Transformation of a Cobalt Carbide (Co₃C) Oxygen Evolution Precatalyst

Abstract

We report on the fabrication of cobalt carbide (Co 3 C) particles via a wet-chemical synthetic procedure and also describe their electrochemical oxidation to amorphous Co oxide particles that can be used as oxygen evolution reaction (OER) catalysts. Metal chalcogenide, carbide, and pnictide materials have been investigated recently, but there is some controversy regarding the composition of the actual electrocatalytic material. Carbides, in particular, have not been heavily studied as OER catalysts, and their catalytic nature is still an open question. In an effort to contribute to the clarification of the catalytic particle composition during OER, we have thoroughly characterized the elemental composition of the cobalt carbide particles at various times during OER testing and found that the particles are first converted to a transitory core−shell structure (Co 3 C core−amorphous Co oxide shell) followed by the gradual but complete conversion to an amorphous Co oxide particle during additional electrochemical OER testing. This amorphous Co oxide particle (derived from Co 3 C) maintains the shape of the original parent Co 3 C particle and exhibits a high electrochemically active surface area (ECSA). Moreover, the amorphous Co oxide particle derived from Co 3 C shows a higher geometric OER activity than either commercial Co oxide particles or Co 3 C−CoO x core−shell particles. We also observe that the fully oxidized Co 3 C shows the same intrinsic activity as commercial Co oxide particles when normalized by the ECSA. Accordingly, the amorphous Co oxide particles produced from Co 3 C possesses a porous nanostructure capable of electrocatalytically oxidizing water within the internal pores of the particles.

The Role of Non‐Metallic and Metalloid Elements on the Electrocatalytic Activity of Cobalt and Nickel Catalysts for the Oxygen Evolution Reaction

“…Mullin's group recently reported the application of cobalt carbide (Co 3 C) for the OER in alkaline solution . It was demonstrated that under electrochemical OER conditions, the Co 3 C particles were fully converted to amorphous cobalt oxide.…”

“…1.0–1.9 V vs RHE) for the Co 3 C sample. Taken from with permission of the American Chemical Society.…”

“…Compounds and alloys of cobalt and nickel with some nonmetals (N, P, S, Se) and metalloids (C, B, C, As, and Te), hereafter denoted as MX (M=Co or Ni, and X is a nonmetal or metalloid), have shown to be very promising pre‐catalysts for the OER in alkaline electrolytes . However, the function of the nonmetals and metalloids, hereafter also referred to as guest elements, in promoting the OER activity of Co and Ni, hereafter also referred to as host elements, remains not well understood.…”

“…It is nonetheless well recognized that during the OER, these MX compounds undergo considerable structural transformation as well as a change in the material composition, the extent of which depending upon the nature of the guest element, structure of MX, size and morphology of the particles, as well as the applied electrochemical conditions. Previous studies have indicated that for relatively large particles, only the surface of MX is oxidized while the core remains intact, leading to the formation of core@shell (MX@MO x ) ‐ type structures, meanwhile, small particles or thin nanosheets tend to suffer total oxidation to yield amorphous metal oxides with complete loss of the guest element . In order to understand the role of the guest elements in enhancing the OER activity of their host 3d transition metal (Co and Ni), the properties and chemical state of the catalyst prior to electrocatalysis, during the OER and after the OER (post‐electrocatalysis) have to be thoroughly probed.…”

Identifying the Intrinsic Relationship between the Restructured Oxide Layer and Oxygen Evolution Reaction Performance on the Cobalt Pnictide Catalyst

“…With respect to the outstanding OER on cobalt pnictides, it is currently argued that the in‐situ evolution of cobalt oxides/(oxy)hydroxides on the surface of cobalt pnictides during OER is a dominant factor for enhancing the catalytic activity. [ 10–14 ] Zhu et al and Li et al have both reported that the high OER activity of CoP nanocrystal results from the significant transition of Co 2+ states to higher oxidation states and the formation of Co oxides/hydroxides during the anodic reaction process. [ 9,15 ] Meanwhile, Fan's group has found that the active CoOOH layer exists in the CoN surface prior to the OER process, and is further oxidized in the OER process to produce the CoO 2 /Co x N, which is a more efficient species for OER.…”