Investigating Gas Promoted Ir Deposition on Pt Black Nanoparticles As Synthesis Method for Bifunctional OER/ORR Catalyst in Acidic Media
Authors: Ruck Stephan , Pham Chuyen , Thiele Simon , Freiberg Anna T.S. 
Published: December 2024

A reversible fuel cell (rFC) integrates a fuel cell (FC) and a water electrolyser (WE) into a single energy conversion device. One potential implementation of an rFC involves operating each electrode with a fixed gas. Specifically, the oxygen electrode would carry out the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) depending on the operation mode. However, a major challenge is the development of a bifunctional catalyst for the oxygen electrode, since the faster hydrogen electrode can be operated with Pt as catalyst for both, hydrogen evolution and oxidation reaction (HER/HOR). State-of-the-art catalyst materials in acidic media, such as Pt and Ir for the ORR and OER, respectively, face challenges due to oxidative and reductive potentials during FC and WE mode operation. Ir undergoes multiple oxidation state changes during the transient between FC and WE operation mode, resulting in dissolution of the material [1]. Pt faces similar oxidation mechanisms and possible dissolution in WE mode [2].
To address these challenges, we propose a Pt@Ir bifunctional catalyst for rFC applications, where Ir is deposited in a non-continuous monolayer shell on Pt particles to maintain active surface area for both reactions. However, in order to control the synthesis of such a composite catalyst, a basic understanding of Pt-Ir interactions has to be established, which is also of interest to cell reversal mitigation catalysts commonly employed in PEM FC.
Herein, we present investigations on Ir deposition on Pt nanoparticles, which builds the foundation for a simple and scalable synthesis method for the proposed Pt@Ir catalyst. We start with investigations on potential-driven deposition of Ir on Pt nanoparticles to study how Ir deposition evolves with applied potential and compare the characteristics with Ir deposition on Ir and Au nanoparticles as reference. We show that Ir deposition on Pt happens at lower cathodic polarization hinting toward a preferential Ir ion reduction on Pt. Further we investigate the hydrogen gas driven deposition of Ir on Pt before transferring the knowledge from a three-electrode set up into a bulk synthesis. The high activity of adsorbed hydrogen gas on Pt surface allows a reduction of metal ions exclusively on Pt surface [3], which seems to be also possible for Ir ions.
We present the successful gas-promoted deposition of Ir on Pt black nanoparticles as bifunctional catalyst for OER/ORR in acidic media. Additionally, we demonstrate the full cell performance of this catalyst in PEM WE and PEM FC operation.
REFERENCES
[1] Geiger, S.; Kasian, O.; Shrestha, B.R.; Mingers, A.M.; Mayrhofer, K.J.J.; Cherevko, S. Activity and Stability of Electrochemically and Thermally Treated Iridium for the Oxygen Evolution Reaction. J. Electrochem. Soc., 2016, 163, F3132-F3138.
[2] Sugawara, Y.; Okayasu, T.; Yadav, A.P.; Nishikata, A.; Tsuru, T. Dissolution Mechanism of Platinum in Sulfuric Acid Solution. J. Electrochem. Soc., 2012, 159, F779-F786.
[3] Loichet Torres, P.A.; El-Sayed, H.A.; Schwämmlein, J.N.; Friedrich, F.; Gasteiger, H.A. Hydrogen Gas Promoted Self-Limiting Copper Monolayer Deposition on Platinum. J. Electrochem. Soc., 2021, 168, 52508.

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