CoP nanosheets in-situ grown on N-doped graphene as an efficient and stable bifunctional electrocatalyst for hydrogen and oxygen evolution reactions

Interplay of Size and Magnetic Effects in Electrocatalytic Water Oxidation Activity of Sub-10 nm NiOx Supported Porous Hard-Carbons

This report describes a systematic approach for precise engineering of a catalyst-metal oxide interface through combining complementary approaches of chemical vapor deposition and atomic layer deposition. Specifically, Chemical Vapor Deposition (CVD) fabricated nanostructured hard-carbon framework (NCF) is employed as synergistic support for precise deposition of NiOx particles through Atomic Layer Deposition (ALD). The three variants of NCF-NiOx system (dimensions ranging from 3-12 nm, surface coverage ranging from 0.14 %-2 %) achieved exhibit unique electrocatalytic water oxidation activities, that are further strongly influenced by an external magnetic field (Hext). This confluence of size engineering and associated magnetic field effects interplay to produce the largest lowering in Rct at Hext=200 mT. A comprehensive analysis of electrocatalytic parameters including the Tafel slope and double layer capacitance establishes further insights on co-relation of size effect and magnetic properties to understand the role of nanocarbon supported transition metal oxides in water electrolysis.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Recent Advances in Carbon-Based Electrodes for Energy Storage and Conversion

Carbon-based nanomaterials, including graphene, fullerenes, and carbon nanotubes, are attracting significant attention as promising materials for next-generation energy storage and conversion applications. They possess unique physicochemical properties, such as structural stability and flexibility, high porosity, and tunable physicochemical features, which render them well suited in these hot research fields. Technological advances at atomic and electronic levels are crucial for developing more efficient and durable devices. This comprehensive review provides a state-of-the-art overview of these advanced carbon-based nanomaterials for various energy storage and conversion applications, focusing on supercapacitors, lithium as well as sodium-ion batteries, and hydrogen evolution reactions. Particular emphasis is placed on the strategies employed to enhance performance through nonmetallic elemental doping of N, B, S, and P in either individual doping or codoping, as well as structural modifications such as the creation of defect sites, edge functionalization, and inter-layer distance manipulation, aiming to provide the general guidelines for designing these devices by the above approaches to achieve optimal performance. Furthermore, this review delves into the challenges and future prospects for the advancement of carbon-based electrodes in energy storage and conversion.

Influence of Nitrogen-Doped Carbon Quantum Dots on the Electrocatalytic Performance of the CoP Nanoflower Catalyst for OER

Cobalt phosphides modified by nitrogen-doped carbon quantum dots (CoP-NCQDs) were successfully constructed by a facile and low-cost hydrothermal treatment, which is expected to replace traditional noble-metal oxygen evolution reaction electrode materials. Detailed experiments and findings show that nitrogen-doped carbon quantum dots (NCQDs) have a significant impact on the morphology of the CoP catalyst, and nitrogen doping can regulate the surface-active sites to obtain the catalyst with abundant structural defects. Simultaneously, nitrogen doping can regulate the content of pyridinic N and pyrrolic N, which exerts positive effects on the formation of the bond structure and electron conduction between NCQDs and CoP.

Synergistic effect of Mn doping and hollow structure boosting Mn-CoP/Co2P nanotubes as efficient bifunctional electrocatalyst for overall water splitting

