Urine, not water for efficient production of green hydrogen

Researchers have developed two unique energy-efficient and cost-effective systems that use urea found in urine and wastewater to generate hydrogen.

The unique systems reveal new pathways to economically generate ‘green’ hydrogen, a sustainable and renewable energy source, and the potential to remediate nitrogenous waste in aquatic environments.

Typically, hydrogen is generated through the use of electrolysis to split water into oxygen and hydrogen. It is a promising technology to help address the global energy crisis, but the process is energy intensive, which renders it cost-prohibitive when compared to extracting hydrogen from fossil fuels (grey hydrogen), itself an undesirable process because of the carbon emissions it generates.

In contrast to water, an electrolysis system that generates hydrogen from urea uses significantly less energy.

Despite this advantage, existing urea-based systems face several limitations, such as the low amounts of hydrogen that are able to be extracted and the generation of undesirable nitrogenous by-products (nitrates and nitrites) that are toxic and compete with hydrogen production, further reducing overall system efficiency.

Researchers from the Australian Research Council Centre of Excellence for Carbon Science and Innovation (COE-CSI) and the University of Adelaide have developed two urea-based electrolysis systems that overcome these problems and can generate green hydrogen at a cost that they have calculated is comparable or cheaper than the cost of producing grey hydrogen.

The research for each system was published in separate papers, one in the journal Angewandte Chemie International Edition, the other in Nature Communications. PhD candidate Xintong Gao was first author on the Angewandte Chemie International Edition paper and is from the University’s team headed by COE-CSI Chief Investigator, Professor Yao Zheng and Professor Shizhang Qiao, Deputy Director and Chief Investigator of COE-CSI who are from the School of Chemical Engineering.

Making hydrogen from pure urea is not new, but the team has found a more accessible and cost-effective process that uses urine as an alternative source to pure urea.

“While we haven’t solved all the problems, should these systems be scaled up, our systems produce harmless nitrogen gas instead of the toxic nitrates and nitrites, and either system will use between 20-27 per cent less electricity than water splitting systems,” says Professor Zheng.

“We need to reduce the cost of making hydrogen, but in a carbon-neutral way. The system in our first paper, while using a unique membrane-free system and novel copper-based catalyst, used pure urea, which is produced through the Haber-Bosch ammonia synthesis process that is energy intensive and releases lots of CO2.

“We solved this by using a green source of urea – human urine – which is the basis of the system examined in our second paper.”

Urine or urea can also be sourced from sewage and other wastewater high in nitrogenous waste. Urine in an electro-catalytic system, however, presents another issue. Chloride ions in urine will trigger a reaction generating chlorine that causes irreversible corrosion of the system’s anode where oxidation and loss of electrons occurs.

“In the first system we developed an innovative and highly efficient membrane-free urea electrolysis system for low-cost hydrogen production. In this second system, we developed a novel chlorine-mediated oxidation mechanism that used platinum-based catalysts on carbon supports to generate hydrogen from urine,” says Professor Qiao.

Platinum is an expensive, precious and finite metal and its increasing demand as a catalytic material is unsustainable. It is a core mission of the ARC Centre of Excellence for Carbon Science and Innovation to enable transformative carbon catalyst technologies for the traditional energy and chemical industries.

The University of Adelaide team will build on this fundamental research by developing carbon-supported, non-precious metal catalysts for constructing membrane-free urine-wastewater systems, achieving lower-cost recovery of green hydrogen while remediating the wastewater environment.