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Stabilizing Silicon
Solar-driven water splitting has potential as an energy storage mechanism to supplement the direct conversion of sunlight to electricity. A submersed integrated device has been proposed both to absorb the light and to catalyze the reaction, but stability has been a problem. Kenney et al. (p. 836; see the Perspective by Turner) found that a nickel coating, thin enough to let light through, could protect a silicon absorber in the alkaline environment of a lithium/potassium borate electrolyte. The nickel also functioned as the oxidation catalyst, and the lithium ions helped to establish a protective film structure in situ.
Abstract
Silicon’s sensitivity to corrosion has hindered its use in photoanode applications. We found that deposition of a ~2-nanometer nickel film on n-type silicon (n-Si) with its native oxide affords a high-performance metal-insulator-semiconductor photoanode for photoelectrochemical (PEC) water oxidation in both aqueous potassium hydroxide (KOH, pH = 14) and aqueous borate buffer (pH = 9.5) solutions. The Ni film acted as a surface protection layer against corrosion and as a nonprecious metal electrocatalyst for oxygen evolution. In 1 M aqueous KOH, the Ni/n-Si photoanodes exhibited high PEC activity with a low onset potential (~1.07 volts versus reversible hydrogen electrode), high photocurrent density, and durability. The electrode showed no sign of decay after ~80 hours of continuous PEC water oxidation in a mixed lithium borate–potassium borate electrolyte. The high photovoltage was attributed to a high built-in potential in a metal-insulator-semiconductor–like device with an ultrathin, incomplete screening Ni/NiOx layer from the electrolyte.