The sluggish kinetic of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) severely hampers the commercial application of electrochemical water splitting, promoting the urgent exploration of high-efficient bifunctional electrocatalysts. Heteroatom doping and structure engineering have been identified as the most effective strategies to boost the catalytic activity of electrocatalysts. Herein, Mn doping and hollow structure were integrated in the design of Co-based transition metal phosphide catalyst to prepare Mn-CoP/Co₂P nanotubes (denoted as Mn-CP NTs) by a facile template-free method. Confirmed by characterization analysis, the introduced Mn species were in high dispersion in the regular CoP/Co₂P hollow tubular framework. Such a favorable design in composition and structure effectively boosted the catalytic activity of Mn-CP NTs toward electrochemical water splitting. The Mn-CP NTs showed superior HER and OER activity demonstrated by the low overpotentials of 82 mV (vs HER) and 309 mV (vs OER) at the current density of 10 mA cm^-2, as well as the satisfactory durability. When used as both cathode and anode in electrolyzer for overall water splitting, only a low cell voltage of 1.67 V was required for the Mn-CP NTs to drive 10 mA cm^-2, accompanied with excellent stability confirmed by over 50 h test.

Effect of cobalt phosphide (CoP) vacancies on its hydrogen evolution activity via water splitting: a theoretical study

Defect engineering plays an important role in improving the performance of catalysts. To clarify the roles of Co and P vacancies in CoP for water splitting, a theoretical study based on density functional theory was carried out in this paper. The geometric and electronic structures, activity and stability of the CoP (101)B surface, CoP (101)B with the Co vacancy (Co_vac) and the P vacancy (P_vac) are investigated. The results indicate that the CoP (101)B surface with P_vac and Co_vac can enhance the electron transfer to the surface. The P_vac will upward shift the Co d-band center near the vacancy site, which promotes the adsorption of H on the Co atom. As a result, the bridge Co-Co sites near the vacancy become the active sites for the hydrogen evolution reaction (HER) (ΔG_H* = 0.01 eV). The loss of the Co atom also results in an upward shift of its d-band center, which will enhance the H adsorption on the adjacent Co sites. The unevenly distributed electrons due to the presence of vacancies on the surface cause spontaneous dissociation of H₂O molecules. Furthermore, the thermodynamic analysis and surface energy find that the CoP (101)B and (101)B facets with Co_vac and P_vac present good stability. The current work has shed light onto the mechanism of water splitting on the surface of phosphide with vacancies. Our study suggests that engineering vacancies on CoP is a feasible route to improve its catalytic activity.

FeNi2P three-dimensional oriented nanosheet array bifunctional catalysts with better full water splitting performance than the full noble metal catalysts

The 3D (three-dimensional) oriented nanosheet array FeNi₂P electrocatalyst grown on carbon cloth (FeNi₂P/CC) is explored in this work. This unique 3D oriented nanosheet array structure can expose more catalytic active sites, promote the penetration of electrolyte solution on the catalyst surface, and facilitate the transfer of ions, thus speeding up the kinetic process of Hydrogen evolution reaction (HER) and Oxygen evolution reaction (OER). At the current densities of 10 mA/cm² in 1 M KOH solution, the HER overpotential (71 mV) of the FeNi₂P/CC self-supporting electrode is very close to that of noble metal HER catalyst of 20% Pt/C (54 mV), and its OER overpotential (210 mV) is 34% lower than that of the precious metal OER catalyst of RuO₂ (318 mV), demonstrating the excellent electrocatalytic performance of the FeNi₂P/CC catalyst. Moreover, the cell voltage for full water splitting (at 10 mA/cm² current densities) of the FeNi₂P/CC bifunctional electrode cell is 1.52 V, which is 3.8% lower than that of the full noble-metal electrode reference cell (RuO₂ || Pt/C, 1.58 V), suggesting that this FeNi₂P/CC bifunctional catalyst is likely to replace precious metals to reduce the costs in full water splitting application. According to density functional theory (DFT) calculation results, the introduction of iron atom can change the electronic structure of the Ni₂P, so it can reduce the adsorption energy of hydrogen and oxygen, and facilitate the adsorption and desorption of hydrogen and oxygen on the surface of the catalyst, improving its performance of HER and OER.

