Acidic Oxygen Evolution Reaction Research Articles

Atomically dispersed hexavalent iridium oxide from MnO2 reduction for oxygen evolution catalysis.

Hexavalent iridium (IrVI) oxide is predicted to be more active and stable than any other iridium oxide for the oxygen evolution reaction in acid; however, its experimental realization remains challenging. In this work, we report the synthesis, characterization, and application of atomically dispersed IrVI oxide (IrVI-ado) for proton exchange membrane (PEM) water electrolysis. The IrVI-ado was synthesized by oxidatively substituting the ligands of potassium hexachloroiridate(IV) (K2IrCl6) with manganese oxide (MnO2). The mass-specific activity (1.7 × 105 amperes per gram of iridium) and turnover number (1.5 × 108) exceeded those of benchmark iridium oxides, and in situ x-ray analysis during PEM operations manifested the durability of IrVI at current densities up to 2.3 amperes per square centimeter. The high activity and stability of IrVI-ado showcase its promise as an anode material for PEM electrolysis.

Magnetic Field Enhanced Cobalt Iridium Alloy Catalyst for Acidic Oxygen Evolution Reaction.

Magnetic field mediated magnetic catalysts provide a powerful pathway for accelerating their sluggish kinetics toward the oxygen evolution reaction (OER) but remain great challenges in acidic media. The key obstacle comes from the production of an ordered magnetic domain catalyst in the harsh acidic OER. In this work, we form an induced local magnetic moment in the metallic Ir catalyst via the significant 3d-5d hybridization by introducing cobalt dopants. Interestingly, CoIr nanoclusters (NCs) exhibit an excellent magnetic field enhanced acidic OER activity, with the lowest overpotential of 220 mV at 10 mA cm-2 and s long-term stability of 120 h under a constant magnetic field (vs 260 mV/20 h without a magnetic field). The turnover frequency reaches 7.4 s-1 at 1.5 V (vs RHE), which is 3.0 times higher than that without magnetization. Density functional theory results show that CoIr NCs have a pronounced spin polarization intensity, which is preferable for OER enhancement.

Boron-incorporated IrO2-Ta2O5 coating as an efficient electrocatalyst for acidic oxygen evolution reaction

The Ir-based electrocatalysts for the acidic oxygen evolution reaction (OER) have demonstrated remarkable durability. Enhancing the Ir-based electrocatalytic activity still remains crucial owing to the scarcity of iridium. Here, a high-temperature sintering technique is employed to fabricate a boron (B)-incorporated IrO2-Ta2O5 coating with an almost perfect rutile-type crystal structure on a corrosion-resistant titanium substrate, ensuring exceptional stability for the acidic OER. The B-incorporated IrO2-Ta2O5 electrode fabricated in a mixed solution of 0.6 M H3BO3, exhibits an overpotential of 210 mV at a current density of 10 mA cm−2 and a lower Tafel slope of 34.2 mV dec−1 in a 0.5 M H2SO4 solution, which is far lower than the 272 mV overpotential and the 45.3 mV dec−1 of the IrO2-Ta2O5/Ti electrode. The electrode possesses a minimal potential increase even after undergoing continuous OER for 400 h at a high current density of 100 mA cm−2 in a 0.5 M H2SO4 solution. The incorporation of B species into IrO2-Ta2O5 effectively fine-tunes the electronic structure of Ir active sites, leading to a substantial enhancement of the intrinsic electrocatalytic activity. This study provides promising prospects for reducing the energy consumption of noble IrO2-based electrocatalysts in the practical application of electrochemical industry for the acidic OER.

Oxygen Radical Coupling on Short-Range Ordered Ru Atom Arrays Enables Exceptional Activity and Stability for Acidic Water Oxidation.

The discovery of efficient and stable electrocatalysts for oxygen evolution reaction (OER) in acid is vital for the commercialization of the proton-exchange membrane water electrolyzer. In this work, we demonstrate that short-range Ru atom arrays with near-ideal Ru-Ru interatomic distances and a unique Ru-O hybridization state can trigger direct O*-O* radical coupling to form an intermediate O*-O*-Ru configuration during acidic OER without generating OOH* species. Further, the Ru atom arrays suppress the participation of lattice oxygen in the OER and the dissolution of active Ru. Benefiting from these advantages, the as-designed Ru array-Co3O4 electrocatalyst breaks the activity/stability trade-off that plagues RuO2-based electrocatalysts, delivering an excellent OER overpotential of only 160 mV at 10 mA cm-2 in 0.5 M H2SO4 and outstanding durability during 1500 h operation, representing one of the best acid-stable OER electrocatalysts reported to date. 18O-labeled operando spectroscopic measurements together with theoretical investigations revealed that the short-range Ru atom arrays switched on an oxide path mechanism (OPM) during the OER. Our work not only guides the design of improved acidic OER catalysts but also encourages the pursuit of short-range metal atom array-based electrocatalysts for other electrocatalytic reactions.

