Green Hydrogen and Polymer Electrolyte Fuel Cells for Our Future Sustainable Growth
Authors: Ota Kenichiro , Nagai Takaaki , Matsuzawa Koichi , Kuroda Yoshiyuki , Mitsushima Shigenori , Ishihara Akimitsu 
Published: February 2020

Considering our future sustainable growth, renewable energies should be the primary energy. In that way we need technologies to store and transport renewable energies. The Green Hydrogen is the hydrogen from water using renewable energies. The environmental impact factor(EIF) was defined as the ratio of the quantity of materials produced by energy consumption of mankind to the natural movement on the earth. Fig.1 shows the relation between the impact factor and the energy consumption density. By the comparison of environmental impact factor, the Green Hydrogen could keep our environment more than 2 orders of magnitude cleaner compared to that of fossil fuels (1). With using Green Hydrogen, we could produce clean electricity through fuel cells without any pollutants. The Green Hydrogen Energy System might be the final energy system for the sustainable growth of human beings.
Polymer electrolyte fuel cells are expected for the residential and transportable applications, due to their high power density and low operating temperature. Low temperature fuel cells have theoretically higher efficiency compared to higher temperature fuel cells such as SOFCs. Polymer electrolyte fuel cells are expected for the residential and transportable applications, due to their high power density and low operating temperature. Many ENEFARMs (micro CHP) are operating and fuel cell vehicles are also commercially available in Japan. However, the estimated amount of Pt reserve is limited and its cost is high. The instability of Pt cathode and carbon might be the big problems to improve the stability of the present PEFC system. A stable non-precious metal oxide cathode with stable metal oxide support might be the final goal for the cathode of PEFC for fuel cell vehicles. In the future energy system fuel cells should be operated at higher efficiency such as 60 %(HHV) since their theoretical efficiency is very high. To get this high efficiency, fuel cells should be operated at 0.9 V or higher. To get this high operation voltage, their operation temperature might be higher than 120 oC for future PEFCs. At these high potential and temperature Pt and carbon are no more stable. We need new materials, such as metal oxides that are stable in acid and oxygen atmosphere.
We have reported that partially oxidized group 4 and 5 metal carbonitrides and organometallic complexes are stable in an acid solution and have definite catalytic activity for the oxygen reduction reaction (ORR) (2,3). In this paper we will report our recent advancement of the group 4 and 5 metal oxide catalyst with a metal oxide support without carbon.
The ORR activity of the TixNbyOz + Ti4O7 is higher than that of the Ti4O7, indicating that the TixNbyOz might have active sites for the ORR. The highest onset potential of the TixNbyOz +Ti4O7 was over 1.1 V vs. RHE (2). No degradation of the ORR performance of TixNbyOz + Ti4O7 was observed during both start-stop and load cycle tests. The ORR activity depended on the heat treatment temperature. Especially we got the highest ORR activity in reducing atmosphere at 800oC. Ti3+ on the surface might affect the ORR reaction. The ORR activity of ZrOx catalyst that was made by the arc-plasma deposition depended on the thickness of the catalysis layer. The maximum ORR current was obtained at the thickness of 2 nm. The tunneling current might help the electron conduction for ORR.
Considering these factors we are going to improve the ORR activity of group 4 and 5 metal oxide cathodes with oxide support.
The authors wish to thank to the New Energy and Industrial Technology Development Organization (NEDO) for their financial support.
REFERENCES
1) K. Ota, et al., Advances in Hydrogen Production, Storage and Distribution, p.32, Woodhead publishing, UK (2014)
2) K. Ota, et al., Polymer Electrolyte Fuel Cells 17, (ECS Trans., 80(8), 717-714 (2017).
3) K. Ota, et al., Polymer Electrolyte Fuel Cells and Electrolyzers 18, (ECS Trans. 86(13)), 549-558 (2018)

Figure 1