Revealing the activity and selectivity of ppm level copper in gas diffusion electrodes towards CO and CO2 electroreduction

Abstract

Cu is a unique metal that catalyzes carbon monoxide/carbon dioxide (CO/CO₂) to form high-order hydrocarbons and oxygenates through the CO/CO₂ reduction reaction (CO/CO₂RR) at decent selectivity and productivity. While this has been shown, the limits of the system are still unknown, i.e., the minimum amount of Cu needed for the CO/CO₂RR, and the maximum activity that trace amounts of Cu can achieve. Here, we have investigated the activity and selectivity of trace Cu with atomic dispersion over a range of loadings below 2 μg cm⁻² and have quantified the mass activity/turnover frequency (TOF) of trace Cu catalysts. A Cu loading of at least 0.042 μg cm⁻² initiates the CORR activity with 30% faradaic efficiency (FE) at a partial current density of 102 mA cm⁻² forming predominantly CH₄. The selectivity moves to C₂-based products (CH₃COO⁻, C₂H₄, and CH₃CH₂OH) with 70% FE as the Cu loading increases to 0.333 μg cm⁻², and increasing the Cu loading to 0.812 μg cm⁻² results in a 78% FE to C₂ with CH₃COO⁻ accounting for 42% of this. The highest mass activity for CH₄ reaches 2435 A mg⁻¹ of Cu, corresponding to a TOF of 267 s⁻¹, while C₂ activity reaches 584 A mg⁻¹ of Cu, leading to a TOF of 145 s⁻¹. Both TOFs are several orders of magnitude higher than the reported values. Different from the CORR, the CO₂RR demands a higher Cu loading and primarily generates C₁ (e.g., CO and CH₄). Metal impurities can be extended to others that are active towards the CO₂RR, such as Zn for CO₂-to-CO conversion. Thus, we suggest that the effect of trace metal impurities must be quantified when developing carbon-based metal-free and coordinated single-atom catalysts for the CO/CO₂RR in order to avoid overestimating their activity.