This is the submitted version of the journal article: Li, Junshan; Xing, Congcong; Zhang, Yu; [et al.]. «Nickel iron diselenide for highly efficient and selective electrocatalytic conversion of methanol to formate». Small, Vol. 17, issue 6 (Feb. 2021), art. 2006623. DOI 10.1002/smll.202006623 This version is available at https://ddd.uab.cat/record/271934 under the terms of the license Nickel Iron Diselenide for Highly Efficient and Selective Electrocatalytic Conversion of Methanol to Formate Junshan Li,a Congcong Xing,b,c Yu Zhang,b Maria Chiara Spadaro,d Ting Zhang,d Qianbao Wu,a Yunan Yi, a Shenlan He,a Jordi Llorca,c Jordi Arbiol,d,e Andreu Cabot,b,e* Chunhua Cuia* a Molecular Electrochemistry Laboratory, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, 610054 Chengdu, P. R. China. b Catalonia Institute for Energy Research - IREC, Sant Adriàde Besòs, Barcelona, 08930, Spain. c Institute of Energy Technologies, Department of Chemical Engineering and Barcelona Research Center in Multiscale Science and Engineering. Universitat Politècnica de Catalunya, EEBE, 08019 Barcelona, Spain. d Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain. e ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain. Abstract The electrooxidation of methanol to formate is an example of the potential use of renewable energies to add value to a biosourced chemical commodity. Additionally, methanol electrooxidation can replace the sluggish oxygen evolution reaction for hydrogen evolution or the electroreduction of other biomass-derived intermediates. Here, we demonstrate a noble metal-free catalyst, Ni1-xFexSe2, with a high potential for an efficient and selective methanol conversion to formate. At its optimum composition, Ni0.75Fe0.25Se2, this diselenide is able to produce 0.47 mmol cm-2 h-1 of formate at 50 mA cm-2 with a Faradaic conversion efficiency of 99 %. We further demonstrate that Ni1-xFexSe2-based catalysts are able to continuously work for over 50,000 s with a minimal loss of efficiency, delivering initial current densities above 50 mA cm-2 and 2.2 A mg-1 in a 1.0 M KOH electrolyte with 1.0 M methanol at 1.5 V vs. RHE. Keywords: electrocatalysis; methanol oxidation reaction; selenide; nanoparticle; formic acid; biorefinery; biomethanol; formate 1 Introduction Electrochemistry can play an important role in future biorefineries by allowing the valorization of biomass-derived chemicals using renewable energies such as wind and solar. It is becoming evident that the electrocatalytic reforming of biomass-derived chemicals is a potential cost effective, CO2-neutral and clean approach to produce hydrogen, biofuels, and value-added chemicals, and it can be even a convenient approach to store renewable energy in chemical bonds.[1–3] Methanol is the most used commodity chemical. It can be obtained from CO2 and different forms of biomass, including municipal, agricultural and forest waste. Methanol is used as a fuel in direct methanol fuel cells, but also as precursor or building block for the synthesis of other chemicals.[4] Currently, close to one million tons of formic acid are produced every year from the combination of methanol with CO at high pressure and the hydrolysis of the resulting methyl formate.[5] This is a cost-effective process because the price per metric ton of formate is a fourfold higher than that of methanol.[6,7] Formate is an essential and highly versatile chemical in many sectors, including chemical, textile, rubber, leather, pharmaceutic and printing industries. Formic acid can be also used as a fuel in direct formic acid fuel cells and for hydrogen storage.[8–11] Among the possible alternative strategies to produce formate, the electrooxidation of (bio)methanol has several advantages, including a high energy efficiency, a potential high selectivity, the use of ambient pressures and temperatures, the potential use of electricity from renewable energy sources, its rapid activation and deactivation that allows using surplus energy when it becomes available, the scalability of the process, and its potential coupling with elecroreduction reactions to generate hydrogen or other valuable fuels or chemicals.[10– 14] The methanol oxidation reaction (MOR) to formate could be even coupled to the oxygen reduction reaction to cogenerate electricity.[15] As a drawback, the electrochemical oxidation of methanol generally relies on noble metal-based and thus high cost electrocatalysts, which limits application at an industrial scale.[16,17] Recently, a great deal of effort has been devoted to the development of Ni- and 2 Co-based catalysts for the MOR.[18] Ni-based catalysts are characterized by particularly outstanding performances in alkaline media. This high performance is correlated with the surface oxidation of Ni to NiOOH, which is considered the MOR active phase.