The efficient use of sunlight to drive the production of solar fuels requires the photogeneration of suitably long-lived charge separated states capable of driving the multi-electron chemistry of ...fuel synthesis. Here we discuss a range of promising material design approaches to increasing charge carrier lifetimes, focusing upon semiconductor photoelectrodes for water photolysis and carbon dioxide reduction. Parallels are drawn between the strategies deployed in the development of such artificial systems and those found in natural photosynthesis. We address the use of a range of junctions, including electrolyte/semiconductor, semiconductor/semiconductor and dye sensitized interfaces to provide the spatial separation of charges. A key consideration of the review is the design of such interfaces to achieve a sufficient increase in charge carrier lifetime with a high quantum yield, whilst minimising the energy loss associated with this lifetime gain.
There is an increasing level of interest in the use of black TiO2 prepared by thermal hydrogen treatments (H:TiO2) due to the potential to enhance both the photocatalytic and the light-harvesting ...properties of TiO2. Here, we examine oxygen-deficient H:TiO2 nanotube arrays that have previously achieved very high solar-to-hydrogen (STH) efficiencies due to incident photon-to-current efficiency (IPCE) values of >90% for photoelectrochemical water splitting at only 0.4 V vs RHE under UV illumination. Our transient absorption (TA) mechanistic study provides strong evidence that the improved electrical properties of oxygen-deficient TiO2 enables remarkably efficient spatial separation of electron–hole pairs on the submicrosecond time scale at moderate applied bias, and this coupled to effective suppression of microsecond to seconds charge carrier recombination is the primary factor behind the dramatically improved photoelectrochemical activity.
Conspectus The electrochemical reduction of CO2 provides a way to sustainably generate carbon-based fuels and feedstocks. Molecular CO2 reduction electrocatalysts provide tunable reaction centers ...offering an approach to control the selectivity of catalysis. Manganese carbonyl complexes, based on Mn(bpy)(CO)3Br and its derivatives (bpy = 2,2′-bipyridine), are particularly interesting due to their ease of synthesis and the use of a first-row earth-abundant transition metal. Mn(bpy)(CO)3Br was first shown to be an active and selective catalyst for reducing CO2 to CO in organic solvents in 2011. Since then, manganese carbonyl catalysts have been widely studied with numerous reports of their use as electrocatalysts and photocatalysts and studies of their mechanism. This class of Mn catalysts only shows CO2 reduction activity with the addition of weak Brønsted acids. Perhaps surprisingly, early reports showed increased turnover frequencies as the acid strength is increased without a loss in selectivity toward CO evolution. It may have been expected that the competing hydrogen evolution reaction could have led to lower selectivity. Inspired by these works we began to explore if the catalyst would work in protic solvents, namely, water, and to explore the pH range over which it can operate. Here we describe the early studies from our laboratory that first demonstrated the use of manganese carbonyl complexes in water and then go on to discuss wider developments on the use of these catalysts in water, highlighting their potential as catalysts for use in aqueous CO2 electrolyzers. Key to the excellent selectivity of these catalysts in the presence of Brønsted acids is a proton-assisted CO2 binding mechanism, where for the acids widely studied, lower pK a values actually favor CO2 binding over Mn–H formation, a precursor to H2 evolution. Here we discuss the wider literature before focusing on our own contributions in validating this previously proposed mechanism through the use of vibrational sum frequency generation (VSFG) spectroelectrochemistry. This allowed us to study Mn(bpy)(CO)3Br while it is at, or near, the electrode surface, which provided a way to identify new catalytic intermediates and also confirm that proton-assisted CO2 binding operates in both the “dimer” and primary (via Mn(bpy)(CO)3−) pathways. Understanding the mechanism of how these highly selective catalysts operate is important as we propose that the Mn complexes will be valuable models to guide the development of new proton/acid tolerant CO2 reduction catalysts.
Graphitic carbon nitride (g-C3N4) synthesised from a urea precursor is an excellent CO2 reduction photocatalyst using Co(bpy)n2+ as a co-catalyst. A five-fold increase in activity for the highly ...polymerised urea derived g-C3N4 is achieved compared to alternative precursors. Transient absorption, time-resolved and steady-state emission studies indicate that the enhanced activity is related to both an increased driving force for photoelectron transfer and a greater availability of photogenerated charges.
