MOF nanosheets electrocatalysts for efficient H2 production from

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MOF nanosheets 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, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06948 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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ACS Applied Materials & Interfaces

MOF nanosheets electrocatalysts for efficient H2 production from methanol solution: methanol assisted water splitting or methanol reforming? Xinfa Wei1, Shun wang1, Zile Hua2, Lisong Chen1*, Jianlin Shi1,2* 1 Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, P. R. China; 2 State

Key

Laboratory

of

High

Performance

Ceramics

and

Superfine

Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China;

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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 greatly potential and extensively studied, is seriously obstructed especially by the anodic oxygen evolution reaction (OER) due to its sluggish kinetics. Herein we report the efficient hydrogen production from methanol solution using facile-synthesized ultrathin 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 RHE) at 10 mA cm-2, which was markedly 232 mV lower than that in conventional water splitting, and moreover, the average turnover frequency (TOF) 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 electro-catalytic hydrogen production from the methanol solution is a methanol-assisted water splitting, rather than a methanol reforming process as claimed in a number of literatures, in which methanol is oxidized 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 being the focuses 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 years4-8. However, the electrochemical water splitting process suffers severely from the high overpotential, especially by the anodic oxygen evolution reaction (OER), which is the rate-determining step of the overall water splitting process9-11. Although many kinds of electro-catalysts have been developed12-15, their performance are still far from being satisfactory, even in the presence of noble metal catalysts (IrO2, RuO2, et al.)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 the cathode. Efforts have been made to integrate the small organic molecules upgrading reactions with decoupled HER in water splitting recently. Various small organic molecules such as ethanol18-20, benzyl alcohol21, 5-hydroxymethylfurfural22-24, and aloe extract25 have been used for H2 production by oxidation. These reactions take place as below: Anode:



RCH2-OH + 4OH- → R-COOH + H2O + 4eOr

RCHO + 2OH- → R-COOH + H2O + 2e-

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(1) (2)

Cathode: 4e- + 4H2O (l) → 4OH-(aq) +2H2 (g)

(3)

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

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 cathode26,27. These small organic molecules oxidation reactions not only show strongly reduced anodic potential, but also eliminate the danger of possible explosion, therefore 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. Firstly, most of the electro-catalysts 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. Secondly, the


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searching for suitable small organic molecules of high hydrogen content, nontoxicity and low cost is still under progress. Last but not the least, hydrogen production mechanism is still controversial to date. Direct evidences are needed on whether the electrochemical hydrogen production process is a small organic molecules-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 catalysis28,29. All metal ions or metal clusters in MOFs are monodisperse, so that almost all metallic catalytically active sites can be exposed and utilized30. As the vast majority of MOFs are nonconductive 30-33, MOFs have been rarely applied in the field of electro-catalysis. The development of two-dimensional 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 ions34. In addition, methanol, as one of the basic organic materials, is of huge production capacity, rather low cost, and importantly, 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 condition by using ultrathin MOF nanosheets as non-noble 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 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 of is probed to be methanol-assisted water splitting rather than methanol-reforming, which is soundly demonstrated by using mass spectroscopy.

RESULTS AND DISCUSSION Materials characterization. The structural and morphological characterizations results of ultrathin bimetal–organic framework nanosheets are shown in Figure 2. 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 Nibased MOFs reported by Mesbah, A. et al (no.985792) 35. According to the structure data reported, we can readily define the crystalline structure of CoCuUMOFNs. 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


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Figure 2: Structural and morphological characterizations of CoCu-UMOFNs. (a) Powder XRD pattern of CoCu-UMOFNs. (b) Crystal structure schematics of CuCoUMOFNs . (c) TEM image of CoCu-UMOFNs. (d) SEM image of CoCu-UMOFNs after drying. (e) AFM image of CoCu-UMOFNs showing the measured thicknesses of individual flakes. (f) SEM-EDS mapping of CoCu-UMOFNs. be identified in the transmission electron microscopic (TEM) image (Figure 2c), which shows the same morphology to Cu-UMOFNs (Figure S1). The SEM image of CuCo-UMOFNs (Figure 2d) after drying shows 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. Due to the aggregation of CoCu-UMOFNs, 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 reactions34. Subsequently energy dispersive spectroscopy (EDS) (Figure S3) and X-ray photoelectron spectroscopy (XPS) (Figure S4) were employed to analyse 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 CuCoUMOFNs without other impurities. All the results above are in consistence with the compositional data of 16.9% Co and 15.7% Cu by inductively coupled plasma spectrometer (ICP). In order to further explore the material's inherent fine structure, X-ray photoelectron spectroscopic (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 three-electrode system (see methods of details in Experimental). Compared to the electrolysis of water for hydrogen production, the anodic potential in methanol solution on CoCuUMOFNs 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 OER process as shown in Figure 3b. Besides, Co-UMOFNs and Cu-UMOFNs (Figure S5) also exhibit similar results. Chronoamperometry experiments were carried


