Researchers at Stanford University have demonstrated solar water splitting by photovoltaic-electrolysis with a solar-to-hydrogen (STH) efficiency of more than 30%—a new record. The prior record was 24.4%. An open-access paper on their work is published in the journal Nature Communications.
The system consists of two polymer electrolyte membrane electrolyzers in series with one InGaP/GaAs/GaInNAsSb triple-junction solar cell, which produces a large-enough voltage to drive both electrolyzers with no additional energy input. The solar concentration is adjusted such that the maximum power point of the photovoltaic is well matched to the operating capacity of the electrolyzers to optimize the system efficiency. The results, the researchers said, demonstrate the potential of photovoltaic-electrolysis systems for cost-effective solar energy storage.
|The PV-electrolysis system consists of a triple-junction solar cell and two PEM electrolysers connected in series. Jia et al. Click to enlarge.|
As the installed capacity of photovoltaics (PVs) continues to grow, cost-effective technologies for solar energy storage will be critical to mitigate the intermittency of the solar resource and to maintain stability of the electrical grid. Hydrogen generation via solar water splitting represents a promising solution to these challenges, as H2 can be stored, transported and consumed without generating harmful byproducts. However, the cost of H2 produced by electrolysis is still significantly higher than that produced by fossil fuels. … To be practical for large-scale deployment, the cost of solar H2 generation must be significantly reduced.
Previous studies have predicted that achieving a high solar-to-hydrogen (STH) efficiency is a significant driving force for reducing the H2 generation cost. … there is significant room for further improvement in the performance of PV-electrolysis system prototypes.
The discrepancy between reported STH efficiencies for PV-electrolysis devices and stand-alone solar-to-electricity PV efficiencies mainly arises from poor matching of the current–voltage (I–V) characteristics of multi-junction PVs with those of water electrolysers. The maximum power-point voltage (VMPP) of a typical commercial triple-junction solar cell is in the range of 2.0–3.5 V under 1–1,000 suns of illumination. However, the thermodynamic minimum voltage required to electrolyse water is only 1.23 V at 300 K, with practical operating voltages in the range of 1.5–1.9 V. Electrolysing water using a voltage in excess of the thermodynamic minimum voltage results in energy wasted as heat rather than stored in H2 chemical bonds. Previously, this limitation was overcome by coupling multiple PV and/or electrolyser units in series, to optimize the match between the voltage characteristics of these device components, although the efficiencies achieved were still far from optimal.—Jia et al.
The team led by Thomas Jaramillo, an associate professor of chemical engineering and of photon science, and James Harris, a professor of electrical engineering, used triple-junction solar cells—each material is tuned to capture blue, green or red light, respectively. Through this precision, triple-junction solar cells convert 39% of incoming solar energy into electricity, compared with roughly 20% for silicon-based, single-junction solar cells found on rooftops worldwide.
Jaramillo and his collaborators built on research they have been conducting on how to improve the performance of catalysts. Much of the catalytic process in the Stanford experiment is built on their previous advances in the area, with one particularly important approach to achieve the record energy capture. Most photovoltaic-powered water-splitting reactions use a single electrolysis device, but this team was able to combine two identical electrolysis devices in such a manner to produce twice as much hydrogen, making use of their higher-efficiency solar cells.
Even with the record efficiency, the cost of the system would still be high for an industrial process. Jaramillo said that they team now has to find ways to get similar results with less expensive materials and devices.
Jieyang Jia, Linsey C. Seitz, Jesse D. Benck, Yijie Huo, Yusi Chen, Jia Wei Desmond Ng, Taner Bilir, James S. Harris & Thomas F. Jaramillo (2016) “Solar water splitting by photovoltaic-electrolysis with a solar-to-hydrogen efficiency over 30%” Nature Communications 7, Article number: 13237 doi: 10.1038/ncomms13237