This is the submitted version of the journal article: Li, Junshan; Tian, Xi; Wang, Xiang; [et al.]. «Electrochemical conversion of alcohols into acidic commodities on nickel sulfide nanoparticles». Inor- ganic Chemistry, Vol. 61, issue 34 (Aug. 2022), p. 13433-13441. DOI 10.1021/acs.inorgchem.2c01695 This version is available at https://ddd.uab.cat/record/270843 under the terms of the license Page 1 of 23 Submitted to Inorganic Chemistry 1 2 3 4 Electrochemical Conversion of Alcohols into Acidic 5 6 7 Commodities on Nickel Sulfide Nanoparticles 8 9 Junshan Li,a Xi Tian,b Xiang Wang,c Ting Zhang,d Maria Chiara Spadaro,d Jordi Arbiol,d,e 10 Luming Li,b* Yong Zuo,c* Andreu Cabotc,e* 11 12 a Institute for Advanced Study, Chengdu University, 610106, Chengdu, China 13 14 b College of Food and Biological Engineering, Chengdu University, Chengdu 610106, China 15 c Catalonia Institute for Energy Research - IREC, Sant Adrià de Besòs, Barcelona, 08930, 16 17 Catalonia, Spain. 18 d Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus 19 UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain. 20 21 e ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain. 22 23 24 Abstract: The electrocatalytic oxidation of alcohols is a potentially cost-effective strategy for 25 26 the synthesis of valuable chemicals at the anode while simultaneously generating hydrogen at 27 28 the cathode. For this approach to become commercially viable, high activity, low-cost, and 29 stable catalysts need to be developed. Herein, we demonstrate an electrocatalyst based on 30 31 earth-abundant nickel and sulfur elements. Experimental investigations reveal the produced 32 33 NiS displays excellent electrocatalytic performance associated with a higher electrochemical 34 35 surface area (ECSA) and the presence of sulfate ions on the formed NiOOH surface in basic 36 37 media. The current densities reached for the oxidation of ethanol and methanol at 1.6 V vs. 38 39 RHE are up to 175.5 and 145.1 mA cm-2, respectively. At these high current densities, the 40 Faradaic efficiency of methanol to formate conversion is 98%, and that of ethanol to acetate is 41 42 81%. Density functional theory calculations demonstrate the presence of the generated sulfate 43 44 groups to modify the electronic properties of the NiOOH surface, improving 45 46 electroconductivity and electron transfer. Besides, calculations are used to determine the 47 48 reaction energy barriers, revealing the dehydrogenation of ethoxy groups to be more 49 50 favourable than that of methoxy on the catalyst surface, which explains the highest current 51 densities obtained for ethanol oxidation. 52 53 54 Keywords: nickel, nickel sulfide; methanol oxidation reaction; ethanol oxidation reaction; 55 56 electrochemical reforming; hydrogen, electrocatalysis. 57 58 1 59 60 ACS Paragon Plus Environment Submitted to Inorganic Chemistry Page 2 of 23 1 2 3 Introduction 4 5 6 With the increasing concern about global warming and environmental pollution, the search 7 8 for clean energy carriers as an alternative to fossil fuels has intensified.1 In this context, 9 hydrogen is considered an ideal energy carrier with zero carbon emissions.2 However, 10 11 hydrogen production still relies on the use of fossil fuels. Thus alternative hydrogen 12 13 production strategies and hydrogen sources are required.3 14 15 Thermodynamics imposes a minimum of 1.23 V to drive the electrocatalytic water splitting, 16 17 but practical water electrolysis requires no less than 1.8 V due to the sluggish kinetics of the 18 19 oxygen evolution reaction (OER) at the anode. Actually, the largest part of the additional 20 21 energy consumed in the process is related to the production of oxygen, which is generally a 22 23 discarded product owing to its low value.4 To increase the process efficiency, more 24 25 favourable anode reactions based on the oxidation of organic molecules, such as alcohols and 26 amines, have been proposed to replace OER.5–8 This strategy not only largely promotes the 27 28 hydrogen evolution reaction (HER) at the cathode, but also generates a valuable side-product 29 30 at the anode. Indeed, the selective oxidation of some of these organic molecules results in 31 32 high purity commodity chemicals. This green chemical synthesis route at ambient temperature 33 34 can be competitive with traditional industrial processes requiring high temperatures and 35 36 pressures to produce some electrocatalylitically-accessible chemicals. This is the case of 37 formate for instance, with an annual demand of 100 Mt.9–11 38 39 40 For the electrochemical process to become cost-effective, noble-metal-free and