Development of Cobalt Hydroxide as a Bifunctional Catalyst for Oxygen Electrocatalysis in Alkaline Solution

First-Row Transition Metals for Catalyzing Oxygen Redox

Rechargeable zinc-air batteries are widely recognized as a highly promising technology for energy conversion and storage, offering a cost-effective and viable alternative to commercial lithium-ion batteries due to their unique advantages. However, the practical application and commercialization of zinc-air batteries are hindered by the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Recently, extensive research has focused on the potential of first-row transition metals (Mn, Fe, Co, Ni, and Cu) as promising alternatives to noble metals in bifunctional ORR/OER electrocatalysts, leveraging their high-efficiency electrocatalytic activity and excellent durability. This review provides a comprehensive summary of the recent advancements in the mechanisms of ORR/OER, the performance of bifunctional electrocatalysts, and the preparation strategies employed for electrocatalysts based on first-row transition metals in alkaline media for zinc-air batteries. The paper concludes by proposing several challenges and highlighting emerging research trends for the future development of bifunctional electrocatalysts based on first-row transition metals.

Recent Advances of Transition Metal Basic Salts for Electrocatalytic Oxygen Evolution Reaction and Overall Water Electrolysis

Electrocatalytic oxygen evolution reaction (OER) has been recognized as the bottleneck of overall water splitting, which is a promising approach for sustainable production of H2. Transition metal (TM) hydroxides are the most conventional and classical non-noble metal-based electrocatalysts for OER, while TM basic salts [M2+(OH)2-x(Am-)x/m, A = CO32-, NO3-, F-, Cl-] consisting of OH- and another anion have drawn extensive research interest due to its higher catalytic activity in the past decade. In this review, we summarize the recent advances of TM basic salts and their application in OER and further overall water splitting. We categorize TM basic salt-based OER pre-catalysts into four types (CO32-, NO3-, F-, Cl-) according to the anion, which is a key factor for their outstanding performance towards OER. We highlight experimental and theoretical methods for understanding the structure evolution during OER and the effect of anion on catalytic performance. To develop bifunctional TM basic salts as catalyst for the practical electrolysis application, we also review the present strategies for enhancing its hydrogen evolution reaction activity and thereby improving its overall water splitting performance. Finally, we conclude this review with a summary and perspective about the remaining challenges and future opportunities of TM basic salts as catalysts for water electrolysis.

Phase-transition tailored nanoporous zinc metal electrodes for rechargeable alkaline zinc-nickel oxide hydroxide and zinc-air batteries

Secondary alkaline Zn batteries are cost-effective, safe, and energy-dense devices, but they are limited in rechargeability. Their short cycle life is caused by the transition between metallic Zn and ZnO, whose differences in electronic conductivity, chemical reactivity, and morphology undermine uniform electrochemical reactions and electrode structural stability. To circumvent these issues, here we propose an electrode design with bi-continuous metallic zinc nanoporous structures capable of stabilizing the electrochemical transition between metallic Zn and ZnO. In particular, via in situ optical microscopy and electrochemical impedance measurements, we demonstrate the kinetics-controlled structural evolution of Zn and ZnO. We also tested the electrochemical energy storage performance of the nanoporous zinc electrodes in alkaline zinc-nickel oxide hydroxide (NiOOH) and zinc-air (using Pt/C/IrO2-based air-electrodes) coin cell configurations. The Zn | |NiOOH cell delivers an areal capacity of 30 mAh/cm2 at 60% depth of discharging for 160 cycles, and the Zn | |Pt/C/IrO2 air cell demonstrates 80-hour stable operation in lean electrolyte condition.

