Heteroatom-Doped Carbon-Encapsulated FeP Nanostructure: A Multifunctional Electrocatalyst for Zinc-Air Battery and Water Electrolyzer

A Dynamic Active Site for OER Catalysis at Nickel-Vanadium-Phosphide Depends on V Surface Position and Fe in Alkaline

Enhancing the rate of the oxygen evolution reaction (OER) by doping Ni-based electrocatalysts with guest metals other than Fe (V in this work) and the stability of the metal site should be assessed independent of Fe traces and in relation to the guest metal activity in solution. We examined OER catalysis and its sustainability at vanadium-doped nickel phosphide (NixPy-V) independent of the role of Fe traces in alkaline. V was included in NixPy by codeposition at cathodic bias (termed Vbulk) or postdeposition during the phosphide-to-hydroxide surface transformation at anodic bias in alkaline spiked with VCl3 (termed Vsurface). Doping with Vsurface strongly promoted OER and reduced the Tafel slope in KOH purified of Fe traces (Fe-free KOH), indicating a Vsurface-path lowering of overpotentials independent of and different from the mechanism via Fe surface sites at NiOxHy. A similar effect was not observed with Vbulk inclusion. Sustaining catalysis via Vsurface-sites required maintaining a sufficiently high activity of V in solution. The prominent role of Vsurface sites and the dependence of stability on solution condition are reminiscent of the behavior of Fe surface sites in NiOxHy. This work points to a trend of behavior of active site dynamics at Ni-based OER catalysts in alkaline and presents a question and insight on how to stabilize the guest-metal site at the solid-liquid interface.

Transition metal phosphide-based oxygen electrocatalysts for aqueous zinc-air batteries

Electrically rechargeable zinc-air batteries (ZABs) are emerging as promising energy storage devices in the post-lithium era, leveraging the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) at the air cathodes. Efficient bifunctional oxygen electrocatalysts, capable of catalyzing both the ORR and OER, are essential for the operation of rechargeable ZABs. Traditional Pt- and RuO2/IrO2-based catalysts are not ideal, as they lack sufficient bifunctional ORR and OER activity, exhibit limited long-term durability, require high overpotentials and are expensive. In contrast, non-precious metal-based catalysts, including transition metal phosphides (TMPs), have gained significant attention for their promising bifunctional catalytic properties, making them attractive candidates for ZABs. Despite encouraging lab-scale achievements, translating these advancements into market-ready applications remains challenging due to suboptimal energy performance. Rationally engineered bifunctional TMPs hold great potential for overcoming these challenges and meeting the requirements of rechargeable ZABs. This feature article reviews recent progress in the development of TMP-based catalysts for ZABs, providing a comprehensive overview of ZAB fundamentals and strategies for catalyst design, synthesis, and engineering. A particular emphasis is placed on widely studied bifunctional Fe, Co, and Ni phosphides, along with approaches to enhance their catalytic performance. Key performance metrics are critically evaluated, including the potential gap (ΔE) between the ORR and the OER, specific capacity, peak power density, and charge-discharge cycling stability. Finally, this feature article discusses the challenges faced in TMP-based ZABs, proposes strategies to address these issues, and explores future directions for improving their rechargeability to meet the demands of commercial-scale energy storage technologies.

A Molecular Catalyst-Driven Sustainable Zinc-Air Battery Assembly

Bidirectional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts are key for molecular oxygen-centric renewable energy transduction via metal-air batteries. Here, a molecular cobalt complex is covalently tethered on a strategically functionalized silica surface that displayed both ORR and OER in alkaline media. The detailed X-ray absorbance spectroscopy (XAS) studies indicate that this catalyst retains its intrinsic molecular features while playing a central role during bidirectional electrocatalysis and demonstrating a relatively lower energy gap between O2/H2O interconversions. This robust molecular catalyst-silica composite (deposited on a porous carbon paper) is assembled along with a zinc foil and polymeric gel membrane to devise an active single-stack quasi-solid zinc-air battery (ZAB) setup. This quasi-solid ZAB assembly displayed impressive power density (60 mW cm-2@100 mA cm-2), specific capacity (818 mAh g-1@ 5mA cm-2), energy density (757 Whkg-1 @5mA cm-2), and elongated charging/discharging life (28 h). An appropriate assembly of these ZAB units is able to power practical electronic appliances, requiring ≈1.6-6.0V potential requirements.

