Oxygen Reduction Kinetics Research Articles

Investigating Cathode Ionomer Content and Assembly Techniques for Anion Exchange Water Electrolysers

An ionomer is often used in catalyst layers to a) adhere catalysts to the membrane or transport layer, b) facilitate ionic conductivity for OH- species through catalyst layers, and c) achieve ideal catalyst dispersions in catalyst inks used in spraying methods[1]. The choice of ionomer plays a crucial role in determining ion conductivity, water management, and mechanical stability within the membrane electrode assembly of anion exchange membrane water electrolysers (AEMWEs). Therefore, optimizing ionomer properties is crucial to achieving high-performance AEMWEs with efficient hydroxide transport.Both low and high ionomer loadings can lead to challenges in charge transport resistance. At low loadings, poor electronic conductivity and reduced contact between the catalyst layer and the membrane surface can occur due to a rough electrode interface[2]. This can result in inefficient charge transfer kinetics and hinder the overall efficiency (Figure 1c). Conversely, high ionomer loadings can lead to increased charge transport resistance, attributed to decreased gas permeability to the catalyst surface. The excessive presence of ionomer within the catalyst layer can block gas-liquid transport pores inside catalyst aggregates, limiting mass transport of reactants to the active sites[2]. This phenomenon can impede gas diffusion and oxygen reduction kinetics, leading to decreased performance and lower efficiency in AEMWEs (Figure 1d). Achieving an optimal ionomer loading balance is therefore essential for promoting efficient charge transfer and gas transport processes (Figure 1e), ultimately enhancing the overall performance and durability of AEMWE devices. The use of a proton conducting ionomer such as Nafion seems counterintuitive for use within AEMWEs, its hydroxide conductivity is comparatively lower, potentially limiting electrolyser performance. However, compared to Nafion, OH- conductive binders (Sustainion and PiperION) typically yield poorer electrodes, with larger catalyst aggregates[3].Two anion exchange ionomers and one ion exchange ionomer are evaluated across a range of loadings, and two different cathode assembly techniques, Catalyst coated substrates (CCS) and catalyst coated membranes (CCM) (Figure 1a). Since different catalyst surfaces are observed to influence ionomer degradation differently[4], commercially available Pt/C was utilised as a catalyst to focus on ionomer content and fabrication techniques. To investigate the effect of using hydrophobic or hydrophilic ionomers on catalyst layer homogeneity, the content of Nafion, Sustainion, and PiperION ionomers in the catalyst layers was varied (Figure 1b). Electrochemical impedance spectroscopy was used to determine the trade-off between high and low ionomer contents, polarisation curves found the in-situ electrocatalytic performance and highlighted the best performing ionomer and fabrication technique. Scanning electron microscopy was used to highlight the differences in catalyst layer structures and homogeneity.[1] Volk et al. Nov. 2023, EES. Catal. 2(1). 109-137. DOI: 10.1039/D3EY00193H.[2] Zhao et al. Feb. 2023, Int. J. Hydrogen Energy. 48(13). 5266-5275. DOI: 10.1016/j.ijhydene.2022.11.057.[3] Jervis et al. Nov. 2017, J. Electrochem. Soc. 164(14). 1551-1555. DOI: 10.1149/2.0441714jes.[4] Li et al. Mar. 2019, ACS Appl. Mater. Interfaces. 11(10). 9696–9701. DOI: 10.1021/acsami.9b00711.Figure 1: Schematic illustrations of: a) Membrane electrode assembly fabrication techniques, catalyst coated membrane and catalyst coated substrate. b) The chemical structures of PiperION A5 by Versogen, Sustainion XA-9 by Dioxide Materials, and Nafion by Liquion. c) Catalyst layer with low ionomer loading. d) Catalyst layer with moderate ionomer loading. e) Catalyst layer with high ionomer loading. Figure 1

Effect of Micro- and Mesopore Size on Electronic Structure and Oxygen Reduction Kinetics of Platinum