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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) A 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 CoCuUMOFNs, Cu-UMOFNs and Co-UMOFNs in N2-saturated 1M KOH solution of 3 M Me-OH. (g) LSV curves in N2-saturated 1M KOH solution of 3 M Me-OH at a scan rate of 5 mVs-1, at 1000th and 2000th cycles. (h) Chronoamperometry responses at a constant potential of 1.383 V for 12

hours. The inset shows the H2 generation on graphite carbon electrodes with no bubbles generated at anode. All polarization curves were used without IR correction. ACS Paragon Plus Environment


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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 shows high performance for HEM. As shown in Figure 3d, the CuCo-UMOFNs exhibits an onset potential (10 mA cm-2) of 1.365 V vs RHE, which is much lower than those of Cu-UMOFNs (1.495 V vs RHE) and Co-UMOFNs (1.427 V vs RHE), so are the potentials at 20, 30, 50 mA cm-2, as listed in supplementary Table S1. In all, bimetallic organic framework com pounds 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 LSV curves were derived to evaluate catalytic kinetics of metal-organic frameworks 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 CuCo-UMOFNs is as low as 46 mV dec-1 (Figure 3e), much smaller than those of Cu-UMOFNs (87 mV dec-1) and Co-UMOFNs (98 mV dec-1), indicating much quicker HEM kinetics. Also, the TOF was measured for the comparison of the intrinsic activity at a current density of 10 mA cm-2 according to equation 4: TOF = J/(4×F×m/M)

(4)


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Where J is the current density (mA cm-2) at a given overpotential (10 mA cm-2), F is the faraday constant (96485 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 centre 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 CoUMOFNs. 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 larger ECSA than Cu-UMOFNs (Figure 3f and S6), though a little lower than CoUMOFNs. Co is proposed to play an important role in enhancing the ECSA. 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 Chronoamperometry test at the applied potential of 1.38 V. No significant

current

Chronoamperometry

density

decrease

experiment

period

was

observed

(Figure

3h).

during Both

12

CV

h and

Chronoamperometry tests confirm the excellent stability of CoCu-UMOFNs towards HEM. To get more information of the interaction between Cu and Co of the catalysts, 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



Figure 4: (a) High resolution Co 2P 3/2 XPS spectra and (b) Cu 2P 3/2 XPS spectra of CoCu-UMOFNs and Co-UMOFNs. (c) Co K-edge EXAFS spectra in R-space of CoCuUMOFNs and Co-UMOFNs and (d) Cu K-edge EXAFS spectra in R-space of CoCuUMOFNs and Cu-UMOFNs. 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 Cu-UMOFNs 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 XANES 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 XAS spectroscopy was employed. As shown in the ex situ EXAFS data in R-space


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(Figure 4c d), all metal atoms are in a single isolated state without metal-metal bonding. Product quantification It is rather debatable in recent years that 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. In order 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 vs RHE in sealed electrolytic pools of solution 0 (H2O solution of 1 M KOH and 3 M MeOH), solution 1(1M KOH and 3M CH3OH dissolved in deuterated water (D2O)) and solution 2(1 M KOH and 3M methanol-4D (C D3OD) dissolved in H2O), and the electro-catalytically produced gases were identified by a mass spectrometer (MS). As can be clearly seen in Figure 5a b c d, D2 and HD can only be obtained from solution 1 containing 3M CH3OH dissolved in deuterated water (D2O), while no D2 and HD were produced from solution 2 containing methanol-4D (D3COD) in non-deuterated H2O. The process of H2 generation at cathode is illustrated in Figure 5e which is similar to that of water splitting, following the Tafel-Heyrovsky mechanism in alkaline condition (formula 5 6 7). H2O (ads) + e- → [H] (ads) + OH-(aq)


(5)


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[H] (ads) + [H] (ads) → H2 (ads)

(6)

H2 (ads) → H2 (g)

(7)

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(1M KOH and 3M Me-OH dissolved in D2O), (c) solution 2(1 M KOH and 3M D3COD dissolved in H2O) in the

first hour, and (d) solution 2 in 12 hour of chronoamperometry measurement at 1.4V vs RHE. (e) Illustration of the reaction mechanism at cathode when deuterated water is employed. (f) The possible reaction mechanism of Me-OH at anode, which contains four sub-steps of dehydrogenation and the oxidation of CO.


