Strategies for Developing Transition Metal Phosphides in Electrochemical Water Splitting

Interfacial engineering of heterostructured CoP/FeP nanoflakes as bifunctional electrocatalyts toward alkaline water splitting

Exploring highly-effective and nonprecious electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is urgent and challenging for developing the hydrogen economy. Interface engineering is a feasible approach for regulating the surface electronic distribution, thereby promoting the catalytic performance. Herein, the CoP/FeP heterostructure is fabricated via the oxidation and phosphating treatments of Fe-decorated Ni(OH)2 nanoflakes. The hierarchically porous nanoflakes can expose more active species, while the formation of CoP/FeP heterojunctions have provided extra catalytic active sites and accelerated the charge transfer process. Theoretical calculations reveal that the interfacial electron coupling between CoP and FeP in the heterostructure has promoted the adsorption of intermediate species on catalytic sites, thereby decreasing the Gibbs free energy during the catalysis. The as-fabricated CoP/FeP catalyst requires small overpotentials of 190 mV and 280 mV to realize a current density of 10 mA cm-2 for alkaline HER and OER, respectively. The electrolytic cell with CoP/FeP as catalyst needs a voltage of 1.61 V to reach 10 mA cm-2, and can run stably for over 25 h. The present study highlights a superiority of interfacial engineering to construct efficient electrocatalysts for water electrolysis.

Unlocking the Potential of High Entropy Alloys in Electrochemical Water Splitting: A Review

The global pursuit of sustainable energy is focused on producing hydrogen through electrocatalysis driven by renewable energy. Recently, High entropy alloys (HEAs) have taken the spotlight in electrolysis due to their intriguing cocktail effect, broad design space, customizable electronic structure, and entropy stabilization effect. The tunability and complexity of HEAs allow a diverse range of active sites, optimizing adsorption strength and activity for electrochemical water splitting. This review comprehensively covers contemporary advancements in synthesis technique, design framework, and physio-chemical evaluation approaches for HEA-based electrocatalysts. Additionally, it explores design principles and strategies aimed at optimizing the catalytic activity, stability, and effectiveness of HEAs in hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting. Through an in-depth investigation of these aspects, the complexity inherent in constituent element interactions, reaction processes, and active sites associated with HEAs is aimed to unravel. Eventually, an outlook regarding challenges and impending difficulties and an outline of the future direction of HEA in electrocatalysis is provided. The thorough knowledge offered in this review will assist in formulating and designing catalysts based on HEAs for the next generation of electrochemistry-related applications.

Surface restructuring Prussian blue analog-derived bimetallic CoFe phosphides by N-doped graphene quantum dots for electroactive hydrogen evolving catalyst

As a crucial stage of electrochemical water splitting, hydrogen evolution reaction (HER) favour catalyst to attain rapid kinetics for its broader application, alternating Pt in the acidic environment. Transition metal phosphides (TMPs) are one kind of earth-abundant, nonprecious-based catalyst which has been classified as a viable alternative and active for HER. While the performance remains inferior to Pt which primarily targets durability under high current density, pinpointing the reconfiguration strategy would be critical to their catalytic competency. Herein, we reported engineered N-doped graphene quantum dots (NGQD) on the surface of bimetallic CoFe phosphide (CoFeP) derived from cobalt iron Prussian blue analogue (CoFePBA) as an efficient HER. By introducing the NGQD, the surface architect and electronic state of the transition metal are altered through an adjusted electronic configuration and thus, improving the electrocatalytic activity for HER. The X-ray absorption spectroscopy (XAS) highlighting the role of NGQD, which successfully induced the electron density of Co atoms, further expedites its conductivity and electroactivity. The optimized NGQD/CoFeP substantially surpasses an overpotential of 70 mV (vs. RHE) at the current density of 10 mA cm-2 in 0.5 M H2SO4. Furthermore, the NGQD/CoFeP maintains its exceptional stability under an extremely high current density of 600 mA cm-2 after 12 h of continuous operation. Our findings show that NGQD/CoFeP might demonstrate as a viable alternative to the conventional Pt electrocatalyst in commercial water splitting for hydrogen generation.

