FeCoP2 Nanoparticles Embedded in N and P Co-doped Hierarchically Porous Carbon for Efficient Electrocatalytic Water Splitting

Co-Fe-Mo Phosphides' Triphasic Heterostructure Loaded on Nitrogen-Doped Carbon Nanofibers by Electrospinning as Efficient Bifunctional Electrocatalysts for Overall Water Splitting

The rational design of efficient and stable bifunctional electrocatalysts for the hydrogen evolution reactions (HER) and oxygen evolution reactions (OER) poses a significant challenge in realizing environmentally friendly hydrogen production through electrocatalytic water splitting. The construction of heterostructure catalysts, coexisting of multiple components, represents a favorable approach for increasing active sites, modulating electronic structure, accelerating charge transfer, decreasing reaction energy barriers, and synergistically enhancing electrocatalytic performance. In this study, a triphasic metal phosphides' heterostructure among CoP, FeP, and MoP4 loaded on nitrogen-doped carbon nanofibers (labeled as CoP-FeP-MoP4@NC) was successfully synthesized through electrospinning and other subsequent steps as a bifunctional electrocatalyst material for water splitting. Benefiting from the strong interaction and synergistic effect among these components, CoP-FeP-MoP4@NC exhibits facile kinetics and high electrocatalytic activity under alkaline conditions with overpotentials (η) of 222 and 75 mV at a current density of 10 mA cm-2 for OER and HER, respectively, as well as a low cell voltage of 1.47 V at 10 mA cm-2 for overall water splitting. Moreover, the catalyst shows great long-term stability at a high current density of about 100 mA cm-2. The density functional theory calculations revealed that the CoP-FeP-MoP4 heterostructure can reduce the Gibbs free energy associated with the H2O dissociation and hydrogen adsorption during HER, as well as the rate-determining step for the OER, increase the electronic states near the Fermi level, and optimize the work function of the electrons, improving electrical conductivity and reaction capacity. This study presents an efficient and stable electrocatalytic material for water splitting, and the design concept provides insights for future rational construction of advanced electrocatalysts.

Hybrid Particle Size Template Method for Controllable Synthesis of Nitrogen-Doped Multilevel Porous Carbon as High-Rate Zn-Ion Hybrid Supercapacitor Cathode Materials

Achieving high rate performance without compromising energy density has always been a critical objective for zinc-ion hybrid supercapacitors (ZHSCs). The pore structure and surface properties of carbon cathode materials play a crucial role. We propose utilizing a hybrid particle size (20 and 40 nm) magnesium oxide templates to regulate the pore structure of nitrogen-doped porous carbon derived from the soybean isolate. The multilevel pore structure enhanced ion transport efficiency while also improving the utilization of micropores. Nitrogen doping and oxygen-containing functional groups enhanced the wettability of carbon materials with aqueous electrolytes and facilitated the chemisorption of Zn2+ on the carbon material surface. The nitrogen-doped multilevel porous carbon material (HT-NMPC-1/1) prepared with a 1 : 1 mass ratio of the two templates exhibited a specific capacity of 146.65 mAh g-1 at 0.2 A g-1. Moreover, the Swagelok cells assembled with HT-NMPC-1/1 and Zn foil achieved a high energy density of 121.5 W h kg-1, high power output of 166 W kg-1, and 93.09 % capacity retention after 8000 cycles at 2 A g-1. Therefore, HT-NMPC-1/1 is a highly promising candidate for ZHSCs cathode materials. Furthermore, the novel pore regulation strategy and straightforward preparation method offer valuable reference points for other porous carbon-based functional materials.

Arrayed metal phosphide heterostructure by Fe doping for robust overall water splitting

Transition metal phosphides (TMPs) show promise in water electrolysis due to their electronic structures, which activate hydrogen/oxygen reaction intermediates. However, TMPs face limitations in catalytic efficiency due to insufficient active sites, poor conductivity, and multiple intermediate steps in water electrolysis. Here, we synthesize a highly efficient bifunctional self-supported electrocatalyst, which consists of an N-doped carbon shell anchored on Fe-doped CoP/Co2P arrays on nickel foam (NC@Fe-CoxP/NF) using hydrothermal and phosphorization techniques. Experimental and theoretical results indicate that the modified morphology, with increased active site density and a tunable electronic structure induced by Fe doping in the CoP/Co2P heterostructure, leads to superior water electrolysis performance. The resulting NC@Fe0.1-CoP/Co2P/NF catalyst exhibits overpotentials of 122 mV for the hydrogen evolution reaction (HER) and 270 mV for the oxygen evolution reaction (OER) at 100 mA cm-2. Furthermore, using NC@Fe0.1-CoP/Co2P/NF as both the cathode and anode in an alkaline electrolyzer enables the cell system to achieve 100 mA cm-2 at a voltage of 1.70 V, while maintaining long-term catalytic durability. This work may pave the way for designing self-supported, highly efficient electrocatalysts for practical water electrolysis applications.