Controllable growth of Fe-doped NiS2 on NiFe-carbon nanofibers for boosting oxygen evolution reaction

The construction of high-efficiency and low-cost electrocatalysts toward oxygen evolution reaction (OER) to improve the overall water decomposition performance is a fascinating route to deal with the clean energy application. Herein, Fe-doped NiS₂ crystals grown on the surface of carbon nanofibers (CNFs) encapsulated with NiFe alloy nanoparticles ((Ni,Fe)S₂/NiFe-CNFs) are fabricated through an electrospinning-calcination-vulcanization process, which has been used as a splendid electrocatalyst for OER. Benefitting from the abundant electrochemical active sites from the incorporation of Fe element in NiS₂ and the synergistic effect between NiFe-CNFs and surface sulfides, the obtained (Ni,Fe)S₂/NiFe-CNFs catalyst exhibits highly electrochemical activities and satisfactory durability toward OER in an alkaline medium with a low overpotential of only 287 mV at a high current density of 30 mA cm^-2, and with a little decline in the current retention after 48 h, suggesting its superior OER performance even compared with some noble metal-based electrocatalysts. Additionally, a two-electrode system conducted by using the (Ni,Fe)S₂/NiFe-CNFs and commercial Pt/C as electrodes, only needs a cell voltage of 1.54 V to afford 10 mA cm^-2 for overall water splitting, which is even much better than the RuO₂||Pt/C electrolyzer. This study offers a promising approach to prepare high-efficiency OER catalysts toward overall water splitting.

A clicking confinement strategy to fabricate transition metal single-atom sites for bifunctional oxygen electrocatalysis

Rechargeable zinc-air batteries call for high-performance bifunctional oxygen electrocatalysts. Transition metal single-atom catalysts constitute a promising candidate considering their maximum atom efficiency and high intrinsic activity. However, the fabrication of atomically dispersed transition metal sites is highly challenging, creating a need for for new design strategies and synthesis methods. Here, a clicking confinement strategy is proposed to efficiently predisperse transitional metal atoms in a precursor directed by click chemistry and ensure successful construction of abundant single-atom sites. Concretely, cobalt-coordinated porphyrin units are covalently clicked on the substrate for the confinement of the cobalt atoms and affording a Co-N-C electrocatalyst. The Co-N-C electrocatalyst exhibits impressive bifunctional oxygen electrocatalytic performances with an activity indicator ΔE of 0.79 V. This work extends the approach to prepare transition metal single-atom sites for efficient bifunctional oxygen electrocatalysis and inspires the methodology on precise synthesis of catalytic materials.

Binder-Free Air Electrodes for Rechargeable Zinc-Air Batteries: Recent Progress and Future Perspectives

Designing an efficient air electrode is of great significance for the performance of rechargeable zinc (Zn)-air batteries. However, the most widely used approach to fabricate an air electrode involves polymeric binders, which may increase the interface resistance and block electrocatalytic active sites, thus deteriorating the performance of the battery. Therefore, binder-free air electrodes have attracted more and more research interests in recent years. This article provides a comprehensive overview of the latest advancements in designing and fabricating binder-free air electrodes for electrically rechargeable Zn-air batteries. Beginning with the fundamentals of Zn-air batteries and recently reported bifunctional active catalysts, self-supported air electrodes for liquid-state and flexible solid-state Zn-air batteries are then discussed in detail. Finally, the conclusion and the challenges faced for binder-free air electrodes in Zn-air batteries are also highlighted.

Investigation of Au/Co3O4 nanocomposites in glycol oxidation by tailoring Co3O4 morphology

The interfacial perimeter of nanogold and supports is often deemed as the catalytically active site for multiple reactions while the geometrical configuration of the interfacial perimeter at atomic scale is less studied. Herein, gold nanoparticles (NPs) of ca. 2.0 nm are dispersed on Co₃O₄ support in the shape of nanocubes (dominant Co₃O₄(001) facet) and nanoplates (Co₃O₄(111)), which forms different Au-Co₃O₄ interfaces with respect to the specific facet of the oxide support. A comparison is made on the basis of the interfacial structures and catalytic behavior of ethylene glycol oxidation. STEM analysis identifies that these metallic Au NPs interact with Co₃O₄ with an orientation relationship of Au/Co₃O₄(001) and Au/Co₃O₄(111). XPS and Raman spectroscopy investigations reveal the important variations in the reactivity of surface oxygen, surface O_ads/O_L ratio, and evolution of surface oxygen vacancies upon variation of the Co₃O₄ shape. Au/Co₃O₄-P exhibits much better catalytic activity than the Au/Co₃O₄-C counterpart in the aerobic oxidation of ethylene glycol, which is promoted by surface oxygen vacancies and intrinsic defects. It has been revealed that the surface oxygen vacancies participate in activating O₂, thus making Co₃O₄-P a superior support for Au NPs in the catalysis of ethylene glycol oxidation.

