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

A Universal Saline-Alkaline Etching Procedure to Enhance the Activity of Oxygen Evolution Catalysts

Production of hydrogen from renewables by water electrolysis is a scalable technology that could compete with fossil fuels if the cost of electrolyzer components was lowered. Herein, we demonstrate a simple and fast method for the several-fold enhancement of mass-activity of oxygen evolution catalysts by oxidatively induced etching facilitated by halide ions in alkaline medium. The performance improvements stem from an increase in the density of active sites on the catalyst surface, allowing the activated materials to maintain the same durability as the original ones. This is exemplified by a one-week stability test at 0.5 A cm-2 following 3-fold activation of an initially highly active nickel-iron sulfide catalyst. Critically, the saline-alkaline activation is universal for a wide range of transition-metal-based catalysts including oxides, layered hydroxides, oxyhydroxides, sulfides, selenides, phosphides, and borides. Hence, beyond water electrolysis, this approach may find use in the development of catalysts for a broad range of applications. </p

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

Mixed Silver–Bismuth Oxides:A Robust Oxygen Evolution Catalyst Operating at Low pH and Elevated Temperatures

Development of catalysts for the oxygen evolution reaction (OER) that are capable of robust operation at low pH and elevated temperatures, but do not contain scarce ruthenium and iridium, presents a challenging yet very attractive strategy in decreasing the high cost of efficient water electrolyzers paired with proton-exchange electrolytes. Toward this aim, combinations of both catalytically active and acid-stable components offer an appealing approach to cost-effective anode catalysis for low-pH water electrolysis. The current work presents an oxygen-evolving [Ag + Bi]Oxcatalyst based on intermixed silver and bismuth oxides, prepared by a simple anodic electrodeposition. We demonstrate that numerous electrode substrates can be functionalized and operate stably with the [Ag + Bi]Oxcatalyst in nominally pure aqueous H2SO4solutions. Moreover, this catalyst maintains robust operation at pH 0.3 and temperatures as high as 80 °C. Under these conditions, the [Ag + Bi]Oxcatalyst can deliver an OER rate of 100 mA cm-2at an overpotential of 0.70 ± 0.02 V vs reversible hydrogen electrode (RHE). In situ X-ray absorption spectroscopic and Fourier transformed alternating current cyclic voltammetric studies of the [Ag + Bi]Oxsystem demonstrate the stabilizing role of the bismuth oxide matrix that facilitates the transformation of silver into a highly oxidized state catalyzing the acidic water electrooxidation.</p

Intrinsic Catalytic Activity for the Alkaline Hydrogen Evolution of Layer-Expanded MoS₂Functionalized with Nanoscale Ni and Co Sulfides

The hydrogen evolution reaction (HER) under alkaline conditions is subject to significant kinetic limitations even with the most active platinum-based catalysts, while more affordable non-noble-metal-based catalytic materials present further challenges in terms of activity and durability in operation. To improve on these aspects, we present a new microwave-assisted synthetic route to fabricate sulfides of nickel and cobalt integrated into a layer expanded molybdenum sulfide (NiSx/MoS2LE and CoSx/MoS2LE), which efficiently catalyze H2 evolution in 1 M KOH. The use of the microwave-synthesis conditions enables the formation of nanoscale Ni and Co sulfides distributed homogeneously within the highly disordered layered molybdenum sulfide, as established using a comprehensive suite of physical methods. Synthesis of FeSx/MoS2LE is also presented, but the resulting material did not exhibit promising properties. Electrocatalytic tests reveal higher activity of the Ni-based catalyst as compared to CoSx/MoS2LE and especially unmodified MoS2LE. The performance of NiSx/MoS2LE at a HER overpotential of 0.15 V at ambient temperature and 60 °C corresponds to specific H2 evolution rates of 28 ± 4 and 58 ± 10 A g-1, respectively. Analysis of the electrokinetic data indicates that the exchange current density of the HER per an electrochemically active surface area of the sulfide-based materials is not high (∼0.001 mA cm-2 at ambient temperature), and that the high performance per unit mass observed here is supported by the well-developed surface area of the material (corresponding to a specific capacitance of ∼71 F g-1). A similar conclusion likely applies to many nickel and cobalt sulfide-based alkaline hydrogen evolution catalysts reported previously. Durability in operation of NiSx/MoS2LE and CoSx/MoS2LE is also demonstrated, in particular through a 2-week-long two-electrode water splitting test. </p

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

Tunable biogenic manganese oxides

Influence of the conditions for aerobic oxidation of Mn-(aq)(2+) 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 Fe-(aq)(2+) makes solids with significant disorder in the structure, while the presence of Ca-(aq)(2+) 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