Theoretical Insights into Heterogeneous (Photo)electrochemical CO₂ Reduction

Electrochemical and photoelectrochemical CO₂ reduction technologies offer the promise of zero-carbon-emission renewable fuels needed for heavy-duty transportation. However, the inert nature of the CO₂ molecule poses a fundamental challenge that must be overcome before efficient (photo)electrochemical CO₂ reduction at scale will be achieved. Optimal catalysts exhibit enduring stability, fast kinetics, high selectivity, and low manufacturing cost. Identifying catalytic mechanisms of CO₂ reduction in (photo)electrochemical systems could accelerate design of efficient catalysts. In recent decades, numerous theoretical studies have contributed to our understanding of CO₂ reduction pathways and identifying rate-limiting steps. Although a significant body of work exists regarding homogeneous electrocatalysis for CO₂ reduction, this review focuses specifically on the theory of heterogeneous (photo)electrochemical reduction. We first give an overview of the relevant thermodynamics and semiconductor physics. We then introduce important, widely used theoretical techniques and modeling approaches to catalysis. Recent progress in elucidating mechanisms of heterogeneous (photo)electrochemical CO₂ reduction is discussed through the lens of two experimental systems: pyridine (Py)-catalyzed CO₂ (photo)electrochemical reduction at p-GaP photoelectrodes and electrochemical CO₂ reduction at Cu electrodes. We close by proposing strategies and principles for the future design of (photo)electrochemical catalysts to improve the selectivity and reaction kinetics of CO₂ reduction.