Highly-Efficient and Durable Carbon Nanotube-Based Anode Electrocatalyst for Water Electrolyzer
Authors: Fujigaya Tsuyohiko , Shi Yilei , Yang Jun , Li Hua , Inada Akiko , Nakashima Naotoshi , Ito Kohei 
Published: February 2020

Water electrolysis is a promising hydrogen production system because of its high efficiency and cleanness in a carbon-neutral energy cycles, which is followed by the hydrogen production and power generation by fuel cells. Especially, proton exchange membrane water electrolyzers (PEMWEs) are currently the most promising candidate due to their compactness, superior efficiency, better load flexibility, and higher purity of hydrogen than alkaline electrolysis. In the PEMWE anode, iridium oxide (IrOx­­) is regarded as the most suitable anode electrocatalyst for the four-electron oxygen evolution reaction (OER) compared to the other catalysts such as ruthenium oxide (RuOx) due to its higher chemical stability even under a high anodic polarization in a strong acidic environment. Since Ir is a very expensive metal and scarce resource, it is highly desired to decrease the amount of Ir in the catalysts. Especially, maximizing the surface area of IrOx by minimizing the size of IrOx is the promising strategy to increase the utilization efficiency of the IrOx. However, it has been considered that a carbon-based conductive support such as carbon blacks (CBs) is difficult to use since the operating conditions of the PEMWE anode (> 1.23 V vs. RHE @ 25 °C) are much more positive than the oxidation potential of carbon (C + 2H2O→CO2 + 4H+ + 4e-, 0.207 V vs. RHE) and, therefore, the carbon can be easily oxidized under PEMWE condition. Therefore, large quantities of bulk IrOx powder without any support material are currently used for the anode catalyst, which increases the cost of the PEMWEs. Recently, non-carbon-based electrocatalyst using semiconducting metal oxides with a high surface area and stability in acid, such as SnO2, Ta2O5, SiO­2, etc., were investigated. However, since the electrical conductivities of these oxides are not as good as carbon materials, further improvement of the conductivity is strongly required. In this study, we chose multi-walled carbon nanotubes (MWNTs) as a conductive supporting material for the IrOx because of the following reasons; i) MWNTs are composed of sp2 carbon and it was pointed out the oxidation potential of the sp2 carbon is much higher than those of the sp3 carbons, which agreed well with the experimental facts that the MWNTs have a superior electrochemical stability compared to other conductive carbons, ii) we have developed a unique and promising technique to immobilize metal nanoparticles without introducing sp3 carbons into MWNTs; that is, the polymer-wrapping method using polybenzimidazole (PBI) as an anchoring layer on the surfaces of the MWNTs,and iii) PBI-wrapped MWNTs decollated by Pt nanoparticles exhibited an extremely high durability even when they are polarized in the high potential region of 1.0-1.5 V vs. RHE in an acidic electrolyte fuel cell tests. Based on such advantages, we newly developed an electrocatalyst based on IrOx nanoparticles loaded on the PBI-wrapped pristine MWNTs (MWNT/PBI/IrOx).As the result, Very high OER mass activity and durability of the MWNT-based electrocatalyst were found in half-cell and single-cell tests.