Cyano-Bridged Bimetallic Polymer Network-Derived Pd3Fe Intermetallic for Aqueous Rechargeable Zinc-Air Batteries

The rational design and synthesis of bifunctionally active and durable oxygen electrocatalysts have garnered significant attention for electrochemical energy conversion and storage. Intermetallic nanostructures are particularly promising for these applications due to their unique catalytic properties and exceptional durability. In this study, we present a fascinating synthetic approach for the direct synthesis of a bifunctional oxygen electrocatalyst based on nitrogen-doped carbon-encapsulated ordered Pd3Fe (o-Pd3Fe@NC) intermetallic, using a cyano-bridged bimetallic single-source precursor tailored for aqueous rechargeable zinc-air batteries (ZABs). Through temperature-controlled annealing of a bipyridine-containing Pd-Fe cyano-bridged polymer network, a catalytically active o-Pd3Fe@NC catalyst is obtained. The spatial confinement of Pd(II) and Fe(II) within the polymer network facilitates the controlled growth of the o-Pd3Fe nanostructure. This intermetallic catalyst exhibits bifunctional activity for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The o-Pd3Fe@NC catalyst achieves an ORR onset potential of 0.98 V and demonstrates remarkable long-term stability, sustaining performance over 30,000 cycles in alkaline electrolytes without noticeable degradation. The rechargeable liquid and flexible ZABs constructed with the o-Pd3Fe@NC air cathode deliver outstanding energy performance, achieving maximum power densities of 212.9 and 109 mW cm-2, respectively. The liquid ZAB delivers a specific capacity of 816 mAh gZn-1 and exhibits excellent charge-discharge cycling stability, maintaining a consistent charge-discharge voltage gap over 200 h. The flexible ZAB retains its charge-storage performance across all bending angles.

Discovery of FeP/Carbon Dots Nanozymes for Enhanced Peroxidase-Like Catalytic and Antibacterial Activity

Iron phosphide/carbon (FeP/C) serving as electrocatalysts exhibit excellent activity in oxygen reduction reaction (ORR) process. H2O2 catalyzed by peroxidase (POD) is similar to the formation of new electron transfer channels and the optimization of adsorption of oxygen-containing intermediates or desorption of products in ORR process. However, it is still a challenge to discover FeP/C with enhanced POD-like catalytic activity in the electrocatalytic database for biocatalysis. The discovery of FeP/carbon dots (FeP/CDs) nanozymes driven by electrocatalytic activity for enhanced POD-like ability is demonstrated. FeP/CDs derived from CDs-Fe3+ chelates show enhanced POD-like catalytic and antibacterial activity. FeP/CDs exhibit enhanced POD-like activities with a specific activity of 31.1 U mg-1 that is double higher than that of FeP. The antibacterial ability of FeP/CDs nanozymes with enhanced POD-like activity is 98.1%. The antibacterial rate of FeP/CDs nanozymes (250 µg mL-1) increased by 5%, 15%, and 36% compared with FeP, Fe2O3/CDs, and Cu3P/CDs nanozymes, respectively. FeP/CDs nanozymes will attract more efforts to discover or screen transition metal phosphide/C nanozymes with enhanced POD-like catalytic activity for biocatalysis in the electrocatalytic database.

