model as a function of time, as shown in Figure 4C.The kinetic constant (k) over the activated CF is 4.75 s −1 , which is 1.8-fold higher than that of the original CF, indicating that the performance of EBA is significantly improved.Nyquist plots of electrochemical impedance spectroscopy (Supporting Information: Figure S14) revealed that the diameter of semicircle decreased for the CF after oxidation, indicating a small charge transfer resistance (R ct ) and faster electrochemical reaction kinetics.From the above results, it can be easily concluded that electrochemical redox activation gives CF-220 a more significant advantage than the original CF in terms of EBA.
The responsive behavior of the CF-220 catalyst for EBA and OER was also tested by the amperometric i-t test, where the current density was recorded before and after BA injection and plotted in Figure 4D.No obvious current density is detected without BA, implying that the OER cannot be triggered at this potential of 1.5 V RHE and the CF-220 has an inert OER responsive behavior.In contrast, the current density increases immediately when 0.05 mol/L of BA is injected, meaning that the electrochemical oxidation of BA is driven and CF-220 is able to catalyze the conversion of BA.The corresponding products in the electrolyte are analyzed by gas chromatography (GC), and the conversion/yield are recorded in Figure 4E.With the increase of reaction time, the conversion of BA gradually increases and finally reaches 98%, whereas the yield of Ph-CHO increases and then decreases, and is oxidized to Ph-COOH finally.This further indicates that the constructed CF-220 has a sensitivity, specificity and favorable selectivity for EBA.Further, the pH of the electrolyte during electrolysis is recorded through in situ test system.As shown in Supporting Information: Figure S15, the pH of the electrolyte decreases from 13.8 to 13.4 within 3 h of continuous electrolysis.This phenomenon can be explained by the fact that part of the alkaline electrolyte reacts with CO 2 in air to form K 2 CO 3 , and Ph-COOH (a typical weak acid) reacts with KOH to form potassium benzoate.In addition, to further check the purity and yield of hydrogen in the electrolyzer, the gas from the electrolyzer was passed into the GC using N 2 as the carrier gas.As shown in the GC results in Supporting Information: Figure S16, a stable signal of H 2 is detected at a potential of 1.5 V RHE .In the absence of potential (black line in Supporting Information: Figure S16), there is also a signal of O 2 , which may be due to the presence of traces of O 2 in N 2 .Meanwhile, the cathodic hydrogen yield in this system (1.5 V RHE with 0.05 mol/L BA) reaches to 90.8 mmol/cm 2 , which is about 3.0-fold higher than that of the original CF (Figure 4F).Thus, both pure hydrogen and anodic organic products can be obtained simultaneously in a single chamber membrane-free electrolytic cell.The electrochemical cycling stability of CF-220 was examined by amperometric i-t test, where the anode Faraday efficiency was also calculated.As shown in the i-t curves of Supporting Information: Figure S17, the activated CF still exhibits stability over six cycles.Meanwhile, the anode Faraday efficiency (Supporting Information: Figure S18) can be kept at about 75% for each cycle, indicative of an efficient charge utilization and favorably electrochemical stability.
