(187g) First Principles Insights into Electrochemical CO2 Reduction on Nickel Phosphide: Formate Leading to Selective Multi-Carbon Product Formation

Designing a selective, active, and stable catalyst for CO₂ reduction reaction (CO₂RR) is crucial to close the carbon cycle and remedy the dire consequences of the climate crisis. However, catalyst design for CO₂RR is challenging because selectivity and activity are often inversely related. Selective formation of multi-carbon products using CO₂RR can provide significant advancement in producing chemical feedstocks. A recent set of experiments showed that nickel phosphides selectively catalyze CO₂RR, producing C₃ and C₄ products while suppressing the hydrogen evolution reaction at 0 V vs. the reversible hydrogen electrode.¹ To elucidate the origin of this selectivity, we investigated the CO₂RR mechanism on Ni₂P using density functional theory (DFT) calculations. We show that multi-carbon products form via formate intermediate which contrasts with the previous mechanisms proposed on other (e.g., Cu) surfaces. Further, we demonstrate that C-C coupling via the self-condensation of formaldehyde intermediates (H₂CO*) leads to the formation of the experimentally-observed C₃ and C₄ products.¹ We find that the surface hydrogen affinity, the consequent surface reconstructions,² and tandem thermal and electrochemical steps are crucial for achieving the high selectivity. Further, we unveil that CO₂ activation step via surface hydride transfer limits the catalytic activity, resulting in the low turnover frequency observed experimentally.¹ The mechanistic knowledge gained here can be used in rendering these selective nickel phosphide-based CO₂RR catalysts more active, and thus industrially viable.

(1) Calvinho, K. U. D.; Laursen, A. B.; Yap, K. M. K.; Goetjen, T. A.; Hwang, S.; Murali, N.; Mejia-Sosa, B.; Lubarski, A.; Teeluck, K. M.; Hall, E. S.; Garfunkel, E.; Greenblatt, M.; Dismukes, G. C. Energy Environ. Sci. 2018, 11(9), 2550–2559

(2) Wexler, R. B.; Martirez, J. M. P.; Rappe, A. M. ACS Catal. 2017, 7 (11), 7718–7725