The urgent demand of scalable hydrogen production has motivated substantial research on low cost, efficient and robust catalysts for water electrolysis. In order to replace noble metals and their derivatives, transition metal (Fe, Co, Ni, Mo, Cu, etc.) selenides have demonstrated promising catalysis on both hydrogen and oxygen evolutions. Very recently, a number of reports have presented a variety of approaches to enhance their electrocatalytic activity. This review summarizes the most recent progress in transition metal selenide electrocatalysts for HER, OER, and overall water splitting. The merits and limitations of metal selenides are also discussed in the aspects of structure and composition. Moreover, we highlight new strategies and future challenges for design and synthesis of high performance electrocatalysts.

Recent progress in transition metal selenide electrocatalysts for water splitting

Highly active and stable catalysts toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is extremely important but challenging for overall water splitting. Herein, a strategy by modulating surface electron densities is adopted to design new catalyst: the Ru-NiFe-P nanosheets on 3D self-supported nickel foam was developed by doping a small quantity of Ru into transition metals phosphates. The designed catalyst displays excellent electrocatalytic performance in 1.0 M KOH, with low overpotentials of 44 mV at 10 mA cm−2 for HER and 242 mV at 100 mA cm−2 for OER. Particularly, it only needs 1.47 V to yield 10 mA cm−2 for overall water splitting. Density functional theory calculations reveal that Ru-incorporated NiFe-P can generate new active sites on Ru atoms and increase the catalytic activity of original P sites leading to an optimal Gibbs free-energy on catalysts surface. This work offers a new perspective for designing admirable electrocatalysts.

Regulating Electron Density of NiFe-P nanosheets Electrocatalysts by a trifle of Ru for High-efficient Overall Water Splitting

Recent advances in cobalt-based electrocatalysts for hydrogen and oxygen evolution reactions

Portable water splitting devices driven by rechargeable metal-air batteries or solar cells are promising, however, their scalable usages are still hindered by lack of suitable multifunctional electrocatalysts. Here, a highly efficient multifunctional electrocatalyst is demonstrated, i.e., 2D nanosheet array of Mo-doped NiCo2 O4 /Co5.47 N heterostructure deposited on nickel foam (Mo-NiCo2 O4 /Co5.47 N/NF). The successful doping of non-3d high-valence metal into a heterostructured nanosheet array, which is directly grown on a conductive substrate endows the resultant catalyst with balanced electronic structure, highly exposed active sites, and binder-free electrode architecture. As a result, the Mo-NiCo2 O4 /Co5.47 N/NF exhibits remarkable catalytic activity toward the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), affording high current densities of 50 mA cm-2 at low overpotentials of 310 mV for OER, and 170 mV for HER, respectively. Moreover, a low voltage of 1.56 V is achieved for the Mo-NiCo2 O4 /Co5.47 N/NF-based water splitting cell to reach 10 mA cm-2 . More importantly, a portable overall water splitting device is demonstrated through the integration of a water-splitting cell and two Zn-air batteries (open-circuit voltage of 1.43 V), which are all fabricated based on Mo-NiCo2 O4 /Co5.47 N/NF, demonstrating a low-cost way to generate fuel energy. This work offers an effective strategy to develop high-performance metal-doped heterostructured electrode.

Non-3d Metal Modulation of a 2D Ni-Co Heterostructure Array as Multifunctional Electrocatalyst for Portable Overall Water Splitting.

Boosting interfacial charge transfer for alkaline hydrogen evolution via rational interior Se modification

Water splitting has long been considered as a promising chemical reaction that can produce clean hydrogen fuel to relieve the energy crisis and environmental pollution. Herein, we report that Co0.75Ni0.25Se/NF formed by two-step growth of metallic cobalt–nickel selenide nanorods on porous nickel foam was used as a bifunctional electrocatalyst. Ni foam serves as a slow-releasing nickel source together with a Co source to form a special proportional cobalt–nickel selenide. Due to its unique rough nanostructure, bimetallic cooperative effects and intrinsic metallic character, the obtained Co0.75Ni0.25Se/NF electrode exhibits a low overpotential of 269 mV (50 mA cm−2) for the oxygen evolution reaction and an overpotential of 106 mV (10 mA cm−2) for the hydrogen evolution reaction. Furthermore, this bifunctional electrocatalyst requires a cell voltage of 1.60 V to achieve a current density of 10 mA cm−2. Besides, based on theoretical calculation, it is further shown that the synergy between Co and Ni elements is beneficial for improving the internal structure of the catalyst, resulting in a high electrical conductivity, and low HER Gibbs free-energy and water adsorption energy. The present results indicate that Co0.75Ni0.25Se/NF exhibits advanced electrocatalytic activity for overall water splitting. This work offers an appropriate methodology and theoretical guidance to synthesize a bimetal–selenide electrocatalyst for water splitting.

Highly conductive and metallic cobalt-nickel selenide nanorods supported on Ni foam as an efficient electrocatalyst for alkaline water splitting.

Exploring highly effective electrocatalysts with heterostructures is significant for sustainable hydrogen production by the hydrogen evolution reaction (HER). However, there are still challenges in improving the HER activities of the heterostructures to achieve efficient hydrogen production. Here, nitrogen-decorated dual transition metal sulphide heterostructures (N-NiS/MoS2) were constructed with an enhanced HER performance in alkaline electrolytes. These novel N-NiS/MoS2 heterostructures exhibited a low overpotential of 71 mV (10 mA cm−2), small Tafel slope of 79 mV dec−1 and favorable stability. In particular, the experimental and theoretical calculation results consistently demonstrated that the introduction of nitrogen can effectively tune the electronic structure of the heterostructures. Furthermore, the synergistic effect between dual-active components N-NiS and N-MoS2 in the N-NiS/MoS2 heterostructures effectively promoted water dissociation and hydrogen formation, leading to remarkable increase in the HER performance in an alkaline medium. This work provides a valuable avenue for the rational modulation of the electronic structure of heterostructures by hetero-atoms for highly efficient HER catalysts.

Mengjie Zhang

Papers

Highly conductive and metallic cobalt-nickel selenide nanorods supported on Ni foam as an efficient electrocatalyst for alkaline water splitting.

Regulating the electron density of dual transition metal sulfide heterostructures for highly efficient hydrogen evolution in alkaline electrolytes