[19–25] Using in-situ infrared spectroscopy together with nuclear magnetic resonance spectroscopy, we recently demonstrated a 100% electrochemical conversion of methanol to formate on branched Ni3C particles.[26] Other groups have demonstrated the combination of Fe and Ni to allow efficient MOR with current densities up to 1.71 A mg-1 at 1.58 V vs. RHE, well above the values obtained for elemental Ni and Fe catalyst.[27] Besides, nickel selenide nanowires grown on nickel foil were demonstrated to deliver much higher current densities than the respective metal, oxide and sulfide, while at the same time showing an excellent stability.[28] We take advantage of recent advances in the field to develop a higher performance catalyst for the cost effective methanol oxidation to formate. We present here an electrocatalysts based on bimetallic nickel-iron diselenide nanorods produced at low temperature in solution. The Ni/Fe ratio within the nanorods is tuned through the full Ni/Fe range to determine the optimum composition. The performance of these materials toward the electrocatalytic oxidation of methanol to formate is analyzed and its activity, selectivity and stability is determined. 3 Experimental Section Chemicals. Nickel (II) acetylacetonate (Ni(acac)2, 96%), iron (III) acetylacetonate (Fe(acac)3, >99%), selenium powder (Se, 200 mesh 99.5%), and oleylemine (OAm, C18H37N, 80-90%) were purchased from Acros Organics. Potassium carbonate (K2CO3, 99.5%), potassium bicarbonate (KHCO3, 99.7%), 1,2-ethanedithiol (EDT, HSCH2CH2SH, >98%), potassium hydroxide (KOH, 85%), oleic acid (OAc, C18H34O2, 99%), carbon black (CB, Vulcan XC72), and Nafion (10 wt.%, perfluorinated ion-exchange resin, dispersion in water) were purchased from sigma Aldrich. Analytical grade hexane and ethanol were obtained from various sources. MilliQ water (18.2 MΩ·cm) was used for ink preparation and IC measurements. All chemicals were used as received, without any purification. Ni1-xFexSe2 precursor. To prepare the precursor required to produce 0.2 g of Ni1-xFexSe2 NRs, (1-x) of mM Ni(acac)2, x mM of Fe(acac)3, and 2 mM of Se powder were placed within a glass vial. Then 5 mL of OAm and 0.5 mL of EDT were added while stirring. Upon EDT incorporation, the color of the solution immediately turned from transparent to light yellow. The mixture was stirred for 2 h at 50 °C. The obtained precursor was then filtered through a 0.2 µm filter before using. All the processes were carried out inside an argon-filled glove box. Ni1-xFexSe2 NRs. 20 ml of OAm was placed in a 50 ml three-neck flask and kept under vacuum first at room temperature for 20 min and then at 80 ºC for 30 min to obtain a clear solution and remove low boiling point impurities. Afterward, the solvent was heated to 220 ºC under Ar, and at this point the Ni1-xFexSe2 precursor solution was injected into the flask. Upon injection, temperature dropped to ~210 ºC. The mixture was allowed to recover the 220 ºC and maintained at this temperature for 10 min. Afterward, the solution was rapidly cooled, ~80 ºC/min, to room temperature using a water bath. During cooling, when the flask reached ~70 ºC, 2 ml of OAc were added to replace the weakly bound OAm molecules. At ambient temperature, the crude solution was mixed with 10 ml of hexane and the product was isolated by centrifugation at 8000 rpm for 5 min. Particles were then dispersed again in hexane and precipitated one more time by centrifugation in the presence of ethanol. The NPs were finally re-dispersed in hexane and kept in the glovebox until their posterior use. 4 Material characterization: The crystal structure of the materials was characterized by X-ray diffraction (XRD) using a Bruker AXS D8 Advance (Cu K radiation: λ = 1.5106 Å). Refinement of the laboratory diffraction patterns were obtained by the Rietveld method with the GSAS-II package. Scanning electron microscopy (SEM) measurements were performed on a Gemini 300 field emission scanning electron microscope (ZEISS, Germany) equipped with an AZtecOne UltimMax40 energy spectrometer. High-resolution transition electron macroscopy (HRTEM) together with scanning TEM (STEM) investigation was performed on a field emission gun FEI Tecnai F20 microscope. High angle annular dark-field (HAADF) STEM was combined with electron energy loss spectroscopy (EELS) in the Tecnai microscope by using a GATAN QUANTUM filter. X-ray photoelectron spectroscopy (XPS) analyses were conducted on a SPECS system. Fourier transform infrared spectroscopy (FTIR) analysis were performed on an Alpha Bruker FTIR spectrometer with a platinum attenuated total reflectance (ATR) single