As discussed in the above section, a mixture of Cu 2 O, Cu(OH) 2, and CuO is formed on the CF surface after electrochemical oxidation and activation.The stability of CF-220 after EBA reaction was assessed by surface morphology and valence.In Supporting Information: Figure S19A, the surface still consists of interconnected nanosheets and wires, which is consistent with the initial structure of the catalyst shown in Figure 1C.The surface valence states of CF-220 after EBA were characterized by Cu LMM AES (Supporting Information: Figure S19B) and the percentage of metallic Cu, Cu(I), and Cu(II) was calculated on the basis of the deconvolution.As shown in Supporting Information: Figure S20, the proportion of copper species is not significantly changed after EBA, indicative of excellent electrochemical stability.
To further confirm the active species for effectively driving the EBA, we assembled an in situ Raman detection system (Supporting Information: Figure S21A).The digital images of the test process and the schematic diagram of the in situ cell are shown in Supporting Information: Figure S21B and Figure 5A.The corresponding in situ Raman processes were tested and recorded at constant potentials (0 and 1.5 V RHE ).For comparison, the Raman signals of the reaction substrate and products were also recorded in Supporting Information: Figure S22.
There is no obvious characteristic peak signal at 0 V RHE (Figure 5B).While at 1.5 V RHE , the new characteristic peaks appear simultaneously at 290, 475, and 551 cm −1 , which belong to CuO, Cu(OH) 2 , and Cu(OH) 4 2− , respectively. 33,34Three characteristic peaks disappear simultaneously when the potential returns to 0 V RHE , which reveals that the reversible conversion of Cu species into CF species occurs during the EBA process.The possible mechanisms for surface reconstruction that contain dissolution/redeposition are also summarized in Supporting Information: which reveals that the reversible conversion.First, the dissolution of metallic Cu produces Cu(OH) 4   2− , which is redeposited on the surface of CF in the form of Cu(OH) 2 phase due to its locally supersaturated concentration (Equation ( 7)), and then dehydrates into stable CuO by thermodynamically favorable reactions (Equation ( 8)).
It is commonly accepted that the adsorption capability of the catalyst for the substrate affects and determines the kinetics of the reaction.Zhao et al. 35 and Pearson 36 reported the enhanced adsorption of BA on the surface of the catalyst through the interaction between Lewis acid and Lewis base.It was confirmed that the alcohol hydroxyl group in the BA molecule contained a lone pair of electrons and can be considered a Lewis base that prefers to adsorb on Lewis acid sites. 37,38To gain insight into the effect of surface acidic sites of activated CF on BA adsorption, the variation of acidic sites was analyzed for pristine CF and CF-220 by temperature-programmed desorption technique with NH 3 as the probe molecule.As shown in Figure 5C, the CF-220 exhibits a peak of NH 3 desorption at 249 °C, while the original CF has no significant peak signal.This result indicates that alkaline NH 3 molecules can bind with acidic sites on the CF-220 surface in the form of covalent bonds.The desorption signal is detected when the adsorbed NH 3 molecules are heated to an energy greater than the activation energy of desorption.As we know, high-valent copper is a typical Lewis acid due to its electron-accepting property. 36herefore, it can be inferred that the Lewis acid sites generated by electrochemical redox activation enhance the adsorption of BA and thus accelerate the EBA process.Overall, this electrochemical redox activation strategy constructs a high-performance CF catalyst resulting in a threefold increase in hydrogen production from the BA-assisted water splitting system.From the theoretical and sustainability point of view, it offers an efficient, economical, and safe strategy for the production of high-purity hydrogen.

