Operational Characteristics and Failure Modes of Protected Si Anodes for Sunlight-Driven Water Oxidation
Authors: Papadantonakis Kimberly , Sun Ke , Lewis Nathan S 
Published: February 2022

The development of a fully integrated photoelectrochemical cell capable of efficiently producing carbon-neutral fuels from sunlight, water, and atmospheric carbon dioxide is challenged by the electrochemical corrosion of photoelectrodes at the potentials required to drive the desired redox reactions. Recent progress in the development of protective coatings that integrate transparent conductive oxides with electrocatalysts has extended the operational lifetime of technologically important semiconductors such as Si, GaAs, and InP for use as photoanodes in aqueous alkaline electrolytes from seconds to hundreds or thousands of hours. Although an understanding of the failure modes for such devices has not yet been developed fully, systematic examinations of failure modes could lead to methods for improving the maximum attainable lifetimes, and would support the development of accelerated stability-testing protocols as well as modeling and simulation efforts to estimate device durability.

This presentation will first provide a brief overview of recent progress in the development of protective coatings and catalyst-placement strategies for the stabilization of semiconductor electrodes for use in integrated solar fuels systems, and will then provide an in-depth examination of the failure mechanisms governing the performance and durability of a model system – Si anodes patterned with micrometer-scale Ni islands operating in contact with 1.0 M KOH (aq). In the patterned-catalyst approach, an insoluble protective oxide layer is grown over areas of the semiconductor not covered by the catalyst islands, and during operation the current is collected at and passed through the catalyst islands. For semiconductors such as Si that form insoluble oxides under water-oxidation conditions, the protective oxide layer can be grown in situ. The patterned-catalyst approach therefore can be considered a model system for protective coatings with pinhole or grain-boundary defects – where stability depends upon on passivation of the semiconductor beneath regions of the coating that allow direct contact with the electrolyte – allowing top-down definition and spatial control over regions analogous to through-film defects that are otherwise difficult to control and vary systematically.

Ex situ and operando electrochemical, microscopic, and spectroscopic techniques were used to investigate the performance and stability of Si anodes patterned with a square array of micrometer-sized Ni islands and operated in contact with 1.0 M KOH (aq). Non-photoactive p+-Si(111) substrates were used to evaluate the stability of the catalyst as well as the formation of the passive SiOx layer. The impact of the diurnal cycle on the stability of the electrodes was evaluated by investigating the behavior of Si(100) substrates under open-circuit conditions. Buried-junction np+-Si(111) substrates were used to evaluate performance and stability under simulated solar illumination. The stability and efficiency of the patterned-catalyst Si electrodes were affected by the filling fraction of the Ni catalyst, the orientation and dopant type of the substrates, and the measurement conditions. The electrochemical behavior at different stages of operation, including Ni catalyst activation, Si passivation, steady-state operation, and device failure was affected by the dynamic processes of anodic formation and isotropic dissolution of SiOx on the exposed Si.

Buried-junction np+-Si(111) photoanodes with an 18.0% filling fraction of a square array of Ni microelectrodes demonstrated performance equivalent to a Ni anode in series with a photovoltaic device having an open-circuit voltage of 538 ± 20 mV, a short-circuit current density of 20.4 ± 1.3 mA cm-2, and a photovoltaic efficiency of 6.7 ± 0.9%. For these samples the photocurrent density at the equilibrium potential for oxygen evolution was 12.7 ± 0.9 mA cm-2, yielding an ideal regenerative cell solar-to-oxygen conversion efficiency of 0.47 ± 0.07%. The photocurrent passed exclusively through the Ni catalyst islands to evolve O2 with nearly 100% faradaic efficiency. However, the passivating layer of SiOx dissolves in KOH, resulting in Si corrosion and SiOx dissolution especially in the dark. The dynamic processes of SiOx formation and etching affect both the electrical stability of the electrochemical and photovoltaic components, as well as the optoelectronic stability of the photovoltaic component. Localized undercutting of catalyst islands and damage to the emitter profile correlated with the current distribution on sample surfaces, suggesting substantial current branching at the location where the active catalysts and the corrosive solution are both present. This work provides evidence of one likely failure mechanism for Si photoanodes protected by transparent catalytic films, specifically, undercutting and removal of the catalysts at defects in protective coatings that can arise during fabrication, deployment and operation.