Neodymium-decorated graphene as an efficient electrocatalyst for hydrogen production

Rare earth (RE) materials such as neodymium (Nd) and others consist of unique electronic configurations which result in unique electronic, electrochemical, and photonic properties. The high temperature (>1100 °C) growth and low active surface areas of REs hinder their use as an efficient electrocatalyst. Herein, different morphologies of Nd were successfully fabricated in situ on the surface of graphene using a double-zone chemical vapor deposition (CVD) method. The morphology of the Nd material on graphene is controlled, which results in the significant enhancement of the large specific surface area and electrochemical active area of the composite material due to the spatial morphology of Nd, thereby improving the hydrogen evolution reaction (HER) performance in an alkaline medium. The significantly enhanced HER activity with an overpotential of 75 mV and a Tafel slope of 95 mV dec^-1 at a current density of 10 mA cm^-2 is observed in Nd-GF. Mainly, a high specific surface area of ∼2217 cm² g^-1 and the porosity of graphene play major roles in the enhancement of activity. Thus, the present work provides a new strategy for the neodymium engineering synthesis of efficient rare earth-graphene composite electrocatalysts with a high electrochemical active area.

Crystal and Electronic Structure Modification of Synthetic Perryite Minerals: A Facile Phase Transformation Strategy to Boost the Oxygen Evolution Reaction

Geometry effect and electronic effect are both essential for the rational design of a highly efficient electrocatalyst. In order to untangle the relationship between these effects and electrocatalytic activity, the perryite phase with a versatile chemical composition, (Ni_xFe_1-x)₈(T_yP_1-y)₃ (T = Si and Ge; 1 ≥ x, y ≥ 0), was selected as a platform to demonstrate the influence of geometry (e.g., atomic size and bond length) and electronic (e.g., bond strength and bonding scheme) factors toward the oxygen evolution reaction (OER). It was realized that the large Ge atom in the perryite phase can expand the unit cell parameters and interatomic distances (i.e., weaken bond strengths), which facilitates the phase transformation into active metal oxyhydroxide during OER. The quaternary perryite phase, Ni₇FeGeP₂, displays excellent OER activity and achieves current densities of 20 and 100 mA/cm² at overpotentials of 239 and 273 mV, respectively. The oxidation state of Ni and Fe in the perryite phase before/after OER was analyzed and discussed. The result suggests that incorporating the Fe element in the system may increase the rate constant of OER (K_OER) and therefore keeps the Ni element in a low valance state (i.e., Ni²⁺). This work indicates that the manipulation of geometry and electronic factors can promote phase transformation as well as OER activity, which exemplifies a strategy to design a promising "precatalyst" for OER.

Hollow CoP/FeP4 Heterostructural Nanorods Interwoven by CNT as a Highly Efficient Electrocatalyst for Oxygen Evolution Reactions

Electrolysis of water to produce hydrogen is crucial for developing sustainable clean energy and protecting the environment. However, because of the multi-electron transfer in the oxygen evolution reaction (OER) process, the kinetics of the reaction is seriously hindered. To address this issue, we designed and synthesized hollow CoP/FeP₄ heterostructural nanorods interwoven by carbon nanotubes (CoP/FeP₄@CNT) via a hydrothermal reaction and a phosphorization process. The CoP/FeP₄@CNT hybrid catalyst delivers prominent OER electrochemical performances: it displays a substantially smaller Tafel slope of 48.0 mV dec⁻¹ and a lower overpotential of 301 mV at 10 mA cm⁻², compared with an RuO₂ commercial catalyst; it also shows good stability over 20 h. The outstanding OER property is mainly attributed to the synergistic coupling between its unique CNT-interwoven hollow nanorod structure and the CoP/FeP₄ heterojunction, which can not only guarantee high conductivity and rich active sites, but also greatly facilitate the electron transfer, ion diffusion, and O₂ gas release and significantly enhance its electrocatalytic activity. This work offers a facile method to develop transition metal-based phosphide heterostructure electrocatalysts with a unique hierarchical nanostructure for high performance water oxidation.

