Enhancing the Performances of P3HT:PCBM–MoS₃‑Based H₂‑Evolving Photocathodes with Interfacial Layers

Organic semiconductors have great potential for producing hydrogen in a durable and economically viable manner because they rely on readily available materials and can be solution-processed over large areas. With the objective of building efficient hybrid organic–inorganic photoelectrochemical cells, we combined a noble-metal-free and solution-processable catalyst for proton reduction, MoS₃, and a poly­(3-hexylthiophene):phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction (BHJ). Different interfacial layers were investigated to improve the charge transfer between P3HT:PCBM and MoS₃. Metallic Al/Ti interfacial layers led to an increase of the photocurrent by up to 8 mA cm^–2 at reversible hydrogen electrode (RHE) potential with a 0.6 V anodic shift of the H₂ evolution reaction onset potential, a value close to the open-circuit potential of the P3HT:PCBM solar cell. A 50-nm-thick C₆₀ layer also works as an interfacial layer, with a current density reaching 1 mA cm^–2 at the RHE potential. Moreover, two recently highlighted figures-of-merit, measuring the ratio of power saved, Φ_saved,ideal and Φ_saved,NPAC, were evaluated and discussed to compare the performances of various photocathodes assessed in a three-electrode configuration. Φ_saved,ideal and Φ_saved,NPAC use the RHE and a nonphotoactive electrode with an identical catalyst as the dark electrode, respectively. They provide different information especially for differentiation of the roles of the photogenerating layer and catalyst. The best results were obtained with the Al/Ti metallic interlayer, with Φ_saved,ideal and Φ_saved,NPAC reaching 0.64% and 2.05%, respectively

Combined Experimental–Theoretical Characterization of the Hydrido-Cobaloxime [HCo(dmgH)₂(P<i>n</i>Bu₃)]

A combined theoretical and experimental approach has been employed to characterize the hydrido-cobaloxime [HCo­(dmgH)₂(P<i>n</i>Bu₃)] compound. This complex was originally investigated by Schrauzer et al. [Schrauzer et al., <i>J. Am. Chem. Soc</i>. <b>1971</b>, <i>93</i>,1505] and has since been referred to as a key, stable analogue of the hydride intermediate involved in hydrogen evolution catalyzed by cobaloxime compounds [Artero, V. et al.<i> Angew. Chem., Int. Ed</i>. <b>2011</b>, <i>50</i>, 7238–7266]. We employed quantum chemical calculations, using density functional theory and correlated RI-SCS-MP2 methods, to characterize the structural and electronic properties of the compound and observed important differences between the calculated ¹H NMR spectrum and that reported in the original study by Schrauzer and Holland. To calibrate the theoretical model, the stable hydrido tetraamine cobalt­(III) complex [HCo­(tmen)₂(OH₂)]²⁺ (tmen = 2,3-dimethyl-butane-2,3-diamine) [Rahman, A. F. M. M. et al.<i> Chem. Commun</i>. <b>2003</b>, 2748–2749] was subjected to a similar analysis, and, in this case, the calculated results agreed well with those obtained experimentally. As a follow-up to the computational work, the title hydrido-cobaloxime compound was synthesized and recharacterized experimentally, together with the Co­(I) derivative, giving results that were in agreement with the theoretical predictions

Phosphine Coordination to a Cobalt Diimine–Dioxime Catalyst Increases Stability during Light-Driven H₂ Production

The combination of cobalt diimine–dioxime complexes with a cyclometalated iridium photosensitizer gives efficient systems for hydrogen generation under visible-light irradiation using triethylamine as a sacrificial electron donor. Interestingly, the addition of triphenylphosphine (PPh₃) to the medium results in a significant improvement of the stability of the system, with up to ∼700 turnovers achieved within 10 h. UV–visible spectroscopic monitoring of the reaction allows identification of a PPh₃-coordinated Co^I intermediate as the active species. Mechanistic issues regarding (i) the photogeneration of the Co^I species, (ii) the nature of the active species, and (iii) the influence of PPh₃ on the H₂-evolution mechanism are discussed

Photoelectrochemical Reduction of CO₂ Coupled to Water Oxidation Using a Photocathode with a Ru(II)–Re(I) Complex Photocatalyst and a CoO_<i>x</i>/TaON Photoanode

