Boosting the Role of Ir in Alleviating Corrosion of Carbon Support By Alloying with Pt
Authors: Bak Junu , Kim Haesol , Lee SangJae , Kim Minjoong , Kim Eom Ji , Roh JeongHan , Shin Jaewook , Choi Chang Hyuck , Cho EunAe 
Published: July 2021

The degradation of PEMFC is a very complex phenomenon and is affected by various factors. Among the components of PEMFC, an electrode composed of a carbon-supported platinum nanoparticle catalyst (Pt/C) plays an important role in determining the durability of PEMFC. Typically, PEMFCs have an operating voltage range of 0.5–1.0V. During long-term operation under these conditions, Pt nanoparticles agglomerate, reducing the active surface area and gradually reducing electrode performance. On the other hand, abnormal conditions such as low fuel and start/shutdown (SU/SD) cycles produce abnormally high potentials above 1.4V at the electrodes. At this high potential, the carbon support corrodes to form carbon dioxide (C + 2H2O → CO2 + 4H + + 4e–, E0 = 0.207 V), leading to loss of supported Pt nanoparticles and rapid decay or catastrophic failure of electrode performance. Automotive PEMFCs, which are more often exposed to the aforementioned abnormal conditions, have a shorter life (almost thousands of hours) than stationary PEMFCs (nearly tens of thousands of hours). This clearly means that these dynamic conditions will decisively affect the degradation of PEMFC.

Extensive research efforts have been made to mitigate the corrosion of carbon supports to improve the durability of PEMFC. First, it improves the corrosion resistance of these materials by modifying the surface properties and crystallinity of carbon. Second, corrosion-resistant and electrically conductive oxides and transition metal carbides have been explored as alternative supports for Pt nanoparticles to remove carbon from the electrode. Electrodes without supports, such as nanostructured thin film electrodes, have also been proposed to remove carbon from the electrode. On the other hand, operating procedures or plant balances (BOPs) have been developed to avoid exposure to cell reversal or reverse current conditions. Despite these studies, the lifetime of automotive PEMFCs is limited due to carbon corrosion, but this is not the only issue. Nevertheless, the mitigation of carbon corrosion remains an important challenge.

With a new approach, iridium oxide (IrO2) is incorporated into the Pt/C electrode as an additive that protects carbon from corrosion with the help of sacrificial oxidation of water (or oxygen evolution reaction, OER) demonstrating improved durability in both half-cell and single-cell tests. However, the function of the OER catalyst was not clear in terms of composition and distribution, which is a prerequisite for further improvement of cathode durability. This study aims to gain a mechanical understanding of OER functional Ir-containing Pt/C electrodes. Carbon-supported Pt–Ir alloy catalysts (PtxIry/C) are synthesized in various compositions and are exposed to potential cycles from 1.0 to 1.5 VRHE compared to Pt/C and mixtures of Pt/C and Ir/C (Pt/C + Ir/C, Pt-Ir ratio 85:15). Pt85Ir15/C has been proven to be more resistant to carbon corrosion than Pt/C as well as Pt/C+Ir/C due to Ir atomically distributed in Pt85Ir15/C. The corrosion behavior of the carbon support is thoroughly investigated by the identical location transmission electron microscopy (IL-TEM), differential electrochemical mass spectrometry (DEMS), and X-ray photoelectron spectroscopy (XPS). As a cathode catalyst, Pt85Ir15/C exhibits much higher oxygen reduction reaction (ORR) activity than Pt/C after potential cycles, which is associated with the formation of high-index facets of Pt as revealed by high-resolution TEM (HR-TEM) and inductively coupled plasma mass spectrometry connected to a scanning flow cell (SFC/ICP-MS). This study suggests that the atomic-level distribution of Ir is a key factor in effectively mitigating carbon corrosion and improving PEMFC durability.