Unveiling Accelerated Oxygen Evolution Reaction through Explosive Reconstruction of Active Species of Coxn@NC
Authors: Choi Hyung Wook , Kim Jiwon , Yoon Dae Ho 
Published: December 2024

Hydrogen (H2) generation through electrolysis has attracted much attention because of growing global energy needs and the demand for green energy strategies. Hydrogen generation through water electrolysis is accompanied by two half reaction, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, OER has the limitation of lowering the energy efficiency of the entire cell due to its sluggish reaction problem and high overpotential. Also, commercial OER electrocatalysts are limited to noble metal oxide such as IrO2 and RuO2, which are scarce and have stability problems in alkaline solution. Therefore, there is a growing desire to develop an OER electrocatalyst that is abundant, stable, and highly efficient for the superior performance of overall water electrolysis.
The doping, alloy, oxide, and nitride of transition metal materials have been reported as abundant materials that exceed the electrolysis efficiency of noble metals during OER. Among them, transition metal nitrides showed superior OER electrocatalytic performance due to their high electrical conductivity, which is derived from their metallic properties. To boost the electrocatalytic activity of transition metal based materials, numerous efforts have focused on modulating electronic structure related to catalytic activity in OER. The d-band center theory which proposed that the position of d-band center (Ed) has close relationship with the catalytic activity has unveiled insights into designing electrocatalysts for OER. However, most researchers have used ion exchange or doping method for modulating d-band center, which requires an additional process and leads to higher difficulty of designing electrocatalysts.
Recently, Prussian Blue Analogues (PBAs) have been widely employed as a self-template material for the energy conversion and storage field such as water electrolysis and batteries. PBAs, which are a type of Metal Organic Frameworks (MOFs) composed of CN groups, have emerged as a promising material due to the following characteristics: 1) numerous interstitial sites with unique 3-dimensional structure due to CN group; 2) advantages of synthesizing various compositions; and 3) chemical stability in solution. Due to these advantages, PBAs have a large specific surface area, fast diffusion kinetics, and structural stability when applied as a precursor to water electrocatalysts. Although PBAs-derived electrocatalysts have high electrical conductivity and electrocatalytic performance, they are prone to aggregate and lose their unique morphology after high temperature process. As a breakthrough of the limitation, polydomamine (PDA) coating has been attracting attention as an encapsulation method to achieve hybridization with carbon materials, as well as enhance electrocatalytic performance. PDA is simply synthesized by the polymerization of Dopamine (DA) in Tris-buffer solution, which is beneficial for mass production. In addition, PDA is advantageous for N-doped carbon formation, because it has high nitrogen content, which improves the charge transfer ability during electrocatalytic reaction. Also, because PDA has thermal stability, agglomeration between particles can be prevented when high-temperature heat treatment is performed. Due to these characteristics of PDA, PDA both prevents the aggregation of PBAs during carbonization at high temperature and improves stability in alkaline solution, while increasing reaction sites by forming a high content of N-doped carbon layer.
Herein, we present a versatile but promising strategy to synthesize cobalt nitride nanoparticles encapsulated in N-doped carbon (Co/Co4N@NC) nanoboxes as highly efficient and stable OER electrocatalysts by modulating d-band center through the simple nitridation of self-template PBAs encapsulated by PDA. The synthesized Co/Co4N@NC nanoboxes have mesoporous and hollow structure, which provides a large specific surface area and fast charge transfer pathways. Especially, the d-band center of CoxN nanoparticles is strategically modulated to be more metallic and heterogeneous to favor electrocatalytic reactions. Furthermore, the highly conductive N-doped carbon shell derived from PDA coating enables electrocatalysts to have not only numerous active reaction sites and fast charge transfer ability but also long-term durability in alkaline solution during OER. The Co/Co4N@NC nanoboxes showed a remarkable overpotential value of 262 mV and 408 mV at 10 mA∙cm-2 and 100 mA∙cm-2, respectively, and a low tafel slope of 130 mV∙dec-1, which is superior to RuO2 (284 mA∙cm-2 and 470 mA∙cm-2). In addition, the Co/Co4N@NC nanoboxes maintained an overpotential value of 92.6 % after 24 h of stability test at 10 mA∙cm-2, as well as showing outstanding durability for 100 h and at high current density of 100 mA∙cm-2. The DFT calculation also identifies advantages of metallic cobalt nitride and heterogeneous structure derived from the modulated electronic structure of Co/Co4N@NC. Finally, the in-situ XANES during OER of Co/Co4N@NC is clear evidence that the modulated electronic structure of Co/Co4N@NC improves the OER catalytic activity due to reconstruction of Co2+ into Co3+.

Figure 1