Effects of Gold Substrates on the Intrinsic and Extrinsic Activity of High-Loading Nickel-Based Oxyhydroxide Oxygen Evolution Catalysts

We systematically investigate the effects of Au substrates on the oxygen evolution activities of cathodically electrodeposited nickel oxyhydroxide (NiOOH), nickel–iron oxyhydroxide (NiFeOOH), and nickel–cerium oxyhydroxide (NiCeOOH) at varying loadings from 0 to 2000 nmol of metal/cm². We determine that the geometric current densities, especially at higher loadings, were greatly enhanced on Au substrates: NiCeOOH/Au reached 10 mA/cm² at 259 mV overpotential, and NiFeOOH/Au achieved 140 mA/cm² at 300 mV overpotential, which were much greater than those of the analogous catalysts on graphitic carbon (GC) substrates. By performing a loading quantification using both inductively coupled plasma optical emission spectrometry and integration of the Ni^2+/3+ redox peak, we show that the enhanced activity is predominantly caused by the stronger physical adhesion of catalysts on Au. Further characterizations using impedance spectroscopy and <i>in situ</i> X-ray absorption spectroscopy revealed that the catalysts on Au exhibited lower film resistances and higher number of electrochemically active metal sites. We attribute this enhanced activity to a more homogeneous electrodeposition on Au, yielding catalyst films with very high geometric current densities on flat substrates. By investigating the mass and site specific activities as a function of loading, we bridge the practical geometric activity to the fundamental intrinsic activity

Understanding activity trends in electrochemical water oxidation to form hydrogen peroxide

Electrochemical production of hydrogen peroxide (H2O2) from water oxidation could provide a very attractive route to locally produce a chemically valuable product from an abundant resource. Herein using density functional theory calculations, we predict trends in activity for water oxidation towards H2O2 evolution on four different metal oxides, i.e., WO3, SnO2, TiO2 and BiVO4. The density functional theory predicted trend for H2O2 evolution is further confirmed by our experimental measurements. Moreover, we identify that BiVO4 has the best H2O2 generation amount of those oxides and can achieve a Faraday efficiency of about 98% for H2O2 production

Single-Source Bismuth (Transition Metal) Polyoxovanadate Precursors for the Scalable Synthesis of Doped BiVO4 Photoanodes.

Single-source precursors are used to produce nanostructured BiVO4 photoanodes for water oxidation in a straightforward and scalable drop-casting synthetic process. Polyoxometallate precursors, which contain both Bi and V, are produced in a one-step reaction from commercially available starting materials. Simple annealing of the molecular precursor produces nanocrystalline BiVO4 films. The precursor can be designed to incorporate a third metal (Co, Ni, Cu, or Zn), enabling the direct formation of doped BiVO4 films. In particular, the Co- and Zn-doped photoanodes show promise for photoelectrochemical water oxidation, with photocurrent densities >1 mA cm-2 at 1.23 V vs reversible hydrogen electrode (RHE). Using this simple synthetic process, a 300 cm2 Co-BiVO4 photoanode is produced, which generates a photocurrent of up to 67 mA at 1.23 V vs RHE and demonstrates the scalability of this approach.We thank the following for financial support: China Scholarship Council (H.L.), the Cambridge Trusts (Vice Chancellor’s Award) and the Winton Programme for the Physics of Sustainability (V.A.), A*STAR Graduate Scholarship (Overseas) (N.L.), Imperial College Research Fellowship (A.R.), Christian Doppler Research Association and the OMV Group (E.R), Herchel Smith Research Fund (S.D.P

Nanostructuring Strategies To Increase the Photoelectrochemical Water Splitting Activity of Silicon Photocathodes

Photoelectrochemical water splitting is a promising route for sustainable hydrogen production. Herein, we demonstrate a photoelectrode motif that enables a nanostructured large-surface area electrocatalyst without requiring a nanostructured semiconductor surface with the goal of promoting electrocatalysis while minimizing surface recombination. We compare the photoelectrochemical H2 evolution activity of two silicon photocathode nanostructuring strategies: (1) direct nanostructuring of the silicon surface and (2) incorporation of nanostructured zinc oxide to increase the electrocatalyst surface area on planar silicon. We observed that silicon photocathodes that utilized nanostructured ZnO supports outperformed nanostructured silicon electrodes by ∼50 mV at open circuit under 1 sun illumination and demonstrated comparable electrocatalytic activity

High Surface Area Transparent Conducting Oxide Electrodes with a Customizable Device Architecture