durable 41 42 electrocatalysts need to be developed. Among other Earth-abundant elements, nickel-based 43 44 catalysts have been reported to exhibit excellent electrocatalytic performances towards a 45 plethora of electrocatalytic reactions, including the electro-oxidation of Cn alcohols (C1 = 46 47 methanol, C2 = ethanol).12–14 48 49 50 Compared with monometallic Ni electrocatalysts, the use of Ni chalcogenides has been 51 52 demonstrated to improve performance toward the OER15–18, methanol oxidation reaction 53 (MOR)19–21 and ethanol oxidation reaction (EOR),22–25 among others.26–28 This improvement 54 55 is generally ascribed to a bulk-driven modulation of the surface electronic structures of the Ni 56 57 58 2 59 60 ACS Paragon Plus Environment Page 3 of 23 Submitted to Inorganic Chemistry 1 2 3 catalyst, accelerating the metal regeneration in an alkaline medium. In addition, the presence 4 5 of the ions produced from the chalcogen oxidation, e.g. sulfates, can change the local 6 7 coordination environment of the active species regulating the adsorption of the 8 9 reactants/intermediates/products.29 10 11 To maximize cost-effectiveness, catalysts need to be produced in the form of nanometric-size 12 13 particles using cost-effective and scalable methods. In this direction, the Brutchey group has 14 15 pioneered an “alkahest solvent” method for the high-throughput synthesis of chalcogenide 16 17 nanoparticles (NPs). The alkahest solvent mixture is composed of ethanedithiol (Edt) and 18 19 ethylenediamine (En) to dissolve a series of V2VI3 chalcogenides, such as As2S3, Sb2S3, 20 Bi2S3, Bi2Se3, Bi2Te3 at room temperature, and then recover them in the form of 21 22 nanostructured materials after annealing.30 Such amine-thiol solution dissolves bulk 23 24 chalcogens resulting in alkyl di-, tri- and tetrachalcogens, with –SR species binding to the 25 26 metal ions, through an amine-catalyzed reaction.30–32 In this amine-thiol system, the main 27 28 difficulty is to prevent the formation of secondary phases, which generally requires proper 29 30 control of the amine-thiol ratio and the reaction conditions.33 Besides the metal precursor 31 counter ions, eg, Cl-, also affect the purity of recovered material, thus pure elemental 32 33 precursors are generally chosen to avoid contamination.33 34 35 36 Herein, we detail the synthesis of nickel sulfur NPs using amine-thiol solutions and study 37 their performance towards ethanol and methanol oxidation at high current density. To gain an 38 39 understanding of the obtained results, we also conduct DFT calculations. We particularly 40 41 analyze the effect of sulfate on the surface electronic structure and compare the mechanism of 42 43 C1 and C2 alcohol oxidation on the same electrocatalysts, which has been very rarely 44 45 reported.13,34–36 The obtained results help to understand why ethanol can be more easily 46 47 oxidized than methanol. 48 49 50 51 52 53 54 55 56 57 58 3 59 60 ACS Paragon Plus Environment Submitted to Inorganic Chemistry Page 4 of 23 1 2 3 Experimental Section 4 5 6 Chemicals. All chemicals, including nickel powder (Ni, spherical, -400 mesh, 98 %, Alfa 7 8 Aesar, nickel(II) acetylacetonate (Ni(acac)2 2H2O, 95%, Sigma-Aldrich), sulfur powder (S, 9 99.98%, Sigma-Aldrich), ethylenediamine (En, 99.5%, Sigma-Aldrich), 1,2-ethanedithiol 10 11 (Edt, 98%, Sigma-Aldrich), potassium hydroxide (KOH, 85%, Fisher), tri-n-octylphosphine 12 13 (TOP, 97%, Strem), oleylamine (OAm, 80-90%, TCI), hydrazine monohydrate (N2H4, 14 15 64-65%, reagent grade, 98%, Sigma-Aldrich), potassium carbonate (K2CO3, 99.5%, Aldrich), 16 17 potassium bicarbonate (KHCO3, 99.7%, Aldrich), trioctylphosphine oxide (TOPO, 99%, 18 19 Sigma-Aldrich), carbon black (CB, Vulcan XC72, Sigma Aldrich), and Nafion (10 wt.%, 20 perfluorinated ion-exchange resin, dispersion in water, Sigma-Aldrich) were used as received 21 22 without any further treatment. All chemical manipulation and ink preparation processes were 23 24 carried out in an Ar-filled glovebox. Analytical grade chloroform and ethanol for the washing 25 26 process were obtained from various sources. MilliQ water (18.2 #J cm) was used for 27 28 electrolyte and ink preparation, and ion chromatography (IC) measurements. Caution: 29 30 1,2-ethanedithiol would release an unpleasant odour and should be carefully handled only in 31 the fume hood and glovebox. 