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。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比恒电位还原的高，且在长时间的电解过程中更稳定。在接下来的研究中，我们从铜基合金中制备了各种电解，并在室温下在CO_2饱和的0.1M KHCO_3溶液中使用脉冲电解考察了CO_2还原产生甲烷和乙烯的法拉第效率。采用单辊快淬法制备了非晶态Cu-Zr和Cu-Ti合金电极。采用溅射法制备了Cu-Ag合金。非晶态Cu-（30，40，60）Zr和Cu-30 Ti合金对甲烷和乙烯的法拉第效率较低，均小于3%。当合金中Ag含量较低（5-7at%）时，用化学镀法制备的晶态Cu-Ag合金电极对甲烷的法拉第效率比纯铜电极高。然而，过量的Ag是有害的。虽然在纯Ag电极上未检测到甲烷和乙烯，但添加少量Cu后，甲烷的降解效率有所提高，但低于纯铜电极上的降解效率。接着，采用气体扩散电极作为阴极。以铜银合金粉、炭黑粉和聚四氟乙烯分散液为原料，制备了气体扩散电极。气体扩散电极仅由炭黑粉末制成，仅产生氢气。然而，负载铜银合金粉末的气体扩散电极产生甲烷和乙烯，总还原电流密度显着较高，但法拉第效率低于板式合金电极。

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.