New Electrocatalyst with the Content of Ir<0.1 Mg/cm2 for Proton Exchange Membrane Water Electrolysis
Authors: Okada Takuya , Nakamura Ryuhei , Li Ailong , Kong Shuang , Fushimi Kazuna 
Published: August 2024

Introduction

Proton exchange membrane (PEM) water electrolysis (WE) is a promising technology for hydrogen production. However, the local environment of the anode in PEMWE is highly acidic and high potential due to the oxygen evolution reaction (OER; 2H2O → O2 + 4H+ + 4e-), and the available anode materials are limited to precious metal elements such as Pt, Ru and Ir. In the current technology, Ir-based materials are the mainstream anode catalysts, and Ir loading of 1-2 mg/cm2 is necessary to achieve both activity and durability. On the other hand, the annual production of Ir is said to be only 5-7 tons. The demand for hydrogen is expected to increase to the GW scale in the future. For example, if water electrolysis is operated at 2 A/cm2 and 2 V with Ir loading of 2 mg/cm2 (0.5 mgIr/W), it is difficult to meet the demand with the existing Ir resources.
Tosoh have been engaged in the electrolytic manganese dioxide (EMD) business. EMD is synthesized thorough anodic oxidation of Mn2+ ion (Mn2+ + 2H2O → MnO2 + 4H+ + 2e-) under acidic conditions, therefore it is potentially suitable for the anode environment of PEMWE. In this study, a very small amount of Ir (<0.1 mg/cm2) was successfully composited with EMD. The resulting composite consisting of Ir and EMD showed more active than commercial Ir oxide with the same Ir content, and operated stably for at least 1000 h.

Experiment

EMD thin film was electrodeposited anodically on Pt-coated Ti fibers (Pt/Ti) thorough electrolyzing a mixed solution of MnSO4 and H2SO4. The obtained EMD-coated Pt/Ti were immersed in an Ir-containing solution, and then annealed to obtain Ir-containing EMD (hereinafter referred to as "Ir-EMD"). The Pt-coated Ti fibers are used as a porous transport layer (PTL) in PEMWE cells, and our developed material serves as both an OER catalyst and a PTL.
OER property was evaluated using a PEMWE cell at 80°C in a two-electrode system. Pt-supported carbon was used as the cathode catalyst and Nafion®-115 as the PEM. See Fig. 1 for PEMWE cell construction flow.

Result and discussion

The Ir content in Ir-EMD was determined to be 0.1 mg/cm2 by ICP-AES. This Ir content is 10-20 times lower than that used in current PEMWE electrolyzers.
The OER activity of Ir-EMD was evaluated by linear sweep voltammetry (LSV) (Fig. 2). For comparison, EMD (with no Ir) and Ir oxide (commercial product) with an Ir content of 0.1 mg/cm2 were evaluated in the same way. The slow OER property of EMD was improved dramatically by introducing Ir into EMD. Also, at the same amount of Ir, Ir-EMD exhibited higher OER activity than that of Ir oxide. The inset in Fig. 2 shows Tafel plots corresponding to LSVs, and the slope of the linear region (Tafel slope) can be used to predict which elementary reaction is the rate-limiting step in the OER. The Tafel slope of Ir-EMD is 58 mV/dec., which is close to the Tafel slope of Ir oxide, 50 mV/dec. This suggests that OER on Ir-EMD proceeds by the same mechanism as Ir oxide.
The durability of Ir-EMD and Ir oxide was tested at a current density of 2 A/cm2 (Fig. 3). Ir oxide was deactivated within only 8 days, and it is confirmed that Ir have disappeared after electrolysis. On the other hand, Ir-EMD has operated stably for more than 42 days (≈1000 h), with a degradation rate of only 13 μV/h. In other words, Ir-EMD has practical durability while reducing the amount of Ir by more than 10 times compared to current technology.
Acknowledgements : This presentation is based on results obtained from a project, JPNP20003, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).

Figure 1