Introduction Oxygen reduction reaction is an indispensable electrochemical reaction to utilize hydrogen or renewable energy in the form of electrochemical devices, i.e., fuel cells or metal-air batteries. However, the reaction overpotential, or activation and concentration overpotentials, must be reduced to raise its expected role in energy conversion. While porous electrodes contribute to decreasing the activation overpotential owing to their high surface area, the effect of pore structure on the reaction kinetics has not been clarified. It is expected that the electronic structures of electrocatalysts change when the pore size is in the range of a few nanometers. In our recent report, we fabricated platinum model electrodes with a uniform pore size of 1.8 nm and controlled electrode thicknesses.1 The intrinsic ORR kinetics was obtained by using a thin model electrode, and the extract activation overpotential was smaller than that of a planar platinum electrode. Herein, to further elucidate the relationship between the pore structure and the ORR kinetics, we present the effect of pore sizes using the platinum model electrodes with controlled pore sizes of 1.3 nm, 1.8 nm, and 3.0 nm (inset in Figure). Our study indicates that the d-band center is shifted downward with decreasing pore size and that ORR kinetics follows a volcano relationship with a maximum at a pore size of 1.8 nm. Experimental Platinum model electrodes were fabricated with the electrodeposition of platinum salt dissolved in a liquid crystalline phase of surfactant.2 The pore diameter was controlled by varying the chain length of the surfactants. The electrode thickness was assumed to be short (~50 nm) enough to mitigate the ORR concentration overpotential.1 Transmission electron microscopy (TEM) was employed to observe the pore structure. The electronic structures of the electrodes were characterized with angle-resolved X-ray photoelectron spectroscopy (ARXPS). The photoelectron takeoff angle was changed to distinguish the local electronic structure of the pore wall from that of the outer surface. Electrochemical measurements were carried out by cyclic voltammetry in N2/O2-saturated 0.1 mol dm–3 KOH solutions with a rotating disk electrode system. Results and discussion The model electrodes exhibited characteristic redox peaks of platinum polycrystalline facets in N2-saturated solution. ORR activities were evaluated in the Tafel plot (Figure), where current densities were normalized by electrochemical surface area and corrected for mass transport. It was found that the activation overpotential was the lowest at a pore size of 1.8 nm. Furthermore, the obtained Tafel slope (ca. –60 mV dec–1) was almost the same, suggesting that the rate-determining step is the same and the concentration activation overpotential is small among the electrodes.1 In ARXPS, the binding energy of the Pt 4f7/2 spectrum exhibited a high energy shift with a decrease in the pore size. The result suggests that the d-band center of platinum is shifted downward with decreasing pore size,3 and that the enhanced ORR kinetics at a pore size of 1.8 nm is attributed to the appropriate adsorption energy of the electrode to oxygen-containing species.4 We will also report the results collected in acidic solutions.

Iron Oxide Clusters as Electron Donors Under Light Enhance Oxygen Reduction Kinetics at Atomically Dispersed Fe for Photocatalytic CH4 Partial Oxidation

AbstractPhotocatalytic CH4 oxidation to CH3OH emerges as a promising strategy to sustainably utilize natural gas and mitigate the greenhouse effect. However, there remains a significant challenge for the synthesis of methanol by using O2 at low temperature. Inspired by the catalytic structure in soluble methane monooxygenase (MMO) and the corresponding reaction mechanism, we prepared a biomimetic photocatalyst with the decoration of Fe2O3 nanoclusters and satellite Fe single atoms immobilized on carbon nitride. The catalyst demonstrates an excellent CH3OH productivity of 5.02 mmol ⋅ gcat−1 ⋅ h−1 with CH3OH selectivity of 98.5 %. Mechanism studies reveal that the synergy between Fe2O3 nanoclusters and Fe single atoms establishes a dual‐Fe site as MMO for O2 activation and subsequent CH4 partial oxidation. Moreover, the light excitation of Fe2O3 nanoclusters with a relative narrow band gap could deliver the electrons and protons to atomic Fe that facilitating the oxygen reduction kinetics for the robust of CH3OH synthesis.

