Stephan Ruck

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. Figure 1

Anion exchange membrane water electrolysis (AEMWE) is highly promising for cost‐effective green hydrogen production due to its basic operating conditions facilitating the use of non‐noble catalysts. While non‐noble Ni/Fe‐based catalysts are utilized at the anode, its cathode catalyst still requires precious Pt. Due to the high cost of Pt and the sluggish hydrogen evolution reaction (HER) at the cathode in basic conditions, developing alternative catalysts to replace Pt is highly important. Here, a synthesis procedure for a Ru‐based catalyst is reported and its high activity toward the HER in alkaline media is demonstrated in both half‐cell and single‐cell tests. The catalyst is synthesized in a two‐step approach. A highly dispersed Ni catalyst is prepared on carbon support in the first step. In the second step, Ru is deposited on its surface using a galvanic displacement reaction. The uniqueness of this method is that Ru is deposited over the entire electrically conductive surface, resulting in an isotropic and homogeneous Ru distribution within the catalyst powder. It is demonstrated that this material remarkably outperforms state‐of‐the‐art Pt/C catalysts in half‐cell and single‐cell tests. The single cell only requires 1.73 V at 1 A cm ⁻² with an overall PGM content of 0.05 mg cm ⁻² .

Establishing anion exchange membrane water electrolysis (AEMWE) as a new technology for efficient hydrogen production requires cost-effective and high-performance catalyst materials. Here, we report the synthesis and comprehensive characterization of carbon supported NiRu alloy nanoparticles as a cost-effective hydrogen evolution reaction catalyst for AEMWEs. Different NiRu catalysts were synthesized using a facile and scalable impregnation method. Half-cell results showed the ‘NiRu’ catalyst with ca. 10 wt.% Ru to exhibit an increased noble metal mass activity and slightly decreased Tafel slope compared to a commercial Pt/C catalyst with 60 wt.% Pt. Further, we report the application of NiRu/C as a cathodic catalyst in AEMWE full cell for the first time. In full cell tests, the synthesized catalysts exhibit 2 A cm⁻² at 1.95 V with a low loading of 0.1 mg_PGM cm⁻² at the cathode.