Improving the lifetime of solar water splitting cells through materials engineering
Keywords: Chemistry.

Solar radiation is a clean and practically inexhaustible source of energy that, if harnessed properly, could replace fossil fuels and sustain humanity indefinitely. However, two main issues need to be addressed before widespread implementation can be achieved. The first issue is the intermittent nature of sunlight and the problems this presents for power generation during the night. The second problem is the uneven distribution of solar power on the Earth's surface. To solve these problems, a process for storing and transporting solar energy must be developed. An attractive process to meet this need is solar water splitting, which uses solar energy to rearrange the bonds of H2O to form H2 and O2. Because this process is endergonic, the conversion of water into hydrogen and oxygen stores the solar energy in the newly formed chemical bonds. While water splitting is relatively straightforward in theory, carrying out the process cheaply and efficiently is a challenge that requires major scientific and engineering breakthroughs. During my Ph.D. research, I have worked with Professor Dai to develop materials and devices to efficiently and economically store light energy as hydrogen fuel, with special focus on material stability. We initially developed Ni-passivated silicon photoanodes and photocathodes that could perform light-assisted water electrolysis for longer periods than previous electrodes in alkaline environments, an important breakthrough for the field. A thin and uniform coating of Ni (2 nm) on n-type silicon allowed the device to carry 10 mA/cm2 of water oxidation current in a potassium borate-based electrolyte (pH = 9.5) for at least 100 hrs without showing decay. In a similar architecture, we found that a dual layer structure (5 nm Ni on 10 nm Ti) on p-type silicon was able to last at least 12 hours in a similar electrolyte. The Ni and Ti layers on the Si electrodes not only provided improved stability and catalytic performance but also formed a solid-state Schottky junction with the semiconductors, allowing for relatively large photovoltages to drive the water splitting reactions. Next, we developed a solar cell-electrolyzer tandem device capable of converting visible light energy to chemical energy (hydrogen fuel) at very high efficiencies. Using a previously reported nickel-iron layered double hydroxide oxygen evolution catalysts and a newly developed hydrogen evolution catalyst consisting of Cr2O3 blended into a Ni-NiO heterostructure (CrNN) on Ni foam, an electrolyzer was constructed that was capable of driving 200 mA/cm2 of current at 1.75 V. When paired with a high-efficiency GaAs solar cell from Alta Devices, the pair was able to split water at an efficiency of 15% under 100 mA/cm2 AM 1.5G illumination (i.e. one sun). In order to lower the operating cost of the electrolyzer, we modified the electrodes to allow them to electrolyze alkaline seawater instead of alkaline purified water (a precious resource). Chloride-mediated anodic corrosion of the OER electrode became a serious problem in our seawater splitting experiments and was solved by the development of a NiSx/NiSO4 passivation layer. This layer allowed for the device to operate at 400 mA/cm2 for at least 1000 hr in 1 M KOH + 0.5 M NaCl (roughly the concentration of NaCl in seawater). Further experiments have also shown that an electrode without the NiSx/NiSO4 coating can be stabilized for 1000 hr by simply adding Na2SO4 into the electrolyte. These findings have important implications for future seawater splitting applications. Finally, we have developed an electrodeposited version of our previously reported CrNN catalyst that is easier to fabricate and shows similar activity. In addition, we have shown that it can also operate in near-neutral phosphate (pH = 7.4) and convert water into hydrogen at extremely low overpotentials. To match the high activity of our neutral HER electrode, we developed a method for in-situ conversion of a nickel-iron foam into a phosphate-intercalated nickel-iron hydroxide. The pair is capable of electrolyzing 1 M phosphate (pH = 7.4) at 50 mA/cm2 for up to 100 hr without significant performance decay. Our research efforts over the last six years have resulted in advancements in photoelectrochemistry, electrolyzer engineering, seawater electrolysis and neutral hydrogen and oxygen evolution catalysis.