Free-Standing Carbon Nanofiber Films with Supported Cobalt Phosphide Nanoparticles as Cathodes for Hydrogen Evolution Reaction in a Microbial Electrolysis Cell

High-performance and cost-efficient electrocatalysts and electrodes are needed to improve the hydrogen evolution reaction (HER) for the hydrogen (H2) generation in electrolysers, including microbial electrolysis cells (MECs). In this study, free-standing carbon nanofiber (CNF) films with supported cobalt phosphide nanoparticles have been prepared by means of an up-scalable electrospinning process followed by a thermal treatment under controlled conditions. The produced cobalt phosphide-supported CNF films show to be nanoporous (pore volume up to 0.33 cm3 g-1) with a high surface area (up to 502 m2 g-1) and with a suitable catalyst mass loading (up to 0.49 mg cm-2). Values of overpotential less than 140 mV at 10 mA cm-2 have been reached for the HER in alkaline media (1 M KOH), which demonstrates a high activity. The high electrical conductivity together with the mechanical stability of the free-standing CNF films allowed their direct use as cathodes in a MEC reactor, resulting in an exceptionally low voltage operation (0.75 V) with a current density demand of 5.4 A m-2. This enabled the production of H2 with an energy consumption below 30 kWh kg-1 H2, which is highly efficient.

Surface-Engineered Ni2P: An Efficient Oxygen Electrocatalyst for Zinc-Air Battery

The surface engineering of electrocatalysts is one of the promising strategies to increase the intrinsic activity of electrocatalysts. It generates anion/cation vacancy defects and increases the electrochemically active surface area. We describe the surface engineering of Ni2P to favorably tune the bifunctional oxygen electrocatalytic activity and the development of a rechargeable zinc-air battery (ZAB). Ni2P encapsulated with N and P-dual doped carbon (Ni2P@NPC) is synthesized using a single-source precursor complex tris-(2,2'-bipyridine)nickel(II) bis(hexafluorophosphate). The surface engineering of the as-synthesized Ni2P@NPC catalyst is achieved by the controlled acid treatment at room temperature. The surface engineering removes the carbon debris and opens the pores, exfoliates the encapsulating carbon layer, increases the P-vacancy in the crystal lattice, and boosts the electrochemically active surface area. The surface-engineered catalyst exhibits enhanced bifunctional activity towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The electrocatalytically active sites of engineered catalysts are highly accessible for facilitated electron transfer kinetics. P-vacancy favors the facile formation of defect-rich OER active metal oxyhydroxide species. The rechargeable ZAB based on the engineered catalyst delivers a specific capacity of 770.25 mA h gZn -1, energy density of 692 Wh kgZn -1, and excellent charge-discharge cycling performance with negligible voltaic efficiency loss (0.6 %) after 100 h.

Single-Atomic Co-N4 Sites with CrCo Nanoparticles for Metal-Air Battery-Driven Hydrogen Evolution

Designing highly active and robust earth abundant trifunctional electrocatalysts for energy storage and conversion applications remain an enormous challenge. Herein, we report a trifunctional electrocatalyst (CrCo/CoN4@CNT-5), synthesized at low calcination temperature (550 °C), which consists of Co-N4 single atom and CrCo alloy nanoparticles and exhibits outstanding electrocatalytic performance for the hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction. The catalyst is able to deliver a current density of 10 mA cm-2 in an alkaline electrolytic cell at a very low cell voltage of ∼1.60 V. When the catalyst is equipped in a liquid rechargeable Zn-air battery, it endowed a high open-circuit voltage with excellent cycling durability and outperformed the commercial Pt/C+IrO2 catalytic system. Furthermore, the Zn-air battery powered self-driven water splitting system is displayed using CrCo/CoN4@CNT-5 as sole trifunctional catalyst, delivering a high H2 evolution rate of 168 μmol h-1. Theoretical calculations reveal synergistic interaction between Co-N4 active sites and CrCo nanoparticles, favoring the Gibbs free energy for H2 evolution. The presence of Cr not only enhances the H2O adsorption and dissociation but also tunes the electronic property of CrCo nanoparticles to provide optimized hydrogen binding capacity to Co-N4 sites, thus giving rise to accelerated H2 evolution kinetics. This work highlights the importance of the presence of small quantity of Cr in enhancing the electrocatalytic activity as well as robustness of single-atom catalyst and suggests the design of the multifunctional robust electrocatalysts for long-term H2 evolution application.

