Controlled Phase and Porosity of Cobalt Disulfide Nanoparticles through Metal-Organic Frameworks As Efficient Bifunctional Electrocatalysts for Overall Water Splitting
Authors: Ahn In-Kyoung , Joo Wonhyo , Lee Ji-Hoon , Kim Hyoung Gyun , Lee So-Yeon , Jung Youngran , Kim Ji-Yong , Lee Gi-Baek , Kim Miyoung , Joo Young-Chang 
Published: May 2020

With growing demands for sustainable energy sources, hydrogen energy is receiving great attention due to their abundant, eco-friendly, and renewable nature. Although water electrolysis is most promising technologies for hydrogen production, it is currently obstructed by the sluggish kinetic of oxygen evolution reaction (OER) compared with hydrogen evolution reaction (HER). Furthermore, according to the demand for simplicity and cost effectiveness of electrolysis, the need for bifunctional electrocatalysts which operate in the same electrolyzer, is rapidly increasing. The cubic pyrite-phase transition metal dichalcogenides, such as FeS2, NiS2, and CoS2, have been proposed as candidates for bifuctional electrocatalysts. Among them, CoS2 has been reported to exhibit high conductivity and high activity for HER and OER in strong alkaline condition.

In this study, while there are many phases of cobalt sulfide (e.g., CoS, Co9S8, and CoS2 etc.), it was successfully controlled cobalt sulfide phase according to the sulfur ratio through predicted synthesis based on thermodynamic. A highly porous CoS2 nanoparticles (NPs) was directly synthesized by controlling the ratio of sulfur and cobalt Prussian blue analogues (Co3[Co(CN)6]2), one of the metal organic frameworks (MOFs). MOF-driven CoS2 could lead to improvements in catalytic activity. The newly-designed MOF-driven CoS2 OER, HER catalysts show distinct strong points. First, CoS2 is as one of transition metal compounds and has high catalytic activity. Furthermore, the CoS2 of this work shows a highly porous and uniform particle size because it was synthesized through MOFs using precipitation.

The Co3[Co(CN)6]2 was used as the starting material before sulfurization. The Co3[Co(CN)6]2 was prepared through a facile precipitation method. The CoS2 NPs were synthesized through gas-solid reaction in vacuum system. A 5 mL size ampoule containing total 45 mg of a 2:1 mixture of Co3[Co(CN)6]2 and sulfur powders was arranged, then the ampoule was vacuumed to 10-2 torr. The prepared ampoule was annealed at 500 ℃ for 2 hr in a furnace at ramping rate of 5 ℃ min-1. The average powder size of synthesized CoS2 NPs was approximately 30 nm with ~ 4nm pores, resulting in a large surface area of 915.6 m2 g-1. Compared to commercial IrO2, CoS2 NPs showed exceptional performance. The synthesized catalysts achieved a catalytic current density of 10 mA cm-2 at overpotentials as low as 300, 200 mV from oxygen, hydrogen evolution polarization curves through linear sweep voltammograms (LSVs). Furthermore, it has outstanding long-term durability under 10 mA cm-2. The electrochemical performance was conducted with three electrode cell using 1.0 M KOH electrolyte and Pt counter electrode, and working electrode is ink type on Ni foam. Furthermore, MOF-driven CoS2 was conducted through a full-cell test of overall water splitting for a practical two-electrode system in 1.0 M KOH. It exhibited an overpotential of 1.65 V at 10 mA cm-2. We introduced MOFs to a new methodology based on thermodynamic processes. It showed the possibility of replacing noble metals such as Ir, Ru in terms of catalytic performance.