Grain‐Boundary‐Rich RuO2 Porous Nanosheet for Efficient and Stable Acidic Water Oxidation

RuO2 has been considered as the most likely acidic oxygen evolution reaction (OER) catalyst to replace IrO2, but its performance, especially long‐term stability under harsh acidic conditions, is still unacceptable. Here, we propose a grain boundary (GB) engineering strategy by fabricating the ultrathin porous RuO2 nanosheet with abundant of grain boundaries (GB‐RuO2) as an efficient acid OER catalyst. The involvement of GB induces significant tensile stress and creates an unsaturated coordination environment, effectively optimizing the adsorption of intermediates and stabilizing active site structure during OER process. Notably, the GB‐RuO2 not only exhibits a low overpotential (η10= 187 mV) with an ultra‐low Tafel slope (34.5 mV dec‐1), but also steadily operates for over 550 h in 0.1 M HClO4. Quasi in situ/operando methods confirm that the improved stability is attributed to GB preventing Ru dissolution and greatly inhibiting the lattice oxygen oxidation mechanism (LOM). A proton exchange membrane water electrolysis (PEMWE) using the GB‐RuO2 catalyst operates a low voltage of 1.669 V at 2 A cm‐2 and operates stably for 100 h at 100 mA cm‐2.

Grain-Boundary-Rich RuO2 Porous Nanosheet for Efficient and Stable Acidic Water Oxidation.

RuO2 has been considered as the most likely acidic oxygen evolution reaction (OER) catalyst to replace IrO2, but its performance, especially long-term stability under harsh acidic conditions, is still unacceptable. Here, we propose a grain boundary (GB) engineering strategy by fabricating the ultrathin porous RuO2 nanosheet with abundant of grain boundaries (GB-RuO2) as an efficient acid OER catalyst. The involvement of GB induces significant tensile stress and creates an unsaturated coordination environment, effectively optimizing the adsorption of intermediates and stabilizing active site structure during OER process. Notably, the GB-RuO2 not only exhibits a low overpotential (η10= 187 mV) with an ultra-low Tafel slope (34.5 mV dec-1), but also steadily operates for over 550 h in 0.1 M HClO4. Quasi in situ/operando methods confirm that the improved stability is attributed to GB preventing Ru dissolution and greatly inhibiting the lattice oxygen oxidation mechanism (LOM). A proton exchange membrane water electrolysis (PEMWE) using the GB-RuO2 catalyst operates a low voltage of 1.669 V at 2 A cm-2 and operates stably for 100 h at 100 mA cm-2.

Unraveling the modulation essence of p bands in Co-based oxide stability on acidic oxygen evolution reaction

Unraveling the modulation essence of p bands in Co-based oxide stability on acidic oxygen evolution reaction

Surface oxidation/spin state determines oxygen evolution reaction activity of cobalt-based catalysts in acidic environment

Co-based catalysts are promising candidates to replace Ir/Ru-based oxides for oxygen evolution reaction (OER) catalysis in an acidic environment. However, both the reaction mechanism and the active species under acidic conditions remain unclear. In this study, by combining surface-sensitive soft X-ray absorption spectroscopy characterization with electrochemical analysis, we discover that the acidic OER activity of Co-based catalysts are determined by their surface oxidation/spin state. Surfaces composed of only high-spin CoII are found to be not active due to their unfavorable water dissociation to form CoIII-OH species. By contrast, the presence of low-spin CoIII is essential, as it promotes surface reconstruction of Co oxides and, hence, OER catalysis. The correlation between OER activity and Co oxidation/spin state signifies a breakthrough in defining the structure-activity relationship of Co-based catalysts for acidic OER, though, interestingly, such a relationship does not hold in alkaline and neutral environments. These findings not only help to design efficient acidic OER catalysts, but also deepen the understanding of the reaction mechanism.

Stabilizing Lattice Oxygen through Mn Doping in NiCo2O4-δ Spinel Electrocatalysts for Efficient and Durable Acid Oxygen Evolution.

