Air Electrode Research Articles

Multiscale nanoengineering fabrication of air electrode catalysts in rechargeable Zn-air batteries

The development of cost-effective, high-activity and stable catalysts to accelerate the sluggish kinetics of cathodic oxygen reduction/evolution reactions (ORR/OER) plays a critical part in commercialization application of rechargeable Zn-air batteries (RZABs). Herein, a multiscale nanoengineering strategy is developed to simultaneously stabilize Co-doped Fe nanoparticles originated from metal-organic framework-derived approach and atomic Fe/Co sites derived from metal nanoparticle-atomized way on N-doped hierarchically tubular porous carbon substrate. Thereinto, metal nanoparticles and single atoms are respectively used to expedite the OER and ORR. Consequently, the final material is acted as an oxygen electrode catalyst, displaying 0.684 V of OER/ORR potential gap, 260 mW cm-2 of peak power density for liquid-state RZAB, 110 mW cm-2 of peak power density for solid-state RZAB, and 1000 charge-discharge cycles without decay, which confirms great potential for energy storage and conversion applications.

An active bifunctional Pd-doped double perovskite air electrode for reversible protonic ceramic electrochemical cells

Reversible protonic ceramic electrochemical cells (R-PCECs) have a good application prospect as the energy conversion and storage devices. The electrode surface kinetics (i.e., oxygen reduction reaction (ORR) and water oxidation reaction (WOR)) are major technical obstacles to developing the R-PCECs. Here, we report a phase-pure Pd-doped double perovskite with a PrBa0.8Ca0.2Co1.95Pd0.05O5+δ (PBCCPd) formula. The PBCCPd air electrode displays a polarization resistance (Rp) of 0.056 Ω cm2 in the BZCYYb-based symmetrical cell at 700 °C, and also performs a peak power density (PPD) of 2.08 W cm−2 using the fuel cell mode, and a current density of -4.42 A cm−2 at 1.3 V using the electrolysis cell mode at 700 °C. Moreover, R-PCECs with PBCCPd air electrodes show excellent stability in fuel and electrolysis cells, and cycling modes with reasonable Faradaic efficiency. It is shown that Pd doping can effectively increase the oxygen vacancy, improve the oxygen surface exchange, bulk diffusion capabilities, and electrocatalytic activity and stability in the air electrode of R-RCECs, indicated by the X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectra (EIS), electrical conductivity relaxation (ECR), and distribution of relaxation time (DRT).

Cross-Validation of the Remarkably High Surface Oxygen Exchange Kinetics of PrBa0.5Sr0.5Co1.5Fe0.5O5+δ: A Combined Thin-Film Mass Relaxation and Bulk Electrical Conductivity Relaxation Study.

In this work, we measure the oxygen kinetic properties of double perovskite PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF), a material widely used as the air electrode in solid oxide electrochemical cells, by mass relaxation (MR) and electrical conductivity relaxation (ECR) experiments. MR studies are carried out using thin films deposited on a gallium phosphate piezocrystal microbalance, and ECR studies are performed using a bulk bar sample with 97% theoretical density. Measurements are performed at 600 °C over the temperature oxygen partial pressure range from 10-4 to 0.21 atm. Despite the differences in experimental formats and surface microstructural features, the ks values extracted from the two methods are found to be in good agreement with one another. The rate constant is found to increase with oxygen partial pressure with a power law dependence, rising from 1.0 × 10-6 cm/s at 3.2 × 10-4 atm to 1.2 × 10-4 cm/s at 0.24 atm, as averaged over the oxidation and reduction directions. The rates in the oxidation direction are observed to be slightly higher than those in the reduction direction for a given pair of pO2 values, suggesting that the final pO2 value controls the overall relaxation behavior. The power law exponent describing the dependence of ks on pO2 is found to be 0.74 ± 0.01. The ECR study of the bulk sample reveals that even with a diffusion length of 1.8 mm, the relaxation process is largely free of diffusion limitations, indicating that PBSCF has the high bulk transport properties required for a double-phase boundary oxidation/reduction pathway.

