Fraunhofer ISE achieves 31.3% record solar-to-hydrogen efficiency in photovoltaic electrolysis

<p class="p1"><span class="s1">Researchers at the German institute built a photovoltaic water electrolysis system based on micro-concentrator photovoltaics coupled to proton exchange membrane electrolysis. Outdoor testing demonstrated a record solar-to-hydrogen efficiency of 31.3%, achieved by a four-junction CPV system driving two PEM cells in series under real operating conditions.</span></p><p>Researchers at the <a href="https://www.pv-magazine.com/2025/11/14/fraunhofer-ise-builds-first-perovskite-silicon-tandem-solar-cell-with-copper-metallization/" rel="noopener" target="_blank">Fraunhofer Institute for Solar Energy Systems</a> (Fraunhofer ISE) in Germany have developed a photovoltaic water electrolysis system that utilizes its own <a href="https://www.pv-magazine.com/2025/06/05/fraunhofer-ise-unveils-micro-cpv-module-with-cost-reducing-design/" rel="noopener" target="_blank">micro concentrator photovoltaics (micro-CPV) technology</a>.</p>
<p>The scientists explained that prior approaches using dual- and triple-junction III-V concentrator cells reached up to 19.8% solar-to-hydrogen efficiency (SHT) outdoors and around 30% indoors, but required careful matching of voltage, current, and system configuration. Their new work demonstrated a four-junction concentrator system driving PEM cells outdoors, achieving a record 31.3% STH efficiency.</p>
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<p><span lang="en-US">&#8220;We are still at low technology readiness level (TRL) and therefore it is hard to say how quickly we can get to a low levelized cost of hydrogen which is competitive. We first need partners to develop the system fully,&#8221; Frank Dimroth told <strong>pv magazine</strong>. &#8220;With <a href="https://www.pv-magazine.com/2025/06/05/fraunhofer-ise-unveils-micro-cpv-module-with-cost-reducing-design/" rel="noopener" target="_blank">Clearsun Energy</a>, we try to create a startup to commercialize concentrating photovoltaics and this solar hydrogen module could be a future generation product for the company.&#8221;</span></p>
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<p><span lang="en-US">The TRL measures the maturity of technology components for a system and is based on a scale from one to nine, with nine representing mature technologies for full commercial application. &#8220;I would say our system is a proof of concept which is TRL3,&#8221; Dimroth added. &#8220;Currently we have no funding to build a pilot system but of course this would be the next step.&#8221;</span></p>
<p>In the paper &#8220;<a href="https://www.nature.com/articles/s44172-026-00610-x" rel="noopener" target="_blank">Photovoltaic water electrolysis reaching 31.3% solar-to-H₂ conversion efficiency under outdoor operating conditions</a>,&#8221; published in <em>communications engineering</em>, the Fraunhofe ISE researchers explained the electrolysis sytem is driven by the propietary HyCon system, which consists of Fresnel lens arrays focusing light onto four parallel-connected 4-junction CPV cells with a size of 7 mm² each, which are in turn electrically and thermally linked to the anode and cathode of two <span>proton exchange membrane</span> (PEM) electrolyzer cells connected in series.</p>
<p>An aluminum frame holds a Fresnel lens array at an 80 mm focal distance from the CPV solar cells, with screw adjustment for fine-tuning alignment. The solar cells are mounted on copper (Cu) substrates fixed to a large copper baseplate, which also supports the overall thermal and structural integration. A series-connected PEM electrolysis stack is attached to the rear of the baseplate, electrically and thermally linked to the CPV system via titanium (Ti) screws and the Cu interface.</p>
<figure class="wp-caption aligncenter" id="attachment_342090" style="width: 600px;"><img alt="" class="size-medium wp-image-342090" height="374" src="https://www.pv-magazine.com/wp-content/uploads/2026/05/Screenshot-2026-05-15-104456-600x374.png" tabindex="0" width="600" /><figcaption class="wp-caption-text">Schematic of the system <p><i>Image: Fraunhofer ISE, communications engineering, CC BY 4.0 </i></p>
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<p>The CPV solar cells are built by wafer-bonding of two dual-junction structures, namely gallium indium phosphide (GaInP)/gallium arsenide (GaAs) and gallium indium arsenide phosphide (GaInAsP)/gallium indium arsenide (GaInAs). &#8220;This 4 J solar cell technology has demonstrated world record solar-to-electricity (STE) conversion efficiencies of up to 47.6% under the concentrated reference AM1.5 direct spectrum,&#8221; the scientists emphasized.</p>
<p>The PEM electrolyzer consists of two machined chlorinated polyvinyl chloride (PVC-C) plates that guide deionized water to the reaction chamber containing the membrane electrode assembly (MEA), which uses a 175 μm perfluorosulfonic acid (PFSA) membrane with a 1.13 cm² active area, coated with iridium at the anode and platinum at the cathode as catalysts. A titanium screw presses a titanium mesh onto the MEA to act as a porous transport layer and flow field for water distribution and product removal.</p>
<p>The whole system was designed to operate the electrolysis stage at elevated temperatures, ideally through thermal coupling with the CPV array.<br />In its current version, however, only limited passive heat transfer was achieved, so additional inlet water heating was required to sustain stable operation and maintain efficiency. &#8220;Hence, active heating will be avoided through an enhanced thermal coupling between the CPV and electrolysis cells in a future design,&#8221; the academics emphasized.</p>
<p>The conducted field testing of the CPV/PEM electrolysis system using a dual-axis solar tracker over 13 summer days in Freiburg, Germany, and found the system can achieve hydrogen production with a solar-to-hydrogen (STH) efficiency of 31.3%. &#8220;This is 5% higher than the best photovoltaic/electrolysis systems reported in literature which range between 20 and 30%,&#8221; the team said.</p>
<p>This peak performance corresponded to operating conditions where the CPV array and PEM electrolysis stack reached efficiencies of 34.7% and 91.1%, respectively. At this operating point, the system operated at a current density of 368 mA/cm² and a cell voltage of 3.25 V. &#8220;No degradation was observed during the 107 hours of operation in which our system went through 13 dynamic cycles,&#8221; the researchers concluded, noting that increasing the capacity factor of the HyCon technology to 35% could enable a levelized cost of hydrogen (LCOH) below $3/kg.</p>
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