Bimetallic Hollow Tubular NiCoOx as a Bifunctional Electrocatalyst for Enhanced Oxygen Reduction and Evolution Reaction

The development of high-efficiency oxygen electrocatalysts with earth-abundant transition metals rather than scarce noble metals has aroused growing interests due to their potential for energy storage and conversion applications. Herein, we developed a facile strategy to synthesize hollow tubular bimetallic Ni-Co oxide rooted with dense nanosheets for enhanced bifunctionality and facilitated redox reaction kinetics. Owing to the rational design of morphology and well-dispersed Ni and Co ions, the bimetallic samples exhibit admirable bifunctional electrocatalytic activities. This bimetallic Ni-Co oxide shows superior oxygen electrocatalytic performance in comparison with the monometallic Ni and Co oxides, according to the electrocatalytic synergistic effect from the bimetallic system. The optimized sample with the specific mass ratio of Ni and Co displays the oxygen reduction reaction (ORR) property comparable to commercial Pt/C and oxygen evolution reaction (OER) performance superior to commercial RuO₂. The electrochemical tests and structural characterizations offer in-depth dissection on the electrocatalytic behaviors, especially the superb stability in both ORR and OER tests, as well as the outstanding resistance to methanol poisoning, representing a promising candidate in the renewable energy field.

Nitrogen-Doped Mixed-Phase Cobalt Nanocatalyst Derived from a Trinuclear Mixed-Valence Cobalt(III)/Cobalt(II) Complex for High-Performance Oxygen Evolution Reaction

Because of a continuous increase in energy demands and environmental concerns, a focus has been on the design and construction of a highly efficient, low-cost, environmentally friendly, and noble-metal free electrocatalyst for energy technology. Herein we report facile synthesis of the mixed-valence trinuclear cobalt complex 1 by the reaction of 2-amino-1-phenylethanol and CoCl₂·6H₂O in methanol as the solvent at room temperature. Further, 1 was reduced by using aqueous N₂H₄ as a simple reducing agent, followed by calcination at 300 °C for 3 h, yielding a nitrogen-doped mixed phase cobalt [β-Co(OH)₂ and CoO] nanocatalyst (N@MPCoNC). Both 1 and N@MPCoNC were characterized by various physicochemical techniques. Moreover, 1 was authenticated by single-crystal X-ray diffraction studies. The hybrid N@MPCoNC reveals a unique electronic and morphological structure, offering a low overpotential of 390 mV for a stable current density of 10 mA cm^-2 with high durability. This N@MPCoNC showed excellent electrocatalytic as well as photocatalytic activity for oxygen evolution reaction compared to 1.

Advanced electrocatalysts based on two-dimensional transition metal hydroxides and their composites for alkaline oxygen reduction reaction

The electrocatalytic oxygen reduction reaction (ORR) is a crucial part in developing high-efficiency fuel cells and metal-air batteries, which have been cherished as clean and sustainable energy conversion devices/systems to meet the ever-increasing energy demand. ORR electrocatalysts currently employed in the cathodes of fuel cells and metal-air batteries are mainly based on high-cost and scarce noble metal elements. It is thus of great importance to develop cheap and earth-abundant ORR electrocatalysts. In this aspect, redox-active transition metal hydroxides, a class of multifunctional inorganic layered materials, have been proposed as prospective candidates on account of their abundance and high ORR activities. In this article, the preparation and structural evolution of transition metal hydroxides, in particular their exfoliation into two-dimensional (2D) nanosheets, as well as compositing/integrating with catalytic active and/or conductive components to overcome the insulating nature of hydroxides in alkaline ORR, are summarized. Recent advances have demonstrated that 2D transition metal hydroxides with carefully tuned compositions and elaborately designed nanoarchitectures can achieve both high activity and high pathway selectivity, as well as excellent stability comparable to those of commercial Pt/C electrocatalysts. To realize the dream of renewable electrochemical energy conversion, new strategies and insights into rational designing of 2D hydroxide-based nanostructures with further enhanced electrocatalytic performance are still to be vigorously pursued.

