Potable water happy byproduct of low-cost green hydrogen technology

The team's hybrid solar distillation-water electrolysis (HSD-WE) device, reported April 9 in Energy and Environmental Science, currently produces 200 milliliters of hydrogen per hour with 12.6% energy efficiency directly from seawater under natural sunlight.

The researchers estimate that within 15 years, the technology could reduce the cost of green hydrogen production to $1 per kilogram -- a key step in achieving net-zero emissions by 2050.

"Water and energy are both critically needed for our everyday life, but typically, if you want to produce more energy, you have to consume more water," said Lenan Zhang, assistant professor and project lead. "On the other hand, we need drinking water, because two-thirds of the global population are facing water scarcity. So, there is a bottleneck in green hydrogen production, and that is reflected in the cost."

Green hydrogen is produced by splitting "high purity" -- i.e., deionized -- water molecules into hydrogen and oxygen through electrolysis. The high cost results from the massive amount of clean water that the process requires; the cost of manufacturing green hydrogen can be roughly 10 times higher than that of regular hydrogen.

"That's why we came up with this technology," Zhang said. "We thought, 'OK, what is the most abundant resource on the Earth?' Solar and seawater are basically infinite resources and also free resources."

Working with researchers from MIT, Johns Hopkins University and Michigan State University, Zhang's team devised a 10 centimeter by 10 centimeter prototype device that leverages one of the drawbacks of photovoltaics: their relatively low efficiency. Most PV cells can only convert up to approximately 30% of solar energy into electricity, and the rest dissipates as waste heat. But the team's device is able to harness most of that waste heat and uses it to warm the seawater until it evaporates.

"Basically, the short-wavelength sunlight interacts with the solar cell to generate electricity, and the longer wavelength light generates the waste heat to power the seawater distillation," Zhang said. "This way, all the solar energy can be fully used. Nothing is wasted."

Once the seawater evaporates, the salt is left behind, and the desalinated vapor condenses into clean water, which passes through an electrolyzer that splits the water molecules into hydrogen and oxygen.

Zhang sees the potential of incorporating the technology into solar farms to cool PV panels, which would improve their efficiency and prolong their lifespan.

"We want to avoid carbon emission, avoid pollution. But meanwhile, we also care about the cost, because the lower cost we have, the higher market potential for large-scale adoption," he said. "We believe there is a huge potential for future installation."

The research was supported by the National Science Foundation.