MWCNTs-CoP hybrids for dual-signal electrochemical immunosensing of carcinoembryonic antigen based on overall water splitting

Great efforts have been made to search for highly active catalysts toward electrochemical water splitting, but double-signal immunosensors have not been reported based on bifunctional water splitting electrocatalysts. We report here a dual-signal electrochemical immunosensor for detecting carcinoembryonic antigen (CEA) using multi-wall carbon nanotubes (MWCNTs)-cobalt phosphide (CoP) as an electrocatalytic label. The preparation of MWCNTs-CoP involves the growth of Co₃O₄ nanoparticles on MWCNTs and low-temperature phosphatization of Co₃O₄ nanoparticles. The MWCNTs-CoP catalyst shows excellent electrocatalytic activities in a neutral medium toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), enabling MWCNTs-CoP as the electrocatalytic label for sensitive immunosensing. The linear range of the sandwich-type immunosensor for detecting CEA based on the HER signal is from 10^-4-100 ng mL^-1, whereas a linear range for detecting CEA based on the OER signal is achieved from 10^-4 to 10 ng mL^-1. The detection limits for detecting CEA using HER and OER signals are 10 and 12 fg mL^-1, respectively. This work can provide a new double-signal immunosensing platform based on a bifunctional water splitting electrocatalyst.

The in situ growth of Cu2O with a honeycomb structure on a roughed graphite paper for the efficient electroreduction of CO2 to C2H4

A Cu2O/NGP self-supporting electrocatalyst is used for the electrocatalytic reduction of CO2 to ethylene to solve environmental and energy problems.

Achieving the robust immobilization of CoP nanoparticles in cellulose nanofiber network-derived carbon via chemical bonding for a stable potassium ion storage

Potassium-ion batteries (KIBs) are currently being investigated as a potential alternative to lithium-ion batteries (LIBs) because of the natural abundance of K resources. Presently, it is crucial yet challenging to explore suitable anode materials for stable K-storage. Herein, a novel robust CoP-carbon composite with highly dispersed CoP nanoparticles (NPs) immobilized in natural cellulose nanofiber network (CNF)-derived carbon (denoted as CoP@CNFC) is synthesized via chemical bonding through a facile hydrothermal and subsequent in situ phosphidation approach. The designed structure can provide diverse merits, including fast reaction kinetics, sufficient active sites and effective accommodation for K⁺ insertion/extraction; thus, CoP@CNFC delivers desired electrochemical performance, including considerable reversible capacity, enhanced rate capability and excellent cycling stability. Additionally, the electrochemical reaction mechanism of CoP@CNFC was clearly revealed by ex situ characterizations and theoretical simulations of cyclic voltammetry (CV) and solid electrolyte interface (SEI) profiles based on first-principles calculations. The achieved deep elucidation of the reversible process of K⁺ insertion and extraction on the surface/interface of the active material during the discharge and charge states clearly highlights its significance for stable K-storage. This work promotes the facile design and deep understanding of nanostructured high-capacity electrodes of transition metal phosphates for rechargeable KIBs.

Noble-Metal-Free Doped Carbon Nanomaterial Electrocatalysts

Electrocatalytic processes, such as oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and carbon dioxide reduction reaction (CO₂ RR), play key roles in various sustainable energy storage and production devices and their optimization in an ecological manner is of paramount importance for mankind. In this inclusive Review, we aspire to set the scene on doped carbon-based nanomaterials and their hybrids as precious-metal alternative electrocatalysts for these critical reactions in order for the research community not only to stay up-to-date, but also to get inspired and keep pushing forward towards their practical application in energy conversion.