| CONCLUSION
In conclusion, we present an electrochemical redox activation strategy to increase the electrocatalytic activity of CF for EBA, achieving a threefold increase in hydrogen production rate for CF-220 after 220 cycles of activation.Systematic insight reveals that the surface reconstruction of CF is greatly dependent on the number of cycles for electrochemical redox activation and the applied potential range.The formed new species derived from reconstruction can enhance hydrophilicity and reduce the R ct of CF-220 surface thus strengthening the electrolyte-catalyst interactions and further promoting charge transport.The successive electrochemical redox activation creates Lewis acid sites that can enhance the adsorption of BA on the CF-220 surface thereby accelerating the reaction kinetics.In situ Raman result further confirms that the as-produced CuO, Cu(OH) 2 , and Cu(OH) 4  2− are possibly active species during the EBA process on CF-220.Resultantly, the required potential to achieve a current density of 10 mA/cm 2 for CF-220 is decreased by 150 mV compared to that of the pristine CF during EBA process.This study provides an effective strategy to improve the catalytic performance of CF, which will also boost the development of copperrelated energy and catalysis fields.

F
I G U R E 1 (A-D) Typical SEM and 2D/3D AFM images of activated CF at different cycles (activation conditions: 1 mol/L KOH electrolyte, the activation potentials: 1.3-1.8V RHE ), where the insets are electrode color evolutionary images of activated CF. 2D, two-dimensional; 3D, three-dimensional; AFM, atomic force microscope; CF, copper foam; RHE, reversible hydrogen electrode; SEM, scanning electron microscope.16.6%, 64.3%, and 19.1%, respectively.After activation for 30 cycles, the corresponding percentages of metallic Cu, Cu(I), and Cu(II) are 57.6%,16.9%, and 25.5%, respectively.As the number of cycles further increases, the proportion of Cu(II) gradually increases, while the proportion of metallic Cu and Cu(I) gradually decreases.When reaching 160-220 cycles, their compositions change slightly and remain basically constant, indicating that the electrochemical oxidation has reached equilibrium.The above results indicate that electrochemical redox activation leads to elevated valence of metallic copper and stable activation of CF-220 surface containing approximately 63.8% Cu(II), 17.1% Cu(I), and 19.1% metallic copper.The process of electrochemical redox activation involves continuous dissolution/reprecipitation of the copper species, leading to a variable composition.The valence states of the inner layer were probed by depth-profiling of Cu LMM AES.As shown in Figure 2F, THE proportions of metallic Cu, Cu(I), and Cu(II) change significantly with the sputtering time for CF-220.Similarly, the proportions of copper species were calculated by deconvolution and recorded in Figure 2G.During the first 30 s, the surface and near surface of CF-220 are still dominated by Cu(II).As the sputtering time increases to 90 s, the Cu(I) gradually dominates, which is similar to the original CF Cu species composition in Figure 2E.When the sputtering time reaches 240 s, the percentage of both Cu(II) and Cu(I) decreases, while the percentage of metallic Cu increases significantly, indicating that the bulk phase of CF has been reached.This result indicates that the activation causes the species of CF to exhibit a gradient distribution from the surface to the bulk, with the largest percentage of Cu(II) species on the surface.

F
I G U R E 2 (A) LSV curves of CF-220 and pristine CF under 1 mol/L KOH.(B) XRD patterns of CF at different cycles of redox activation.(C) Corresponding enlarged XRD patterns.(D,E) The Cu LMM AES of CF with different cycles of activation and the ratios of metallic Cu, Cu(I), and Cu(II) derived from the peak area calculations.(F) Depth Cu LMM AES of CF-220.(G) The composition ratio of metallic Cu, Cu(I), and Cu(II) metals as a function of sputtering time.(H) Raman spectra of pristine CF and CF-220.AES, auger electron spectra; CF, copper foam; LSV, linear sweep voltammetry; XRD, X-ray diffraction.
The TEM image of CF-220 and HR-TEM images of CF-220 in the regions highlighted by the wireframes.(B) Corresponding FFT pattern of CF-220.(C-E) The images of high-angle annular dark field-TEM and elemental mapping of CF-220.CF, copper foam; FFT, fast Fourier transform; HR-TEM, high-resolution transmission electron microscopy.

F
I G U R E 4 (A) Electrochemical behavior of BA oxidation at different concentrations over CF-220.(B) LSV curves of the pristine CF and CF-220 for OER and EBA in 1 mol/L KOH.(C) The first-order reaction kinetic model of BA electrooxidation was tested by pristine CF and CF-220 electrodes.(D) The amperometric i-t curve of CF-220 electrode in 1 mol/L KOH at 1.5 V. (E) During the electrooxidation of BA, the conversion of BA and the yield of its oxidation products change on the CF-220 electrode.(F) The current density and hydrogen yield comparison of the original CF and CF-220.BA, benzyl alcohol; CF, copper foam; EBA, electrocatalytic benzyl alcohol oxidation; LSV, linear sweep voltammetry; OER, oxygen evolution reaction.

F
I G U R E 5 (A) Configuration diagram of the in situ reaction cell for in situ Raman system.(B) In situ Raman spectra of during EBA process over CF-220.(C) NH 3 -TPD curves of pristine CF and CF-220.CF, copper foam; EBA, electrocatalytic benzyl alcohol oxidation.