Devisable POM/Ni Foam Composite: Precisely Control Synthesis toward Enhanced Hydrogen Evolution Reaction at High pH

Abstract Polyoxometalates (POMs) are promising catalysts for the electrochemical hydrogen production from water owing to their high intrinsic catalytic activity and chemical tunability. However, poor electrical conductivity and easy detachment of the POMs from the electrode cause significant challenges under operating condition. Herein, a simple one‐step hydrothermal method is reported to synthesize a series of Dexter–Silverton POM/Ni foam composites (denoted as NiM‐POM/Ni; M=Co, Zn, Mn), in which the stable linkage between the POM catalysts and the Ni foam electrodes lead to high activity for the hydrogen evolution reaction (HER). Among them, the highest HER performance can be observed in the NiCo‐POM/Ni, featuring an overpotential of 64 mV (at 10 mA cm−2, vs. reversible hydrogen electrode), and a Tafel slope of 75 mV dec−1 in 1.0 m aqueous KOH. Moreover, the NiCo‐POM/Ni catalyst showed a high faradaic efficiency ≈97 % for HER. Post‐catalytic of NiCo‐POM/Ni analyses showed virtually no mechanical or chemical degradation. The findings propose a facile and inexpensive method to design stable and effective POM‐based catalysts for HER in alkaline water electrolysis.


Introduction
Hydrogeni sa na ttractive sustainable energy carriert oa ddress the challenges related with fossil fuel. [1,2] One of the most promising routes to generate hydrogen is electrochemical water splitting. The hydrogen evolution reaction (HER) reaction is at ypical two-electron transfer reaction, and involves interfacial proton-coupled electron transfer and concomitant hydrogen evolution. [3][4][5] Compared with the electrolysis process in acidic media, HER in alkaline electrolysis is more favourable for electrochemical water splitting due to the robustness of electrode materials, long lifetime of catalysts, cheap electrolyser construction and less equipment corrosion. [6][7][8] However, HER reactionr ate for most catalysts in alkalines olution is 2-3 orders of magnitude lower than that in acidic solution, [9,10] and HER application is largelyl imited by sluggish kinetics in alkaline solution. Therefore, significant studies are focusing on the development of electrocatalysts to overcome the high energy barriers and slow kinetics of HER reaction in alkaline solution. To date, the platinum-based catalysts are still widely used as HER catalysts, which combine low overpotential with high operational stability. [11][12][13][14] However,t he low abundance and high cost of Pt-based electrocatalysts cause seriousp roblems for widespreadc ommercialization of HER technology. [15] As such, academic and industrial communities spare no efforts in exploring efficient, stable and easily accessible HER catalysts based on earth-abundant metals.
Polyoxometalates (POMs), as ac lass of anionic transition metal oxides, which show ar ich diversity of structural types and reversiblem ulti-electronr edoxb ehaviours. [16][17][18] Owing to their unique electron sponge behaviour,P OMs exhibit protoncoupled multi-electron transfers. [19][20][21][22] More importantly,t he electrocatalytic efficiency can be tuned by adjusting the chemical compositiona nd microenvironment of POMs. [23][24][25][26] In terms of POMs applicationinHER, several POM-based electrocatalysts for HER in acidic solution were reporteds of ar.F or example, Zhang etal. presented the P 8 W 48 /rGO nanomaterial as an efficient electrocatalyst for the HER in acidic aqueous solution. [27] Ma et al. synthesized two POM-encapsulated twenty-nuclear silver-tetrazole nanocage frameworks, whiche xhibited high activity for HERi n0 .5 m H 2 SO 4 aqueous solution. [28] In contrast, few studies can be found on POM-based electrocatalysts for HER in alkaline media. Under such circumstances, development of highly active and stable electrocatalysts forH ER at high pH by technologically viable and scalable syntheticr outes remains highly challenging.
In this paper,w eh ave successfully constructed as eries of mixed metal polyoxometalate microcrystals on commercial Ni foam electrodes for high-performance HER in alkaline media.
