Metal-air Batteries Research Articles

Atomically Dispersed Metal Catalysts for Oxygen Reduction Reaction: Two-Electron vs. Four-Electron Pathways.

Developing eco-friendly electrochemical devices for electrosynthesis, fuel cells (FCs), and metal-air batteries (MABs) requires precisely designing the electronic pathway in the oxygen reduction reaction (ORR) process. Understanding the principle of designing low-cost, highly active, and stable catalysts helps to reduce the usage of noble metals in ORR. Atomically dispersed metal catalysts (ADMCs) emerge as promising alternatives to replace commercial noble metals due to their high utilization of active metal atoms, high intrinsic activity, and controllable coordination environments. In this review, the research tendency and reaction mechanisms in ORR are first summarized. The basic principles concerning the geometric size of the catalysts for both two-electron ORR (2e ORR) and four-electron ORR (4e ORR) were then discussed, aiming to outline the material design differences for 2e ORR and 4e ORR. Subsequently, the recent advances concentrated on the ADMCs, including coordination numbers, light heteroatom doping, dual-metal atoms-based coordination, and interaction between nanoparticle (NPs)/nanoclusters (NCs) and ADMs are documented. Finally, the key challenges and opportunities in the future design of ADMCs for the ORR are highlighted.

Atomically Dispersed Metal Catalysts for Oxygen Reduction Reaction: Two‐Electron vs. Four‐Electron Pathways

Developing eco‐friendly electrochemical devices for electrosynthesis, fuel cells (FCs), and metal‐air batteries (MABs) requires precisely designing the electronic pathway in the oxygen reduction reaction (ORR) process. Understanding the principle of designing low‐cost, highly active, and stable catalysts helps to reduce the usage of noble metals in ORR. Atomically dispersed metal catalysts (ADMCs) emerge as promising alternatives to replace commercial noble metals due to their high utilization of active metal atoms, high intrinsic activity, and controllable coordination environments. In this review, the research tendency and reaction mechanisms in ORR are first summarized. The basic principles concerning the geometric size of the catalysts for both two‐electron ORR (2e ORR) and four‐electron ORR (4e ORR) were then discussed, aiming to outline the material design differences for 2e ORR and 4e ORR. Subsequently, the recent advances concentrated on the ADMCs, including coordination numbers, light heteroatom doping, dual‐metal atoms‐based coordination, and interaction between nanoparticle (NPs)/nanoclusters (NCs) and ADMs are documented. Finally, the key challenges and opportunities in the future design of ADMCs for the ORR are highlighted.

Synergy of Pyridinic-N and Co Single Atom Sites for Enhanced Oxygen Redox Reactions in High-Performance Zinc-Air Batteries.

Cobalt single-atom catalysts (SACs) have the potential to act as bi-functional electrocatalysts for the oxygen-redox reactions in metal-air batteries. However, achieving both high performance and stability in these SACs has been challenging. Here, a novel and facile synthesis method is used to create cobalt-doped-nitrogen-carbon structures (Co-N-C) containing cobalt-SACs by carbonizing a modified ZIF-11. HAADF-STEM images and EXAFS spectra confirmed that the structure with the lowest cobalt concentration contains single cobalt atoms coordinated with four nitrogen atoms (Co-N₄). Electrochemical tests showed that this electrocatalyst performed exceptionally well in both oxygen reduction reaction (ORR) (E1/2≈0.859V) and oxygen evolution reaction (OER) (Ej = 10: 1.544V), with excellent stability. When used as a bi-functional electrocatalyst in the air cathode of a rechargeable zinc-air battery (ZAB), a peak power density of 178.6.1mWcm-2, a specific capacity of 799mAhgZn -1 and a cycle-life of 1580 is achieved. Density functional theory (DFT) calculations revealed that the concentration and the position of the pyridinic nitrogen with Co play a critical role in determining the overpotential of this electrocatalyst for oxygen-redox reactions. The unprecedented performance of this electrocatalyst can bring paradigm changes in the practical realization and application of metal-air batteries.

Modulating the Oxygen Evolution Reaction of Single-Crystal Cobalt Carbonate Hydroxide via Surface Fe Doping and Facet Dependence.