Design the electrocatalysts without noble metal is still a challenge for oxygen evolution reaction (OER) in acid media. Herein, we reported the manganese (Mn) doping method to decrease the concentration of oxygen vacancy (VO) and form the Mn-O structure adjacent octahedral sites in spinel NiCo2O4-δ (NiMn1.5Co3O4-δ), which highly enhanced the activity and stability of spinel NiCo2O4-δ with a low overpotential (η) of 280 mV at j=10 mA cm-2 and long-term stability of 80 h in acid media. The isotopic labelling experiment based on differential electrochemical mass spectrometry (DEMS) clearly demonstrated the lattice oxygen in NiMn1.5Co3O4-δ is more stable due to strong Mn-O bond and shows synergetic adsorbate evolution mechanism (SAEM) for acid OER. Density functional theory (DFT) calculations reveal highly increased oxygen vacancy formation energy (EVO) of NiCo2O4-δ after Mn doping. More importantly, the highly hydrogen bonding between Mn-O and *OOH adsorbed on adjacent Co octahedral sites promote the formation of *OO from *OOH due to the greatly enhanced charge density of O in Mn substituted sites.

Ruthenium-Based Binary Alloy with Oxide Nanosheath for Highly Efficient and Stable Oxygen Evolution Reaction in Acidic Media.

Traditional noble metal oxide, such as RuO2, is considered a benchmark catalyst for acidic oxygen evolution reaction (OER). However, its practical application is limited due to sluggish activity and severe electrochemical corrosion. In this study, Ru-Fe nanoparticles loading on carbon felt (RuFe@CF) is synthesized via an ultrafast Joule heating method as an active and durable OER catalyst in acidic conditions. Remarkably low overpotentials of 188 and 269mV are achieved at 10 and 100mA cm-2, respectively, with a robust stability up to 620 h at 10mA cm-2. When used as an anode in a proton exchange membrane water electrolyzer, the catalyst shows more than 250 h of stability at a water-splitting current of 200mA cm-2. Experimental characterizations reveal the presence of a Ru-based oxide nanosheath on the surface of the catalyst during OER tests, suggesting a surface reconstruction process that enhances the intrinsic activity and inhibits continuous metal dissolution. Moreover, density functional theory calculations demonstrate that the introduction of Fe into the RuFe@CF catalyst reduces the energy barrier and boosts its activities. This work offers an effective and universal strategy for the development of highly efficient and stable catalysts for acidic water splitting.

NdMn1.5Ru0.5O5, a high-performance electrocatalyst with low Ru content for acidic oxygen evolution reaction

A mixed oxide with the crystal structure of the DyMn2O5 family, namely NdMn1.5Ru0.5O5, is reported active for the oxygen evolution reaction (OER) in acidic media. NdMn1.5Ru0.5O5 displays high OER activity of 500 A gRu−1 at 1.5 V. Moreover, is more stable than most Ru oxides reported to date, remaining active for more than 500 cycles between 1.1. and 1.7 V at low scan rate of 10 mV s−1. The high activity and stability are attributed to the Ru cations, as NdMn2O5 exhibits very low OER activity. NdMn1.5Ru0.5O5 has particularly short Ru–Ru distances of 2.60 Å, a value close to the Ru–Ru metallic distances around 2.642 Å. The high activity and durability of NdMn1.5Ru0.5O5 for the OER are also demonstrated in a proton exchange membrane water electrolysis cell by producing a low-loaded anode electrode with 0.5 mgRucm−2. The cell achieves 1.97 V at 0.5 A cm−2, consistent with the performances reported for Ru-based catalysts but with lower Ru loading. This performance is maintained during 100 h of operation. Additionally, NdMn1.5Ru0.5O5 displays visible ORR activity in acidic media, recording an onset potential of 0.85 V at 0.1 mA cm−2. It is noteworthy to highlight the extreme rarity of bifunctional ORR/OER catalysts acidic media.

Acidic Oxygen Evolution Reaction: Fundamental Understanding and Electrocatalysts Design.

Water electrolysis driven by "green electricity" is an ideal technology to realize energy conversion and store renewable energy into hydrogen. With the development of proton exchange membrane (PEM), water electrolysis in acidic media suitable for many situations with an outstanding advantage of high gas purity has attracted significant attention. Compared with hydrogen evolution reaction (HER) in water electrolysis, oxygen evolution reaction (OER) is a kinetic sluggish process that needs a higher overpotential. Especially in acidic media, OER process poses higher requirements for the electrocatalysts, such as high efficiency, high stability and low costs. This review focuses on the acidic OER electrocatalysis, reaction mechanisms, and critical parameters used to evaluate performance. Especially the modification strategies applied in the design and construction of new-type electrocatalysts are also summarized. The characteristics of traditional noble metal-based electrocatalysts and the noble metal-free electrocatalysts developed in recent decades are compared and discussed. Finally, the current challenges for the most promising acidic OER electrocatalysts are presented, together with a perspective for future water electrolysis.

