Electrodeposition of Ni-Ga Alloy Catalyst from BMIM-Otf Ionic Liquid Electrolyte for Electroreduction of Carbon Dioxide

Abstract

To achieve a low-carbon economy, it is important to convert and use carbon-based energy sources efficiently. Electroreduction of CO2 to fuel and chemical feedstock is an attractive and prospective route for simultaneous conversion of CO2 and renewable power sources [1]. Ni5Ga3 compound was thought as the most active composition to take place of conventional Cu/ZnO/Al2O3 catalyst for the synthesis of methanol using CO2 and H2 [2,3]. Compared with chemical synthesis method, electrodeposition is an in-situ, convenient, chemical residue-free and easily controllable way to prepare catalyst. BMIM-OTF (1-Butyl-3-methylimidazolium trifluoromethanesulfonate) ionic liquid mixing with propyl alcohol in volume ratio of 6:1 was used as the electrolyte to electrodeposit Ni-Ga catalysts. The concentrations of nickel chloride and gallium chloride in the electrolyte are 50~80mmol/L and of 50~160mmol/L, respectively. Cyclic voltammetry (CV) of Ni, Ga and co-deposition of Ni-Ga alloy were scanned at speed of 100mV/s using a glassy carbon electrode (GCE) as working electrode, a platinum sheet as counter electrode and a platinum wire as reference electrode. The GCE was polished by Al2O3 powder of 1.0μm and ultrasonic washed in deionized water for 3 times before measuring. As seen from CV curves, the cathodic potential peak of Ni-Ga alloy is more positive than those of Ga and Ni, which indicates that the co-deposition of Ni-Ga is easier than that of single metal. The co-deposition of Ni-Ga is a diffusion controlled irreversible process. Gray-white deposits of Ni-Ga were electrodeposited at potential range of -1.4V to -2.0V vs. Pt. Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) were employed to observe the micromorphologies of Ni-Ga alloy. The Ni-Ga deposits are loose and composed by small particle clusters. The particle size of Ni-Ga alloy is about 100nm, which is bigger than that prepared by chemical synthesis method. There may be coexist of crystalline and amorphous structures in Ni-Ga alloy. The contents of Ni and Ga in the alloy were measured by Energy disperse spectroscopy (EDS). With the increase of concentration of gallium chloride in the electrolyte and the negative shift of potential, the contents of Ga in the Ni-Ga deposits increase. Catalytic activity of Ni-Ga alloy on conversion of CO2 were evaluated by CV curves measured in CO2 saturated KHCO3 aqueous solution of 0.1mol/L at scan rate of 100mV/S. Ni-Ga deposit shows a relative bigger catalytic activity compared to GC and nickel electrode. In conclusion, Ni-Ga alloy catalyst in size of 100nm were electrodeposited at -1.8V from the BMIM-OTF ionic liquid electrolyte, which has catalytic activity on electroreduction of CO2. The future work will focus on the preparation of nano size particles to present higher catalytic activity.