Self-Supported Hierarchical IrO2@NiO Nanoflake Arrays as an Efficient and Durable Catalyst for Electrochemical Oxygen Evolution

Interfacial engineering of heterostructured CoP/FeP nanoflakes as bifunctional electrocatalyts toward alkaline water splitting

Exploring highly-effective and nonprecious electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is urgent and challenging for developing the hydrogen economy. Interface engineering is a feasible approach for regulating the surface electronic distribution, thereby promoting the catalytic performance. Herein, the CoP/FeP heterostructure is fabricated via the oxidation and phosphating treatments of Fe-decorated Ni(OH)2 nanoflakes. The hierarchically porous nanoflakes can expose more active species, while the formation of CoP/FeP heterojunctions have provided extra catalytic active sites and accelerated the charge transfer process. Theoretical calculations reveal that the interfacial electron coupling between CoP and FeP in the heterostructure has promoted the adsorption of intermediate species on catalytic sites, thereby decreasing the Gibbs free energy during the catalysis. The as-fabricated CoP/FeP catalyst requires small overpotentials of 190 mV and 280 mV to realize a current density of 10 mA cm-2 for alkaline HER and OER, respectively. The electrolytic cell with CoP/FeP as catalyst needs a voltage of 1.61 V to reach 10 mA cm-2, and can run stably for over 25 h. The present study highlights a superiority of interfacial engineering to construct efficient electrocatalysts for water electrolysis.

A Porous Perovskite Nanofiber with Reinforced Aerophobicity for High-Performance Anion Exchange Membrane Water Splitting

Perovskite oxides stand out as emerging oxygen evolution reaction (OER) catalysts on account of their effective electrocatalytic performance and low costs. Nevertheless, perovskite oxides suffer from severe bubble overpotential and inhibited electrochemical performance in large current densities due to their small specific surface areas and structural compactness. Herein, the study highlights the electrospun nickel-substituted La0.5 Sr0.5 FeO3-δ (LSF) porous perovskite nanofibers, that is, La0.5 Sr0.5 Fe1-x Nix O3-δ (denoted as ES-LSFN-x, x = 0, 0.1, 0.3, and 0.5), as high-performance OER electrocatalysts. The most effective La0.5 Sr0.5 Fe0.5 Ni0.5 O3-δ (ES-LSFN-0.5) nanofibers suggest a larger specific surface area, higher porosity, and faster mass transfer than the counterpart sample prepared by conventional sol-gel method (SG-LSFN-0.5), showing notably increased geometric and intrinsic activities. The bubble visualization results demonstrate that the enriched and nano-sized porosity of ES-LSFN-0.5 enables reinforced aerophobicity and rapid detachment of oxygen bubbles, thereby reducing the bubble overpotential and enhancing the electrochemical performance. As a result, the ES-LSFN-0.5-based anion exchange membrane water electrolysis delivers a superior stability of 100 h while the SG-LSFN-0.5 counterpart degrades rapidly within 20 h under a current density of 100 mA cm-2 . The results highlight the advantage of porous electrocatalysts in optimizing the performance of large current density water electrolysis devices by reducing the bubble overpotential.

One-step chemical vapor deposition fabrication of Ni@NiO@graphite nanoparticles for the oxygen evolution reaction of water splitting

NiO combined with conductive materials is a practicable way to improve its catalytic property for the oxygen evolution reaction (OER) by enhancing its electrical conductivity. Herein, Ni@NiO@graphite nanoparticles less than 20 nm in average diameter were synthesized by a one-step chemical vapor deposition process. Due to the deliberately controlled lack of oxygen, Ni particles and carbon clusters decomposed from NiCp2 precursors were oxidized incompletely and formed Ni@NiO core-shell nanoparticles coated by a graphite layer. The thickness of the graphite layer and the content of Ni were controlled by varying deposition temperature. The electrochemical activity towards the oxygen evolution reaction was assessed within alkaline media. Compared with commercial NiO powder, the Ni@NiO@graphite nanoparticles with the unique core-shell microstructure exhibit excellent OER performance, i.e., an overpotential of 330 mV (vs. RHE) at 10 mA cm-2 and a Tafel slope of 49 mV dec-1, due to the improved electrical conductivity and more active sites. This work provides a facile and rapid strategy to produce nanoparticles with unique microstructures as highly active electrocatalysts for the OER.

Morphology-oxygen evolution activity relationship of iridium(iv) oxide nanomaterials

This work demonstrated shape tuning of IrO2 nanoparticles to nanocube and nanorods in molten salt and demonstrated the exemplary performance of IrO2 nanorods as an electrocatalyst for oxygen evolution reaction even surpassing commercial IrO2.

In situ construction of FeNi2Se4-FeNi LDH heterointerfaces with electron redistribution for enhanced overall water splitting

The abundant heterogeneous interfaces between the FeNi2Se4 and FeNi LDH can provide enriched active sites and accelerate reaction kinetics, which improves the overall water splitting performance.