The oxygen evolution reaction (OER) is a critical half-reaction in water splitting and metal-air cells. The sensitivity of the OER to the composition and structure of the electrocatalyst presents a significant challenge in elucidating the structure-property relationship. In this study, highly stable single-crystal cobalt carbonate hydroxide [Co2(OH)2CO3, CoCH] was used as a model to investigate the correlations among structure, composition, and reactivity. Single-crystal CoCH nanowires (denoted as CoCH NWs) and Fe-doped CoCH nanowires (denoted as Fe-CoCH NWs) with an exposed (210) facet and Fe-doped CoCH nanosheets (denoted as Fe-CoCH NSs) with an exposed (2-13) facet were synthesized using electrochemical and one-step hydrothermal strategies, respectively. Their OER activity decreased in the following order: Fe-CoCH NWs > Fe-CoCH NSs > CoCH NWs. Theoretical investigation suggested that the doped Fe sites serve as active sites, and the crystal-facet dependence can finely adjust the 3d configuration of Fe sites, resulting in the optimal adsorption strengths and energy barriers for potential-determining steps on the (210) facet of CoCH. This renders the as-prepared Fe-CoCH NWs as some of the most promising Co-based OER catalysts.

Different Metal–Air Batteries as Range Extenders for the Electric Vehicle Market: A Comparative Study

Different Metal–Air Batteries as Range Extenders for the Electric Vehicle Market: A Comparative Study

Cucurbit[n]uril-Derived Electrocatalysts for Oxygen Evolution, Oxygen Reduction, and Hydrogen Evolution Reactions.

Various sustainable energy conversion techniques like water electrolyzers, fuel cells, and metal-air battery devices are promising to alleviate the issues in fossil fuel consumption. However, their broad employment has been mainly inhibited by the lack of advanced electrocatalysts to accelerate the sluggish kinetics of the three involved half-reactions including oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER). Recent advances have witnessed the cucurbit[n]uril (CB[n])-directed strategy as a prominent tool to develop high performance electrocatalysts with either OER, ORR, or HER activities. In this review, the recent progress on CB[n]-derived electrocatalysts ranging from molecular complexes to heterogeneous nanostructures and single-atoms for the three half-reactions are reviewed, and future opportunities are discussed. A concise introduction to the fundamentals of CB[n]s regarding their synthesis, structure, and chemistry is given first. Subsequently, the systematic summary of CB[n]-derived electrocatalysts and their performance for the OER/ORR/HER are discussed in detail, with a specific emphasis on correlating their structure and activities by combining diverse physiochemical characterizations, electrochemical experiments, and theory simulations. Finally, a brief conclusion and perspective for future opportunities regarding CB[n]-derived electrocatalysts for many other electrocatalytic applications are proposed.

Conventional versus Unconventional Oxygen Reduction Reaction Intermediates on Single Atom Catalysts.

The oxygen reduction reaction (ORR) stands as a pivotal process in electrochemistry, finding applications in various energy conversion technologies such as fuel cells, metal-air batteries, and chlor-alkali electrolyzers. Hereby, a comprehensive density functional theory (DFT) investigation is presented into the proposed conventional and unconventional ORR mechanisms using single-atom catalysts (SACs) supported on nitrogen-doped graphene (NG) as model systems. Several reaction intermediates have been identified that appear to be more stable than the ones postulated in the conventional mechanism, which follows the *OOH, *O, and *OH intermediates. This finding particularly holds for adsorbed *O2, which can have different adsorption geometries, ranging from η1Ο2 or η2Ο2 superoxo complexes as well as sin and anti complexes, with the two O-related ligands binding on the same or opposite sides, respectively. In the case of M@NG (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pt), the ORR follows these unconventional *O2 intermediates, whereas for Cr@NG and Cu@NG classical and unconventional *O2 intermediates compete. We approximate the electrocatalytic activity using the concept of the thermodynamic overpotential and demonstrate that the conventional mechanism gives rise to a smaller overpotential compared to mechanisms following unconventional intermediates during the four proton-coupled electron transfer steps. Our trend study indicates that transition metals with fewer d electrons reveal smaller electrocatalytic activity due to a larger thermodynamic overpotential. Among the investigated SAC systems, Co emerges as a promising candidate, with thermodynamic overpotential and limiting potential values of 0.38 and 0.85 V vs the standard hydrogen electrode, respectively, with the conventional mechanism being favored, and with Cu appearing as the second-best candidate.

