The electrochemical CO₂ reduction reaction (CO₂RR) to multi-carbon products, such as ethanol and ethylene, offers a means to produce fuels and chemicals, all powered using renewable (a.k.a. green) electricity. The most selective and active CO₂RR electrolyzers to date have relied on locally alkaline conditions such as in alkaline flow cells and anion exchange membrane (AEM) electrolyzers. Even the most carbon-efficient of these, however, suffer low carbon utilization, as well as severe crossover of liquid products. Both CO₂ crossover and product crossover lead to the need for more costly product separation.  

CO₂RR in acidic media is known to eliminate carbonate formation; relying on acidic electrolytes enables the use of proton exchange membranes that block liquid crossover. However, the competing hydrogen evolution reaction in acidic solution militates against both selectivity and activity of CO₂RR, especially for CO₂-to-multi-carbon products.

We therefore sought routes to suppress the competing hydrogen evolution reaction (HER). Metals with a strong binding affinity toward CO* weaken H* binding affinity due to adsorbate-adsorbate interactions. Using DFT calculations, we screened different bimetallic X-Cu (X = Cr, Mo, W, Mn, Re, Fe, Ru, Co, Rh, Ir, Ni, Pt, and Pd) catalysts and found that Pd-Cu offers the lowest ∆G_OCCOH* and ∆G_OCCOH* - ∆G_CHO*. The adsorption energy of H* is 0.2 eV weaker on Pd–Cu compared to on Cu, pointing toward the suppression of HER.

Experimentally, we synthesized Pd–Cu bimetallic catalysts for implementation in an acidic CO₂RR electrolyzer employing an acidic bulk environment and operating under conditions that produce a mildly alkaline local environment at the catalyst surface. We found that electrolytes with pH 2 led to a locally mildly alkaline environment under practical operating current densities: this enabled both high CO₂RR activity and yet sufficient acidity in the bulk liquid in order to convert carbonate back into CO₂.

We report as a result liquid product crossover < 0.05%  with a CO₂-to-C₂₊ Faradaic efficiency (FE) of (89 ± 4)% and single-pass carbon efficiency of (60 ± 2)% to C₂₊ (total CO₂ utilization of (68 ± 4)% when considering the sum of C₁ and C₂₊ products) at 500 mA cm^-2 in acidic media.

Fig. 1 | (a) Schematic of acidic CO₂RR electrolyser. (b) Modeled pH changes along the catalyst’s surface at solution with various pH under applied current density of 500 mA cm^-2 . (c) Two-dimensional C₂₊ activity (∆G_OCCOH*) and C₂₊ vs. C₁ selectivity (∆G_OCCOH* − ∆G_CHO*) plot of CO₂RR to C₁ and C₂₊ products. (d) FEs and single-pass carbon efficiency (SPCE_C2+) of CO₂-to-C₂₊ on 6.2% Pd–Cu with different CO₂ flow rates (applied current density = 500 mA cm^-2). (e) Comparison of CO₂RR partial current density, C₂₊ product FE, liquid product crossover fraction, pH of bulk electrolyte, and single-pass carbon efficiency (SPCE) of the 6.2%-Pd-Cu with state-of-art CO₂RR catalysts.

https://www.nature.com/articles/s41929-022-00788-1