Boron and Nitrogen Co-Doped Porous Carbon Materials Derived from Orange Peels As an Electrocatalyst for the All-Vanadium Redox Flow Batteries

Nowadays, intensive efforts have been achieved to overcome the drawbacks of carbon felts by using surface functionalities for abundant active sites or increasing electron conductivities. In this regard, various catalyst materials have been reported, which is summarized by metal- or carbon-based materials. The metal-based catalysts (Pt, Bi, and Ir) have demonstrated improved cycle life and catalytic activity towards the vanadium redox reactions. Pacific Northwest National Laboratory (PNNL) has reported novel electrocatalysts that employ a bismuth nanoparticle or niobium oxide nanorod. In contrast, carbon-based catalysts have been highlighted because of its stability in concentrated-acidic condition and high electrical conductivity. Modified multi-walled carbon nanotubes (MWCNTs), carbon nanotubes/nanofibers (CNTs/CNFs) composite, N-doped CNTs, N-doped graphene, graphene oxide (GO), GO/MWCNT have been developed. However, most of them use expensive materials such as CNTs, which is not cost-effective.Cost-effective catalysts derived from biomass for energy storage materials such as ORR catalysts, supercapacitors, and VRFBs have been reported using peanut shell, coconut shell, walnut shell, bamboo, sunflower seed shell and so on. Recently, varieties of porous carbon materials that are derived from plant biomass have been synthesized. For example, Chen et al. used Typha orientalis to synthesize nitrogen-doped nanoporous carbon nanosheets for oxygen redox reaction (ORR) catalyst. Gao et al. presented nitrogen-doped carbon by using amaranthus for ORR catalyst. Madhavi et al. used coconut shell as the mesoporous carbon electrode for VRFBs. Yet, the orange peels have not been introduced as a bio-derived carbon precursors. Orange processing in the United States produces ~700,000 tons of peel wastes annually. A orange peel has unique heterogeneous structure, including cellulose, hemicelluloses, lignins, pectins, free sugars, proteins. Lignin can play a key role in order to convert orange peel into porous carbons through pyrolysis. Proteins such as ceramide in orange peels also exclude the need for external nitrogen sources such as NH₃gas. Heteroatom doping methods have been widely used for various applications such as ORR catalysts or supercapacitors electrodes. In general, heteroatom doping on carbon framework enhances electron conductivity, electrolyte permeability, and ion-accessibility on the electrode surface. In case of nitrogen atoms, the defective active sites and charge delocalization in carbon atoms can be formed by doped nitrogen atoms. As for boron atoms, the catalytic activity of the carbon-based catalysts can be attributed to the asymmetric spin density of carbon atoms induced by electron-deficient boron doping. This positively charged boron atom is favorable to capture of the reactants. In particular, the co-doping with two elements (e.g. B, P, or S with N) enhances much more catalytic activities than singly-doped catalysts. Difference in electronegativity (x) induces asymmetric spin density of atoms, enhancing the catalytic activities. Thus, the synergetic effects among different heteroatoms can be expected by using boron atoms with lower electronegativity (x = 2.04) and/or nitrogen atoms with higher electronegativity (x = 3.04) than carbon atoms (x= 2.55).

To find optimized conditions for boron and nitrogen co-doping as an electrocatalyst for vanadium redox reactions, we suggest environmentally friendly and cost effective method for porous carbon catalysts by a self-contained nitrogen in orange peels with additional boron co-doping. This heteroatom co-doped porous carbon coated with the ketjenblack nanoparticles (KB-BNPC) includes defect-rich active sites. Improved catalytic activities toward vanadium redox couples of V²⁺/V³⁺ and VO₂⁺/VO²⁺ were demonstrated, exhibiting increased redox onset potentials and peak current. Notably, we controlled the doping ratio of B and N atoms, suggesting that B/N ratio of 0.65 gives rise to synergistic effect of the heteroatoms for vanadium redox reactions. We believe that the optimized porous structures and defects in the KB-BNPC catalyst facilitate the absorption and desorption of the vanadium ions by enhancing electron and mass transfer kinetics.