Elucidating the CO2 Electroreduction Mechanism of a Metalloporphyrin PCN-222 Metal–Organic Framework

Abstract

The electrochemical CO2 reduction reaction (CO2RR) has enormous potential for reducing CO2 emissions and generating fuels, plastics, and other commodity chemicals from renewable resources. Porphyrin-based metal–organic frameworks (MOFs) have recently been explored for CO2RR electrocatalysis due to their modular nature, high surface area, and facile chemical tunability. Herein, we report the synthesis and characterization of an iron-metalated PCN-222 MOF, demonstrate its efficacy for CO2-to-CO conversion, and probe its CO2RR mechanism. We fit linear regression models on CO2RR electrochemical activity as a function of bicarbonate concentration and CO2 partial pressure to generate species-dependence rate equations for CO2RR at different potentials. We find that CO2RR on PCN-222(Fe) proceeds by a stepwise electron transfer–proton transfer mechanism, with the initial electron transfer (ET) constituting the rate-determining step (RDS) of the reaction. Next, we apply traditional linear and novel nonlinear regression models to estimate Tafel slopes from the electrochemical data, which are compared against Tafel slope cardinal values representing distinct reaction mechanisms under strict ideality assumptions. Only two of the three linear models produce Tafel slope estimates corresponding to cardinal values consistent with the mechanistic assessment made from the species-dependence experiments — that ET is the RDS of electrochemical CO2RR on PCN-222(Fe). Analysis of asymptotic 95% confidence intervals (CIs) of the Tafel slope parameters reveals that CIs of only the linear models capture the Tafel slope cardinal value for ET as the RDS. Therefore, we contest the standard practice in electrocatalysis of drawing mechanistic insights from Tafel slope estimates mapped to cardinal values.