Publications
7 results found
Hillman SAJ, Sprick RS, Pearce D, et al., 2022, Why do sulfone-containing polymer photocatalysts work so well for sacrificial hydrogen evolution from water?, Journal of the American Chemical Society, Vol: 144, Pages: 19382-19395, ISSN: 0002-7863
Many of the highest-performing polymer photocatalysts for sacrificial hydrogen evolution from water have contained dibenzo[b,d]thiophene sulfone units in their polymer backbones. However, the reasons behind the dominance of this building block are not well understood. We study films, dispersions, and solutions of a new set of solution-processable materials, where the sulfone content is systematically controlled, to understand how the sulfone unit affects the three key processes involved in photocatalytic hydrogen generation in this system: light absorption; transfer of the photogenerated hole to the hole scavenger triethylamine (TEA); and transfer of the photogenerated electron to the palladium metal co-catalyst that remains in the polymer from synthesis. Transient absorption spectroscopy and electrochemical measurements, combined with molecular dynamics and density functional theory simulations, show that the sulfone unit has two primary effects. On the picosecond timescale, it dictates the thermodynamics of hole transfer out of the polymer. The sulfone unit attracts water molecules such that the average permittivity experienced by the solvated polymer is increased. We show that TEA oxidation is only thermodynamically favorable above a certain permittivity threshold. On the microsecond timescale, we present experimental evidence that the sulfone unit acts as the electron transfer site out of the polymer, with the kinetics of electron extraction to palladium dictated by the ratio of photogenerated electrons to the number of sulfone units. For the highest-performing, sulfone-rich material, hydrogen evolution seems to be limited by the photogeneration rate of electrons rather than their extraction from the polymer.
Pulignani C, Mesa C, Hillman S, et al., 2022, Rational design of carbon nitride photoelectrodes with high activity toward organic oxidations, Angewandte Chemie International Edition, ISSN: 1433-7851
Carbon nitride (CNx) is a scalable polymeric light-absorber with excellent performance in photocatalytic suspension systems, but the activity of CNx photoelectrodes has remained low. Here, cyanamide-functionalized CNx (NCNCNx) has been co-deposited with ITO nanoparticles on a 1.8 Å thick alumina-coated FTO-glass electrode. Transient spectroscopy and impedance measurements support that ITO acts as conductive binder and improves the electron extraction from the NCNCNx, whilst the alumina underlayer reduces the electrical resistance between the ITO and the FTO-coated electrode. The Al2O3|ITO:NCNCNx electrode displays a new benchmark performance for CNx-based photoanodes with a remarkably low onset of –0.4 V vs the reversible hydrogen electrode (RHE) and an outstanding 1.4 ± 0.2 mA cm–2 at 1.23 V vs RHE for the selective oxidation of 4-methylbenzyl alcohol to the corresponding aldehyde. This facile assembly will enable the exploration of CNx in fundamental and applied PEC studies, paving the way for the development of high-performance photoelectrodes using other semiconductor powders
Bozal-Ginesta C, Rao RR, Mesa CA, et al., 2021, Redox-state kinetics in water-oxidation IrOx electrocatalysts measured by operando spectroelectrochemistry, ACS Catalysis, Vol: 11, Pages: 15013-15025, ISSN: 2155-5435
Hydrous iridium oxides (IrOx) are the best oxygen evolution electrocatalysts available for operation in acidic environments. In this study, we employ time-resolved operando spectroelectrochemistry to investigate the redox-state kinetics of IrOx electrocatalyst films for both water and hydrogen peroxide oxidation. Three different redox species involving Ir3+, Ir3.x+, Ir4+, and Ir4.y+ are identified spectroscopically, and their concentrations are quantified as a function of applied potential. The generation of Ir4.y+ states is found to be the potential-determining step for catalytic water oxidation, while H2O2 oxidation is observed to be driven by the generation of Ir4+ states. The reaction kinetics for water oxidation, determined from the optical signal decays at open circuit, accelerates from ∼20 to <0.5 s with increasing applied potential above 1.3 V versus reversible hydrogen electrode [i.e., turnover frequencies (TOFs) per active Ir state increasing from 0.05 to 2 s–1]. In contrast, the reaction kinetics for H2O2 is found to be almost independent of the applied potential (increasing from 0.1 to 0.3 s–1 over a wider potential window), indicative of a first-order reaction mechanism. These spectroelectrochemical data quantify the increase of both the density of active Ir4.y+ states and the TOFs of these states with applied positive potential, resulting in the observed sharp turn on of catalytic water oxidation current. We reconcile these data with the broader literature while providing a unique kinetic insight into IrOx electrocatalytic reaction mechanisms, indicating a first-order reaction mechanism for H2O2 oxidation driven by Ir4+ states and a higher-order reaction mechanism involving the cooperative interaction of multiple Ir4.y+ states for water oxidation.