reflection module. Electron paramagnetic resonance (EPR) spectroscopy was conducted on a Bruker EMXplus instrument (Bruker, Germany) with a microwave frequency of 9.40 GHz at 100 K. Around 10 mg of sample was loaded in a quartz EPR tube for each EPR measurement. Electrochemical characterization. Electrochemical performance was measured on a Bio-logic SP-200 potentiostat at room temperature. All measurements were carried out using a conventional three-electrode system: a Pt wire as counter electrode (CE), a 5 mm diameter glassy carbon as working electrode (WE) and an Ag/AgCl as reference electrode (RE). To prepare the WE, 5 mg NRs and 10 mg CB were dispersed in 3 ml of MilliQ water/ethanol (v/v = 1:1) and 100 μL of a 10 wt% Nafion. After vigorously sonicating for 1 hour, 3 μL of the prepared ink was drop-casted on a carefully cleaned glassy carbon (GC) electrode, and then dried naturally in open air. The prepared alkaline electrolyte was bubbled with high-purity N2 gas for half an hour before exploring the electrocatalytic performance. Cyclic voltammetry (CV), chronoamperometry (CA), and chronopotentiometry (CP) techniques were applied to study the activity and stability of the electrocatalyst. The measured potential (E) vs. reversible hydrogen electrode (RHE) was obtained using the following equation: 0 𝐸𝑣𝑠.𝑅𝐻𝐸 = 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙 + 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙 + 0.059 × 𝑝𝐻 5 where 𝑬𝑨𝒈/𝑨𝒈𝑪𝒍 is the measured potential E vs. RHE, 𝑬𝟎𝑨𝒈/𝑨𝒈𝑪𝒍 is the potential of the RE vs. RHE (0.21 V according to the manufacturer’s web site), and pH is the theoretical value of 1.0 M KOH. Product quantification. The product of the methanol electrochemical oxidation was characterized by ion chromatography (IC, analysis lab). A freshly prepared 4.5 mM KCO3 and 0.8 mM KHCO3 solution was used as leachate solution. During the CP measurement at a constant current output, 0.5 mL solution was collected at the same time intervals and diluted into MilliQ water with a ratio of 1:19 for the IC measurement. The produced amount of the product was calibrated based on standard solution with known concentrations. Then, the Faradaic efficiency (FE) was calculated using the following equation: 𝑚𝑜𝑙 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 × 𝑛 × 𝐹 𝐹𝐸(%) = × 100% 𝑡𝑜𝑡𝑎𝑙 𝑐ℎ𝑎𝑟𝑔𝑒 𝑝𝑎𝑠𝑠𝑒𝑑 where n is the electron transfer number and F is the Faradaic constant (96485 C mol-1). 6 Results and discussion Nickel iron diselenide, Ni1-xFexSe2, particles were produced over their entire compositional range via a two-step solution-based approach. The process involved the dissolution of Ni and Fe acetylacetonates and Se powder in an EDT solution, and the subsequent reaction of the mixture in OAm at 220 ºC (see scheme in Figure 1a and experimental section for details). The reaction product consisted of particles with elongated morphology, with an average width of ca. 10 nm and an average length of ca. 50 nm (Figures 1b and S1). The Ni/Fe ratio was tuned by varying composition of the initial precursor solution (Table 1). Because the measured composition was in good agreement with the nominal composition, in the following we use the later to refer to the different samples. a b Ni0.75Fe0.25Se2 ink preparation injection Fe(acac)3 Ni(acac)2 ethylenediamine ethanedithiol V/V 10:1 Se powder Heating 100 nm Figure 1. (a) Scheme of the solution-based approach used to prepare Ni1-xFexSe2. (b) Representative TEM macrograph of Ni0.75Fe0.25Se2 nanoparticles with elongated geometry. XRD patterns obtained from NiSe2 and FeSe2 particles were properly indexed considering the cubic pa-3 and orthorhombic pnnm crystal structure, respectively (Figure 2a). Ni0.75Fe0.25Se2 was found to crystalize in the cubic NiSe2 phase. Rietveld refinement of the XRD pattern of this sample showed a unit cell with a=b=c of 5.9449 Å, slightly above the 5.9195 Å measured for NiSe2 (Figure S2). When adding larger amounts of Fe, XRD peaks in the two-theta region of 28o-38o and 48o-56o became visible, evidencing the change from the cubic NiSe2 to the orthorhombic FeSe2 phase. Table 1. Nominal and EDS Ni/Fe/Se atomic ratio of the Ni1-xFexSe2 NRs. Ni1-xFexSe2 Atomic ratio by EDS（％） Fe* (x) Ni Fe Se (x) 0 33 0 67 0.00 0.25 22 7 71 0.24 7 0.50 15 15 69 0.50 0.75 9 23 68 0.72 1.0 0 32 67 1.00 * The atomic amount of Fe (x) was calculated by fixing the Se composition at 2 in Ni1-xFexSe2. Figures 2bc and S3 display representative HRTEM micrographs and the indexed power spectra of Ni0.75Fe0.25Se2 and Ni0.50Fe0.50Se2 samples. In both cases, particles were imaged along their [010] zone axis. STEM-EELS chemical analyses showed Fe, Ni, and Se to be homogeneously distributed within the particles (Figure 2b). STEM-EELS