Iron Oxide Clusters as Electron Donors Under Light Enhance Oxygen Reduction Kinetics at Atomically Dispersed Fe for Photocatalytic CH4 Partial Oxidation.

Photocatalytic CH4 oxidation to CH3OH emerges as a promising strategy to sustainably utilize natural gas and mitigate the greenhouse effect. However, there remains a significant challenge for the synthesis of methanol by using O2 at low temperature. Inspired by the catalytic structure in soluble methane monooxygenase (MMO) and the corresponding reaction mechanism, we prepared a biomimetic photocatalyst with the decoration of Fe2O3 nanoclusters and satellite Fe single atoms immobilized on carbon nitride. The catalyst demonstrates an excellent CH3OH productivity of 5.02 mmol ⋅ gcat -1 ⋅ h-1 with CH3OH selectivity of 98.5 %. Mechanism studies reveal that the synergy between Fe2O3 nanoclusters and Fe single atoms establishes a dual-Fe site as MMO for O2 activation and subsequent CH4 partial oxidation. Moreover, the light excitation of Fe2O3 nanoclusters with a relative narrow band gap could deliver the electrons and protons to atomic Fe that facilitating the oxygen reduction kinetics for the robust of CH3OH synthesis.

Characterization of Oxygen Diffusion and Catalytic Behavior of Composite Materials in Solid Oxide Fuel Cells Using the Non-destructive Adler-Lane-Steele Method

Solid oxide fuel cells (SOFCs) are technologically interesting because they provide high-efficiency energy conversion via an electrochemical reaction within the cells. To bring SOFC to market, researchers must develop highly electroactive, low-cost devices that are chemically stable in both high (800 – 1000 °C) and low (600 – 800 °C) temperatures, as well as optimize oxygen reduction in electrodes. The electrochemical performance of SOFC is highly dependent on the catalytic activity (or catalyst behaviours) of the electrode materials involved in the oxygen reduction reaction due to the slower oxygen reduction kinetics at lower temperatures. Understanding the fundamental properties of oxygen self-diffusion in solid-state ionic systems is critical for developing nextgeneration electrolyte and electrode material compositions and microstructures that enable SOFCs to operate at lower temperatures more effectively, durably, and affordably. To date, the most common method for evaluating electrocatalytic performance is electrochemical impedance spectroscopy (EIS), which causes sample damage due to its set-up procedure. As a result, modelling approaches have been used to better understand the reaction mechanism, oxygen diffusion coefficient, and kinetics of SOFC electrode reactions, including the mixed ionic electronic conductor (MIEC)-based electrode. Several models have recently been developed to represent the reaction mechanisms of SOFCs, with Adler-Lane-Steele (ASL) mathematical theory believed to be suitable for predicting oxygen gas diffusion through the pores of the MIEC-based materials. Hence, the aim of this paper is to validate the area-specific resistance of composite electrodes using the ASL modeling technique rather than the standard EIS analysis. The findings are significant in contributing to the development of a new non-destructive method for analyzing the catalytic behavior of SOFC components.

Cobalt Nitride-Implanted PtCo Intermetallic Nanocatalysts for Ultrahigh Fuel Cell Cathode Performance.

Stable and active oxygen reduction electrocatalysts are essential for practical fuel cells. Herein, we report a novel class of highly ordered platinum-cobalt (Pt-Co) alloys embedded with cobalt nitride. The intermetallic core-shell catalyst demonstrates an initial mass activity of 0.88 A mgPt-1 at 0.9 V with 71% retention after 30,000 potential cycles of an aggressive square-wave accelerated durability test and loses only 9% of its electrochemical surface area, far exceeding the US Department of Energy 2025 targets, with unprecedented stability and only a minimal voltage loss under practical fuel cell operating conditions. We discover that regulating the atomic ordering in the core results in an optimal lattice configuration that accelerates the oxygen reduction kinetics. The presence of cobalt nitride decorated within PtCo superlattices guarantees a larger barrier to Co dissolution, leading to the excellent endurance of the electrocatalysts. This work brings up a transformative structural engineering strategy for rationally designing high-performing Pt-based catalysts with a unique atomic configuration for broad practical uses in energy conversion technology.