Hematite is currently considered one of the most promising materials for the conversion and storage of solar energy
via
the photoelectrolysis of water. Whilst there has been extensive research and ...much progress in the development of hematite structures with enhanced photoelectrochemical (PEC) activity, relatively limited information has been available until recently concerning the dynamics of photogenerated charge carriers in hematite and their impact upon the efficiency of water photoelectrolysis. In this perspective we present an overview of our recent studies of the dynamics of photoinduced charge carrier processes in hematite, derived primarily from transient absorption spectroscopy of nanostructured photoanodes. The relationship between PEC activity and transient measurements are discussed in terms of a phenomenological model which rationalizes the observations and in particular the impact of external potential bias on the relative rates of charge carrier trapping, recombination and interfacial transfer in hematite photoanodes for water oxidation.
In this perspective we present an overview of our recent studies of the dynamics of photoinduced charge carrier processes in hematite, derived primarily from transient absorption spectroscopy of nanostructured photoanodes.
We report a strategy for efficient suppression of electron–hole recombination in hematite photoanodes. Acid‐treated hematite showed a substantially enhanced photocurrent density compared to untreated ...samples. Electrochemical impedance spectroscopy studies revealed that the enhanced photocurrent is partly due to improved efficiency of charge separation. Transient absorption spectroscopic studies coupled to electrochemical measurements indicate that, in addition to improved bulk electrochemical properties, acid‐treated hematite has significantly decreased surface electron–hole recombination losses owing to a greater yield of the trapped photoelectrons being extracted to the external circuit.
A simple acid treatment method is reported to increase the photoelectrochemical activity of hematite photoanodes. The enhanced photocurrent was due to the combination of improved efficiency of charge separation and suppressed electron–hole recombination, resulting in a greater yield of trapped photoelectrons being extracted to the external circuit.
Direct photocatalytic water splitting is an attractive strategy for clean energy production, but multicomponent nanostructured systems that mimic natural photosynthesis can be difficult to fabricate ...because of the insolubility of most photocatalysts. Here, a solution‐processable organic polymer is reported that is a good photocatalyst for hydrogen evolution from water, either as a powder or as a thin film, suggesting future applications for soluble conjugated organic polymers in multicomponent photocatalysts for overall water splitting.
Multicomponent nanostructured systems that mimic natural photosynthesis can be difficult to fabricate because of the insolubility of most photocatalysts. Here, a solution‐processable organic polymer is reported that is a good photocatalyst for hydrogen evolution from water, either as a powder or as a thin film, suggesting future applications for soluble conjugated organic polymers in multicomponent photocatalysts for overall water splitting.
This paper addresses the origin of the decrease in the external electrical bias required for water photoelectrolysis with hematite photoanodes, observed following surface treatments of such ...electrodes. We consider two alternative surface modifications: a cobalt oxo/hydroxo-based (CoO ₓ) overlayer, reported previously to function as an efficient water oxidation electrocatalyst, and a Ga ₂O ₃ overlayer, reported to passivate hematite surface states. Transient absorption studies of these composite electrodes under applied bias showed that the cathodic shift of the photocurrent onset observed after each of the surface modifications is accompanied by a similar cathodic shift of the appearance of long-lived hematite photoholes, due to a retardation of electron/hole recombination. The origin of the slower electron/hole recombination is assigned primarily to enhanced electron depletion in the Fe ₂O ₃ for a given applied bias.
The scaling-up of electrochemical CO2 reduction requires circumventing the CO2 loss as carbonates under alkaline conditions. Zero-gap cell configurations with a reverse-bias bipolar membrane (BPM) ...represent a possible solution, but the catalyst layer in direct contact with the acidic environment of a BPM usually leads to H2 evolution dominating. Here we show that using acid-tolerant Ni molecular electrocatalysts selective (>60%) CO2 reduction can be achieved in a zero-gap BPM device using a pure water and CO2 feed. At a higher current density (100 mA cm–2), CO selectivity decreases, but was still >30%, due to reversible product inhibition. This study demonstrates the importance of developing acid-tolerant catalysts for use in large-scale CO2 reduction devices.
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