A Highly Active and Robust Copper-Based Electrocatalyst toward

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A highly active and robust copper-based electrocatalyst toward hydrogen evolution reaction with low overpotential in neutral solution Jialei Du, Jianying Wang, Lvlv Ji, Xiaoxiang Xu, and Zuofeng Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09975 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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ACS Applied Materials & Interfaces

A highly active and robust copper-based electrocatalyst toward hydrogen evolution reaction with low overpotential in neutral solution Jialei Du, Jianying Wang, Lvlv Ji, Xiaoxiang Xu, and Zuofeng Chen* Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China

ABSTRACT Although significant progress has been made recently, copper-based materials had long been considered to be ineffective catalysts toward hydrogen evolution reaction (HER), in most cases, requiring high overpotentials more than 300 mV. We report here that a Cu(0)-based nanoparticle film electrodeposited in situ from a Cu(II) oxime complex can act as a highly active and robust HER electrocatalyst in neutral phosphate buffer solution. The as-prepared nanoparticle film is of poor crystallization, which incorporates significant amounts of oxime ligand residues and buffer anions PO43–. The proposed mechanism suggests that the Cu(0)-based nanoparticle film is activated with incorporated or adsorbed PO43– anions and the PO43– anionsanchored sites might serve as the actual catalytic active sites with efficient proton transport mediators. Catalysis occurs with a low onset overpotential (η) of 65 mV and a current density of 1 mA/cm2 can be achieved with η = 120 mV. The nanoparticle film shows an excellent catalytic durability with slightly rising current density during electrolysis, presumably due to further incorporation or adsorption of PO43– anions in the process. This electrocatalyst not only forms in

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situ from earth-abundant materials but also operates in neutral water with low overpotential and high stability. KEYWORDS hydrogen evolution reaction; heterogeneous electrocatalysis; Cu-based nanoparticle film; electrodeposition; neutral pH

INTRODUCTION Growing concerns over the limited reserves of fossil fuels and global climate change have driven extensive researches in clean and renewable energy sources technologies. Hydrogen has attracted considerable interest because of its cleanability and sustainability. The photocatalytic/electrocatalytic reduction of protons/water to hydrogen, i.e. hydrogen evolution reaction (HER), represents one of the long-studied and promising scenarios to store solar or electric energy in the form of molecular fuels.1-4 As a zero-emission fuel, the reverse hydrogen consumption reactions, when burned with oxygen or used in a fuel cell, not only regenerate energy but produce clean water as the sole product. To achieve high efficiencies of solar or electric energy conversion to hydrogen, HER electrocatalyts are required which should possess low overpotential, low Tafel slope and high durability in neutral aqueous solutions. For scalable hydrogen technologies, HER catalysts should be made from earth-abundant and nontoxic elements. Precious metals, such as Pt and its alloys5-10 display remarkable hydrogen evolution efficiency, but their low natural abundance and high cost have largely obstructed their widespread applications. Over the past decade, major efforts have been devoted to search for efficient and inexpensive HER electrocatalysts especially based on the first-row transition metals owing to their high availability and low toxicity.11 As an earth-abundant


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and biologically relevant metal, Cu was expected to be an important candidate for this application. In solar fuels production, copper metal or its complexes have been the focus of most CO2 reduction studies.12-16 Surprisingly, the application with these materials for the HER was much less explored.17-33 In comparison with numerous literature reports on the oxygen activation and reduction by Cu(II) complexes,34-36 the development of homogeneous catalysts for the HER is largely hampered by dissociation of Cu(II) complexes due to more negative potentials required for the latter. It is only recently that several molecular copper complexes have been shown to electrocatalyze the HER, most of which however requires large overpotentials more than 450 mV for functioning17-21. On the other hand, earlier experiments on well-defined single-crystal copper electrodes showed that the activities of various facets of Cu toward the HER are generally low with an onset overpotential of at least 300 mV.22-25 The differences in the rate constants between various facets of Cu are not large, of the order of a factor of five, with the Cu(111) surface bearing the highest activity toward the HER. These fundamental research findings seem to have discouraged researchers from further investigations of heterogeneous copper-based HER catalysts. Nevertheless, several examples of Cu(0)-based nanoparticle film or their mixture with Cu2O/CuO/Cu(OH)2 have been reported recently which showed enhanced catalytic activity over the bulk copper metal.26-31 Some Cu-containing bimetallic materials such as the highly crystalline layered Cu2MoS4 compound and Cu-CuxO-Pt nanoparticles were also developed for the HER, which exhibit superior catalytic performance over electrocatalysts with only a single component.32,33


