Hematite Photoanodes for Solar Water Splitting: A Detailed Spectroelectrochemical Analysis on the pH-Dependent Performance

Abstract

The effect of the electrolyte pH on the performance of metal oxide photoanodes for solar water oxidation has not been fully resolved, and contrasting views have been presented in recent reports. Herein, a comprehensive set of spectroelectrochemical techniques (impedance spectroscopy, intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS), and operando UV–vis spectroscopy) are deployed to clearly uncover the role of pH on the performance of hematite photoanodes. Our results reveal that, despite the presence of high-valent iron-oxo active sites over a wide pH range (7–13.6), the observed performance improvement with increasing pH is mainly driven by the reduction of surface accumulated charges, in the form of reactive intermediate species, that alleviates Fermi level pinning (FLP). Interestingly, IMPS data provides compelling evidence that the mitigation of FLP originates from changes in the reaction mechanism which boost the rate of charge transfer reducing, in turn, the surface charging. Additionally, we present a phenomenological analysis of the IMVS response which brings to light the additional impact of the electrolyte pH on the surface-related recombination dynamics. Our work identifies the pH-dependent kinetics of water oxidation as the key step governing the performance, defining not only the efficiency of charge transfer across the interface but also the degree of FLP that determines both the photocurrent magnitude and onset potential.

In Situ Electrochemical Oxidation of Cu2S into CuO Nanowires as a Durable and Efficient Electrocatalyst for Oxygen Evolution Reaction

“…76 As shown in Figure S11 (Figure 7e, inset). 77,79,80 Fitting results unambiguously revealed a clearly enhanced charge transfer process on Cu2S/CM_200CVs, with a of 146 , than on CuO/CM-AN_100CVs, with an order of magnitude higher , 1288 .…”

Understanding Surface Recombination Processes Using Intensity‐Modulated Photovoltage Spectroscopy on Hematite Photoanodes for Solar Water Splitting

“…In the absence of a sacrificial agent, even though a net exchange of current through the semiconductor/electrolyte interface is not established under open circuit conditions, surface intermediate states can apparently still be formed (that is, due to the adsorption/desorption of hydroxide and/or high valent Fe 4+ species) . Such states can act as surface recombination centers and have previously been characterized by electrochemical impedance spectroscopy, operando UV‐Vis spectroscopy, and operando infrared spectroscopy . Thus we consider that the characteristic frequency at the maximum point of the Nyquist plot ( f max ) links to the time constant that associates to surface intermediate states ( τ int ): …”

“…Hematite thin films were prepared by a hydrothermal/annealing technique . Fluorine doped tin oxide (FTO) coated glass substrates ( Solaronix TCO10‐10 , 8 Ω/sq.)…”

Heteroatom Doping Strategy for Establishing Hematite Homojunction as Efficient Photocatalyst for Accelerating Water Splitting

“…Hematite (α-Fe 2 O 3 ) is one of the promising photocatalysts for water oxidation, owing to its small band gap, high theoretical solar-to-hydrogen (STH) efficiency, as well as abundant and low cost properties. [1][2][3] To improve the photocatalytic ability of hematite on water oxidation, it is necessary to solve the low electrical conductivity and slow oxygen evolution kinetics problems. Several strategies have been applied in past decades, such as design well-defined structure, [4][5][6] doping heteroatoms [5][6][7][8][9][10][11] as well as establishing heterojunction [12][13][14] and homojunction [15][16][17][18] to enhance the electrical conductivity and increase the carrier density.…”

“…Hematite (α‐Fe 2 O 3 ) is one of the promising photocatalysts for water oxidation, owing to its small band gap, high theoretical solar‐to‐hydrogen (STH) efficiency, as well as abundant and low cost properties [1–3] . To improve the photocatalytic ability of hematite on water oxidation, it is necessary to solve the low electrical conductivity and slow oxygen evolution kinetics problems.…”