32 33 34 NiS synthesis. Three steps were involved in the synthesis of NiS NPs (Scheme 1): i) Ink 35 36 Preparation. To prepare a L molecular ink, elemental Ni (1 mmol) and S (1 mmol) 37 powders were weighed and added to a glass vial (10 mL). En (3.3 mL) and Edt (0.3 mL) were 38 39 sequentially added to the vial. The mixture was stirred at 750 rpm at room temperature for 24 40 41 hrs. These operations were conducted in the Ar-filled glovebox. ii) Solvent evaporation to 42 43 obtain a dry powder. The obtained Ni-S molecular ink was filtered using a PTFE filter (0.45 44 45 µm) to remove the insoluble particles, and swiftly injected into a preheated (250 °C) 46 47 three-neck flask. Ar was introduced into the flask from one neck, while the other neck was 48 connected to the vacuum to quickly remove the vapour coming from the ink. A safety flask 49 50 containing water/ethanol was placed between the vacuum and reaction flask to collect the 51 52 vapour emanating from the ink solution. Upon ink injection, the temperature decreased and 53 54 then increased slowly to 250 °C. The flask was then kept at this temperature to obtain a dry 55 56 powder (generally within 10 min). The product was then grounded into fine powder for the 57 58 4 59 60 ACS Paragon Plus Environment Page 5 of 23 Submitted to Inorganic Chemistry 1 2 3 following annealing treatment. iii) Annealing of dry powder. The collected dry powder was 4 5 placed in a tubular furnace under Ar atmosphere at 300 °C for 1 h, and then at 400 °C for 6 7 additional 30 mins. A ramp rate of 3 °C/min was adopted. The annealed powder was collected 8 9 and stored in the glovebox for further use. 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Scheme 1. Protocol for the synthesis of NiS NPs through an amine-thiol system. 30 31 Ni synthesis: Ni NPs were prepared following the procedure described in previous 32 33 literature.37 Typically, a three-neck flask was loaded with a magnetic bar, 1 mmol Ni(acac)2, 7 34 ml OAm, 0.4 mmol TOP and 0.25 mmol TOPO, and the temperature was kept at 130 °C for 35 36 30 min with a gentle flow of argon. Then, the flask was quickly heated to 215 °C and 37 38 maintained at this temperature for 45 min. Subsequently, the flask was cooled down to room 39 40 temperature using a water bath. The black precipitate was separated through centrifugation 41 42 after adding ethanol. To further remove the residual ligands on the surface before exploring 43 44 the electrochemical performance, the precipitated NPs were dispersed in a mixture containing 45 28 mL acetonitrile and 0.8 mL hydrazine hydrate and stirred for at least 2 hours. The NPs 46 47 were then collected by centrifugation and washed with acetonitrile 3 times. Finally, the Ni 48 49 NPs were dried under vacuum. 50 51 52 Characterization: X-ray diffraction (XRD) was used to study the crystal structure on a 53 Bruker AXS D8 Advance (Cu K radiation: Q = 1.5106 Å). Transition electron microscopy 54 55 (TEM) images were obtained using ZEISS LIBRA 120, operating at 120 kV. High-resolution 56 57 58 5 59 60 ACS Paragon Plus Environment Submitted to Inorganic Chemistry Page 6 of 23 1 2 3 TEM (HRTEM) together with scanning TEM (STEM) was investigated on a field emission 4 5 gun FEI Tecnai F20 microscope. High angle annular dark-field (HAADF) STEM was 6 7 combined with electron energy loss spectroscopy (EELS) in the Tecnai microscope by using a 8 9 GATAN QUANTUM filter. The representative atomic model has been created with the 10 11 Rhodius software.38–40 X-ray photoelectron spectroscopy (XPS) analyses were conducted on a 12 SPECS system. 13 14 15 Electrochemical measurements: Electrochemical performance was studied using a 16 17 conventional three-electrode system, including a Hg/HgO reference electrode (RE), a Pt wire 18 19 as the counter electrode (CE), and a 5 mm diameter glassy carbon (GC) as the working 20 electrode (WE). To prepare the WE, firstly 5 mg of dried NPs and 10 mg of CB were 21 22 dispersed in a vial containing 2 ml of MilliQ water/ethanol (v/v = 1:1) and 100 T of a 10 wt% 23 24 Nafion solution, followed by 1 h vigorous sonication. Afterwards, 5 T of prepared ink were 25 26 drop-casted on the top of a carefully polished GC electrode, and then allowed to dry naturally 27 28 in the open air. Before the measurement, the alkaline electrolyte was bubbled with high-purity 29 30 N2 gas for half an hour. Conventional voltammetry techniques, including cyclic voltammetry 31 (CV) and chronoamperometry (CA), were used to study the electrocatalytic property of the 32 33 NPs. Electrochemical impedance spectra (EIS) were obtained in 1 M KOH with or without 34 35 the presence of 1 M alcohols from 105 to 10-1 Hz. All obtained potentials (vs. Hg/HgO) were 36 37 converted to the reversible hydrogen electrode (RHE) reference according to the Nernst 38 39 equation: ERHE = EHg/HgO + 0.059xpH + EVHg/HgO, where EHg/HgO is the measured potential, 40 41 EVHg/HgO