Carbon-assisted hydrogen production via electrolysis at intermediate temperatures: Impact of mineral composition, functional groups, and membrane effects on current density

•CAWE used low-calorific lignites for hydrogen production.•CAWE utilized lower potential and temperature with a membrane.•Membrane pre-treatment boosted proton transport, increasing current by 60%.•High current densities were due to functional groups, not just carbon content. Hydrogen production by electrolysis of materials such as lignite and leonardite, which do not have efficient calorific value for use in energy systems, was examined in this study. The effects of functional group structures and mineral composition on the oxidation process and current density were assessed using FT-IR analysis, SEM-EDX and XRF analyses which identified Tunçbilek lignite as having the highest coked and carbonaceous functional groups and leonardite as having the highest mineral content. Potentiostatic studies at 1 V potential with a Zn/Zn electrode at 50 °C showed that the highest current density values were achieved with Tunçbilek lignite, reaching 2.84 mA/cm2. The presence of minerals, particularly calcite, was found to negatively impact the electrooxidation process. Additionally, pre-treatment of the membranes with H2SO4 improved protonation through sulfonic acid groups, enhancing proton transport and ionic conductivity, which led to a current density increase of up to 60 % compared to without using a membrane. Gas analyses revealed that all carbon-based samples produced 100 % pure hydrogen from the cathode compartment. At a constant potential of 2 V, the hydrogen production rate was measured at approximately 0.3 mL∙min−1 for all samples.