Oxidation mechanism for coal-assisted water electrolysis for hydrogen production: Evolution of different structures in coal molecules with reaction depth.

• Coal particles with different reaction depths are obtained by temperature control. • Further oxidation of the carbonyl group is the controlling step of CAWE. • The C-O structure of the surface layer of coal has highest oxidation activity. • The oxidation of coal particles mainly occurs on the external surface and pores. Coal-assisted water electrolysis can be used to produce pure hydrogen under mild conditions and reduce the electricity consumption required for water electrolysis by 40–60% while achieving clean utilization of coal. However, the kinetics for the process are very slow, and the mechanism has not yet been fully understood. Investigation of the oxidation mechanism for coal particles in the electrolysis process can help to clarify the control steps for the entire oxidation process so that research can be conducted in a more focused manner. In this study, coal particles were subjected to electrolysis with different extents of reaction using different reaction temperatures, and various analytical characterization methods, such as 13C NMR, were used to study the trends for multiple characteristic structures of coal molecules during the electrolysis process. The results show that the oxidation of coal particles mainly occurs on the external surface and pores, and its temperature coefficient is higher than that for Fe2+. Upon increasing the reaction temperature to 90 ℃, the reaction time can be reduced by more than half compared to room temperature. Aliphatic carbons, especially methylene-related structures, are the main oxidation sites in coal molecules, while aromatic carbons produce less oxidation. The oxidation rate for the C-O structure is faster, especially at the surface layer of coal molecules, and its content decreases gradually with increasing oxidation depth, while the oxidation rate for C = O and COOH is slower, which represents the main control step for the entire process of coal oxidation. [ABSTRACT FROM AUTHOR]