Recycling alkali lignin-derived biochar with adsorbed cadmium into cost-effective CdS/C photocatalyst for methylene blue removal

Cadmium (Cd)-enriched adsorbents wastes possess great environmental risk due to their large-scale accumulation and toxicity in the natural environment. Recycling spent Cd-enriched adsorbents into efficient catalysts for advanced applications could address the environmental issues and attain the carbon neutral goal. Herein, a facile strategy is developed for the first time to reutilize the alkali lignin (AL)-derived biochar (ALB) absorbed with Cd into cadmium sulphide (CdS)/C composite for the efficient methylene blue (MB) removal. The ALB is initially treated with Cd-containing solution, then the recycling ALB samples with adsorbed Cd are converted to the final CdS/C composite using NaS2 as the sulphurizing reagent for vulcanization reaction. The optimal ALB400 demonstrates a high adsorption capacity of 576.0 mg g-1 for Cd removal. Then the converted CdS/C composite shows an efficient MB removal efficiency of 94%. The photodegradation mechanism is mainly attributed to carbon components in the CdS/C composite as electron acceptor promoting the separation of photoelectrons/holes and slowing down the abrasion of CdS particles. The enhanced charge transfer and contact between the carrier and the active site thus improves the removal performance and reusability. This work not only develops a method for removing Cd from wastewater effectively and achieving the waste resource utilization but also further offers a significant guidance to use other kinds of spent heavy metal removal adsorbents for the construction of low-cost and high value-added functional materials.

Enhanced hydrogen evolution reaction activity through samarium-doped nickel phosphide (Ni2P) electrocatalyst

Hydrogen evolution reaction (HER) stands out among conventional hydrogen production processes by featuring excellent advantages. However, the uncompetitive production cost due to the low energy efficiency has hindered its development, necessitating the introduction of cost-effective electrocatalysts. In this study, we introduced samarium doping as a high-potential approach to improve the electrocatalytic properties of nickel phosphide (Ni2P) for efficient HER. Samarium-doped Ni2P was synthesized via a facile two-step vapor-solid reaction technique. Different physical and electrochemical analyses showed that samarium doping significantly improved pure Ni2P characteristics, such as particle size, specific surface area, electrochemical hydrogen adsorption, intrinsic activity, electrochemical active surface area, and charge transfer ability in favor of HER. Namely, Ni2P doped with 3%mol of samarium (Sm0.03Ni2P) with a Tafel slope of 67.8 mV/dec. and overpotential of 130.6 mV at a current density of 10 mA/cm2 in 1.0 M KOH solution exhibited a notable performance, suggesting Sm0.03Ni2P and samarium doping as a remarkable electrocatalyst and promising promoter for efficient HER process, respectively.

Rational design and construction of hierarchical porous quasi-hexagonal Co2P nanosheets/Co heterostructures as highly efficient bifunctional electrocatalysts for overall water splitting

Constructing heterostructured electrocatalysts has proven effective in enhancing intrinsic catalytic activity. Herein, under guidance of theoretical calculations, hierarchical porous quasi-hexagonal Co2P nanosheets/Co heterostructures supported on carbon cloth (Co2P/Co/CC) with a high surface area were rationally designed and elaborately constructed through electroless Co plating, electrochemical oxidation, and phosphidation process, which showed significant electrocatalytic performance toward water electrolysis. Specifically, theoretical calculations revealed that the Co2P/Co heterostructure adjusted the electronic structure of Co2P and Co, reducing the energy barrier for target reactions and thereby boosting electrocatalytic activities for the hydrogen evolution reaction (HER). Notably, the typical Co2P/Co/CC catalyst demonstrated impressive HER performance, with low overpotentials of only 52 and 48 mV to achieve a current density of 10 mA/cm2 in 0.5 M H2SO4 and 1.0 M KOH solutions, respectively. The remarkable electrocatalytic performance of the catalyst can be attributed to the improved intrinsic activity resulting from the Co2P/Co heterostructures and the highly exposed active sites provided by the hierarchical porous structures. Furthermore, the Co2P/Co/CC catalyst exhibited excellent oxygen evolution reaction (OER) performance in alkaline electrolyte, requiring a low overpotential of only 306 mV to achieve a current density of 100 mA/cm2. Additionally, a two-electrode electrolyzer assembled with the Co2P/Co/CC electrodes achieved a current density of 10 mA/cm2 at a low cell voltage of 1.54 V and demonstrated excellent long-term stability. This work presents a novel and feasible strategy for constructing hierarchical heterostructured electrocatalysts that enable efficient water electrolysis. By combining rational design and theoretical guidance, our approach offers promising prospects for advancing the field of electrocatalysis and facilitating sustainable energy conversion.

Next-Generation Green Hydrogen: Progress and Perspective from Electricity, Catalyst to Electrolyte in Electrocatalytic Water Splitting

Green hydrogen from electrolysis of water has attracted widespread attention as a renewable power source. Among several hydrogen production methods, it has become the most promising technology. However, there is no large-scale renewable hydrogen production system currently that can compete with conventional fossil fuel hydrogen production. Renewable energy electrocatalytic water splitting is an ideal production technology with environmental cleanliness protection and good hydrogen purity, which meet the requirements of future development. This review summarizes and introduces the current status of hydrogen production by water splitting from three aspects: electricity, catalyst and electrolyte. In particular, the present situation and the latest progress of the key sources of power, catalytic materials and electrolyzers for electrocatalytic water splitting are introduced. Finally, the problems of hydrogen generation from electrolytic water splitting and directions of next-generation green hydrogen in the future are discussed and outlooked. It is expected that this review will have an important impact on the field of hydrogen production from water.

