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

首先，我们比较了稳态恒电位法和脉冲法在CO_2电化学还原中的应用，发现铜电极脉冲电还原CO_2生成甲烷和乙烯的法拉第效率高于恒电位法，且在长期电解中更稳定。在接下来的研究中，我们对铜基合金进行了不同的电解，并考察了在室温下，在CO_2饱和的0.1M KHCO3溶液中，脉冲电解CO_2还原产生甲烷和乙烯的法拉第效率。采用单辊熔体旋压法制备了非晶态铜-锆和铜-钛合金电极。采用溅射法制备了铜银合金。非晶态的Cu-(30，40，60)Zr和Cu-30Ti合金对甲烷和乙烯的法拉第效率很低，这两种效率都低于3%。溅射沉积的晶态铜银合金电极在含少量银(5-7at%)时，甲烷的法拉第效率比纯铜电极高。然而，过量的银是有害的。虽然在纯银电极上没有检测到甲烷和乙烯，但添加少量铜后甲烷的效率有所提高，但效率低于纯铜电极。其次，采用气体扩散电极作为阴极。用溅射沉积的铜银合金粉末、炭黑粉末和聚四氟乙烯分散体制备了气体扩散电极。气体扩散电极只由碳黑粉末组成，只产生氢气。然而，铜银合金粉状气体扩散电极产生甲烷和乙烯，总还原电流密度非常高，但其法拉第效率低于板状合金电极。

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.