Advances in Oxygen Evolution Reaction Electrocatalysts via Direct Oxygen-Oxygen Radical Coupling Pathway.

Oxygen evolution reaction (OER) is a cornerstone of various electrochemical energy conversion and storage systems, including water splitting, CO2/N2 reduction, reversible fuel cells, and rechargeable metal-air batteries. OER typically proceeds through three primary mechanisms: adsorbate evolution mechanism (AEM), lattice oxygen oxidation mechanism (LOM), and oxide path mechanism (OPM). Unlike AEM and LOM, the OPM proceeds via direct oxygen-oxygen radical coupling that can bypass linear scaling relationships of reaction intermediates in AEM and avoid catalyst structural collapse in LOM, thereby enabling enhanced catalytic activity and stability. Despite its unique advantage, electrocatalysts that can drive OER via OPM remain nascent and are increasingly recognized as critical. This review discusses recent advances in OPM-based OER electrocatalysts. It starts by analyzing three reaction mechanisms that guide the design of electrocatalysts. Then, several types of novel materials, including atomic ensembles, metal oxides, perovskite oxides, and molecular complexes, are highlighted. Afterward, operando characterization techniques used to monitor the dynamic evolution of active sites and reaction intermediates are examined. The review concludes by discussing several research directions to advance OPM-based OER electrocatalysts toward practical applications.

Gas-Diffusion Composite Electrode Containing Zeolite and Carbon in the Gas-Diffusion Layer

Abstract A composite gas-diffusion electrode containing carbon and zeolite, for rechargeable metal-air batteries was developed. To ensure the uniform distribution of the clinoptiolite particles in the carbon matrix, as well as mechanical stability of the gas-diffusion layer high-energy milling (5000 rpm for 1 min) and subsequent pressing of the mixture (300 kg.cm-2 at 280oC) were applied. By partially replacing the teflonized carbon black in the gas-diffusion layer with clinoptiolite the necessary hydrophobicity could be attained while maintaining the high gas permeability of the zeolite material. The teflonized carbon black: zeolite mass ratio was optimized in terms of the mechanical stability of the electrode and its electrochemical performance. The catalytic layer comprised a bimetallic catalyst consisting of Ag and γ-MnO2 in a 1:1 mass ratio. The morphology of the catalytic layer, as well as the pore size in the gas-diffusion layer, were studied using scanning electron microscopy and the Brunnauer-Emmett-Teller method. The determined pore size below 10 nm implies mainly the Knudsen diffusion through the gas-diffusion layer. In the preliminary durability tests high mechanical, chemical, and electrochemical stability (more than 750 cycles) was obtained for the newly developed composite gas-diffusion electrode.

Covalent Organic Frameworks with Carbon-Centered Radical Sites for Promoting the 4e- Oxygen Reduction Reaction.

Radical covalent organic frameworks (RCOFs) have demonstrated significant potential in redox catalysis and energy conversion applications. However, the synthesis of stable RCOFs with well-defined neutral carbon radical centers is challenging due to the inherent radical instability, limited synthetic methods and characterization difficulties. Building upon the understanding of stable carbon radicals and structural modulations for preparing crystalline COFs, herein we report the synthesis of a crystalline carbon-centered RCOF through a facile post-oxidation process. Moreover, the RCOF demonstrated outstanding catalytic activity in the 4e- oxygen reduction reaction (ORR) with a half-wave potential of 0.82 V (vs. RHE) and electron transfer number of 3.98, among the highest in reported COF-based electrocatalysts. The promoted 4e- and suppressed 2e- ORR pathway (99.5 % vs. 0.5 %) can be attributed to facilitated reaction initiation and smoother transition steps at the carbon radical sites, which is practically beneficial for minimizing peroxide formation, thus contributing to safer and more sustainable fuel cell and metal-air battery applications. Overall, our study not only provided a facile strategy for preparing stable RCOFs with well-defined neutral carbon radical centers but also demonstrated their capability to fine-tune the redox catalytic activity of COF materials, which could be potentially useful in electrocatalysis and energy conversion applications.

Synthesis of high-performance multifunctional electrode material using sweetwood lignin as a precursor

The N-doped lignin-based carbon material has a promising activity and stability for its application as supercapacitor as well as non-noble metal-based catalysts for fuel cells, metal–air batteries.

