Impact of the Synthesis Route on the Water Oxidation Kinetics of Hematite Photoanodes

Abstract

Operando spectroelectrochemical analysis is used to determine the water oxidation reaction kinetics for hematite photoanodes prepared using four different synthetic procedures. Whilst these photoanodes exhibit very different current / voltage performance, their underlying water oxidation kinetics are found to be almost invariant. Lower photoanode performance was found to correlate with the observation of optical signals indicative of charge accumulation in mid-gap oxygen vacancy states, indicating these states do not contribute directly to water oxidation. Photoelectrochemical water splitting is attracting extensive interest as a promising solar-to-fuel process to store solar energy in chemical bonds (i.e., hydrogen). In solar-driven water splitting, it is widely accepted that the oxygen evolution reaction (OER) is the most kinetically demanding process, especially when using earth abundant metal-oxide photoanodes. 1,2 Consequently, one of the primary limitations to efficiency in such photoanodes is the kinetic mismatch between the lifetimes of photogenerated charges, limited by recombination processes on ps to ms timescales, and the slow kinetics of OER catalysis, occurring usually on the ms-s timescale. 3-5 The efficiency of such photoanodes is strongly dependent on not only the selection of metal oxide, but also the methodology used to synthesize the photoanode, attributed to variations in nanomorphology, surface facet, doping and surface state densities and surface / co-catalyst treatments. 2 However, it is often unclear whether such variations in photoelectrochemical performance result from differences in the underlying kinetics of OER catalysis or rather from differences in competing bulk or surface recombination processes. In this study, we address this issue for one of the most widely studied metal oxides for light driven oxygen evolving photoanodes, hematite (a-Fe2O3). The kinetic mismatch between charge recombination and water oxidation is particularly severe in hematite. Consequently, analyzing the connection between overall performance and the underlying OER kinetics in hematite photoanodes synthesized by different deposition methods, yielding different morphologies, may inform strategies to further narrow the kinetic mismatch between reaction and recombination. One of the key considerations for water oxidation on metal oxides, including hematite, is the potential role of intra-bandgap surface states. 6-9 Such surface states have often been related to oxygen vacancies and structural imperfections / defect sites. 10,11 Surface holes on hematite have been assigned to Fe IV =O species, with these states being proposed as the first intermediate species of the OER. 12 Some studies, including electrochemical impedance analyses, have provided evidence that mid-gap surface states participate as intermediates in the OER catalysis on such photoanodes. 13-16 Other studies, including transient absorption analyses, have suggested OER catalysis is driven by valence band holes localized at the metal oxide surface, w...

Oxygen Evolution Catalysts at Transition Metal Oxide Photoanodes: Their Differing Roles for Solar Water Splitting

“…Recently reported spectroelectrochemical measurements suggest highly similar reaction kinetics (perhaps mechanisms too) at sufficiently high bias across hematite samples which were synthesized from various routes, such as atmospheric pressure chemical vapor deposition, atomic layer deposition, and solution‐based regrowth, and treated at different temperatures ranging from 230 to 800 °C ( Figure 18a ). [ 40 ] The kinetic plots for hematite also resemble those for BiVO 4 and TiO 2 in terms of reaction order (Figure 17b). Nevertheless, these electrodes do have fundamentally distinct photoresponses.…”

“…[ 37 ] It has also been reported that stronger illumination intensities that give higher surface hole densities (above ≈1 hole nm ‐1 ) can transform the reaction mechanism from first to third order, and the third‐order reaction regime is more efficient for the OER. [ 38–40 ] Whether operating conditions have a fundamental impact on the OER mechanism at OEC‐decorated photoanodes remains an open and important question. A Marcus theory‐based theoretical microkinetic model recently proposed for the OER has opened up a new path in this direction.…”

“…Similar results have been also observed previously with potential modulated absorption spectroscopy and light modulated absorption spectroscopy, where these features are subdued upon Al 2 O 3 coating but unchanged with various bulk thicknesses. [ 105 ] Notably, the signal at 580 nm has also been assigned to r‐ss, [ 40 ] which seems to be slightly conflicting. However, in one of our ongoing studies, we have observed different types of signals near 580 nm on the same electrode that are active and inactive toward OER using operando spectroelectrochemical measurements.…”

“…[ 109,110 ] Second, since V O are balanced by Fe 2+ for α‐Fe 2 O 3 and V 4+ for BiVO 4 , these ions would easily trap holes. [ 40,111 ] Third, new phases such as Fe 3 O 4 for hematite may form, which close the bandgap and can also function as recombination sites. [ 107 ] Compared to the widely received attention on oxygen vacancies, research, especially experimental, on metal vacancies is scarce in spite of their high abundancy in the bulk.…”

“…Reproduced with permission. [ 40 ] Copyright 2020, American Chemical Society. b) Rate law analysis of WO 3 , α‐Fe 2 O 3 , TiO 2 , and BiVO 4 based on PIA and TPS experiments.…”