High‐Alkaline Water‐Splitting Activity of Mesoporous 3D Heterostructures: An Amorphous‐Shell@Crystalline‐Core Nano‐Assembly of Co‐Ni‐Phosphate Ultrathin‐Nanosheets and V‐ Doped Cobalt‐Nitride Nanowires

Abstract

Introducing amorphous and ultrathin nanosheets of transition bimetal phosphate arrays that are highly active in the oxygen evolution reaction (OER) as shells over an electronically modulated crystalline core with low hydrogen absorption energy for an excellent hydrogen evolution reaction (HER) can boost the sluggish kinetics of the OER and HER in alkaline electrolytes. Therefore, in this study, ultrathin and amorphous cobalt‐nickel‐phosphate (CoNiPO x ) nanosheet arrays are deposited over vanadium (V)‐doped cobalt‐nitride (V 3% ‐Co 4 N) crystalline core nanowires to obtain amorphous‐shell@crystalline‐core mesoporous 3D‐heterostructures (CoNiPO x @V‐Co 4 N/NF) as bifunctional electrocatalysts. The optimized electrocatalyst shows extremely low HER and OER overpotentials of 53 and 270 mV at 10 mA cm −2 , respectively. The CoNiPO x @V 3% ‐Co 4 N/NF (+/−) electrolyzer utilizing the electrocatalyst as both anode and cathode demonstrates remarkable overall water‐splitting activity, requiring a cell potential of only 1.52 V at 10 mA cm −2 , 30 mV lower than that of the RuO 2 /NF (+)/20%‐Pt/C/NF (−) electrolyzer. Such impressive bifunctional activities can be attributed to abundant active sites, adjusted electronic structure, lower charge‐transfer resistance, enhanced electrochemically active surface area (ECSA), and surface‐ and volume‐confined electrocatalysis resulting from the synergistic effects of the crystalline V 3% ‐Co 4 N core and amorphous CoNiPO x shells boosting water splitting in alkaline media.

Cerium-Doped Nickel Phosphide Nanosheet Arrays as Highly Efficient Electrocatalysts for the Hydrogen Evolution Reaction in Acidic and Alkaline Conditions

“…The number of active sites can be determined by a normal electrochemical method (Figure S8a). − Obviously, as shown in Figure S8b, Ce 0.10 -Ni 2 P@NF has much larger TOF values compared to the pure Ni 2 P in the whole potential range. Consequently, it is obvious to conclude that the intrinsic activity of Ni 2 P for the HER in 1 M KOH can be enhanced by the introduction of cerium heteroatoms.…”

“…It can be calculated based on the following equation where j is the current in ampere (A) obtained during LSV measurement, F is the Faraday constant (C mol –1 ), and n is the number of active sites (mol). We used an electrochemical method to get the number of active sites at the surface considering its bulk feature. − Specifically, electrochemical tests were conducted to get the CV curves in a 1 M PBS electrolyte (pH = 7) at a scan rate of 50 mV s –1 in the potential range from −0.2 to 0.6 V versus the RHE. Assuming a one-electron process for both reduction and oxidation, the active-site density can be directly derived from the absolute components of the voltammetric charge over the entire potential range divided by two (cathodic and anodic) and the Faraday constant.…”

In Situ Growth of Self-Supporting MOFs-Derived Ni2P on Hierarchical Doped Carbon for Efficient Overall Water Splitting

“…A similar trend was observed as the area normalized LSV curves, in which Ni 2 P/NPC/NF-6 h also demonstrated the most superior catalytic performance among all synthesized materials. In order to further estimate the intrinsic catalytic activity of Ni 2 P/NPC/NF, the turnover frequency (TOF) for HER and OER was investigated [39][40][41]. The relationships between the hydrogen and oxygen generation rate and the number of active sites through the CV test are shown in Figures S3a and S4a.…”

Recent advances of cobalt-based nitride catalysts in solar energy conversion

“…V-doped Co 4 N nanowires have been prepared by Singh et al and used as a substrate for CoNiPO x deposition. 81…”

“…V-doped Co 4 N nanowires have been prepared by Singh et al and used as a substrate for CoNiPO x deposition. 81 Deposition of non-metal atoms on the surface of host materials is expected to be an effective avenue to modulate the surface electronic structure while maintaining the bulk phase stability. Se-Co 4 N nanowires have been synthesized via a lowtemperature selenium sublimation strategy.…”