Multistage Electron Distribution Engineering of Iridium Oxide by Codoping W and Sn for Enhanced Acidic Water Oxidation Electrocatalysis

Microkinetic Modelling of Electrochemical Oxygen Evolution Reaction on Ir(111)@N-Graphene Surface

We have explored the thermodynamics and microkinetic aspects of oxygen evolution catalysis on low loading of Ir(111) on nitrogen-doped graphene at constant potential. The electronic modification induced by N-doping is the reason for the reduced overpotential of OER. The N-induced defect in the charge density is observed with increasing charge-depleted region around the Ir atoms. The lattice contraction shifts the d-band center away from the Fermi level, which increases the barrier for OH* and O* formation on Ir(111) supported on NGr (Ir(111)@NGr). Thus, highly endothermic O* formation reduces the OOH* formation, which is the potential determining step. For comparison, all electronic and binding energy calculations were also performed against Ir NP supported on Gr (Ir(111)@Gr). The stepwise potential-dependent activation barrier (G a ${{G}_{a}}$ ) was obtained using the charge extrapolation method. The third step remains the RDS in all ranges of water oxidation potentials. The potential dependentG a ${{G}_{a}}$ is further applied to the Eyring rate equation to obtain the current density (j O E R ${{j}_{OER}}$ ) and correlation betweenj O E R ${{j}_{OER}}$ and pH dependence, i. e., OH- concentration. The microkineticj O E R ${{j}_{OER}}$ progression leads to a Tafel slope value of 30 mV dec-1 at pH=14.0, requiringη k i n e t i c = 0 . 33 V ${{\eta }_{kinetic}=0.33\ V}$ .

Lattice-Doped Ir Cooperating with Surface-Anchored IrOx for Acidic Oxygen Evolution Reaction with Ultralow Ir Loading

Reducing iridium (Ir) loading while maintaining efficiency and stability is crucial for the acidic oxygen evolution reaction (OER). In this study, we develop a synthetic method of sequential electrochemical deposition and high-temperature thermal shock to produce an IrOx/Ir-WO3 electrocatalyst with ∼1.75 nm IrOx nanoparticles anchoring on Ir-doped WO3 nanosheets. The IrOx/Ir-WO3 electrocatalyst with a low Ir loading of 0.035 mg cm-2 demonstrates a low overpotential of 239 mV to achieve a current density of 10 mA cm-2 and a mass activity of 6.6 × 104 A gIr-1 @1.75 V vs RHE in 0.5 M H2SO4. IrOx/Ir-WO3 on carbon paper as the anode and Pt/C as the cathode work stably for 40 h at 30 mA cm-2 in a proton exchange membrane water electrolyzer. It is found that the cooperation of lattice-doped Ir and surface-anchored IrOx enhances the activity and stability of IrOx/Ir-WO3 for acidic OER. Specifically, the doped Ir reduces the electron density of the anchored IrOx, thus optimizing the adsorption energy of oxygen-containing intermediates and the kinetic barrier of H2O dissociation, leading to an enhanced activity of IrOx/Ir-WO3. Also, the Ir-WO3 support provides electrons to retard the overoxidation and dissolution of Ir atoms from the anchored IrOx during acidic OER.

Controlled Structural Activation of Iridium Single Atom Catalyst for High-Performance Proton Exchange Membrane Water Electrolysis

Iridium single atom catalysts are promising oxygen evolution reaction (OER) electrocatalysts for proton exchange membrane water electrolysis (PEMWE), as they can reduce the reliance on costly Ir in the OER catalysts. However, their practical application is hindered by their limited stability during PEMWE operation. Herein, we report on the activation of Ir-doped CoMn2O4 in acidic electrolyte that leads to enhanced activity and stability in acidic OER for long-term PEMWE operation. In-depth material characterization combined with electrochemical analysis and theoretical calculations reveal that activating Ir-doped CoMn2O4 induces controlled restructuring of Ir single atoms to IrOx nanoclusters, resulting in an optimized Ir configuration with outstanding mass activity of 3562 A gIr-1 at 1.53 V (vs RHE) and enhanced OER stability. The PEMWE using activated Ir-doped CoMn2O4 exhibited a stable operation for >1000 h at 250 mA cm-2 with a low degradation rate of 0.013 mV h-1, demonstrating its practical applicability. Furthermore, it remained stable for more than 400 h at a high current density of 1000 mA cm-2, demonstrating long-term durability under practical operation conditions.

