Performance Promotion of Multipurpose Catalysts Using Increased Oxygen Vacancy Amounts by Charge-Mismatched Doping

Modulating the oxygen vacancy (V0) in nanostructures has opened a new avenue for efficient catalyst design, facilitating biomass oxidation reactions and electrocatalytic properties. In this study, we have investigated the properties of NiO-based catalysts with varying degrees of V0 achieved through ion doping of the catalyst with cations of different oxidation states (TM3+) or the same valence state (TM2+) as Ni2+ in the NiO matrix. By introducing charge-mismatched dopants, we enhanced the concentration of V0 in the NiO catalyst, resulting in remarkable selectivity (∼50%) for the conversion of 2,5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA), as well as a lower overpotential in the oxygen evolution reaction (OER). We believe that charge-mismatched doping offers a novel avenue for optimizing defect engineering in oxide-based catalysts, which can enable more efficient biomass conversion and water splitting. These findings have made a significant contribution to the field of multipurpose catalysis and hold the potential to inspire new catalyst designs that would usher in a more sustainable future.

Breaking the Activity and Stability Bottlenecks of Electrocatalysts for Oxygen Evolution Reactions in Acids

Oxygen evolution reaction (OER) is a cornerstone reaction for a variety of electrochemical energy conversion and storage systems such as water splitting, CO2 /N2 reduction, reversible fuel cells, and metal-air batteries. However, OER catalysis in acids suffers from extra sluggish kinetics due to the additional step of water dissociation along with its multiple electron transfer processes. Furthermore, OER catalysts often suffer from poor stability in harsh acidic electrolytes due to the severe dissolution/corrosion processes. The development of active and stable OER catalysts in acids is highly demanded. Here,  the recent advances in OER electrocatalysis in acids are reviewed and the key strategies are summarized to overcome the bottlenecks of activity and stability for both noble-metal-based and noble metal-free catalysts, including i) morphology engineering, ii) composition engineering, and iii) defect engineering. Recent achievements in operando characterization and theoretical calculations are summarized which provide an unprecedented understanding of the OER mechanisms regarding active site identification, surface reconstruction, and degradation/dissolution pathways. Finally, views are offered on the current challenges and opportunities to break the activity-stability relationships for acidic OER in mechanism understanding, catalyst design, as well as standardized stability and activity evaluation for industrial applications such as proton exchange membrane water electrolyzers and beyond.

Boosting oxygen evolution electrocatalysis via CeO2 engineering on Fe2N nanoparticles for rechargeable Zn-air batteries

In the process of developing low-cost and high-performance bifunctional electrocatalysts, rational selection of catalytic components and tuning of their electronic structures to achieve synergistic effects is a feasible approach. In this work, CeO₂ was composited into Fe/N-doped carbon foam by a molten salt method to improve the electrocatalytic performance of the composite catalyst for the oxygen evolution reaction (OER). The results showed that the excitation of oxygen vacancies in CeO₂ accelerated the migration of oxygen species and enhanced the oxygen storage/release capacity of the as-prepared catalyst. Meanwhile, the size effect of CeO₂ particles enabled the timely discharge of gas bubbles from the reaction system and thus improved the OER kinetics. In addition, a large number of pyridine-N species were induced by CeO₂-doping and sequentially anchored in the carbon matrix. As a result, the Fe₂N active state was formed through the strengthened binding of Fe-N elements. Benefiting from the strong electronic interaction between Fe₂N and CeO₂ components, the optimal CeO₂-Fe₂N/NFC_-2 catalyst sample showed a good OER performance (E_j=10 = 266 mV) and ORR electrocatalytic activity (E_1/2 = 0.87 V). The practical feasibility tests indicated that the Zn-air battery assembled by the CeO₂-Fe₂N/NFC_-2 catalyst exhibited a large energy density and an excellent long-term cycling stability.