Temperature-controlled in-situ construction of composition-tunable nanoparticle-decorated SOFC cathodes with enhanced oxygen reduction kinetics and CO2 tolerance

High oxygen reduction reaction (ORR) catalytic activity and CO2 resistance of the cathode are fundamental to the commercial application of solid oxide fuel cells (SOFCs). Therefore, we develop a temperature-driven reduction-reoxidation strategy to in-situ construct heterostructured perovskite cathodes decorated with different nanoparticles by controlling the reduction temperature. For (Pr0.4Sr0.6)0.95Co0.2Fe0.8-xNixO3-δ (PSCFN, x = 0.05, 0.1), reduction (@700 °C)-reoxidation results in the exsolution of a ComFenNi3-m-nO4 spinel phase on the perovskite scaffold surface, while reduction (@750 °C)-reoxidation leads to the formation of both ComFenNi3-m-nO4 spinel phase and NiO nanoparticles. The exsolution of these highly active species increases the quantity of oxygen reduction active sites and effectively suppresses Sr segregation. The simultaneous formation of ComFenNi3-m-nO4 spinel phase and NiO nanoparticles induces B-site ion vacancies in the main phase, therefore facilitates the formation of oxygen vacancies. Additionally, the presence of ComFenNi3-m-nO4/NiO/PSCFN heterointerfaces promotes oxygen adsorption and transfer. The strong interactions among ComFenNi3-m-nO4, NiO, and PSCFN significantly enhance the structural stability. At 800 °C, Reo2-PSCFN0.1 achieves an output performance of 1.12 W cm−2, representing a 36.6 % enhancement compared to PSCFN0.1. Moreover, the Rp of Reo2-PSCFN0.1 is merely 0.0186 Ω cm2, marking a 40.4 % decrease relative to PSCFN0.1. This temperature-driven reduction-reoxidation strategy shows great promise as a novel approach for creating high-performance IT-SOFC cathodes.

Highly efficient nitrogen-doped three-dimensional interconnected porous carbon supported heterogeneous Co/MnO nanoparticles oxygen electrocatalysts for rechargeable zinc air batteries

Rechargeable zinc-air batteries (RZABs) show great promising in the current energy storage system because of their high energy density, low cost and high safety. However, RZABs’ cycling performance is still unsatisfactory due to the sluggish oxygen reduction and oxygen evolution reaction kinetics of air electrode. Transition Metal Oxides (TMOs) are considered to be highly promising oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts for RZABs by high activity, robust stability and low cost. Here, the nitrogen-doped three-dimensional interconnected porous carbon supported heterogeneous Co/MnO nanoparticles composite (Co/MnO-NCs) as the air cathode in RZAB is synthesized via annealing method. The NCs not only provide efficient electron transfer channels but also significantly increases the specific surface area of the catalyst, exposing more reaction sites. MnO can effectively facilitate charge transfer and accelerate the decomposition reactions. The heterogeneous Co/MnO interface provides efficient reaction sites for the ORR/OER. Electrochemical results show that the prepared Co/MnO-NCs composite not only exhibits excellent ORR/OER performance but also demonstrates good stability. Furthermore, RZABs assembled by Co/MnO-NCs air cathode exhibit high open circuit voltage, high specific capacity and durable cyclic stability. This strategy has broad versatility and can be generalized to other TMOs electrocatalysts for RZABs.

Fluorine-Doped Carbon Support Enables Superfast Oxygen Reduction Kinetics by Breaking the Scaling Relationship.