Characterization and Electrochemical Behaviour of Nanoscale Hydrotalcite-Like Compounds toward the Reduction of Nitrate

In this research, nano Cu/Al–HTLCs, Co/Al–HTLCs and Cu/Co/Al–HTLCs were synthesized, characterized, and tested in electrolytic reduction nitrate. Experimental results showed that Cu/Al–HTLCs were less stable than Co/Al–HTLCs due to the Jahn–Teller effect. However, electrocatalytic activity of copper was superior to that of cobalt; thus, Cu/Co/Al–HTLCs were selected based on their stable crystalline structure and electrochemical activity. The optimized Cu₂CoAl–HTLC was highly active in nitrate reduction, with two peaks for nitrate and nitrite reduction, respectively. Ammonia, nitrite and N-containing gases were found to be the final products of constant potential electrolysis at −0.54 and −0.74 V.

Synthesis of an Ultrafine CoP Nanocrystal/Graphene Sandwiched Structure for Efficient Overall Water Splitting

A CoP/graphene composite was synthesized through a coprecipitation and in situ phosphorization protocol using α-Co(OH)₂ and graphene oxide as precursors. The similar two-dimensional layered structures ensured evenly attached α-Co(OH)₂ nanosheets on the graphene oxide support and the formation of a sandwich-like structure. The sequential in situ phosphorization strategy not only generated a high density of ultrafine CoP nanocrystals but also simultaneously reduced the graphene oxide support. The enough exposed active sites combined with a highly conductive matrix resulted in an excellent electrochemical catalyst for overall water splitting. The overpotential is only 125 mV at 10 mA·cm² in 0.5 M H₂SO₄. Good electrocatalytic performance was also exhibited in alkaline conditions for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The overpotential is 119 mV for HER and 374 mV for OER at 10 mA·cm² in 1 M KOH. More importantly, the composite exhibited much higher exchange current densities during HER processes (1.64 × 10^-4 A·cm^-2 in 0.5 M H₂SO₄ and 2.93 × 10^-4 A·cm^-2 in 1 M KOH) when compared with similar materials reported before. This low-cost, simple, and efficient approach is suitable for mass production and practical applications.

FeNi3-modified Fe2O3/NiO/MoO2 heterogeneous nanoparticles immobilized on N, P co-doped CNT as an efficient and stable electrocatalyst for water oxidation

As a rate-determining step, electrocatalytic water oxidation acts a pivotal role in the water splitting process. As a consequence, it is of great significance to explore low-cost, efficient and durable electrocatalysts for the oxygen evolution reaction (OER) to promote electrocatalytic splitting water. Herein, for the first time, FeNi₃-modified Fe₂O₃/NiO/MoO₂ heterogeneous nanoparticles immobilized on N, P co-doped CNT matrix materials (FNM/NPCNT) are synthesized via a facile solid-phase grinding of the precursor, composed of nickel hexacyanoferrate/phosphomolybdic acid/CNT, and subsequently pyrolyzing under nitrogen atmosphere without any further post-processing. Due to its significant enhancement of the charge transfer efficiency and prevention of the metallic-based catalysts from being corroded, the as-prepared FNM/NPCNT hybrid electrocatalyst shows a high OER activity with a low overpotential of 282 mV vs. RHE at 10 mA cm^-2 and a small Tafel slope of 46.2 mV dec^-1 in an alkaline electrolyte. Moreover, the as-prepared FNM/NPCNT hybrid delivers a large mass activity of 327.6 A g^-1 at the potential of 1.7 V and excellent stability (more than 20 h). This study opens up a new approach to design and synthesize non-precious transition metal-based composites immobilized N, P co-doped CNT materials as OER catalysts with high efficiency and long-term stability for promoting water splitting.

