Biomass-Derived Carbon Supported on Nickel Oxide for Efficient Urea Oxidation Reaction

Abstract

In recent years, there has been a fast-growing trend in developing urea (CO(NH2)2) as a substitute H2 carrier in energy conversion due to its high energy density, nontoxicity, stability and non-flammability.[1] Urea, a byproduct in the metabolism of proteins and a frequent contaminant in wastewater, is an abundant compound that has demonstrated favorable characteristics as a hydrogen-rich fuel source with 6.7 wt.% in gravimetric hydrogen content.Also, there is 2-2.5 wt.% urea from mammal urine, therefore, 0.5 million tons of additional fuels will be produced per year just from human urine (240 million tons each year). Electrochemical oxidation has been recognized as an efficient strategy for urea conversion and wastewater remediation. Thus, the chemical energy harvested from urea/urine can be converted to electricity via urea oxidation reaction (UOR). Moreover, the removal of urea from water is a priority for improving drinking water quality and presents an opportunity for UOR. However, the transition of UOR from theory and laboratory experiments to real-world applications is largely limited by the conversion efficiency, catalyst cost and the feasibility of wide-spread usage.Recent studies implementing common transition metals and their oxides, particularly nickel, have found similar success while having much lower material costs. For instance, Luo et al. prepared ultrathin and porous nickel hydroxide nanosheets for efficient UOR, and found that 1.82 V (vs. RHE, Reversible Hydrogen Electrode) was needed to achieve a large current density of 298 mA cm-2.[2] However, there is still a sluggish kinetics of UOR at the anodic area owing to the multi-electron transfer and multiple gas-adsorption/desorption procedures. To address this key issue, the coordination of high surface area and, conductive materials are considered beneficial. Biomass-derived carbon materials have been increasingly implemented in electrochemical energy conversion and detection owing to their low-price, porous structure and high conductivity. Hierarchical porous activated carbons are favored in particular due to their variety in pore size and volume, the potential for modification, and synthesis from waste biomaterials. One promising material that has demonstrated notable electrochemical properties is the eggshell membrane (ESM). ESM is a thin, protein-based membrane functioning as a gas-exchange interface for the embryo within the egg to the outside world via its abundant micro- and nano-sized pores. Aside from the traditional methods of waste management, ESM has been used in the production of clean energy where it replaces coal, oil or natural gases to generate electricity through fuel cell devices. ESM collected from waste eggshells has demonstrated excellent electrochemical behavior on its own in energy storage and conversion as well an ability to be infused with different transition metal oxides for sensing purposes.In this abstract, we reported a low-cost UOR electrocatalyst (C@NiO), composed of nickel oxide nanoparticles anchored on the porous carbon derived from the biowaste eggshell membrane via hydrothermal synthesis and pyrolysis strategy[3]. Benefiting from the strong synergistic effect between nickel oxide and the porous carbon, the as-prepared electrode only needs 1.36 V versus RHE to realize 10 mA cm-2 in 1.0 M alkali solution containing 0.33 M urea, and delivers 25 mA cm-2 at 1.46 V. In addition, in the viewpoint of the theoretical calculations, its intermediate (C@NiOOH), which formed from C@NiO in alkaline solution, made this electrocatalyst possessing the ability to effectively hinder “CO2 poisoning”, as well as ensuring its superior performance for UOR.