It is well-established that Pt-based catalysts suffer from the unfavorable linear scaling relationship (LSR) between *OOH and *OH (ΔG(*OOH)=ΔG(*OH)+3.2±0.2 eV) for the oxygen reduction reaction (ORR), resulting in a great challenge to significantly reduced ORR overpotentials. Herein, we propose a universal and feasible strategy of fluorine-doped carbon supports, which optimize interfacial microenvironment of Pt-based catalysts and thus significantly enhance their reactive kinetics. The introduction of C-F bonds not only weakens the *OH binding energy, but also stabilizes the *OOH intermediate, resulting in a break of LSR. Furthermore, fluorine-doped carbon constructs a local super-hydrophobic interface that facilitates the diffusion of H2O and the mass transfer of O2. Electrochemical tests show that the F-doped carbon-supported Pt catalysts exhibit over 2-fold higher mass activities than those without F modification. More importantly, those catalysts also demonstrate excellent stability in both rotating disk electrode (RDE) and membrane electrode assembly (MEA) tests. This study not only validates the feasibility of tuning the electrocatalytic microenvironment to improve mass transport and to break the scaling relationship, but also provides a universal catalyst design paradigm for other gas-involving electrocatalytic reactions.

Fluorine‐Doped Carbon Support Enables Superfast Oxygen Reduction Kinetics by Breaking the Scaling Relationship

AbstractIt is well‐established that Pt‐based catalysts suffer from the unfavorable linear scaling relationship (LSR) between *OOH and *OH (ΔG(*OOH)=ΔG(*OH)+3.2±0.2 eV) for the oxygen reduction reaction (ORR), resulting in a great challenge to significantly reduced ORR overpotentials. Herein, we propose a universal and feasible strategy of fluorine‐doped carbon supports, which optimize interfacial microenvironment of Pt‐based catalysts and thus significantly enhance their reactive kinetics. The introduction of C−F bonds not only weakens the *OH binding energy, but also stabilizes the *OOH intermediate, resulting in a break of LSR. Furthermore, fluorine‐doped carbon constructs a local super‐hydrophobic interface that facilitates the diffusion of H2O and the mass transfer of O2. Electrochemical tests show that the F‐doped carbon‐supported Pt catalysts exhibit over 2‐fold higher mass activities than those without F modification. More importantly, those catalysts also demonstrate excellent stability in both rotating disk electrode (RDE) and membrane electrode assembly (MEA) tests. This study not only validates the feasibility of tuning the electrocatalytic microenvironment to improve mass transport and to break the scaling relationship, but also provides a universal catalyst design paradigm for other gas‐involving electrocatalytic reactions.

Co/CoTe heterostructure internal hairy fibers as high-efficiency oxygen electrocatalyst for Zn-air batteries

The design of highly efficient catalysts to enhance the kinetics of oxygen reduction (OER) and oxygen evolution (ORR) reactions is the key issue for the development of high-performance Zn-air battery. In this work, we report the design of Co-CoTe heterostructured fibers as the bifunctional oxygen catalyst for Zn-air battery. Firstly, the theoretical analysis was carried out on Co-CoTe heterostructure. The large work function difference is favorable to construct strong interfacial built-in electric field (BIEF), and the low energy barrier endows high catalytic activities. Moreover, the in-situ grown carbon shell was designed to build “core–shell” Co-CoTe/C unit to realize its high performance. They assemble the Co-CoTe@HFS fiber with good self-supporting and flexible features. Taken the advantages of the strong BIEF, the “core–shell” basic unit, and the freestanding substrate, the Co-CoTe@HFS fiber achieves the good electrocatalytic properties and high reliability. The full Zn-air battery (ZAB) with the Co/CoTe@HFS air cathode achieves the high peak power density and cycling stability over long-term cycling. Therefore, this work provides a clue to design bifunctional oxygen catalysts for high-performance ZABs.

