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Hydrogen is a key starting point for the climate-friendly reorganisation of industrial processes. However, an energy source that burns without releasing CO2 should, if possible, also be produced without a CO2 footprint. A classic process for this is electrolysis, in which water is broken down into hydrogen and oxygen using electricity. If the electricity required for electrolysis comes from renewable sources such as photovoltaics, this is known as green hydrogen. The disadvantage is that the electrolysers required for this process are usually large and highly complex systems. What's more, the costly and high-maintenance equipment is in danger of becoming a scarce commodity in view of current global and climate policy.

Solar hydrogen production

Direct solar water splitting, or photoelectrochemical cell (PEC), therefore offers an exciting alternative. In the Neo-PEC joint project, researchers from three Fraunhofer Institutes have developed a modular solution for this, which should enable highly flexible hydrogen production and supply using solar energy. The centrepiece of the Fraunhofer development is a tandem PEC module. It is similar to its classic photovoltaic counterpart - with one key difference: the electricity is not generated to be electrolysed elsewhere later. The entire process takes place in one and the same unit. But caution is required: As hydrogen and oxygen are produced in the process, the structure must be designed in such a way that these elements remain strictly separated from each other.

High-purity semiconductor materials ensure increased hydrogen yield

For the tandem cell, the experts coat commercially available float or flat glass on both sides with semiconducting materials. When exposed to sunlight, one side of the module absorbs the short-wave light. At the same time, the long-wave light penetrates the upper glass layer and is absorbed on the reverse side. In the process, the module releases hydrogen on the reverse or cathode side and oxygen on the upper, anode side. Over the three-year duration of the project, the Fraunhofer scientists researched and developed high-purity semiconductor materials, which they applied using particularly gentle coating processes. This enables them to increase the hydrogen yield of the process.

A turbo that accelerates activity

"We use the gas phase to build up nanometre-thick layers on the glass. The resulting structures have a major influence on the reactor activity, in addition to the actual material properties, which we have also optimised," explains Dr Arno Görne, Group Manager 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 an additional voltage: it acts like a turbo that accelerates the activity and further increases the efficiency.

Square, practical - safe

The result is a reactor with an active surface area of half a square metre. Separated from the oxygen, it produces the hydrogen, which can be collected and quantified immediately. Currently, a single module with European solar radiation produces more than 30 kilos of hydrogen per year per 100 square metres. With this yield, a hydrogen car, for example, could cover 15,000 to 20,000 kilometres.

From single modules to large areas

"The dimensions of the tandem cell are limited by the fact that our module splits the water directly, but electricity also has to pass from one side to the other. As the module area increases, the increasing resistances have an unfavourable effect on the system. The current format has proven to be optimal. It is stable, robust and significantly larger than all comparable solutions," emphasises Görne. The compact elements can be interconnected as required without any negative side effects, from a single module to large areas - a key advantage of the Fraunhofer solution.

Linking competences

According to those responsible, the project is also a successful example of cross-institute collaboration and the combination of complementary Fraunhofer competences: As part of the project, which has now been finalised, Fraunhofer IKTS researched materials and processing for the photoactive layer. Colleagues from the Fraunhofer Institute for Surface Engineering and Thin Films IST contributed their experience in large-area coating using physical vapour deposition. The reactor design, cost-effective and reliable production and subsequent evaluation of the modules were in the hands of the experts at the Fraunhofer Centre for Silicon Photovoltaics CSP.

Cooperation with companies desired

The project partners have already proven in numerous field tests that the module and the interconnection work stably and smoothly. However, the Fraunhofer teams, who successfully presented their reactor to the public for the first time in June 2024, have long been planning the next steps: On the one hand, they aim to continue their successful institute collaboration in a follow-up project; on the other, they plan to further develop their solution in various directions in cooperation with companies - for direct, safe and efficient decentralised hydrogen production and supply.