Titanium Dioxide/N-Doped Graphene Composites as Non-Noble Bifunctional Oxygen Electrocatalysts

Abstract

Bifunctional oxygen electrocatalysts are essential in the development of low-temperature unitized regenerative fuel cells (URFCs), as a promising alternative for storing energy via hydrogen. TiO 2 , as a semiconductor material, is commonly not established as an active electrocatalyst for oxygen reduction and oxygen evolution due to its poor electrical conductivity and low reactivity. Here, we demonstrated that composites composed of TiO 2 and N-doped graphene can be active in oxygen reduction and evolution reactions in an alkaline environment. Combination factors such anatase/rutile interaction, N-doping graphene, and the presence of Ti 3+ /Ti–N species raise the active sites and improve the electrochemical activity. Our results may afford an opportunity to develop a non-noble and promising electrocatalyst in energy storage technology.

Sulfur-Doped Carbon Nanofibers as Support for Tantalum Oxides Bifunctional Catalysts for the Oxygen Reduction and Evolution Reactions

“…The two S-doped TaOx/CNF catalysts have less OER activity than the TaOx/CNF in terms of current at a fixed potential (j 1.65 V vs RHE ) or overpotential (η), with exception of TaOx/CNF-S250 at low current density. By comparing the two doped samples, the one treated at 250 • C presents a better OER activity as reflected by its lower overpotential and higher current density, although the oxygen efficiency is higher for the catalyst doped at 400 • C. Compared to a benchmark commercial IrO 2 catalyst (η = 370 mV at 10 mA cm − 2 [7]), the TaOx catalysts present between only 35 and 120 mV higher overpotential.…”

“…For example, when combining sodium tantalate (Na 2 Ta 8 O 21 ) with tantalum oxide (Ta 2 O 5 ) and tantalum nitride (Ta 3 N 5 ), the ORR activity is much higher (E onset = 0.9 V vs. RHE in 0.1 M KOH) [40]. Compared to bifunctional catalysts, titanium oxide (another group 4 metal) combined with N-doped graphene [7], or some other combinations of metal oxides (Fe, Co, Ni) with carbon nanofilaments (nanofibers, nanotubes) [23,26,42], present much better ORR activity (E onset above 0.9 V vs. RHE in alkaline conditions), which is closer to benchmark commercial Pt/C catalyst (E onset = 1.01 V vs. RHE [7]). In any case, the results of this work offer a Tafel plots are shown in Fig.…”

“…The reversibility of published bifunctional catalysts in terms of ΔE is variable and its determination depends on the electrolyte used and the criteria for calculation. For example, Luque-Centeno et al reported ΔE of 846 mV for a catalyst based on titanium supported on N-doped graphene, at 5 mA cm − 2 in 0.1 M NaOH [7]. Other transition metal-based bifuncional catalysts from the state of the art exhibit ΔE values above 800 mV (spinels or metals combined with N-doped carbons) or above 900 mV (perovskites), with nanocomposites presenting the best bifunctional behavior with ΔE in the range 700-800 mV, similar to the best noble metal combinations [44][45][46].…”

“…However, their main shortcoming is related to the weak resistance to corrosion and oxidation of carbon materials during OER, which restricts their use as bifunctional oxygen catalysts. To solve this issue, advanced carbon nanostructures, such as graphene, carbon nanofibers or carbon nanotubes, have shown promising performance and stability as bifunctional catalysts [5][6][7]. Among them, one dimensional carbon materials like carbon nanofibers (CNFs) have shown great potential because of their graphite-like structure together with more exposed active sites [8].…”

Fe/N/S Co-doped Porous Carbon from the Co-processing Residue of Coal and Heavy Oil for an Efficient Oxygen Reduction Reaction

“…Recently, the conversion of carbon-containing industrial wastes into high value-added carbon-based materials and then as ORR electrocatalysts has attracted wide attention from researchers. − These raw materials usually contain heteroatoms (N, S, P, etc. ), which can further reduce the dosage of toxic and polluting dopants and increase the effective utilization rate of resources. − Unfortunately, these materials often have the problems of pore structure collapse or insufficient active sites in the preparation of ORR electrocatalysts. − It is necessary to further pore-form and introduce active metal components with redox activity and electronic regulation , to improve the electrocatalytic performance and apply it to the actual operation of fuel cells. , …”