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/cm2

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 H2 assisted by the anodic Fe2+ oxidation (Fe2+→Fe3+), while the heterogeneous HS supplies Fe2+ via the reduction of Fe3+ by coal slurry (Fe3+→Fe2+). Results showed that key operating parameters significantly affected the Fe3+ 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/cm2 and remain 136 mA/cm2 after 5 cycles, at a low cell voltage of 1.2 V, corresponding to a low power consumption of 2.87 kWh/Nm3(H2).