Iron Active Center Coordination Reconstruction in Iron Carbide Modified on Porous Carbon for Superior Overall Water Splitting

In this work, a novel liquid nitrogen quenching strategy is engineered to fulfill iron active center coordination reconstruction within iron carbide (Fe3C) modified on biomass-derived nitrogen-doped porous carbon (NC) for initiating rapid hydrogen and oxygen evolution, where the chrysanthemum tea (elm seeds, corn leaves, and shaddock peel, etc.) is treated as biomass carbon source within Fe3C and NC. Moreover, the original thermodynamic stability is changed through the corresponding force generated by liquid nitrogen quenching and the phase transformation is induced with rich carbon vacancies with the increasing instantaneous temperature drop amplitude. Noteworthy, the optimizing intermediate absorption/desorption is achieved by new phases, Fe coordination, and carbon vacancies. The Fe3C/NC-550 (550 refers to quenching temperature) demonstrates outstanding overpotential for hydrogen evolution reaction (26.3 mV at -10 mA cm-2) and oxygen evolution reaction (281.4 mV at 10 mA cm-2), favorable overall water splitting activity (1.57 V at 10 mA cm-2). Density functional theory (DFT) calculations further confirm that liquid nitrogen quenching treatment can enhance the intrinsic electrocatalytic activity efficiently by optimizing the adsorption free energy of reaction intermediates. Overall, the above results authenticate that liquid nitrogen quenching strategy open up new possibilities for obtaining highly active electrocatalysts for the new generation of green energy conversion systems.

Carbon-Based Composites for Oxygen Evolution Reaction Electrocatalysts: Design, Fabrication, and Application

The four-electron oxidation process of the oxygen evolution reaction (OER) highly influences the performance of many green energy storage and conversion devices due to its sluggish kinetics. The fabrication of cost-effective OER electrocatalysts via a facile and green method is, hence, highly desirable. This review summarizes and discusses the recent progress in creating carbon-based materials for alkaline OER. The contents mainly focus on the design, fabrication, and application of carbon-based materials for alkaline OER, including metal-free carbon materials, carbon-based supported composites, and carbon-based material core-shell hybrids. The work presents references and suggestions for the rational design of highly efficient carbon-based OER materials.

Co-Adjusting d-Band Center of Fe to Accelerate Proton Coupling for Efficient Oxygen Electrocatalysis

The problem in d-band center modulation of transition metal-based catalysts for the rate-determining steps of oxygen conversion is an obstacle to boost the electrocatalytic activity by accelerating proton coupling. Herein, the Co doping to FeP is adopted to modify the d-band center of Fe. Optimized Fe sites accelerate the proton coupling of oxygen reduction reaction (ORR) on N-doped wood-derived carbon through promoting water dissociation. In situ generated Fe sites optimize the adsorption of oxygen-related intermediates of oxygen evolution reaction (OER) on CoFeP NPs. Superior catalytic activity toward ORR (half-wave potential of 0.88 V) and OER (overpotential of 300 mV at 10 mA cm-2) express an unprecedented level in carbon-based transition metal-phosphide catalysts. The liquid zinc-air battery presents an outstanding cycling stability of 800 h (2400 cycles). This research offers a newfangled perception on designing highly efficient carbon-based bifunctional catalysts for ORR and OER.

Recent progress in hierarchical nanostructures for Ni-based industrial-level OER catalysts

Exploring efficient and low-cost oxygen evolution reaction (OER) electrocatalysts reaching the industrial level current density is crucial for hydrogen production via water electrolysis. In this feature article, we summarize the recent progress in hierarchical nanostructures for the industrial-level OER. The contents mainly concern (i) the design of a hierarchical structure; (ii) a Ni-based hierarchical structure for the industrial current density OER; and (iii) the surface reconstruction of the hierarchical structure during the OER process. The work provides valuable guidance and insights for the manufacture of hierarchical nanomaterials and devices for industrial applications.

Waste biomass-derived N, P co-doping carbon aerogel-coated CoxFe1-xP with modulated electron density for efficient electrooxidation of contaminants

Developing low-cost, green, high-performing electrode materials to address environmental pollutants and the energy crisis is significant but challenging. Herein, the bimetallic iron cobalt phosphide coated in waste biomass-derived N, P co-doping carbon (CoxFe1-xP@NPC) is constructed. Furthermore, the active site density and the water decomposition energy barrier of surface-coated NPC are modulated by optimizing the electronic structure of CoxFe1-xP via doping engineering. The Fe-modulated CoxFe1-xP@NPC exhibits a hierarchical porous self-supporting structure and excellent physical & chemical properties with excellent electrooxidation performance, achieving over 95% removal of TCH within 60 min. The density functional theory (DFT) calculations further confirms that N carries more positive charge and P carries more negative charge in the NPC of CoxFe1-xP@NPC with Fe modulation, which can promote the adsorption and dissociation of water molecules. Of note, Co0.75Fe025P@NPC displays a low water dissociation energy barrier to produce ·OH and a high energy barrier to produce O2 than its counterparts. This study offers new insight into controllable modulation of biomass carbon-based composite electrode catalytic activity for high-efficiency degradation of contaminants.

