Separating hydrogen from water molecules through electrolysis requires energy, and ideally, this energy should come from renewable sources. Scientists at the Fraunhofer Institute for Solar Energy Systems (ISE) have developed a photovoltaic/electrolysis combination module that can efficiently use solar energy to produce hydrogen directly. The principle involves using the high photovoltage of multijunction solar cells to split water molecules, with these solar cells being electrically and thermally connected to the electrolyzer cells. A demonstrator tested outdoors by the research team can convert up to 31.3% of solar energy into chemical energy, based on the higher heating value of hydrogen.

"For the photovoltaic/electrolysis module we developed, we employed what is known as concentrator photovoltaics technology," explains Dr. Juan Francisco Martínez Sánchez, project manager at Fraunhofer ISE. "An array of Fresnel lenses focuses direct sunlight onto high-efficiency III-V solar cells, which in this application achieve an open-circuit voltage of more than 4 volts." III-V solar cells are the world's most efficient solar cells, proven over many years and long-term stable. Currently, they are primarily used in space applications, but they can also generate electricity cost-effectively on Earth in concentrator photovoltaic modules.

"To produce hydrogen, we connected these solar cells directly to the cathode and anode of two series-connected PEM electrolyzer cells, achieving a perfect match between their electrical characteristics," adds Dr. Tom Smolinka, head of the Membrane Electrolysis Department at Fraunhofer ISE. As a result, the electricity generated by the solar cells is used directly for hydrogen production without any intermediate conversion steps, thereby avoiding losses. In field testing, the new photovoltaic/electrolysis module demonstrator, with a lens area of 64 square centimeters, converted up to 31.3% of solar energy into chemical energy, based on the higher heating value of hydrogen.

"Our new record demonstrates that sunlight can be used to produce hydrogen directly and efficiently," states Dr. Frank Dimroth, head of the III-V Photovoltaics and Concentrator Technology Department at Fraunhofer ISE. "This development is still in its early stages, and it is difficult to say how soon we can achieve a competitive system. To further advance this concept, we are seeking investors for our planned spin-off startup, Clearsun Energy."