Mesoporous NiCo2O4 Nanosheets Grown on Stainless Steel Meshes As Binder Free Electrodes for Urea Electrolysis

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.

References

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