Herein, we report the development of optically transparent high surface area electrodes (HSEs) made of indium tin oxide (ITO) using a facile, template-free, scalable wet chemical synthetic approach. The transparent HSEs are electronically conductive and physically robust, with tunable electrochemically accessible roughness factors from 1 through ∼100. These transparent HSEs can serve as a broadly functionalizable scaffold ideally suited to bridge the gap between the need to minimize ionic and electronic transport lengths within a device and the need to achieve high loadings of active material per surface area (e.g. for high capacity as needed for batteries or high optical density as required for electrochromics or photovoltaics). This gap currently stands as a major hurdle for the utilization of many nanomaterials in electronic and optoelectronic devices. The synthetic approach described here for ITO is transferable to other transparent conducting oxide (TCO) materials. This is the first report of a large-pore, tunable, high surface area TCO electrode with excellent optical and conductive properties

Tandem core-shell Si-Ta3-N5 photoanodes for photoelectrochemical water splitting

Nanostructured core–shell Si–Ta3N5 photoanodes were designed and synthesized to overcome charge transport limitations of Ta3N5 for photoelectrochemical water splitting. The core–shell devices were fabricated by atomic layer deposition of amorphous Ta2O5 onto nanostructured Si and subsequent nitridation to crystalline Ta3N5. Nanostructuring with a thin shell of Ta3N5 results in a 10-fold improvement in photocurrent compared to a planar device of the same thickness. In examining thickness dependence of the Ta3N5 shell from 10 to 70 nm, superior photocurrent and absorbed-photon-to-current efficiencies are obtained from the thinner Ta3N5 shells, indicating minority carrier diffusion lengths on the order of tens of nanometers. The fabrication of a heterostructure based on a semiconducting, n-type Si core produced a tandem photoanode with a photocurrent onset shifted to lower potentials by 200 mV. CoTiOx and NiOx water oxidation cocatalysts were deposited onto the Si–Ta3N5 to yield active photoanodes that with NiOx retained 50–60% of their maximum photocurrent after 24 h chronoamperometry experiments and are thus among the most stable Ta3N5 photoanodes reported to date

Preferred Orientation in Sputtered TiO₂ Thin Films and Its Effect on the Photo-Oxidation of Acetaldehyde

Crystal orientation is not typically considered when investigating the reactivity of thin films. We propose that accounting for the preferred crystallographic orientation may serve as an indirect measure of the active sites along the solid–solid interface that are difficult to measure with direct techniques. The goal of this work is to identify the preferred orientation, examine its evolution as a function of synthesis parameters, and determine its effect on photoreactivity. We examine the effect of substrate radio frequency (RF) bias and reactive gas partial pressure on the structure and photoreactivity of TiO₂ films synthesized by reactive direct current (DC) magnetron sputtering. We characterize these films using ellipsometry, scanning electron microscopy (SEM), grazing incidence X-ray diffraction (GIXRD), and pole figure scans, and test their photoreactivity with the degradation of acetaldehyde under 365 nm UV light. We find that, in the parameter space investigated, changes in RF bias strongly influence both film texture and reactivity, and that the orientation of the crystallites is the best predictor of photoreactivity. Under the synthesis conditions tested, we observe an optimum RF bias of −50 V at which the films exhibit biaxial texture with the <i>c</i>-axis parallel to the surface with maximum crystallinity and degree of orientation, corresponding to a maximum in the reactivity as well. Beyond this point a change in the preferred orientation is observed, and the films transition to a fiber texture with the <i>c</i>-axis normal to the film surface and the appearance of small amounts of rutile. The effect of texture on reactivity is discussed

General Pyrolysis for High-Loading Transition Metal Single Atoms on 2D-Nitro-Oxygeneous Carbon as Efficient ORR Electrocatalysts

Single-atom catalysts (SACs) possess the potential to involve the merits of both homogeneous and heterogeneous catalysts altogether and thus have gained considerable attention. However, the large-scale synthesis of SACs with rich isolate-metal sites by simple and low-cost strategies has remained challenging. In this work, we report a facile one-step pyrolysis that automatically produces SACs with high metal loading (5.2–15.9 wt %) supported on two-dimensional nitro-oxygenated carbon (M1-2D-NOC) without using any solvents and sacrificial templates. The method is also generic to various transition metals and can be scaled up to several grams based on the capacity of the containers and furnaces. The high density of active sites with N/O coordination geometry endows them with impressive catalytic activities and stability, as demonstrated in the oxygen reduction reaction (ORR). For example, Fe1-2D-NOC exhibits an onset potential of 0.985 V vs RHE, a half-wave potential of 0.826 V, and a Tafel slope of −40.860 mV/dec. Combining the theoretical and experimental studies, the high ORR activity could be attributed its unique FeO-N3O structure, which facilitates effective charge transfer between the surface and the intermediates along the reaction, and uniform dispersion of this active site on thin 2D nanocarbon supports that maximize the exposure to the reactants