A novel Co-free and efficient Pr0.5Ba0.5Fe0.8Cu0.2O3-δ nanofiber cathode material for intermediate temperature solid oxide fuel cells

Fe-based perovskite without Co has a low thermal expansion coefficient and can be used as a solid oxide fuel cell (SOFC) cathode to ensure the thermal compatibility with the electrolyte; however, its further advancement is impeded by sluggish oxygen reduction reaction (ORR) dynamics. In this work, the electrospinning method and Cu doping strategy are employed to improve the microstructure and oxygen reduction kinetics of Pr0.5Ba0.5FeO3-δ cathodes. The results show that Pr0.5Ba0.5Fe0.8Cu0.2O3-δ has a unique fiber structure and high specific surface area, and the introduction of Cu facilitates the release of lattice oxygen, effectively increasing the oxygen vacancy content. The relaxation time distribution analysis and oxygen reduction reaction dynamics indicate that the oxygen adsorption-dissociation process is the main rate-limiting step in the oxygen reduction reaction of Pr0.5Ba0.5Fe0.8Cu0.2O3-δ electrode materials. At 700 °C, the area-specific resistance (ASR) of the symmetric cell of Pr0.5Ba0.5Fe0.8Cu0.2O3-δ fiber material is 0.071 Ω cm2, and the power density of the single cell reaches 988 mW cm−2, which shows good electrochemical output performance and long-term operational stability. The preparation of nanofiber cathodes with high specific surface area and porosity by electrospinning provides an effective strategy to enhance the electrocatalytic activity of the IT-SOFC cathode.

Rationalizing the catalytic surface area of oxygen vacancy‐enriched layered perovskite LaSrCrO4 nanowires on oxygen electrocatalyst for enhanced performance of Li–O2 batteries

AbstractEfficient electrocatalysis at the cathode is crucial to addressing the limited stability and low rate capability of Li−O2 batteries. This study examines the kinetic behavior of Li−O2 batteries utilizing layered perovskite LaSrCrO4 nanowires (NWs) composed of lower oxidation states. Layered perovskite LaSrCrO4 NWs exhibited improved rate capability over a wide range of current densities and longer cycle life in Li−O2 batteries than V‐based layered perovskite (LaSrVO4) and simple perovskite (La0.8Sr0.2CrO3) NWs. X‐ray photoelectron spectroscopy and electrochemical surface area analyses showed that the observed performance variations primarily stemmed from active sites such as oxygen vacancies. In situ Raman analysis showed that these active sites significantly modulate the kinetics of oxygen reduction and evolution, which are related to LiO2 intermediate adsorption. Electrochemical impedance spectroscopy showed that the active sites in layered perovskite LaSrCrO4 NWs contributed to their high charge transfer capability and reduced polarization. This study presents an appealing method for the precise fabrication and analysis of Cr‐based layered perovskites, aimed at achieving highly efficient and stable bifunctional oxygen electrocatalysis.

An Active and Stable High‐Entropy Ruddlesden‐Popper Type La1.4Sr0.6Co0.2Fe0.2Ni0.2Mn0.2Cu0.2O4±δ Oxygen Electrode for Reversible Solid Oxide Cells

AbstractInsufficient catalytic activity and stability of oxygen electrodes are major challenges for the widespread use of reversible solid oxide cells (Re‐SOCs). Here, a Ruddlesden‐Popper‐structured high‐entropy La1.4Sr0.6Co0.2Fe0.2Ni0.2Mn0.2Cu0.2O4±δ (RP‐LSCFNMC) oxygen electrode with fast oxygen reduction and emission reaction kinetics, inhibited Sr segregation and favorable thermal expansion efficient is reported. A Re‐SOC with the RP‐LSCFNMC oxygen electrode achieves an encouraging peak power density of 1.74 W cm−2 in the fuel cell mode and a remarkable current density of 2.10 A cm−2 at 1.3 V in the water electrolysis mode at 800 °C. The Re‐SOC also shows excellent stability, with no Sr segregation observed after 120 h of testing in both the fuel cell and electrolysis modes at 750 °C. Furthermore, the improved activity and stability of the RP‐LSCFNMC oxygen electrode are confirmed through a combination of experiments and density functional theory‐based calculations. These findings make the high‐entropy RP‐LSCFNMC oxide a promising oxygen electrode candidate for advanced Re‐SOCs.

Fluid-Induced Piezoelectric Field Enhancing Photo-Assisted Zn-Air Batteries Based on a Fe@P(V-T) Microhelical Cathode.

