Investigation of electrocatalysts for CO_2 reduction to methane using gas-diffusion electrode and mechanism of enhanced activity

首先比较了稳态常规恒电位法和脉冲法在CO_2电化学还原中的应用。结果表明，铜电极脉冲电还原co_2制甲烷和乙烯的法拉第效率高于恒电位还原法，且在长期电解中更为稳定。在接下来的研究中，我们对铜基合金进行了各种电解，并在室温下，在CO_2饱和的0.1 M KHCO_3溶液中，用脉冲电解法测定了CO_2还原制甲烷和乙烯的法拉第效率。采用单辊熔融纺丝法制备了Cu-Zr和Cu-Tialloy非晶电极。采用溅射法制备Cu-Ag合金。非晶态Cu-(30,40,60)Zr和Cu- 30ti合金产甲烷和乙烯的法拉第效率均低于3%。溅射沉积结晶Cu-Ag合金电极，当合金中含有少量Ag （5-7at%）时，与纯Cu电极相比，具有更高的甲烷法拉第效率。然而，过量的银是有害的。虽然在纯Ag电极上没有检测到甲烷和乙烯，但随着少量Cu的加入，甲烷的效率有所提高，但效率低于纯Cu电极。其次，采用气体扩散电极作为阴极。以溅射沉积的Cu-Ag合金粉末、炭黑粉末和聚四氟乙烯分散体为材料制备了气体扩散电极。气体扩散电极仅由炭黑粉末构成，只产生氢气。而Cu-Ag合金粉末负载气体扩散电极产生甲烷和乙烯，总还原电流密度显著高，但法拉第效率低于板型合金电极。

First of all, we have compared the steady state conventional potentiostatic method with pulsed method in the electrochemical reduction of CO_2. It was found that the faradaic efficiencies for the pulsed electroreduction ofCO_2 to methane and ethylene using copper electrode are higher than those for the potentiostatic reduction and more stable in a long-term electrolysis. In the next study, we have made various electrolysis from copper-based alloys and examined the faradaic efficiencies for methane and ethylene production from CO_2 reduction using pulsed electrolysis in CO_2 saturated 0.1 M KHCO_3 solution at room temperature. Amorphous Cu-Zr and Cu-Tialloy electrodes were prepared using a single-roller melt-spinning method. The sputtering method was applied to prepare Cu-Ag alloys. The amorphous Cu-(30, 40, 60)Zr and Cu-30Ti alloys showed considerably low faradaic efficiencies for methane and ethylene production and both efficiencies were less than 3 %. The sputter-deposited crystalline Cu-Ag alloy electrodes, when the alloys contain small amount of Ag (5-7at%), have a higher faradaic efficiency of methane in comparison with pure Cu electrode. However, excess amount of Ag was detrimental. Although methane and ethylene were not detected on pure Ag electrode, the efficiency of methane increased with the addition of a small amount of Cu but the efficiency was lower than that on pure Cu electrode. Next, gas-diffusion electrodes have been employed as the cathode. The gas-diffusion electrodes have been prepared from sputter-deposited Cu-Ag alloyed powder, carbon black powder and polytetrafluoroethylene dispersion. Thegas-diffusion electrode only made up of carbon black powder produced exclusively hydrogen gas. However, the Cu-Ag alloy powder-loaded gas-diffusion electrodes produced methane and ethylene and the total reduction current density was remarkably high, but their faradaic efficiencies were lower than those on the plate-type alloyelectrodes.