Understanding the Impact of Different Types of Surface States on Photoelectrochemical Water Oxidation: A Microkinetic Modeling Approach

Abstract

The oxygen evolution reaction (OER) has been identified as one of the performance-limiting processes in solar water splitting using photoelectrochemical (PEC) cells. One of the reasons for the low OER performance is related to the existence of different types of surface states at the semiconductor–electrolyte interface: recombining surface states (r-SS) and surface states due to intermediate species (i-SS). Since the impact of surface states on OER is still under debate, we investigate how different types of surface states affect PEC water oxidation and how they impact experimental measurements. In a new computational approach, we combine a microkinetic model of the OER on the semiconductor surface with the charge carrier dynamics within the semiconductor. The impact of r-SS and i-SS on the current–voltage curves, hole flux, surface state capacitance, Mott–Schottky plots, and chopped light measurements is systematically investigated. It is found that (a) r-SS results in a capacitance peak below the OER onset potential, while i-SS results in a capacitance peak around the onset potential; (b) r-SS leads to an increase in the OER onset potential and a decrease in the saturation current density; (c) r-SS leads to Fermi-level pinning before the onset potential, while i-SS does not result in Fermi-level pinning; and (d) a smaller capacitance peak of i-SS can be an indication of the lower catalytic performance of the semiconductor surface. Our approach in combination with experimental comparison allows distinguishing the impact of r-SS and i-SS in PEC experiments. We conclude that r-SS reduces the OER performance and i-SS mediates the OER.

Calculation of the Tafel slope and reaction order of the oxygen evolution reaction between pH 12 and pH 14 for the adsorbate mechanism

“…This is mainly caused by the fact that oxygen evolution reaction is a complex process that requires the transfer of four electrons and four protons. Most of our knowledge about likely mechanisms is obtained from theoretical works, either by density functional theory or kinetic modeling or a combination of both [7][8][9][10][11][12][13][14][15][16][17][18]. The pH dependence of the calculated properties has thus far received little attention.…”

Computational Study Revealing the Influence of Surface Phenomena in p-GaAs Water-Splitting Cells

“…Photoelectrochemical (PEC) water splitting is a promising pathway to store solar energy in a chemical form, making it possible to produce carbon neutral fuel. − Since its discovery and early developments in the 1970s, , it has drawn increasing attention, both in terms of major improvements in efficiency and durability and in terms of the theoretical understanding of the physics behind the phenomenon . While the basic operational principles of a photoelectrochemical cell are well-known, understanding the dynamics of the charge transport in the photoactive material and the charge transfer to the electrolyte is still under investigation. , While the nominal free energy required to split water is 237.24 kJ/mol, resulting in a minimum electron potential of 1.23 eV, various losses must be taken into account . Both hydrogen and oxygen evolution reactions have overpotentials that depend on the quality of the catalyst used to drive the reaction, and some of the photoexcited electron–hole pairs are lost in the recombination processes both in the bulk semiconductor and at the surface. , …”

“…GaAs is one of the oldest and most studied semiconductor materials applicable to water splitting , and to other light-harvesting devices, , thanks to its suitable band gap of 1.4 eV and good alignment of the valence and conduction band edges with the water-splitting reaction potentials. They have been shown to achieve high efficiencies in water splitting while retaining relatively simple structure − and, on occasion, also high stability. , In order to accurately estimate the overpotentials, knowledge of the potential of the valence and conduction band edges with respect to the reaction potentials is crucial. , Both experimental and theoretical investigations are extensively available in the literature, ,, and a significant amount of work exists concerning modeling the carrier dynamics and the current–voltage response in photoelectrochemical cells, in general, utilizing a variety of different approaches. ,,,− Recently, also simulating electrochemical impedance spectroscopy (EIS) or Mott–Schottky response of the cell has gained some attraction, but these are so far limited to analytic models. , …”

“…6 While the basic operational principles of a photoelectrochemical cell are wellknown, 1 understanding the dynamics of the charge transport in the photoactive material and the charge transfer to the electrolyte is still under investigation. 6,7 While the nominal free energy required to split water is 237.24 kJ/mol, resulting in a minimum electron potential of 1.23 eV, various losses must be taken into account. 8 Both hydrogen and oxygen evolution reactions have overpotentials that depend on the quality of the catalyst used to drive the reaction, and some of the photoexcited electron−hole pairs are lost in the recombination processes both in the bulk semiconductor and at the surface.…”

“…6,8,26,28−30 Recently, also simulating electrochemical impedance spectroscopy (EIS) or Mott−Schottky response of the cell has gained some attraction, but these are so far limited to analytic models. 7,31 T h i s c o n t e n t i s We present this work that focuses on investigating the band structure of p-doped GaAs at the solid−electrolyte interface, by combining experimental measurements of the photoelectrochemical cell with a numerical model describing the photoinduced carrier generation, mass transfer in the semiconductor, and carrier transfer dynamics at the semiconductor−electrolyte interface. The simulation combines a fully self-consistent solution of the drift-diffusion equations with an optical model that accounts for illumination losses.…”