Vanadium-induced synthesis of amorphous V-Co-P nanoparticles for an enhanced hydrogen evolution reaction

The hydrogen evolution reaction (HER) plays a vital role for the production of pure hydrogen with zero carbon release. Developing high efficiency non-noble metal electrocatalysts could reduce its cost. Here, vanadium doped cobalt phosphide grown on carbon cloth (CC) was synthesized by the low temperature electrodeposition-phosphorization method. The influence of V dopants on the structural, morphological, and electrocatalytic performance of V_x-Co_1-x-P composites was also investigated in-depth. Impressively, the optimized amorphous V_0.1-Co_0.9-P nano-electrocatalyst exhibits outstanding catalytic activity with a low overpotential of 50 mV at a current density of 10 mA cm^-2 and a small Tafel value of 48.5 mV dec^-1 in alkaline media. The results showed that V dopants in the composite change its crystal structure from the crystalline phase to the amorphous phase, resulting in the introduction of V-O sites, which regulate the electron density of the active sites and the exposure of surface active sites and thus promote the electrocatalytic HER process. This work provides a novel idea for the fabrication of high-efficiency metal phosphide based electrocatalysts.

Bio-inspired design of NiFeP nanoparticles embedded in (N,P) co-doped carbon for boosting overall water splitting

The design and synthesis of cost-effective and stable bifunctional electrocatalysts for water splitting via a green and sustainable fabrication way remain a challenging problem. Herein, a bio-inspired method was used to synthesize NiFeP nanoparticles embedded in (N,P) co-doped carbon with the added carbon nanotubes. The obtained Ni_0.8Fe_0.2P-C catalyst displayed excellent hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performances in both alkaline and alkaline simulated seawater solutions. The optimal Ni_0.8Fe_0.2P-C/NF only needs overpotentials of 45 and 242 mV to reach the current density of 10 mA cm^-2 under HER and OER working conditions in 1.0 M KOH solution, respectively. First-principles calculations revealed the presence of a strong interaction between the carbon layer and metal phosphide nanoparticles. Benefiting from this and carbon nanotubes modification, the fabricated Ni_0.8Fe_0.2P-C presents impressive stability, working continuously for 100 h without collapse. A low alkaline cell voltage of 1.56 V for the assembled Ni_0.8Fe_0.2P-C/NF//Ni_0.8Fe_0.2P-C/NF electrocatalyzer could afford a current density of 10 mA cm^-2. Moreover, when integrated with a photovoltaic device, the bifunctional Ni_0.8Fe_0.2P-C electrocatalyst demonstrates application potential for sustainable solar-driven water electrolysis.

Bioinspired and Bioderived Aqueous Electrocatalysis

The development of efficient and sustainable electrochemical systems able to provide clean-energy fuels and chemicals is one of the main current challenges of materials science and engineering. Over the last decades, significant advances have been made in the development of robust electrocatalysts for different reactions, with fundamental insights from both computational and experimental work. Some of the most promising systems in the literature are based on expensive and scarce platinum-group metals; however, natural enzymes show the highest per-site catalytic activities, while their active sites are based exclusively on earth-abundant metals. Additionally, natural biomass provides a valuable feedstock for producing advanced carbonaceous materials with porous hierarchical structures. Utilizing resources and design inspiration from nature can help create more sustainable and cost-effective strategies for manufacturing cost-effective, sustainable, and robust electrochemical materials and devices. This review spans from materials to device engineering; we initially discuss the design of carbon-based materials with bioinspired features (such as enzyme active sites), the utilization of biomass resources to construct tailored carbon materials, and their activity in aqueous electrocatalysis for water splitting, oxygen reduction, and CO2 reduction. We then delve in the applicability of bioinspired features in electrochemical devices, such as the engineering of bioinspired mass transport and electrode interfaces. Finally, we address remaining challenges, such as the stability of bioinspired active sites or the activity of metal-free carbon materials, and discuss new potential research directions that can open the gates to the implementation of bioinspired sustainable materials in electrochemical devices.

FeCoP nanosheets on NiO nanoparticles as electrocatalysts: tuning and stabilizing active sites for water splitting

Herein, we demonstrate a novel strategy for tailoring and stabilizing the interface of active sites on hierarchical three-dimensional (3D) iron-cobalt phosphide (Fe_1-xCo_xP) nanosheets on nickel oxide nanoparticles for overall water splitting. The developed bifunctional electrode required an overpotential of only ∼158 mV and ∼74 mV to attain 10 mA cm^-2 for oxygen evolution and hydrogen evolution reactions, respectively, with excellent durability over 100 h in 1.0 M KOH via engineering interfacial active sites, revealing the progress in the development of electrocatalytic activity.

Porous Polymer Materials for CO2 Capture and Electrocatalytic Reduction

Efficient capture of CO₂ and its conversion into other high value-added compounds by electrochemical methods is an effective way to reduce excess CO₂ in the atmosphere. Porous polymeric materials hold great promise for selective adsorption and electrocatalytic reduction of CO₂ due to their high specific surface area, tunable porosity, structural diversity, and chemical stability. Here, we review recent research advances in this field, including design of porous organic polymers (POPs), porous coordination polymers (PCPs), covalent organic frameworks (COFs), and functional nitrogen-containing polymers for capture and electrocatalytic reduction of CO₂. In addition, key issues and prospects for the optimal design of porous polymers for future development are elucidated. This review is expected to shed new light on the development of advanced porous polymer electrocatalysts for efficient CO₂ reduction.