The accurate structure of the NiCo-POM was further investigated by X-ray absorption near edge structure (XANES). The Co K-edge XANESs pectra of NiCo-POM/Ni, Co 3 O 4 and Co foil were presented in Figure4.B yc omparing the Co K-edge of NiCo-POM/Ni sample with that of Co and Co 3 O 4 ,t he NiCo-POM/Nie xhibitedaslight shiftt ol ower energy,i ndicating the + 2o xidation state of Co in the NiCo-POM. [37,38] It was consistent with that of XPS results. The Co K-edge k 3 c(k)o scillation curve ( Figure 4c)d isplayed that the oscillation amplitude of NiCo-POM/Ni was smaller than those observedi nt he Co 3 O 4 and Co foil, indicating structurald ifference in the coordination environment surrounding the Co atoms. As shown in Figure 4d,t he corresponding Fourier transformed R-space spectra of NiCo-POM/Nie xhibited two peaks at % 1.6 and % 3.3 .T he peak at 1.6 corresponded to the first octahedralc oordination of Co(1)ÀOs hell, while the peak at 3.3 was attributedt ot he scattering between the Co and the Wi nt he second nearest neighbour shell. Notably,aweak peak at 2.6 was assigned to Co(2) coordination with OH anions( bond distance 3.08 ) ( Figure 4a). [32,39,40] Furthermore, the XANES fitting of Dexter-Silverton NiCo-POM/Ni indicated that the Co centres were six-coordinate and occupied the nearly octahedral environment in the NiCo-POM/Ni.B ased on the XPS, XANES and ICP-AES data ( The NiM-POM/Ni composites were investigated as electrocatalysts for HER in alkaline aqueous KOH as electrolyte. In order to evaluatethe HER performance of the Ni-POM/Ni, NiCo-POM/ Ni, NiZn-POM/Ni,a nd NiMn-POM/Ni,atypicalt hree-electrode system in an N 2 -saturated 1.0 m KOH aqueous solution was applied by using the correspondingc omposites as working electrodes, as aturated calomel electrode as reference electrode and ac arbon rod as counter electrode. All potentials werer eferenced to the reversible hydrogen electrode (RHE), and the data were corrected for internal resistances (IR). Linear sweep voltammetry (LSV) of the composites (Figure 5a)s howed that all NiM-POM/Ni composites exhibited excellent electrocatalytic HER performance. Amongt hem, the NiCo-POM/Ni exhibited the best performance with al ow onset potential of 8mVa s well as al ow overpotential of 64 mV at ac urrent density j = 10 mA cm À2 .I ncontrast, other composites showeds lightly highero verpotential of 68 mV for NiMn-POM/Ni,7 4mVf or NiZn-POM/Ni,a nd 83 mV for Ni-POM/Ni. The overpotential of the NiM-POM/Ni electrodes are comparable to that of most Nibased materials for HER in alkaline solution (Table 1).
Figure5bd isplayed the Tafel plots of NiM-POM/Ni electrocatalysts,w hich provided deep insight into the HER reaction pathway on the electrocatalyst. The NiCo-POM/Ni composite electrode possessed aT afel slope of 75 mV dec À1 ,w hich was lower than that of NiMn-POM/Ni (79 mV dec À1 ), NiZn-POM/Ni (87 mV dec À1 ), and Ni-POM/Ni (98 mV dec À1 ), respectively.T hese results suggested that the hydrogen evolution process followed the Volmer-Heyrovsky mechanism and the rate-determining step was the hydrogen generation. [41] NiCo-POM/Ni  composite exhibited relativelye xcellent performance may attribute to the presence of Co sites to providem ore accessible H adsorption sites. [42] Potentiostatic cathodic electrolysis was measured by maintaining NiCo-POM/Ni electrode at the overpotentialo f2 00 mV versusR HE for 60 min. The NiCo-POM/Ni electrode exhibited % 97 %f aradaic efficiency for HER ( Figure S6), suggesting ahigh selectivity to hydrogenwith minimal faradaic losses. [43,44] To have ab etter understanding of the catalytic behaviour and internal resistance of the composite electrodes, we performed electrochemical impedance spectroscopy (EIS) for NiM-POM/Nii n1 .0 m KOH solution. The Nyquist plots (Figure 5c)i ndicatedt he expected semicircular features associated with charge transfer resistances in the HER process. As revealed by the charge transfer resistance (R ct )i nT able2,t he NiCo-POM/Ni electrode displayed the R ct of 11.9 W,w hich was lower than those of NiMn-POM/Ni (12.6 W), NiZn-POM/Ni( 15.1 W)a nd Ni-POM/Ni( 13.6 W). This result indicated that the NiCo-POM/Ni possessed the lowest charge-transfer resistance and superior conductivity,w hiche nabled efficient interfacial electron transport without ab inder or conductive additive for HER. [45] To rationalize whethert his finding was correlated to the electrochemically active surfacea reas (ECSA) of the electrodes, the ECSA values was determined based on the double-layer capacitance (C dl )c alculated from voltammetric data in the non-faradaic region.A ss hown in Figure 5d,t he NiCo-POM/Ni showed the largest C dl of 4.56 mF cm À2 ,w hile the C dl of NiMn-POM/Ni (1.47 mF cm À2 )a nd NiZn-POM/Ni (0.69 mF cm À2 )w ere higher when compared with the monometallic Ni-POM/Ni (0.39 mF cm À2 )a nd Ni foam (0.09 mF cm À2 ). Theh igh C dl of NiCo-POM/Ni wasr elatedt ot he enhanced ECSA (Table 2), indicating the increaseo ft he accessible catalytic sites for HER. Therefore, the highest ECSA of NiCo-POM/Ni can be beneficial to water adsorption.