is the reference potential of 0.098 V, and pH is 13.6 as it corresponds to the actual 42 value in 1 M KOH solution. To analyze the product of the oxidation reaction at the anode and 43 44 calculate the faradaic efficiency, 0.5 mL of the solution was collected, diluted in 8 mL MilliQ 45 46 water, and then analyzed by IC. A freshly prepared solution containing 4.5 mM KCO3 and 0.8 47 48 mM KHCO3 was used as leachate solution during the IC measurement. The amount of 49 50 product was quantified with a standard curve based on the known ion concentrations. Then, 51 × × 52 the Faradaic efficiency (FE) was calculated using the equation: (%) = 53 54 × 100%, where n is the electron transfer number, F is the Faradaic constant (96485 C mol-1), 55 56 and the total charge passed is the electrons passed through the anode during the testing period. 57 58 6 59 60 ACS Paragon Plus Environment Page 7 of 23 Submitted to Inorganic Chemistry 1 2 3 Computational method: All the density functional theory (DFT) calculations were 4 5 performed on the Vienna ab initio package (VASP) within the generalized gradient 6 7 approximation (GGA) using the PBE formulation.41–43 The projected augmented wave (PAW) 8 9 potential was used to describe the ionic cores and valence electrons were taken into account 10 11 using a plane-wave basis set with a kinetic energy cutoff of 400 eV.44,45 Partial occupancies of 12 the H L orbitals were allowed using the Gaussian smearing method and a width of 13 14 0.05 eV. The electronic energy was considered self-consistent when the energy change was 15 16 smaller than 10L4 eV. Geometry optimization was considered convergent when the force 17 18 change was smaller than 0.02 eV/Å. Grimme’s DFT-D3 methodology was used to describe 19 20 the dispersion interactions.46 21 22 Using a 7×11×9 Monkhorst-Pack k-point grid for Brillouin zone sampling, the equilibrium 23 24 lattice constants of the monoclinic NiOOH unit cell were optimized to be a=5.168 Å, b=2.847 25 26 Å, c=4.516 Å, at YC7BO ZC B3* O [C7BO* They were used to construct a NiOOH(001) surface 27 28 model with p(2×3) periodicity in the x and y directions and 1 stoichiometric layer in the 29 30 z-direction separated by a vacuum layer in the depth of 15 Å to separate the surface slab from 31 its periodic duplicates. One O atom was removed to create an oxygen vacancy. During 32 33 structural optimizations, the gamma point in the Brillouin zone was used for k-point sampling, 34 35 and all atoms were allowed to relax. 36 37 The free energy of a gas phase molecule or an adsorbate on the surface was calculated by the 38 39 equation G = E + ZPE L TS, where E is the total energy, ZPE is the zero-point energy, T is 40 41 the temperature in kelvin (298.15 K), and S is the entropy. 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 7 59 60 ACS Paragon Plus Environment Submitted to Inorganic Chemistry Page 8 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment Page 9 of 23 Submitted to Inorganic Chemistry 1 2 3 along its hexagonal [2-1-13] zone axis. The corresponding atomic supercell model of the NiS 4 NP is also displayed and its 1*1*1 unit crystal model is also identified. 5 6 XPS spectra were collected to study the surface composition and atomic valence states 7 8 (Figure S2). The Ni 2p3/2 XPS spectrum displayed three peaks, at 852.2 eV, 855.0 eV and 9 10 860.6 eV, which were associated with Ni within the NiS lattice, a Ni oxide or sulfoxide phase 11 12 and a satellite peak, respectively.47 The S 2p XPS spectrum displayed three doublets with the 13 14 S 2p peak at 167.5 eV, 163.1 eV, and 160.9 eV that were assigned to sulfur oxides, NiS and 15 surface sulfur or S-C, respectively.48 These fitting characteristics of the XPS spectra are in 16 17 good agreement with previous reports based on nickel and sulfur elements.27,49–51 18 19 20 To compare the electrocatalytic performance of NiS NPs with monometallic Ni, spherical Ni 21 22 NPs with an average diameter of 11 nm were synthesized according to a previous report.37 23 The XRD pattern and representative TEM images of the obtained particles can be found in 24 25 Figure S3. 26 27 28 Working electrodes were prepared by drop-casting an ink containing a composite of Ni or NiS 29 30 NPs and carbon black on a glassy carbon electrode. Using a conventional three-electrode 31 system, the electrocatalytic activity of both electrodes was first studied in 1 M KOH solution 32 33 via CV at a scan rate of 50 mV s-1 in the applied potential range of 0.9-1.7 V vs. RHE (black 34 35 curves in Figure 2a for NiS, and Figure S4 for Ni). 