Cong S, Creamer A, Fei Z, et al., 2020, Tunable control of the hydrophilicity and wettability of conjugated polymers by a postpolymerization modification approach., Macromolecular Bioscience, Vol: 20, Pages: 1-8, ISSN: 1616-5187
A facile method to prepare hydrophilic polymers by a postpolymerization nucleophillic aromatic substitution reaction of fluoride on an emissive conjugated polymer (CP) backbone is reported. Quantitative functionalization by a series of monofunctionalized ethylene glycol oligomers, from dimer to hexamer, as well as with high molecular weight polyethylene glycol is demonstrated. The length of the ethylene glycol sidechains is shown to have a direct impact on the surface wettability of the polymer, as well as its solubility in polar solvents. However, the energetics and band gap of the CPs remain essentially constant. This method therefore allows an easy way to modulate the wettability and solubility of CP materials for a diverse series of applications.
Woods DJ, Hillman S, Pearce D, et al., 2020, Side-chain tuning in conjugated polymer photocatalysts for improved hydrogen production from water, Energy & Environmental Science, Vol: 13, Pages: 1843-1855, ISSN: 1754-5692
Structure–property–activity relationships in solution processable polymer photocatalysts for hydrogen production from water were probed by varying the chemical structure of both the polymer side-chains and the polymer backbone. In both cases, the photocatalytic performance depends strongly on the inclusion of more polar groups, such as dibenzo[b,d]thiophene sulfone backbone units or oligo(ethylene glycol) side-chains. We used optical, spectroscopic, and structural characterisation techniques to understand the different catalytic activities of these systems. We find that although polar groups improve the wettability of the material with water in all cases, backbone and side-chain modifications affect photocatalytic performance in different ways: the inclusion of dibenzo[b,d]thiophene sulfone backbone units improves the thermodynamic driving force for hole transfer to the sacrificial donor, while the inclusion of oligo ethylene glycol side-chains aids the degree of polymer swelling and also extends the electron polaron lifetime. The best performing material, FS-TEG, exhibits a HER of 72.5 μmol h−1 for 25 mg photocatalyst (2.9 mmol g−1 h−1) when dispersed in the presence of a sacrificial donor and illuminated with λ > 420 nm light, corresponding to a hydrogen evolution EQE of 10% at 420 nm. When cast as a thin film, this HER was further boosted to 13.9 mmol g−1 h−1 (3.0 mmol m−2 h−1), which is among the highest rates in this field.
Yang W, Godin R, Kasap H, et al., 2019, Electron accumulation induces efficiency bottleneck for hydrogen production in carbon nitride photocatalysts, Journal of the American Chemical Society, Vol: 141, Pages: 11219-11229, ISSN: 1520-5126
This study addresses the light intensity dependence of charge accumulation in a photocatalyst suspension, and its impact on both charge recombination kinetics and steady-state H2 evolution efficiency. Cyanamide surface functionalized melon-type carbon nitride (NCNCNx) has been selected as an example of emerging carbon nitrides photocatalysts because of its excellent charge storage ability. Transient spectroscopic studies (from ps to s) show that the bimolecular recombination of photogenerated electrons and holes in NCNCNx can be well described by a random walk model. Remarkably, the addition of hole scavengers such as 4-methylbenzyl alcohol can lead to ∼400-fold faster recombination kinetics (lifetime shortening to ∼10 ps). We show that this acceleration is not the direct result of ultrafast hole extraction by the scavenger, but is rather caused by long-lived electron accumulation in NCNCNx after hole extraction. The dispersive pseudo-first order recombination kinetics become controlled by the density of accumulated electrons. H2 production and steady-state spectroscopic measurements indicate that the accelerated recombination caused by electron accumulation limits the H2 generation efficiency. The addition of a reversible electron acceptor and mediator, methyl viologen (MV2+), accelerates the extraction of electrons from the NCNCNx and increases the H2 production efficiency under one sun irradiation by more than 30%. These results demonstrate quantitatively that while long-lived electrons are essential to drive photoinduced H2 generation in many photocatalysts, excessive electron accumulation may result in accelerated recombination losses and lower performance, and thus highlight the importance of efficient electron and hole extraction in enabling efficient water splitting photocatalysts.
Sachs M, Sprick RS, Pearce D, et al., 2018, Understanding structure-activity relationships in linear polymer photocatalysts for hydrogen evolution, Nature Communications, Vol: 9, ISSN: 2041-1723
Conjugated polymers have sparked much interest as photocatalysts for hydrogen production. However, beyond basic considerations such as spectral absorption, the factors that dictate their photocatalytic activity are poorly understood. Here we investigate a series of linear conjugated polymers with external quantum efficiencies for hydrogen production between 0.4 and 11.6%. We monitor the generation of the photoactive species from femtoseconds to seconds after light absorption using transient spectroscopy and correlate their yield with the measured photocatalytic activity. Experiments coupled with modeling suggest that the localization of water around the polymer chain due to the incorporation of sulfone groups into an otherwise hydrophobic backbone is crucial for charge generation. Calculations of solution redox potentials and charge transfer free energies demonstrate that electron transfer from the sacrificial donor becomes thermodynamically favored as a result of the more polar local environment, leading to the production of long-lived electrons in these amphiphilic polymers.
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