A solar water heater on a rooftop.
“Lowering the cost of storing solar power with chemistry”
by MainGate Staff
In the Chemistry Building near the northwestern rim of campus, hidden among the trees, sits Lara Halaoui’s lab, the site of exciting new scholarship on renewable energy. Halaoui, a chemist, is considering how the energy captured in solar cells might be stored for future use in a cost-effective way. She is focusing on one of the more promising approaches, which relies on a process known as solar water splitting: the solar cell sits in water, absorbing the sun’s rays until enough energy is produced to split the water molecule into its two components, hydrogen and oxygen. The hydrogen may then be captured and stored (as a gas or liquid or in materials) to be used as a clean fuel when needed in the future. The process of producing hydrogen by photoelectrochemical solar water splitting is, according to the Office of Renewable Energy, “a long-term technology pathway, with the potential for low or no greenhouse gas emissions.”
“Solar water splitting mimics natural photosynthesis, storing solar energy in high energy-density chemical bonds for use on demand,” Halaoui says. “The oxygen evolution reaction (OER), one of the two half-reactions for water splitting, a four-electron four-proton process, is the more kinetically demanding and has been traditionally catalyzed by precious metal iridium and ruthenium oxides.” Without catalysts, the water splitting reaction occurs too slowly and generates too little hydrogen to be economical. But traditional iridium and ruthenium oxide catalysts for OER are rare and expensive.
“In recent years, bimetallic Nickel-Iron-oxides/hydroxides have emerged as viable low-cost alternative catalysts,” Halaoui says. Unlike iridium and ruthenium, nickel and iron are cheap and abundant. Still, the exact mechanism by which iron acts to promote catalysis in nickel oxides remains elusive.
Understanding exactly how iron manages to enhance catalysis of the OER in this catalyst is the focus of Halaoui’s latest paper, published in ACS Catalysis, a journal of the American Chemical Society, and selected as ACS Editors’ Choice. She has conducted, along with graduate student Rida Farhat, and research assistant Jihan Dhainy, a detailed electrochemical study of catalysis of the half-reaction by which oxygen evolves from the water molecule at Ni-Fe oxo/hydroxide. The team investigated the catalytic activity of the nickel-iron-oxo/hydroxide catalyst in the presence and absence of iron in solution and found the presence of iron in solution to be necessary to the sustainability of the catalytic activity. This finding and others in the paper offer insight into this class of catalysts in a water splitting reaction, an important step for rational catalyst design and for long-term operation of these catalysts.
“A fraction of the energy received by our planet from the sun can eliminate all our dependence on fossil fuel,” Halaoui explains. But that solar energy is applied unevenly and inconsistently as the seasons and weather change and vary with the diurnal cycle, which makes an optimized conversion and storage process so important. Halaoui’s work may ultimately inform the design of a catalyst for solar water splitting that makes solar energy conversion into storable hydrogen fuel economical and widespread.
There are currently no commercially available photoelectrochemical solar water splitting systems. At the time of this writing they exist only in labs. This subject is really at the forefront of research on renewable energy. It’s high priority and many scientists in the US and around the globe are studying this process.