Understanding the Low-Overpotential Production of CH4 from CO2 on Mo2C Catalysts

详细信息    查看全文

• 作者：Seok Ki Kim ; Yin-Jia Zhang ; Helen Bergstrom ; Ronald Michalsky ; Andrew Peterson
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：March 4, 2016
• 年：2016
• 卷：6
• 期：3
• 页码：2003-2013
• 全文大小：590K
• ISSN：2155-5435

文摘

While Cu is the only electrocatalyst that converts CO₂ into meaningful quantities of CH₄ fuel, it requires significant overpotentials (onset potential of ∼−0.80 V vs RHE), decreasing energy conversion efficiencies. We report that Mo₂C is capable of catalyzing CO₂ into CH₄ at low potentials (onset potential of ∼−0.55 V vs RHE), where Cu electrocatalysts do not convert CO₂. This low-overpotential catalyst was first identified as a candidate by electronic structure calculations, which indicated the free energetics of CO hydrogenation to be more favorable than that on conventional transition metals such as Cu. Despite the low onset potential for CH₄, the CH₄ has a steep Tafel slope (∼−280 mV/dec), resulting in most of the current passing through the Mo₂C electrocatalysts being utilized for the competitive hydrogen evolution reaction. We conducted a detailed theoretical analysis on the basis of density functional theory calculations, microkinetic analysis, and simulated Pourbaix diagrams to suggest the reasons for these characteristics. These analyses suggest that the potential-limiting step in CH₄ evolution is the clearing of OH from the surface, while the rate-limiting step is the nonelectrochemical C–O bond scission, resulting in a high OH coverage and a high Tafel slope. Our calculations suggest that this high coverage weakens H binding, causing enhancement of the H₂ evolution reaction in comparison to that under CO₂-free conditions. This analysis shows that the detailed interaction of theory and experiment can be used to design and analyze operational electrocatalysts for CO₂ reduction and other complicated electrocatalytic reactions.