Mesoporous NiCo2O4 Nanosheets Grown on Stainless Steel Meshes As Binder Free Electrodes for Urea Electrolysis
Authors: Wang Dan , Botte Gerardine G 
Published: February 2020

To alleviate the requirement of non-renewable fossil fuels and satisfy the increasing global energy demands, it is important to seek and develop the sustainable and environmental friendly energy sources. Recently, urea (CO(NH2)2) has been demonstrated as a hydrogen carrier for long-term sustainable energy supply due to its intrinsic properties, such as non-toxic, non-flammable, and easy storage.1-3

Urea electrolysis with an inexpensive bulk nickel catalyst showed great promising in directly converting urea into valuable energy product (H2) and a non-toxic product (N2).1 Nanostructured nickel catalysts enhanced the electrocatalytic oxidation of urea due to their large surface areas and controlled low dimensional structures.4 However, the introduction of polymer binders for the nanostructured nickel powder based electrode preparation unavoidably decreased the conductivity of catalysts and thus reduced the electrocatalytic performance of urea electrolysis. In addition, previous research showed that the incorporation of cobalt into nickel catalysts decreased the overpotential of urea oxidation and improved the reaction rate considerably.5 Nickel cobaltite (NiCo2O4) is well-known electrode materials for supercapacitors and lithium batteries and nanostructured NiCo2O4 exhibited their improved electrochemical performance.6,7 Therefore, it’s interesting to extend the applications of nanostructured NiCo2O4 to urea electrolysis for hydrogen production.

Within this context, this work focuses on directly synthesizing mesoporous NiCo2O4 nanosheets on conductive substrates, such as stainless steel mesh, and using it as binder free electrode for urea electrolysis. TEM image (Figure 1 a) shows the morphology of two dimensional NiCo2O4 nanosheets. XRD pattern (Figure 1 b) shows the crystalline structure of NiCo2O4 nanosheets. Figure 1 d (curve 2) shows a strong oxidation current starting at ca. 0.38 V vs. Hg/HgO when 0.33 M urea was present in the KOH solution. As a comparison, the urea oxidation at the bulk Ni(OH)2 powder electrode (Figure 1 c) started at ca. 0.46 V vs. Hg/HgO. Thus the NiCo2O4 nanosheets electrode reduced the onset potential of the oxidation process of urea by 80 mV. Furthermore, the NiCo2O4 nanosheet electrode sharply increased the urea oxidation current density compared to bulk Ni(OH)2electrode.  

Figure 1. (a) TEM image of mesoporous NiCo2O4 nanosheets; (b) XRD pattern of NiCo2O4; (c) Cyclic voltammograms of bulk Ni(OH)2 electrode in 5 M KOH solution in the absence (curve 1) and presence (curve 2) of 0.33 M urea. (d) CV of NiCo2O4 nanosheet electrode in 5 M KOH solution in the absence (curve 1) and presence (curve 2) of 0.33 M urea. The scan rate was 10 mVs-1.

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Figure 1