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Herein, we report that Cu(0)-based nanoparticle film generated from in situ electrodeposition using a pentadentate oxime Cu(II) complex can serve as a highly efficient and robust HER catalyst in neutral phosphate buffer solution. The resulted film was characterized by various techniques in comparison with the copper film prepared from inorganic Cu(II) salt, Cu(ClO4)2. The proposed mechanism suggests that the PO43– anions incorporated in or adsorbed on the copper nanoparticles of poor crystallization might serve as efficient proton transport mediators to vicinity of the catalytic sites that activates the HER. The electrocatalytic tests confirm that this Cu(0)-based nanomaterial combining both high activity and stability is among the most promising Cu-based electrocatalysts in practical applications for hydrogen production.

RESULTS AND DISCUSSION Electrocatalysis. The oxime ligand and its mononuclear Cu(II) complex (Figure 1A) could be synthesized by procedures described in Scheme S1 (Supporting Information)37. The 1H NMR and

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C NMR spectra of the oxime ligand are shown in Figures S1 and S2

(Supporting Information), respectively. Ultraviolet-visible (UV-Vis) spectra shows that the synthetic Cu(II) oxime complex has a characteristic Cu(II) d-d absorption at 635 nm with a molar absorptivity of εmax ~ 100 M–1 cm–1 in 0.5 M phosphate buffer solution (PBS) at pH 7, Figure 1B. In comparison with that of Cu(II) in de-ionized (DI) water, the increase in light absorption by Cu(II) d-d transitions and the shift of λmax toward lower wavelength is consistent with coordination by oxime. In the absence of oxime, Cu(II) precipitates immediately as Cu3(PO4)2 (Ksp = 1.40 × 10–37)38 in 0.5 M PBS and the UV-


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Vis spectrum is relatively featureless with an elevated baseline due to the light scattering by the Cu3(PO4)2 or Cu(OH)2 precipitate.

Figure 1. (A) Schematic structure of the Cu(II) oxime complex. (B) UV-Vis spectra of Cu(II) ions in DI water (black), Cu(II) ions in 0.5 M PBS at pH 7 (blue), and the synthetic Cu(II) complex in 0.5 M PBS at pH 7 (red). The catalytic Cu(0)-based film can be prepared by cathodic deposition of the Cu(II) oxime complex. Figure 2A shows cyclic voltammogram (CV) of the Cu(II) oxime complex in 0.5 M PBS at pH 7 using a glassy carbon (GC, 0.071 cm2) as the working electrode. Prior to the measurement, the solution was degassed by bubbling Ar for 10 min to remove dissolved oxygen and a slow constant flow of Ar was maintained over the solution during the measurement. In the absence of Cu(II) and oxime, no apparent Faraday current was observed at potentials prior to ‒0.85 V vs. RHE (reversible hydrogen electrode). With adding 1 mM ligand, a small diffusional reductive wave appears beginning from ‒0.3 V, which is consistent with reduction of the C=N bond in the free oxime39,40. With adding 1 mM synthetic Cu(II) oxime complex, the wave for reduction of the free oxime disappears; instead, a dramatic current enhancement was observed above background beginning from –0.42 V, which is due to the catalytic water reduction by the


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electrodeposited copper (see below). During the reverse scan, there is clear evidence for the re-oxidation of the interfacial copper at 0.35 - 0.7 V (Figure 2A inset). To achieve continuous electrodeposition of copper on the electrode surface, constant potential electrolysis was conducted in the same solution at –0.8 V, Figure 2B. The background current at the applied potential was negligible with a current density of ~0.02 mA/cm2. By contrast, addition of 1 mM Cu(II) oxime complex resulted in cumulative charge and rising current density that reached 32 C and 56 mA/cm2 after 3 h, respectively. The feature with rising current density during electrolysis/electrodeposition is indicative of formation of a catalytically active surface-bound precipitate and water reduction by interfacial catalysis. Indeed, during the process there was visible evidence for formation of a precipitated Cu(0)-based film with a dull red color on the GC electrode surface, Figure S3A.