Pd nanoparticles deposited on Co(OH)2 nanoplatelets as a bifunctional electrocatalyst and their application in Zn-air and Li-O2 batteries

The development of affordable electrocatalysts for both oxygen reduction and evolution reactions (ORR/OER) has received great interest due to their importance in metal-air batteries and regenerative fuel cells. We developed a high-performance bifunctional oxygen electrocatalyst based on Pd nanoparticles supported on cobalt hydroxide nanoplatelets (Pd/Co(OH)₂) as an air cathode for metal-air batteries. The Pd/Co(OH)₂ shows remarkably higher electrocatalytic activity in comparison with commercial catalysts (Pt/C, IrO₂), including an ORR half-wave potential (E_1/2) of 0.87 V vs. RHE and an OER overpotential of 0.39 V at 10 mA cm^-2 in aqueous alkaline medium. The Zn-air battery constructed with Pd/Co(OH)₂ presents stable charge/discharge voltage (ΔE_OER-ORR = 0.69 V), along with durable cycling stability for over 30 h. Also, this cathode exhibits a maximum discharge capacity of 17 698 mA h g^-1, and stable battery operation over 50 cycles at a fixed capacity of 1000 mA h g^-1, as an efficient air electrode for Li-O₂ batteries, indicating that Pd/Co(OH)₂ can be a potential candidate for both aqueous and non-aqueous metal-air batteries.

The design of zinc-substituted cobalt (pyro)phosphates as efficient bifunctional electrocatalysts for zinc-air batteries

In an effort to rationally design economic electrocatalysts, zinc-substituted cobalt phosphate and pyrophosphate were prepared using facile template-free combustion synthesis. They act as efficient stable bifunctional electrocatalysts due to the tuning of oxygen affinity by zinc substitution and catalytically active cobalt sites. Exploiting their bifunctional activity, these cobalt (pyro)phosphates were incorporated into a zinc-air battery in an alkaline electrolyte.

Electrodeposited Sulfur and CoxS Electrocatalyst on Buckypaper as High-Performance Cathode for Li-S Batteries

Lithium-sulfur batteries (LSBs) are considered as the next generation of advanced rechargeable batteries because of their high energy density. In this study, sulfur and Co_xS electrocatalyst are deposited on carbon nanotube buckypaper (S/Co_xS/BP) by a facile electrodeposition method and are used as a binder-free high-performance cathode for LSBs. Elemental sulfur is deposited on buckypaper by electrooxidation of a polysulfide solution (~ S₆^2-). This approach substantially increased the current and time efficiency of sulfur electrochemical deposition on conductive material for LSBs. S/Co_xS/BP cathode could deliver an initial discharge capacity as high as 1650 mAh g^-1 at 0.1 C, which is close to the theoretical capacity of sulfur. At current rate of 0.5 C, the S/Co_xS/BP has a capacity of 1420 mAh g^-1 at the first cycle and 715 mAh g^-1 after 500 cycles with a fading rate of 0.099% per cycle. The high capacity of S/Co_xS/BP is attributed to both the homogeneous dispersion of nanosized sulfur within BP and the presence of Co_xS catalyst. The sodium dodecyl sulfate (SDS) pretreatment of BP renders it polarity to bind polysulfides and thus facilitates the good dispersibility of nanosized sulfur within BP. Co_xS catalyst accelerates the kinetics of polysulfide conversion and reduces the presence of polysulfide in the cathode, which suppresses the polysulfide diffusion to anode, i.e., the shuttle effect. The mitigation of the active material loss improves not only the capacity but also the cyclability of S/Co_xS/BP.

The NHx Group Induced Formation of 3D α-Co(OH)2 Curly Nanosheet Aggregates as Efficient Oxygen Evolution Electrocatalysts

Recently, the curly structure attracts researchers' attention due to the strain effect, electronic effect, and improved surface area, which exhibits enhanced electrocatalytic activity. However, the synthesis of metastable curved structures is very difficult. Herein, a simple room temperature coprecipitation method is proposed to synthesize 3D cobalt (Co) hydroxide (α-Co(OH)₂ ) electrocatalysts that consist of curly 2D nanosheets. The formation process of curly nanosheets is elaborated systematically and the results demonstrate that the NH_x group has great effect on the formation of curly structure. Combining the advantage of 2D curly nanosheet and 3D aggregate structure, the as-prepared α-Co(OH)₂ curly nanosheet aggregates show the best water oxidation activity with an overpotential of 269 mV at j = 10 mA cm^-2 in 1.0 m KOH. The electrocatalytic process studies demonstrate that the formation of Co^IV O species is the rate-determining step. Theoretical calculations further confirm the beneficial effect of the bent structure on the conductivity, the adsorption of OH^- and the formation of OOH* species.

