Reactions Related with CO2 Reduction

The chemical reactivity of CO2 is low. However, the equilibrium potentials of CO2 reduction are not very negative as compared with that of the hydrogen evolution reaction (HER) in aqueous electrolyte solutions. For example, electrochemical reduction of CO2 to HCOO in aqueous solution is given below together with the standard electrode potential at pH 7.0 at 25°C with respect to the standard hydrogen electrode (SHE). 

The standard electrode potential of HER at pH 7.0 is -0.414 V vs. SHE at 25°C. The CO2 reduction in aqueous solution involves H2O and OH, and the equilibrium potential varies in accordance 

Nevertheless, CO2 reduction does not take place easily, and the actual electrolysis potentials for CO2 reduction are much more negative in most cases than the equilibrium ones. The reason is that the intermediate species CO2, formed by an electron transfer to a CO2 molecule, proceeds as the first step at highly negative potential, such as -2.21 V vs. saturated calomel electrode (SCE) measured in dimethyl fonnamide (DMF), as discussed later in detail. 

The standard potentials for CO2 reduction and HER, estimated from thermodynamic data, decrease similarly with pH as is apparent in Fig. 1. However, the actual potential of CO2 reduction does not depend on pH as described later, whereas that of HER is proportional to proton activity. Thus HER prevails over CO2 reduction in acidic solutions, and most studies of CO2 reduction in aqueous media have been made in neutral pH region. 

Such pH potential diagrams may be given for all the products of CO2 reduction. Only the standard electrode potentials of CO2 reduction are given below for formation of CO, CH4, C2H4 C2H5OH, and C3H7OH for brevity. The values are estimated from thermodynamic data, in aqueous media at 25°C with respect to SHE. The standard potentials are conveniently given at pH 7.0, where most of actual CO2 reductions are measured. 

---