Metal–Organic Framework Nanosheet Electrocatalysts for Efficient H2

Jul 10, 2018 - Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of ... Engineering, East China Normal University, Shanghai 20...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 25422−25428

Metal−Organic Framework Nanosheet Electrocatalysts for Efficient H2 Production from Methanol Solution: Methanol-Assisted Water Splitting or Methanol Reforming? Xinfa Wei,† Shun Wang,† Zile Hua,‡ Lisong Chen,*,† and Jianlin Shi*,†,‡ †

ACS Appl. Mater. Interfaces 2018.10:25422-25428. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 08/25/18. For personal use only.

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, P. R. China ‡ State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China S Supporting Information *

ABSTRACT: Hydrogen (H2) is presently one of the most promising clean and renewable energy sources, but the conventional hydrogen production by electrochemical water-splitting, though of great potential and extensively studied, is seriously obstructed especially by the anodic oxygen evolution reaction because of its sluggish kinetics. Herein, we report the efficient hydrogen production from methanol solution using facile-synthesized ultrathin 2D bi-metal−organic framework nanosheets (UMOFNs) as a precious metal-free anodic catalyst. The prepared UMOFNs showed a much lowered anodic potential of 1.365 (V vs reversible hydrogen electrode) at 10 mA cm−2, which was markedly 232 mV lower than that in conventional water splitting, and moreover, the average turnover frequency reached 19.62 s−1. Benefiting from nearly 100% Faraday efficiency of H2 production on the counter graphite carbon electrodes without additional electrocatalysts, high-purity hydrogen was produced with enhanced efficiency. More importantly, the anodic electro-reaction mechanism has been evidenced experimentally: the electrocatalytic hydrogen production from the methanol solution is a methanol-assisted water splitting, rather than a methanol-reforming process as claimed in a number of literature studies, in which methanol is oxidized as a sacrificing agent in place of water oxidization in pure water. KEYWORDS: hydrogen production, electrocatalysts, methanol-assisted water splitting, metal−organic framework nanosheets, isotope labeling

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INTRODUCTION The increasing energy shortage and environment pollution are becoming more and more serious, which have been the focus of global attention. Searching for clean and renewable energy resources is one of the top priorities of sustainable societies. Hydrogen is considered one of the most promising alternatives for its excellent intrinsic properties, such as high calorific value, pollution-free, and so on.1−3 Electrochemical water splitting is a striking H2 production approach in recent years.4−8 However, the electrochemical water splitting process suffers severely from the high overpotential, especially by the anodic oxygen evolution reaction (OER), which is the ratedetermining step of the overall water splitting process.9−11 Although many kinds of electrocatalysts have been developed,12−15 their performance are still far from being satisfactory, even in the presence of noble metal catalysts (IrO2, RuO2, etc.).16,17 O2 produced at the anode is of less value as it can be obtained facilely from air; moreover, there is a great hidden danger of explosion because the O2 produced at the anode will be unavoidably mixed with the H2 produced at © 2018 American Chemical Society

the cathode. Efforts have been made recently to integrate the small organic molecules upgrading reactions with decoupled hydrogen evolution reaction (HER) in water splitting. Various small organic molecules such as ethanol,18−20 benzyl alcohol,21 5-hydroxymethylfurfural,22−24 and aloe extract25 have been used for H2 production by oxidation. These reactions take place as below Anode: RCH 2−OH + 4OH− → R−COOH + H 2O + 4e−

(1)

RCHO + 2OH− → R−COOH + H 2O + 2e−

(2)

or

Cathode: 4e− + 4H 2O(l) → 4OH−(aq) + 2H 2(g)

(3)

