The replacement of fossil fuels by a clean and renewable energy source is one of the most urgent and challenging issues our society is facing today, which is why intense research has been devoted to this topic recently. Nature has been using sunlight as the primary energy input to oxidise water and generate carbohydrates (solar fuel) for over a billion years. Inspired, but not constrained, by nature, artificial systems can be designed to capture light and oxidise water and reduce protons or other organic compounds to generate useful chemical fuels. This tutorial review covers the primary topics that need to be understood and mastered in order to come up with practical solutions for the generation of solar fuels. These topics are: the fundamentals of light capturing and conversion, water oxidation catalysis, proton and CO2 reduction catalysis and the combination of all of these for the construction of complete cells for the generation of solar fuels.

Water oxidation is the key kinetic bottleneck of photoelectrochemical devices for fuel synthesis. Despite advances in the identification of intermediates, elucidating the catalytic mechanism of this multi-redox reaction on metal-oxide photoanodes remains a significant experimental and theoretical challenge. Here we report an experimental analysis of water oxidation kinetics on four widely studied metal oxides, focusing particularly upon hematite. We observe that hematite is able to access a reaction mechanism third order in surface hole density, assigned to equilibration between three surface holes and M(OH)-O-M(OH) sites. This reaction exhibits a remarkably low activation energy (Ea ~ 60 meV). Density functional theory is employed to determine the energetics of charge accumulation and O-O bond formation on a model hematite 110 surface. The proposed mechanism shows parallels with the function of oxygen evolving complex of photosystem II, and provides new insights to the mechanism of heterogeneous water oxidation on a metal oxide surface.

Oxygen vacancies are ubiquitous in metal oxides and critical to performance, yet the impact of these states upon charge carrier dynamics important for photoelectrochemical and photocatalytic applications, remains contentious and poorly understood. A key challenge is the unambiguous identification of spectroscopic fingerprints which can be used to track their function. Herein, we employ five complementary techniques to modulate the electronic occupancy of states associated with oxygen vacancies in situ in BiVO4 photoanodes, allowing us to identify a spectral signature for the ionisation of these states. We obtain an activation energy of ̴ 0.2 eV for this ionisation process, with thermally activated electron de-trapping from these states determining the kinetics of electron extraction, consistent with improved photoelectrochemical performance at higher temperatures. Bulk, un-ionised states however, function as deep hole traps, with such trapped holes being energetically unable to drive water oxidation. These observations help address recent controversies in the literature over oxygen vacancy function, providing new insights into their impact upon photoelectrochemical performance.

Ni/Fe oxyhydroxides are the best performing Earth-abundant electrocatalysts for water oxidation. However, the origin of their remarkable performance is not well understood. Herein, we employ spectroelectrochemical techniques to analyse the kinetics of water oxidation on a series of Ni/Fe oxyhydroxide films: FeOOH, FeOOHNiOOH, and Ni(Fe)OOH (5% Fe). The concentrations and reaction rates of the oxidised states accumulated during catalysis are determined. Ni(Fe)OOH is found to exhibit the fastest reaction kinetics but accumulates fewer states, resulting in a similar performance to FeOOHNiOOH. The later catalytic onset in FeOOH is attributed to an anodic shift in the accumulation of oxidised states. Rate law analyses reveal that the rate limiting step for each catalyst involves the accumulation of four oxidised states, Ni-centred for Ni(Fe)OOH but Fe-centred for FeOOH and FeOOHNiOOH. We conclude by highlighting the importance of equilibria between these accumulated species and reactive intermediates in determining the activity of these materials.

for this application. [1][2][3][4][5][6] Recently, however, new classes of materials are being investigated for hydrogen evolution from water, such as nitrogen doped graphene oxide, [7] covalent organic frameworks, [8][9][10] conjugated polymer networks, [11][12][13][14] and linear conjugated polymers. [15][16][17][18][19][20] With the notable exception of graphitic carbon nitride, [21] most organic semiconductors have required an additional metal co-catalyst to produce an appreciable H 2 evolution rate, often introduced by Pt photo deposition, [22] or the addition of a molecular catalyst. [8,23,24] However, recent reports of photocatalysis employing both linear and cross-linked conjugated polymers suggest that these systems are able to produce H 2 under visible light irradiation without any added co-catalyst, at a much faster rate than commercial graphitic carbon nitride, even when the latter is subjected to Pt photodeposition. [11] Activity without the need for co-catalysts, which are often precious metals or molecular complexes with limited stability, [25] could be a very attractive feature of such polymer photocatalysts with the potential to vastly reduce the cost and complexity of current photocatalytic systems.A feature of these linear and cross-linked conjugated polymers that has not yet been systematically studied is the presence of significant quantities of residual Pd originating from their synthesis via Pd catalyzed polycondensation reactions. [11,12,15,26] It has previously been shown that the molecular Pd catalysts used in these reactions can decompose via deligation and subsequent Ostwald ripening to form metallic Pd 0 particles, which are strongly retained within the polymer matrix and are difficult to remove via classical purification techniques. [27][28][29][30][31] Similar to Pt 0 , Pd 0 is a highly active proton reduction electrocatalyst. [32][33][34] Therefore, it is possible that the residual Pd, dispersed within these polymer structures, facilitates H 2 evolution and removes the need for additional co-catalysts. Much uncertainty remains about the nature of the active site in conjugated polymer systems, and there are conflicting views in the literature on whether or not the residual amounts of Pd play a significant role. No correlation was seen between Pd concentration and hydrogen evolution reaction (HER) rate in a range of conjugated micro porous polymers, [11] and in a series of linear polymersThe effect of residual Pd on hydrogen evolution activity in conjugated polymer photocatalytic systems is systematically investigated using colloidal poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) nanoparticles as a model system. Residual Pd, originating from the synthesis of F8BT via Pd catalyzed polycondensation polymerization, is observed in the form of homogeneously distributed Pd nanoparticles within the polymer. Residual Pd is essential for any hydrogen evolution to be observed from this polymer, and very low Pd concentrations (<40 ppm) are sufficient to have a significant effect on the h...