Ruthenium-Doping-Induced Amorphization of VS4 Nanostructures with a Rich Sulfur Vacancy for Enhanced Hydrogen Evolution Reaction in a Neutral Electrolyte Medium

The generation of pure H₂ from a neutral electrolyte solution represents a transformative route with low cost and environmentally friendly nature. However, the complex kinetics of hydrogen evolution reaction (HER) via water electrolysis make its practical application to be difficult. Herein, we have reported Ru-doping-induced formation of VS₄ nanostructures with a rich S vacancy for neutral HER in a 0.2 M phosphate buffer solution. The Ru-doped VS₄ demands an overpotential value of 160 mV at 10 mA/cm² current density with a lower catalyst loading of 0.1 mg/cm², while pristine VS₄ demands a 374 mV overpotential with the same mass loading. 60 hours of chronoamperometric study reveals the excellent stability of Ru-doped VS₄ materials, which is the highest amount of time ever reported for neutral HER. The marginal degradation of a catalyst under a long-term stability study was confirmed through inductively coupled plasma mass spectrometry (ICP-MS) analysis. The introduction of Ru to the VS₄ lattice leads to a 4.35-fold increase in the turnover-frequency values compared to those of bare VS₄ nanostructures. The higher HER activity of S-vacancy-enriched VS₄ materials is thought to originate through effective water adsorption in S vacancy and Ru³⁺ sites followed by the dissociation of a H₂O molecule, and S₂^2- efficiently converts H_ad to H₂. Also, post-HER characterization reveals that the transformation of some Ru³⁺ to Ru⁰ additionally favored the HER by providing a better H adsorption site under a static cathodic potential.

Mn-Dopant Differentiating the Ru and Ir Oxidation States in Catalytic Oxides Toward Durable Oxygen Evolution Reaction in Acidic Electrolyte

Designing an efficient and durable electrocatalyst for the sluggish oxygen evolution reaction (OER) at the anode remains the foremost challenge in developing proton exchange membrane (PEM) electrolyzers. Here, a highly active and durable cactus-like nanoparticle with an exposed heterointerface between the IrO₂ and the low oxidation state Ru by introducing a trace amount of Mn dopant is reported. The heterostructure fabrication relies on initial mixing of the Ru and Ir phases before electrochemical oxidation to produce a conjoined Ru/IrO₂ heterointerface. Benefitting from electron transfer at the heterointerface, the low oxidation state Ru species shows excellent initial activity, which is maintained even after 180 h of continuous OER test. In a half-cell test, the Mn-doped RuIr nanocactus (Mn-RuIr NCT) achieves a mass activity of 1.85 A mg_Ir+Ru ^-1 at 1.48 V_RHE , which is 139-fold higher than that of commercial IrO₂ . Moreover, the superior electrocatalytic performance of Mn-RuIr NCT in the PEM electrolysis system ensures its viability in practical uses. The results of the excellent catalytic performance for acidic OER indicate that the heterostructuring robust rutile IrO₂ and the highly active Ru species with a low oxidation state on the catalyst surface drive a synergistic effect.

Evolution of Cationic Vacancy Defects: A Motif for Surface Restructuration of OER Precatalyst

Defects have been found to enhance the electrocatalytic performance of NiFe-LDH for oxygen evolution reaction (OER). Nevertheless, their specific configuration and the role played in regulating the surface reconstruction of electrocatalysts remain ambiguous. Herein, cationic vacancy defects are generated via aprotic-solvent-solvation-induced leaking of metal cations from NiFe-LDH nanosheets. DFT calculation and in situ Raman spectroscopic observation both reveal that the as-generated cationic vacancy defects tend to exist as V_M (M=Ni/Fe); under increasing applied voltage, they tend to assume the configuration V_MOH , and eventually transform into V_MOH-H which is the most active yet most difficult to form thermodynamically. Meanwhile, with increasing voltage the surface crystalline Ni(OH)_x in the NiFe-LDH is gradually converted into disordered status; under sufficiently high voltage when oxygen bubbles start to evolve, local NiOOH species become appearing, which is the residual product from the formation of vacancy V_MOH-H . Thus, we demonstrate that the cationic defects evolve along with increasing applied voltage (V_M → V_MOH → V_MOH-H ), and reveal the essential motif for the surface restructuration process of NiFe-LDH (crystalline Ni(OH)_x → disordered Ni(OH)_x → NiOOH). Our work provides insight into defect-induced surface restructuration behaviors of NiFe-LDH as a typical precatalyst for efficient OER electrocatalysis.