quantitative analysis of the Ni and Fe content were in good agreement with SEM-EDS analysis, with the metal composition varying in the ranges 75-80% for Ni and 20-25% for Fe in Ni0.75Fe0.25Se2 (Figure 2b), and in the range 45-55% for both elements in Ni0.5Fe0.5Se2 (Figure S3a). HRTEM analysis confirmed the Ni1-xFexSe2 crystal structure to change symmetry when increasing the Fe amount, from cubic NiSe2 pa-3 with space group 205 to orthorhombic FeSe2 pnnm, space group 58, in good agreement with XRD analyses (Figure 2a). According to HRTEM analysis, Ni0.50Fe0.50Se2 already crystallized in the orthorhombic structure and its power spectrum could be simulated using orthorhombic FeSe2 with a 50% replacement of Fe by Ni (Figure S3b). High resolution XPS spectra of the Ni 2p3/2, Fe 2p3/2 and Se 3d regions of Ni0.75Fe0.25Se2 are displayed in Figures 2c and S4. Figure S5 displays XPS data obtained from NiSe2. The main contribution to the XPS spectra of the two metals provided from surface oxidized states, NiOx and FeOx, although bands corresponding to Ni and Fe within a selenide environment where also clearly visible.[29] Surface oxidation took place during sample handling and transportation, as commonly observed in related materials.[30,31] The Se 3d spectra also showed two well defined contributions associated to an oxide component and to Se within a metal selenide environment.[32] The addition of a second minor oxidized component was required to properly fit the experimental data. These XPS spectra are in good agreement with data published in previous nickel iron selenides reports.[33–35] 8 a FeSe2 Ni0.75Fe0.25Se2 measured 1 calculated Intensities (a. u.) background 0.75 difference Orthorhombic (pnnm) 0.50 NiSe2 0.25 0 20 40 60 80 2 theta () NiSe2 20 30 40 50 60 70 80 28 35 48 56 Fe Ni Se 2 theta () Cubic (Pa-3) b 100% 75% c Ni. % 25% Fe. % 0% (03-1) (021) 50 nm (013) Fe Ni Se [100] NiSe2 PA-3 d Ni 3P3/2 Se 3d e 0.25 Intensities (a. u.) Intensities (a. u.) Intensities (a. u.) 1 NiOx Se-Ni(Fe) Se-O 0.75 Ni-Se sat. 0 868 864 860 856 852 64 60 56 52 1500 3000 4500 6000 Binding energy (eV) Binding energy (eV) Magnetic field (G) Figure 2. (a) XRD pattern of Ni1-xFexSe2, Rietveld refinement of the Ni0.75Fe0.25Se2 pattern (GOF = 1.27. Rw = 9.44%) and scheme of the crystal structure of NiSe2 and FeSe2. (b) HAADF-STEM image and EELS-STEM maps for Ni0.75Fe0.25Se2. Maps were obtained using the Fe L-edges at 708 eV (green), Ni L-edges at 855 eV (red) and Se L edge at 1436 eV (blue). Relative composition maps are also displayed. (c) HRTEM micrographs and indexed power spectra obtained from Ni0.75Fe0.25Se2 particles. (d) Ni 2p3/2, and Se 3d high resolution XPS spectra from Ni0.75Fe0.25Se2. (e) EPR spectra of Ni1-xFexSe2, labels indicate x. 9 EPR analysis was used to evidence the electronic interaction between Ni and Fe within Ni1-xFexSe2. Typically, Fe3+, Ni2+, and Ni0 have associated EPR signals, while Fe2+, Fe0, and Ni3+ do not.[36–38] As displayed in Figure 2e, no obvious resonance signal was detected from NiSe2 (black line). On the other hand, broad EPR signals were obtained for all the samples containing Fe, with the highest signal intensity being observed for the sample containing the lowest amount of Fe: Ni0.75Fe0.25Se2. The fluctuation of the signal intensity and position from sample to sample revealed the change of the metal valences as a function of the Ni/Fe ratio and the existence of a strong interaction between the two metals.[38] Prior to the electrocatalyst preparation, Ni1-xFexSe2 particles were precipitated and redispersed multiple times using ethanol and hexane. FTIR spectroscopy displayed the final particles to contain minimal amounts of organics on their surface (Figure S6). Electrocatalysts were prepared by mixing the cleaned Ni1-xFexSe2 particles with CB and Nafion in DI water and ethanol (see experimental section for details). The electrochemical performance of Ni1-xFexSe2-based electrodes was initially tested using a conventional three-electrode system in the potential window 1.0-1.7 V vs. RHE with a scan rate of 50 mV s-1 in an Ar-bubbled 1.0 M KOH electrolyte solution (Figure 3a). During the forward scan, a first current density peak for NiSe2 and Ni0.75Fe0.25Se2 electrodes at around 1.392 V was attributed to the transformation of Ni(OH)2 to NiOOH.[39] At a potential higher than ca. 1.55 V, a dramatic current rise associated to the production of oxygen was obtained. During the backward scan, the reduction peak at around 1.31 V, both for NiSe2 and for Ni0.75Fe0.25Se2, was associated to the Ni reduction Ni3+Ni2+. Increasing the Fe composition, the redox peaks shifted positively and became less obviously, while no redox peaks were obtained with the FeSe2 electrodes below the OER potential. 