Two-step coal-assisted water electrolysis for energy-saving hydrogen production at cell voltage of 1.2 V with current densities larger than 150 mA/cm 2

The application of water electrolysis for hydrogen production is inhibited by high energy consumption, primarily due to the sluggish anodic oxygen evolution reaction (OER). Replacing OER with a thermodynamically favorable reaction is significant to decrease the energy consumption. Coal-assisted water electrolysis (CAWE), with a theoretical potential of only 0.21 V, received special attention. However, the conventional CAWE suffers from poor stability to maintain high current, the continuous collision of coal particles with electrodes and membranes also causes the abrasion of these components. Here, a two-step CAWE system with the electrolysis system (ES) and hydrothermal system (HS) was proposed. The homogeneous ES generates H 2 assisted by the anodic Fe 2+ oxidation (Fe 2+ →Fe 3+ ), while the heterogeneous HS supplies Fe 2+ via the reduction of Fe 3+ by coal slurry (Fe 3+ →Fe 2+ ). Results showed that key operating parameters significantly affected the Fe 3+ reduction by coal in HS. Additionally, a homemade continuous proton exchange membrane electrolyzer was used to test the two-step CAWE concept. The current density can achieve 154 mA/cm 2 and remain 136 mA/cm 2 after 5 cycles, at a low cell voltage of 1.2 V, corresponding to a low power consumption of 2.87 kWh/Nm 3 (H 2 ).