Received: May 4, 2018 Accepted: July 10, 2018 Published: July 10, 2018 25422

DOI: 10.1021/acsami.8b06948 ACS Appl. Mater. Interfaces 2018, 10, 25422−25428

Research Article

ACS Applied Materials & Interfaces The hydroxyl or aldehyde groups of small organic molecules lose electrons and convent to carboxyl groups at the anode without O2 production, while the donated electrons react with water and produce H2 at the cathode.26,27 These small organic molecules oxidation reactions not only show strongly reduced anodic potential but also eliminate the danger of possible explosion; therefore, they are undoubtedly promising for practical applications and interesting for researchers. There are at least three issues which should be investigated before its large-scale applications. First, most of the electrocatalysts reported for this kind of reactions are precious metal-based materials. The high cost and scarcity of precious metals will prevent them from practical application. Second, the searching for suitable small organic molecules of high hydrogen content, nontoxicity, and low cost is still under progress. Last but not the least, the hydrogen production mechanism is still controversial to date. Direct evidences are needed on whether the electrochemical hydrogen production process is a small organic molecule-assisted water splitting or small organic molecules reforming. As a new type of porous and high surface area material, MOF has shown great potential in the field of catalysis.28,29 All metal ions or metal clusters in MOFs are monodisperse, so that almost all metallic catalytically active sites can be exposed and utilized.30 As the vast majority of MOFs are nonconductive,30−33 MOFs have been rarely applied in the field of electrocatalysis. The development of two-dimensional (2D) materials has evoked the interest of scholars to be engaged in electrochemical research in layered MOF materials because of its much improved conductivities of both electron and ions.34 In addition, methanol, as one of the basic organic materials, has huge production capacity, rather inexpensive, and importantly, has the largest hydrogen content. Therefore, methanol is of great potential for hydrogen electro-production. Thus, ultrathin 2D bi-metal−organic framework nanosheets (UMOFNs) were synthesized and used for the electrocatalytic hydrogen production in methanol aqueous. Here, we report the electrocatalytic H2 evolution from methanol aqueous under alkaline conditions by using ultrathin MOF nanosheets as a nonnoble anodic catalyst. Accompanying the methanol cracking at the anode with a relatively low potential, H2 is produced on the cathode graphite electrode (Figure 1). The only gaseous product of this reaction is H2 without O2 generation at the anode, which eliminates the possible explosion risk in conventional water splitting electrolyzers and makes the following gas purification totally unnecessary. What excited us the most is that the onset potential (10 mV cm−2) of anode reaction is more than 200 mV lower than that for electrolytic water splitting. Therefore, it is expected that the present strategy will provide an innovative and highly promising way for large-quantity production of high purity H2. Finally, the electrocatalytic hydrogen evolution mechanism in the aqueous solution is probed to be methanolassisted water splitting rather than methanol-reforming, which is soundly demonstrated by using mass spectroscopy.

Figure 1. Schematic illustration of electrocatalytic H2 production from methanol solution.

According to the structure data reported, we can readily define the crystalline structure of CoCu-UMOFNs. The pseudo octahedral, in which every metal atom is coordinated by six O atoms, prefers to grow along the (200) crystallographic plane to form a 2D structure as shown in Figure 2b. The 2D ultrathin morphology of UMOFNs can be identified in the transmission electron microscopic (TEM) image (Figure 2c), which shows the same morphology as Cu-UMOFNs (Figure S1). The scanning electron microscopy (SEM) image of CuCoUMOFNs (Figure 2d) after drying shows a clearly layered structure. From the atomic force microscopic (AFM) image (Figure 2e), it can be seen that the thickness of single flake is approximately 3 nm. Because of the aggregation of CoCuUMOFNs, slit-like mesopores can be found from the nitrogen sorption isotherms (Figure S2). The 2D layered structure tends to form incomplete coordination between benzenedicarboxylic acid (BDC) and the metal atoms with large numbers of coordination-unsaturated metal sites being exposed on the surfaces, which will play an important role in catalytic reactions.34 Subsequently, energy-dispersive spectroscopy (EDS) (Figure S3) and X-ray photoelectron spectroscopy (XPS) (Figure S4) were employed to analyze the surface composition of samples. The results of EDS mapping (Figure 2f) show the uniform elemental distributions of C, O, Cu, and Co elements in CuCo-UMOFNs without other impurities. All the results mentioned above are consistent with the compositional data of 16.9% Co and 15.7% Cu obtained using an inductively coupled plasma spectrometer. To further explore the material’s inherent fine structure, XPS and X-ray absorption fine structure (XAFS) spectroscopic analyses were carried out, which will be discussed below. Evaluation of Electrochemical Activity. The electrocatalytic properties of the materials for hydrogen evolution from methanol solution (HEM) were evaluated in a threeelectrode system (see methods of details in the Experimental Section). Compared to the electrolysis of water for hydrogen production, the anodic potential in methanol solution on CoCu-UMOFNs is significantly reduced by 232 mV (Figure 3a) at the same current density of 10 mA cm−2. The anodic potentials in methanol solution on the current densities of 20, 50, and 100 mA cm−2 are also much lower than those of the OER process as shown in Figure 3b. Besides, Co-UMOFNs

