High Salt Electrolyte Solutions Challenge the Electrochemical CO2 Reduction Reaction to Formate at Indium and Tin Cathodes

Electrochemical feeding of salt-loving microorganisms: Halophilic microorganisms are promising for bioproduction from formate. Electrochemical CO₂ reduction to formate necessitates high reaction kinetics, selectivity, and overall efficiency in saline conditions. Starting from a growth media for halophiles adjusting the concentration and composition of salts and buffers in electrolyte solutions enabled higher electrochemical production of formate.

Abstract

Formate is a promising product of the electrochemical CO₂ reduction reaction (eCO₂RR) that can serve as feedstock for biological syntheses. Indium (In) has been shown as a selective electrocatalyst of eCO₂RR with high coulombic efficiency (CE) for formate production at small scale at biocompatible non-halophilic that is low salt conditions. Ohmic losses and challenges on potential/current distribution arise for scaling-up, where higher salt loads are advantageous for minimizing these. Higher salt concentration within the solution or halophilic conditions also enable the use of halophilic biocatalysts. We optimized eCO₂RR with halophilic media by introducing tin (Sn) as a more sustainable alternative to In. At 3 % NaCl providing a catholyte conductivity ( of 70 mS cm⁻¹, the maximum specific formate production rates (r _formate) of 0.143±0.030 mmol cm⁻² h⁻¹ and 0.167±0.027 mmol cm⁻² h⁻¹ were achieved at In and Sn electrocatalysts, respectively. Decrease in r _formate and CE, in addition to higher variation between replicates was observed with further increase in NaCl concentration above 3 % ( >70 mS cm⁻¹) up to 10 % ( =127 mS cm⁻¹). This study sets the foundation for integrated microbial synthesis by halophiles.

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