Enhancing Hydrogen Evolution Electrocatalytic Performance in Neutral Media via Nitrogen and Iron Phosphide Interactions

Identification of true active sites in N-doped carbon-supported Fe2P nanoparticles toward oxygen reduction reaction

Transition metal-based nanoparticles (NPs) are emerging as potential alternatives to platinum for catalyzing the oxygen reduction reaction (ORR) in zinc-air batteries (ZAB). However, the simultaneous coexistence of single-atom moieties in the preparation of NPs is inevitable, and the structural complexity of catalysts poses a great challenge to identifying the true active site. Herein, by employing in situ and ex situ XAS analysis, we demonstrate the coexistence of single-atom moieties and iron phosphide NPs in the N, P co-doped porous carbon (in short, Fe-N4-Fe2P NPs/NPC), and identify that ORR predominantly proceeds via the atomic-dispersed Fe-N4 sites, while the presence of Fe2P NPs exerts an inhibitory effect by decreasing the site utilization and impeding mass transfer of reactants. The single-atom catalyst Fe-N4/NPC displays a half-wave potential of 0.873 V, surpassing both Fe-N4-Fe2P NPs/NPC (0.858 V) and commercial Pt/C (0.842 V) in alkaline condition. In addition, the ZAB based on Fe-N4/NPC achieves a peak power density of 140.3 mW cm-2, outperforming that of Pt/C-based ZAB (91.8 mW cm-2) and exhibits excellent long-term stability. This study provides insight into the identification of true active sites of supported ORR catalysts and offers an approach for developing highly efficient, nonprecious metal-based catalysts for high-energy-density metal-air batteries.

Double loading of nickel phosphide surface for efficient hydrogen evolution reaction

Metal phosphide, as a highly conductive, chemically stable catalyst material, modulating its hydrogen adsorption is crucial to enhance hydrogen evolution reaction (HER) activity. In this study, we propose a double loading strategy to build Ag and AgP2 heterogeneous structures on Ni2P nanosheets (Ag-AgP2/Ni2P). This is the first application of AgP2 materials in HER. This innovative synthesis was achieved by liquid-phase adsorption of precursors and heat-treatment phosphorization, surface adsorbed AgNO3 is converted to Ag-AgP2 double loading at the same time as Ni2P formation. Density functional theory (DFT) calculations reveal that the double loading structure optimizes charge distribution and d-band center. Its hydrogen adsorption free energy is closer to electroneutrality than that of single loading and simple heterostructures. Benefiting from the special structure, Ag-AgP2/Ni2P exhibits excellent HER performance in alkaline media, requiring only 78 mV overpotential to reach 10 mA cm-2 and stability up to 200 h. This dual loading strategy broadens the perspective of heterogeneous electrocatalyst development.

Interfacial Regulation of Rice-Grain-like Iron-Nickel Phosphide Nanorods on Phosphorus-Doped Graphene Architectures as Bifunctional Electrocatalysts for Water Splitting

The design of bimetallic metal-organic frameworks (MOFs) with a hierarchical structure is important to improve the electrocatalytic performance of catalysts due to their synergistic effect on different metal ions. In this work, the catalyst comprises bimetallic iron-nickel MOF-derived FeNi phosphides, intricately integrated with phosphorus-doped reduced graphene oxide architectures (FeNi2P-C/P-rGA) through the hydrothermal and phosphating treatments. The hierarchical architecture of the catalyst is beneficial for exposing active sites and facilitating electron transfer. The FeNi2P-C/P-rGA catalyst exhibits excellent performance in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline electrolytes. Notably, FeNi2P-C/P-rGA requires only the overpotential of 93 and 210 mV to achieve a current density of 10 mA cm-2 for the HER and OER with small values of Tafel slope and charge transfer resistance, respectively. Furthermore, the catalyst exhibits boosted activity for overall water splitting with a low potential of 1.56 V. This work can be considered to extend the design of multilevel catalysts in the application of water splitting.