Modulating the electronic structure of ternary transition metal phosphide for enhanced hydrogen evolution activity

Rationally designing ternary transition-metal phosphides (TMPs) for the hydrogen evolution reaction (HER) is desirable but remains a significant challenge. Herein, ternary FeCoNiP encapsulated in a porous carbon shell, coupled with N-doped carbon nanotubes (FeCoNiP@NCNTs) are synthesized via a simple pyrolysis-phosphatization strategy derived from FeCoNi-MOF-100@dicyandiamide. Because Co/Ni enters the FeP lattice, FeCoNiP@NCNTs show a favorable catalytic performance towards the HER with low overpotential values of 86.7 and 233.5 mV at 10 mA cm^-2 in acidic and alkaline media, respectively, surpassing the HER performance of FeP@NCNTs, FeCoP@NCNTs, and FeNiP@NCNTs. Impressively, FeCoNiP@NCNTs display adequate acid-resistance capacity during the HER process, with nearly negligible decay due to the thin graphitized carbon shell structure with a thickness of 11.5-20.3 nm. The results of experiments, structural characterization, and density functional theory (DFT) calculations demonstrate that Co/Ni co-doping can modulate the adsorption and dissociation processes of H⁺ and downshift the d-band center of FeP. This work proposes a strategy for fabricating ternary TMP catalysts with heterogeneous structures for the HER.

One-Dimensional CoMoP Nanostructures as Bifunctional Electrodes for Overall Water Splitting

As high-quality substitutes for conventional catalysts, the bifunctional catalytic properties of the coating of transition-metal-based materials are pivotal for improving water-splitting efficiency. Herein, cobalt-molybdenum bimetallic phosphide nanofibers (CoMoP NFs) were synthesized via a series of facile strategies, which are divided into pyrolysis electrospun PAN and metal salts, to obtain one-dimensional morphology and a gas-solid phosphating precursor. The obtained CoMoP NFs catalyst has superior catalytic activity performance in 1M KOH. Serving as an oxygen evolution reaction (OER) catalyst, the electrode of the CoMoP NFs affords different kinds of current densities at 50 mA cm^-2 and 100 mA cm^-2, with low overpotentials of 362 and 391 mV, respectively. In addition, the hydrogen evolution reaction (HER) performance of the CoMoP NFs mainly shows when under a low overpotential of 126 mV, which can deliver a current density of 10 mA cm^-2. In order to further detect the stability of the catalyst, we used multiple cyclic voltammetry and chronopotentiometry tests for OERs and HERs, which maintain performance and carry a current density of 10 mA cm^-2 for longer. As an integrated high-performance bifunctional electrode for overall water splitting, the CoMoP NFs only require 1.75 V@10 mA cm^-2 for 40 h. This work highlights a facile method to create an electrocatalyst with fiber nanostructures which possesses excellent activity as an alkaline electrolyte.

Heterogeneous Ni3P/Ni nanoparticles with optimized Ni active sites anchored in N-doped mesoporous nanofibers for boosting pH-universal hydrogen evolution

Developing low-cost, environmentally friendly and efficient non-precious metal electrocatalysts as alternatives to noble metals for the hydrogen evolution reaction (HER) is highly essential for the sustainable advancement of green hydrogen energy. Herein, a novel heterostructured Ni₃P/Ni nanoparticle anchored in nitrogen-doped mesoporous carbon nanofibers (Ni₃P/Ni@N-CNFs) is prepared by a facile solid-phase calcination protocol. The results demonstrated that benefiting from the intensive electronic coupling effect at the interface of the Ni₃P/Ni heterostructure, the electron configuration of the Ni active site is optimized and thus the favorable HER activity. Furthermore, the N-doped carbon nanofiber scaffold with an extensive mesoporous structure endows Ni₃P/Ni@N-CNFs with abundant electrochemically active sites together with excellent conductivity and stability, contributing to fast electron/mass transport. As expected, the resultant Ni₃P/Ni@N-CNF electrocatalyst exhibited exceptional HER catalytic properties under universal pH conditions, driving a current density of 10 mA cm^-2 at pretty low overpotentials of 121 mV, 145 mV and 187 mV in acidic, basic and neutral solutions, respectively, and retaining the catalytic stability for over 60 h. This intriguing work represents a fresh perspective for designing and exploiting highly advanced phosphide electrocatalysts for green hydrogen fuel production.

