Bifunctional Pt-IrO2 Catalysts for the Oxygen Evolution and Oxygen Reduction Reactions: Alloy Nanoparticles vs. Nanocomposite Catalysts

Abstract

<p>In the present study different concepts for the development of bifunctional oxygen reduction reaction / oxygen evolution reaction (ORR / OER) electrocatalysts are explored and compared. Bifunctional ORR / OER catalysts are often suggested to improve the stability during startup and shutdown of fuel cells. Furthermore, they are proposed for so-called unitized regenerative fuel cells (URFCs) that would allow a closed loop system to use and produce hydrogen on demand. We compare the electrocatalytic performance of conventional Pt<sub>x</sub>Ir<sub>y</sub> alloy nanoparticles (NPs) with Pt – IrO<sub>2</sub> NP composites (nanocomposites), both immobilized onto a commercial carbon support. The Pt – IrO<sub>2</sub> nanocomposites thereby consist of a mixture of Pt NPs and IrO<sub>2</sub> NPs. By probing the electrocatalytic performance before and after exposing the electrocatalysts to accelerated degradation tests (ADTs) it is shown that the Pt – IrO<sub>2</sub> nanocomposite concept offers advantages but also some disadvantages over the conventional alloy concept. In particular it is shown that while the nanocomposites are initially less active for the ORR due to an interparticle effect, their performance is less affected by the ADTs. However, all tested catalysts experience a decline of the Ir / Pt ratio upon the ADTs treatment, highlighting the challenging stability requirements for URFCs.</p>

Elucidating Pt-Based Nanocomposite Catalysts for the Oxygen Reduction Reaction in Rotating Disk Electrode and Gas Diffusion Electrode Measurements

“…The ORR activity of Pt-based catalysts is in general considered to be correlated to the adsorption energies of the O * , OH * , and OOH * intermediates [45], here referred to as oxophilicity. In agreement with our previous study [27], we therefore assume that the reduced ORR activity of the Pt-IrO2/C nanocomposite can be ascribed to the more oxophilic nature of Pt NPs adjacent to IrO2 particles. Concurrently, the improved activity of Pt-Au/C suggests that the electronic properties of Pt NPs are affected in a different way (i.e., less oxophilic) by the adjacent Au NPs.…”

“…The trend in stability observed in the RDE measurements is less straightforward and depends on the applied ADT protocol. Based on literature and our previous work [27,47], one might expect an improvement in stability when adding Ir NPs to the Pt/C benchmark catalyst. Such improvement is indeed observed, when choosing an ADT protocol that applies highly oxidizing conditions, see Figure 2.…”

“…It should be mentioned though that the ex situ determined WL intensity of the non-activated Pt-IrO2/C nanocomposite is comparable to the one of the Pt/C benchmark (Figure S6) and that for a more detailed study in situ XANES are required. [27]. The ORR activity is determined from negative sweeping polarization curve at 0.85 VRHE, a scan rate of 50 mV s -1 and a rotation speed of 1600 rpm are applied in a potential region of 0.05 -1.10 VRHE.…”

“…The third protocol was designed of a combination of the above two protocols, cycling the electrode potential between 0.40 and 1.40 VRHE with a scan rate of 1000 mV s -1 , the total treatment lasted for 3600 cycles [34]. The fourth protocol was performed in current control alternating reductive and oxidative currents and was adapted from our previous stability study of bifunctional ORR -OER catalysts [27]. The measurements were conducted in the O2 saturated electrolyte maintaining a rotation of 1600 rpm.…”

Beyond Active Site Design: A Surfactant‐Free Toolbox Approach for Optimized Supported Nanoparticle Catalysts

“…With the development of new methods to test catalysts [58] and the demonstration of the toolbox approach with alternative precious metal catalyst like Ir [59] or Ru, [38] it is expected that this approach will be key to explore more complex materials like bimetallic catalysts, nanocomposites, [60] and more complex phenomenon like the joint effect of size and interparticle distance at the nanoscale, in order to develop more performant supported NP catalysts.…”