Synergistic interaction of P and N co-doping EDTA with controllable active EDTA-cobalt sites as efficient electrocatalyst for oxygen reduction reaction
Graphical abstract
Introduction
Electrocatalyst for the oxygen reduction reaction (ORR) is crucial in next generation storage such as metal-air battery [1], [2], [3]. The cathode is composed of air and, hence, the efficiency of energy storage depends on the reactivity with oxygenn [4]. However, slow steps in ORR and sluggishness due to additional reactions, limit the performance of next generation storage such as metal-air batteries. Electrocatalysts are therefore essential for reducing the activation energy of enhancing the ORR performance and the sluggish kinetics [5]. To date, platinum (Pt)-based catalysts have been considered the most effective for ORR, because of the fast onset potential and theoretically high current density [6], [7], [8]. Nevertheless, factors including poor durability, high cost, and limited materials, have hindered the use of Pt-based catalysts in large-scale industrial applications. Consequently, carbon-based catalysts, such as graphene, porous carbon and carbon nanotubes (CNTs) have attracted significant attention as substitutes for non-metal catalysts [9], [10], [11], [12], [13]. Recently, a research of controlling the bonding site by combining a metal to a carbon-based material with single-metal-atom has been conducted [8], [13] (Scheme 1).
However, a catalyst consisting solely of carbon materials exhibits lower ORR performance than commercial Pt. To overcome this drawback and enhance the performance, previous catalysts have been doped with a hetero-atom (nitrogen (N), sulfur (S), boron (B), phosphorus (P)) singleness [14], [15], [16], [17], [18], [19], [20]. Among these dopants in doped carbon nanostructures, N as an essential doping material has a similar atomic size to that of carbon as a matrix and 5 valence electrons that provide a strong covalent bond with the carbon structure [19], [21]. In addition, studies of the M-N-C structure containing nitrogen (N) and metals (M) have been demonstrated several times with high electrochemical activity [20], [22]. This doping can lead to changes in the spin charge density and atomic charge density, as determined via density functional theory (DFT) calculations [15]. The corresponding redistribution of electron can alter the O2 chemisorption behavior of the catalyst surface and undermine the bond between oxygen, thereby favoring ORR [23]. Among them, nitrogen has the greatest electronegativity with carbon and many studies have been conducted [24], [25], [26]. In addition, the difference in the electronegativities of boron, sulfur and phosphorus is low, but this difference induces changes in the charge density, thereby creating an ORR-active site [27].
Phosphorus has the same number of valence electrons as nitrogen and often exhibits similar chemical properties. As the atomic radius is large and the electron-donating ability is high, phosphorus is considered a wise choice as a dopant for the carbon material and a dopant-induced improvement in the catalytic activity is expected [28]. Moreover, when two heteroatoms are doped together on the carbon, the synergistic effect between the two heteroatoms improves the ORR effect [29]. The atomic-radius difference between phosphorus and carbon and the electronegativity difference between other atoms and nitrogen are combined to change the spin density and the charge density, thereby improving the ORR performance [30]. In addition, three electron-isolated pairs of phosphorus atoms contribute to the chemical adsorption of oxygen. But, despite this high catalytic potential of phosphorus, reported studies of ethylenediaminetetraacetic acid (EDTA) based nitrogen, phosphorus co-doping catalysts are lacking.
Nitrogen and Phosphorus co-doped mesoporous carbon (NPMPC), a highly functional electrocatalyst for ORR, was successfully synthesized via calcination (under purging N) of EDTA-, cobalt (Co)- and P-containing complex. Transition metal- based catalysts are considered an alternative electrocatalyst, owing to their variable valency state and structurally active sites of oxygen catalytic activity [31]. Cobalt represents one of the most extensively studied transition metal-based catalysts [32]. For example, Yin et al. reported that cobalt-doped carbon materials exhibit better ORR activity than carbon-only materials [33]. It is characterized by atomically dispersed sites throughout the metal-organic framework of metal-containing nodes and organic links. EDTA has an octahedral structure through six coordination bonds with the transition metal and is mesoporous after calcination, and this porosity yields a uniform nanostructure and high specific area. The high surface area and morphology of the mesoporous carbon offer a three-phase (gas-liquid-solid) region favorable for the transfer of oxygen in the electrolyte and cathode [34]. The doped nitrogen and phosphorus ratios and chemical states were easily controlled by annealing at a temperature of 700 °C. When used as cathodic electrocatalysts, the NPMPC samples exhibited better ORR performance, durability and safety than Pt/C under alkaline electrolyte conditions.
Section snippets
Preparation of materials
The NPMPC was synthesized from EDTA powder, using a simple human’s method. 2 g of ethylenediaminetetraacetic acid, 1 g of triphenylphospine, 1 g of potassium hydroxide, 1 g of melamine and the required amount of Co(Cl)2∙6H2O were mixed until all components were evenly mixed and transferred in an agate mortar. The mixture was filled to an alumina boat and calcined (heating rate: 10 °C/min) to 700 °C for 3 h under nitrogen flow in a tube furnace. The mixture was then cool down at room temperature.
Characterization of NPMPCs
The morphology and structure property of NPMPC were examined via FE-SEM and FE-TEM. Low magnification SEM images (Fig.1(a)) of the NPMPC-0 show that carbon particles formed spacious planks (average width: 20 μm, height: 10 μm and thickness: 2 μm) similar to a plate. The high magnification image in Fig. 1(b) reveals the flat and clean board shape of the NPMPC-0, i.e., the state without added impurities. Low magnification images of the NPMPC-0.6 reveal the very rough and porous surface area (see
Conclusion
We successfully synthesized a catalyst including Co oxides supported on N and P co-doped mesoporous carbon boards via simple one-step pyrolysis of EDTA and melamine in the presence of KOH and Co(Cl) 2∙6H2 O. The resulting NPMPC, with a suitable amount of N and P doping as a highly efficient catalyst, exhibited outstanding electrocatalytic activity and high durability for ORR in alkaline solution. In addition, the coordination between Co and EDTA, along with the shape of the spacious board
Conflict of interest
None.
Acknowledgements
This work was supported by the Human Resources Development (No. 20184030202070) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.
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