Tunable product selectivity on demand: a mechanism-guided Lewis acid co-catalyst for CO2 electroreduction to ethylene glycol†

Abstract

Bioinspired nickel phosphide electrocatalysts can produce more complex multi-carbon products than natural photosynthetic enzymes but controlling C-product selectivity and suppressing H₂ evolution remain open challenges. Here, we report a significant shift in the CO₂RR product distribution on Ni₂P in the presence of boric acid/borate, a soluble Lewis acid/base co-catalyst. Using Ni₂P without a co-catalyst, CO₂ reduction produces a mixture of methyl glyoxal (C₃) > 2,3-furnadiol (C₄) and formic acid (C₁) with 100% Faradaic efficiency for carbon products. Addition of boric acid/borate shifts product selectivity to ethylene glycol (EG) with an 85% CO₂-Faradaic efficiency (at 10 mM, 0 V vs. RHE), with the balance being the aforementioned C₁, C₃ and C₄ products. The mechanism of EG formation is proposed to occur by the co-catalyst activating a reaction between surface *hydride and *glycolaldehyde on Ni₂P, while suppressing the aldol C–C coupling reaction that forms the C₃ and C₄ products. The formation of an intermediate borate-EG-diester, [(OCH₂CHO)₂B]⁻, is detected by ¹¹B-NMR, which hydrolyzes to release the EG product. Extended electrolysis of boric acid modifies the surface of Ni₂P by forming *BO₃–Ni₂P, as shown by XPS. CO₂ electro-reduction on *BO₃–Ni₂P in the absence of free boric acid produces exclusively ethylene oxide (EO), which slowly hydrolyzes to EG in the bicarbonate electrolyte. The combined Faradaic efficiencies for CO₂RR products EO + EG with free boric acid as the co-catalyst and *BO₃–Ni₂P as the cathode reaches 88% (at 0 V vs. RHE), a record carbon selectivity. This work illustrates the feasibility of using Lewis acid/base co-catalysts to change the established chemical reaction mechanism of an electrocatalyst to form a new, chemically predictable, more valuable product in high yield.