Photoelectrochemical Performance of the Ag(III)-Based Oxygen-Evolving Catalyst

Selective Catalytic Electro-Oxidation of Water with Cobalt Oxide in Ion Impermeable Reduced Graphene Oxide Porous Electrodes

The direct electrolysis of seawater is greatly inhibited by the oxidation of Cl^- to free chlorine, an undesirable, corrosive byproduct. To suppress the parasitic interference of Cl^- and any other ion, we developed a freestanding, electrically conducting, 3D macroporous reduced graphene oxide (rGO) scaffold with cobalt oxide particles selectively deposited on the internal walls of its closed pores (with an average diameter of ∼180 μm). The pore walls act as membranes composed of stacked rGO flakes; the nanochannels between rGO layers (size <1 nm) are permeable to water and gases while preventing the diffusion of dissolved ions such as Cl^-. Due to this, the catalytic particles are selectively accessible to water molecules but not to ions, allowing electrolysis to occur without chlorine evolution. The electrodes developed exhibit a stable generation of O₂ from simulated seawater at pH 14, reaching a specific current density of up to 25 A g^-1 during continuous electrolysis with 89-98% Faradaic efficiency, while chlorine generation is below 6 ppm h^-1, the sensitivity limit of the detection method employed. The strategy here proposed can be generalized to build electrodes that are inherently selective thanks to their architecture, with catalytically active particles loaded into closed pores with selective ion transport properties.

AgO-decorated multi-dimensional chrysanthemum-like NiCo2O4 mounted on nickel foam as a highly efficient and stable electrocatalyst for the oxygen evolution reaction

AgO nanoparticles were successfully integrated into NiCo2O4 nanosheets for enhanced electrochemical catalysis ability and stability in the oxygen evolution reaction (OER). The chrysanthemum-like NiCo2O4/AgO composites mounted on nickel foam (NF) were synthesized by a hydrothermal-calcination method. AgO upgraded the ratio of Co3+/Co2+ and thus regulated the intrinsic activity of the species. The highly hierarchical structure of NiCo2O4/AgO composed of 0D AgO nanoparticles, 1D NiCo2O4 needles, 2D NiCo2O4 nanosheets, and 3D chrysanthemum-like bundles grown on NF bestowed the high surface area and mesoporous structure for the easy evolution of O2. The Ni atoms in NiCo2O4 originating in situ from NF in the process of AgO formation produced an integrated electrode of the active component of NiCo2O4 bound on NF with a superb highway for charge transfer. AgO significantly tuned the structure and physicochemical properties of NiCo2O4. As a result, NiCo2O4/AgO/NF exhibited excellent OER performance with an overpotential of 232 mV to obtain a current density of 10 mAcm-2 in an alkaline electrolyte, and the catalyst showed a small loss of the initial catalyst activity for 50 h and over 5000 cycles. This study provides a pathway for developing high-performance OER electrocatalysts.

Interfacial Coupling Effect on Electron Transport in Hierarchical TaON/Au/ZnCo-LDH Photoanode with Enhanced Photoelectrochemical Water Oxidation

Rational design and construction of interface heterostructures, which can simultaneously accelerate the photoinduced carrier separation and enhance the surface water oxidation kinetics, is of great necessity for photoelectrochemical (PEC) water oxidation. Herein, we report a new strategy for boosting the PEC water oxidation by introducing Schottky junction and semiconductor/water oxidation cocatalysts (SC/WOCs) junction into the TaON photocatalyst. Compared with pristine TaON photoanode, the hierarchical TaON/Au/ZnCo-LDH (LDH = layered double hydroxide) photoanode reveals a cathodic shift of 156 mV for the onset potential and 17.3-fold photocurrent density enhancement at 1.23 V vs RHE, as well as improved long-term stability. Diagnostic efficiencies of the TaON/Au/ZnCo-LDH photoanode demonstrate that the enhanced PEC performance is not dominated by surface electrochemical water oxidation kinetics but largely contributed by the improved charge separation and transfer, indicative of synergistic effects of Au and ZnCo-LDH. Theoretical calculations further reveal that the midgap states introduced by Au and ZnCo-LDH in TaON electronic structures bring about photoexcited electrons concentrated on TaON, while holes accumulated on ZnCo-LDH to achieve efficiently spatial charge separation, which is responsible for the boosted PEC water oxidation performance. The present work highlights the importance and elucidates the mechanism of interface heterojunction in PEC water oxidation, which can provide an efficient approach to design and fabricate a new structural photoanode.

Rational Design and Construction of Cocatalysts for Semiconductor-Based Photo-Electrochemical Oxygen Evolution: A Comprehensive Review

Photo-electrochemical (PEC) water splitting, as an essential and indispensable research branch of solar energy applications, has achieved increasing attention in the past decades. Between the two photoelectrodes, the photoanodes for PEC water oxidation are mostly studied for the facile selection of n-type semiconductors. Initially, the efficiency of the PEC process is rather limited, which mainly results from the existing drawbacks of photoanodes such as instability and serious charge-carrier recombination. To improve PEC performances, researchers gradually focus on exploring many strategies, among which engineering photoelectrodes with suitable cocatalysts is one of the most feasible and promising methods to lower reaction obstacles and boost PEC water splitting ability. Here, the basic principles, modules of the PEC system, evaluation parameters in PEC water oxidation reactions occurring on the surface of photoanodes, and the basic functions of cocatalysts on the promotion of PEC performance are demonstrated. Then, the key progress of cocatalyst design and construction applied to photoanodes for PEC oxygen evolution is emphatically introduced and the influences of different kinds of water oxidation cocatalysts are elucidated in detail. Finally, the outlook of highly active cocatalysts for the photosynthesis process is also included.