Figure 2. (A) CVs of a bare GC, oxime, and Cu(II) oxime complex in 0.5 M PBS at pH 7 at 100 mV/s. Inset: a magnified view of the CVs. (B) As in (A), electrolysis at –0.8 V without or with 1 mM Cu(II) oxime complex. Inset: the corresponding current density plot. The surface-bound precipitate is highly active toward the HER in blank 0.5 M PBS at pH 7, Figure 3A. Scanning the electrode negatively leads to a dramatic current


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enhancement above background with a very low onset overpotential (η) of 65 mV, which is among the lowest values reported for the known Cu-based HER catalysts in neutral aqueous solution17-31. In comparison with the initial CV with Cu(II) oxime complex precursor in Figure 2A, the shift of the catalytic onset from –0.42 V to –0.065 V is highly remarkable. It is consistent with the rising current density for film formation during electrolysis/electrodeposition. In a control experiment, a Cu(0) film was also prepared by electrodeposition of Cu(ClO4)2 in 0.5 M NaClO4 (pH 7) at 0.56 V vs. RHE. Note that electrodeposition by addition of Cu(ClO4)2 in 0.5 M PBS (pH 7) is not feasible because of formation of the Cu3(PO4)2 precipitate. Unlike the dull red film prepared with the Cu(II) oxime complex, the film electrodeposited from Cu(ClO4)2 is in a bright red color with metallic luster, Figure S3B. The CV curve of this Cu(0) film shows an onset overpotential of 350 mV toward the HER in 0.5 M PBS at pH 7, which is ~290 mV more sluggish than that of the film prepared with the Cu(II) oxime complex, Figure 3A. The sharp contrast in the catalytic activity is related with the different nature of the nanoparticle films which will be discussed below. The electrodeposition time has an important effect on the catalytic activity of the film prepared with the Cu(II) oxime complex precursor. The CVs in Figure 3B show that both the onset and peak potentials of the HER shift positively by prolonging the electrodeposition time. The maximum activity was achieved at the film formed after 3 h electrodeposition, beyond which there was a slight decrease in the catalytic activity (see SEM images below for further discussion). Therefore, the film formed by 3 h electrodeposition was chosen for the electrocatalytic study.


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To test the capability for sustained H2 production, constant potential electrolysis using the as-prepared Cu(0)-based film was first performed at –0.35 V in 0.5 M PBS at pH 7. The catalytic current was sustained and slightly increased reaching ~6.25 mA/cm2 after 8 h, Figure 3C. Effervescence from the coating was vigorous. Measurement of hydrogen evolved by gas chromatography gave ~60 µmol of H2 over an electrolysis period of 8 h with a Faradaic efficiency close to unit for H2 production. CVs of the catalytic film before and after 8 h electrolysis in the blank solution were compared. Figure S4 shows that the CV profile is generally unchanged after 8 h electrolysis; moreover, the HER onset shifts positively by approximately 10 mV. The positive shift of the catalytic potential indicates further activation of the catalytic film during electrolysis, which is consistent with the slight increase in the catalytic current density during electrolysis. By contrast, using the film prepared by electrodeposition of Cu(ClO4)2, electrolysis under the same experimental condition produces a current density that is approximately 25 times less than that of the film prepared with the Cu(II) oxime complex. In order to confirm the capability to catalyze H2 generation at even lower overpotentials, additional electrolysis experiments were carried out at −0.19 V, −0.14 V and −0.09 V, respectively. Sustained catalytic current was also obtained and H2 could be produced with current yield close to unit. Similarly, the catalytic current densities were also increased during electrolyses at these applied potentials, consistent with further activation of the electrocatalyst. These results indicate that the as-prepared Cu(0)-based film can really catalyze H2 production at very low overpotentials. The Tafel experiment was conducted at the Cu(0)-based film that experienced 8 h electrolysis activation at –0.35 V to achieve the maximum activity. The steady current