Hypercrosslinked polymer-mediated fabrication of binary metal phosphide decorated spherical carbon as an efficient and durable bifunctional electrocatalyst for rechargeable Zn-air batteries

Bifunctional oxygen catalysts with excellent catalytic activity and durability towards both oxygen reduction and oxygen evolution reactions (ORR/OER) are pivotal for long-term rechargeable Zn-air batteries. Herein, we report a spherical carbon decorated with FeP and CoP nanoparticles (denoted as FeCoP/NPC) as an ORR/OER bifunctional electrocatalyst for rechargeable Zn-air batteries. HCTCz@Fe/Co-PA is first produced by the modification of phytic acid (PA) onto (into) a porous cross-linked polymeric sphere of poly(bis(N-carbazolyl)-1,2,4,5-tetrazine) (HCTCz), followed by chelating with metal ions (i.e., Fe³⁺ and Co²⁺). The subsequent pyrolysis yields FeCoP/NPC, which shows prominent activity and reliability for the ORR and OER due primarily to the synergistic effect of FeP and CoP active sites and N/P co-doped carbon. The aqueous Zn-air battery assembled with FeCoP/NPC provides high specific capacity and peak power density. Notably, the constructed Zn-air battery can be repetitively charged and discharged for 1200 h at 5 mA cm^-2. In addition, a flexible solid-state Zn-air battery made from FeCoP/NPC exhibits a power density of 74 mW cm^-2 and repeatedly works for 90 h at 2 mA cm^-2. This work opens up an avenue for the preparation of highly efficient bifunctional electrocatalysts for Zn-air batteries considering the extensive N-rich polymer precursors and various metal phosphide nanoparticles.

The surface structure, stability, and catalytic performances toward O2 reduction of CoP and FeCoP2

The systematic atomistic level investigation of low-index surface structures, stabilities, and catalytic performances of CoP and FeCoP₂ towards the O₂ reduction reaction (ORR) is vital for their applications. Employing first-principles calculations, it is revealed that CoP and FeCoP₂ present the same surface stability in the order of (101) ≈ (011) > (111) > (001) > (110) > (010) > (100). They also possess a similar Wulff equilibrium crystal shape with (101) and (011) exposing the largest surface area. From the electronic view, FeCoP₂ presents improved electronic conductivity compared with CoP. From the energy view, whether FeCoP₂ delivers improved electrocatalytic activity toward the ORR with respect to CoP depends on the reactive surfaces and sites. Among the 4 surfaces considered, only CoP(101), FeCoP₂(101) and FeCoP₂(011) delivered ORR performances theoretically when the bridge metal-metal site acts as the reactive center, which makes CoP(011) the only exception. CoP(101)-b_Co-Co and FeCoP₂(011)-b_Fe-Co exhibit a larger thermodynamic limiting potential than FeCoP₂(101)-b_Co-Co, suggesting their higher performances toward the ORR. The last step of HO* desorption as the rate-limiting step accounts for 3/4. The third step of transformation from O* to HO* as the most sluggish step accounts for 1/4. The work function, d-band center, Bader charge, and electronic localization function calculations are performed to reveal the HO adsorption nature. The present work provides fundamental insight into the effect of Fe doping into CoP, the determination of the catalyst surface and the key species adsorption nature to guide the rational design of high-performance materials.

Janus bimetallic materials as efficient electrocatalysts for hydrogen oxidation and evolution reactions

The development of hydrogen energy is limited by the high cost of platinum group metals (PGM). There is an urgent need to design efficient PGM-free electrocatalysts in the hydrogen electrode. Herein, Janus Ni/W bimetallic materials are proposed as an effective PGM-free bifunctional hydrogen electrocatalyst. By constructing the bimetallic materials, a synergistic effect is realized to enhance the reaction kinetics and improve the catalytic performance. In general, Ni can provide excellent H_ad sites, and W serves as OH_ad sites. Therefore, the synergistic effect of Ni and W can improve the kinetics of hydrogen evolution reaction and the hydroxide oxidation reaction. Ni/W@NF can obtain the hydrogen evolution reaction current density of 10 mA cm^-2 with an overpotential of only 62.6 mV, and the exchange current density of hydroxide oxidation reaction can reach 1.83 mA cm^-2. This work provides a new idea for the design of high-efficiency and low-cost PGM-free bifunctional hydrogen electrocatalysts.

Tiny Ni Nanoparticles Embedded in Boron- and Nitrogen-Codoped Porous Carbon Nanowires for High-Efficiency Water Splitting

The integration of nickel (Ni) nanoparticle (NP)-embedded carbon layers (Ni@C) into the three-dimensional (3D) hierarchically porous carbon architectures, where ultrahigh boron (B) and nitrogen (N) doping is a potential methodology for boosting Ni catalysts' water splitting performances, was achieved. In this study, the novel 3D ultrafine Ni NP-embedded and B- and N-codoped hierarchically porous carbon nanowires (denoted as Ni@BNPCFs) were successfully synthesized via pyrolysis of the corresponding 3D nickel acetate [Ni(AC)₂·4H₂O]-hydroxybenzeneboronic acid-polyvinylpyrrolidone precursor networks woven by electrospinning. After optimizing the pyrolysis temperatures, various structural and morphological characterization analyses indicate that the optimal Ni@BNPCFs-900 networks own a large surface area, abundant micro/mesopores, and vast carbon edges/defects, which boost doping a large amount of B (5.81 atom %) and N (5.84 atom %) dopants into carbon frameworks with 6.36 atom % of BC₃, pyridinic-N (pyridinic-N-Ni), and graphitic-N active sites. Electrochemical measurements demonstrate that Ni@BNPCFs-900 reveals the best hydrogen evolution reaction (HER) and oxygen reduction reaction catalytic activities in an alkaline solution. The HER potential at 10 mA cm^-2 [E₁₀ = -164.2 mV vs reversible hydrogen electrode (RHE)] of the optimal Ni@BNPCFs-900 is just 96.2 mV more negative than that of the state-of-the-art 20 wt % Pt/C (E₁₀ = -68 mV vs RHE). In particular, the OER E₁₀ and Tafel slope of the optimal Ni@BNPCFs-900 (1.517 V vs RHE and 19.31 mV dec^-1) are much smaller than those of RuO₂ (1.557 V vs RHE and 64.03 mV dec^-1). For full water splitting, the catalytic current density achieves 10 mA cm^-2 at a low cell voltage of 1.584 V for the (-) Ni@BNPCFs-900||Ni@BNPCFs-900 (+) electrolysis cell, which is 10 mV smaller than that of the (-) 20 wt % Pt/C||RuO₂ (+) benchmark (1.594 V) under the same conditions. The synergistic effects of 3D hierarchically porous structures, advanced charge transport ability, and abundant active centers [such as Ni@BNC, BC₃, pyridinic-N (pyridinic-N-Ni), and graphitic-N] are responsible for the excellent water-splitting catalytic activity of the Ni@BNPCFs-900 networks. Especially, because of the remarkable structural and chemical stabilities of 3D hierarchically porous Ni@BNPCFs-900 networks, the (-) Ni@BNPCFs-900||Ni@BNPCFs-900 (+) water electrolysis cell displays an excellent stability.