The formation of unsaturated IrOx in SrIrO3 by cobalt-doping for acidic oxygen evolution reaction

Electrocatalytic water splitting is a promising route for sustainable hydrogen production. However, the high overpotential of the anodic oxygen evolution reaction poses significant challenge. SrIrO3-based perovskite-type catalysts have shown great potential for acidic oxygen evolution reaction, but the origins of their high activity are still unclear. Herein, we develop a Co-doped SrIrO3 system to enhance oxygen evolution reaction activity and elucidate the origin of catalytic activity. In situ experiments reveal Co activates surface lattice oxygen, rapidly exposing IrOx active sites, while bulk Co doping optimizes the adsorbate binding energy of IrOx. The Co-doped SrIrO3 demonstrates high oxygen evolution reaction electrocatalytic activity, markedly surpassing the commercial IrO2 catalysts in both conventional electrolyzer and proton exchange membrane water electrolyzer.

Research Advances of Non-Noble Metal Catalysts for Oxygen Evolution Reaction in Acid.

Water splitting is an important way to obtain hydrogen applied in clean energy, which mainly consists of two half-reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, the kinetics of the OER of water splitting, which occurs at the anode, is slow and inefficient, especially in acid. Currently, the main OER catalysts are still based on noble metals, such as Ir and Ru, which are the main active components. Hence, the exploration of new OER catalysts with low cost, high activity, and stability has become a key issue in the research of electrolytic water hydrogen production technology. In this paper, the reaction mechanism of OER in acid was discussed and summarized, and the main methods to improve the activity and stability of non-noble metal OER catalysts were summarized and categorized. Finally, the future prospects of OER catalysts in acid were made to provide a little reference idea for the development of advanced OER catalysts in acid in the future.

Engineering the orbital splitting and electronic reoccupation at alkali-metal-doped perovskites with face-shared IrO6 octahedral dimers

The intriguing catalysts with excellent catalytic activity and durable stability in acidic oxygen evolution reaction (OER) are extremely desirable for efficient energy conversion. Herein, engineering the orbital splitting and electronic reoccupation at metal sites of alkali-metal-doped perovskites with face-shared IrO6 octahedral dimers is realized through structural distortion and symmetry breaking in an effort to boost the sluggish reaction kinetics and mitigate the degradation of the Ir-based catalysts in acidic electrolyte as well. Doping Na or K element can increase the oxidation state of Ir at IrO6 octahedral dimers, which not only displaces the Ir d-band centers for higher catalytic activity but also leads to a good stability of Ir centers with higher dissolution potentials. This work provides a new insight into enhancing the reactive activity by doping alkali metals.

Layered PbO2 electrode decorated with Ni-doped Mn5O8 peach-like microspheres for reinforced acidic oxygen evolution reaction in zinc electrowinning

Developing anodic electrocatalysts with superior activity and durability suitable for acidic oxygen evolution reaction (OER) remains a challenge to overcome. Herein, a layered Pb-(0.76 wt%)Ag/α-PbO2/β-PbO2 electrode modified with Ni-doped Mn5O8 particles (PA/PbO2-NMO) is successfully synthesized by a coprecipitation coupling with anodic codeposition process. The physical characterization demonstrated that the surface of electrode is composed by β-PbO2 and portion NMO that embedded in the deposit. In addition, it presents a layer structure, in which PA serving as support, α-PbO2 as the mid layer enforcing the adherence, and β-PbO2-NMO function as active layer. Benefiting from the above unique layered structure, the as-fabricated PA/PbO2-NMO electrode with a layered structure exhibits the outstanding acidic OER performance (583 mV, 139.04 mV dec–1) at 50 mA cm−2 in 1.53 M H2SO4. Meanwhile, the responding accelerated corrosion life at 1.5 A cm−2 is up to 64 h, indicating its robust anticorrosion ability. As for the applied performance in zinc electrowinning simulation electrolyte (1.53 M H2SO4 + 0.31 M ZnSO4), the current efficiency of the PA/PbO2 electrode is promoted to 93.8 % after the modification with NMO. Moreover, the cell voltage and one-ton zinc energy consumption of the modified PA/PbO2 electrode are diminished by 14.32 %, 17.75 % compared with the industrial Pb-(0.76 wt%) Ag (PA) electrode. Therefore, this study offers a new horizon on the development of the anodic electrocatalyst for both strongly acidic watersplitting and non-ferrous metals electrowinning.