Facile one-step synthesis of Ru doped NiCoP nanoparticles as highly efficient electrocatalysts for oxygen evolution reaction

Transition metal phosphides (TMPs) as ever-evolving electrocatalytic materials have attracted increasing attention in water splitting reactions owing to their cost-effective, highly active and stable catalytic properties. This work presents a facile synthetic route to NiCoP nanoparticles with Ru dopants which function as highly efficient electrocatalysts for oxygen evolution reaction (OER) in alkaline media. The Ru dopants induced a high content of Ni and Co vacancies in NiCoP nanoparticles, and the more defective Ru doped NiCoP phase than undoped NiCoP ones led to a greater number of catalytically active sites and improved electrical conductivity after undergoing electrochemical activation. The Ru doped NiCoP catalyst exhibited high OER catalytic performance in alkaline media with a low overpotential of 281 mV at 10 mA cm^-2 and a Tafel slope of 42.7 mV dec^-1 .

Enabling and Inducing Oxygen Vacancies in Cobalt Iron Layer Double Hydroxide via Selenization as Precatalysts for Electrocatalytic Hydrogen and Oxygen Evolution Reactions

Production of hydrogen by water electrolysis is an environment-friendly method and comparatively greener than other methods of hydrogen production such as stream reforming carbon, hydrolysis of metal hydride, etc. However, sluggish kinetics of the individual half-cell reactions hinders the large-scale production of hydrogen. To minimize this disadvantage, finding an appropriate, competent, and low-cost catalyst has attracted attention worldwide. Layer double hydroxide (LDH)-based materials are promising candidates for oxygen evolution reaction (OER) but not fruitful and their hydrogen evolution reaction (HER) activity is very poor, due to the lack of ionic conductivity. The inclusion of chalcogenide and generation of inherent oxygen vacancies in the lattice of LDH lead to improvement of both OER and HER activities. The presence of rich oxygen vacancies was confirmed using both the Tauc plot (1.11 eV, vacancy induction) and the photoluminescence study (peak at 426 nm, photoregeneration of oxygen). In this work, we have developed vacancy-enriched, selenized CoFe-LDH by the consequent wet-chemical and hydrothermal routes, respectively, which was used for OER and HER applications in 1 M KOH and 0.5 M H₂SO₄ electrolytes, respectively. For OER, the catalyst required only 251 mV overpotential to reach a 50 mA/cm² current density with a Tafel slope value of 47 mV/dec. For HER, the catalyst demanded only 222 mV overpotential for reaching a 50 mA/cm² current density with a Tafel slope value of 126 mV/dec. Hence, generating oxygen vacancies leads to several advantages from enhancing the exposed active sites to high probability in obtaining electrocatalytically active species and subsequent assistance in oxygen and hydrogen molecule cleavage.

Cu2O@Fe-Ni3S2 nanoflower in situ grown on copper foam at room temperature as an excellent oxygen evolution electrocatalyst

Herein, we have synthesized successfully a three-dimensional/two dimensional (3D/2D) core-shell Cu₂O@Fe-Ni₃S₂ nanoflower on copper foam at room temperature. Remarkably, by virtue of rich active sites and vacancies, large surface area, high conductivity and close contact with the electrolyte, the Cu₂O@Fe-Ni₃S₂ catalyst exhibits superior stability and oxygen evolution reaction performance.