Phase engineering of a donor-doped air electrode for reversible protonic ceramic electrochemical cells

Reversible protonic ceramic electrochemical cells (R-PCECs) demonstrate great feasibility for efficient energy storage and conversion. One critical challenge for the development of R-PCECs is the design of novel air electrodes with the characteristics of high catalytic activity and acceptable durability. Here, we report a donor doping of Hf into the B-site of a cobalt-based double perovskite with a nominal formula of PrBa0.8Ca0.2Co1.9Hf0.1O5+δ (PBCCHf0.1), which is naturally reconfigured to a double perovskite PrBa0.8-xCa0.2Co1.9Hf0.1-xO5+δ (PBCCHf0.1-x) backbone and nano-sized BaHfO3 (BHO) on the surface of PBCCHf0.1−x. The air electrode demonstrates enhanced catalytic activity and durability (a stable polarization resistance of 0.269 ​Ω ​cm2 for ∼100 ​h at 600 ​°C), due likely to the fast surface exchange process and bulk diffusion process. When employed as an air electrode of R-PCECs, a cell with PBCCHf0.1 air electrode demonstrates encouraging performances in modes of the fuel cell (FC) and electrolysis (EL) at 600 ​°C: a peak power density of 0.998 ​W ​cm−2 and a current density of −1.613 ​A ​cm−2 ​at 1.3 ​V (with acceptable Faradaic efficiencies). More importantly, the single-cell with PBCCHf0.1 air electrode demonstrates good cycling stability, switching back and forth from FC mode to EL mode ±0.5 ​A ​cm−2 for 200 ​h and 50 cycles.

Carbon Fiber Film with Multi-Hollow Channels to Expedite Oxygen Electrocatalytic Reaction Kinetics for Flexible Zn-Air Battery.

The high oxygen electrocatalytic overpotential of flexible cathodes due to sluggish reaction kinetics result in low energy conversion efficiency of wearable zinc-air batteries (ZABs). Herein, lignin, as a 3D flexible carbon-rich macromolecule, is employed for partial replacement of polyacrylonitrile and constructing flexible freestanding air electrodes (FFAEs) with large amount of mesopores and multi-hollow channels via electrospinning combined with annealing strategy. The presence of lignin with disordered structure decreases the graphitization of carbon fibers, increases the structural defects, and optimizes the pore structure, facilitating the enhancement of electron-transfer kinetics. This unique structure effectively improves the accessibility of graphitic-N/pyridinic-N with oxygen reduction reaction (ORR) activity and pyridinic-N with oxygen evolution reaction (OER) activity for FFAEs, accelerating the mass transfer process of oxygen-active species. The resulting N-doped hollow carbon fiber films (NHCFs) exhibit superior bifunctional ORR/OER performance with a low potential difference of only 0.60V. The rechargeable ZABs with NHCFs as metal-free cathodes possess a long-term cycling stability. Furthermore, the NHCFs can be used as FFAEs for flexible ZABs which have a high specific capacity and good cycling stability under different bending states. This work paves the way to design and produce highly active metal-free bifunctional FFAEs for electrochemical energy devices.

Modulating Lattice Oxygen Activity of Iron-Based Triple-Conducting Nanoheterostructure Air Electrode via Sc-Substitution Strategy for Protonic Ceramic Cells.

Iron-based perovskite air electrodes for protonic ceramic cells (PCCs) offer broad application prospects owing to their reasonable thermomechanical compatibility and steam tolerance. However, their insufficient electrocatalytic activity has considerably limited further development. Herein, oxygen-vacancy-rich BaFe0.6Ce0.2Sc0.2O3-δ (BFCS) perovskite is rationally designed by a facile Sc-substitution strategy for BaFe0.6Ce0.4O3-δ (BFC) as efficient and stable air electrode for PCCs. The BFCS electrode with an optimized Fe 3d-eg orbital occupancy and more oxygen vacancies exhibits a polarization resistance of ≈ 0.175Ωcm2 at 600°C, ≈ 1/3 of the BFC electrode (≈0.64Ωcm2). Simultaneously, BFCS shows favorable proton uptake with a low proton defect formation enthalpy (- 81kJmol-1). By combining soft X-ray absorption spectroscopy and electrical conductivity relaxation studies, it is revealed that the enhancement of Fe4+-O2- interactions in BFCS promotes the activation and mobility of lattice oxygen, triggering the activity of BFCS in both oxygen reduction and evolution reactions (ORR/OER). The single cell achieves encouraging output performance in both fuel cell (1.55 Wcm-2) and electrolysis cell (-2.96Acm-2 at 1.3V) modes at 700°C. These results highlight the importance of activating lattice oxygen in air electrodes of PCCs.