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density for H2 evolution (j), which was obtained from the linear sweep voltammogram (LSV) plot at a very slow scan rate (0.1 mV/s, Figure S5), was measured as a function of the overpotential (η), Figure 3D. A plot of log j vs. η produces a slope of ~63 mV/dec. An appreciable catalytic current of 0.1 mA cm2 was observed from the plot at η = 65 mV and a current density of 1 mA/cm2 requires η = 120 mV. Although such a catalytic activity is inferior to that of the Pt/C catalysts,5-10 the low overpotential and small Tafel slope make the present Cu(0)-based electrocatalyst comparable to other well-known earth-abundant metal-based materials for the HER in neutral aqueous media, Table S1.

Figure 3. (A) CVs of a bare GC, and Cu(0)-based films prepared from Cu(ClO4)2 or Cu(II) oxime complex; scan rate, 100 mV/s. (B) CVs of Cu(0)-based films formed by electrodeposition with the Cu(II) oxime complex for different time; scan rate, 100 mV/s. (C) Electrolyses at –0.35


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V, –0.19 V, –0.14 V and –0.09 V. (D) Tafel plot, η = E – (Vappl + iR) (where Vappl is the applied potential and corrected for the iR drop of the solution), of a Cu(0)-based film that experienced further activation by 8 h electrolysis at –0.35 V. Solution, 0.5 M PBS at pH 7. Characterization. Figure 4A displays the scanning electron microscopy (SEM) image of the Cu(0)-based film by 3 h electrodeposition of Cu(II) oxime complex. The electrodeposited material consists of irregular nanoparticles with diameters in the tens of nanometers with some coalesced into larger ones. Figure S6 shows additional SEM images of the film prepared by electrodeposition for 0.5 and 5 h, respectively. In general, an initial increase in the electrodeposition time builds up the film with growing nanoparticles; however, prolonging the electrodeposition time to 5 h results in a severe aggregation generating large nanospheres of 100 - 300 nm. This would decrease the specific surface area of the film leading to decreased catalytic current density as shown in Figure 3B. For comparison, Figure 4B displays the SEM image of the film prepared by electrodeposition of Cu(ClO4)2 at 0.56 V vs. RHE. The resulted film consists of concrete crystals, which ironically possesses a very poor catalytic activity toward the HER as demonstrated above. Electrodeposition of Cu(ClO4)2 was also conducted at a more negative potential of –0.35 V and –0.8, respectively, where in situ hydrogen evolution from the electrodeposited copper nanoparticles is getting involved. The SEM images in Figure S7A,C show generally less regular crystal forms generated at these potentials presumably because of the interference from the concomitant hydrogen evolution. Although the shape of the electrodeposited nanoparticles is somewhat dependent on the applied potentials, the corresponding CVs in Figure S7B,D show a consistently poor


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catalytic activity toward the HER by the films prepared from Cu(ClO4)2 at different potentials. Figures 4C-F show transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images of the nanoparticles scraped from the Cu(0)-based film prepared from Cu(II) oxime complex or Cu(ClO4)2. In general, the TEM images of low magnifications (Figure 4C and 4D) also show different crystallization between the two samples which are similar with those by SEM images (Figure 4A and 4B). The HRTEM image of the nanoparticles by Cu(II) oxime complex (Figure 4E) displays only barely discernible lattice fringes. By contrast, the HRTEM image of the nanoparticles by Cu(ClO4)2 (Figure 4F) demonstrates clear lattice fringes with d-spacing of 0.21 nm, which is in good agreement with that of the (111) plane of Cu.