Co-Fe-P Nanosheet Arrays as a Highly Synergistic and Efficient Electrocatalyst for Oxygen Evolution Reaction

The rational design and synthesis of highly efficient electrocatalysts for oxygen evolution reaction (OER) is of critical importance to the large-scale production of hydrogen by water electrolysis. Here, we develop a bimetallic, synergistic, and highly efficient Co-Fe-P electrocatalyst for OER, by selecting a two-dimensional metal-organic framework (MOF) of Co-ZIF-L as the precursor. The Co-Fe-P electrocatalyst features pronounced synergistic effects induced by notable electron transfer from Co to Fe, and a large electrochemical active surface area achieved by organizing the synergistic Co-Fe-P into hierarchical nanosheet arrays with disordered grain boundaries. Such features facilitate the generation of abundant and efficiently exposed Co³⁺ sites for electrocatalytic OER and thus enable Co-Fe-P to deliver excellent activity (overpotential and Tafel slope as low as 240 mV and 36 mV dec^-1, respectively, at a current density of 10 mA cm^-2 in 1.0 M KOH solution). The Co-Fe-P electrocatalyst also shows great durability by steadily working for up to 24 h. Our work thus provides new insight into the development of highly efficient electrocatalysts based on nanoscale and/or electronic structure engineering.

Nonmetallic Active Sites on Nickel Phosphide in Oxygen Evolution Reaction

Efficient and durable catalysts are crucial for the oxygen evolution reaction (OER). The discovery of the high OER catalytic activity in Ni12P5 has attracted a great deal of attention recently. Herein, the microscopic mechanism of OER on the surface of Ni12P5 is studied using density functional theory calculations (DFT) and ab initio molecular dynamics simulation (AIMD). Our results demonstrate that the H2O molecule is preferentially adsorbed on the P atom instead of on the Ni atom, indicating that the nonmetallic P atom is the active site of the OER reaction. AIMD simulations show that the dissociation of H from the H2O molecule takes place in steps; the hydrogen bond changes from Oa-H⋯Ob to Oa⋯H-Ob, then the hydrogen bond breaks and an H+ is dissociated. In the OER reaction on nickel phosphides, the rate-determining step is the formation of the OOH group and the overpotential of Ni12P5 is the lowest, thus showing enhanced catalytic activity over other nickel phosphides. Moreover, we found that the charge of Ni and P sites has a linear relationship with the adsorption energy of OH and O, which can be utilized to optimize the OER catalyst.

AlP-regulated phosphorus vacancies over Ni-P compounds promoting efficient and durable hydrogen generation in acidic media

Engineered anion vacancy catalysts exhibit speedy activity in the field of electrocatalysis due to their tunable electronic structure and moderate free energy of adsorbed intermediates. Herein, we demonstrate a facile process of preparing multiphase phosphides with abundant phosphorus vacancies (P_V) supported on nanoporous Ni(Al). X-ray diffraction (XRD), electron paramagnetic resonance (EPR) and high-resolution transmission electron microscopy (HRTEM) reveal that the as-obtained material has ample P_V induced by the AlP phase. The optimized catalyst also equips with aligned nanoflakes grown in situ on np-Ni(Al) skeletons/ligaments, thereby exposing a large specific surface area for hydrogen evolution reactions (HERs) in acidic media. Benefitting from its unique hierarchical structure and sufficient P_V, the P_V-np-Ni(Al)-40 electrode displays a low overpotential of 36 mV at a cathodic current density of 10 mA cm^-2 and an outstanding long-term operational stability for up to 94 h with a slight decay. Density functional theory (DFT) calculations confirm that P_V could induce the redistribution of electrons and significantly reduce the Gibbs free energy (ΔG_H*) of 2P_V-NiP₂ on the P site close to P_V (-0.055 eV). Moreover, the P_V is beneficial for enriching the electronic states nearby the Fermi level, thereby improving the conductivity of NiP₂ to achieve superior HER activity. This finding skillfully utilizes Al elements to not only create porous structures but also regulate the P_V concentration, opening up an accessible route to obtain P_Vvia dealloying-phosphorization, and boosting the development of high-performance HER electrocatalyst.