Reversible characterization of power generation and steam electrolysis for protonic ceramic cells with bi-layer electrolyte of BaZr0.8Yb0.2O3-δ and BaZr0.1Ce0.7Y0.1Yb0.1O3-δ

This work presents the reversible operation between fuel cell and steam electrolysis modes in a protonic ceramic cell (PCC) with a bi-layer electrolyte structure composed of BaZr0.8Yb0.2O3-δ and BaZr0.1Ce0.7Y0.1Yb0.1O3-δ. PCC using the bi-layer electrolyte demonstrated a slightly less leakage current and an improved performance than other samples using only BaZr0.8Yb0.2O3-δ electrolyte. In the fuel cell mode, the cell exhibited the maximum power density of 0.61 W cm−2 at 600 °C. Meanwhile, in steam electrolysis mode at 600 °C, the cell showed a current density of 0.94 A cm−2 and a Faraday efficiency 46% when operating in thermoneutral voltage. However, when evaluated in steam electrolysis mode at 600 °C with a current density of 0.6 A cm−2 for 250 h, it was confirmed that the air electrode had deteriorated, causing a decrease in Faraday efficiency from 46% to 25% after 250 h of operation, indicating a degradation in the efficacy of leak current mitigation during extended operating conditions. After 250 h operating a distribution of relaxation times analysis revealed a characteristic peak at low frequencies that could be associated with variation in the ionic transference number of the electrolyte after long-term operation.

Impact of Sweep Gas on the Degradation of an La0.6Sr0.4Co0.8Fe0.8O3 Anode in a Solid Oxide Electrolysis Cell

The degradation of solid oxide electrolysis (SOE) cells with different anode sweep gases was studied in 1000 h-long measurements in order to investigate the impact of sweep gas composition on cell performance. Cathode-supported electrolysis cells with an La0.6Sr0.4Co0.2Fe0.8O3 air electrode (active area of 4 × 4 cm2) were tested under a constant current (−0.25 A/cm2) in the electrolysis mode while supplying the cathode side with 70% H2O–30% H2 mixtures at 800 °C and using oxygen, nitrogen, and steam as sweep gases. It was demonstrated that the degradation of the anode in steam conditions resulted in more than a 2-fold increase in both, polarization and ohmic resistance (from 0.20–0.25 to 0.6–0.65 Ω cm2 compared to relatively stable values of 0.15–0.2 Ω cm2 for N2), as a consequence of the phase decomposition. Strontium played an important role in steam-induced degradation, migrating from the volume of the electrode layer to the surface of the electrolyte. As a result, the Sr-enriched layer demonstrated susceptibility to Cr poisoning. The cell purged with N2 demonstrated enhanced performance, while the use of oxygen led to degradation originating from the well-described delamination process. DRT analysis demonstrated some similarity of the spectra for steam and N2, namely the presence of a slow process at τ≈0.5 s, which might be associated with hindered oxygen transport due to point defect association in the perovskite structure. The results of this study showed that Sr-containing materials likely cannot be used as an SOE anode in high humidity conditions.

