α‐Co(OH)₂ Thin‐Layered Cactus‐Like Nanostructures Wrapped Ni₃S₂ Nanowires: A Robust and Potential Catalyst for Electro‐oxidation of Hydrazine

Abstract

It is of significant urgency to fabricate highly active catalysts for electro‐oxidation of hydrazine for application in direct hydrazine fuel cells (DHFCs). Thus, in this work we grow three‐dimensional (3‐D) α‐Co(OH)2 thin‐layered cactus‐like nanostructures (shell) on the surface of Ni3S2 nanowires (core) enveloped nickel (Ni) foam substrate. HRTEM images clearly reveal that the α‐Co(OH)2 thin‐layered cactus‐like nanostructures (shell) are evenly surrounded on the surface of Ni3S2 nanowires (core). Notably, α‐Co(OH)2/Ni3S2 thin‐layered cactus‐like nanowires modified Ni foam displays an enhanced electro‐oxidation of hydrazine (340 mA current response and the onset oxidation potential of −1.10 V) than α‐Co(OH)2 sponge‐like, and Ni3S2 nanowire structures. The obtained enhanced electro‐catalytic response is mainly due to its electron rich Ni active centers, larger electrochemically active surface area, higher electrical conductivity, and porous surface‐structure formed by growing α‐Co(OH)2 thin‐layered cactus‐like nanostructures on the Ni3S2 nanowires, henceforth increasing the inherent activity and the number of available active sites/spots. Further, the α‐Co(OH)2/Ni3S2 thin‐layered cactus‐like nanowire catalyst shows notable stability (stable for more than a week) towards electro‐oxidation of hydrazine. Thus, as fabricated cactus‐like thin‐layered α‐Co(OH)2/Ni3S2 core‐shell nanowires catalyst can be considered as potential and robust catalyst for electro‐oxidation of hydrazine.

Co Nanoparticle-Encapsulated Nitrogen-Doped Carbon Nanotubes as an Efficient and Robust Catalyst for Electro-Oxidation of Hydrazine

“…The electro-oxidation of hydrazine was noticeably enhanced with respect to the increasing concentrations of the NaOH solution (Figure 5C; curves a-e); however, the hydrazine onset oxidation potential slightly shifted to the positive side due to the high density of available OH − ions. In other words, the efficient diffusion of hydrazine molecules was hindered due to a large amount of OH − ions, signifying that the oxidation of hydrazine is purely subject to the optimized concentration of hydroxide ions (OH − ); since an optimal concentration of OH − ions (1.0 M NaOH) supports the catalytic oxidation reaction that converts hydrazine into nitrogen and water in 1.0 M NaOH (Scheme 1), this inference is in agreement with earlier studies [9,10]. Using LSV polarization curves, the Tafel plots (Figure S3) were obtained for NCNTs (Figure 5D; curve a), Co@NCNTs-2 (Figure 5D; curve b), and Co@NCNTs-1 (Figure 5D; curve c) GCEs in 1.0 M NaOH and 0.1 M Hydrazine solution.…”

“…Comprehensively, the adsorption of OH − and hydrazine (N 2 H 4 ) on the surface of Co@NCNTs-1 catalyst can be considered as the rate-determining step (N 2 H 4ads + OH − ads /N 2 H 3ads + H 2 O + e − ) [45], and the adsorption of OH − ions on the surface of Co sites might be the principal step for the electro-oxidation reaction of hydrazine. The Co@NCNTs-1 catalyst displays large available Co sites, as revealed by the Co 2p XPS spectra, which improves adsorption of the OH − ions and facilitates a superior polarization reaction with a lower onset potential for electro-oxidation reaction of hydrazine [9,10]. Ω for NCNTs (curve b), 17.7 Ω for Co@NCNTs-2 (curve c), and 8.4 Ω for Co@NCNTs-1 (curve d), demonstrating that a faster interfacial electron transfer reaction takes place during the hydrazine electro-oxidation at Co@NCNTs-1 due to its higher electronic conductivity, its large active surface area, the presence of several micro-and mesopores, and the inherent synergistic activity between N-doped CNTs vs. Co nanoparticles [46].…”

“…OHand hydrazine (N2H4) on the surface of Co@NCNTs-1 catalyst can be considered as the rate-determining step (N2H4ads + OHads/N2H3ads + H2O + e -) [45], and the adsorption of OHions on the surface of Co sites might be the principal step for the electro-oxidation reaction of hydrazine. The Co@NCNTs-1 catalyst displays large available Co sites, as revealed by the Co 2p XPS spectra, which improves adsorption of the OHions and facilitates a superior polarization reaction with a lower onset potential for electro-oxidation reaction of hydrazine [9,10]. The Co@NCNTs-2 catalyst (without CaCl2) was also used to examine the electro-oxidation reaction of hydrazine.…”

“…Though, the incomplete oxidation of small organic molecules can be accumulated on intermediate products, thereby considerably diminishing fuel cell efficacy. Thus, direct hydrazine fuel cell (DHFC) technology was adopted for the use of a greener, carbon-less fuel known as hydrazine (N 2 H 4 ), which has a higher hydrogen content (12.5 wt.%) than sodium borohydride (10.6 wt.%), and is comparable to methanol; hydrazine has received considerable recognition in fuel cell technology [8][9][10][11][12][13]. Furthermore, hydrazine shows significant benefits, including a larger theoretical cell-voltage (1.56 V vs. SHE), environmentally harmless final-products (only N 2 and H 2 O), a higher energy density (5500 W h L −1 ), and easy transportation and storage [14][15][16].…”

Prior oxidation of Ni substrates increases the number of active sites in Ni3S2 obtained by sulfidation and enhances its multifunctional electrocatalytic activity

“…This was true for all the reaction studied here. In addition to this, some of the recent reports of the OER, MOR, GOR, and HzOR with Ni and other 3d metal-based catalysts 40–52 are compared in Table 1 and benchmarked against our catalyst (Ni 3 S 2 –O/Ni) which proves that our strategy of pre-oxidation/hydroxylation is superior to the other methods for obtaining such electrocatalysts. From Table 1, it can be noted that our catalyst outperformed most of them in terms of activity while having relatively lower catalyst loading and containing no noble metals.…”