Towards Light Driven Water Splitting

Energy storage technologies to overcome the intermittent nature of renewable power sources are essential for economic and environmental security. The conversion of energy from solar to fuel follows the blueprint of evolution, which has demonstrated sustainability over a three billion year timescale and inspired researchers for over one hundred years. There are major challenges to efficient solar fuel synthesis leading to it being described as a Holy Grail of science. <br>    The synthesis of a fuel occurs via the endothermic reduction of a suitable reactant (e.g., H⁺, CO₂, N₂), thus an an electron source is required with H₂O the ideal molecule to be oxidised due to its abundance and innocuous nature. However, the oxidation of water (the oxygen evolution reaction, OER) is kinetically complex and active catalysts are required for it to proceed in an energy efficient manner. An objective of global scale artificial photosynthesis also necessitates that the catalysts and all other components are composed of Earth abundant elements. <br>    First row transition metal oxides (<i>i.e.</i> Mn, Co, Ni) have demonstrated high catalytic activity towards the OER and have been studied in detail as electrocatalysts. The focus herein is on metal oxides prepared through oxidative electrodeposition, specifically non-stoichiometric Co-oxide (CoO_x). CoO_x electrosynthesised from neutral and near-neutral, pH buffered electrolyte solutions has been demonstrated as an active water oxidation catalyst and received tremendous attention within the literature, including integration into devices. <br>    Herein, ultra-thin and transparent CoO_x films are described, with cobalt complexes used as precursors to allow conformal catalyst coatings. This facilitates integration of CoO_x films onto photoanodes <i>via</i> photo-electrodeposition and addresses previously identified limitations from opaque CoO_x layers. Systematically varying the stability of the precursor allows control of the deposition rate at po- tentials more positive than the OER. The precursor can thus be tailored to the photopotential of an <i>n</i>-type light harvester. Furthermore, the films with lower Co loadings demonstrated higher activity per metal amount at high positive poten-tials, which is due to proton transport becoming the limiting step in thick catalyst films. <br>    The mechanism of electrocatalysis and quantification of parameters was then herein examined using large amplitude Fourier transformed alternating current (ac) voltammetry. In contrast to typical direct current (dc) voltammetry, ac voltam- metry filtered out water oxidation dc current and allowed the resolution of redox transformations of CoO_x, MnO_x and NiO_x coupled to catalytic water oxidation. A general water oxidation model was then developed for these surface confined redox transformations coupled to a catalytic reaction with a substrate in solution. The model emphasises the role of the Brønsted base in proton abstraction within a proton coupled electron transfer mechanism. Comprehensive comparisons be- tween experimental and theoretical data revealed comparable effective reversible potentials of 1.9-2.1 V <i>vs</i> the reversible hydrogen electrode for each type of catalyst, i.e. for the formation of the species that undergoes catalytic turnover accompanied by the evolution of O₂. The pseudo-first order forward rate constants for these species were also determined to be between 2 ·10³ to5 ·10⁴ s^–1 for all three metal oxides, which is higher than any previously reported OER catalyst. <br>    Retaining a focus on CoO_x, the relationship between structural disorder and catalytic activity was examined with samples synthesised in systematic steps through a bulk chemical oxidation. Electrochemical testing revealed the more disordered cobalt oxides were less active for OER catalysis but stronger oxidants, <i>viz</i>. they were more readily involved in non-catalytic chemical reactions. This supports more disordered CoO_x being less thermodynamically stable, but challenges previously proposed correlations about disorder in metal oxide OER catalysts being beneficial to catalytic activity. <br>    Finally, effective pairing of Ni electrodes to a III/V type photovoltaic (PV) allowed overall water splitting at 22% solar to fuel power conversion efficiency (SFE) using commercially available components, while avoiding the use of Pt group metals and eclipsing the 12% SFE demonstrated in the previous year. The approach was defined with PV-electrolyser pairing parameters that demonstrate the point of power loss from photon to current through to SFE and identify the directions for improvement. Additionally, the parameters allow the maximum SFE of a system to be determined from characterisation of the PV

A novel concept of photosynthetic soft membranes: a numerical study.