10 120 200 a b 1.0 M KOH + 0.5 M MeOH Current density (mA cm ) Current density (mA cm ) -2 -2 1.0 M KOH 160 90 0.25 0.25 120 60 0.50 0.50 80 0.75 1 0 30 40 -2 10 mA cm 0.75 0 0 0 1 1.20 1.35 1.50 1.65 1.80 1.05 1.20 1.35 1.50 1.65 1.80 E (V vs. RHE) E (V vs. RHE) Figure 3. CV curves in 1.0 M KOH without (a) and in presence of 0.5 M methanol (b) in the potential range 1.0 - 1.7 V vs. RHE and at a scan rate of 50 mV s-1. When adding 0.5 M methanol in the alkaline media, a sharp rise in the current density for NiSe2 and Ni0.75Fe0.25Se2 electrodes was obtained at around 1.40 V, i.e. after NiOOH formation (Figure 3b). On the other hand, a moderate increase of current density was obtained for the three electrodes containing larger amounts of Fe. Figure S7 displays the dependence of the MOR current density at 1.5 V on composition, after subtracting the current density delivered in 1.0 M KOH. When increasing the Fe content, the current density increased from the 15.5mA obtained for NiSe2 to the 57.6 mA cm-2 for Ni0.75Fe0.25Se2 electrodes, to later decrease to 10.9, and 1.8 mA cm-2 for Ni0.5Fe0.5Se2 and Ni0.25Fe0.25Se2 electrodes, respectively. FeSe2 electrodes delivered no MOR current. Nyquist plots of the impedance response of a Ni0.75Fe0.25Se2 electrode measured at 1.6 V and 1.5 V in 1.0 M KOH with the presence and absence of 0.5 M methanol are shown in Figure S8. At 1.6 V, where oxygen was effectively generated, this electrode exhibited a smaller semicircle in the presence of methanol (Rct of 6.5 ) than in its absence (Rct of 10.9 ). As listed in Table S1, the fitting results at 1.5 V indicated a clearly enhanced charge-transfer process in the presence of 1.0 M methanol with a Rct of only 5.4 , much smaller than that in 1.0 M KOH, 121.5 . The improved impedance responses indicated that the electrooxidation of methanol was favored over the production of oxygen at the same potential. 11 0.018 0.015 2.0 0.012 a 100 mV s -1 b 1.5 c Current (mA) 0.012 Ep Current (mA) 2 current (mA) R =0.99 1.0 0.006 0.009 0.5 Ni0.75Fe0.25Se2 0.000 10 mV s -1 0.006 0.0 -0.006 0.003 -0.5 -1 -0.012 NiSe2 50 mV s 0.000 -1.0 1.02 1.05 1.08 1.11 1.14 0 20 40 60 80 100 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 -1 E (V vs. RHE) scan rate (mV s ) E (V vs.RHE) 2.0 2.0 6.0 d 1.5 e 1.5 f Peak current (mA) Peak current (mA) 1.0 1.0 2 Current (mA) 4.5 100 mV s -1 R = 0.99  0.5 2 0.5 Ipa = 0.163 - 0.23 3.0 R = 0.99 0.0 Ipa= 0.015 + 0.20 0.0  1.5 -1 Ipc = -0.176 + 0.75 10 mV s -0.5 -0.5 2 2 R = 0.99 0.0 R = 0.99 -1.0 Ipc = -0.012 + 0.09 -1.0 -1.5 -1.5 -1.5 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 10 20 30 40 50 8 9 10 -1 1/2 -1 1/2 E (V vs. RHE) scan rate (mV s ) scan rate (mV s ) Figure 4. Electrochemical kinetics of a Ni0.75Fe0.25Se2-based electrode in 1.0 M KOH. (a) CV curves in 1.0 M KOH in the non-faradaic potential range with a scan rate of 10-100 mV s-1. (b) Linear fitting of the current as a function of scan rate. (c) CV curves in the potential range from 1.0 V to 1.60 V at a scan rate of 50 mV s-1. The curve obtained from a NiSe2-based electrode is also plotted as reference. (d) CV curves in the potential range from 1.0 V to 1.60 V with variable scan rate of 10-100 mV s-1. (e) Linear fitting of the peak current as a function of the scan rate in the scan rate range from 10 to 50 mV s-1. (f) Linear fitting of the peak current as a function of the square root of scan rate in the scan rate range from 60 to 100 mV s-1. The electrochemically active surface area (ECSA) of the catalysts was determined using the double-layer capacitance (Cdl) on the basis of CVs measured at a series of variable scan rates in the non-faradaic potential range.[40] Generally, Cdl can be calculated from the linear fit of the current (i) with the scan rate (ν): 𝑖𝑐 = 𝜈 𝐶𝑑𝑙 Then ECSA can be determined by dividing Cdl by the specific capacitance: 𝐸𝐶𝑆𝐴 = 𝐶𝑑𝑙 ⁄𝐶𝑠 where Cs is 0.04 mF cm-2 based on values reported for metal electrodes in aqueous alkaline solution.[41] Ni0.75Fe0.25Se2 electrodes displayed the largest ECSA values among the series tested, 62.5 cm2 g-1, threefold above the value obtained for NiSe2 electrodes, 18.75 cm2 g-1 (Figure S9). These values were below those reported for branched Ni3C particles, 108.6 cm2 g-1, [26] and 12 Ni97Bi3 aerogels, 176.6 cm2 g-1, [42] but well above those of branched Ni0.75Cu0.25, 0.12 cm2 g-1, [43] and NiO nanotubes, 36.25 m2 g‒1. [44] In the faradaic region, the potential difference between the Ni(OH)2NiOOH redox peaks (∆𝐸𝑝) is related to the electron transfer kinetics between the electrode surface and the active centers.