Tuning the particle size of the POM microcrystals can be critical for controlling the number of surface-accessible HER reaction sites. [55] Ta king the NiCo-POM/Ni as an example, we examined the effect of average crystal size on HER activity.T he NiCo-POM microcrystal particles ize can be easily controlled by variationo ft he hydrothermalr eactiont ime. Particle formation can be monitored using powder XRD (Figure 6a), where formation of the cubic NiCo-POM phases tarteda tr eactiont ime of 2hours. The size growth of NiCo-POM on Ni foam can be observedu pt om aximum reaction time of 8hours and no impurities were observedi nt he XRD. SEM images ( Figure S1) of the NiCo-POM/Ni composite obtained at different reactiont ime clearlyr evealed the growth process of the NiCo-POM microcrystalso nt he Ni foam surface.
HER electrocatalysis studies (Figure 6b)o ft hese composites showed that the catalytic performance was related to the mi-  crocrystal size. The lower HER overpotentials (64-78 mV) were obtained for NiCo-POM/Nie lectrodes with average particle size of 2-4 mma tr eactiont ime 3-6 h, while significant increasei n overpotential can be found for microcrystal size with smaller than 1 mma nd/orl arger than 5 mmp article size. In addition, the NiCo-POM/Ni with large particle-size revealed increased Ta fel slope, suggesting ak inetically favoured HER processes for the smaller particles (Figure 6c). Furthers tudies were carried out by examiningthe electrochemical double-layer capacitance (C dl ) ( Figure 6d). Herein, the NiCo-POM/Ni with 2 mma verage size exhibited the largest C dl ,i ndicating that the NiCo-POM/Ni featured the highest number of electrochemically active surface sites. It was furthers upported by EIS analyses ( Figure S7). The NiCo-POM/Ni with 2 mmp article size exhibited the lowest charge-t ransfer resistance, leading to efficient electron transport at the NiCo-POM/Ni interface. [56] The long-term stability and degradation resistance of the NiCo-POM/Ni electrode (particles ize 2 mm, reactiont ime 3h) were assessed by CV cycling at scan rate 5mVs À1 in 1.0 m KOH. Figure 7b showedt hree LSV curveso ft he electrode after 0, 1000 and 2000 cycles.I tw as found that almost negligible decreaseh ighlighted the long-term stability of the NiCo-POM/ Ni electrode. Ap ost-catalytic SEM image (Figure 7a)i ndicated that the morphologyo ft he NiCo-POM catalyst was mostly retained, and no mechanical detachmento ft he catalyst from the electrode was observed. In addition, powder XRD analysis ( Figure S8 a) and FTIR spectra (Figure S8 b) of the post-catalytic electrode provided furthers upport for the remarkable stability of the NiCo-POM/Ni composite during sustained HER process in alkaline condition.

