[Science Column] Producing Hydrogen from Toxic Waste

Overcoming the Limits of Solar Hydrogen Production Efficiency
Through Hydrazine Purification

A photoelectrochemical (PEC) system that purifies toxic industrial waste while simultaneously producing clean hydrogen energy has been developed by a domestic research team. The key innovation is replacing the slow oxygen evolution reaction (OER), which has been a bottleneck in hydrogen production, with the oxidation of toxic waste, thereby significantly improving efficiency.

The National Research Foundation of Korea announced on January 20 that Professor Jo Inseon’s team at Ajou University has developed a new system that can selectively oxidize hydrazine, a toxic industrial waste, while greatly enhancing the efficiency of solar-based PEC hydrogen production. This technology is noteworthy in that it enables simultaneous waste purification and hydrogen production within a single process.

Schematic of Ti-doped hematite photoelectrode with aligned and hierarchical porous structure and hydrazine oxidation-based PV-PEC hydrogen production. Through multiple growth and flame annealing processes, Ti-doped hematite (Fe2O3) photoanode with (110) crystal orientation alignment and hierarchical porous structure was realized, and the schematic shows its application in a PV-PEC system combining hydrazine oxidation reaction (HzOR) and hydrogen evolution reaction (HER). Provided by the research team

PEC technology, which produces hydrogen directly from water using sunlight, has been considered a next-generation eco-friendly energy technology with zero carbon emissions. However, in practical systems, the slow rate of the oxygen evolution reaction, along with the low charge transport and separation properties of photoanode materials, has posed structural limitations to improving efficiency. In particular, hematite (α-Fe₂O₃), which has attracted attention as a low-cost and highly stable material, has faced significant challenges in commercialization due to these material constraints.

The research team developed the ‘MGFA process,’ which combines multiple solution growth and high-temperature flame annealing, to realize a hematite photoanode with a hierarchically porous structure and aligned crystal orientation. This approach shortened charge transport pathways and maximized the reaction interface, enabling stable water oxidation for over 100 hours without the need for a co-catalyst.

Furthermore, the team introduced the ‘hydrazine oxidation reaction (HzOR),’ which rapidly oxidizes highly toxic industrial waste hydrazine, in place of the oxygen evolution reaction that had been a bottleneck in PEC systems. This strategy significantly increased photocurrent at the same potential and effectively bypassed the reaction barrier that had previously limited hydrogen production efficiency.

In an unassisted PV-PEC tandem system that combines the developed photoanode with a commercial silicon solar cell, the team achieved a solar-to-hydrogen (STH) conversion efficiency of 8.7% under sunlight-only operation without any external power source. This is the highest performance among iron oxide-based PV-PEC systems and demonstrates that it is possible to achieve both environmental purification and high-efficiency hydrogen production using low-cost materials.

Professor Jo Inseon stated, “This study demonstrates that toxic waste can be converted into an energy source while simultaneously producing hydrogen. It is significant in that it presents a new direction for achieving environmental purification and clean energy production within a single system.”

This research was supported by the Mid-Career Research Program funded by the Ministry of Science and ICT and the National Research Foundation of Korea. The results were published online in the international journal Nano-Micro Letters, specializing in nano- and micro-structured materials, on January 8.

The title of the paper is “Textured and Hierarchically Porous Ti-Doped Hematite Photoanodes Enabled by Multiple Growth and Flame Annealing for Efficient Unassisted Solar Hydrogen Production.”