Sulfur-Doped Dicobalt Phosphide Outperforming Precious Metals as a Bifunctional Electrocatalyst for Alkaline Water Electrolysis

Abstract

Metallic dicobalt phosphide (Co 2 P) is doped with electronegative sulfur (S:Co 2 P) by using an economical and eco-friendly thiourea-phosphate-assisted strategy. Density functional theory calculation in conjunction with X-ray photoelectron spectroscopy reveals that S-doping decreases the electron density near the Fermi level to reduce the metallic nature of Co 2 P. Thus, a more positive charge is induced onto Co to balance between hydride (Co δ+ H δ− ) and proton (S/P δ− H δ+ ) acceptors. As a result, it increases the number of active Co 2+ sites as well as the turnover frequency of a single site. The hybrid electrodes obtained by loading S:Co 2 P nanoparticles on N-doped carbon cloth or nickel foam (NF) exhibit an outstanding activity and stability of hydrogen and oxygen evolution reactions in alkaline electrolytes outperforming conventional, precious metal-based Pt/C and IrO 2 catalysts, and most of the other state-of-the-art nonprecious metal electrocatalysts reported so far. An alkaline electrolyzer with S:Co 2 P@NF as both cathode and anode produces a stable current density of 100 mA/cm 2 at 1.782 V, which is superior to the IrO 2 −Pt/C electrolyzer (1.823 V).

Multifunctional Transition Metal‐Based Phosphides in Energy‐Related Electrocatalysis

“…It was evident that in c‐CoSe 2 the poisoning effect led to significant degradation of HER performance, while this condition was totally different in o‐CoSe 2 |P. Successively, Lee and co‐workers fabricated the sulfur‐doped dicobalt phosphide (S:Co 2 P) by a facile thiourea‐phosphate‐assisted strategy . This S:Co 2 P demonstrated higher HER and OER performance in contrast with pure Co 2 P. Interestingly, in view of its high OER activity, they estimated the density of Co 2+ active sites and further calculated the turnover frequency (TOF).…”

“…Copyright 2018, Nature Publishing Group. e) The calculated number of active sites (left) and TOF (right) in S:Co 2 P. Reproduced with permission . Copyright 2018, American Chemical Society.…”

The Effect of Heteroatom Doping on Nickel Cobalt Oxide Electrocatalysts for Oxygen Evolution and Reduction Reactions

“…[32,40,60] Recently, heteroatoms (S, N, P) doped transition metal have been reported to be an efficient approach for enhancing the catalytic activities. [61,62] The pronounced OER performance is linked to the main Ni 3 + oxidation state of Ni on SÀ NiCoO and also to the presence of sulfide which increase the number of active sites, change the chemical as well as the coordination of Ni/Co. [63]…”

Comprehensively Probing the Contribution of Site Activity and Population of Active Sites toward Heterogeneous Electrocatalysis

“…[3,9.10] In connection with advanced projects for significantly increasing the efficiency of water electrolysis, two noticeable research domains are adopted. The first one is to create high surface areas of electrocatalysts for improving the number of exposed active sites, while the second one is to design a new electrocatalyst for enhancing intrinsic activity (Table ) . Loading or in‐situ generation of electrocatalysts on two‐dimensional/three‐dimensional frameworks, such as graphene derivatives or metal‐organic frameworks, bears large specific area for improving the exposure of active sites.…”

“…Controlling nanostructured shape, array and size of electrocatalysts is another strategy to increase the population of active sites . On the other hand, doping elements, tuning ratio of transition metal complexes, replacing substrate or anchoring atoms to modulate local coordination environments have been a frequent approach for promoting the activity of electrocatalysts …”