A Little Nickel Goes a Long Way: Ni Incorporation into Rh2P for Stable Bifunctional Electrocatalytic Water Splitting in Acidic Media

In acidic media, many transition-metal phosphides are reported to be stable catalysts for the hydrogen evolution reaction (HER) but typically exhibit poor stability toward the corresponding oxygen evolution reaction (OER). A notable exception appears to be Rh2P/C nanoparticles, reported to be active and stable toward both the HER and OER. Previously, we investigated base-metal-substituted Rh2P, specifically Co2-xRhxP and Ni2-xRhxP, for HER and OER as a means to reduce the noble-metal content and tune the reactivity for these disparate reactions. In alkaline media, the Rh-rich phases were found to be most active for the HER, while base-metal-rich phases were found to be the most active for the OER. However, Co2-xRhxP was not stable in acidic media due to the dissolution of Co. In this study, the activity and stability of our previously synthesized Ni2-xRhxP nanoparticle catalysts (x = 0, 0.25, 0.50, 1.75) toward the HER and OER in acidic electrolyte are probed. For the HER, the Ni0.25Rh1.75P phase was found to have comparable geometric activity (overpotential at 10 mA/cmgeo2) and stability to Rh2P. In contrast, for OER, all of the tested Ni2-xRhxP phases had similar overpotential values at 10 mA/cmgeo2, but these were >2x the initial value for Rh2P. However, the activity of Rh2P fades rapidly, as does Ni2P and Ni-rich Ni2-xRhxP phases, whereas Ni0.25Rh1.75P shows only modest declines. Overall water splitting (OWS) conducted using Ni0.25Rh1.75P as a catalyst relative to the state-of-the-art (RuO2||20% Pt/C) revealed comparable stabilities, with the Ni0.25Rh1.75P system demanding an additional 200 mV to achieve 10 mA/cmgeo2. In contrast, a Rh2P||Rh2P OWS cell had a similar initial overpotential to RuO2||20% Pt/C, but is unstable, completely deactivating over 140 min. Thus, Rh2P is not a stable anode for the OER in acidic media, but can be stabilized, albeit with a loss of activity, by incorporation of nominally modest amounts of Ni.

Recent advances in amorphous electrocatalysts for oxygen evolution reaction

Oxygen evolution reaction (OER) has attracted great attention as an important half-reaction in the electrochemical splitting of water for green hydrogen production. However, the inadequacy of highly efficient and stable electrocatalysts has impeded the development of this technology. Amorphous materials with long-range disordered structures have exhibited superior electrocatalytic performance compared to their crystalline counterparts due to more active sites and higher structural flexibility. This review summarizes the preparation methods of amorphous materials involving oxides, hydroxide, phosphides, sulfides, and their composites, and introduces the recent progress of amorphous OER electrocatalysts in acidic and alkaline media. Finally, the existing challenges and future perspectives for amorphous electrocatalysts for OER are discussed. Therefore, we believe that this review will guide designing amorphous OER electrocatalysts with high performance for future energy applications.

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.

A Facile Design of Solution-Phase Based VS2 Multifunctional Electrode for Green Energy Harvesting and Storage

This work reports the fabrication of vanadium sulfide (VS₂) microflower via one-step solvo-/hydro-thermal process. The impact of ethylene glycol on the VS₂ morphology and crystal structure as well as the ensuing influences on electrocatalytic hydrogen evolution reaction (HER) and supercapacitor performance are explored and compared with those of the VS₂ obtained from the standard pure-aqueous and pure-ethylene glycol solvents. The optimized VS₂ obtained from the ethylene glycol and water mixed solvents exhibits a highly ordered unique assembly of petals resulting a highly open microflower structure. The electrode based on the optimized VS₂ and exhibits a promising HER electrocatalysis in 0.5 M H₂SO₄ and 1 M KOH electrolytes, attaining a low overpotential of 161 and 197 mV, respectively, at 10 mA.cm^-2 with a small Tafel slope 83 and 139 mVdec^-1. In addition, the optimized VS₂ based electrode exhibits an excellent electrochemical durability over 13 h. Furthermore, the superior VS₂ electrode based symmetric supercapacitor delivers a specific capacitance of 139 Fg^-1 at a discharging current density of 0.7 Ag^-1 and exhibits an enhanced energy density of 15.63 Whkg^-1 at a power density 0.304 kWkg^-1. Notably, the device exhibits the capacity retention of 86.8% after 7000 charge/discharge cycles, demonstrating a high stability of the VS₂ electrode.