Optimisation of electrochemical performance of Sr(Fe,Sb)O3-δ air electrodes for intermediate-temperature solid oxide cells through spray-pyrolysis processing

The effects of replacing iron with cobalt and employing either Pechini or spray pyrolysis synthesis routes have been studied for the perovskite system SrFe0.9-xCoxSb0.1O3-δ (x = 0; 0.45; 0.9) to optimise sustainable air electrodes for intermediate-temperature solid oxide cells. The electrochemical performance of Pechini-prepared compositions improves with increasing cobalt content, attributable to higher electronic conductivity and oxygen-vacancy concentration, and lower processing temperature from 1300 ᵒC, for SrFe0.9Sb0.1O3-δ, to 1200 and 1100 ᵒC, for SrFe0.45Co0.45Sb0.1O3−δ and SrCo0.9Sb0.1O3−δ, respectively, with SrCo0.9Sb0.1O3-δ exhibiting an area specific resistance (ASR) of 0.084 Ω cm2 at 700 °C. The electrochemical stability of Fe-containing Pechini-based compositions is impaired, most likely due to a higher processing temperature. However, the very low processing temperature of 800 °C for spray-pyrolysed electrodes produces similar electrochemical performances for Co-containing and Co-free compositions (ASR≈ 0.065 Ω cm2 at 700 °C for SrFe0.9Sb0.1O3-δ) and high electrochemical stability with practically unaltered electrode polarisation resistance on thermal cycling.

Architecting N-doped Carbon Nanotube-Rich Carbon Nanofibers with Biomimetic Vine-Leaf-Whisker Structure as Robust Bifunctional Electrocatalysts for Rechargeable Zn-Air Batteries.

Efficient and durable bifunctional catalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are urgently desirable but challenging for rechargeable Zn-air batteries (ZABs), especially flexible wearable ZABs. Inspired by the vine-leaf-whisker structure in nature, we proposed a three-dimensional (3D) hierarchical bifunctional catalyst (denoted as Co-Fe-Zn@N-CNT/CNF) consisting of N-doped carbon nanotubes embedded with abundant CoFe alloy nanoparticles, leaf-shaped N-doped carbon nanoflakes, and porous carbon fibers for rechargeable ZABs. The special biomimetic structure provides a large specific surface area, allowing for high exposure of the active site and ensuring fast mass transport/charge transfer. The close combination of CoFe bimetallic alloys and N-doped carbon nanotubes delivers high electrocatalytic activity, while the coexistence of various active sites such as metal nanoparticles (NPs), metal-Nx, doped N species, and their synergistic interactions endows the catalysts with more active sites. As such, the Co-Fe-Zn@N-CNT/CNF catalyst achieves superior bifunctional catalytic activities for the ORR (a half-wave potential of 0.84 V) and the OER (an overpotential of 326 mV at 10 mA cm-2) in alkaline media, comparable to commercial Pt/C and RuO2. Remarkably, both aqueous and solid-state ZABs assembled with Co-Fe-Zn@N-CNT/CNF catalysts as air electrodes demonstrate excellent charging/discharging performance, high peak power density, and robust long-term cycling stability. More interestingly, the flexible ZAB performs well even under bending conditions, displaying satisfactory device stability and mechanical flexibility. This study presents a new collective morphological-composition-structural engineering strategy for exploiting the efficient bifunctional oxygen electrocatalysts, which is of great significance for high-performance rechargeable ZABs and wearable energy storage devices.

MoNP-doped Defective Carbon Fibers with Bark-like Nanosurface as Effective Bifunctional Electrocatalysts for Zn-air Batteries.

The flexible air electrode with high oxygen electrocatalytic performance and outstanding stability under various deformations plays a vital role in high-performance flexible Zn-air batteries (ZABs). Herein, a self-supported Mo, N, and P co-doped carbon cloth (CC) denoted as MoNP@CC with bark-like surface structure is fabricated by a facile two-step approach via a one-pot method and pyrolysis. The surface of the electrode shows a nanoscale "rift valley" and uniformly distributed active sites. Taking advantage of the nano-surface as well as transition metal and heteroatom doping, the self-supported electrocatalysis air electrode exhibits considerable oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) performance in terms of low overpotential (388 mV at 10 mA cm-2 ) for OER and a much positive potential (0.74 V) at 1.0 mA cm-2 for ORR. Furthermore, MoNP@CC is further used for the flexible ZAB to demonstrate its practical application. The MoNP@CC-based ZAB displays a good cycling performance for 2800 min and an open-circuit voltage of 1.44 V. This work provides a new approach to the construction of a high-performance, self-supported electrocatalysis electrode used for a flexible energy storage device.