Photo-assisted Zn-air batteries can accelerate the kinetics of oxygen reduction and oxygen evolution reactions (ORR/OER); however, challenges such as rapid charge carrier recombination and continuous electrolyte evaporation remain. Herein, for the first time, piezoelectric catalysis is introduced in a photo-assisted Zn-air battery to improve carrier separation capability and accelerate the ORR/OER kinetics of the photoelectric cathode. The designed microhelical catalyst exploits simple harmonic vibrations to regenerate the built-in electric field continuously. Specifically, in the presence of the low-frequency kinetic energy that occurs during water flow, the piezoelectric-photocoupling catalyst of poly(vinylidene fluoride-co-trifluoroethylene)@ferric oxide(Fe@P(V-T)) is periodically deformed, generating a constant reconfiguration of the built-in electric field that separates photogenerated electrons and holes continuously. Further, on exposure to microvibrations, the gap between the charge and discharge potentials of the Fe@P(V-T)-based photo-assisted Zn-air battery is reduced by 1.7 times compared to that without piezoelectric assistance, indicating that piezoelectric catalysis is highly effective for enhancing photocatalytic efficiency. This study provides a thorough understanding of coupling piezoelectric polarization and photo-assisted strategy in the field of energy storage and opens a fresh perspective for the investigation of multi-field coupling-assisted Zn-air batteries.

Non-metal doping enhsances oxygen reduction kinetics and CO2 tolerance of SrFeO3-δ perovskite as high-performance cathodes for solid oxide fuel cells

The sluggish of the oxygen reduction reaction (ORR) kinetics and the susceptibility to CO2 interactions are major barriers to the widespread application of solid oxide fuel cells (SOFCs). In this study, a non-metal cation doping strategy was employed to enhance the ORR catalytic activity and CO2 tolerance of SrFeO3-δ cathode. SrFe1-xPxO3-δ (SFPx, x = 0, 0.05, 0.1, 0.15, 0.2) materials were synthesized by substituting phosphorus (P) for Fe in SrFeO3-δ. The doping of P facilitated the formation of oxygen vacancies in SrFeO3-δ, enhancing oxygen adsorption, dissociation, diffusion, and charge transfer processes, leading to a notable increase in ORR catalytic activity. At 800 ℃, the polarization resistance of SFP0.15 was 0.08 Ω cm2, a reduction of 42.86 % compared to undoped SrFeO3-δ. Additionally, SFP0.15 achieved a peak power density of 749.36 mW cm2, representing a 91.99 % enhancement over SrFeO3-δ. These findings suggest that non-metal cation doping is an effective approach to boost the performance of SrFeO3-δ perovskite cathode.

Accelerated oxygen reduction kinetics in BaCo0.4Fe0.4Zr0.2O3-δ cathode via doping with a trace amount of tungsten for protonic ceramic fuel cells

Protonic ceramic fuel cells (PCFCs), which are based on proton conducting electrolytes, are a class of power generation devices that can efficiently operate at the intermediate (500–700 °C), even low (450 °C) temperatures, but their development is hindered by sluggish cathodic kinetics at reduced temperatures. At the PCFC cathode, the absorbed oxygen molecules react with oxygen vacancies, protons and electrons to generate water and release electrical energy. Thus, the PCFC cathode materials require percolation network for proton, oxide-ion, and electron carriers simultaneously. In this work, we developed a triple ionic-electronic conductor BaCo0.4Fe0.4Zr0.15W0.05O3-δ as a new cathode material for the PCFCs using BaZr0.1Ce0.7Y0.1Yb0.1O3-δ as the electrolyte. Anode supported cells were fabricated and the performances were investigated. Compared with the parent oxide, tungsten doping can significantly improve the electrochemical reduction kinetic of oxygen. The BaCo0.4Fe0.4Zr0.15W0.05O3-δ based PCFC obtained the peak power density of 688 mW ⋅ cm−2 at 600 °C, which is 1.29 time higher than that of BaCo0.4Fe0.4Zr0.2O3-δ based PCFC (the peak power density of 534 mW ⋅ cm−2 at 600 °C). Moreover, BaCo0.4Fe0.4Zr0.15W0.05O3-δ has a better thermal expansion matching with the electrolyte material BaZr0.1Ce0.7Y0.1Yb0.1O3-δ.