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RESULTS AND DISCUSSION Materials Characterization. The structural and morphological characterization results of UMOFNs are shown in Figure 2. The powder X-ray diffraction (PXRD) pattern of synthesized UMOFNs is shown in Figure 2a, which suggests that the as-prepared CoCu-UMOFNs are isostructural to the Ni-based MOFs reported by Mesbah et al. (no. 985792).35 25423

DOI: 10.1021/acsami.8b06948 ACS Appl. Mater. Interfaces 2018, 10, 25422−25428

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Figure 2. Structural and morphological characterizations of CoCu-UMOFNs. (a) PXRD pattern of CoCu-UMOFNs. (b) Crystal structure schematics of CuCo-UMOFNs. (c) TEM image of CoCu-UMOFNs. (d) SEM image of CoCu-UMOFNs after drying. (e) AFM image of CoCuUMOFNs showing the measured thicknesses of individual flakes. (f) SEM−EDS mapping of CoCu-UMOFNs.

Figure 3. Electrochemical performances of samples. (a) LSV curves for the methanol anodic oxidation catalyzed by CoCu-UMOFNs at a scan rate of 5 mV s−1 in 1 M KOH with and without 3 M methanol (Me−OH). (b) Comparison of the anodic potentials to achieve varied current densities by CoCu-UMOFNs in 1 M KOH with and without 3 M Me−OH. (c) Comparison of experimental H2 production volume with theoretical calculation, showing a near 100% Faradaic efficiency for the H2 evolution on graphite carbon electrodes in 1 M KOH with 3 M Me−OH. (d) LSV curves and (e) Tafel plots derived from corresponding LSV plots, (f) ECSAs of CoCu-UMOFNs, Cu-UMOFNs, and Co-UMOFNs in N2-saturated 1 M KOH solution of 3 M Me−OH. (g) LSV curves in N2-saturated 1 M KOH solution of 3 M Me−OH at a scan rate of 5 mV s−1, at 1000th and 2000th cycles. (h) Chronoamperometry responses at a constant potential of 1.383 V for 12 h. The inset shows the H2 generation on graphite carbon electrodes with no bubbles generated at the anode. All polarization curves were used without IR correction.

and Cu-UMOFNs (Figure S5) also exhibit similar results. Chronoamperometry experiments were carried out at 1.421 V in methanol solution to measure the Faradaic efficiency for hydrogen evolution. As shown in Figure 3c, the hydrogen production amount is very close to the calculated theoretical

result, which means that the Faradaic efficiency for the hydrogen evolution in methanol solution is up to 100%. Therefore, HEM is a cost-effective strategy for the electrochemical production of hydrogen, and CuCo-UMOFNs show high performance for HEM. As shown in Figure 3d, the CuCo25424