Polarized Ultrathin BN Induced Dynamic Electron Interactions for Enhancing Acidic Oxygen Evolution

AbstractDeveloping ruthenium‐based heterogeneous catalysts with an efficient and stable interface is essential for enhanced acidic oxygen evolution reaction (OER). Herein, we report a defect‐rich ultrathin boron nitride nanosheet support with relatively independent electron donor and acceptor sites, which serves as an electron reservoir and receiving station for RuO2, realizing the rapid supply and reception of electrons. Through precisely controlling the reaction interface, a low OER overpotential of only 180 mV (at 10 mA cm−2) and long‐term operational stability (350 h) are achieved, suggesting potential practical applications. In situ characterization and theoretical calculations have validated the existence of a localized electronic recycling between RuO2 and ultrathin BN nanosheets (BNNS). The electron‐rich Ru sites accelerate the adsorption of water molecules and the dissociation of intermediates, while the interconnection between the O‐terminal and B‐terminal edge establishes electronic back‐donation, effectively suppressing the over‐oxidation of lattice oxygen. This study provides a new perspective for constructing a stable and highly active catalytic interface.

Size optimization of IrOx nanoparticles synthesized by Br mediation for enhanced PEM water electrolysis

We have creatively developed a hydrothermal approach to synthesize ultra-small IrOx nanoparticles with precise size control at 1.6 nm by carefully modulating KBr concentrations. Br ions are adsorbed onto the surface of IrOx and modulate the free energy through adsorption, thereby controlling the nanoparticle size to achieve ultra-small IrOx. The tailored IrOx nanoparticles are aimed to be high-performance electrocatalysts for efficient oxygen evolution reaction. The optimized nanoparticles exhibited an ultra-low overpotential of 230 mV to reach 10 mA/cm2 for oxygen evolution reaction in acid and showed negligible degradation after 50 h operation at 10 mA/cm2. When assembled in a proton exchange membrane electrolyzer, the catalyst enabled the device to reach 1 A/cm2 at only 1.69 V with excellent durability over 100 h at 1 A/cm2. Through detailed cost accounting, it was determined that 1 kg of hydrogen could be produced for a remarkably low, significantly below prevailing cost targets. This work not only develops an effective strategy to enhance oxygen evolution reaction efficiency by tailoring IrO2 particle size, but also shows the great potential for scalable application in industrial water electrolysis.

Reconstructing Hydrogen-Bond Network for Efficient Acidic Oxygen Evolution.

Developing highly active oxygen evolution reaction (OER) catalysts in acidic conditions is a pressing demand for proton-exchange membrane water electrolysis. Manipulating proton character at the electrified interface, as the crux of all proton-coupled electrochemical reactions, is highly desirable but elusive. Herein we present a promising protocol, which reconstructs a connected hydrogen-bond network between the catalyst-electrolyte interface by coupling hydrophilic units to boost acidic OER activity. Modelling on N-doped-carbon-layer clothed Mn-doped-Co3O4 (Mn-Co3O4@CN), we unravel that the hydrogen-bond interaction between CN units and H2O molecule not only drags the free water to enrich the surface of Mn-Co3O4 but also serves as a channel to promote the dehydrogenation process. Meanwhile, the modulated local charge of the Co sites from CN units/Mn dopant lowers the OER barrier. Therefore, Mn-Co3O4@CN surpasses RuO2 at high current density (100 mA cm-2 @ ~538 mV).

Reconstructing Hydrogen‐Bond Network for Efficient Acidic Oxygen Evolution

AbstractDeveloping highly active oxygen evolution reaction (OER) catalysts in acidic conditions is a pressing demand for proton‐exchange membrane water electrolysis. Manipulating proton character at the electrified interface, as the crux of all proton‐coupled electrochemical reactions, is highly desirable but elusive. Herein we present a promising protocol, which reconstructs a connected hydrogen‐bond network between the catalyst‐electrolyte interface by coupling hydrophilic units to boost acidic OER activity. Modelling on N‐doped‐carbon‐layer clothed Mn‐doped‐Co3O4 (Mn−Co3O4@CN), we unravel that the hydrogen‐bond interaction between CN units and H2O molecule not only drags the free water to enrich the surface of Mn−Co3O4 but also serves as a channel to promote the dehydrogenation process. Meanwhile, the modulated local charge of the Co sites from CN units/Mn dopant lowers the OER barrier. Therefore, Mn−Co3O4@CN surpasses RuO2 at high current density (100 mA cm−2 @ ~538 mV).