[45] As shown in Figure 4c, Ni0.75Fe0.25Se2 electrodes displayed the lowest ∆𝐸𝑝, 65 mV compared with the 91 mV obtained for NiSe2, demonstrating the addition of Fe within the NiSe2 lattice to improve the reaction kinetics. Figure 4d displays CV curves at different scan rates in the range 10-100 mV s-1 for a Ni0.75Fe0.25Se2 electrode. While the peak current increased with the scan rate, the anodic peak potential shifted positively and the cathodic negatively. Qualitatively similar changes in the peak current and potential were observed for NiSe2 electrodes (Figure S10a). The shift of the peak potential is attributed to an electrochemical polarization and the limited reaction kinetics that control the formation of NiOOH species.[45] The electrochemical performance of Ni-based electrodes is generally improved with the surface coverage of Ni(OH)2/NiOOH redox species (∗ ).[46] From the linear fit of the anodic and cathodic peak current (Ip) with the sweep rate (𝑣) from 10 to 50 mV s-1 in the potential range of 1.0-1.6 V, the surface coverage of redox species ( ∗ ) was estimated using the following equation: 𝑛2 𝐹 2 𝐼𝑝 = ( ) 𝐴 ∗ 𝑣 4𝑅𝑇 where n is the number of transferred electrons (assumed to be 1), F is the Faraday constant (96,845 C mol-1), R is the gas constant (8.314 J K-1 mol-1), T is absolute temperature (295 K) and A is the geometric surface area of the GC electrode (0.196 cm2). Averaging the results obtained from the forward and reverse scans, ∗ of Ni0.75Fe0.25Se2 electrodes was 7.3 × 10-8 mol cm-2 (Figure 4e), slightly larger than ∗ obtained for NiSe2, 5.7 × 10-8 mol cm-2 (Figure S10b). These values of ∗ were in agreement with those reported previously for NiCo2Sn2 (4.1× 10-8 mol cm-2),[47] Ni97Bi3 (5.6 × 10-7 mol cm-2),[42] Ni-C-30 (8.9 ×10-8 mol cm-2),[48] and Ni nanoparticles (1.9× 10-7 mol cm-2). [49] 13 We further investigated the electrochemical activities based on the proton diffusivity (D) within the electrodes according to the Randles–Sevcik equation.[50] For Ni-based electrodes, the proton diffusion is generally regarded as a rate-limiting step that controls the Ni(OH)2NiOOH redox reaction.[51] In the scan rate range 60-100 mV s-1 (Figure 4f), the anodic and cathodic peak current (Ip) was found to be a linear function of the square root of scan rate (v1/2), inferring a diffusion-limited redox reaction. Thus, the diffusion coefficient (D) could be determined from the following equation: 𝐼𝑝 = 2.69 × 105 𝑛3⁄2 𝐴𝐷1⁄2 𝐶𝑣 1⁄2 where n and C are the electron transfer number (assumed to be 1) and the initial concentration of redox species, respectively. Using this equation, the proton diffusion coefficient for the Ni0.75Fe0.25Se2 electrode was estimated at 2.2×10-10 cm2 s-1, an order of magnitude above that of the NiSe2 electrode, 5.7×10-11 cm2 s-1 (Figure S10c), which pointed at a much faster diffusion of the redox limiting specie in the sample containing Fe. The diffusion coefficient calculated for Ni0.75Fe0.25Se2 was in the same order of magnitude as that of branched Ni3C particles (6.0 × 10-9 cm2 s-1),[26] branched Ni0.75Cu0.25 (2.9 × 10-8 cm2 s-1),[43] Ni-B nanoparticles on nanoporous Cu electrode (4.8 × 10-9 cm2 s-1),[52] and nanoporous Ni-Cu-P alloys (3.65 ×10-10 cm2 s-1).[53] 80 80 a b c Current density (mA cm ) Current density (mA cm ) -2 -2 Cmethanol 1.80 60 lg(J[mA cm )] 1.0 M 60 -2 40 1.65 40 0.1 M 2 R =0.989 20 1.50 [1.0 M KOH] 20 -1 lg(J)=0.56lg(Cmethanol)+2.02 0 50 mV s @1.5V vs. RHE 1.35 0 1.0 1.1 1.2 1.3 1.4 1.5 1.6 0.0 0.5 1.0 1.5 2.0 2.5 3.0 -0.9 -0.6 -0.3 0.0 0.3 E (V vs. RHE) Cmethanol (M) lg[Cmethanol(M)] 1.60 1.0 36 105 d e 10000s f Faradaic Efficiency (%) 1.55 0.8 33 100 5 mA - Potential (mV) E (V vs. RHE) HCOO Current (mA) 1.50 98.2% 98.7% 98.0% 99.4% 99.2% 0.6 30 95 1.45 2000s 0.4 27 90 1.40 5.0 5.5 Time (min) 1.35 1.48 V 0.2 24 85 - - 2- H2 O F Cl SO4 1.30 0.0 21 80 0 2 4 6 8 10 0 5 10 15 20 25 s s s 0s s 00 00 00 00 00 3 Time (10 s) Time (min) 40 60 80 20 10 Figure 5. Performance of the Ni0.75Fe0.25Se2-based electrode. (a) CV curves in 1.0 M KOH electrolyte in the potentials range from 1.0 V to 1.5 V at a scan rate of 50 mV s-1 with the 14 presence of different methanol concentration from 0.1 M to 1.0 M. (b) Current density at 1.5 V in 1.0 M KOH containing 0.1 M to 3.0 M methanol. (c) Logarithmic dependence of the current density at 1.50 V in 1.0 M KOH solution with the methanol concentration from 0.1 M to 3.0 M. (d) CP profile at a constant current of 5 mA in 1.0 M KOH containing 1.0 M methanol and CA response at 1.48 V during 10,000 s testing. (e) IC profile in 1.0 M KOH containing 1.0 M methanol during 10,000 s CP testing (5 mA) obtained at 2,000 s intervals. (f) Calculated Faradaic efficiency for the methanol-to-formate conversion at 2,000 s, 4,000 s, 6,000 s, 8,000 s, and 10,000 s times during CP test. Figure 5a displays CVs (50 mV s-1) of