Facile Deficiency Engineering in a Cobalt-Free Perovskite Air Electrode to Achieve Enhanced Performance for Protonic Ceramic Fuel Cells.

As a crucial component responsible for the oxygen reduction reaction (ORR), cobalt-rich perovskite-type cathode materials have been extensively investigated in protonic ceramic fuel cell (PCFC). However, their widespread application at a commercial scale is considerably hindered by the high cost and inadequate stability. In response to these weaknesses, the study presents a novel cobalt-free perovskite oxide, Ba0.95La0.05(Fe0.8Zn0.2)0.95O3-δ (BLFZ0.95), with the triple-conducting (H+|O2-|e-) property as an active and robust air electrode for PCFC. The B-site deficiency state contributes significantly to the optimization of crystal and electronic structure, as well as the increase in oxygen vacancy concentration, thus in turn favoring the catalytic capacity. As a result, the as-obtained BLFZ0.95 electrode demonstrates exceptional electrochemical performance at 700 °C, representing extremely low area-specific resistance of 0.04 Ω cm2 in humid air (3 vol.% H2O), extraordinarily high peak power density of 1114mW cm-2, and improved resistance against CO2 poisoning. Furthermore, the outstanding long-term durability is achieved without visible deterioration in both symmetrical and single cell modes. This study presents a simple but crucial case for rational design of cobalt-free perovskite cathode materials with appreciable performance via B-site deficiency regulation.

Unveiling dual functional catalytic performance of Pr1.5La0.5Ni0.8Cu0.2O4+δ air electrode for solid oxide cells

As a promising energy conversion device, solid oxide cell can realize efficient conversion between chemical energy and electrical energy under solid oxide fuel cell and solid oxide electrolysis cell mode. In this work, a Ruddlesden-Popper type oxide Pr1.5La0.5Ni0.8Cu0.2O4+δ (PLNC) with good mixed ionic-electronic conductivity is investigated as a dual functional air electrode for SOC. The results confirm that the peak power densities of the LSGM electrolyte supported cell with PLNC-GDC composite air electrode can reach 1.127, 0.911, 0.675 and 0.446 W cm−2 at 800, 750, 700 and 650 °C, respectively. Meanwhile, the cell possesses excellent durability for nearly 200 h under a constant current density of 400 mA cm−2 at 700 °C. The cell also shows superior electrolysis current density of 1.54 A cm−2 under electrolysis voltage of 1.3 V at 800 °C. The study demonstrates that PLNC is a promising dual functional air electrode material with excellent oxygen reduction reaction and oxygen evolution reaction activity for oxygen electrocatalytic reaction.

Efficient and Stable Dual-Active-Site of Core-Shell NiFe-Layered Double Hydroxide Anchored on FeMnON-N-Doped Carbon Nanotubes as Bifunctional Oxygen Electrocatalysts for Zn-Air Batteries

The bifunctional air electrodes with numerous dual-active sites and low cost are desirable to modify the performance of Zn-air batteries (ZABs). Metal–oxygen-nitrogen–carbon substrate (M = Mn, Fe, Ni, etc.) and NiFe-layered double hydroxide (NiFe-LDH) nanosheets are excellent catalysts in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes, respectively. Hereby, we investigate a bifunctional electrocatalytic substrate with a 3D core–shell hierarchical architecture by anchoring high OER-active NiFe-LDH on ORR-active FeMnZIF-8@gC3N4-derived FeMnON-N doped carbon nanotubes bamboo like (NiFe-LDH@FeMnON-NC). This nanocomposite has unique features such as robust synergistic effects, high conductivity, balance, and optimization of surface chemical valences of Fe, Mn, and Ni atoms to boost the bifunctional ORR and OER properties and stability in ZABs. The NiFe-LDH@FeMnON-NC nanocomposite not only exhibited superior OER electroactivity with a low onset overpotential of 235 mV (10 mA cm−2) but also had excellent ORR activity with a current density of −5.48 mA cm−2 and onset potential of 1.04 V, which is better than or comparable to those of commercial Pt/C and RuO2. Rechargeable ZABs constructed by bifunctional NiFe-LDH@FeMnON-NC have a peak power density (235.41 mW cm−2), open-circuit potential (OCV) (1.53 V), small discharge/charge band gap of 0.74 V and excellent discharge stability.