One-Step Synthesis of N, P-Codoped Carbon Nanosheets Encapsulated CoP Particles for Highly Efficient Oxygen Evolution Reaction

Oxygen electrocatalysis, especially oxygen evolution reaction (OER), is a central process during the actual application of rechargeable metal-air battery. It is still challenging to develop ideal electrocatalysts to substitute the commercial noble metal-based materials. In this work, we have constructed a new material, CoP nanoparticles, which are encapsulated by a biomolecule-derived N, P-codoped carbon nanosheets via a simple and facile one-step strategy. The as-prepared material releases a high electrocatalytic activity and stability for OER, with an overpotential of 310 mV to achieve 10 mA/cm2 in 1 M KOH. Importantly, we found that the phosphoric acid can not only introduce phosphorus dopant into 2D N-doped carbon nanosheets and play a role of pore-forming agent, but also participate in the formation of active center (cobalt phosphide). Moreover, the coverage of N, P-doped carbon can prevent the CoP nanoparticles from corrosion under the harsh reaction medium to achieve high and stable activity. We believe that our strategy can offer a novel pathway to synthesize new transition metal-based catalysts for electrocatalysis or other heterogeneous catalysis.

In situ growth of urchin-like cobalt–chromium phosphide on 3D graphene foam for efficient overall water splitting

The CoCr-P@3DGF composite shows excellent activity and durability for overall water splitting due to Cr-doping and the 3DGF substrate.

3D hollow Co-Fe-P nanoframes immobilized on N,P-doped CNT as an efficient electrocatalyst for overall water splitting

The rational design of nonprecious and high-efficiency bifunctional electrocatalysts with advanced structural and compositional preponderance for water electrolysis is of paramount importance for the generation of sustainable and clean energy. Herein, for the first time, a novel three-dimensional (3D) hollow hybrid electrocatalyst, Co-Fe-P nanoframe immobilized on N,P-doped carbon nanotubes (CoFeP NFs/NPCNT), was synthesized by selectively etching a CNT-composited Co,Fe-based Prussian blue analogue and subsequent phosphorization. Benefiting from its unique 3D hollow nanoarchitecture, which offers rich porosity and abundant catalytically active sites and guarantees excellent conductivity and structural stability, the hollow CoFeP NFs/NPCNT hybrid delivered pronounced catalytic activity for oxygen evolution (or hydrogen evolution) in alkaline electrolyte, with a low overpotential of 278 (or 132) mV at 10 mA cm-2, small Tafel slope of 39.5 (or 62.9) mV dec-1 and prominent long-term stability. Therefore, when CoFeP NFs/NPCNT was employed as the cathode and anode toward overall water-splitting, it required a quite small cell voltage of only 1.56 V to afford a current density of 10 mA cm-2, and displayed outstanding electrocatalytic stability over 60 h, greatly approaching the performance of the commercial Pt/C(-)//RuO2(+) electrolyzer and outperforming most other non-noble-based electrolyzers.

Hierarchically Porous W-Doped CoP Nanoflake Arrays as Highly Efficient and Stable Electrocatalyst for pH-Universal Hydrogen Evolution

It is still challenging to develop high-efficiency and low-cost non-noble metal-based electrocatalysts for hydrogen evolution reaction (HER) in pH-universal electrolytes. Herein, hierarchically porous W-doped CoP nanoflake arrays on carbon cloth (W-CoP NAs/CC) are synthesized via facile liquid-phase reactions and a subsequent phosphorization process. The W-CoP NAs/CC hybrid can be directly employed as a binder-free electrocatalyst and delivers superior HER performance in pH-universal electrolytes. Especially, it delivers very low overpotentials of 89, 94, and 102 mV to reach a current density of 10 mA cm^-2 in acidic, alkaline, and neutral electrolytes, respectively. Furthermore, it shows a nearly 100% Faradaic efficiency as well as superior long-term stability with no decreasing up to 36 h in pH-universal electrolytes. The outstanding electrocatalytic performance of W-CoP NAs/CC can be mainly attributed to the porous W-doped nanoflake arrays, which not only afford rich exposed active sites, but also accelerate the access of electrolytes and the diffusion of H₂ bubbles, thus efficiently promoting the HER performance. This work provides a new horizon to rationally design and synthesize highly effective and stable non-noble metal phosphide-based pH-universal electrocatalysts for HER.