Hierarchical sheet-on-sheet heterojunction array of a β-Ni(OH)2/Fe(OH)3 self-supporting anode for effective overall alkaline water splitting

Rationally designing high-performance non-noble metal electrocatalysts is of essence to improve energy conversion efficiency in water splitting. Herein, a unique 3D hierarchical sheet-on-sheet heterojunction between Fe(OH)₃ and β-Ni(OH)₂ on pretreated Ni foam (NiFe-HD/pre-NF) was fabricated by a two-step strategy involving the interfacial hydrolysis-deposition of Fe²⁺ and electrodeposition of Ni²⁺. The presence of the Ni-O-Fe bridge at the Fe(OH)₃/β-Ni(OH)₂ heterointerface can induce interfacial electronic redistribution to form Ni³⁺ in NiFe-HD/pre-NF, and further strengthen the adsorption of OH^- and weaken the O-H bond to change the rate-determining step (RDS) for accelerating OER kinetics. Benefiting from the sheet-on-sheet architecture and dual-phase synergism on NiFe-HD/pre-NF, the optimal NiFe-HD/pre-NF exhibits excellent OER performance with a lower overpotential of 256 mV at 100 mA cm^-2, a small Tafel slope of 81 mV dec^-1, high intrinsic activity and robust stability. Alkaline water-splitting using NiFe-HD/pre-NF as the anode requires ultralow cell voltages of 1.62 V and 1.83 V at current densities of 100 mA cm^-2 and 400 mA cm^-2, respectively, which are comparable with commercial alkaline water electrolysis, and operates steadily at a current density of 100 mA cm^-2 for 85 h without decay. This work proposes a facile strategy for constructing heterojunctions and modulating electronic interaction to develop electrocatalysts with new architectures.

Enhancing Effect of Fe2+ Doping of Ni/NiO Nanocomposite Films on Catalytic Hydrogen Generation

Highly active and stable non-noble metal catalysts are expected to play a critical role in future hydrogen storage and conversion applications. The design of active sites with composite oxides provides a new approach for developing high-performance catalysts. In this study, an Fe-doped Ni/NiO nanocomposite film was constructed on an ionic liquid/water interface to promote hydrogen generation. The optimized Ni/FeNiO_x-25 catalyst showed excellent catalytic activity toward ammonia borane hydrolysis, with a turnover frequency of 72.3 min^-1. The enhancing effect of Fe²⁺ doping on Ni/NiO films was confirmed by the improved intrinsic activity and theoretical simulations. Fe ion doping stabilized NiO and prevented NiO from becoming Ni. The interfacial Ni-Fe²⁺ dual active sites on the FeNiO_x and Ni interfaces participated in the targeted adsorption and effective activation of water and NH₃BH₃ molecules, respectively. The sufficiently exposed plane surface of the nanofilms provided abundant active sites for catalytic reactions. This significant advance will inspire development in the ambient liquid hydrogen storage field.

Single-Atom Doping and High-Valence State for Synergistic Enhancement of NiO Electrocatalytic Water Oxidation

The NiO-based electrocatalytic oxygen evolution reaction (OER) of water splitting is recognized as a promising approach to produce clean H₂ fuel. However, the OER performance is still low, and especially, the overpotential is larger than 200 mV at the current density of 10 mA cm^-2 . Herein, an Ir@IrNiO sample is prepared with single-atom (SA) Ir⁴⁺ doping and surface metallic Ir nanoparticles loaded onto the NiO. Owing to the bonding of the loaded Ir with surface-exposed Ni²⁺ , the nearby Ni atoms exist in the +3 valence state, that is, the surface-loaded Ir particles behave like a stabilizer for the Ni³⁺ sites. Under the synergistic effect of SA Ir⁴⁺ and high-valance-state Ni³⁺ , the Ir@IrNiO nanostructure effectively reduces the overpotential to 195 mV at a current density of 10 mA cm^-2 . Moreover, it gives an Ir-content-normalized current density of 0.0457 A mg_Ir ^-1 , 72.1 times higher than that of the best commercialized IrO₂ (6.33 × 10^-4 A mg_Ir ^-1 ), under the condition of 1.5 V versus reversible hydrogen electrode. Operando Raman and X-ray absorption fine-structure (XAFS) measurements reveal that there are more surface-active species of Ni³⁺ , which adsorb and activate water molecules to form Ni³⁺ -*OH at low voltage, the intermediate of Ni⁴⁺ -•O is then formed at a relatively high bias voltage, and then the •O is transferred to the SA Ir⁴⁺ sites to generate Ir⁴⁺ -O-O with OH at increased voltage. This work can help design more SA-based highly active OER materials.

