Adv. Energy Mater. http://doi.org/f3mrbx (2016)
Photoelectrochemical devices for solar-driven water splitting harness energy from the Sun to produce fuel, in the form of hydrogen. Typically the same electrolyte, which is often a strong acid or alkali, is used in both the cathodic chamber and the anodic chamber of the device where hydrogen and oxygen are generated, respectively. Finding earth-abundant, active components for the electrodes that are stable in the same electrolyte is challenging and many devices can suffer from poor stability. Chengxiang Xiang, Nathan Lewis and colleagues at the California Institute of Technology now report a safe, stable, integrated device with a large photoactive area (>1 cm2) that makes use of a bipolar membrane to allow solutions of different pH to be used in each chamber, achieving a solar-to-hydrogen efficiency of 10%.
By matching the electrode components to the appropriate electrolyte in terms of chemical stability (cathode — acid; anode — alkali), the researchers were able to demonstrate stable operation for over 100 hours for a device consisting of an oxide-protected tandem-junction photoanode (GaAs/InGaP) coupled with a nickel catalyst for oxygen evolution, separated from the cobalt phosphide-based cathode by a commercially available bipolar membrane. The membrane maintains a steady pH gradient and due to its low permeability to both hydrogen and oxygen, there is little crossover between the products, mitigating the risk of formation of explosive gas mixtures.