Bifunctional Catalysts for Reversible Oxygen Evolution Reaction and Oxygen Reduction Reaction

Metal-air batteries (MABs) and reversible fuel cells (RFCs) rely on the bifunctional oxygen catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Finding efficient bifunctional oxygen catalysts is the ultimate goal and it has attracted a great deal of attention. The dilemma is that a good ORR catalyst is not necessarily efficient for OER, and vice versa. Thus, the development of a new type of bifunctional oxygen catalysts should ensure that the catalysts exhibit high activity for both OER and ORR. Composites with multicomponents for active centers supported on highly conductive matrices could be able to meet the challenges and offering new opportunities. In this Review, the evolution of bifunctional catalysts is summarized and discussed aiming to deliver high-performance bifunctional catalysts with low overpotentials.

CoFe–OH Double Hydroxide Films Electrodeposited on Ni-Foam as Electrocatalyst for the Oxygen Evolution Reaction

Cobalt-iron double hydroxide (CoFe–OH) films were electrochemically deposited on 3D Ni foam electrodes for the oxygen evolution reaction (OER). The dependence of the OER activity on film composition and thickness was evaluated, which revealed an optimal Fe:Co ratio of about 1:2.33. The composition of the catalyst film was observed to vary with film thickness. The electrodeposition parameters were carefully controlled to yield microstructured Ni-foam decorated with CoFe–OH films of controlled thickness and composition. The most active electrode exhibited an overpotential as low as 360 mV OER at an industrial scale current density of 400 mA cm−2 that remained stable for at least 320 h. This work contributes towards the fabrication of practical electrodes with the focus on the development of stable electrodes for electrocatalytic oxygen evolution at high current densities.

Hybrid-atom-doped NiMoO4 nanotubes for oxygen evolution reaction

Doping with hybrid atoms can narrow the band gap of NiMoO₄ nanotubes, improving their performance in the oxygen evolution reaction.

Amorphous nickel–iron hydroxide films on nickel sulfide nanoparticles for the oxygen evolution reaction

The development of earth-abundant and low-cost electrocatalysts with high performance toward the oxygen evolution reaction (OER) plays a key role in water splitting.

Heterolayered Ni-Fe Hydroxide/Oxide Nanostructures Generated on a Stainless-Steel Substrate for Efficient Alkaline Water Splitting

Highly active and inexpensive anode materials are required for large-scale hydrogen production using alkaline water electrolysis (AWE). Here, heterolayered nanostructures of Ni-Fe hydroxides/oxides with high activity for the oxygen evolution reaction (OER) were synthesized on a 316 stainless steel (SS) substrate through constant current density electrolysis. The thicknesses, morphologies, and compositions of the nanostructures, generated through dealloying and surface oxidation of the SS elements with severe oxygen microbubble evolution, were dependent on the electrolysis time. Nanostructural analyses showed that the heterolayered Ni-Fe hydroxide/oxide nanostructures were generated during the initial stage of electrolysis, growing nanofiberlike Ni-Fe hydroxide layers with increasing electrolysis time of up to 5 h. The prolonged electrolysis resulted in densification of the nanofiber structures. The OER overpotential at 10 mA/cm² was estimated to be 254 mV at 20 °C, demonstrating better performance than a standard OER catalyst, for example, Ir oxide, and obtaining the value of the Ni-Fe layered double hydroxide (LDH). Furthermore, the OER property surpassed the Ni-Fe LDH catalysts at high current density regions greater than 100 mA/cm². Moreover, stable electrolysis was achieved for 20 h under conditions similar to that of the practical AWE of 400 mA/cm² in 20 and 75 °C solution. Therefore, the simple surface modification method could synthesize highly active nanostructures for alkaline water splitting anodes.