36 37 In alkaline media, the surface of the Ni and NiS is hydroxidized into Ni(OH)2 (equation 1).52 38 39 Upon increasing the external potential, the Ni(OH)2 oxidizes to NiOOH (equation 2). This 40 41 oxidation took place at 1.32 V for both Ni- and NiS-based electrodes. Subsequently, a distinct 42 43 peak was recorded at 1.34 V for the NiS (Figure 3a), and 1.43 V for the Ni electrode (Figure 44 45 S4). When increasing the potential above ca. 1.65 V, oxygen started to evolve (equation 3). 46 47 Ni + 2OH- Ni(OH)2 + 2e- (1) 48 49 50 Ni(OH)2 + OH- NiOOH + H2O + e- (2) 51 52 NiOOH + OH- Ni(OH)2 + O2 + e- (3) 53 54 55 By adding 1 M methanol into the 1 M KOH electrolyte (olive curves), for NiS a sharp 56 increase in the current density was obtained at ca. 1.342 V, just after NiOOH was formed. In 57 58 9 59 60 ACS Paragon Plus Environment Submitted to Inorganic Chemistry Page 10 of 23 1 2 3 the presence of 1 M ethanol (blue curves), the current density started to increase at ca. 1.311 4 5 V in NiS, also along with the oxidation of Ni(OH)2 to NiOOH. With the reference Ni 6 7 electrode, the oxidation potential of ethanol was also lower than that of methanol for both 8 9 electrodes (Figure S4).53 10 11 NiS NPs 12 200 13 [1 M] 160 EtOH 160 MeOH a b 1.0 M c 14 150 1.0 M J (mA cm ) EtOH 120 120 -2 J (mA cm ) J (mA cm ) 15 -2 -2 100 80 80 16 MeOH 40 40 17 50 -2 0.1 M 0.1 M KOH 18 10 mA cm 0 [1 M KOH] 0 [1 M KOH] 0 19 -40 -40 1.0 1.2 1.4 1.6 1.8 1.0 1.2 1.4 1.6 1.8 1.0 1.2 1.4 1.6 1.8 20 E (V vs. RHE) E (V vs. RHE) E (V vs. RHE) 21 300 22 160 d EtOH 2.2 e f lg(J[mA cm )] 23 240 -2 ROEtOH=0.84 120 2.0 J (mA cm ) -Z'' ( ) 24 1 M KOH -2 MeOH 180 10 1 M EtOH 1 M MeOH 25 80 1.8 -Z'' ( ) ROMeOH=0.66 120 5 26 40 1.6 60 27 @1.6V vs RHE 0 6 Z' ( ) 12 0 1.4 0 28 0.0 0.2 0.4 0.6 0.8 1.0 -0.9 -0.6 -0.3 0.0 0 60 120 180 240 300 29 CR-OH (M) lg[CR-OH(M)] Z' ( ) 30 31 Figure 2. Electrochemical performance of a NiS-based electrode in 1 M KOH electrolyte. (a) 32 CV curves without and with 1 M methanol or ethanol at a scan rate of 50 mV s-1. (b-c) CV 33 curves as a function of methanol and ethanol concentration from 0.1 M to 1 M. (d-e) Current 34 35 density at 1.6 V vs RHE as a function of methanol and ethanol concentration, and the 36 corresponding reaction order. (f) Nyquist plots of the impedance data obtained from a NiS 37 electrode in 1 M KOH with and without 1 M methanol and ethanol at 1.5 V. Note that the 38 current densities (J) were referred to the measured current (i) with the geometrical area of the 39 40 GC electrode, i.e. 0.196 cm-2. 41 42 For the NiS-based electrode, the potential required to generate a current density of 10 43 44 mA cm-2 in 1 M KOH was 1.635 V for OER. For MOR and EOR, the potentials required to 45 generate a current density of 10 mA cm-2 in the presence of 1 M methanol and ethanol were 46 47 1.368 V and 1.339 V, respectively. As expected, the presence of alcohols in the alkaline 48 49 media largely reduced the required potential, demonstrating the replacement of the sluggish 50 51 OER at the cathode. In this context, much less energy was required to produce the same 52 53 amount of hydrogen by coupling the EOR and MOR to the HER in alkaline media. These 54 55 values were significantly lower than those obtained from the Ni-based electrode, 1.723 V in 1 56 M KOH, and 1.401 V and 1.360 V in the presence of 1 M methanol and ethanol, respectively. 57 58 10 59 60 ACS Paragon Plus Environment Page 11 of 23 Submitted to Inorganic Chemistry 1 2 3 Thus the use of NiS precursor NPs to produce the active NiOOH significantly improved the 4 5 electrochemical activity. 6 7 8 Figures 2bc displays the CVs of 1 M KOH containing different concentrations of methanol 9 and ethanol, from 0.1 M to 1 M. Both curves show a rapid increase of current density with the 10 11 increase of the alcohol concentration. However, as observed in Figure 2d, where the current 12 13 density at 1.6 V is plotted, a plateau is reached for concentrations around 0.5 and 0.6 M. The 14 15 plateau is reached at smaller concentrations of ethanol than methanol, which is consistent with 16 17 the higher activities obtained with ethanol, thus resulting in a saturation of the active sites at 18 19 small concentrations. A linear dependence between the logarithmic current density and 20 concentration at low concentrations was observed. From this linear region, a reaction order 21 22 (RO) of 0.84 for ethanol and 0.66 for methanol were determined.54 Similar values were 23 24 obtained from the Ni electrode, 0.71 for ethanol and 0.64 for methanol (Figure S5), which 25 26 were consistent with those obtained in previous reports.11,55,56 27 28 The Nyquist plots of the impedance response measured at 1.5 V versus RHE on the NiS-based 29 30 electrode in 1 M KOH with and without ethanol or methanol are shown in Figure 2f. 31 32 Compared with the semicircle radius in 1 M KOH, a smaller one for ethanol and methanol 33 34 was exhibited. Impedance responses were fitted with an equivalent circuit (Figure 2f, inset). 