In situ characterization techniques: main tools for revealing OER/ORR catalytic mechanism and reaction dynamics

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are some of the most important reactions in electrochemical energy technologies such as fuel cells and metal–air cells.

Facile Encapsulation Strategy for Uniformly-Dispersed Catalytic Nanoparticle/Carbon Nanofiber Toward Advanced Zn-Air Battery

The catalyst-loaded carbon nanofibers (CNFs) have been a promising platform to enhance the efficiency of key electrochemical reactions, crucial for the operation of fuel cells, metal-air batteries, and the reduction...

Modulating single-atom M-N-C electrocatalysts for the oxygen reduction: the insights beyond the first coordination shell

The oxygen reduction reaction (ORR) is a pivotal process in electrochemical energy systems such as fuel cells and metal-air batteries. Recent advancements have highlighted the single-atom metal-nitrogen-carbon (M-N-C) catalysts for their exceptional ORR electrocatalytic performance. However, further exploration is needed for the optimization methods of single atomic active sites. Significantly, the modulation of coordination environment emerges as a pivotal technique for the enhancement of M-N-C catalysts, while extending this modulation beyond the first coordination shell has ignited substantial investigation. This review delves into the frontier of M-N-C optimization by transcending the first coordination shell, presenting a comprehensive analysis of innovative strategies that modulate the electronic structure and reactivity of MN4 sites. The primary focus lies in three regulation approaches: regulating atomic entities, introducing metallic species and tailoring non-metallic modulators. These strategies are scrutinized for their ability to fine-tune the ORR activity and stability at the atomic level. By providing a clearer trajectory for future research, this review should be able to inspire novel designs of high-performance M-N-C ORR catalysts.

Beyond conventional structures: emerging complex metal oxides for efficient oxygen and hydrogen electrocatalysis.

The core of clean energy technologies such as fuel cells, water electrolyzers, and metal-air batteries depends on a series of oxygen and hydrogen-based electrocatalysis reactions, including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), which necessitate cost-effective electrocatalysts to improve their energy efficiency. In the recent decade, complex metal oxides (beyond simple transition metal oxides, spinel oxides and ABO3 perovskite oxides) have emerged as promising candidate materials with unexpected electrocatalytic activities for oxygen and hydrogen electrocatalysis owing to their special crystal structures and unique physicochemical properties. In this review, the current progress in complex metal oxides for ORR, OER, and HER electrocatalysis is comprehensively presented. Initially, we present a brief description of some fundamental concepts of the ORR, OER, and HER and a detailed description of complex metal oxides, including their physicochemical characteristics, synthesis methods, and structural characterization. Subsequently, we present a thorough overview of various complex metal oxides reported for ORR, OER, and HER electrocatalysis thus far, such as double/triple/quadruple perovskites, perovskite hydroxides, brownmillerites, Ruddlesden-Popper oxides, Aurivillius oxides, lithium/sodium transition metal oxides, pyrochlores, metal phosphates, polyoxometalates and other specially structured oxides, with emphasis on the designed strategies for promoting their performance and structure-property-performance relationships. Moreover, the practical device applications of complex metal oxides in fuel cells, water electrolyzers, and metal-air batteries are discussed. Finally, some concluding remarks summarizing the challenges, perspectives, and research trends of this topic are presented. We hope that this review provides a clear overview of the current status of this emerging field and stimulate future efforts to design more advanced electrocatalysts.

Exploration of potential-limited protocols to prevent inefficiencies in Li-O2 batteries during charge.

Metal-air batteries are promising energy storage systems with high specific energy density and low dependence on critical materials. However, their development is hindered by slow kinetics, low roundtrip efficiency, deficient capacity recovery, and limited lifetime. This work explores the effect of cycling protocols on the lifetime of Li-O2 cells, and the interplay between electrolyte composition and the upper cut-off voltage during charge. Our results suggest that constant-current-constant-voltage (CCCV) protocols accommodate the slower kinetics at the end of charge in Li-O2 cells better than the constant-current (CC) protocols majorly used in the field. These results suggest that CCCV protocols should be standardised to assess performance improvements in Li-O2 cells.

