Precipitation of (bi)carbonate salts during the electrochemical CO₂ reduction (CO₂RR) has been identified as a major cause of degradation and one of the main challenges to be overcome before implementing this technology on the industrial scale. Recently, the use of acidic electrolytes has been suggested as a promising strategy to avoid this undesirable precipitation of carbonates and maximize the conversion of CO₂ into valuable products. However, direct experimental evidence supporting this claim, as well as an understanding of degradation mechanisms in acidic electrolytes, is currently missing. In this study, we use operando synchrotron wide-angle X-ray scattering to provide unambiguous experimental proof that substantial (bi)carbonate precipitation takes place in Ag and Cu gas diffusion electrodes (GDEs) during CO₂RR even in strongly acidic electrolytes (pH 1). The (bi)carbonate signal intensity increases over time, correlating with a loss of selectivity for carbon products and an increased level of H₂ production. Electrochemical measurements coupled with products analysis also show that a minimum current is required to activate the CO₂RR and suppress hydrogen evolution and that this current is higher for more acidic electrolytes. These findings provide experimental evidence for high local pH close to the catalyst even in acidic electrolytes. Our results confirm that the CO₂RR performed in an acidic electrolyte can yield high selectivity to carbon products, provided sufficiently large current densities are applied to provoke local proton depletion in the vicinity of the catalyst, but that detrimental (bi)carbonate precipitation and selectivity losses occur even under these conditions.

中文翻译：

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强酸性电解质中电化学 CO2 还原过程中碳酸氢盐沉淀的操作观察

电化学CO ₂还原(CO ₂ RR)过程中碳酸(氢)盐的沉淀已被确定为降解的主要原因，也是在工业规模上实施该技术之前需要克服的主要挑战之一。最近，使用酸性电解质被认为是一种有前途的策略，可以避免这种不希望的碳酸盐沉淀，并最大限度地将CO ₂转化为有价值的产品。然而，目前缺乏支持这一说法的直接实验证据，以及对酸性电解质降解机制的理解。在本研究中_，我们使用操作同步加速器广角 X 射线散射提供明确的实验证据，证明即使在强酸性电解质（pH 1）。碳酸（氢）盐信号强度随着时间的推移而增加，与碳产物选择性的损失和H ₂产量水平的增加相关。电化学测量与产物分析还表明，激活CO ₂ RR并抑制氢气析出需要最小电流，并且对于酸性电解质，该电流更高。这些发现为即使在酸性电解质中也接近催化剂的高局部 pH 值提供了实验证据。我们的结果证实，在酸性电解质中进行的 CO ₂ RR 可以对碳产物产生高选择性，前提是施加足够大的电流密度以引起催化剂附近的局部质子耗尽，但是碳酸氢盐沉淀和选择性是有害的即使在这些条件下也会发生损失。