Durable and Cost Effective Oxygen Electrode Prepared By Electrodeposition for Proton Exchange Membrane Water Electrolyzer (PEMWE)
Authors: Choe Seunghoe , Lee Hye Jin , Lim Ahyoun , Park Hyun-Seo , Lee Kyu Hwan , Jang Jong Hyun 
Published: February 2020

Hydrogen gas has been regarded as future energy medium for the zero-carbon, hydrogen-based energy ecosystem. For decades, however, majority of hydrogen gas (96%) has been produced by petrochemical processes that emit CO2 into atmosphere. As an effort to facilitate carbon-free energy ecosystem, the water electrolysis (WE), a technology to split water into hydrogen and oxygen using renewable sources, has been received much attention. Currently, alkaline water electrolyzer (AWE) has dominated the commercial market due to its technical maturity that provides low price. However, the drawbacks such as a need for large-scale plant and the use of toxic solvent has limited its further application on small size WE. As an alternative, the proton exchange membrane water electrolyzer (PEMWE) has been researched and recently commercialized. Advantages of this technology include compact design, high energy efficiency and high H2 purity, which is suitable to small- and medium-scale electrolyzer (<300 kW).

PEMWE running at high temperature (100-150℃), also referred as to high temperature PEMWE (HT-PEMWE) is beneficial in both thermodynamic and kinetic points of views relative to conventional PEMWE (60–90℃). The better cell efficiency is associated with fast electrode reactions due to better electrode kinetics and lower equilibrium voltages. The drawbacks of high temperature operation is relatively inferior cell stability resulting from high operating temperature, which inevitably increases the stack replacement cost. The cell is severely degraded during HT-PEMWE operation, and the degradation rate become faster when the operating temperature increases. One of the major degradation source is a contact loss between IrO2 and Ti DL. The recent studies showed that Ti is rapidly passivated and corroded at high temperature (>100℃) and anodic potential (>1.0 VSHE). This process would severely degrade the DL and BP, though the impact of Ti corrosion has not been experimentally explored yet.

In this study, electrodeposited IrO2 is suggested as a corrosion-protective film that suppresses the passivation of the inner Ti substrate, while also acting as a cost-effective catalyst layer for OER. An IrO2 film of submicron thickness was uniformly deposited over the entire surface of porous Ti mesh via anodic electrodeposition. The properties of the IrO2 film were examined and the performance and stability of this electrode were assessed at the single-cell level.

Membrane electrode assembly (MEA) was prepared by placing the oxygen (IrO­2/Ti mesh) and hydrogen electrodes (Pt/C) onto either sides of nafion membrane (NR-212, Dupont). Oxygen electrode was prepared via anodic electrodeposition of IrO2 onto porous Ti diffusion layer in the bath of 10 mM iridium chloride hydrate (IrCl4H2O), 100 mM hydrogen peroxide H2O2, 40 mM oxalic acid (COOH22H2O, 340 mM potassium carbonate (KCO3). Ti mesh and standard calomel electrode (SCE) were employed as counter and reference electrodes, respectively. Hydrogen electrode was prepared via spraying the catalyst ink composed of 46.5wt% Pt/C (Johnson Matthey), 5 wt% nafion solution, isopropyl alcohol (IPA, J.T. Baker), and deionized water on commercial carbon paper (39BC, SGL carbon).

As shown in Fig. 1, electrodeposited IrO2 film played dual roles as not only the catalyst layer toward oxygen evolution reaction (OER) but also the corrosion-protection layer preventing inner Ti from oxidation. The e-IrO2/Ti resulted in high performance (0.97 A/cm2) despite low IrO2 loading (0.4 mg/cm2) in single cell test conducted at 1.6 V and 120oC, which was comparable to that of conventional electrodes with more catalyst loading. Furthermore, the corrosion polarization test revealed that the coated IrO2 film physically blocked the exposure of Ti diffusion layer, and thus reduced the corrosion of Ti in acidic condition. Therefore, the low degradation rate (1.5 mA/cm2-hr (0.11 %/hr)) was obtained in aging experiment at 120℃ and 1.72 V (voltage efficiency : 85%), confirming the excellent stability of this electrode.

Figure 1