Optimal Electrocatalyst Design Strategies for Acidic Oxygen Evolution

Hydrogen, a clean resource with high energy density, is one of the most promising alternatives to fossil. Proton exchange membrane water electrolyzers are beneficial for hydrogen production because of their high current density, facile operation, and high gas purity. However, the large-scale application of electrochemical water splitting to acidic electrolytes is severely limited by the sluggish kinetics of the anodic reaction and the inadequate development of corrosion- and highly oxidation-resistant anode catalysts. Therefore, anode catalysts with excellent performance and long-term durability must be developed for anodic oxygen evolution reactions (OER) in acidic media. This review comprehensively outlines three commonly employed strategies, namely, defect, phase, and structure engineering, to address the challenges within the acidic OER, while also identifying their existing limitations. Accordingly, the correlation between material design strategies and catalytic performance is discussed in terms of their contribution to high activity and long-term stability. In addition, various nanostructures that can effectively enhance the catalyst performance at the mesoscale are summarized from the perspective of engineering technology, thus providing suitable strategies for catalyst design that satisfy industrial requirements. Finally, the challenges and future outlook in the area of acidic OER are presented.

Recent Research on Iridium-Based Electrocatalysts for Acidic Oxygen Evolution Reaction from the Origin of Reaction Mechanism

As the anode reaction of proton exchange membrane water electrolysis (PEMWE), the acidic oxygen evolution reaction (OER) is one of the main obstacles to the practical application of PEMWE due to its sluggish four-electron transfer process. The development of high-performance acidic OER electrocatalysts has become the key to improving the reaction kinetics. To date, although various excellent acidic OER electrocatalysts have been widely researched, Ir-based nanomaterials are still state-of-the-art electrocatalysts. Hence, a comprehensive and in-depth understanding of the reaction mechanism of Ir-based electrocatalysts is crucial for the precise optimization of catalytic performance. In this review, the origin and nature of the conventional adsorbate evolution mechanism (AEM) and the derived volcanic relationship on Ir-based electrocatalysts for acidic OER processes are summarized and some optimization strategies for Ir-based electrocatalysts based on the AEM are introduced. To further investigate the development strategy of high-performance Ir-based electrocatalysts, several unconventional OER mechanisms including dual-site mechanism and lattice oxygen mediated mechanism, and their applications are introduced in detail. Thereafter, the active species on Ir-based electrocatalysts at acidic OER are summarized and classified into surface Ir species and O species. Finally, the future development direction and prospect of Ir-based electrocatalysts for acidic OER are put forward.

Regulating Strain and Electronic Structure of Indium Tin Oxide Supported IrOx Electrocatalysts for Highly Efficient Oxygen Evolution Reaction in Acid

The development of proton exchange membrane water electrolysis is a promising technology for hydrogen production, which has always been restricted by the slow kinetics of the oxygen evolution reaction (OER). Although IrOx is one of the benchmark acidic OER electrocatalysts, there are still challenges in designing highly active and stable Ir-based electrocatalysts for commercial application. Herein, a Ru-doped IrOx electrocatalyst with abundant twin boundaries (TB-Ru0.3Ir0.7Ox@ITO) is reported, employing indium tin oxide with high conductivity as the support material. Combing the TB-Ru0.3Ir0.7Ox nanoparticles with ITO support could expose more active sites and accelerate the electron transfer. The TB-Ru0.3Ir0.7Ox@ITO exhibits a low overpotential of 203 mV to achieve 10 mA cm-2 and a high mass activity of 854.45 A g-1noble metal at 1.53 V vs RHE toward acidic OER, which exceeds most reported Ir-based OER catalysts. Moreover, improved long-term stability could be obtained, maintaining the reaction for over 110 h at 10 mA cm-2 with negligible deactivation. DFT calculations further reveal the activity enhancement mechanism, demonstrating the synergistic effects of Ru doping and strains on the optimization of the d-band center (εd) position and the adsorption free energy of oxygen intermediates. This work provides ideas to realize the trade-off between high catalytic activity and good stability for acidic OER electrocatalysts.

Atomically Dispersed Iridium on Polyimide Support for Acidic Oxygen Evolution

Designing a high-performing iridium (Ir) single-atom catalyst is desired for acidic water electrolysis, which shows enormous potential given its high catalytic activity toward acidic oxygen evolution reaction (OER) with minimum usage of precious Ir metal. However, it still remains a substantial challenge to stabilize the Ir single atoms during the OER operation without sacrificing the activity. Here, we report a high-performing OER catalyst by immobilizing Ir single atoms on a polyimide support, which exhibits a high mass activity on a carbon paper electrode while simultaneously achieving outstanding stability with negligible decay for 360 h. The resulting electrode (denoted as Ir1-PI@CP) reaches a 49.7-fold improvement in mass activity compared to the counterpart electrode prepared without polyimide support. Both our experimental and theoretical results suggest that, owing to the strong metal-support interactions, the polyimide support can enhance the Ir 5d states of Ir single atoms in Ir1-PI@CP, which can tailor the adsorption energies of intermediates and decrease the thermodynamic barrier at the rate-determining step of the OER, but also facilitate the proton-electron-transfer process and improve the reaction kinetics. This work offers an alternative avenue for developing single-atom catalysts with superior activity and durability toward various catalytic systems and beyond.