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HER is almost impossible to proceed at cathode under alkaline condition without the electron supply from anode where methanol or water is oxidized. The participation of methanol electro-oxidation at 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. The possible reactions on the anode catalyst surface contain the breakings of C-H, C-O and O-H bonds. However the breaking of C-O bond is extremely difficult due to its high bond strength, and O-H is more likely to break under alkaline condition. The anodic oxidation steps of methanol are schematically illustrated in Figure 5f. Firstly methanol molecules are adsorbed on the surface of 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 cathode for HER. In all, the six sub-steps of the electrochemical reactions that occur at anode can be written as: Dehydrogenation reactions of methanol: CH3OH (ads) → CH3O (ads) + [H] (ads)

(8)

CH3O (ads) → H2CO (ads) + [H] (ads)

(9)



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H2CO (ads) → HCO (ads) + [H] (ads)

(10)

HCO (ads) → CO (ads) + [H] (ads)

(11)

CO and hydrogen oxidation reactions for electron donations from anode to cathode: CO (ads) + 2OH- → CO2 + H2O + 2e4[H] (ads) + 4OH- → 4H2O + 4e-

(12) (13)

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

CONCLUSIONs In summary, we have demonstrated a novel strategy of electro-catalytic pure H2 production through a methanol-assisted water splitting approach, by using CoCu-UMOFNs as a non-noble metal anodic catalyst for methanol oxidation and graphite cathodic electrode for ultra-pure 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 232mV lower than that of OER at 10


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mA cm-2 with a nearly 100% Faradic 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 of has been probed to be a methanol-assisted water splitting process rather than a methanol-reforming 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.

EXPERIMENTAL Chemicals：CuCl2·2H2O (99.99%, AR, grade), CoCl2·6H2O (99.99% AR, grade), potassium hydroxide, benzenedicarboxylic acid (BDC) and N, Ndimethylformamide (DMF) were bought from Macklin. All chemicals were used as received without any further purification. 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 BDC was dissolved into the mixed solution under ultrasonication. Subsequently, 0.089 g CoCl2·6H2O and 0.064 g CuCl2·2H2O were added. After Cu2+ and Co2+ salts were dissolved, 0.8 ml TEA was added into the solution. Then, the solution was stirred for 5 min to obtain a uniform colloidal suspension. Afterwards, 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 (2mL) of water and ethanol (Vwater/Vethanol = 1/1). Then 50 µL 5 wt% Nafion solution was added. The mixed solution followed by ultrasonication for 30 min to obtain a homogeneous catalyst suspension.5 µL catalyst ink was transferred onto 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 water, 1 mL ethanol, 50 µL 5 wt% Nafion solution and 5 mg material followed 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. Linear sweep voltammetry (LSVs) was measured at a scan rate of 5 mV s–1. All potentials were referenced to an Ag/AgCl (sat. KCl) reference electrode, and carbon rod was used as the counter electrode in all measurements. All potentials were calibrated with respect to reversible


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hydrogen electrode (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 which both the working electrode and the counter electrode are platinum electrodes and Ag/AgCl (sat. KCl) electrode as reference electrode. The measurement results of E RHE vs Ag/AgCl are show in Figure S10. All polarization curves in this manuscript were used without IR correction. Estimation of Effective Electrochemical Active Surface Area (ESCA): To evaluate the ECSA, cyclic voltammetry (CV) could be carried out to probe the electrochemical double layer capacitance (Cdl) of various samples at nonFaradaic region, identified from CV in quiescent solution. Measurement of the Faraday Efficiency: A piece of CuCo-UMOFNs on Ni foam (surface area 0.5 cm2) was used as the working electrode. The hydrogen concentrations were measured using gas chromatography (Ramiin GC2060) equipped with packed column and thermal conductivity detector. And 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 as 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 vs RHE. After 19.2 C charge was calculated by electrochemical workstation, the 2.445 mL H2 was generated on the cathode, which was measured on gas chromatography, which approaches the theoretical value of 2.447 mL.



The source of the hydrogen: In order 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: Powder XRD data were acquired on a RigakuD/MAX 2550 diffractometer with Cu Ka radiation (l = 1.5418 Ǻ). SEM measurements were carried out on a Hitachi S-4800 scanning electron microscope at an accelerating voltage of 3 KV. Transmission electron microscope (TEM) images and acquired on a JEM-2100F equipped with operated at 200 kV. Nitrogen isotherms corresponding to the Barrett-JoynerHalenda (BJH) pore distribution and Brunauer-Emmett-Teller (BET) surface area were collected on a quadrasorb SI at 77 K after degassing the samples at 150 °C for 5 h. Co and Cu K-edge X-ray absorption fine structure (XAFS) spectra of the CoCu-UMOFNs Co-UMOFNs and Cu-UMOFNs were recorded in 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. X-ray photoelectron spectroscopy (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 C1s 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


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valance band XPS (VBXPS) were aligned by using the Fermi level of the XPS instrument. The quantitative analysis of element contents in CoCuMOFNs sample was performed with Agilent 700 Series inductively coupled plasma optical emission spectrometry (ICP-OES). The deuterated hydrogen was detected by PROLINE, America. ASSOCIATED CONTENT Supporting Information TEM , BET ,EDX and XPS of three different materials; Polarization curves of contrasts in two electrolytes; CVs result of ECSA measurement; Data of Co K-edge Cu K-edge; Reference electrode calibration curves in two electrolytes. Standard curve of H2 production measured by gas chromatography. Anode voltages (E) of different samples. AUTHOR INFORMATION Corresponding authors *E-mail address: [email protected] *E-mail address: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements 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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MOF nanosheets electrocatalysts for efficient H2 production from

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