Acknowledgements: Erwin Reisner is gratefully acknowledged for useful discussions on photochemical reactions and electron transfer mechanisms. The authors acknowledge all the partners of the SoFiA consortium for fruitful discussions.We focus on a novel concept of photosynthetic soft membranes, possibly able to allow the conversion of solar energy and carbon dioxide (CO[Formula: see text]) into green fuels. The considered membranes rely on self-assembled functional molecules in the form of soap films. We elaborate a multi-scale and multi-physics model to describe the relevant phenomena, investigating the expected performance of a single soft photosynthetic membrane. First, we present a macroscale continuum model, which accounts for the transport of gaseous and ionic species within the soap film, the chemical equilibria and the two involved photocatalytic half reactions of the CO[Formula: see text] reduction and water oxidation at the two gas-surfactant-water interfaces of the soap film. Second, we introduce a mesoscale discrete Monte Carlo model, to deepen the investigation of the structure of the functional monolayers. Finally, the morphological information obtained at the mesoscale is integrated into the continuum model in a multi-scale framework. The developed tools are then used to perform sensitivity studies in a wide range of possible experimental conditions, to provide scenarios on fuel production by such a novel approach

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

New dye anchoring system that sustains intimate electronic coupling while addressing the notorious instability of dye-sensitised electrodes in aqueous media is introduced.</p

Origin of Photoelectrochemical Generation of Dihydrogen by a Dye-Sensitized Photocathode without an Intentionally Introduced Catalyst

Dye-sensitized photocathodes have been observed on several occasions to sustain light-driven H 2 generation without intentionally introduced catalysts. Herein, plausible mechanisms addressing this phenomenon are probed by a combination of long-term photoelectrochemical measurements with concurrent gas chromatography, transient absorption spectroscopy, and inductively coupled mass spectrometry using a perylenemonoimide-sexithiophene-triphenylamine (PMI-6T-TPA) sensitized NiO electrode. The experimental evidence obtained discounts the possibility for direct reduction of hydrogen by the dye and demonstrates that the availability of interfaces between dye molecules and any electrically disconnected NiO particles exposed to the electrolyte solution is critical for photoelectrocatalytic H 2 generation. These interfaces are postulated to serve as photoactive sites for the formation of a hydrogen evolution catalyst, e.g., metallic nickel, which can accept photogenerated electrons from the excited dye molecules. The Ni 0 catalyst can form via photoelectroreduction of Ni 2+ , which has been found to slowly dissolve from the NiO support into the solutions during the photoelectrochemical measurements. Additionally, dependence of the H 2 generation rate on the anion within the electrolyte has been identified, with the highest rates of 35-40 nmol h -1 cm -2 achieved with acetate. The origin of this dependence remains unsolved at this stage but is clearly demonstrated to be not associated with the different rates of dissolution of NiO, the presence of other transition metal contaminants, nor electronic impacts of the anion on the NiO valence band. Overall, the results herein demonstrate that the effects of the chemical nature of the electrolyte, metallic nickel deposited from dissolved Ni 2+ , and availability of the interfaces between disconnected NiO and adsorbed dye should be considered when interpreting the photoelectrocatalytic performance of dye-sensitized photocathodes for dihydrogen evolution

Parameterization of Water Electrooxidation Catalyzed by Metal Oxides Using Fourier Transformed Alternating Current Voltammetry

Detection and quantification of redox transformations involved in water oxidation electrocatalysis is often not possible using conventional techniques. Herein, use of large amplitude Fourier transformed ac voltammetry and comprehensive analysis of the higher harmonics has enabled us to access the redox processes responsible for catalysis. An examination of the voltammetric data for water oxidation in borate buffered solutions (pH 9.2) at electrodes functionalized with systematically varied low loadings of cobalt (CoO_<i>x</i>), manganese (MnO_<i>x</i>), and nickel oxides (NiO_<i>x</i>) has been undertaken, and extensive experiment-simulation comparisons have been introduced for the first time. Analysis shows that a single redox process controls the rate of catalysis for Co and Mn oxides, while two electron transfer events contribute in the Ni case. We apply a “molecular catalysis” model that couples a redox transformation of a surface-confined species (effective reversible potential, <i>E</i>_eff⁰) to a catalytic reaction with a substrate in solution (pseudo-first-order rate constant, <i>k</i>₁^f), accounts for the important role of a Brønsted base, and mimics the experimental behavior. The analysis revealed that <i>E</i>_eff⁰ values for CoO_<i>x</i>, MnO_<i>x</i>, and NiO_<i>x</i> lie within the range 1.9–2.1 V vs reversible hydrogen electrode, and <i>k</i>₁^f varies from 2 × 10³ to 4 × 10⁴ s^–1. The <i>k</i>₁^f values are much higher than reported for any water electrooxidation catalyst before. The <i>E</i>_eff⁰ values provide a guide for in situ spectroscopic characterization of the active states involved in catalysis by metal oxides

