Transition Metals on N-doped Graphene-ZrO۲ Composite as ORR Electrocatalystin Alkaline Media

There have been challenges with energy conversion and storage systems due to slow oxygen reductionreactions [۱,۲]. Since Pt exhibits high activity and durability in acidic and alkaline media, atremendous number of precious metals is used as electrocatalysts. However, problems such as the highprice, scarce reserve, and poor durability of Pt-based electrocatalysts have led researchers to substitutethem with nonprecious metal catalysts (NPMCs) such as metal-nitrogen-carbon composites. Due to apotential synergistic effect between these two compounds, composite materials like nitrogen-dopedcarbonaceous structures with transient metal oxide (TMO) can significantly change this field. Dopingdifferent transition metals like Ni, Zn, Fe, Co, and Cr into electrocatalysts further enhances ORRcatalytic activity by modulating the surface adsorption energies of the intermediates [۳].Generally, one-dimensional nanotubes and two-dimensional graphene are considered suitable supportsbecause of their low cost, high surface area, excellent mechanical and electrical properties, andsufficient stability under harsh conditions, and easy operation [۴]. On the other hand, when nitrogenand carbon atoms are bonded, nitrogen pulls the carbon electrons toward it. It induces a positive chargeon the carbon atoms, facilitating oxygen atom absorption by the nanostructure [۲]. Furthermore, theredox properties of TMOs' NPs, such as TiO۲, WOx, Fe۳O۴, SnO۲, and ZrO۲, and the ease with whichthey can be synthesized, have attracted considerable attention [۵]. In the meantime, zirconium (Zr) is atransition metal with excellent corrosion resistance.Among different approaches to dope metals into the structure of carbonaceous carbon the polyolmethod is the best method of saturation and deposition of metal alloys on carbon. This matter isbecause of generating higher dispersion of metal particles (Pt, Fe, and Co), achieving optimal metalparticle sizes on the carbonaceous support, and having better catalytic performance [۶]. Therefore, inthis work, two electrocatalysts were prepared to investigate the effect of N-doping in carbonaceous supports and the deposition of transient metal NPs on the supports surface. First, a composite of CNTwith ZrO۲ and N-doped graphene with ZrO۲ were prepared to study the effect of heteroatom-doping.Second, additional transition metals Fe-Co NPs were deposited on these supports. According to CVand LSV results (cathodic peak at -۰.۳ VSCE and Eonset=-۰.۱۳ VSCE), the graphene support doped withnitrogen which then decorated with Fe and Co NPs (Fe-Co/ZrO۲-NG) could represent better activitycompared to its counterpart (Fe-Co/ZrO۲-CNT) (Figure ۱). As a result of the chronoamperometry test,this optimum electrocatalyst retained ۹۲% of its initial current density, which is comparable tocommercial Pt/C electrocatalysts.

Keywords:

Transition metal-carbon nanocomposite , Oxygen reduction reaction (ORR) , Nitrogendopedgraphene , ZrO۲ , Nonprecious metals