Iridium Oxide Nanoparticles Supported on Polybenzimidazole-Wrapped Multi-Wall Carbon Nanotube As Efficient and Durable Electrocatalysts for Proton Exchange Membrane Water Electrolyzers
Authors: Fujigaya Tsuyohiko , Yang Jun , Ito Kohei , Shi Yilei , Nakashima Naotoshi 
Published: February 2020

Water electrolysis technology has attracted much attention because it serves as the key part of a carbon-neutral energy storage system as follows: electricity generated from renewable resources such as sunlight, water power, and wind, is used to produce hydrogen by water electrolysis. Then, the hydrogen is supplied to fuel cell system for power generation, which only generates electricity and water. Proton exchange membrane water electrolyzers (PEMWEs) are currently the most promising candidate because polymer electrolyte membrane provides higher ionic conductivity, better load flexibility, and purer hydrogen than alkaline electrolyte solution, anion exchange membrane, and solid oxide electrolyte, etc.

The key challenge for PEMWEs is to develop efficient anode electrocatalyst because of the high catalytic efficiency loss related with the oxygen evolution reaction (OER) on anode. In addition, the use of PEM requires the anode material durable under long-term anodic polarization in acid environment, which significantly limits the options of materials. At present stage, iridium oxide (IrO2­) is widely regarded as the most promising anode electrocatalyst for PEMWEs owing to its high activity for OER and chemical stability in strong acid environment. Considering the price and scarcity of Ir, however, it is highly desired to lower the amount of Ir in the anode. In view of this, many approaches have focused on enhancing the mass activity of Ir-based catalysts towards OER by designing novel nanostructures. Another effective way is to disperse IrO2 on electronic conductive support with large surface area. Unfortunately, the most common support material for electrocatalysts, carbon materials, are regarded as not appropriate for PEMWEs because it would encounter severe degradation at high oxidizing potential region. Instead, metal oxides with large surface area and resistance to acid are used. However, their low electrical conductivity limits the electrochemical performance for OER.

In previous studies, our group wrapped pristine multi-wall carbon nanotubes (MWNTs) with polybenzimidazole (PBI). The thin layer of PBI (thickness<1 nm) provides abundant binding sites for cations. As a result, uniform Pt nanoparticles could be deposited homogeneously on the PBI-wrapped pristine MWNTs. The resultant composite catalyst exhibited ehnhanced electrocatalytic activity and extraordinary long-term durability even polarized at high potential region of 1.0-1.5 V vs. RHE because (1) the aggregation of Pt nanoparticles was significantly suppressed due to the constraint effect of polymer and (2) the highly crystallized graphitic surface of pristine MWNTs refrained the electro-oxidation of carbon. The results provide a new insight into carbon nanotubes as an excellent support material with high corrosion-resistance under anodic polarization in acid environment.

In present work, a composite catalyst of pristine MWNT, PBI, and IrO2 prepared by similar strategy. For comparison, Ir was also deposited on carbon black (CB) and PBI-wrapped CB. The products were characterized by thermogravimetry (TG), X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and scanning transmission electron microscope (STEM). The electrochemical properties of the catalysts were investigated by half-cell tests.