DOI: 10.1021/acsami.8b06948 ACS Appl. Mater. Interfaces 2018, 10, 25422−25428

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ACS Applied Materials & Interfaces UMOFNs exhibit an onset potential (10 mA cm−2) of 1.365 V versus reversible hydrogen electrode (RHE), which is much lower than those of Cu-UMOFNs (1.495 V vs RHE) and CoUMOFNs (1.427 V vs RHE); so are the potentials at 20, 30, and 50 mA cm−2, as listed in Table S1. In all, bimetallic organic framework compounds show significantly lower anodic potential compared to those of two single-metal organic frameworks, as will be discussed below. The Tafel plots obtained from the corresponding linear sweep voltammetry (LSV) curves were derived to evaluate catalytic kinetics of MOFs in N2-saturated 1 M KOH solution with 3 M methanol. Owing to the isolated distributions of Co and Cu atoms in CoCu-UMOFNs, the Tafel slope of CuCoUMOFNs is as low as 46 mV dec−1 (Figure 3e), which is much smaller than those of Cu-UMOFNs (87 mV dec−1) and CoUMOFNs (98 mV dec−1), indicating much quicker HEM kinetics. Also, the turnover frequency (TOF) was measured for the comparison of the intrinsic activity at a current density of 10 mA cm−2 according to eq 4 TOF = J /(4 × F × m /M )

Figure 4. (a) High-resolution Co 2p3/2 XPS spectra and (b) Cu 2p3/2 XPS spectra of CoCu-UMOFNs and Co-UMOFNs. (c) Co K-edge EXAFS spectra in R-space of CoCu-UMOFNs and Co-UMOFNs and (d) Cu K-edge EXAFS spectra in R-space of CoCu-UMOFNs and Cu-UMOFNs.

(4)

where J is the current density (mA cm−2) at a given overpotential (10 mA cm−2), F is the Faraday constant (96 485 C mol−1), m is the mass loading of the catalyst material (mg cm−2), and M is the molecular weight of the catalyst material unified with one active center per formula unit. The TOF value of CuCo-UMOFNs is as high as 19.62 s−1, which is much larger than those of Co-UMOFNs and Co-UMOFNs. To gain more insight into the electrocatalytic activity of the HEM electrocatalysis, we further measured the electrochemical surface area (ECSA) of CuCo-UMOFNs. It can be noticed that the CuCo-UMOFNs show a larger ECSA than CuUMOFNs (Figures 3f and S6), though a little lower than CoUMOFNs. Co is proposed to play an important role in enhancing the ECSA. Cyclic voltammetries (CVs) for 2000 times and chronoamperometry tests were performed to measure the stability of CoCu-UMOFNs, whose LSV curve (Figure 3g) and morphology (Figure S7) remained almost unchanged after 2000 CVs. As shown in the inset in Figure 3h, H2 bubbles can be clearly seen on the surface of graphite carbon electrodes during the chronoamperometry test at the applied potential of 1.38 V. No significant current density decrease was observed during the 12 h chronoamperometry experiment period (Figure 3h). Both CV and chronoamperometry tests confirm the excellent stability of CoCu-UMOFNs toward HEM. To get more information of the interaction between Cu and Co of the catalysts, the XPS spectrum of obtained samples were obtained. The shifts of binding energy levels of Cu and Co species provide a clear evidence of the interaction between the Co2+ and Cu2+ ions (Figure 4a,b). After hybridization, the binding energy of Co2+ 2p3/2 become higher, from 781.9 eV for Co-UMOFNs to 782.2 eV for CoCu-UMOFNs, and that of Cu2+ 2p3/2 peaks become lower, from 935.8 eV for CuUMOFNs to 935.5 eV for CoCu-UMOFNs, which implies the electron transfer from Co2+ to Cu2+. The same result can be obtained from the ex situ comparative X-ray absorption nearedge spectroscopy data of CoCu-UMOFNs, Co-UMOFNs, and Cu-UMOFNs (Figure S8). The interactions between two kinds of metal ions in the ordered structure further contribute to the HEM performance. To further reveal the coordination nature of surface metal atoms in CoCu-UMOFNs, ex situ Xray absorption spectroscopy was employed. As shown in the ex

