System Design Rules for Intensifying the Electrochemical Reduction of CO₂ to CO on Ag Nanoparticles

Abstract

Electroreduction of CO 2 (eCO 2 RR) is a potentially sustainable approach for carbon-based chemical production. Despite significant progress, performing eCO 2 RR economically at scale is challenging. Here we report meeting key technoeconomic benchmarks simultaneously through electrolyte engineering and process optimization. A systematic flow electrolysis studyperforming eCO 2 RR to CO on Ag nanoparticles as a function of electrolyte composition (cations, anions), electrolyte concentration, electrolyte flow rate, cathode catalyst loading, and CO 2 flow rate -resulted in partial current densities of 417 and 866 mA/cm 2 with faradaic efficiencies of 100 and 98 % at cell potentials of À 2.5 and À 3.0 V with full cell energy efficiencies of 53 and 43 %, and a conversion per pass of 17 and 36 %, respectively, when using a CsOH-based electrolyte. The cumulative insights of this study led to the formulation of system design rules for high rate, highly selective, and highly energy efficient eCO 2 RR to CO.[a] S.

Impact of Alkali Cation Identity on the Conversion of HCO3− to CO in Bicarbonate Electrolyzers

“…These trends are similar to experimental results obtained from gas-fed CO 2 electrolyzers. [32,36] The next step was to determine whether the cation identity affects the rate of i-CO 2 formation [Equation (2)], the rate of the CO2RR [Equation (3)], or both. To resolve this question, we quantified the rates of i-CO 2 formation for each [MHCO 3 ] (M = Li, Na, K, Cs) electrolyte at a constant applied current density of 100 mA cm À 2 (Figure 3a).…”

“…[16,[18][19][20] The CO2RR studies that use gaseous CO 2 or CO 2 dissolved in water as feedstocks have shown that increasing the ionic size of the alkali-metal cation from Li + to Cs + increases CO2RR product selectivity. [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] This CO2RR literature, however, provides little direction as to how to optimize the composition of a bicarbonate feedstock to enhance CO formation rates in a bicarbonate electrolyzer. Some reports have shown that the cation identity influences the local pH [and therefore acid-base reactions such as Equation (2)], [23,24,[28][29][30] while others have shown that the cation affects the CO2RR [Equation (3)] by modulating the electrical double-layer at the silver cathode surface.…”

Operando cathode activation with alkali metal cations for high current density operation of water-fed zero-gap carbon dioxide electrolysers

“…Considering the results of previous studies on the promoting effect of alkali cations in electrochemical CO 2 R [37][38][39][40][41][42][43][44] , cation permeation through the membrane from the anolyte might contribute to achieving high reaction rates (this is absent in the case of water-based operation). In microfluidic electrolysers operating with liquid catholyte, alkali ions are inherently present around the cathode catalyst, which might (partly) explain why these systems outperform their zero-gap counterparts in terms of the achievable current densities 13,45 . Furthermore, a long activation period (~60 min) was observed for zero-gap cells operated with dilute alkaline anolytes (Supplementary Fig.…”

“…In microfluidic cells, highly alkaline conditions allow high current density (>1 A cm −2 ) production of CO 13 , ethylene 14,15 , methane 16,17 and multi-carbon products 18 . While these results are very promising, the scale-up of microfluidic electrolysers seems challenging 2 .…”

High carbonate ion conductance of a robust PiperION membrane allows industrial current density and conversion in a zero-gap carbon dioxide electrolyzer cell

“…Instead, the overall trend is that zero-gap electrolyzers tend to operate with smaller current densities, but the total cell voltage and single pass conversion values are similar to studies using liquid catholyte. 7,[12][13][14][15][16][17] In general, j CO remains below 400 mA cm À2 in most studies with zero-gap electrolyzers (there is only one single report on j CO over 400 mA cm À2 employing the Sustainion s membrane 18 ). Since very similar cathode GDEs (containing Ag nanoparticle catalysts), anode and anolytes are employed in most studies (irrespective of the cell architecture), we can assume that something else has been limiting the achievable current density in the zero-gap design.…”