Hydrogen Evolution in Neutral Media by Differential Intermediate Binding at Charge-Modulated Sites of a Bimetallic Alloy Electrocatalyst

The energy barrier to dissociate neutral water has been lowered by the differential intermediate binding on the charge-modulated metal centers of Co85Mo15 sheets supported on Ni-foam (NF), where the overpotential for hydrogen evolution reaction (HER) in 1 M phosphate buffer solution (PBS) is only 50±9 mV at -10 mA cm-2. It has a turnover frequency (TOF) of 0.18 s-1, mass activity of 13.2 A g-1 at -200 mV vs. reversible hydrogen electrode (RHE), and produces 16 ml H2 h-1 at -300 mV vs. RHE, more than double that of 20 % Pt/C. The Moδ+ and Coδ- sites adsorb OH*, and H*, respectively, and the electron injection from Co to H-O-H cleaves the O-H bond to form the Mo-OH* intermediate. Operando spectral analyses indicate a weak H-bonded network for facilitating the H2O*/OH* transport, and a potential-induced reversal of the charge density from Co to the more electronegative Mo, because of the electron withdrawing Co-H* and Mo-OH* species. Co85Mo15/NF can also drive the complete electrolysis of neutral water at only 1.73 V (10 mA cm-2). In alkaline, and acidic media, it demonstrates a Pt-like HER activity, accomplishing -1000 mA cm-2 at overpotentials of 161±7, and 175±22 mV, respectively.

Shape-Preserved CoFeNi-MOF/NF Exhibiting Superior Performance for Overall Water Splitting across Alkaline and Neutral Conditions

This study reported a multi-functional Co0.45Fe0.45Ni0.9-MOF/NF catalyst for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting, which was synthesized via a novel shape-preserving two-step hydrothermal method. The resulting bowknot flake structure on NF enhanced the exposure of active sites, fostering a superior electrocatalytic surface, and the synergistic effect between Co, Fe, and Ni enhanced the catalytic activity of the active site. In an alkaline environment, the catalyst exhibited impressive overpotentials of 244 mV and 287 mV at current densities of 50 mA cm-2 and 100 mA cm-2, respectively. Transitioning to a neutral environment, an overpotential of 505 mV at a current density of 10 mA cm-2 was achieved with the same catalyst, showing a superior property compared to similar catalysts. Furthermore, it was demonstrated that Co0.45Fe0.45Ni0.9-MOF/NF shows versatility as a bifunctional catalyst, excelling in both OER and HER, as well as overall water splitting. The innovative shape-preserving synthesis method presented in this study offers a facile method to develop an efficient electrocatalyst for OER under both alkaline and neutral conditions, which makes it a promising catalyst for hydrogen production by water splitting.

Nickel-Cobalt Hydrogen Phosphate on Nickel Nitride Supported on Nickel Foam for Alkaline Seawater Electrolysis

Developing high-performance non-noble bifunctional catalysts is pivotal for large-scale seawater electrolysis but remains a challenge. Here we report a sandwichlike NiCo(HPO₄)₂@Ni₃N/NF (denoted by NiCoHPi@Ni₃N/NF) catalyst. Vertical Ni₃N nanosheet arrays are first grown and supported on nickel foam, and then a bimetallic NiCoHPi coating is decorated on Ni₃N nanosheets by one-step electrodeposition. The hierarchical sandwich like structure offers a large surface area and plenty of catalytic active sites, and the coupling of interconnected Ni₃N and NiCoHPi accelerates the electron transfer. Moreover, the surficial hydrogen phosphate ions contribute to a proper OH^- absorption capacity due to the Lewis acid-base reaction. As a result, the NiCoHPi@Ni₃N/NF catalyst exhibits good OER and HER activity, requiring overpotentials of 365 mV (for OER) and 174 mV (for HER) to deliver 100 mA cm^-2 in the alkaline simulated seawater electrolyte. When assembled the NiCoHPi@Ni₃N/NF catalyst as both the anode and cathode, it only needs 1.86 V to reach 100 mA cm^-2 in alkaline simulated seawater electrolyte. This work may inspire the design and exploration of self-supported hierarchical composite electrocatalysts for hydrogen production from the electrolysis of seawater.

Recent Advances in Design of Electrocatalysts for High-Current-Density Water Splitting

Electrochemical water splitting technology for producing "green hydrogen" is important for the global mission of carbon neutrality. Electrocatalysts with decent performance at high current densities play a central role in the industrial implementation of this technology. This field has advanced immensely in recent years, as witnessed by many types of catalysts designed and synthesized toward industriallyrelevant current densities (>200 mA cm^-2 ). By discussing recent advances in this field, several key aspects are summarized that affect the catalytic performance for high-current-density electrocatalysis, including dimensionality of catalysts, surface chemistry, electron transport path, morphology, and catalyst-electrolyte interplay. The multiscale design strategy that considers these aspects comprehensively for developing high-current-density electrocatalysts are highlighted. The perspectives on the future directions in this emerging field are also put forward.

