Design and multilevel regulation of transition metal phosphides for efficient and industrial water electrolysis

Renewable energy electrolysis of water to produce hydrogen is an effective measure to break the energy dilemma. However, achieving activity and stability at a high current density is still a key problem in water electrolyzers. Transition metal phosphides (TMPs), with high activity and relative inexpensiveness, have become excellent candidates for the production of highly pure green hydrogen for industrial applications. In this mini-review, multilevel regulation strategies including nanoscale control, surface composition and interface structure design of high-performance TMPs for hydrogen evolution are systematically summarized. On this basis, in order to achieve large-scale hydrogen production in industry, the hydrogen evolution performance and stability of TMPs at a high current density are also discussed. Peculiarly, the practical application and requirements in proton exchange membrane (PEM) or anion exchange membrane (AEM) electrolyzers can guide the advanced design of regulatory strategies of TMPs for green hydrogen production from renewable energy. Finally, the challenges and prospects in the future development trend of TMPs for efficient and industrial water electrolysis are given.

Ultrafast Porous Carbon Activation Promises High-Energy Density Supercapacitors

Activated porous carbons (APCs) are traditionally produced by heat treatment and KOH activation, where the production time can be as long as 2 h, and the produced activated porous carbons suffer from relatively low specific surface area and porosity. In this study, the fast high-temperature shock (HTS) carbonization and HTS-KOH activation method to synthesize activated porous carbons with high specific surface area of ≈843 m² g^-1 , is proposed. During the HTS process, the instant Joule heating (at a heating speed of ≈1100 K s^-1 ) with high temperature and rapid quenching can effectively produce abundant pores with homogeneous size-distribution due to the instant melt of KOH into small droplets, which facilitates the interaction between carbon and KOH to form controllable, dense, and small pores. The as-prepared HTS-APC-based supercapacitors deliver a high energy density of 25 Wh kg^-1 at a power density of 582 W kg^-1 in the EMIMBF₄ ionic liquid. It is believed that the proposed HTS technique has created a new pathway for manufacturing activated porous carbons with largely enhanced energy density of supercapacitors, which can inspire the development of energy storage materials.

Freestanding trimetallic Fe-Co-Ni phosphide nanosheet arrays as an advanced electrode for high-performance asymmetric supercapacitors

Transition metal phosphides hold great promise for high performance battery-type electrode materials due to their superb electrical conductivity and high theoretical capacity. Unfortunately, the electrochemical properties of single metal or bimetallic phosphides are unsatisfactory owing to their low energy density and poor cyclic stability, and one feasible approach is to introduce heteroatoms to form trimetallic phosphides. Here, novel Fe-Co-Ni-P nanosheet arrays are in situ synthesized on a flexible carbon cloth substrate via an electrodeposition method followed by a phosphorization treatment. Due to the presence of abundant redox active sites, large specific surface area with mesoporous channels, desirable electrical conductivity, modified electronic structure, and synergistic effect of Fe, Co, and Ni ions, the as-prepared Fe-Co-Ni-P electrode displays significantly enhanced electrochemical performance when compared to bimetallic phosphides Fe-Co-P and Fe-Ni-P. Remarkably, the Fe-Co-Ni-P electrode exhibits a large specific capacity of 593.0 C g^-1 at 1 A g^-1, exceptional rate performance (80.3% capacity retention at 20 A g^-1), and good cycling stability (84.2% capacity retention after 5000cycles). Besides, an asymmetric supercapacitor device with Fe-Co-Ni-P electrode as a positive electrode and a hierarchical porous carbon as a negative electrode shows a high energy density of 57.1 Wh kg^-1 at a power density of 768.5 W kg^-1 as well as excellent cyclability with 88.4% of initial capacity after 10,000cycles. This work manifests that the construction of trimetallic phosphides is an effective strategy to solve the shortcomings of single or bimetallic phosphides for high-performance supercapacitors.