Chromium-Induced High Covalent Co-O Bonds for Efficient Anodic Catalysts in PEM Electrolyzer

The proton exchange membrane water electrolyzer (PEMWE), crucial for green hydrogen production, is challenged by the scarcity and high cost of iridium-based materials. Cobalt oxides, as ideal electrocatalysts for oxygen evolution reaction (OER), have not been extensively applied in PEMWE, due to extremely high voltage and poor stability at large current density, caused by complicated structural variations of cobalt compounds during the OER process. Thus, the authors sought to introduce chromium into a cobalt spinel (Co3O4) catalyst to regulate the electronic structure of cobalt, exhibiting a higher oxidation state and increased Co-O covalency with a stable structure. In-depth operando characterizations and theoretical calculations revealed that the activated Co-O covalency and adaptable redox behavior are crucial for facilitating its OER activity. Both turnover frequency and mass activity of Cr-doped Co3O4 (CoCr) at 1.67 V (vs RHE) increased by over eight times than those of as-synthesized Co3O4. The obtained CoCr catalyst achieved 1500 mA cm-2 at 2.17 V and exhibited notable durability over extended operation periods - over 100 h at 500 mA cm-2 and 500 h at 100 mA cm-2, demonstrating promising application in the PEMWE industry.

Key role of subsurface doping in optimizing active sites of IrO2 for the oxygen evolution reaction

The oxygen evolution reaction (OER) over a family of metal-doped rutile IrO2 catalysts is theoretically investigated by controlling the species and position of doped elements. The subsurface substitution doping is demonstrated to efficiently regulate the eg-filling of surface iridium sites and lower the adsorption strength of oxygen intermediates, improving the catalytic activity for the OER. Finally, based on screening, subsurface Cu- and Li-doped IrO2 models stand near the top of the volcano plot and display high levels of structural stability toward acidic OER.

Co3 O4 Supported on β-Mo2 C with Different Interfaces for Electrocatalytic Oxygen Evolution Reaction

Interface engineering is an effective strategy for improving the activity of catalysts in electrocatalytic oxygen evolution reaction (OER). Herein, Co3 O4 supported on β-Mo2 C with different interfaces were investigated for electrocatalytic OER. The morphological diversity of β-Mo2 C supports allowed different Co3 O4 -Mo2 C interactions. Various techniques characterized the composition and microstructure of the interface in the composites. Due to the strong interaction between Co3 O4 nanoparticles and β-Mo2 C nanobelts with opposing surface potentials, compact interface was observed between Co3 O4 active species and β-Mo2 C nanobelt support. The compact interface enhanced the conductivity of the material and also regulated the interfacial electron redistribution of Mo and Co atoms, promoting the charge transfer process during OER. In addition, the surface loading of Co3 O4 can effectively improve the hydrophilicity of the surface. β-Mo2 C has the capability in dissociating H2 O molecules. Thus, an example has been carefully demonstrated for interface engineering in electrocatalytic OER.

IrPd Nanoalloy-Structured Bifunctional Electrocatalyst for Efficient and pH-Universal Water Splitting

The lack of high efficiency and pH-universal bifunctional electrocatalysts for water splitting to hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) hinders the large-scale production of green hydrogen. Here, an IrPd electrocatalyst supported on ketjenblack that exhibits outstanding bifunctional performance for both HER and OER at wide pH conditions is presented. The optimized IrPd catalyst exhibits a specific activity of 4.46 and 3.98 A mg_Ir ^-1 in the overpotential of 100 and 370 mV for HER and OER, respectively, in alkaline conditions. When applied to the anion exchange membrane electrolyzer, the Ir₄₄ Pd₅₆ /KB catalyst shows a stability of >20 h at a current of 250 mA cm^-2 for water decomposition, indicating promising prospects for practical applications. Beyond offering an advanced electrocatalyst, this work also guides the rational design of desirable bifunctional electrocatalysts for HER and OER by regulating the microenvironments and electronic structures of metal catalytic sites for diverse catalysis.