Electrochemical reduction of CO₂ (CO₂RR) to value-added liquid fuels or chemical feedstocks using off-peak electricity or intermittent renewable energy sources is a green approach for energy storage and carbon recycling. However, the difficult activation of CO₂ to form the CO₂^∙− intermediate and the competitive hydrogen evolution (HER) in aqueous electrolytes make the Faradaic efficiency and selectivity for the CO₂RR remain very low. From another perspective, CO₂ reduction to CO, with co-production of H₂ from water, is very appealing as the mixture of CO and H₂ with controlled ratio can be used as syngas for methanol synthesis and Fischer-Tropsch reactions. In this approach, catalyst search is therefore no longer limited to metals with low HER activity such as Au and Ag. Materials with fair HER activity can be potentially considered for the CO₂RR. Interestingly, Pd, a good HER catalyst, has been found to continuously produce CO in the CO₂RR, which is however counterintuitive to that Pd is prone to CO poisoning.[1] In this work, we have re-examined Pd for the CO₂RR using in situ XAS and in situ XRD, and have found that Pd transforms into β-PdH under the CO₂RR condition. DFT calculations suggest that CO adsorption energy on the PdH surface is greatly reduced compared to that on the Pd surface, which is most likely the cause for high selectivity of CO₂RR to CO when using Pd as the electrocatalyst.[2]

[1] Y. Hori, Modern Aspects of Electrochemistry, Springer, New York, 2008, vol. 42, ch. 3, pp. 89–189.

[2] Wenchao Sheng, Shyam Kattel, Siyu Yao, Binhang Yan, Zhixiu Liang, Christopher J. Hawxhurst, Qiyuan Wu and Jingguang G. Chen, Energy & Environmental Science, 10, 1180-1185 (2017)