situ EXAFS data in R-space (Figure 4c,d), all metal atoms are in a single isolated state without metal−metal bonding. Product Quantification. It has been rather debatable in recent years whether electrochemical hydrogen production from small organic molecule solutions, such as methanol and ethanol solutions, is a small organic molecule-reforming process or small organic molecule-assisted water splitting. However, direct experimental evidence is still absent. To clarify this, the types of produced hydrogen isotopes from deuterated or nondeuterated aqueous solutions of methanol were verified by using mass spectroscopy. Chronoamperometry was employed at a potential of 1.5 V versus RHE in sealed electrolytic pools of solution 0 (H2O solution of 1 M KOH and 3 M Me−OH), solution 1 [1 M KOH and 3 M CH3OH dissolved in deuterated water (D2O)], and solution 2 [1 M KOH and 3 M methanol-4D (CD3OD) dissolved in H2O], and the electrocatalytically produced gases were identified by a mass spectrometer. As can be clearly seen in Figure 5a−d, D2 and HD can only be obtained from solution 1 containing 3 M CH3OH dissolved in deuterated water (D2O), while no D2 and HD were produced from solution 2 containing methanol-4D (D3COD) in nondeuterated H2O. The process of H2 generation at the cathode is illustrated in Figure 5e which is similar to that of water splitting, following the Tafel−Heyrovsky mechanism in alkaline conditions (formula 5−7). H 2O(ads) + e− → [H](ads) + OH−(aq)

(5)

[H](ads) + [H](ads) → H 2(ads)

(6)

H 2(ads) → H 2(g)

(7)

HER is almost impossible to proceed at the cathode under alkaline conditions without the electron supply from the anode where methanol or water is oxidized. The participation of methanol electro-oxidation at the anode significantly lowers the anodic potential and promotes the chemical to electric energy conversion, which drives the sluggish HER process to take place at enhanced rates. Therefore, the mechanism analysis of methanol anodic oxidation is particularly beneficial. 25425

DOI: 10.1021/acsami.8b06948 ACS Appl. Mater. Interfaces 2018, 10, 25422−25428

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ACS Applied Materials & Interfaces

Figure 5. Mass spectroscopic analyses of the produced gases from (a) solution 0 (H2O solution of 1 M KOH and 3 M Me−OH), (b) solution 1 (1 M KOH and 3 M Me−OH dissolved in D2O), (c) solution 2 (1 M KOH and 3 M D3COD dissolved in H2O) in the first hour, and (d) solution 2 in 12 h of chronoamperometry measurement at 1.4 V vs RHE. (e) Illustration of the reaction mechanism at the cathode when deuterated water is employed. (f) Possible reaction mechanism of Me−OH at the anode, which contains four sub-steps of dehydrogenation and the oxidation of CO.

process: methanol is oxidized at the anode, which is kinetically faster than the water oxidation (OER) at the anode in traditional water splitting electrolysers, which greatly promote the water reduction (HER) at the cathode for hydrogen production. It is believed that the demonstrated strategy using CoCu-UMOFNs as the nonnoble metal catalysts may provide an efficient way for the scalable hydrogen production from methanol solution.

The possible reactions on the anode catalyst surface contain the breakings of C−H, C−O, and O−H bonds. However, the breaking of the C−O bond is extremely difficult because of its high bond strength, and O−H is more likely to break under alkaline conditions. The anodic oxidation steps of methanol are schematically illustrated in Figure 5f. First, methanol molecules are adsorbed on the surface of the catalyst with the immediate breaking of O−H and C−H bonds, generating formaldehyde and hydrogen atoms. Then, formaldehyde is further dehydrogenated stepwise into carbon monoxide (CO). There are four hydrogen atoms [H] dehydrogenated from Me−OH throughout the whole processes. Finally, the CO is oxidized to carbon dioxide at a lowered potential, thanks to the synergistic effect between Cu and Co atoms in CoCu-UMOFNs. The oxidation process supplies electrons to the cathode for HER. In all, the six substeps of the electrochemical reactions that occur at the anode can be written as: Dehydrogenation reactions of methanol CH3OH(ads) → CH3O(ads) + [H](ads)

(8)

CH3O(ads) → H 2CO(ads) + [H](ads)

(9)

H 2CO(ads) → HCO(ads) + [H](ads)

(10)

HCO(ads) → CO(ads) + [H](ads)

(11)