Ni0.75Fe0.25Se2-based electrodes in a 1.0 M KOH electrolyte containing different methanol concentrations, from 0.1 M to 1.0 M. The current density associated to the MOR clearly increased with the methanol concentration, until ca. 0.3 M. Figure 5b displays current density vs. methanol concentration at 1.5 V. The current density initially increased with the methanol concentration, from 28.6 mA cm-2 at 0.1 M, up to 51.8 mA cm-2 at 0.3 M, and it was stabilized at around this value for higher methanol concentrations, up to 3.0 M. A linear fit of the logarithmic dependence of the current density vs. methanol concentration in the low concentration range indicated the methanol reaction order to be ca. 0.6 (Figure 5c). Averaging the current density obtained from 5 Ni0.75Fe0.25Se2–based electrodes fabricated from different batches of particles, we calculated a current density of 59.7 mA cm-2 at 1.5 V, which decreased to 53.5 mA cm-2 after subtracting the current density in 1.0 M KOH (Figure S11). This value corresponds to 2.2 A mg-1 (considering the mass of catalyst loaded on the GC), which is the highest MOR activity reported for a Ni-based electrode to date (Table 2). Table 2. Electrocatalytic performance comparison between the Ni0.75Fe0.25Se2 electrodes and other Ni-based electrodes previously reported Activities Electrocatalyst Electrolyte (1.5 V vs RHE) Reference -2 -1 mA cm A mg Ni/rGO 1.0 M KOH + 1.0 M methanol 1.5 [54] Ni-CNT/SiC 1.0 M NaOH + 1.0 M methanol 31.2 1.9 [55] (110)-faceted Ni 1.0 M KOH + 1.0 M methanol 41.1 1.4 [49] Ni/beta- zeolite 0.1 M NaOH + 0.1 M methanol 7.5 0.2 [56] NiO layer/CNT 1.0 M KOH + 0.5 M methanol 56.7 [57] defect-rich NiO 1.0 M KOH + 0.5 M methanol 24.3 0.1 [44] NiO/Ni heterostructures 1.0 M KOH + 2.0 M methanol 9.4 1.8 [58] 15 Ni2Co2 1.0 M NaOH + 0.5 M methanol 18.5 [19] Ni2.5Co0.5Sn2 1.0 M KOH + 1.0 M methanol 38.6 0.6 [47] NiCo/NiO-CoO/carbon 0.5 M NaOH + 0.5 M methanol 75.6 [59] NiMoO4 1.0 M KOH + 2.0 M methanol 11.2 [60] Ni0.75Cu0.25 1.0 M NaOH + 0.5 M methanol 23.6 0.1 [43] Cu-Ni/CNT 1.0 M NaOH + 3.0 M methanol 0.3 [21] 1D Cu/NiCu 1.0 M KOH + 1.0 M methanol 25.1 0.6 [61] Fe-Ni core/shell 1.0 M NaOH + 1.0 M methanol 6.5 1.4 [27] Ni0.97Bi0.03 1.0 M KOH + 1.0 M methanol 0.8 [42] Ni-B amorphous alloy/Cu 1.0 M KOH + 0.3 M methanol 66.1 [52] 3D Ni-P-O 0.5 M KOH + 1.0 M methanol 1.1 [62] Branched Ni3C 1.0 M KOH + 1.0 M methanol 51.2 1.6 [26] NiSe/Ni 1.0 M KOH + 0.5 M methanol 132 [28] Ni0.75Fe0.25Se2 1.0 M KOH + 1.0 M methanol 53.5 2.2 This work Figure 5d displays results from CP test at 5 mA in a 1.0 M KOH electrolyte containing 1.0 M methanol. A relatively stable potential, in the range 1.45-1.47 V, was required to maintain a stable current during the 10,000 s test. Figure 5d displays the low current obtained during a CA test at 1.48 V in 1.0 M KOH without methanol, demonstrating the current measured during CP to be directly related to the methanol oxidation. The reaction products were quantified by analyzing the ion type and concentration during CP test by means of IC (Figure 5d). As shown in Figure 5e, several peaks were identified at 2.7, 4.1, 5.2, 6.9, and 21 min, which were ascribed to water, F-, HCOO-, Cl-, and SO42- respectively. Notice how the largest peak corresponded to formate, which generation involved 4 electrons in the overall reaction in alkaline media:[6] 𝐶𝐻3 𝑂𝐻 + 5𝑂𝐻 − → 𝐻𝐶𝑂𝑂− + 𝐻2 𝑂 + 4𝑒 − As displayed in the inset in figure 5e, the formate concentration increased with the CP time, i.e. with the total amount of charge flown (Table S1). From these values, the Faradaic efficiency of the methanol-to-formate conversion was calculated to be close to 99% (Figure 5f). In theseconditions, the production rate of formate was 0.23 mmol cm-2 h-1. The stability of Ni0.75Fe0.25Se2 electrodes was further investigated by means of CP, CA and continuous CVs in 1.0 M KOH electrolyte with 1.0 M methanol. Figure 6a displays a CP 16 profile at 50 mA cm-2. The initial voltage was 1.46 V, and after 50,000 s CP test this value increased by just ca. 50 mV. After this test, 1.27 mmol formate had been electrochemically produced through the injection of 500 C of charge, which corresponded to a Faradaic efficiency of 98.2% and a production rate of 0.47 mmol cm-2 h-1. CA analysis at 1.50 V displayed the current density of the Ni0.75Fe0.25Se2 electrode to gradually decrease. While a 27.4% lower current density was obtained after 50,000 s, the retained current density (38.9 mA cm-2) was still well above the initial current densities of most Ni-based MOR catalysts previously reported (Table 2). As shown in the inset of Figure 6b, after the long-term electrocatalytic process (50,000 s CA) in alkaline media and at high potential, the catalyst particle size and morphology remained constant.