Fe3 C/Fe Decorated N-doped Carbon Derived from Tetrabutylammonium tetrachloroferrate Complex as Bifunctional Electrocatalysts for ORR, OER and Zn-Air Batteries in Alkaline Medium.

The emergence of non-precious metal-based robust and economic bifunctional oxygen electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for the rational design of commercial rechargeable Zn-air batteries (RZAB) with safe energy conversion and storage systems. Herein, a facile strategy to fabricate a cost-efficient, bifunctional oxygen electrocatalyst Fe3 C/Fe decorated N doped carbon (FeC-700, the catalyst prepared at carbinization temperature of 700 °C) with a unique structure has been developed by carbonization of a single source precursor, tetrabutylammonium tetrachloroferrate(III) complex. The ORR and OER activity revealed excellent performance (ΔE=0.77 V) of the FeC-700 electrocatalyst, comparable to commercial Pt/C and RuO2, respectively. The designed temperature-tuneable structure provided sufficiently accessible active sites for the continuous passage of electrons by shortening the mass transfer pathway, leading to extremely durable electrocatalysts with high ECSA and amazing charge transfer performance. Remarkably, the assembled Zn-air batteries with the FeC-700 catalyst as the bifunctional air electrode delivers gratifying charging-discharging ability with an impressive power density of 134 mW cm-2 with a long lifespan, demonstrating prodigious possibilities for practical application.

Improved Catalytic Properties of Fluorine-Doped La0.6Sr0.4Co0.2Fe0.8O3-δ for Air Electrode with High-Performance Metal-Air Batteries

La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), a perovskite material, is widely recognized as an excellent catalyst for the oxygen evolution reaction (OER). An anion doping strategy was implemented to enhance the presence of highly oxidation-active O2−/O− species crucial for the electrochemical reaction, effectively replacing oxygen. The introduction of 5 mol% fluorine to LSCF resulted in improved OER performance, comparable to that of commercial noble catalysts. Furthermore, we confirmed that fluorine-doped LSCF enhanced the oxygen reduction reaction (ORR) performance, establishing its effectiveness as a bifunctional catalyst. Moreover, when utilized as an air electrode in a homemade zinc-air battery cell, the electrochemical performance of the doped LSCF remained stable after repeated charge/discharge tests. These findings underscore the potential application of anion doping in electrochemical devices.Graphical

A Superior Catalytic Air Electrode with Temperature-Induced Exsolution toward Protonic Ceramic Cells.

Protonic ceramic cells merit extensive exploration, attributed to their innate capabilities for potent and environmentally benign energy conversion. In this work, a temperature-induced exsolution methodology to synthesize SrCo0.5Nb0.5O3-δ (SCN) nanoparticles (NPs) with notably elevated activity on the surface of PrSrCo1.8Nb0.2O6-δ (PSCN) is proposed, directly addressing the extant challenge of restrained catalytic activity prevalent in air electrode materials. In situ assessments reveal that SCN NPs commence exsolution from the matrix at temperatures surpassing 900 °C during straightforward calcination processes and maintain stability throughout annealing. Notably, the resultant SCN-PSCN interface facilitates vapor adsorption and protonation processes, which are poised to enhance surface reaction kinetics pertaining to the proton-involved oxygen reduction and evolution reaction (P-ORR and P-OER). A fuel-electrode-supported protonic ceramic cell leveraging SCN-PSCN as the air electrode manifests compelling performance, attaining a peak power density of 1.30 W·cm-2 in the fuel cell modality and a current density of 1.91 A·cm-2 at 1.3 V in the electrolysis mode, recorded at 650 °C. Furthermore, density functional theory calculations validate that the introduction of SCN NPs onto the PSCN surface conspicuously accelerates electrode reaction rates correlated with P-ORR and P-OER, by significantly mitigating energy barriers associated with surface oxygen and vapor dissociation.