Probing the fate of Mn complexes in Nafion: a combined multifrequency EPR and XAS study

Multifrequency electron paramagnetic resonance (EPR, 9.4 and 263 GHz) and X-ray absorption spectroscopy (XAS) were employed to study structural and electrochemical changes of selected Mn complexes in contact with dried Nafion films upon electro-oxidation and after long-term illumination. It was found that when in contact with Nafion the Mn-Me₃TACN complexes are reduced into Mn(II) complexes with an octahedral geometry (Me₃TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane). The reduction process involves an intermediate product in which Mn has been reduced from the initial +III or +IV state of the precursor Mn complex but is still coordinated to the TACN ligand. Electro-oxidation yields a MnOₓ mineral with a birnessite structure, in which the Mn(III) or Mn(IV) ions exhibit very strong magnetic coupling. Long-term illumination of the oxide-containing Nafion film while it is exposed to an aqueous electrolyte partially decomposes the mineral and forms a Mn(II) species with octahedral coordination

Multi-Variable Multi-Metric Optimization of Self-Assembled Photocatalytic CO2 Reduction Performance Using Machine Learning Algorithms.

Publication status: PublishedThe sunlight-driven reduction of CO2 into fuels and platform chemicals is a promising approach to enable a circular economy. However, established optimization approaches are poorly suited to multivariable multimetric photocatalytic systems because they aim to optimize one performance metric while sacrificing the others and thereby limit overall system performance. Herein, we address this multimetric challenge by defining a metric for holistic system performance that takes multiple figures of merit into account, and employ a machine learning algorithm to efficiently guide our experiments through the large parameter matrix to make holistic optimization accessible for human experimentalists. As a test platform, we employ a five-component system that self-assembles into photocatalytic micelles for CO2-to-CO reduction, which we experimentally optimized to simultaneously improve yield, quantum yield, turnover number, and frequency while maintaining high selectivity. Leveraging the data set with machine learning algorithms allows quantification of each parameter's effect on overall system performance. The buffer concentration is unexpectedly revealed as the dominating parameter for optimal photocatalytic activity, and is nearly four times more important than the catalyst concentration. The expanded use and standardization of this methodology to define and optimize holistic performance will accelerate progress in different areas of catalysis by providing unprecedented insights into performance bottlenecks, enhancing comparability, and taking results beyond comparison of subjective figures of merit

Electrosynthesis of highly transparent cobalt oxide water oxidation catalyst files 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

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

Photo-electrocatalytic hydrogen generation at dye-sensitised electrodes functionalised with a heterogeneous metal catalyst

Dye-sensitised photocathodes promoting hydrogen evolution are usually coupled to a catalyst to improve the reaction rate. Herein, we report on the first successful integration of a heterogeneous metal particulate catalyst, viz., Pt aggregates electrodeposited from acidic solutions on the surface of a NiO-based photocathode sensitised with a p-type perylenemonoimid-sexithiophene-triphenylamine dye (PMI-6T-TPA). The platinised dye-NiO electrodes generate photocurrent density of ca -0.03 mA cm-2 (geom.) with 100% faradaic efficiency for the H2 evolution at 0.059 V vs. reversible hydrogen electrode under 1 sun visible light irradiation (AM1.5G, 100 mW cm-2, \u3e 400 nm) for more than 10 hours in 0.1 M H2SO4 (aq.). The Pt-free dye-NiO and dye-free Pt-modified NiO cathodes show no photo-electrocatalytic hydrogen evolution under these conditions. The performance of these Pt-modified PMI-6T-TPA-based photoelectrodes compares well to that of previously reported dye-sensitised photocathodes for H2 evolution