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Figure 4. (A,B) SEM, (C,D) TEM and (E,F) HRTEM of Cu(0)-based nanoparticle films electrodeposited from the Cu(II) oxime complex (A,C,E) and Cu(ClO4)2 (B,D,F). The SEM and TEM morphologies (Figure 4) combining the visual observation (Figure S3) indicates quite different crystallization of the films prepared from the Cu(II) oxime complex and Cu(ClO4)2, which is further confirmed by the X-ray diffraction (XRD) pattern. Figure 5A shows that the nanoparticles film prepared from Cu(ClO4)2 displays


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strong diffraction peaks which can be ascribed to the Cu(0) phase with a cubic structure (PDF#04-0836). By contrast, the XRD pattern with the Cu(II) oxime complex displays only barely discernible diffraction peaks above the GC substrate at the close positions. The composition of the as-prepared Cu(0)-based film was further investigated by both the energy-dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS). In Figure 5B, the EDX data suggested that the deposited solid from Cu(II) oxime complex is a mixture containing Cu/C/O/P/N with Cu:P:N ~ 40:2.5:1. The non-metal elements were either from the oxime ligand or from the electrolyte buffer. These nonmetal elements cannot be removed even after carefully washing the sample with water for several times, indicating that they are trapped inside or encapsulated by the nanoparticles. By contrast, the dominant element in the deposited solid prepared from Cu(ClO4)2 is Cu with negligible non-metal components. The absence of detectable “impurities” is consistent with the ideal crystal morphology of this sample. Figures S8 shows the XPS survey spectra of both samples prepared from the Cu(II) oxime complex and Cu(ClO4)2. All of the XPS peaks are correspondingly assignable based on the elements detected by the EDX technique. In both samples, the presence of metallic Cu is confirmed by the binding energies of Cu 2p3/2 at ~932.6 eV and 2p1/2 at ~952.3 eV, Figure 5C and 5D.41 The shoulder peaks at slightly higher binding energy with the small rounded peaks at 943.6 and 962.4 eV can be attributed to the presence of divalent Cu specie.41,42 While the surface Cu oxide is inevitably present in both samples, the much larger divalent Cu signal in the film prepared from the Cu(II) oxime complex indicates that the Cu nanoparticles of smaller size and poor crystallization are more ready to be oxidized. In addition, the divalent Cu signal could also partially arise from Cu3(PO4)2


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because of the observation of significant P signal (i.e. PO43–) in this sample. Figure S9 provides additional high resolution XPS spectrum of the sample prepared from the Cu(II) oxime complex in the region of Pt(0) 4f7/2 and 4f5/2,which indicates no involved Pt impurity and thus eliminates the influence of Pt for HER performance.

Figure 5. (A) XRD, (B) EDX, and (C,D) high resolution Cu 2p XPS spectra of Cu(0)-based film electrodeposited from the Cu(II) oxime complex and Cu(ClO4)2. Discussion. The SEM, TEM, XRD, EDX and XPS results in the above section illustrate that electrodeposition in 0.5 M PBS at pH 7 containing the Cu(II) oxime complex precursor results in formation of highly active Cu(0)-based nanoparticles film of poor crystallization, which incorporates significant amounts of oxime ligand residues and electrolyte buffer anions PO43–. In contrast to the well-crystalized nanoparticles film electrodeposited from Cu(ClO4)2, the poor crystallization could be mainly attributable to


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the chelation effect of the oxime ligand that inhibits the growth of the copper atoms into an ideal crystal form presumably by N-atom adsorption onto the copper atoms. The crystal growth process may be further disturbed by the concomitant hydrogen evolution during electrodeposition at the applied potential. The dissociation of Cu(II) oxime complexes by reduction generates Cu(0) atoms that are exposed in phosphate media. The chemical adsorption of phosphate anions on the crystalline Cu electrode surfaces has been known.43 In comparison with the electrodeposition of Cu(ClO4)2 in NaClO4 solution, the growth of the copper nanoparticles with the chemically adsorbed phosphate anions would finally encapsulate a significant amount of phosphate anions inside the nanoparticles, which would further degrade the crystallization of the nanoparticles. The different catalytic performance between the two Cu(0)-based films prepared from Cu(II) oxime complex and Cu(ClO4)2 may be directly related with their different composition and crystallization. In addition to the incorporated phosphate anions, Cu(0)based composite prepared from the Cu(II) oxime complex would also be more favorable for the adsorption of phosphate anions because of its larger surface area (rougher morphology from SEM images). In addition, the presence of higher content of surface copper oxide could presumably act as anchoring sites for the chemical adsorption of phosphate anions, for example, producing CuII-O-PO3 species. The earlier study27 on the Cu/Cu2O catalysis of HER has suggested that the chemically adsorbed phosphate anions could act as proton transport mediators and contribute to efficiency of transferring protons to vicinity of the catalytic sites on the electrode surface, leading to enhanced catalytic activity of the Cu/Cu2O electrode.