Surface Tailoring-Modulated Bifunctional Oxygen Electrocatalysis with CoP for Rechargeable Zn-Air Battery and Water Splitting

The transition metal phosphide (TMP)-based functional electrocatalysts are very promising for the development of electrochemical energy conversion and storage devices including rechargeable metal-air batteries and water electrolyzer. Tuning the electrocatalytic activity of TMPs is one of the vital steps to achieve the desired performance of these energy devices. Herein, we demonstrate the modulation of the bifunctional oxygen electrocatalytic activity of nitrogen-doped carbon-encapsulated CoP (CoP@NC) nanostructures by surface tailoring with ultralow amount (0.56 atomic %) of Ru nanoparticles (2.5 nm). The CoP at the core and the Ru nanoparticles on the shell have a facile charge transfer interaction with the encapsulating NC. The strong coupling of Ru with CoP@NC boosts the electrocatalytic performance toward oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions. The surface-tailored catalyst requires only 35 mV to deliver the benchmark current density of 10 mA·cm-2 for HER. A small potential gap of 620 mV between ORR and OER is achieved, making the catalyst highly suitable for the development of rechargeable zinc-air batteries (ZABs). The homemade ZAB delivers a specific capacity of 780 mA·hgZn-1 and peak power density of 175 mW·cm-2 with a very small voltaic efficiency loss (1.1%) after 300 cycles. The two-electrode water splitting cell (CoP@NC-Ru||CoP@NC-Ru) delivers remarkably low cell voltage of 1.47 V at the benchmark current density. Stable current density of 25 mA·cm-2 for 25 h without any significant change is achieved. Theoretical studies support the charge transfer interaction-induced enhanced electrocatalytic activity of the surface-tailored nanostructure.

Synergistic Promotion of Large-Current Water Splitting through Interfacial Engineering of Hierarchically Structured CoP-FeP Nanosheets with Rich P Vacancies

The development of hydrogen evolution reaction (HER) catalysts with high performance under large current density is still a challenge. Introducing P vacancies in heterostructure is an appealing strategy to enhance HER kinetics. This study investigates a CoP-FeP heterostructure catalyst with abundant P vacancies (Vp-CoP-FeP/NF) on nickel foam (NF), which was prepared using dipping and phosphating treatment. The optimized Vp-CoP-FeP catalyst exerted prominent HER catalytic capability, requiring an ultra-low overpotential (58 mV @ 10 mA cm-2 ) and displaying robust durability (50 h @ 200 mA cm-2 ) in 1.0 M KOH solution. Furthermore, the catalyst demonstrated superior overall water splitting activity as cathode, demanding only cell voltage of 1.76 V at 200 mA cm-2 , outperforming Pt/C/NF(-) || RuO2 /NF(+) . The catalyst's outstanding performance can be attributed to the hierarchical structure of porous nanosheets, abundant P vacancies, and synergistic effect between CoP and FeP components, which promote water dissociation and H* adsorption and desorption, thereby synergically accelerating HER kinetics and enhancing HER activity. This study demonstrates the potential of HER catalysts with phosphorus-rich vacancies that can work under industrial-scale current density, highlighting the importance of developing durable and efficient catalysts for hydrogen production.

Fabricating Ru Atom-Doped Novel FeP4/Fe2PO5 Heterogeneous Interface for Overall Water Splitting in Alkaline Environment