Photoelectrochemical CO₂ reduction activity of a hybrid photocathode, based on a Ru­(II)–Re­(I) supramolecular metal complex photocatalyst immobilized on a NiO electrode (NiO–<b>RuRe</b>), was confirmed in an aqueous electrolyte solution. Under half-reaction conditions, the NiO–<b>RuRe</b> photocathode generated CO with high selectivity, and its turnover number for CO formation reached 32 based on the amount of immobilized <b>RuRe</b>. A photoelectrochemical cell comprising a NiO–<b>RuRe</b> photocathode and a CoO_<i>x</i>/TaON photoanode showed activity for visible-light-driven CO₂ reduction using water as a reductant to generate CO and O₂, with the assistance of an external electrical (0.3 V) and chemical (0.10 V) bias produced by a pH difference. This is the first example of a molecular and semiconductor photocatalyst hybrid-constructed photoelectrochemical cell for visible-light-driven CO₂ reduction using water as a reductant

Pump-Flow-Probe X‑ray Absorption Spectroscopy as a Tool for Studying Intermediate States of Photocatalytic Systems

A new setup for pump-flow-probe X-ray absorption spectroscopy has been implemented at the SuperXAS beamline of the Swiss Light Source. It allows recording X-ray absorption spectra with a time resolution of tens of microseconds and high detection efficiency for samples with sub-millimolar concentrations. A continuous wave laser is used for the photoexcitation, with the distance between laser and X-ray beams and velocity of liquid flow determining the time delay, while the focusing of both beams and the flow speed profile define the time resolution. This method is compared with the alternative measurement technique that utilizes a 1 kHz repetition rate laser and multiple X-ray probe pulses. Such an experiment was performed at beamline 11ID-D of the Advanced Photon Source. Advantages, limitations, and potential for improvement of the pump-flow-probe setup are discussed by analyzing the photon statistics. Both methods with Co K-edge probing were applied to the investigation of a cobaloxime-based photocatalytic reaction. The interplay between optimizing for efficient photoexcitation and time resolution as well and the effect of sample degradation for these two setups are discussed

Enhanced Light Trapping in GaAs/TiO₂‑Based Photocathodes for Hydrogen Production

Photoelectrochemical cells (PEC) are appealing devices for the production of renewable energy carriers. In this context, III–V semiconductors such as GaAs are very promising materials due to their tunable band gaps, which can be appropriately adjusted for sunlight harvesting. Because of the high cost of these semiconductors, the nanostructuring of the photoactive layer can help to improve the device efficiency as well as drastically reduce the amount of material needed. III–V nanowire-based photoelectrodes benefit from the intrinsically high aspect ratio of nanowires, their enhanced ability to trap light, and their improved charge separation and collection abilities and thus are particularly attractive for PECs. However, III–V semiconductors often suffer from corrosion in aqueous electrolytes, preventing their utilization over long periods under relevant working conditions. Here, photocathodes of GaAs nanowires protected with thin TiO2 shells were prepared and studied under simulated sunlight irradiation to assess their photoelectrochemical performances in correlation with their structural degradation, highlighting the advantageous nanowire geometry compared to its thin-film counterpart. Morphological and electronic parameters, such as the aspect ratio of the nanowires and their doping pattern, were found to strongly influence the photocatalytic performances of the system. This work highlights the advantageous combination of nanowires featuring a buried radial p–n junction with Co nanoparticles used as a hydrogen evolution catalyst. The nanostructured photocathodes exhibit significant photocatalytic activities comparable with previous noble-metal-based systems. This study demonstrates the potential of a GaAs nanostructured semiconductor and its reliable use for photodriven hydrogen production

Cobaloxime-Based Artificial Hydrogenases

Cobaloximes are popular H₂ evolution molecular catalysts but have so far mainly been studied in nonaqueous conditions. We show here that they are also valuable for the design of artificial hydrogenases for application in neutral aqueous solutions and report on the preparation of two well-defined biohybrid species via the binding of two cobaloxime moieties, {Co­(dmgH)₂} and {Co­(dmgBF₂)₂} (dmgH₂ = dimethylglyoxime), to apo <i>Sperm-whale</i> myoglobin (<i>Sw</i>Mb). All spectroscopic data confirm that the cobaloxime moieties are inserted within the binding pocket of the <i>Sw</i>Mb protein and are coordinated to a histidine residue in the axial position of the cobalt complex, resulting in thermodynamically stable complexes. Quantum chemical/molecular mechanical docking calculations indicated a coordination preference for His93 over the other histidine residue (His64) present in the vicinity. Interestingly, the redox activity of the cobalt centers is retained in both biohybrids, which provides them with the catalytic activity for H₂ evolution in near-neutral aqueous conditions