National Renewable Energy Laboratory (NREL) scientists have recaptured the world record for the highest efficiency in solar hydrogen production.
This was made possible via a photoelectrochemical (PEC) water-splitting process.
The new solar-to-hydrogen (STH) efficiency record of 16.2 percent topped a reported 14 percent efficiency in 2015 by an international team made up of researchers from Helmholtz-Zentrum Berlin, TU Ilmenau, Fraunhofer ISE and the California Institute of Technology.
Both the old and new PEC processes employ stacks of light-absorbing tandem semiconductors that are immersed in an acid/water solution (electrolyte) where the water-splitting reaction occurs to form hydrogen and oxygen gases. But unlike the previous device, the new PEC cell is grown upside-down, from top to bottom, resulting in a so-called inverted metamorphic multijunction (IMM) device.
This IMM advancement allowed the NREL researchers to substitute indium gallium arsenide (InGaAs) for the conventional GaAs layers, improving the device efficiency considerably.
A second key distinguishing feature of the new advancement was depositing a very thin aluminum indium phosphide (AlInP) “window layer” on top of the device, followed by a second thin layer of GaInP2. These extra layers served both to eliminate defects at the surface that otherwise reduce efficiency and to partially protect the critical underlying layers from the corrosive electrolyte solution that degrades the semiconductor material and limits the lifespan of the PEC cell.
The scientists nevertheless caution that before the PEC technology can be commercially viable, the cost of hydrogen production needs to come down to meet DOE’s target of less than $2 per kilogram of hydrogen. Continued improvements in cell efficiency and lifetime are needed to meet this target.
Further enhanced efficiency would increase the hydrogen production rate per unit area, which decreases hydrogen cost by reducing balance-of-system expenditures. In conjunction with efficiency improvements, durability of the current cell configuration needs to be significantly extended beyond its several hours of operational life to dramatically bring down costs.
NREL researchers are actively pursuing methods of increasing the lifespan of the PEC device in addition to further efficiency gains.
Content and image: R&D Magazine