Developing bifunctional electrocatalysts with low-content noble metals and high activity and stability is crucial for water splitting. Herein, we reported a novel Ru doped FeP4/Fe2PO5 heterogeneous interface catalyst (Ru@FeP4/Fe2PO5) for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) by heat treatment coupling electrodeposition strategy. Experiments disclosed that Ru@FeP4/Fe2PO5 proclaimed excellent catalytic activity for the OER (249 mV@100 mA cm-2) and HER (49 mV@10 mA cm-2) in a 1 M KOH environment. More importantly, the mass activity and turnover frequency of Ru@FeP4/Fe2PO5 were 117 and 108 times higher than that of commercial RuO2 at an overpotential of 300 mV during the OER, respectively. In addition, the assembled Ru@FeP4/Fe2PO5 || Ru@FeP4/Fe2PO5 system could retain superior durability in a two-electrode system for 134 h at 300 mA cm-2. Further mechanism studies revealed that Ru atoms in Ru@FeP4/Fe2PO5 act in a key role for the excellent activity during water splitting because the electronic structure of Ru sites could be optimized by the interaction between Ru and Fe atoms at the interface to strengthen the adsorption of reaction intermediates. Besides, the introduction of Ru atoms could also enhance the charge transfer, which effectually accelerates the reaction kinetics. The strategy of anchoring Ru atom on novel heterostructure provides a promising path to boost the overall activity of electrocatalysts for water splitting.

FeNiCrCoMn High-Entropy Alloy Nanoparticles Loaded on Carbon Nanotubes as Bifunctional Oxygen Catalysts for Rechargeable Zinc-Air Batteries

An efficient and stable bifunctional oxygen catalyst is necessary to complete the application of the rechargeable zinc-air battery. Herein, an economical and convenient process was adopted to successfully coat high-entropy alloy Fe₁₂Ni₂₃Cr₁₀Co_55-xMn_x nanoparticles on carbon nanotubes (CNTs). In 0.1 M KOH solution, with a bifunctional oxygen overpotential (ΔE) of only 0.7 V, the catalyst Fe₁₂Ni₂₃Cr₁₀Co₃₀Mn₂₅/CNT exhibits excellent bifunctional oxygen catalytic performance, exceeding most catalysts reported so far. In addition, the air electrode assembled with this catalyst exhibits high specific capacity (760 mA h g^-1) and energy density (865.5 W h kg^-1) in a liquid zinc-air battery, with a long-term cycle stability over 256 h. The density functional theory calculation points out that changing the atomic ratio of Co/Mn can change the adsorption energy of the oxygen intermediate (*OOH), which allows the ORR catalytic process to be accelerated in the alkaline environment, thereby increasing the ORR catalytic activity. This article has important implications for the progress of commercially available bifunctional oxygen catalysts and their applications in zinc-air batteries.

In Situ Surface Restructuring of Amorphous Ni-Doped CoMo Phosphate-Based Three-Dimensional Networked Nanosheets: Highly Efficient and Durable Electrocatalyst for Overall Alkaline Water Splitting

Developing cost-efficient bifunctional electrocatalysts with high efficiency and durability for the production of green hydrogen and oxygen is a demanding and challenging research area. Due to their high earth abundance, transition metal-based electrocatalysts are alternatives to noble metal-based water splitting electrocatalysts. Herein, binder-free three-dimensional (3D) networked nanosheets of Ni-doped CoMo ternary phosphate (Pi) were prepared using a facile electrochemical synthetic strategy on flexible carbon cloth without any high-temperature heat treatment or complicated electrode fabrication. The optimized CoMoNiPi electrocatalyst delivers admirable hydrogen (η₁₀ = 96 mV) and oxygen (η₁₀ = 272 mV) evolution performances in 1.0 M KOH electrolyte. For overall water splitting in a two-electrode system, the present catalyst demands only 1.59 and 1.90 V to reach current densities of 10 and 100 mA/cm², respectively, which is lower than that of the Pt/C||RuO₂ couple (1.61 V @ 10 mA/cm², 2 V > @ 100 mA/cm²) and many other catalysts reported previously. Furthermore, the present catalyst delivers excellent long-term stability in a two-electrode system continuously over 100 h at a high current density of 100 mA/cm², exhibiting nearly 100% faradic efficiency. The unique 3D amorphous structure with high porosity, a high active surface area, and lower charge transfer resistance provides excellent overall water splitting. Notably, the amorphous structure of the present catalyst favors the in situ surface reconstruction during electrolysis and generates very stable surface-active sites capable of long-term performance. The present work provides a route for the preparation of multimetallic-Pi nanostructures for various electrode applications that are easy to prepare and have superior activity, high stability, and low cost.