35 36 Fitting results revealed an enhanced charge-transfer process in ethanol and methanol with a 37 Rct of 4.2 J and 5.3 J (Table S1), two orders of magnitude lower than that obtained in 1 M 38 39 KOH (462.9 J.* 40 41 42 The electrochemical activity in the alkaline media, without the presence of alcohols, reveals 43 44 the intrinsic properties of the catalyst itself. The measurement of CVs at different scan rates in 45 the non-faradaic region allows for estimating the electrochemical surface area (ECSA).57 As 46 47 shown in Figure 3a, a double layer was observed in the 0.9-1.0 V potential window at the scan 48 49 rate of 10-100 mV s-1. Figure 3b presents a linear fitting between the current (i) at 0.95 V and 50 51 scan rates with a slope of 3.1 and 2.1×104 at the forward and backward sweep, yielding its 52 53 capacitance (Cdl). For both materials, ECSA was calculated by dividing Cdl by a specific 54 55 capacitance (Cs) of 0.04 mF cm-2, a typical number based on values reported for metal 56 electrodes in an aqueous NaOH solution.58 The ECSA value for NiS-based electrode was 57 58 11 59 60 ACS Paragon Plus Environment Submitted to Inorganic Chemistry Page 12 of 23 1 2 3 found to be 6.4 cm2, slightly above that of Ni NPs electrode (5.5 cm2, Figure S6). It should be 4 5 noted that the difference in the ECSA value between the NiS and Ni NPs could be slightly 6 7 modified if considering different values for the specific capacitance.57 8 9 0.06 0.06 [0.95 V] 3 [50 mV s ] -1 10 a b 2 c Current (mA) 0.04 0.04 Current (mA) Current (mA) -4 11 Sf=3.1 10 0.02 0.02 1 12 13 0.00 0.00 0 14 -0.02 10 mV s -1 -0.02 -1 15 100 mV s -1 Sb=-2.1 10 -4 -2 Ni NiS 16 -0.04 -0.04 0.90 0.93 0.96 0.99 0 20 40 60 80 100 1.0 1.2 1.4 1.6 17 E (V vs. RHE) -1 (mV s ) E (V vs. RHE) 18 3 3 4 19 d e f Peak current (mA) Peak current (mA) 100 mV s -1 2 2 Current (mA) 2 20 1 ipa=0.285 1/2 -0.019 21 1 ipa=0.306 +0.481 10 mV s -1 0 0 22 0 -1 23 -1 -2 -2 24 -2 ipc=-0.239 -0.305 ipc=-0.165 1/2 -0.346 25 1.0 1.2 1.4 1.6 -3 10 20 30 -1 40 50 -4 7.7 8.4 9.1 9.8 10.5 26 E (V vs. RHE) (mV s ) -1 1/2 (mV s ) 27 28 Figure 3. Electrochemical activity of a NiS-based electrode in 1 M KOH. (a) CV curves in the 29 potential range of 0.9-1.0 V at different scan rates. (b) Current collected at 0.95 V in the 30 forward and backward scan as a function of scan rates, and the corresponding linear fitting. 31 32 (c) Comparison of the CV curve of NiS and Ni electrodes in the applied potential window of 33 0.9-1.6 V at the scan rate of 50 mV s-1. (d) CV curves in the potential range of 0.9-1.6 V at 34 different scan rates. (e) Linear fitting between the peak current and scan rates from 10 to 50 35 mV s-1. (f) Linear fitting between the peak current and square root of scan rates from 60 to 36 37 100 mV s-1. 38 39 Figure 3c compares the CV curve for NiS and Ni electrodes in 1 M KOH. As can be seen, the 40 41 NiOOH on both electrodes starts to form at the same potential, ca. 1.309 V, but reached its 42 maximum at clearly different potential values. A maximum current of 1.961 mA at 1.337 V 43 44 was obtained for the NiS electrode, and 2.118 mA at 1.408 V for the Ni one. In the cathodic 45 46 scan, the current peaks were located at 1.465 V and 1.204 V, with current values of -1.249 47 48 mA and -1.764 mA, for NiS and Ni electrodes, respectively. The potential difference of redox 49 50 peaks -_ p) for NiS electrode was determined to be 0.086 V, significantly below the 0.204 V 51 52 obtained for the Ni electrode. These results indicate a much faster rate of electron transfer 53 between the electrode surface and the active centers in NiS-based compared with Ni-based 54 55 electrodes.59 56 57 58 12 59 60 ACS Paragon Plus Environment Page 13 of 23 Submitted to Inorganic Chemistry 1 2 3 The surface coverage of active species and the diffusion coefficient are important parameters 4 5 that determine electrocatalytic performance. As shown in Figures 3d and S7a, a set of CVs 6 7 was recorded at different scanning rates. As the scanning rate increases, larger 8 9 oxidation/reduction peaks of Ni(OH)2 NiOOH were measured.54 Using the small scanning 10 11 rate range, by plotting the peak current as a function of the scanning rate, the active surface 12 coverage of NiOOH -`$. can be calculated (see the calculations in SI). For the NiS electrode, 13 14 `$ was determined to be 1.48×10-6 mol cmL6 (Figure 3e), an order magnitude above that of 15 16 the Ni electrode (1.61×10-7 mol cmL6, Figure S7b). In the high scanning rate range, the peak 17 18 oxidation/reduction current densities can be linearly fitted with the square root of the scan 19 20 rate. This linear dependence demonstrates the Ni(OH)2/NiOOH redox reaction at NiS and Ni 21 22 electrodes to be diffusion-limited (Figure 3f).54 From the Randles equation, the diffusion 23 coefficient was evaluated to be 3.78×10-7 cm2 s-1 for NiS, slightly higher than that obtained 24 25 for Ni, 3.13×10-7 cm2 s-1 (Fig. S7c). 