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CONCLUSIONS In summary, we have demonstrated a novel strategy of electrocatalytic pure H2 production through a methanolassisted water splitting approach, by using CoCu-UMOFNs as a nonnoble metal anodic catalyst for methanol oxidation and graphite cathodic electrode for ultrapure H2 evolution under alkaline conditions. The obtained samples show much lowered anodic potential (1.365 V vs RHE at 10 mA cm−2) in methanol solution, which is 232 mV lower than that of OER at 10 mA cm−2 with a nearly 100% Faradaic efficiency. Methanol oxidation on the anode necessitates much reduced potential, and therefore eliminates potential danger of explosion by substituting the oxygen evolution with methanol oxidation. More significantly, the electrocatalytic hydrogen evolution in the aqueous solution has been probed to be a methanolassisted water splitting process rather than a methanolreforming process, as clearly evidenced by mass spectroscopy. The present study may provide an effective approach for the electrocatalytic H2 production from Me−OH aqueous solution for commercially achievable hydrogen energy utilization.

CO and hydrogen oxidation reactions for electron donations from the anode to the cathode CO(ads) + 2OH− → CO2 + H 2O + 2e−

(12)

4[H](ads) + 4OH− → 4H 2O + 4e−

(13)

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EXPERIMENTAL SECTION

Chemicals. CuCl2·2H2O (99.99%, AR, grade), CoCl2·6H2O (99.99% AR, grade), potassium hydroxide, BDC, and N,Ndimethylformamide (DMF) were bought from Macklin. All chemicals were used as received without any further purification.

Taking together the reactions at the anode and the cathode, the hydrogen production in methanol solution is an methanolassisted water splitting rather than methanol-reforming 25426

DOI: 10.1021/acsami.8b06948 ACS Appl. Mater. Interfaces 2018, 10, 25422−25428

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ACS Applied Materials & Interfaces Synthesis of Materials. First, DMF (32 mL), ethanol (2 mL), and water (2 mL) were mixed in a 100 mL silk mouth reagent bottles. Next, 0.125 g of BDC was dissolved into the mixed solution under ultrasonication. Subsequently, 0.089 g of CoCl2·6H2O and 0.064 g of CuCl2·2H2O were added. After Cu2+ and Co2+ salts were dissolved, 0.8 mL of tetraethylammonium was added into the solution. Then, the solution was stirred for 5 min to obtain a uniform colloidal suspension. Afterward, the colloidal solution was continuously ultrasonicated for 8 h (40 kHz). Finally, the products were obtained via centrifugation, washed with ethanol 3 times, and dried at room temperature. Preparation of Working Electrodes. The catalyst was dropped on the glass carbon electrode: 5 mg UMOFNs was dispersed in mixed solution (2 mL) of water and ethanol (Vwater/Vethanol = 1/1). Then, 50 μL 5 wt % Nafion solution was added. The mixed solution was followed by ultrasonication for 30 min to obtain a homogeneous catalyst suspension. Catalyst ink (5 μL) was transferred onto a glass carbon electrode (diameter = 3 mm) yielding a mass loading of 0.18 mg cm−2. It was dried at ambient temperature before electrochemical measurements. Catalysts were coated on copper foam: the catalyst ink was prepared using mixture of 1 mL of water, 1 mL of ethanol, 50 μL of 5 wt % Nafion solution, and 5 mg material followed by ultrasonication for 30 min. The homogeneous catalyst ink (72 μL) was transferred onto a piece of clean Cu foam (surface area 1 cm2), yielding a mass loading of 0.18 mg cm−2. It was dried at ambient temperature before electrochemical measurements. Electrochemical Measurements. All measurements were performed in an electrochemical glass cell with a CHI 760E electrochemical workstation at room temperature in the N2-saturated solution (1 M KOH) for OER and solution (1 M KOH + 3 M methanol) for HEM. LSVs were measured at a scan rate of 5 mV s−1. All potentials were referenced to an Ag/AgCl (sat. KCl) reference electrode, and a carbon rod was used as the counter electrode in all measurements. All potentials were calibrated with respect to RHE. Evs RHE = Evs Ag/AgCl − ERHE vs Ag/AgCl. ERHE vs Ag/AgCl was measured in an electrochemical glass cell at room temperature in the H2-saturated solution. The measurement uses a three-electrode system where both the working electrode and the counter electrode are platinum electrodes and the reference electrode is the Ag/AgCl (sat. KCl) electrode. The measurement results of ERHE vs Ag/AgCl are shown in Figure S10. All polarization curves in this manuscript were used without IR correction. Estimation of Effective Electrochemical Surface Area. To evaluate the ECSA, CV could be carried out to probe the electrochemical double layer capacitance (Cdl) of various samples at the non-Faradaic region, identified from CV in quiescent solution. Measurement of the Faraday Efficiency. A piece of CuCoUMOFNs on Cu foam (surface area 0.5 cm2) was used as the working electrode. The hydrogen concentrations were measured using gas chromatography (Ramiin GC2060) equipped with a packed column and a thermal conductivity detector. In addition, the amount of hydrogen was quantified by the external standard method. The Faraday efficiency is nH2 (experimental)/nH2 (theoretical) × 100%. The standard curve of H2 is shown in Figure S9. To calculate the Faradaic efficiencies of cathodic reduction for H2 production, the HEM was carried out at a potential 1.473 V versus RHE. After 19.2 C charge was calculated by electrochemical workstation, the 2.445 mL H2 was generated on the cathode, which was measured using gas chromatography, which approaches the theoretical value of 2.447 mL. Source of the Hydrogen. To verify the source of hydrogen, deuterated water (D2O), methanol-4D (D3COD), and methanol-D (H3COD) were employed to participate in the reaction, according to the mass spectroscopic analyses of the produced gases from electrolytes with different deuterium reagent. Materials Characterization. PXRD data were acquired on a Rigaku D/Max 2550 diffractometer with Cu Kα radiation (l = 1.5418 Å). SEM measurements were carried out on a Hitachi S-4800 scanning electron microscope at an accelerating voltage of 3 kV. TEM images are acquired on a JEM-2100F equipped with operated at 200