Priority Occupation of C-Sites by N-Confining P-Implantation in Pyrrodic N-Sites in NCNT@P,N-Mo2C for Highly Efficient Electrocatalytic Hydrogen Evolution

Optimizing the electronic configuration of Mo₂C by activating heteroatom(s)-neighboring carbon atoms to enhance the activity of hydrogen evolution reaction (HER) has been demonstrated. However, the development of heteroatom-doped Mo₂C to fabricate a water electrolyzer is still a challenge because of the limitation of a well-defined electronic structure of hybridization of Mo with heteroatom(s). Here, nitrogen (N) and phosphor (P) codoped Mo₂C embedded carbon nanotubes (NCNT@P,N-Mo₂C) with the priority occupation of C-sites by N, which well confines the P-implantation at the pyrrodic N-sites and brings out N-O bonding on the surface, which favorably modifies the electronic configuration of adjacent Mo, resulting in highly efficient pH-tolerant HER activity. This study not only presents a potential HER electrocatalyst candidate but also provides a strategy for the construction of a well-defined electronic structure of heteroatom(s)-neighboring carbon-based materials.

Interfacial Engineering and a Low-Crystalline Strategy for High-Performance Supercapacitor Negative Electrodes: Fe2P2O7 Nanoplates Anchored on N/P Co-doped Graphene Nanotubes

Developing novel hybrid negative electrode materials with high specific capacity, rate capacitance, and long-term cycle stability is a key factor for pushing large-scale application of supercapacitors. However, construction of robust interfaces and low-crystalline active materials plays a crucial role in realizing the target. In this paper, a one-step phosphorization approach was employed to make low-crystalline Fe₂P₂O₇ nanoplates closely bonded to N/P-co-doped graphene nanotubes (N/P-GNTs@b-Fe₂P₂O₇) through interfacial chemical bonding. The N and P heteroatoms as substitutions for C in GNT skeletons can introduce rich electronic centers, which induces Fe₂P₂O₇ to fix the surface of N/P-GNTs through Fe-N and Fe-P bonds as confirmed by the characterizations. Moreover, the low-crystalline active materials own a disordered internal structure and numerous defects, which not only endows with excellent conductivity but also provides many active sites for redox reactions. Benefiting from the synergistic effects, the prepared N/P-GNTs@b-Fe₂P₂O₇ can not only deliver a high capacity of 257 mA h g^-1 (927 F g^-1) at 1 A g^-1 but also present an excellent rate capability of 184 mA h g^-1 (665 F g^-1) at 50 A g^-1 and outstanding cycle stability (∼90.6% capacity retention over 40,000 cycles). Furthermore, an asymmetric supercapacitor was assembled using the obtained N/P-GNTs@b-Fe₂P₂O₇ as electrode materials, which can present the energy density as high as 83.3 W h kg^-1 at 791 W kg^-1 and long-term durability. Therefore, this strategy not only offers an effective pathway for achieving high-performance negative electrode materials but also lays a foundation for further industrialization.

Organic/Inorganic Hybrid Fibers: Controllable Architectures for Electrochemical Energy Applications

Organic/inorganic hybrid fibers (OIHFs) are intriguing materials, possessing an intrinsic high specific surface area and flexibility coupled to unique anisotropic properties, diverse chemical compositions, and controllable hybrid architectures. During the last decade, advanced OIHFs with exceptional properties for electrochemical energy applications, including possessing interconnected networks, abundant active sites, and short ion diffusion length have emerged. Here, a comprehensive overview of the controllable architectures and electrochemical energy applications of OIHFs is presented. After a brief introduction, the controllable construction of OIHFs is described in detail through precise tailoring of the overall, interior, and interface structures. Additionally, several important electrochemical energy applications including rechargeable batteries (lithium-ion batteries, sodium-ion batteries, and lithium-sulfur batteries), supercapacitors (sandwich-shaped supercapacitors and fiber-shaped supercapacitors), and electrocatalysts (oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction) are presented. The current state of the field and challenges are discussed, and a vision of the future directions to exploit OIHFs for electrochemical energy devices is provided.