Conclusions
In conclusion, as eries of NiM-POM clusters weres uccessfully fabricatedo nt he porous Ni foam by hydrothermal methods. The as-prepared NiM-POM/Ni composites showedo utstanding reactivitya nd stabilityf or HER in alkaline aqueous solution. Amongt hem, the NiCo-POM/Ni exhibited the best performance with low overpotential of 64 mV togetherw ith Tafel slope of 75 mV dec À1 at 10 mA cm À2 .B esides, the chemically stable deposition of NiCo-POM on the Ni foam led to their high reactivity and long-term stability. Such excellent HER performance of the NiCo-POM/Ni can be due to the intrinsic reactivity of POM microcrystals with abundant activity sites. To the best of our knowledge,t he NiCo-POM/Ni composite outperforms the most known Ni-based HER electrocatalysts in alkaline media. Further work will expand this study to relateds ystems to enablea ccess to an ew class of mixed-metalo xides for energy-relevant high-performancee lectrocatalysts.

Experimental Section
Materials and Characterization: All chemicals were of analytical grade and were used as received without any further purification. Na 3 [PW 12 O 40 ]·6H 2 Ow as synthesized according to al iterature method. [57] Fourier transform infrared (FTIR) spectra were recorded on aB ruker Vector 22 infrared spectrometer by using the KBr pellet method. Scanning electron microscopy (SEM) images were obtained by using aZ eiss Supra 55 SEM. The powder X-ray diffraction (XRD) analysis was carried out on aB ruker D8 diffractometer with highintensity Cu Ka radiation. High-resolution transmission electron microscopy (HRTEM) was conducted on aJ EOL JEM-2100 equipment under an accelerating voltage of 400 kV.T he XPS spectra were acquired with PHI Quantera SXM Al with an monochromatized Al cathode (hn = 1486.6 eV) as the X-ray source set at 100 Wa nd a pass energy of 26.00 eV.W ide and detailed spectra were collected at 08 take-off angle, using fixed analyser transmission mode with channel widths of 1.0 and 0.1 eV,r espectively.T he binding energies were calibrated based on C1s( 284.8 eV). Raman spectroscopy was performed on aR enishaw Raman spectrometer at al aser excitation wavelength of 532 nm. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis was performed using aS himadzu ICPS-7500 spectrometer.X -ray absorption near edge structure (XANES) measurements were obtained from the 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF). The EXAFS data were processed and fitted in R-space according to the standard procedures using the ATHENA and ARTEMIS modules implemented in the IFEFFIT software packages.
Synthesis of the NiCo-POM/Ni electrode: The POM-based electrode material was prepared by as imple hydrothermal process. The commercial Ni foam was cut into a1 4cmb lock, washed with acetone, aqueous HCl solution (2.0 m), deionized water and ethanol. 0.29 gC o(NO 3 ) 2 ·6H 2 Oa nd 3.18 gN a 3 [PW 12 O 40 ]·6H 2 Ow ere dissolved in 40 mL deionized water in a5 0mLT eflon autoclave. Then Ni foam was immersed in above solution and the reaction was heated to 180 8Cf or 3h ours to give the NiCo-POM/Ni electrode.
Electrochemical Measurements: Electrochemical measurements were performed with an Instrument CHI 660E in 1.0 m aqueous KOH solution at room temperature in three-electrode setup (working electrode:N i M-POM/Ni, Reference electrode:s tandard calomel electrode (SCE), counter electrode:c arbon rod). Linear sweep voltammetry was performed with ascan rate of 5mVs À1 .EIS measurements were carried out in 1.0 m aqueous KOH at different potentials in the frequency range 0.1 to 105 Hz with an amplitude of 10 mV.T he potential versus SCE was converted to the reversible Where n H 2 theoretical ðÞ is the theoretical number of moles of H 2 produced, Q is the charge passed through the electrodes, F is the Faradaic constant (96485 Cmol À1 ). The evolved amounts of H 2 n H 2 experimental ðÞ were obtained by aw ater-gas displacing method, in which the H 2 volume fraction (V %) of the hydrogen was analysed by gas chromatography (GC, Shimadzu GC-2014C). V cell is the initial volume of electrolyte, V electrolyte is the residual volume of electrolyte in the electrolyser. P is the atmospheric pressure (105 kPa), T is the temperature (298 K), and R is the molar gas constant (8.314 Jmol À1 K À1 ). And c ðsolu H2Þ is the solubility of H 2 in water.