26 27 28 To evaluate the electrochemical stability of NiS and Ni catalysts, long-term CA tests were 29 30 carried out in 1 M KOH solution with 1 M ethanol and methanol (Figures 4a and S8a). The 31 current density for NiS was found to decrease relatively rapidly during the hour, to later 32 33 approximately stabilize. The initial EOR and MOR current densities were 175.5 and 145.2 34 35 mA cm-2 on the NiS electrode. After a 10,000 s reaction, the current density was still 87.6 and 36 37 82.5 mA cm-2 for EOR and MOR. A similar decay trend was observed with the Ni electrode, 38 39 maintaining 75.7 and 45.2 mA cm-2 for ethanol and methanol after the 10,000 s test, 40 41 respectively (Figure S8a). Both the initial and the retained current density for the NiS-based 42 electrode were higher than that of the reference Ni-based electrode and most previously 43 44 reported catalysts (Table S2). In terms of stability, a severe decay in the activity was seen for 45 46 both NiS- and Ni-based electrodes, which is also consistent with previous Ni-related studies 47 48 (Table S2). 49 50 As shown in Figure S9, the XPS spectra of the NiS electrode after the long-term CA test 51 52 demonstrates that a larger amount of sulfur oxides (SOx) remain attached to the NiS surface 53 54 (Figure S2). Besides, the high-resolution Ni 2p XPS spectrum shows the surface of the NiS to 55 56 be completely oxidized to Ni(OH)2 or NiOOH. DFT calculations show the optimized 57 58 13 59 60 ACS Paragon Plus Environment Submitted to Inorganic Chemistry Page 14 of 23 1 2 3 NiOOH-SOx structural model (Figure S10a). Compared with that of pristine NiOOH, the d 4 5 electrons of Ni in the SOx-decorated NiOOH significantly concentrate around the Fermi 6 7 level, thus improving the electroconductivity and electron transfer (Figure S10b). Thus, the 8 9 higher performance of the NiS-based electrode may be in part associated with the presence of 10 11 SOx species during the electrocatalytic process, in agreement with previous 12 publications.29,60,61 13 14 15 To calculate the Faradaic efficiency for the ethanol and methanol conversion process, the 16 17 electrolyte obtained at the end of CA testing was analyzed by the means of IC. As shown in 18 19 Figures 4b and S8b, a downward peak for water was presented at 2.8 min, and upward peaks 20 at 4.3 min and 4.8 min, corresponding to the acetate and formate, were observed as the main 21 22 products of the ethanol and methanol oxidation, respectively.11 By comparing with a standard 23 24 curve, 0.47 mmol acetate and 0.45 mmol formate were electrochemically produced for the 25 26 NiS-based electrode. Thus, the FE was determined to be 98.0% for MOR and 80.9% for EOR. 27 28 For monometallic Ni-based electrodes, over 10,000 s test in 1 M KOH with 1 M methanol, 29 30 0.27 mmol formate was produced with a FE of 96.6% (Figure 4c). For electrooxidation of 31 ethanol, 0.32 mmol acetate was detected and the calculated FE was 76.7% (Figure 4c). Very 32 33 close methanol-to-formate and ethanol-to-acetate FEs were obtained from both electrodes, 34 35 thus the presence of S did not change the reaction selectivity. 36 37 200 70 a b c MOR Faradaic Efficiency (%) 38 120 EOR Potentials (mV) 60 formate 39 150 100 J (mA cm ) -2 40 100 50 water 80 41 40 60 42 50 40 43 30 20 0 44 20 acetate 0 45 0 2 4 6 3 8 10 0 2 4 6 8 NiS NPs Ni NPs Time ( 10 s) Time (s) 46 47 48 Figure 4. (a) CA test towards MOR (blue), EOR (green), and comparative curve (black, 49 without alcohols) on NiS NPs electrode over 10,000 s testing period in 1 M KOH at 1.6 V.(b) 50 IC curve of the solution at the end of CA testing. (c) Comparison of Faradaic efficiency 51 between the NiS and Ni-electrodes. 