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In our case, we speculate that the phosphate anions incorporated in the Cu(0)-based composite or chemically adsorbed on its surface may be also involved in the actual catalytically active sites for the HER which contributes to the unprecedented activities of our copper electrocatalyst. The XPS measurements on the electrocatalyst before and after 8 h electrolysis show that the content of P increases by approximately 30%, which presumably accounts for the catalytic current increase during electrolysis in Figure 3C; it in turn also further confirms the important role of phosphate anions. Additional control experiment was carried out with 0.5 M NaHCO3 (pH 7) as electrolyte buffers for electrodeposition of Cu(II) oxime complexes. In contrast to 0.5 M PBS (pH 7), Cu(II) ions are soluble in 0.5 M NaHCO3 (pH 7) because of the coordination effect of HCO3–/CO32– anions.38 Therefore, unlike PO43–, incorporation of HCO3–/CO32– anions by Cu(II) precipitation followed by dissociation of Cu(II) oxime complexes might be less favorable. In addition, the chemical adsorption of HCO3–/CO32– anions on the surface Cu oxide may occur accompanied with coordination dissolution of the surface Cu oxide by HCO3–/CO32– anions. In consistency with these unfavorable effects of HCO3– /CO32– anions, Figure S10 shows that the film prepared in 0.5 M NaHCO3 (pH 7) has an onset overpotential more than 280 mV, which is much higher than that prepared in 0.5 M PBS (pH 7). This result sheds light on the choice of electrolyte buffer in activating the Cu(0)-based nanoparticle film.

CONCLUSIONS In summary, the lack of highly active, robust and low-cost hydrogen evolution electrocatalysts continues to be one of the bottlenecks to realize large-scale water


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splitting. The results reported here highlight the use of a Cu(0)-based electrocatalyst for hydrogen evolution which could be prepared facilely by in-situ electrodeposition from a Cu(II) oxime complex precursor. The dissociation induced Cu(II) reduction makes the nanoparticles poorly crystallized, which incorporates significant amounts of oxime ligand residues and buffer anions PO43–. The incorporated and adsorbed PO43– anions may serve as proton transport mediators that activates the HER reaction. The catalyst reported here has many remarkable features, including (i) its in-situ formation from earth-abundant metalbased precursor, (ii) incorporated or chemically adsorbed buffer anions as efficient proton transport mediators, (iii) the generation of H2 at low overpotential at neutral pH under ambient conditions, and (iv) further activation during electrolysis to maintain high catalytic stability. These appealing features suggest that this nanomaterial may serve as a promising electrocatalyst in practical applications for hydrogen production.

ASSOCIATED CONTENT Supporting Information Experimental section, 1H NMR and

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C NMR spectra of the oxime ligand, photographs, survey

XPS spectra and high resolution Pt 4f XPS spectra of the Cu(0) films, SEM images and CVs of the Cu(0) films prepared under different experimental conditions and after long-term electrolysis, LSV of the Cu(0) film at a scan rate of 0.1 mV/s, a table comparing our Cu(0)based electrocatalyst with other earth-abundant metal-based materials for the HER in neutral aqueous media. AUTHOR INFORMATION Corresponding Author


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[email protected] (Z.-F. C.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Z.-F.C. thanks The National Natural Science Foundation of China (21405114, 21573160, 21401142), The Science & Technology Commission of Shanghai Municipality (14DZ2261100), and The Recruitment Program of Global Youth Experts by China for support.

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A Highly Active and Robust Copper-Based Electrocatalyst toward

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