[63–65] CV profiles showed a gradual current density decay in the potential window 1.0-1.5 V upon long term cycling (figure 6c). The current density at 1.5 V retained 91.9%, 83.9%, 74.0%, 67.7%, and 61.5% of the initial value after 200, 400, 600, 800, and 1000 cycles, respectively (Figure 6d). 1.6 70 a b Current density (mA cm ) 1.51 V -2 -2 60 1.5 [50 mA cm ] 50 E (V vs. RHE) 40 Potential (mV) 80 1.27 mmol 1.4 1.46 V 60 30 40 20 1.3 20 4.9 5.6 10 100 nm Time (min) 1.2 0 1 2 3 4 5 0 1 2 3 4 5  Time ( s) 4 Time (10 s) 80 80 c d Current density (mA cm ) Current density (mA cm ) [1.0 M methanol] -2 -2 @1.5 V vs. RHE 60 60 st 40 1 CV 100% 40 91.9% 83.9% 74.0% 67.7% 20 61.5% 1000 CV th 20 0 100 mV s -1 0 st th th th th th 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1 200 400 600 800 1000 E (V vs. RHE) Number of CVs Figure 6. Stability test of the Ni0.75Fe0.25Se2-based electrode in 1.0 M KOH with 1.0 M methanol. (a) CP profile during 50,000 s operation at a constant current density of 50 mA cm-2 The IC profile obtained from the solution after completing the CP test is also plotted as inset. 17 (b) CA response at 1.5 V during 50,000 s test. The inset shows the morphology of the catalyst after the test. (c) Continuous 1,000 CVs cycling in the potential range of 1.0-1.5 V. (d) The corresponding current density change at the 200th, 400th, 600th, 800th, and 1000th CV compared with the initial CV at 1.5 V. Conclusion In summary, we reported a fast solution-based route to produce nickel iron diselenide nanorods. The materials were thoroughly characterized and their performance toward MOR was tested. We found the incorporation of iron into the NiSe2 structure to modify the material electronic structure and to significantly improve the electrocatalytic performance, including ECSA, surface coverage of active species for MOR, and diffusivity of limiting species in alkaline media. The optimal composition was found to be Ni0.75Fe0.25Se2. Electrodes based on Ni0.75Fe0.25Se2 nanorods delivered a current density up to 53.5 mA cm-2, corresponding to 2.2 A mg-1, in 1.0 M KOH electrolyte containing 1.0 M methanol at 1.5 V. These electrodes were able to produce 0.47 mmol cm-2 h-1 of formate at 50 mA cm-2 with a Faradaic conversion efficiency of 99 % and to continuously work for over 50,000 s with a low efficiency loss. Taking into account the economic advantages of using a noble-metal-free catalyst, its high Faradaic efficiency, and ambient operation, the electrochemical conversion of methanol to formate might indeed become an efficient approach to add value to biomass-derived chemicals. Author Information Corresponding Author * Prof. Andreu Cabot: acabot@irec.cat * Prof. Chunhua Cui: chunhua.cui@uestc.edu.cn Author contributions The manuscript was contributed and authorized by all authors. C. Cui and A. Cabot conceived and guided the project, and supervised the work. J. Li designed the experiments, performed electrochemical measurements, and wrote the first draft. C. Xing and Y. Zhang produced the NRs, conducted XRD. M. C. Spadaro, T. Zhang, and J. Arbiol conducted structural and 18 compositional NRs characterization by means of HRTEM and EELS, and discussed the results. Q. Wu and Y. Yu provided assistance to the EPR and IC measurement, respectively. S. He significantly joined the results discussion. J. Llorca measured and discussed XPS data. This manuscript was corrected and improved by all authors. Acknowledgements J. Li obtained International Postdoctoral Exchange Fellowship Program (Talent-Introduction program) in 2019 and is grateful for the project (2019M663468) funded by the China Postdoctoral Science Foundation. This work was supported from the UESTC start-up funding and the Recruitment Program of Thousand Youth Talents. It was also supported by the European Regional Development Funds and by the Spanish Ministerio de Economí a y Competitividad through the project SEHTOP (ENE2016-77798-C4-3-R) and VALPEC (ENE2017-85087-C3). C. Xing, Y. Zhang, T. Zhang, and X. Wang are grateful for the China Scholarship Council (CSC) for scholarship support. M. C. Spadaro, T. Zhang, and J. Arbiol acknowledge funding from Generalitat de Catalunya 2017 SGR 327. ICN2 acknowledges support from the Severo Ochoa Programme (MINECO, Grant no. SEV-2013-0295). IREC and ICN2 are funded by the CERCA Programme / Generalitat de Catalunya. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program. J. Llorca is a Serra Húnter Fellow and is grateful to MICINN/FEDER RTI2018-093996-B-C31, GC 2017 SGR 128 and to ICREA Academia program. Conflict of interest The authors declare no competing financial interest. Reference [1] Y. Gogotsi, P. 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