Mechanism investigation of carboxyl functional groups catalytic oxidation in coal assisted water electrolysis cell

Highlights: • Coal assisted water electrolysis lower the electric energy input for H₂ production. • Oxidation mechanism of carboxyl groups was investigated by preparing a model compound. • Carboxyl groups can be completely oxidized to CO₂ and CO at 90 °C on electrode. • Fe³⁺ was able to oxidize carboxyl groups at 90 °C and reduced the cell voltage. • The results verified that the carboxyl groups played an important role in the process. The electric energy consumption for H₂ production can be significantly reduced by integrating chemical energy of coal in the coal assisted water electrolysis (CAWE) process. While the oxidation mechanism of coal has not been comprehensively demonstrated. In this work, the oxidation mechanism of carboxyl group, a typical oxygen-functional group of coal, was systematically investigated by preparing a model compound of carboxylated graphene. For direct oxidation of carboxylated graphene (GCM), the onset potential was close to 1.2 V at ambient temperature. The apparent activation energy for hydrogen production was dramatically diminished by electrolysis of GCM slurry. The products analysis indicated that carboxyl groups can be oxidized to CO and CO₂ at 90 ^°C around 1.6 V Fe³⁺ was sufficient to oxidize carboxyl groups to generate Fe²⁺ at 90 ^°C. The chemical oxidation of carboxyl groups was a first-order reaction accompanied by adsorption, oxidization and desorption stages. In the presence of Fe²⁺/Fe³⁺, electrolysis initiated at about 0.5 V. A gentle slope of voltage from 0.6 V to 1.2 V was observed in the continuous electrolysis and the regeneration of Fe²⁺ became the limiting step for a long period of time. The results are essential to guide the application of CAWE.