Shannon A. Bonke

Renewable fuels from concentrated solar power: towards practical artificial photosynthesis

There is intense interest in the solar driven conversion of water to hydrogen as a means of achieving the sustainable generation of a practical fuel. It is widely considered that such “Artificial Photosynthesis” processes need to achieve an energy conversion efficiency exceeding 10% to have practical impact. Although some solar-driven fuel generating systems have reached efficiencies as high as 18%, they are often based on precious metal catalysts, or offer only limited stability. We describe here a system that utilises concentrated solar power, which is inexpensive to produce, and an electrolyser module based on Earth-abundant materials capable of operating under benign conditions. This system delivers the highest efficiency reported to date, in excess of 22%. The electrolyser functions in electrolytes with pH values ranging from neutral to alkaline, including river water, allowing implementation in a variety of geographic locations. Testing over multiple diurnal cycles confirmed the long-term stability of performance. We also describe an analysis of the efficiency in terms of the critical cell-matching parameters and thereby understand the key directions for further optimisation. This simple and adaptable system addresses key criteria for the large-scale deployment of an artificial photosynthesis device.

Electronic structural insights into efficient MnOx catalysts

Soft X-ray absorption and resonant inelastic X-ray scattering at the Mn L-edge are established as tools for gaining electronic structural insights into water oxidation catalysis. The MnOx catalyst with the lowest d–d transitions, strongest charge transfer and a higher proportion of Mn3+ over Mn2+/4+ produces itinerant electrons that contribute to a higher catalytic activity.

Electrosynthesis of Highly Transparent Cobalt Oxide Water Oxidation Catalyst Films from Cobalt Aminopolycarboxylate Complexes

Efficient catalysis of water oxidation represents one of the major challenges en route to efficient sunlight-driven water splitting. Cobalt oxides (CoOx ) have been widely investigated as water oxidation catalysts, although the incorporation of these materials into photoelectrochemical devices has been hindered by a lack of transparency. Herein, the electrosynthesis of transparent CoOx catalyst films is described by utilizing cobalt(II) aminopolycarboxylate complexes as precursors to the oxide. These complexes allow control over the deposition rate and morphology to enable the production of thin, catalytic CoOx films on a conductive substrate, which can be exploited in integrated photoelectrochemical devices. Notably, under a bias of 1.0 V (vs. Ag/AgCl), the film deposited from [Co(NTA)(OH2 )2 ](-) (NTA=nitrilotriacetate) decreased the transmission by only 10 % at λ=500 nm, but still generated >80 % of the water oxidation current produced by a [Co(OH2 )6 ](2+) -derived oxide film whose transmission was only 40 % at λ=500 nm.

Back-contacted hybrid organic–inorganic perovskite solar cells

A novel architecture for quasi-interdigitated electrodes (QIDEs) allows for the fabrication of back-contacted perovskite solar cells. The devices showed a stable power output of 3.2%. The design of the QIDEs avoids the defects that cause short-circuiting in conventional IDEs, while enhancing the collection area of the electrodes. Photoluminescence and photocurrent mapping is used to probe the charge generation and transport properties of the perovskite solar cells.

Engineering Disorder at a Nanoscale: A Combined TEM and XAS Investigation of Amorphous versus Nanocrystalline Sodium Birnessite

The term amorphous metal oxide is becoming widely used in the catalysis community. The term is generally used when there are no apparent peaks in an X-ray diffraction pattern. However, the absence of such features in X-ray diffraction can mean that the material is either truly amorphous or that it is better described as nanocrystalline. By coprecipitating a sodium birnessite-like phase with and without phosphate (1.5 %), we are able to engineer two very similar but distinct materials – one that is nanocrystalline and the other that is amorphous. The two closely related phases were characterized with both Mn K-edge X-ray absorption spectroscopy and high-resolution transmission electron microscopy. These structural results were then correlated with catalytic and electrocatalytic activities for water oxidation catalysis. In this case, the amorphous phosphate-doped material was less catalytically active than the nanocrystalline material.

Screen‐Printing of ZnO Nanostructures from Sol–Gel Solutions for Their Application in Dye‐Sensitized Solar Cells

Diblock copolymers have been used in sol-gel synthesis to successfully tailor the nanoscale morphology of thin ZnO films. As the fabrication of several-micron-thick mesoporous films such as those required in dye-sensitized solar cells (DSSCs) was difficult with this approach, we exploited the benefits of diblock-copolymer-directed synthesis that made it compatible with screen printing. The simple conversion of the diblock copolymer ZnO precursor sol to a screen-printing paste was not possible as it resulted in poor film properties. To overcome this problem, an alternative route is proposed in which the diblock copolymer ZnO precursor sol is first blade coated and calcined, then converted to a screen-printing paste. This allows the benefits of diblock-copolymer-directed particle formation to be compatible with printing methods. The morphologies of the ZnO nanostructures were studied by SEM and correlated with the current density-voltage characteristics.