Boosting the performance of La0.5Sr0.5Fe0.9Mo0.1O3-δ oxygen electrode via surface-decoration with Pr6O11 nano-catalysts

Cobalt-free La0.5Sr0.5Fe0.9Mo0.1O3-δ (LSFM) oxide is particularly investigated as a durable and efficient oxygen electrode material for the reversible solid oxide cells (RSOCs) in this study. Pr6O11 nano-catalysts are introduced into LSFM oxygen electrode as synergistic catalysts to enhance and optimize the electrode reactions. Results demonstrate an effectively promoted oxygen reduction and evolution reaction kinetics of the Pr6O11 decorated LSFM electrode. Polarization resistances (Rp) of LSFM electrode are dramatically reduced with the increasing Pr6O11 nano-catalysts and the lowest Rp of 0.039 Ω cm2 is obtained at 800 °C, which is ∼97% lower than that of the LSFM electrode. The Pr6O11 decorated LSFM oxygen electrode also displays a superior durability under cyclic anodic and cathodic polarizations. Furthermore, both the maximum power density and electrolytic current density of the Pr6O11 decorated cell are more than 2 times that of the LSFM cell. Results make clear reliability of the Pr6O11 decorated LSFM to be a promising oxygen electrode for RSOCs.

Engineering rich defects in nitrogen-doped porous carbon to boost oxygen reduction reaction kinetics

Metal-free carbon-based materials offer a promising alternative to Pt-based catalysts for the oxygen reduction reaction (ORR). However, challenges persist due to its sluggish kinetics and poor acid ORR performance. Here, we introduce a novel nitrogen-doped porous carbon with rich defects sites (such as pentagons, edge and vacancy defects) (PV/HPC) via a simple etching strategy. The PV/HPC demonstrates long-term stability and exceptional catalytic activity with half-wave potential of 0.9 V and average electron transfer number of 3.98 in alkaline solution while 0.78 V and 3.78 in acidic solution, indicating its efficiency and robustness as an ORR catalyst. Additionally, it achieves a higher kinetic current density of 91.9 mA cm−2 at 0.8 V, which is 1.75 times that of Pt/C (52.5 mA cm−2). Furthemore, it enables Al-air battery to attain a maximum power density of 487 mW cm−2, compared to 477 mW cm−2 for the Pt/C catalyst. Density functional theory (DFT) calculations elucidate that the introduction of multifunctional defects in nitrogen-doped porous carbon collectively reduces the reaction energy barrier of the departure of OH* and boosts the oxygen reduction reaction kinetics. This work presents a simple method to design durable and effective carbon-based ORR catalysts.

Advanced electrocatalytic performance of the configuration entropy cobalt-free Bi0.5Sr0.5FeO3–δ cathode catalysts for solid oxide fuel cells

The medium-entropy perovskite oxide Bi0.5Sr0.5Fe0.85Nb0.05Ta0.05Sb0.05O3–δ (BSFNTS) is evaluated as a potential cathode catalyst for solid oxide fuel cells (SOFCs). The crystal structure, electrocatalytic activity, oxygen reduction kinetics, and CO2 tolerance are systematically investigated. At 700 °C, the BSFNTS cathode exhibits excellent electrochemical performance with a polarization resistance as low as 0.095 Ω cm2. The maximal power density of the fuel cell with the BSFNTS cathode is 900 mW cm−2. Furthermore, the rate control step for the oxygen reduction reaction (ORR) of the electrode is primarily identified as the adsorbed and diffusion process of the molecule oxygen. The BSFNTS electrode presents excellent CO2 tolerance and durability in a CO2-containing atmosphere, which is related to the high acidity of Bi, Nb, Ta, and Sb cations and the larger average bonding energy of BSFNTS. The preliminary results indicate that BSFNTS medium-entropy oxide is an attractive cathode electrocatalyst for SOFCs.