Solar photovoltaic cells (PV) provide a renewable method of both generating electric current and storing fuels as hydrogen gas. The existence of this technology offers the hope of replacing our increasingly scarce fossil fuels with a dependable, infinite alternative; however, the monetary cost of manufacturing these cells has blocked their implementation. Traditionally, the PV process required the use of “prohibitively expensive light-absorbing materials [e.g., (Al)GaAs and GaInP], and/or fuel-forming catalysts (e.g., Pt, RuO2, IrO2), and strongly acidic or basic reaction media, which are corrosive and expensive to manage over the large areas required for light harvesting” (Reece et. al 2011). The aim of Reece and his colleagues is to mimic the photosynthetic process with earth-abundant materials under neutral pH conditions. They were successful in achieving this goal for both wired and wireless solar photo-electrochemical systems (PEC), replicating leaf photosynthesis to fix hydrogen and oxygen gas. For the wired system they were able to obtain an efficiency of 4.7%, while the wireless system had an efficiency of 2.5%. The continual development of this technology offers greater potential for solar energy to replace traditional sources due not only to necessity, but due to market factors as well.—Donald Hamnett
The specific compounds Reece and his colleagues discovered that make this process work are: a cobalt oxygen evolving catalyst (Co-OEC), a nickel-molybdenum-zinc hydrogen evolving catalyst (NiMoZn), and a triple junction amorphous silicon (3jn-a-Si) interface between the catalysts which is coated with indium tin oxide (ITO). In the wired case, the NiMoZn catalyst was deposited on a nickel mesh substrate, which was wired to the 3jn-a-Si electrode. In the wireless case, the NiMoZn catalyst was deposited directly onto the silicone electrode’s adjacent stainless steel surface. Baseline values for the 3jn-a-Si cell were obtained by operating the cell in a three-electrode voltammetry setup. Under a no-light condition, the cell had a current of less than .05 mA/cm2, a low value. Upon illumination of 1 sun (100mW/cm2) of air mass (AM) of 1.5, the cell produced a current of .39 mA/cm2 at a potential of 0.55V when using a 1M potassium borate electrolyte (pH 9.2). To increase the efficiency in this example, they added the 0.25 mM Co2+(aq) catalyst. The Co-OEC increased the current to 4.17 mA/cm2. Bubbles at both electrodes indicated the formation of oxygen and hydrogen gas. The experiment went on to study the effect of differing thicknesses of Co-OEC films on the electrodes’ surfaces. It was found that at a 5 minute deposition time, the thin (85 nm) film created photoanodes with optimum performance. There is a give and take between Co-OEC presence and performance of the cell, because though it increases activity, the built up film also blocks incident radiation. When they tested the wireless cell, it was found to be stable for 10 hours. It was discovered that the stability of the wireless cell is dependent on the type of conductive oxide barrier layer used.
Though there is still work to be done to increase the efficiency and stability of these PEC cells, Steven Reece and his team has succeeded in replicating the photosynthetic functions of a leaf. Additionally, the fact that they have done so in a relatively neutral pH means that hydrogen and oxygen fuels could be generated without a membrane, as the two gases are very insoluble at a neutral pH. Lastly, the most notable aspect of this research is that they have replicated photosynthesis with low-cost, earth-abundant materials. The continued research in this area may very well spark an energy revolution, as the decreased overhead will incentivize investment in solar technology