Stainless-Steel Scrap as Electrodes and a Reverse Osmosis Reject as Electrolytes for Cost-Efficient H ₂ Production: A Green and Sustainable Approach

Hydrogen production via water electrolysis holds great promise for revolutionizing the field of hydrogen energy, provided that the challenges related to electrode material availability and the demand for fresh water as the electrolyte are addressed. In this study, we propose the use of in situ-activated stainless-steel 434L scrap (SS scrap), commonly used as dishwashing scrubbers, as both the anode and cathode, with an alkalified reverse osmosis (RO) reject serving as the electrolyte. This approach simultaneously tackles the issue of electrode material scarcity and eliminates reliance on fresh water, which is becoming increasingly limited. The activation of SS scrap is primarily driven by the presence of chloride ions in the RO reject, which promotes the removal of the passive Cr2O3 layer. A full water-splitting cell constructed using a pair of activated SS scrap electrodes (0.2 g each) and an alkalified RO reject at pH 14 as the electrolyte delivered stable current densities of 85 mA g–1 and 195 mA g–1 at 2.0 and 2.25 V, respectively. While these values may not rival state-of-the-art performance, the utilization of waste materials (both SS scrap and RO reject) as the electrode and electrolyte components demonstrates the practical feasibility of large-scale hydrogen production via water electrolysis. Our calculations indicate that with just 30 g of SS-434L scrap, 1.60 L of H2 can be generated at 2.25 V using an alkalified RO reject (pH 14) as the electrolyte in 1 h. This study could encourage the exploration of other sustainable electrocatalysts and support the broader adoption of RO reject as a viable alternative to fresh water in industrial-scale water electrolysis. Moreover, a critical concern in the field of RO water purification technology is the increasing salinity of groundwater and seawater, a consequence of discharging salt-rich RO reject generated from the treatment of hard water and seawater. By employing this salt-rich RO reject as an alternative electrolyte in water electrolysis, this escalating environmental challenge can also be directly mitigated.