52 53 Concerning that the current density of C2 ethanol oxidation is higher than that of C1 methanol 54 55 oxidation both on the NiS and Ni electrode (Figures 2a and S4), it is interesting to compare 56 57 58 14 59 60 ACS Paragon Plus Environment Page 15 of 23 Submitted to Inorganic Chemistry 1 2 3 the dehydrogenation steps during the electrochemical process. Thus, we conducted DFT to 4 5 understand and compare the EOR and MOR pathways. Considering that the NiOOH is the 6 7 main active phase for the methanol and ethanol electrooxidation, we built a NiOOH(001) 8 9 surface model to calculate the energy variation at steps of stable adsorption configurations of 10 11 intermediates (as shown Figure 5a). According to previous reports, the alcohol oxidation 12 process on the surface of Ni-based catalysts in alkaline media, which is accompanied by the 13 14 reduction of NiOOH to Ni(OH)2, takes place through the following steps:21 15 16 17 NiOOH + OH- + HCH2OH* a HCH2O* + Ni(OH)2 + H2O + e (4) 18 19 NiOOH + OH- + HCH2O* HCHO* + Ni(OH)2 + H2O + e (5) 20 21 22 NiOOH + OH- + HCHO* a HCO* + Ni(OH)2 + H2O + e (6) 23 24 NiOOH + OH- + HCO* a HCOOH* + Ni(OH)2 + H2O + e (7) 25 26 As shown in Figure 5b, the adsorption energy for methanol is larger than that of ethanol, 27 28 (-0.25 eV for CH3OH, and -0.14 eV for CH3CH2OH adsorption from the gas phase). As a 29 30 second step, a reaction energy of 0.39 eV for the first dehydrogenation from CH3OH to 31 32 methoxy HCH2O was calculated. This value is significantly lower than that of CH3CH2OH to 33 34 ethoxy CH3CH2O, 0.82 eV. The second dehydrogenation step of R-CH2O is exothermic by 35 36 -0.45 eV for ethanol, CH3CH2O to CH3CHO, while endothermic by 0.30 eV for methanol, 37 HCH2O to HCHO. The next step requires a reaction energy of 0.16 eV for the 38 39 dehydrogenation of CH3CHO to CH3CO, well below that of the dehydrogenation of HCHO to 40 41 HCO, 0.45 eV. Lastly, the adsorbed CH3CO and HCO are endothermically released when 42 43 interacting with OH- ions in the alkaline media to form CH3COOH and HCOOH, 44 45 respectively. Overall, based on the calculated energy landscape illustrated in Figure5, the 46 47 EOR requires overcoming total reaction energy of 0.82 eV, while MOR involves 3 48 consecutive endodermal dehydrogenation HCH2OH to HCO with total reaction energy of 1.25 49 50 eV, which explains the higher current densities measured for EOR compared with MOR. 51 52 53 54 55 56 57 58 15 59 60 ACS Paragon Plus Environment Submitted to Inorganic Chemistry Page 16 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment Page 17 of 23 Submitted to Inorganic Chemistry 1 2 3 transfer. Besides, DFT calculations demonstrate a more favourable reaction pathway for the 4 5 decomposition of ethanol with a total required reaction energy of 0.82 eV, leading to higher 6 7 performance than that of methanol decomposition involving 3 consecutive endodermal 8 9 dehydrogenation HCH2OH to HCO with total reaction energy of 1.25 eV. Overall, this study 10 11 provides a better understanding of the electrocatalytic alcohol oxidation mechanism and paves 12 the way for further advancing the catalyst in related fields. 13 14 15 Author Information 16 17 Corresponding Author 18 19 20 *Dr. Luming Li: liluming@cdu.edu.cn 21 22 *Dr. Yong Zuo: yongzuo16@gmail.com 23 24 25 *Prof. Andreu Cabot: acabot@irec.cat 26 27 Supporting Information 28 29 30 The Supporting Information is available free of charge at XXXXXX. 31 Structure and surface characterization including SEM, EDS, XPS, XRD, and TEM for Ni 32 33 and/or NiS NPs, additional electrochemical measurements, fitting parameters for the Nyquist 34 35 curve for NiS NPs, comparison of electrocatalytic performance, IC curve for Ni NPs, and 36 37 DFT calculation including optimized structural model and the DOS (PDF). 38 39 40 41 Acknowledgements 42 43 This work was supported by the start-up funding at Chengdu University and also from the 44 45 Natural Science Foundation of Sichuan (NSFSC) project funded by the Science and 46 47 Technology Department of Sichuan Province (2022NSFSC1229). It was also supported by the 48 European Regional Development Funds and by the Spanish Ministerio de Ciencia e 49 50 Innovación through the project COMBENERGY (PID2019-105490RB-C32). T. Zhang is 51 52 grateful to the China Scholarship Council (CSC) for scholarship support. T. Zhang, M.C. 53 54 Spadaro and J. Arbiol acknowledge funding from Generalitat de Catalunya 2017 SGR 327. 55 56 ICN2 acknowledges support from the Severo Ochoa Programme (MINECO, Grant no. 57 58 17 59 60 ACS Paragon Plus Environment Submitted to Inorganic Chemistry Page 18 of 23 1 2 3 SEV-2013-0295). T. Zhang has received funding from the CSC-UAB PhD scholarship 4 5 program. M. C. Spadaro has received funding from the postdoctoral fellowship Juan de la 6 7 Cierva from MICINN (IJCI-2019) and the Severo Ochoa programme. ICN2 authors thank 8 9 support from the project NANOGEN (PID2020-116093RB-C43), funded by MCIN/ 10 11 AEI/10.13039/501100011033/. 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