kV. Nitrogen isotherms corresponding to the Barrett−Joyner− Halenda pore distribution and the Brunauer−Emmett−Teller (BET) surface area were collected on QUADRASORB SI at 77 K after degassing the samples at 150 °C for 5 h. Co and Cu K-edge XAFS spectra of the CoCu-UMOFNs Co-UMOFNs and CuUMOFNs were recorded in the transmission mode at the BL14W1 beam line of the Shanghai Synchrotron Radiation Facility (SSRF), China. The corresponding data analysis was carried out according to the standard procedures. XPS experiments were carried out on a Thermo Scientific ESCALAB 250 spectrometer with a monochromated Al Kα source (hν = 1486.6 eV). Accurate binding energies (±0.1 eV) are determined with respect to the position of the adventitious C 1s peak at 284.8 eV. The deconvolution of the Co and Cu was performed through a software XPSPEAK version 4.1, and the energy scales of valance band XPS were aligned by using the Fermi level of the XPS instrument. The quantitative analysis of element contents in the CoCu-MOFNs sample was performed with Agilent 700 Series inductively coupled plasma optical emission spectrometry. The deuterated hydrogen was detected by Proline, America.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b06948.

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TEM, BET, EDX, and XPS of three different materials; polarization curves of contrasts in two electrolytes; CV results of ECSA measurement; data of Co K-edge Cu Kedge; reference electrode calibration curves in two electrolytes; standard curve of H2 production measured by gas chromatography; and anode voltages (E) of different samples (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.C.). *E-mail: [email protected] (J.S.). ORCID

Jianlin Shi: 0000-0001-8790-195X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51702099), Shanghai Sailing Program (17YF1403800), China Postdoctoral Science Foundation funded project (2017M611500), the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL201702SIC). The authors thank beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.

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REFERENCES

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DOI: 10.1021/acsami.8b06948 ACS Appl. Mater. Interfaces 2018, 10, 25422−25428

Research Article

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DOI: 10.1021/acsami.8b06948 ACS Appl. Mater. Interfaces 2018, 10, 25422−25428