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

Progress in non-precious metal oxide-based cathode for polymerelectrolyte fuel cellsPresented BySuchith SadanandContents of Presentation:IntroductionTransition metal nitrides and carbonitrides as cathode catalystsStability of oxides in acid electrolyteNon-precious metal oxide-based cathode catalystsStability of group 4 and 5 metal oxide-based catalystsFormation of complex oxide layer containing active sitesSubstitutional doping of nitrogenIntroduction of surface oxygen defectsPartial oxidation of carbonitridesConclusionsIntroductionPolymer electrolyte fuel cells (PEFCs) are expected as power sources for residential and transport applicationsPt is generally used as a cathode catalyst in the present PEFCsThese catalysts have poor stability in an acid media such as H2SO4 and polymer electrolytesRu and Rh are precious metals and their resources are very limitedTransition metal nitrides and carbonitrides are used as anticorrosion and coating materialssurface modificationsFormation of complex oxide layer containing active sitesSubstitutional doping of nitrogenIntroduction of surface oxygen defectPartial oxidation of carbonitrides.Transition metal nitrides and carbonitrides as cathode catalysts

Electrochemical reduction of oxygen was carried out on amorphous cobalt-nitride thin film electrode prepared by the reactive RF sputteringFeCN and CoCN thin films prepared by sputtering with metal and carbon target under N2 were mainly investigated because Fe and Co were thought to act as active sites in macrocyclic complexesFe dissolved out of FeCN, the activity remained after the acid exposure

Dependence of onset potential for the ORR on N content of FeCN and CoCN prepared by combinatorial magnetron sputter deposition in an Ar/N2 gas mixture followed by subsequent heat treatmentStability of oxides in acid electrolyte

Platinum were thermo chemically unstable in acid electrolyte under O2In the present PEFCs, carbon-supported Pt or Pt-alloy electro catalysts are widely usedThe carbon corrosion due to oxidation under the cathode condition causes a serious degradation of the electro catalystsThe oxides of group 4 elements such as titanium, zirconium, and hafnium are thermo chemically unstable in acid electrolyte

Band structure of groups 4 and 5 metal oxidesNon-precious metal oxide-based cathode catalysts 4.1.Stability of group 4 and 5 metal oxide-based catalysts(i)Before the discussion on the catalytic activity for the ORR, the stability of the oxide-based catalysts should be considered.(ii)Table 2 shows the solubility of oxide-based catalysts prepared by various methods in 0.1 mol dm

(iii)In addition, cyclic voltammograms of these compounds in 0.1 mol dm3 H2 SO4 in inert atmosphere under the potential range of PEFC operation reached steady state immediately, and the anodic charge was cor- responded to the cathodic one.

4.2. Formation of complex oxide layer containing active sites

Tungsten carbide has platinum-like electronic structure [52] and exhibits platinum-like behavior for hydrogen chemisorption and isomerization of 2,2-dimethypropane These results showed that tung- sten and other carbides were unstable at the high potential region in an acid solution.We examined the effect of addition of tantalum to tungsten car- bide using thin lms prepared by sputtering [65,66]. Fig. 3 shows cyclic voltammograms of pure tungsten carbide and tungsten car- bide with tantalum addition on a polymer electrolyte under N2 or O2 atmosphere.Therefore, they concluded that TaNiC ternary system preparedBy the dc magnetron sputtering method was not a good system for the ORR catalyst. It seemed that the carbon was also important although Burstein and Dahn did not emphasize the effect of the carbon. In addition, long-term durability should be investigated.

4.3. Substitutional doping of nitrogenThe density functional theory calculations show that the valence band is composed mainly of the anion 2p orbitals hybridized with metal d states. In case of TaON, the upper energy level of the valence band is dominated by N 2p states . The pure WC catalyst showed a signicant anodic current above 0.5 V (vs. dynamic hydrogen electrode; DHE). The tungsten in the WC was oxidized to tungsten oxide (WO3 ) with the evolution of CO2.However, the onset potential of their best catalysts (Ta0.26 Ni0.73 C0.01 and Ta0.01 Ni0.16 C0.83 ) was about 0.45 V vs. RHE. Therefore, they concluded that TaNiC ternary system prepared by the dc magnetron sputtering method was not a good system for the ORR catalyst.

4.4. Introduction of surface oxygen defects

Adsorption of oxygen molecules on the surface was required as the rst step to proceed the ORR. Many researches reveals that the presence of surface defects sites is required to adsorb the oxygen molecules on the surface of the oxides such as TiO2 (11 0) In addition, these oxides were inactive for methanol oxidation. Fig. 6 shows the potential-current curves of ZrO2 x and TiO in 0.1 mol dm3 H2 SO4 under O2 with or without 0.1 mol dm3 methanol.Titanium oxide prepared by the heat treatment of titanium sheets under nitrogen gas including a trace of oxygen had also some catalytic activity for the ORRAn important nding from these studies is that the surface of the oxides with high ORR activity was almost completely oxidized although these oxides were expected to have the defects on the sur- face.

4.5. Partial oxidation of carbonitridesThe surface oxidation state would be changed by the control of a partial oxidation of non-oxides. We used group 4 and 5 car- bonitrides as starting materials for a partial oxidation.TaC0.58 N0.42 was heat-treated under a trace amount of oxygen gas at 1000 C. The XRD patterns of the heat-treated specimens showed both Ta2 O5 and TaCN peaks. It was noted that the mixture of the car- bon black such as Ketjen Black as a current collector was required to obtain a sufcient electrical conductivity because the surface of the partially oxidized specimens considerably oxidized.

Although few differences were observed between a partially oxidized TaCx Ny and a commercial Ta2 O5 by XPS, the catalytic activities were quite different and they depended on the ioniza-

Conclusions

In the present review we have summarized development of nonprecious metal oxide-based cathode for polymer electrolyte fuel cell.

The modifications were effective to improve the catalytic activity to the oxides with a high stability