Theoretical screening of highly efficient multifunctional single-atom catalysts supported by pc-C3N2 monolayers for the electrocatalytic HER, OER and ORR

Two-dimensional pc-C3N2 monolayer anchored Rh atoms can serve as a superior HER/OER/ORR trifunctional electrocatalyst for overall water splitting and rechargeable metal–air batteries with ultralow overpotentials.

Impact of diffusion layer structure of air electrodes on their performance in neutral zinc/iron - air batteries

Neutral metal-air batteries are of promising practical significance due to their excellent safety, environmental friendliness and strong resistance to carbon dioxide. Among them, the electrochemical performance and stability of air cathode will become a crucial factor restricting the development of neutral metal-air batteries. Herein, gas diffusion layer of an air electrode was constructed by applying different carbon materials including acetylene black (AB), carbon nanotubes (CNT), graphite (GP), ball milled graphite (m-GP) and their mixture with different mass ratios. Corresponding air electrodes were fabricated based on the MnO2/C as the catalyst. For the air electrode AB2@CNT8, its gas diffusion layer was obtained by using the mixture of AB and CNT with a mass ratio of 2:8, and it reveals good oxygen reduction reaction (ORR) performance and high discharge stability when applied to neutral zinc/iron - air batteries. In the neutral chloride electrolyte system, the discharge life of the AB2@CNTCNT8 zinc-air battery reaches 381.39h and its specific discharge capacity is 765 mAh·gZn-1. For the neutral AB2@CNTCNT8 iron-air battery, its specific discharge capacity reaches 863 mAh·gFe-1, and it can last a long working time of over 1384h. In addition, the performance of the air electrode AB2@CNTCNT8 that undergone a long-term operation can be quickly restored by simple hot water treatment. Results indicate that the air electrode AB2@CNTCNT8 has good performance, high stability and durability in the neutral chloride system, and is expected to be a potential candidate for the air cathode of neutral zinc/iron - air batteries.

Efficient Bifunctional Electrocatalysts for Oxygen Evolution/Reduction Reactions in Two-Dimensional Metal-Organic Frameworks by a Constant Potential Method.

The evolution of bifunctional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts that are highly active, stable, and conductive is crucial for advancing metal-air batteries and fuel cells. We have here thoroughly explored the OER and ORR performance for a category of two-dimensional (2D) metal-organic frameworks (MOFs) called TM3(HADQ)2, and Rh3(HADQ)2 exhibits a promising bifunctional OER/ORR activity, with an overpotential of 0.31 V for both OER and ORR. The d-band center (εd) and crystal orbital Hamilton populations (COHP) are utilized to study the relationship between OER/ORR activity and the electronic structure of catalysts, and it is found that the elementary d-electron number (Ne) of the central TM for TM3(HADQ)2, as well as the electronegativity of the ligand TM-N4 and the intermediate O atom, are the main reason that affects the catalytic activity of OER/ORR. Additionally, Rh3(HADQ)2 can be proven through the constant potential method (CPM) and microkinetics method that it is an acidic OER/ORR bifunctional catalyst. Rh3(HADQ)2 has a high toxicity tolerance, making it a potential bifunctional catalyst. Our research contributes to both the rational design of SACs for various catalytic processes and the fabrication of bifunctional, cost-effective oxygen-electric catalysts.

FeCu-N6-C Diatomic Sites Catalyst for the Boosted Oxygen Reduction Reactions in Zinc-Air Batteries.

Due to the high catalytic activity and stability for oxygen reduction reaction, N-coordinated Fe-Cu dual-metal doped carbon material (FeCu-N-C) is considered to be one of the promising electrode materials for metal-air battery and fuel cells. Herein, FeCu-N-C dual-metal catalysts was synthesized by an adsorption-calcination strategy. The prepared FeCu-N-C exhibited high activity and stability both in alkaline and acidic media. In alkaline/acid medium, the half-wave potential reaches to 0.90/0.80 V, which is better than Fe-N-C catalyst. The power density for FeCu-N-C in zinc-air battery reaches to 220 mW cm-2 and shows high electrochemical stability for more than 600 hours in charge/discharge cycles, much higher than 130 hours for Pt/C (40 %) and 100 hours for Fe-N-C. Density-functional theory calculations showed that the FeCu-N-C dual-metal catalysts got lower overpotential of 0.50 V than Fe-N-C (0.53 V), which improved the ORR activity. The results are helpful for the deep understanding of high-performance diatomic catalysts.