A Bidirectional Nanomodification Approach for Synthesizing Hierarchically Architected Mixed Oxide Electrodes for Oxygen Evolution

Several transition-metal oxides and hydroxides based on earth-abundant elements, such as Fe, Ni, and Co, have emerged as a new generation of oxygen evolution reaction (OER) catalysts due to their low cost, favorable activity, and multifunctional behavior. However, the relatively complicated surface structuring methods, high Tafel slope, and low stability hinder their practical applications to replace the conventional Ir- and Ru-based catalysts. Herein, a strategy to construct hierarchically architected mixed oxides on conductive substrates (e.g., ITO and Ni foam) via a nanosheet (NS) deposition and subsequent bidirectional nanomodification approach, with metal salts in an aprotic polar solvent (e.g., acetone) as the primary modifying reactants is reported. This strategy is used to prepare NiO-based NSs with nanopores, nanobranches, or a combination of both, containing up to four transition metal elements. Record-low Tafel slope (22.3 mV·dec^-1 , ≈lowest possible by computational predictions) and week-long continuous operation durability are achieved by FeMnNi-O NSs supported on Ni foams. Taken together, properly designed hierarchical mixed oxide electrodes may provide a cost-effective route to generating high, reliable, and stable OER catalytic activities, paving the way for both new electrocatalyst design and practical water-splitting devices.

A Simple Method for Synthesizing Highly Active Amorphous Iridium Oxide for Oxygen Evolution under Acidic Conditions

Water splitting for hydrogen production has been recognized as a promising approach to store sustainable energy. The performance of this method is limited by the oxygen-evolution reaction. Herein, an approach for synthesizing a highly active oxygen-evolving catalyst by a one-step, low-cost, environmentally friendly, and easy-to-perform method is presented, which works by using iridium metal as the anode at a relatively high potential. The obtained IrO_x /Ir interface showed an overpotential of 250 mV at 10 mA cm^-2 in 0.1 m HClO₄ and remained stable under electrochemical conditions. The IrO_x that was mechanically separated from the surface of IrO_x /Ir metal after operation showed a threefold increase in activity compared to the current benchmark IrO₂ catalyst. Various characterization analyses were used to identify the structure and morphology of the catalyst, which suggested nanosized, porous, and amorphous IrO_x on the surface of metallic Ir. This synthetic approach can inspire a variety of opportunities to design and synthesize efficient metal oxide-based electrocatalysts for sustainable energy conversion and utilization.

Continuous Synthesis of Hollow High-Entropy Nanoparticles for Energy and Catalysis Applications

Mixing multimetallic elements in hollow-structured nanoparticles is a promising strategy for the synthesis of highly efficient and cost-effective catalysts. However, the synthesis of multimetallic hollow nanoparticles is limited to two or three elements due to the difficulties in morphology control under the harsh alloying conditions. Herein, the rapid and continuous synthesis of hollow high-entropy-alloy (HEA) nanoparticles using a continuous "droplet-to-particle" method is reported. The formation of these hollow HEA nanoparticles is enabled through the decomposition of a gas-blowing agent in which a large amount of gas is produced in situ to "puff" the droplet during heating, followed by decomposition of the metal salt precursors and nucleation/growth of multimetallic particles. The high active sites per mass ratio of such hollow HEA nanoparticles makes them promising candidates for energy and electrocatalysis applications. As a proof-of-concept, it is demonstrated that these materials can be applied as the cathode catalyst for Li-O₂ battery operations with a record-high current density per catalyst mass loading of 2000 mA g_cat. ^-1 , as well as good stability and durable catalytic activity. This work offers a viable strategy for the continuous manufacturing of hollow HEA nanomaterials that can find broad applications in energy and catalysis.

Hierarchical 3D Oxygenated Cobalt Molybdenum Selenide Nanosheets as Robust Trifunctional Catalyst for Water Splitting and Zinc-Air Batteries

The development of hierarchical nanostructures with highly active and durable multifunctional catalysts has a new significance in the context of new energy technologies of water splitting and metal-air batteries. Herein, a strategy is demonstrated to construct a 3D hierarchical oxygenated cobalt molybdenum selenide (O-Co_{1- x} Mo_x Se₂ ) series with attractive nanoarchitectures, which are fabricated by a simple and cost-effective hydrothermal process followed by an exclusive ion-exchange process. Owing to its highly electroactive sites with numerous nanoporous networks and plentiful oxygen vacancies, the optimal O-Co_0.5 Mo_0.5 Se₂ could catalyze the hydrogen evolution reaction and oxygen evolution reaction effectively with a low overpotential of ≈102 and 189 mV, at a current density of 10 mA cm^-2 , respectively, and exceptional durability. Most importantly, the O-Co_0.5 Mo_0.5 Se₂ ||O-Co_0.5 Mo_0.5 Se₂ water splitting device only entails a voltage of ≈1.53 V at a current density of 10 mA cm^-2 , which is much better than benchmark Pt/C||RuO₂ (≈1.56 V). Furthermore, O-Co_0.5 Mo_0.5 Se₂ air cathode-based zinc-air batteries exhibit an excellent power density of 120.28 mW cm^-2 and exceptional cycling stability for 60 h, superior to those of state-of-art Pt/C+RuO₂ pair-based zinc-air batteries. The present study provides a strategy to design hierarchical 3D oxygenated bimetallic selenide-based multifunctional catalysts for energy conversion and storage systems.