Porous organic polymers for electrocatalysis

Porous organic polymers (POPs) composed of organic building units linked via covalent bonds are a class of lightweight porous network materials with high surface areas, tuneable pores, and designable components and structures. Owing to their well-preserved characteristics in terms of structure and composition, POPs applied as electrocatalysts have shown promising activity and achieved considerable advances in numerous electrocatalytic reactions, including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO₂ reduction reaction, N₂ reduction reaction, nitrate/nitrite reduction reaction, nitrobenzene reduction reaction, hydrogen oxidation reaction, and benzyl alcohol oxidation reaction. Herein, we present a systematic overview of recent advances in the applications of POPs in these electrocatalytic reactions. The synthesis strategies, specific active sites, and catalytic mechanisms of POPs are summarized in this review. The fundamental principles of some electrocatalytic reactions are also concluded. We further discuss the current challenges of and perspectives on POPs for electrocatalytic applications. Meanwhile, the possible future directions are highlighted to afford guidelines for the development of efficient POP electrocatalysts.

3D nanoporous NiCoP as a highly efficient electrocatalyst for the hydrogen evolution reaction in alkaline electrolyte

3D nanoporous NiCoP properly inherits the dealloyed double-continuous nanoporous structure, enables fast charge transfer, and fully reflects its inherent activity.

MOF-Derived Cu3P nanoparticles Coated by N-doped carbon for Nitrogen Fixation

Electrochemical nitrogen reduction is a significant alternative route to synthesize ammonia, while constructing efficient catalysts for electrochemical nitrogen fixation still faces tough challenges. In this work, the Cu3P@NC (NC, nitrogen-doped...

Carbon-incorporated bimetallic phosphides nanospheres derived from MOFs as superior electrocatalysts for hydrogen evolution

Preparing low-cost and highly efficient electrocatalysts for hydrogen evolution reaction in a simple strategy is still facing challenges. In this work, we proposed a facile phosphating process to successfully transform...

Accurately manipulating hierarchical flower-like Fe2P@CoP@nitrogen-doped carbon spheres as an efficient carrier material of Pt-based catalyst

Fabrication of hierarchical porous catalysts with a large specific surface area and tunable architecture provides an effective strategy to promote the catalytic performance of Pt-based catalysts. Herein, we design and construct hierarchical flower-like Fe₂P@CoP@nitrogen-doped carbon (Fe₂P@CoP@NDC) through a facile method, and synthesize Pt/Fe₂P@CoP@NDC porous spheres via acid pickling and depositing of Pt NPs. The morphology of Fe₂P@CoP@NDC is precisely manipulated by controlling the synthesis conditions, including the reaction time and the addition of a protective agent, and the protective growth mechanism of the hierarchical flower-like Fe₂P@CoP@NDC spheres is mentioned. Significantly, the Pt/Fe₂P@CoP@NDC catalyst exhibits 3.29 and 2.36 times higher mass activity and specific activity than those of commercial Pt/C for methanol oxidation, respectively. Furthermore, its residual mass activity after 1000 cycles is 5.77 times as much as that of the commercial Pt/C catalyst in acidic electrolytes. Based on exploration of the reaction kinetics of the Pt/Fe₂P@CoP@NDC catalyst, the excellent catalytic activity and durability are attributed to the unique porous structure with relatively open area and enlarged specific surface area, which can promote fast electron transport and charge transfer, resulting in quick reaction kinetics. Moreover, metal phosphides can effectively accelerate the oxidative removal of intermediates, accordingly improving the catalytic activity. Therefore, the Pt/Fe₂P@CoP@NDC material with these compositional and structural features is expected to be a promising electrochemical catalyst.

Self-driven Ru-modified NiFe MOF nanosheet as multifunctional electrocatalyst for boosting water and urea electrolysis

Urea electro-oxidation reaction (UOR) has been a promising strategy to replace oxygen evolution reaction (OER) by urea-mediated water splitting for hydrogen production. Naturally, rational design of high-efficiency and multifunctional electrocatalyst towards UOR and hydrogen evolution reaction (HER) is of vital significance, but still a grand challenge. Herein, an innovative 3D Ru-modified NiFe metal-organic framework (MOF) nanoflake array on Ni foam (Ru-NiFe-x/NF) was elaborately designed via spontaneous galvanic replacement reaction (GRR). Notably, the adsorption capability of intermediate species (H*) of catalyst is significantly optimized by Ru modification. Meanwhile, rich high-valence Ni active species can be acquired by self-driven electronic reconstruction in the interface, then dramatically accelerating the electrolysis of water and urea. Remarkably, the optimized Ru-NiFe-③/NF (1.6 at% of Ru) only requires the overpotential of 90 and 310 mV to attain 100 mA cm^-2 toward HER and OER in alkaline electrolyte, respectively. Impressively, an ultralow voltage of 1.47 V is required for Ru-NiFe-③/NF to deliver a current density of 100 mA cm^-2 in urea-assisted electrolysis cell with superior stability, which is 190 mV lower than that of Pt/C-NF||RuO₂/NF couple. This work is desired to explore a facile way to exploit environmentally-friendly energy by coupling hydrogen evolution with urea-rich sewage disposal.