Investigation into stability of iridium catalyzed water oxidation photoanodes
Keywords: Materials Science and Engineering.

As various forms of renewable energy see increased adoption in power grids worldwide, effectively harnessing solar energy has revealed itself to be a matter of upmost importance. The conversion of solar energy into chemical fuels by photoelectrochemical means is an attractive method of utilizing this abundant resource, and has been researched extensively in recent years. While these technologies are still far from commercial implementation, the ability to convert renewable energy into clean chemical fuels will be a key component of a sustainable energy economy. Metal-insulator-semiconductor (MIS) junctions are an attractive device architecture for the photoelectrochemical conversion of solar energy into chemical fuels, coupling intrinsically unstable semiconductor absorbers to efficient catalyst systems. Critical to the success of such devices was the development of atomic layer deposited (ALD) TiO$_2$ as a protection layer, which ensured the survival of the underlying semiconductor while electronically coupling it to the overlying catalyst layer. In order for any such device to be competitive with existing technologies, it must be stable and efficient enough to be cost competitive. While many current works demonstrate MIS photoelectrochemical cells with high efficiencies, few are stable enough to be considered commercially. In this work, I examine issues surrounding the stability of MIS photoelectrochemical cells, as well as the application of the MIS device paradigm to the chlorine evolution reaction. Generally the failure of photoelectrochemical cells protected by ALD layers has been attributed to corrosion of the underlying substrate, a case where the protection layer was unable to isolate the substrate from the electrolyte. In the first part of this work, I investigate the interactions at the catalyst-protection layer interface and unravel the failure modes associated with it. Here I demonstrate that the ALD-TiO$_2$ protection layer is effective at completely preventing oxidative attack of the underlying substrate, and that device failure is primarily dictated by the catalyst/protection layer adhesion. In the case of an iridium catalyst and ALD-TiO$_2$ protection layer, the adhesion energy is more than double that of the unprotected case, resulting in significantly improved electrochemical stability. These results indicate the importance of catalyst adhesion to an interposed protection layer in promoting the operational stability of MIS water splitting photoanodes. Second, I studied the iridium catalyst layer itself, unraveling the dynamics of a reversible activity decay that occurs within minutes of performing the oxygen evolution reaction. The potential dependence of the activity was explored, and a brief excursion ($< 1$ second) to 0.0 V vs NHE was found to be an effective potential to recover the initial water oxidation performance. Iridium thin films on rotating disk electrodes were used to rule out limited diffusion in the electrolyte as a mechanism of activity decay and to establish that the decay is independent of film thickness. Careful examination of the time dependence of the activity decay from the 2 second to 10$^2$ seconds time scales reveals that it is well described by a t$^{-1/4}$ functional dependence across multiple electrochemical cell geometries. The Tafel behavior is analyzed by normal pulse voltammetry, suggesting that, after 10 minutes of activity decay, the iridium catalyst exhibits a five-fold decrease in active site density while the mechanism of water oxidation is not altered. Third, I showed these same iridium catalyzed MIS photoandoes could also affect the photo-oxidation of chloride to chlorine gas in a selective and stable manner. These photoanodes represent a nanoscale analog to the dimensionally-stable anode, which is the industry standard for the chlorine oxidation reaction. Under 1 sun irradiation, a photovoltage of 0.566 V is achieved. Although this n-Si/TiO$_2$/Ir photoanode is only one half of an eventual tandem cell needed for photosynthetic brine splitting, its applied bias photon to current efficiency for chloride oxidation is 1.42\%, roughly 28 times that for water oxidation. The illuminated n-Si/TiO$_2$/Ir photoanode remained stable at 1 mA cm$^{-2}$ during a six-day chronopotentiometry test.