Engineering disorder into heterogenite-like cobalt oxides by phosphate doping: implications for the design of water-oxidation catalysts

Amongst the most promising materials designed to catalyse water oxidation from earth‐abundant materials are the metal oxides. Despite the success of these materials, understanding the relationship of structure to function has been very challenging. It has been noted that many metal oxide water‐oxidation catalysts function best in a proton‐accepting electrolyte, such as a borate or phosphate buffer. However, these same electrolytes are known to significantly affect the metal oxide structures by imparting a level of “disorder” or “molecular nature” to the materials. The most well‐known case is that of Noceras Co–Pi catalyst (Pi: inorganic phosphorus). In this study, we have synthesised a series of “heterogenite‐like” cobalt oxides with different levels of phosphate doping (0–9 %P). Our synthetic method enables us to make “bulk materials”, the structural properties of which (as observed by X‐ray absorption spectroscopy and transmission electron microscopy) mimic those observed directly on electrode surfaces. The changes made to the bulk phases were directly correlated with the reactivity for water‐oxidation catalysis and the ability of the CoOx materials to act as sacrificial oxidants. The most disordered materials were most reactive for sacrificial oxidation but were less effective as water‐oxidation catalysts. These results help us understand how disorder changes the thermodynamic stability of metal oxides and how this impacts on efficiency for water oxidation.

Cooperative silanetriolate-carboxylate sensitiser anchoring for outstanding stability and improved performance of dye-sensitised photoelectrodes

Photosensitising dyes require anchoring groups for attachment to a metal oxide support surface. When a dye-sensitised electrode is designed for applications in aqueous media, e.g. for photoelectrocatalytic water splitting, these anchoring groups must be highly stable towards hydrolysis while retaining sufficient electrical conductivity to sustain efficient transfer of photogenerated charges. With this motivation, we introduce herein a cooperative silanetriolate-carboxylate anchoring combination. A ruthenium(II) tris-bipyridine dye has been functionalised with this dual-anchor through a facile peptide coupling reaction. The photoelectrooxidation performance and stability of mesoporous TiO2 electrodes sensitised with this new dye have been systematically examined in solutions with pH 1–9 in the absence and presence of a sacrificial electron donor and/or buffering electrolyte following a specifically designed testing protocol, which involves comprehensive characterisation of the electrodes and electrolyte solutions by ICP-MS and UV-vis spectroscopy. When compared to the state-of-the-art phosphonate anchoring group, the silanetriolate-carboxylate combination enables approximately 4- and 8-fold enhancements in the rate of sulphite oxidation by the Ru(II) (bipyridine)3-sensitised TiO2 photoanodes under 1 sun irradiation after 2 and 24 hours of reaction, respectively. In the absence of a sacrificial electron donor, photoanodes based on carboxylate- and phosphonate-anchored dyes undergo continuous and rapid degradation under all conditions examined, leading to almost completely bleached electrodes within less than an hour of operation. Conversely, silanetriolate-carboxylate anchoring provides quasi-stable operation with typically less than 10% ruthenium loss from the electrode surface under the same conditions. The analysis undertaken here unambiguously attests to the significantly improved long-term performance of the dye-sensitised photoelectrodes provided by the cooperative anchoring system where silanetriolate provides high stability and carboxylate sustains efficient charge transfer. This furnishes a practical pathway towards the synthesis of photosensitisers capable of stable and efficient operation within photoelectrochemical devices in aqueous environments.

Tunable biogenic manganese oxides

Influence of the conditions for aerobic oxidation of Mn2+(aq) catalysed by the MnxEFG protein complex on the morphology, structure and reactivity of the resulting biogenic manganese oxides (MnOx ) is explored. Physical characterisation of MnOx includes scanning and transmission electron microscopy, and X-ray photoelectron and K-edge Mn, Fe X-ray absorption spectroscopy. This characterisation reveals that the MnOx materials share the structural features of birnessite, yet differ in the degree of structural disorder. Importantly, these biogenic products exhibit strikingly different morphologies that can be easily controlled. Changing the substrate-to-protein ratio produces MnOx either as nm-thin sheets, or rods with diameters below 20 nm, or a combination of the two. Mineralisation in solutions that contain Fe2+(aq) makes solids with significant disorder in the structure, while the presence of Ca2+(aq) facilitates formation of more ordered materials. The (photo)oxidation and (photo)electrocatalytic capacity of the MnOx minerals is examined and correlated with their structural properties.