While the concept of using sunlight to split water to produce hydrogen (and oxygen) without the interface of electricity (called photoelectrochemical process) is not entirely new, the German Fraunhofer Institute has come up with its own design, which uses semiconductors. Researchers from the institute have collaborated to create a modular solution that enables highly flexible hydrogen generation and supply solar energy for it.
At the heart of this technology is a tandem PEC module. It’s similar to its traditional photovoltaic counterpart, but with one crucial difference: the electricity is not generated for purposes of later electrolysis elsewhere. The entire process takes place in one unit. Caution is needed throughout — since the process results in hydrogen and oxygen, the structure must be designed to maintain a strict separation between the two elements during generation and after.
“To produce the tandem cell, experts coated standard commercially available float or plate glass with semiconducting materials on both sides,” notes a press release from the institute. When the sunlight hits the glass, one side of the module absorbs the short-wavelength light. Meanwhile, the long-wavelength light passes through the upper layer of glass and is absorbed on the reverse side. The module releases hydrogen on the reverse or cathode side and oxygen on the upper/anode side.
Over the project’s three-year term, the Fraunhofer scientists researched and developed high-purity semiconductor materials, which they apply using ultra-gentle coating methods. This allows them to increase the method’s hydrogen yield.
“We use the vapour phase to form layers that are just a few nanometres thick on the glass. The structures created in the process have a huge impact on reactor activity, in addition to the actual material properties, which we have also optimised,” explains Dr Arno Görne, group manager of Functional Materials for Hybrid Microsystems at the Fraunhofer Institute for Ceramic Technologies and Systems IKTS. The photovoltaic elements linked in the module supply the system with additional voltage — that accelerates activity and boosts efficiency.
The result is a reactor with an active surface area of half a square metre. Separated from the oxygen, it generates hydrogen, which can be captured and quantified. Right now, a single module exposed to sunlight under European conditions can generate over 30 kilograms of hydrogen per year over 100 square metres.
“In terms of the dimensions of the tandem cell, we are limited by the fact that our module splits water directly. But it is also necessary for electricity to get from one side to the other to achieve this. As the module area increases, the rising resistance has an unfavourable effect on the system. Currently, the existing format has proven to be optimal. It is stable, robust and significantly larger than any comparable solution,” Görne notes. The compact elements can be connected as needed without any negative side effects, from a single module to large areas – a significant advantage, the release says.