Morphology and Composition Regulation of FeCoNi Prussian Blue Analogues to Advance in the Catalytic Performances of the Derivative Ternary Transition‐Metal Phosphides for OER

Hollow Structure Derived Phosphide Nanosheets for Water Oxidation

Avoiding the stacking of active sites in catalyst structural design is a promising route for realizing active oxygen evolution reaction (OER). Herein, using a CoFe Prussian blue analoge cube with hollow structure (C-CoFe PBA) as a derived support, a highly effective Ni2P-FeP4-Co2P catalyst with a larger specific surface area is reported. Benefiting from the abundant active sites and fast charge transfer capability of the phosphide nanosheets, the Ni2P-FeP4-Co2P catalyst in 1 m KOH requires only overpotentials of 248 and 277 mV to reach current density of 10 and 50 mA cm-2 and outperforms the commercial catalyst RuO2 and most reported non-noble metal OER catalysts. In addition, the two-electrode system consisting of Ni2P-FeP4-Co2P and Pt/C is able to achieve a current density of 10 and 50 mA cm-2 at 1.529 and 1.65 V. This work provides more ideas and directions for synthesizing transition metal catalysts for efficient OER performance.

Well-defined ternary metal phosphide nanowires with stabilized Pt nanoclusters to boost alkaline hydrogen evolution reaction

Designing low-content and high-activity Pt-based catalysts with the high durability for the electrochemical hydrogen production remains a challenge. In this study, a ternary metal phosphide (NiCoP) with 1D nanowire (NW) and 2D nanosheet (NS) morphologies incorporating Pt clusters (denoted as Ptcluster-NiCoP@NF NWs and Ptcluster-NiCoP@NF NSs, respectively) was prepared using a hydrothermal-phosphorization-electrodeposition method. Based on the "tip effect" of NWs and a high electrochemical surface area, the as-prepared Ptcluster-NiCoP@NF NWs display better hydrogen evolution reaction (HER) performance, with a low overpotential of 65 mV at a high current density of 100 mA cm-2 and a low Tafel slope of 38.86 mV dec-1, than the Ptcluster-NiCoP@NF NSs, with an overportential of 95 mV at 42.53 mV dec-1. This indicates that the NiCoP NW-based support exhibits faster HER kinetics. The mass activity (11.47 A mgPt-1) of the Ptcluster-NiCoP@NF NWs is higher than that of commercial Pt/C catalysts. Significantly, the Ptcluster-NiCoP@NF NWs display excellent cyclic stability with negligible losses for 5000 cycles and 30-h tests at a high current of 500 mA cm-2.

Enhanced Electrochemical Water Oxidation Activity by Structural Engineered Prussian Blue Analogue/rGO Heterostructure

Prussian blue analogue (PBA), with a three-dimensional open skeleton and abundant unsaturated surface coordination atoms, attracts extensive research interest in electrochemical energy-related fields due to facile preparation, low cost, and adjustable components. However, it remains a challenge to directly employ PBA as an electrocatalyst for water splitting owing to their poor charge transport ability and electrochemical stability. Herein, the PBA/rGO heterostructure is constructed based on structural engineering. Graphene not only improves the charge transfer efficiency of the compound material but also provides confined growth sites for PBA. Furthermore, the charge transfer interaction between the heterostructure interfaces facilitates the electrocatalytic oxygen evolution reaction of the composite, which is confirmed by the results of the electrochemical measurements. The overpotential of the PBA/rGO material is only 331.5 mV at a current density of 30 mA cm⁻² in 1.0 M KOH electrolyte with a small Tafel slope of 57.9 mV dec⁻¹, and the compound material exhibits high durability lasting for 40 h.

Metal - organic frameworks derived Ni5P4/NC@CoFeP/NC composites for highly efficient oxygen evolution reaction

As an efficient non-precious metal catalyst for the oxygen evolution reaction (OER), phosphides suffer from poor electrical conductivity, so it is still a challenge to reasonably design their structures to further improve their conductivity and OER performances. Here, we present a novel Ni₅P₄/N-doped carbon@CoFeP/N-doped carbon composite (Ni₅P₄/NC@CoFeP/NC) as electrocatalysts for OER. This elaborate structure consists of Ni₅P₄/NC derived from Ni-MOF and CoFeP/NC derived from CoFe-Prussian blue analog MOF (Co-Fe PBA). The cube-like CoFeP/NC are scattered and uniformly coated on the sheet of Ni₅P₄/NC flowers. Among them, NC can enhance the conductivity of phosphides, while CoFeP/NC can increase the electrochemical active area, which benefit the properties of Ni₅P₄/NC@CoFeP/NC. Notably, the Ni₅P₄/NC@CoFeP/NC catalyst possesses outstanding OER performances with a low overpotential of 260 and 303 mV at a current density of 10 and 100 mA·cm^-2, an ultra-low Tafel slope of 31.1 mV·dec^-1 and excellent stability in 1 M KOH. XPS analysis shows that proper chemical composition promotes the oxidation of transition metal species and the chemisorption of OH^-, thus accelerating the OER kinetics. Therefore, this work provides a hopeful method for designing and preparing transition metal phosphide/carbon composite as OER electrocatalysts.

Strategies for Developing Transition Metal Phosphides in Electrochemical Water Splitting

Electrochemical water splitting involving hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is a greatly promising technology to generate sustainable and renewable energy resources, which relies on the exploration regarding the design of electrocatalysts with high efficiency, high stability, and low cost. Transition metal phosphides (TMPs), as nonprecious metallic electrocatalysts, have been extensively investigated and proved to be high-efficient electrocatalysts in both HER and OER. In this minireview, a general overview of recent progress in developing high-performance TMP electrocatalysts for electrochemical water splitting has been presented. Design strategies including composition engineering by element doping, hybridization, and tuning the molar ratio, structure engineering by porous structures, nanoarray structures, and amorphous structures, and surface/interface engineering by tuning surface wetting states, facet control, and novel substrate are summarized. Key scientific problems and prospective research directions are also briefly discussed.

Prussian Blue Analogs and Their Derived Nanomaterials for Electrochemical Energy Storage and Electrocatalysis

Prussian blue analogs (PBAs), the oldest artificial cyanide-based coordination polymers, possess open framework structures, large specific surface areas, uniform metal active sites, and tunable composition, showing significant perspective in electrochemical energy storage. These electrochemically active materials have also been converted to various functional metal containing nanomaterials, including carbon encapsulated metals/metal alloys, metal oxides, metal sulfides, metal phosphides, etc. originating from the multi-element compositions as well as elaborate structure design. In this paper, a comprehensive review will be presented on the recent progresses in the development of PBA frameworks and their derivatives based electrode materials and electrocatalysts for electrochemical energy storage and conversion. In particular, it will focus on the synthesis of representative nanostructures, the structure design, and figure out the correlation between nanomaterials structure and electrochemical performance. Lastly, critical scientific challenges in this research area are also discussed and perspective directions for the future research in this field are provided, in order to provide a brand new vision into the further development of novel active materials for the next-generation advanced electrochemical devices.