Efficient Phosphorus Doping into the Surface Oxide Layers on TiN to Enhance Oxygen Reduction Reaction Activity in Acidic Media

Abstract

Oxide catalysts have been developed in polymer electrolyte fuel cell cathodes to replace platinum-group-metal catalysts while retaining moderate oxygen reduction reaction (ORR) activity. Recently, carbon support-free titanium oxide (TiO 2 ) catalysts formed on the outer surface of titanium nitride (TiN) have been shown to significantly increase ORR activity. In this study, ORR activity was further enhanced by increasing the surface phosphorus content on the disordered TiO 2 layer by using hypophosphorous acid as a phosphorus source. Both the half-wave potential and limiting current density in acidic media were maximized to 0.66 V and 5.18 mA cm −2 , respectively. Besides, the durability against high potential cycles between 1.0 and 1.5 V versus the reversible hydrogen electrode, which is critical to be used in vehicles, was enhanced. The decrease in half-wave potential during the 5000 cycles was suppressed to 0.08 V, which is 0.03 V lower than that in a previous study in which phosphoric acid was used. Furthermore, nitrogen doping on the catalyst under an NH 3 flow improved the 4-electron ORR selectivity and reduced the hydrogen peroxide yield to less than half of the value with undoped catalysts. Although some phosphorus and nitrogen atoms were removed from the catalyst under high potential cycles, the surface oxides protected inner TiN for 5000 cycles. Codoping the TiO 2 surface with large amounts of both phosphorus and nitrogen atoms is necessary to use this catalyst without relying on a high-cost system in which the potential is kept below 1.0 V.

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

“…However, the E g peak position of the Ti 0.8 Zr 0.2 O x N y catalyst displayed in Figure b­(ii) is ∼404 cm –1 , almost the same as that of 2% Fe 3+ -doped TiO 2 , 402.5 cm –1 , which is estimated to contain 1% of oxygen vacancies by assuming the charge neutrality. The observed E g peak at ∼404 cm –1 from the Ti 0.8 Zr 0.2 O x N y catalyst is lower than that of the previously reported phosphorus and nitrogen co-doped rutile TiO 2 covered on TiN, TiO x N y P z catalyst, ∼412 cm –1 , indicative of the larger number of oxygen vacancies on the surface of Ti 0.8 Zr 0.2 O x N y than TiO x N y P z . In addition to the three rutile TiO 2 peaks, two more peaks appear near 300 cm –1 when d ≥ 0.3, as shown in Figure b­(iii) and b­(iv).…”

“…This agrees with the lattice distance calculated from the TiN(1 1 1) peak in the corresponding XRD pattern in Figure a­(ii). This value is slightly larger than that for zirconium-free TiN, 0.24 nm, due to lattice expansion via zirconium doping. The outermost surface of the catalyst is disordered, as indicated by the black arrows.…”

“…The catalyst was synthesized using a two-step heating process; it was first heated under N 2 at 1173 K for 2 h and then heated under NH 3 at 1073 K for 3 h. Before and after the 5000 cycles, scans were performed under N 2 and O 2 atmospheres using a rotation speed of 1500 rpm and a scan rate of −5 mV s −1 (cathodic) to evaluate the activity. For reference, RDE voltammograms of the TiO x N y P z catalyst before (dash-double-dotted curve) and after (dashed curve) 5000 cycles 44 are shown.…”

“…Ti 1– d Zr d O x N y catalysts were synthesized via a modified combustion route − without using supports. First, 0.49 g of titanium oxysulfate (TiOSO 4 · n H 2 O; n ≑ 1–2, Kishida Chemical Co. Ltd., Osaka, Osaka, Japan) and various amounts of zirconium chloride (ZrCl 4 , Fuji Film Wako Pure Chemical Co., Osaka, Osaka, Japan) powders were dispersed in 371 cm 3 of distilled water by stirring in a polytetrafluoroethylene (PTFE) beaker with a PTFE-coated magnetic bar.…”

“…As mentioned above, oxide compounds exhibit poor conductivity. Because of this, nitrogen-doped TiO 2 layer (TiO x N y ) catalysts were formed on conductive TiN particles without the use of carbon supports. − Some control experiments revealed that both the ORR activity and TiO x N y conductivity are driven by TiN rather than by urea-derived carbon residues. , Recently, phosphorus atoms were doped into the surface oxide layers. The resulting materials exhibited the highest mass activities of any oxide/oxynitride catalysts, while some H 2 O 2 molecules were formed as byproducts during the ORR .…”