Peng Fei Liu

Homogeneously dispersed, multimetal oxygen-evolving catalysts

Modulating metal oxides The more difficult step in fuel cells and water electrolysis is the oxygen evolution reaction. The search for earth-abundant materials to replace noble metals for this reaction often turns to oxides of three-dimensional metals such as iron. Zhang et al. show that the applied voltages needed to drive this reaction are reduced for iron-cobalt oxides by the addition of tungsten. The addition of tungsten favorably modulates the electronic structure of the oxyhydroxide. A key development is to keep the metals well mixed and avoid the formation of separate phases. Science, this issue p. 333 The addition of tungsten to iron cobalt oxides lowers the overpotential required for the evolution of oxygen from water. Earth-abundant first-row (3d) transition metal–based catalysts have been developed for the oxygen-evolution reaction (OER); however, they operate at overpotentials substantially above thermodynamic requirements. Density functional theory suggested that non-3d high-valency metals such as tungsten can modulate 3d metal oxides, providing near-optimal adsorption energies for OER intermediates. We developed a room-temperature synthesis to produce gelled oxyhydroxides materials with an atomically homogeneous metal distribution. These gelled FeCoW oxyhydroxides exhibit the lowest overpotential (191 millivolts) reported at 10 milliamperes per square centimeter in alkaline electrolyte. The catalyst shows no evidence of degradation after more than 500 hours of operation. X-ray absorption and computational studies reveal a synergistic interplay between tungsten, iron, and cobalt in producing a favorable local coordination environment and electronic structure that enhance the energetics for OER.

Atomically isolated nickel species anchored on graphitized carbon for efficient hydrogen evolution electrocatalysis

Hydrogen production through electrochemical process is at the heart of key renewable energy technologies including water splitting and hydrogen fuel cells. Despite tremendous efforts, exploring cheap, efficient and durable electrocatalysts for hydrogen evolution still remains as a great challenge. Here we synthesize a nickel–carbon-based catalyst, from carbonization of metal-organic frameworks, to replace currently best-known platinum-based materials for electrocatalytic hydrogen evolution. This nickel-carbon-based catalyst can be activated to obtain isolated nickel atoms on the graphitic carbon support when applying electrochemical potential, exhibiting highly efficient hydrogen evolution performance with high exchange current density of 1.2 mA cm−2 and impressive durability. This work may enable new opportunities for designing and tuning properties of electrocatalysts at atomic scale for large-scale water electrolysis.

Electrochemical etching of α-cobalt hydroxide for improvement of oxygen evolution reaction

Exploring low-cost and high-efficiency electrocatalysts for oxygen evolution reaction (OER) is of paramount importance to develop large-scale alkaline water-splitting electrolyzers (AWEs). Cobalt-based materials have been widely studied because of their promising activity and abundance. However, exposing more active sites for cobalt-based electrocatalysts with enhanced activity and durability still remains as a great challenge. Herein, we demonstrate that chlorine intercalated α-type cobalt hydroxide can be electrochemically etched to form a highly active OER catalyst, achieving a current density of 10 mA cm−2 at an OER overpotential of ∼320 mV. The improvement in the OER activity can be ascribed to the dechlorination induced defective structures and in situ formation of cobalt oxyhydroxide fragments as active sites via anodic polarization. Our findings suggest a novel strategy of electrochemical etching of chlorine containing hydroxide catalysts to obtain defective oxyhydroxides for continuous electrochemical water splitting.

Mn3O4 nano-octahedrons on Ni foam as an efficient three-dimensional oxygen evolution electrocatalyst

Large-scale electrolysis of water for hydrogen production requires efficient and earth-abundant oxygen evolution catalysts. Here, we demonstrate a facile method to prepare catalytically active Mn3O4 octahedral crystals on Ni foam as a composite electrode, which exhibits outstanding activity and stability for electrocatalytic oxygen evolution.

