Partially Oxidized Niobium Carbonitride as Non-Platinum Cathode for PEFC

Abstract

Partially oxidized niobium carbonitride has been examined to apply to a non-platinum cathode for polymer electrolyte fuel cells ͑PEFCs͒. NbC 0.5 N 0.5 powder was partially oxidized in an inert gas containing 30 or 100 ppm O 2 at 1000°C for 10 h. NbO 2 or Nb 2 O 5 was observed in the X-ray diffraction patterns of the specimens. The catalytic activity for the oxygen reduction reaction ͑ORR͒ of the specimen that contained Nb 2 O 5 was higher than that of the specimens with NbO 2 and as-prepared NbC 0.5 N 0.5 . X-ray photoelectron spectra suggested that the highest oxidation state of Nb affected the improvement of the catalytic activity for the ORR.Polymer electrolyte fuel cells ͑PEFCs͒ have drawn much attention because they have many advantages as power sources for residential and transportation applications such as high power density, low operating temperature, and high energy-conversion efficiency. However, PEFCs still have serious problems to commercialize widely. A large amount of Pt nanoparticles was used in both the anode and cathode electrodes of PEFCs. In particular, the Pt nanoparticles at the cathode have a large overpotential of oxygen reduction reaction ͑ORR͒ and dissolve at high potentials. In addition, the natural resource of Pt is too limited to make many fuel cell vehicles. Therefore, development of an alternative material with low cost, high stability, and high catalytic activity for the ORR is vital. It is reported that transition metal oxide-based compounds have high stability and some catalytic activity for the ORR. 1-6 In particular, partially oxidized tantalum carbonitride͑Ta-CN͒ has the onset potential for the ORR of 0.83 V vs a reversible hydrogen electrode ͑RHE͒ in 0.1 mol dm −3 H 2 SO 4 at 30°C. 3 Although partially oxidized Ta-CN would be one of the possible candidates as a non-platinum cathode, the resource of tantalum is as limited as that of platinum. The resource of niobium, 4.4 ϫ 10 3 kton, is 2 orders of magnitude greater than that of tantalum, ͑43 kton͒. Ohnishi et al. reported that oxidebased niobium compounds prepared by reactive sputtering have some catalytic activity for the ORR. 7 In this study, we evaluated the partially oxidized niobium carbonitride ͑Nb-CN͒ as a non-platinum cathode for PEFCs. Nb-CN was synthesized by a mixture of metal Nb and carbon powder at 1500°C in N 2 atmosphere. The effect of the partial oxidation of the Nb-CN on the ORR activity was examined. ExperimentalNb-CN powder was synthesized by using a mixture of Nb powder ͑Ͻ45 m, 99.9%, High Purity Chemicals, Japan͒ and carbon black ͑average particle size of Ͻ1 m, 99.95%, SEC Corp.͒ as starting materials. The Nb and carbon black powders were mixed with a composition of Nb:C = 1:0.55 in molar ratio using a mortar and pestle. A cylindrical-shaped tablet ͑10 mm ϫ 5 mm͒ of their mixed powder was made by uniaxial pressing at 2 MPa, and then the tablet was heat-treated in an alumina tube furnace in N 2 atmosphere at 1500°C for 10 h, resulting in the formation of Nb-CN powder. The partial oxidation of Ta-CN was very...

Catalytically active single-atom niobium in graphitic layers

“…Several important studies on niobium carbonitride with partial oxidation, associated with Nb 2 O 5 species on the surface, have suggested that the surface oxide is the origin of catalysis 27,28 . However, the atomic-scale characterization indicates that there is no surface oxide on the samples and, thus, the origin of catalysis in our materials is the niobium-carbon complexes rather than the oxide.…”

Progress in non-precious metal oxide-based cathode for polymer electrolyte fuel cells

“…We used group 4 and 5 carbonitrides as starting materials for a partial oxidation [94][95][96][97][98]. We have mainly investigated partially oxidized tantalum carbonitride [94,97,98].…”

“…We have developed the oxide-based non-Pt cathode catalysts [22,[48][49][50]65,66,72,[76][77][86][87][88][94][95][96][97][98]103]. Fig.…”

Electrochemical and physicochemical properties of titanium Oxy-nitride electrocatalyst prepared by sol-gel methods for the oxygen reduction reaction purposes

“…They have high chemical stability in acidic media [21,22]. In addition, it has been reported that these compounds have some electrocatalytic activity for the ORR [23,24]. Transition metal oxides, such as zirconium oxide [25][26][27], titanium oxide [28][29][30], manganese oxide [31], tungsten oxide [32,33], niobium oxide [28,34,35], cobalt, and tin oxide [35] have been evaluated as electrocatalyst or catalyst-support for the ORR.…”