Mechanistic Insights into CO₂ Electroreduction on Ni₂P: Understanding Its Selectivity toward Multicarbon Products

Abstract

Recently, nickel phosphides (Ni x P y ) have been reported to enable selective electrochemical formation of multicarbon products (C 3 and C 4 ) via the CO 2 reduction reaction (CO 2 RR); nevertheless, their activities remain low. In order to understand the roots of their high selectivity and low activity and to direct the design of more active Ni x P y -based CO 2 RR catalysts, we investigate the CO 2 RR mechanism on Ni 2 P using density functional theory (DFT) calculations. We reveal that the reaction proceeds through the formate pathway, followed by formaldehyde (H 2 CO*) formation and self-condensation. Moreover, we demonstrate that surface hydride transfer steps, along with surface-mediated C−C coupling, are essential in order to avoid C 1 product formation and boost selectivity toward multicarbon products. In addition, we find that the thermal surface hydride transfer from the surface to the physisorbed CO 2 is one of the key rate-limiting steps, and since it is not electroactive, it cannot be accelerated by applying an overpotential. Finally, our results also show that the hydrogen affinity of the surface and the dynamic surface reconstruction via H adsorption facilitate selective CO 2 reduction and C−C coupling on Ni 2 P. These findings provide an impetus for exploring materials design space to identify the physical principles that govern the thermodynamics of rate-limiting thermal steps in electrocatalytic processes.

Surface Hydrides on Fe2P Electrocatalyst Reduce CO2 at Low Overpotential: Steering Selectivity to Ethylene Glycol

“…Figure S7 details all the steps we considered and discuss next. To form ethylene glycol, methylglyoxal, and 2,3-furandiol, the reaction must undergo carbon−carbon coupling through formaldehyde, which was first proposed and experimentally verified in our previous work on Ni 2 P 40 and confirmed by DFT calculations by Rappe et al 64 Our GC-DFT calculations reveal that Fe 2 P can catalyze CO 2 RR via two possible pathways to form formaldehyde, but with very different energetics for the intermediates, as summarized in Figure 5.…”

“…This reaction step is highly exergonic by 0.83− 0.96 eV, depending on the geometry of *H 2 CO, which can readily undergo carbon−carbon coupling with other formaldehyde molecules on nearby Fe sites to form C 2 and higher products, as illustrated in Figure 4. The carbon−carbon coupling step was previously shown to be energetically favorable on Ni 2 P, 64 although the calculations were performed using the computational hydrogen electrode (CHE) model and require further validation, for example by GC-DFT.…”

Boosting CO2 electroreduction towards C2+ products via CO* intermediate manipulation on copper-based catalysts

“…270,271 6) To date, the most widely reported C 2+ -producing catalysts are based on Cu. However, some new materials, including Ni 2 P (FE C 2 +C 3 = 99%), 272 Ni 2 P/Ho 2 O 3 core/shell nanoparticles (FE (CH 3 ) 2 CO = 25.4%), 273 hexagonal cobalt nanosheet (FE CH 3 CHO = 60%), 274 nitrogen-doped mesoporous carbon (FE ethanol = 77%), 148 nitrogen-doped nanodiamond Environmental Science: Nano Critical review (FE CH 3 COO − = 91.8%), 275 B-and N-co-doped nanodiamond (FE ethanol = 93.2%), 276 and N-doped graphene quantum dots (FE ethanol = 31%), 277 have exhibited a high performance for the production of C 2+ . The C 2+ product distribution and production rate on non-copper materials are comparable to that obtained using copper-based electrocatalysts.…”

The Role of Hydrogen Adsorption Site Diversity in Catalysis on Transition Metal Phosphide Surfaces

“…94,95 DFT studies have shown that C-C coupling on Ni2P via *HCOO self-condensation is exergonic due to the presence of many types of H-binding sites, which results in selectivity toward multicarbon products. 103 It may be intuitive to think that the many H-adsorption sites promote higher activity for HER; however, it instead provides facile hydrogenation of CO2 intermediates. This phenomenon demonstrates that the complexity of TMP surfaces can be used to our advantage when tuning reaction selectivity.…”

“…83 Considering intermediates other than hydrogen is important in electrochemical CO2 reduction as well, where a common theory is that a binary surface can provide local hydride sources for CO2 hydrogenation (Figure 4). 93,103 Li et al demonstrated that AgP2 has an onset potential of -0.22 V vs RHE, over 0.3 V lower than that of Ag (-0.59 V vs RHE). They attribute this partially to the strong adsorption of H on phosphorus sites that can suppress HER and serve as a local hydride source for CO2 hydrogenation.…”