Co3+ -Rich Na1.95 CoP2 O7 Phosphates as Efficient Bifunctional Catalysts for Oxygen Evolution and Reduction Reactions in Alkaline Solution

Implementing sustainable energy conversion and storage technologies is highly reliant on crucial oxygen electrocatalysis, such as the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). However, the pursuit of low cost, energetic efficient and robust bifunctional catalysts for OER and ORR remains a great challenge. Herein, the novel Na-ion-deficient Na_2-x CoP₂ O₇ catalysts are proposed to efficiently electrocatalyze OER and ORR in alkaline solution. The engineering of Na-ion deficiency can tune the electronic structure of Co, and thus tailor the intrinsically electrocatalytic performance. Among the sodium cobalt phosphate catalysts, the Na_1.95 CoP₂ O₇ (NCPO5) catalyst exhibits the lowest ΔE (E_J10,OER -E_J-1,ORR ) of only 0.86 V, which favorably outperforms most of the reported non-noble metal catalysts. Moreover, the Na-ion deficiency can stabilize the phase structure and morphology of NCPO5 during the OER and ORR processes. This study highlights the Na-ion deficient Na_2-x CoP₂ O₇ as a promising class of low-cost, highly active and robust bifunctional catalysts for OER and ORR.

FeNi-Based Coordination Crystal Directly Serving as Efficient Oxygen Evolution Reaction Catalyst and Its Density Functional Theory Insight on the Active Site Change Mechanism

Although most metal-organic coordination materials are promising materials used as templates to develop highly efficient electrocatalysts via pyrolysis in situ, few studies have explored the use of these materials for direct catalysis of oxygen evolution reaction (OER). Herein, inspired by the natural synthesis and the inherent properties of metal-organic coordination materials, the FeNi-tannic acid coordination crystal was in situ grown on Ni foam ((FeNi)-Tan/NF) to directly catalyze the OER. It was found that (FeNi)-Tan/NF exhibited predominant OER activity, which required a low overpotential of 208 mV to reach a current density of 50 mA·cm^-2 under a small Tafel slope of 33.5 mV·dec^-1, and it possessed robust stability. Density functional theory (DFT) calculations demonstrated that the active site change from Ni in Ni-Tan to the Fe atom in (FeNi)-Tan may provide a more favorable OER catalytic route. This application of such polyphenol coordination materials is promising for stimulating the exploration of functional metal-organic coordination materials toward applications in the energy conversion field.

Hierarchically Structured Co(OH)2/CoPt/N-CN Air Cathodes for Rechargeable Zinc-Air Batteries

For the realization of the large-scale deployment of rechargeable Zn-air batteries, it is crucial to develop cost-effective, efficient, and stable bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this work, an integrated electrocatalyst consisting of Co(OH)₂/CoPt/N-CN was developed to enable both ORR and OER reactions for Zn-air batteries. The hierarchical Co(OH)₂/CoPt/N-CN electrocatalyst has desirable electrochemical properties, with comparable activity and better durability than commercial Pt/C for ORR and improved activity and long-term stability than commercial IrO₂ catalyst for OER. When implemented as air-cathode for rechargeable Zn-air batteries, Co(OH)₂/CoPt/N-CN exhibited a high power-density of 171 mW cm^-2, a specific capacity of 812 mA h g^-1, and a robust cycling life. Interestingly, the hierarchical structure remained intact upon charge and discharge tests, suggesting potential long-term use in the Zn-air battery technology. The material development strategy presented here can enrich the toolbox for the design and construction of cost-effective, efficient, and robust bi-functional electrocatalysts for ORR and OER toward rechargeable Zn-air battery applications.

Chlorine-doped α-Co(OH)2 hollow nano-dodecahedrons prepared by a ZIF-67 self-sacrificing template route and enhanced OER catalytic activity

A ZIF-67-self-sacrificing template strategy was designed for the synthesis of undoped/Cl-doped α-Co(OH)₂ hollow nano-dodecahedrons with enhanced OER performance.