D-Orbital steered FeN4 moiety through N, S dual-site adjustation for zinc-air flow battery

The implementation of pristine covalent organic polymer (COP) with well-defined structure as air electrode may spark fresh vitality to rechargeable zinc-air flow batteries (ZAFBs), but it still remains challenges in synergistically regulating their electronic states and structural porosity for the great device performance. Here, we conquer these issues by exploiting N and S co-doped graphene with COP rich in metal–ligand nitrogen to synergistically construct an effective catalyst for oxygen reduction reaction (ORR). Among them, the N and S co-doped sites with high electronegativity properties alter the number of electron occupations in the d orbital of the iron centre and form electron-transfer bridges, thereby boosting the selectivity of the ORR-catalysed four-electron pathway. Meanwhile, the introduction of COP materials aids the formation of pore interstices in the graphene lamellae, which both adequately expose the active sites and facilitate the transport of reactive substances. Benefiting from the synergistic effect, as-prepared catalyst exhibits excellent half-wave potentials (E1/2 = 912 mV) and stability (merely 8.8% drop after a long-term durability test of 50000 s). Further, ZAFBs assembled with the N/SG@COP catalyst demonstrate exceptional power density (163.8 mW cm−2) and continuous charge and discharge for approximately 140 h at 10 mA cm−2, outperforming the noble-metal benchmarks.

Direct power to hydrogen to power based on solid oxide cells with a high-performance Sr-Ba free air electrode

Reversible Solid Oxide Cell (RSOC) exhibits considerable potential for direct power to hydrogen and hydrogen to power with high efficiency. The Sr-Ba free cobalt-based air electrodes with suitable physicochemical characterization and considerable electrocatalytic activity are promising candidates for RSOC. However, the research concerning this material is limited. Herein, La0.6Ca0.4Fe0.8Co0.2O3-δ (LCFC) is explored as a Sr-Ba-free perovskite air electrode for RSOC in this work, and it exhibits a suitable value of TEC (14.66 × 10−6 K−1), excellent chemical stability, and promising conductivity. When fueled in H2(3%H2O) at 800 °C for power generation, the cell demonstrates a maximum power density of 1.59 W·cm−2, and it also demonstrates a high current density for power to hydrogen (2.89 A·cm−2 @800 °C) under 1.5 V with 50%H2O-50%H2. The performance of the cell in the two modes is also comparable to or even exceeds other cells reported in the literature. Besides, during a short-term reversible operation of 140 h in both hydrogen to power (SOFC) and power to hydrogen (SOEC) modes, the cell demonstrates promising reversibility and durability during different kinds of tests. All the results confirm the LCFC is an ideal candidate air electrode for RSOC.

Advanced Architectures of Air Electrodes in Zinc-Air Batteries and Hydrogen Fuel Cells.

The air electrode is an essential component of air-demanding energy storage/conversion devices, such as zinc-air batteries (ZABs) and hydrogen fuel cells (HFCs), which determines the output power and stability of the devices. Despite atom-level modulation in catalyst design being recently achieved, the air electrodes have received much less attention, causing a stagnation in the development of air-demanding equipment. Herein, the evolution of air electrodes for ZABs and HFCs from the early stages to current requirements is reviewed. In addition, the operation mechanism and the corresponding electrocatalytic mechanisms of ZABs are summarized. In particular, by clarifying the air electrode interfaces of ZABs at different scales, several approaches to improve the air electrode in rechargeable ZABs are reviewed, including innovative electrode structures and bifunctional oxygen catalysts. Afterward, the operating mechanisms of proton-exchange-membrane fuel cells (PEMFCs) and anion-exchange-membrane fuel cells (AEMFCs) are explained. Subsequently, the strategies employed to enhance the efficiency of the membrane electrode assembly (MEA) in PEMFCs and AEMFCs, respectively, are highlighted and discussed in detail. Last, the prospects for air electrodes in ZABs and HFCs are considered by discussing the main challenges. The aim of this review is to facilitate the industrialization of ZABs and HFCs.