Electrical energy input efficiency limitations in CO2‐to‐CO electrolysis and attempts for improvement

The electrical input energy efficiency in CO₂ electrolysis is limited by the inherent CO₂ transport under alkaline operating conditions. Different production routes towards CO are examined and experimentally validated. CO₂ electrolysis was conducted under acidic electrolyte conditions (pH = 0–6). FE_HCOOH was stable down to pH ≈ 1. Even under very acidic bulk electrolytes, no change in the reduction mechanism could be forced.

Abstract

Electrochemical CO₂ reduction is a potentially up-coming technology to convert anthropogenic emitted CO₂ into chemical feedstock. Due to alkaline operating conditions of CO₂-electrolyis in gas diffusion electrodes, exothermal hydroxide ion neutralization with the excess of supplied CO₂ leads to unavoidable electricity-to-heat conversion at the cathode, therefore limiting electrical energy input efficiency. The decomposition reaction of carbonates by protons from water oxidation completes the inherent CO₂ transport at the anode. In this work, different production routes to CO are thermodynamically examined and experimentally validated. Using formic acid as an intermediate towards CO the electrical energy input efficiency can rise to 71% on a thermodynamical basis. Additionally, the possibility of altering the mechanism of CO₂ reduction under acidic conditions is investigated, which would lead to even larger electrical energy input efficiencies. The concept was investigated by pH series measurements (pH = 0–6) at 50 mA/cm² where Pb derived from Pb₃O₄ was used as a CO₂ reduction catalyst. The reduction to formic acid under acidic bulk electrolyte pH is stable at FE_HCOOH = 70% down to pH ≈ 1, while HER is becoming dominant below. Even under such acidic bulk electrolyte conditions no change in reduction mechanism could be forced, which is reflected in invariant cell voltages in the model experiment.

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