Nickel-Based Bicarbonates as Bifunctional Catalysts for Oxygen Evolution and Reduction Reaction in Alkaline Media

Oxygen electrocatalysis, including the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), is one of the most important electrochemical processes for sustainable energy conversion and storage technologies. Herein, nickel-based bicarbonates are, for the first time, developed as catalysts for oxygen electrocatalysis, and demonstrate superior electrocatalytic performance in alkaline media. Iron doping can significantly tune the real valence of nickel ions, and consequently tailor the electrocatalytic ability of bicarbonates. Among the nickel-based bicarbonates, Ni_0.9 Fe_0.1 (HCO₃ )₂ exhibits the highest bifunctional catalytic activity, with a potential difference of 0.86 V between the OER potential at a current density of 10 mA cm^-2 and the ORR potential at a current density of -1 mA cm^-2 , which outperforms most of the reported precious-metal-free catalysts. The present work provides new insights into exploring efficient catalysts for oxygen electrocatalysis, and it suggests that, in addition to the extensively studied transition metal hydroxides and oxides, bicarbonates and carbonates also show great potential as precious metal-free catalysts.

Recent Advances in Materials and Design of Electrochemically Rechargeable Zinc-Air Batteries

The century-old zinc-air (Zn-air) battery concept has been revived in the last decade due to its high theoretical energy density, environmental-friendliness, affordability, and safety. Particularly, electrically rechargeable Zn-air battery technologies are of great importance for bulk applications like electric vehicles, grid management, and portable electronic devices. Nevertheless, Zn-air batteries are still not competitive enough to realize widespread practical adoption because of issues in efficiency, durability, and cycle life. Here, following an introduction to the fundamentals and performance testing techniques, the latest research progress related to electrically rechargeable Zn-air batteries is compiled, particularly new key findings in the last five years (2013-2018). The strategies concerning the development of Zn and air electrodes are in focus. The design of other battery components, namely electrolytes and separators are also discussed. Poor performance of O₂ electrocatalysts and the lack of the long-term stability of Zn electrodes and electrolytes remain major challenges. Finally, recommendations regarding the testing routines and materials design are provided. It is hoped that this up-to-date account will help to shape the future research activities toward the development of practical electrically rechargeable Zn-air batteries with extended lifetime and superior performance.

Emerging Materials in Heterogeneous Electrocatalysis Involving Oxygen for Energy Harvesting

Water-based renewable energy cycle involved in water splitting, fuel cells, and metal-air batteries has been gaining increasing attention for sustainable generation and storage of energy. The major challenges in these technologies arise due to the poor kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER), besides the high cost of the catalysts. Attempts to address these issues have led to the development of many novel and inexpensive catalysts as well as newer mechanistic insights, particularly so in the last three-four years when more catalysts have been investigated than ever before. With the growing emphasis on bifunctionality, that is, materials that can facilitate both reduction and evolution of oxygen, this review is intended to discuss all major families of ORR, OER, and bifunctional catalysts such as metals, alloys, oxides, other chalcogenides, pnictides, and metal-free materials developed during this period in a single platform, while also directing the readers to specific and detailed review articles dealing with each family. In addition, each section highlights the latest theoretical and experimental insights that may further improve ORR/OER performances. The bifunctional catalysts being sufficiently new, no consensus appears to have emerged about the efficiencies. Therefore, a statistical analysis of their performances by considering nearly all literature reports that have appeared in this period is presented. The current challenges in rational design of these catalysts as well as probable strategies to improve their performances are presented.

Effect of Intrinsic Properties of Anions on the Electrocatalytic Activity of NiCo2O4 and NiCo2O x S4-x Grown by Chemical Bath Deposition

Electrochemical water (H₂O) splitting is one of the most promising technologies for energy storage by hydrogen (H₂) generation but suffers from the requirement of high overpotential in the anodic half-reaction (oxygen evolution), which is a four-electron process. Though transition-metal oxides and oxysulfides are increasingly researched and used as oxygen evolution electrocatalysts, the bases of their differential activities are not properly understood. In this article, we have synthesized NiCo₂O₄ and NiCo₂O _x S_4-x by a chemical bath deposition technique, and the latter has shown better oxygen evolution performance, both in terms of stability and activity, under alkaline conditions. Comprehensive analysis through time-dependent cyclic voltammetry, microscopy, and elemental analysis reveal that the higher activity of NiCo₂O _x S_4-x may be attributed to the lower metal-sulfur bond energy that facilitates the activation process to form the active metal hydroxide/oxyhydroxide species, higher electrochemically active surface area, higher pore diameter and rugged morphology that prevents corrosion. This work provides significant insights on the advantages of sulfur-containing materials as electrochemical precatalysts over their oxide counterparts for oxygen evolution reaction.