Structure disorder of graphitic carbon nitride induced by liquid-assisted grinding for enhanced photocatalytic conversion

Graphitic-C3N4 with a disordered structure was processed for the first time by a liquid-assisted planetary ball milling approach. Through this simple and effective mechanochemistry method, the milled samples displayed outstanding visible-light photoactivity and the optimized one showed 7-fold higher H2 evolution rate than the bulk one.

Cobalt Covalent Doping in MoS2 to Induce Bifunctionality of Overall Water Splitting

The layer-structured MoS2 is a typical hydrogen evolution reaction (HER) electrocatalyst but it possesses poor activity for the oxygen evolution reaction (OER). In this work, a cobalt covalent doping approach capable of inducing HER and OER bifunctionality into MoS2 for efficient overall water splitting is reported. The results demonstrate that covalently doping cobalt into MoS2 can lead to dramatically enhanced HER activity while simultaneously inducing remarkable OER activity. The catalyst with optimal cobalt doping density can readily achieve HER and OER onset potentials of -0.02 and 1.45 V (vs reversible hydrogen electrode (RHE)) in 1.0 m KOH. Importantly, it can deliver high current densities of 10, 100, and 200 mA cm-2 at low HER and OER overpotentials of 48, 132, 165 mV and 260, 350, 390 mV, respectively. The reported catalyst activation approach can be adapted for bifunctionalization of other transition metal dichalcogenides.

Band-aligned C3N4−xS3x/2 stabilizes CdS/CuInGaS2 photocathodes for efficient water reduction

Compared with bare CIGS films and CdS-modified CIGS films (CdS/CIGS), C3N4−xS3x/2/CdS/CIGS electrodes exhibit a reduction of 200 mV in the onset potential. They also show a doubled photocurrent at 0 V versus the RHE, and a broadband 20% enhancement of incident photon to current efficiency (IPCE) in noble-metal-free systems. Remarkably, the C3N4−xS3x/2/CdS/CIGS electrode shows ≤5% loss over 20 hours of continuous operation, whereas bare CdS/CIGS shows a rapid degradation within the first 4 hours.

Simple Cadmium Sulfide Compound with Stable 95 % Selectivity for Carbon Dioxide Electroreduction in Aqueous Medium

A simple cadmium sulfide nanomaterial is found to be an efficient and stable electrocatalyst for CO2 reduction in aqueous medium for more than 40 h with a steady CO faradaic efficiency of approximately 95 %. Moreover, it can realize a current density of -10 mA cm-2 at an overpotential of -0.55 V on a porous substrate with similar selectivity. Theoretical and experimental results confirm that the high selectivity for CO2 reduction is due to its (0 0 0 2) face with sulfur vacancies that prefers CO2 molecule reduction in aqueous medium.

Amorphous ferric oxide as a hole-extraction and transfer layer on nanoporous bismuth vanadate photoanode for water oxidation

Abstract An amorphous ferric oxide layer was prepared on a bismuth vanadate photoanode. This resulted in improved charge carrier separation and surface catalytic performance compared with the photoanode without the oxide layer. The photocurrent of the oxide-layer-containing photoanode was 2.52 mA/cm 2 at 1.23 V versus the reversible hydrogen electrode, in potassium phosphate buffer (0.5 mol/L, pH = 7.0). The amorphous ferric oxide layer on the photoanode contained low-valence-state iron species (Fe II ), which enabled efficient hole extraction and transfer.

Bottom-Up Enhancement of g-C3N4 Photocatalytic H2 Evolution Utilising Disordering Intermolecular Interactions of Precursor

Disordered intermolecular interaction carbon nitride precursor prepared by water-assisted grinding of dicyandiamide was used for synthesis of g-C3N4. The final sample possesses much looser structure and provides a broadening optical window for effective light harvesting and charge separation efficiency, which exhibits significantly improved H2 evolution by photocatalytic water splitting. The bottom-up mechanochemistry method opens new vistas towards the potential applications of weak interactions in the photocatalysis field and may also stimulate novel ideas completely different from traditional ones for the design and optimization of photocatalysts.