Manganese Dioxide-Based Catalysts with Low Ir Content for High-Durability PEM Water Electrolysis
Authors: Kitade Naoya , Okada Takuya , Nakamura Ryuhei , Li Ailong , Kong Shuang , Fushimi Kazuna 
Published: December 2024

Introduction

Proton exchange membrane water electrolysis (PEMWE) is a promising technology for hydrogen production. However, the local environment of the anode in PEMWE systems is highly acidic, primarily due to the oxygen evolution reaction (OER; 2H2O → O2 + 4H+ + 4e−). Furthermore, the available anode materials are limited to Ir-based materials. Current technology necessitates a high Ir loading of 1−2 mg/cm2 to achieve activity and durability. Nevertheless, the future demand for hydrogen is anticipated to increase to the GW scale, posing challenges in meeting this demand with existing Ir resources (5~7 tons/year) when the Ir loading is 2 mg/cm2 (0.5 mgIr/W).
Meanwhile, Tosoh has been actively involved in the electrolytic manganese dioxide (EMD) industry. EMD is synthesized through the anodic oxidation of Mn2+ ions (Mn2+ + 2H2O → MnO2 + 4H+ + 2e−) under acidic conditions, making it potentially suitable for PEMWE. In this study, a very small amount of Ir (<0.1 mg/cm2) was composited with EMD to reduce the Ir content. The resulting composite exhibited high activity, achieving a current density of 3.2 A/cm2 at 2 V. Furthermore, the composite demonstrated stable operation for over 3800 h at 2 A/cm2.

Experiments

EMD was electrodeposited onto Pt-coated Ti (Pt/Ti) fibers by electrolyzing a Mn2+-containing electrolyte. Subsequently, the EMD-coated Pt/Ti was immersed in an Ir-containing solution, followed by annealing to obtain Ir-containing EMD, referred to as "IrMnOx." In general, Pt/Ti fibers are used as a porous transport layer (PTL) in PEMWE systems, and our developed material serves dual roles as an OER catalyst and a PTL.
Furthermore, the OER characteristic was evaluated using a PEMWE cell operating at 80℃ in a two-electrode system. Pt-supported carbon served as the cathode catalyst, with Nafion®-115 as the PEM.

Results and discussion

Using inductively coupled plasma atomic emission spectroscopy (ICP-AES), the Ir content in IrMnOx was determined to be 0.1 mg/cm2. The OER activity of IrMnOx was evaluated by linear sweep voltammetry (LSV). For comparison, commercial Ir oxide with an Ir content of 0.1 mg/cm2 was evaluated using the same method.
As depicted in Figure 1, IrMnOx demonstrated superior OER activity compared to Ir oxides with similar Ir content. Notably, the OER activity of IrMnOx reached 3.2 A/cm2 at 2 V. This could be attributed to EMD serving as a high specific surface area support and the highly dispersed Ir, thereby augmenting the catalytic sites for the OER.
Additionally, the constant current durability of IrMnOx and Ir oxide was tested at a current density of 2 A/cm2 (Fig. 2). Ir oxide became deactivated within 200 h, whereas IrMnOx remained stable for over 3800 h without degradation. Essentially, IrMnOx exhibited practical OER activity and durability while reducing the required Ir amount by over tenfold compared to current technologies.
Acknowledgments: This presentation is based on results obtained from project, JPNP20003, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).

Figure 1