Twofold Effects of Zirconium Doping into TiN on Durability and Oxygen Reduction Reactivity in an Acidic Environment

Abstract

Carbon-support-free TiN-based catalysts have recently been developed to avoid both carbon oxidation and the use of scarce platinum group metals in acidic polymer electrolyte fuel cell cathodes. However, providing sufficient durability at high potentials above 1.0 V remains a challenge. Herein, zirconium doping is revealed as a new route that enhances catalyst activity and selectivity toward the four-electron oxygen reduction reaction (ORR) in an acidic environment. The TiN surface is oxidized to form rutile TiO 2 layers, and some zirconium atoms are dissolved into both the bulk TiN and the surface rutile layers. The zirconium atoms distort the rutile lattice to increase the number of oxygen vacancies which are critical for the ORR, whereas some others segregate to form monoclinic and tetragonal ZrO 2 phases to inhibit the TiN crystallite growth. The optimized Ti 0.8 Zr 0.2 O x N y catalysts exhibit excellent durability during 5000 start-up and shut-down cycles between 1.0 and 1.5 V versus the reversible hydrogen electrode in 0.1 mol dm −3 H 2 SO 4 solution. The decrease in the halfwave potential during the 5000 cycles is only 0.04 V, which is half that of the previous best phosphorus-doped TiN catalyst.

Enhancement of oxygen reduction reactivity on TiN by tuning the work function via metal doping

“…20 We have previously shown that TiN on the surface of Ti 0.8 Zr 0.2 O x N y evaluated by visible Raman spectroscopy with a 532 nm laser is oxidised to a rutile TiO 2 phase by moisture in the air. 15 The UV Raman spectrum shown in Fig. 1(b)-(i) is clearly different from that of rutile TiO 2 powder measured under the same conditions, indicated by the dashed line.…”

“…14 Both the activity and durability of TiN have recently been enhanced via doping with zirconium (Zr) as Zr (1) distorts the TiN lattice to create active sites and (2) inhibits crystallite growth to increase surface area during high-temperature synthesis. 15 The catalyst with the optimised composition, Ti 0.8 Zr 0.2 O x N y delivered high durability during 5000 startup and shutdown cycles to display only 0.04 V decrease in halfwave potential. 15 The durability is the merit of this catalyst when compared with M/N/C catalyst which significantly degraded due to the oxidation of carbon species.…”

“…15 The catalyst with the optimised composition, Ti 0.8 Zr 0.2 O x N y delivered high durability during 5000 startup and shutdown cycles to display only 0.04 V decrease in halfwave potential. 15 The durability is the merit of this catalyst when compared with M/N/C catalyst which significantly degraded due to the oxidation of carbon species. 16 However, the initial activity should be enhanced further for the practical use.…”

“…6,7 Recently, the use of PGMs for the metal site such as Ir/N/C and Rh/N/C has been proposed. 8 The other is oxide catalysts with group IV or V metals, [9][10][11][12][13][14][15] which have the advantage of high chemical stability and do not dissolve in the acidic environment of PEFCs. 10 However, they exhibit poor conductivity that inhibits their activity evaluation, even in screening the catalyst candidates in a half cell in liquid electrolytes.…”

“…Zr, niobium (Nb), nickel (Ni) and vanadium (V) were selected as TiN dopants, with all of the dual nitrides synthesised via combustion, [13][14][15] as detailed alongside the characterisation of the materials in S1 (ESI †). As the surface of TiN is oxidised by moisture in the air, 18 the catalysts were denoted as Ti 1Àd -M d O x N y , with the doping level d of M (Zr, Nb, Ni or V) fixed at 0.2, the optimum value for the reactivity of Ti 1-d Zr d O x N y .…”