Organophosphoric acid-derived CoP quantum dots@S,N-codoped graphite carbon as a trifunctional electrocatalyst for overall water splitting and Zn-air batteries

Developing highly efficient, low-cost, and multifunctional electro-catalysts to replace noble metals is of significant importance for energy storage and conversion systems. Herein, we demonstrate a facile strategy for the preparation of CoP quantum dots (QDs) embedded in S,N-codoped graphite carbon (CoP@SNC) by using organophosphoric acid as both phosphorus and carbon sources. Benefiting from the strong coupling and synergistic effect between CoP QDs and highly conductive S,N-codoped carbon, well-structured porosity and high specific surface area, the resulting CoP@SNC exhibits excellent activities for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR), making it a trifunctional electro-catalyst for overall water splitting and rechargeable Zn-air batteries. When CoP@SNC is used for overall water splitting, a cell voltage as low as 1.64 V is required to reach the current density of 10 mA cm-2; the obtained rechargeable Zn-air battery with CoP@SNC as the air cathode exhibits a high open-circuit voltage of 1.45 V, a very low discharge-charge voltage gap (0.83 V at 10 mA cm-2), and a long cycle life (up to 180 cycles). This work not only offers a new strategy for the synthesis of CoP@SNC but also presents its huge potential as a trifunctional electro-catalyst for clean energy systems.

Superior stability of a bifunctional oxygen electrode for primary, rechargeable and flexible Zn-air batteries

Central to commercializing metal-air batteries is the development of highly efficient and stable catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this study, a composite catalyst with a unique interpenetrating network (denoted as NiCo2O4@MnO2-CNTs-3) was synthesized and exhibited better bifunctional activity (ΔE = 0.87 V) and durability than both Pt/C and Ir/C catalysts. The improved performance arises from three factors: (i) MnO2 promotes the ORR while NiCo2O4 facilitates the OER; (ii) carbon nanotubes improve the electronic conductivity; and (iii) the highly porous structure enables the adsorption-desorption of O2 and enhances the structural stability. As a result, the primary and rechargeable Zn-air battery affords a high power density and specific capacity (722 mA h g-1), an outstanding discharge stability (255 mW cm-2 after 1000 cycles) and a high cycling stability (over 2280 cycles). Electron microscopy and electrochemical analysis revealed that the degradation of the rechargeable Zn-air battery performance resulted from the damage of the air electrode and the hydrogen evolution reaction on the zinc electrode. A flexible Zn-air battery employing a solid-state electrolyte showed an exciting stability (540 cycles) and high power density (85.9 mW cm-2), suggesting that the anion exchange membrane effectively prevents the migration of Zn2+ ions and the deposition of carbonates.

3D Networks of CoFePi with Hierarchical Porosity for Effective OER Electrocatalysis

A series of amorphous 3D Co-based phosphate networks with hierarchical porosity, including the CoPi, the binary CoM₁ Pi and the trinary CoM₁ M₂ Pi (M_i  = Ni^II , Fe^III , Ce^III ) are produced via a novel bitemplate coprecipitation approach at room temperature. Interestingly, the integration of Fe^III and Co^II in the same network is found to significantly influence both the porosity and the electronic state of Co^II . The CoFePi with a Fe^III to Co^II mole ratio of 0.91 has a specific surface area of 170 m² g^-1 and average pore size of 12.3 nm, larger than those of the CoPi network; furthermore, the Co^II within such CoFePi exhibits a higher oxidation state than that in the CoPi. Due to such structural and compositional merits, the binary CoFePi network shows superior oxygen evolution reaction (OER) electrocatalytic activity, which gives an overpotential as low as 0.315 V at 10 mA cm^-2 and a Tafel slope of 33 mV dec^-1 in 0.10 m KOH. Additionally, the trinary CoFeNiPi demonstrates similar OER catalytic performance. The two phosphate networks also exhibit remarkable catalytic stability. In view of their easy preparation, superior activity, high stability, and low cost, such transition metal phosphate networks are promising catalysts for practical OER processes.