# Tuning surface chemistry and self-assembly to increase current density in bio-inspired hydrogen fuel cell anodes with ionomer

Nathan Coutard, Ahmed Ghedjatti, Sandrine Lyonnard, Hanako Okuno,
Laure Guétaz, Vincent Artero, Pascale Chenevier

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Nathan Coutard, Ahmed Ghedjatti, Sandrine Lyonnard, Hanako Okuno, Laure Guétaz, et al. Tuning surface chemistry and self-assembly to increase current density in bio-inspired hydrogen fuel cell anodes with ionomer. E.MRS 2021 - European Materials Research Society fall meeting, European Materials Research Society, Sep 2021, Varsovie (Online), Poland. hal-04924286

HAL Id: hal-04924286

https://hal.science/hal-04924286v1

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# Tuning surface chemistry and self-assembly to increase current density in bio-inspired hydrogen fuel cell anodes with ionomer

N. Coutard,ª A. Ghedjatti,ª,ᵇ S. Lyonnard,ᵇ H. Okuno,ᶜ L. Guetaz,ᵈ V. Artero,ª* P. Chenevierᵇ*

ª Univ. Grenoble Alpes, CEA, CNRS, IRIG, Laboratoire de Chimie et Biologie des Métaux, 38000 Grenoble, France

ᵇ Univ. Grenoble Alpes, CEA, CNRS, IRIG, Systèmes Moléculaires et Matériaux pour l’Energie et la Santé, 38000 Grenoble, France

ᶜ Univ. Grenoble Alpes, CEA, IRIG, Modélisation et Exploration des Matériaux, 38000 Grenoble, France

ᵇ Univ. Grenoble Alpes, CEA, LITEN, DTNM, Lab. Nanocaractérisation et Nanosécurité, 38000 Grenoble, France

Commercial proton exchange membrane fuel cells (PEMFCs) use platinum as the catalyst, a too scarce and precious metal for sustainable energy supply through PEMFCs. Pt-free catalysts are actively searched for, both at the cathode and at the anode. With huge advancements in current density, lower overpotential and higher stability in the past few years, molecular engineered bio-inspired catalysts hold promise for the next generation of PEMFC [1, 2]. Yet, their implementation in catalytic layers faces nanocomposite formulation issues [3]. Here, we use carbon nanotube immobilized DuBois nickel catalysts to exemplify how self-assembly at the mesoscale affects performances of H₂ oxidation anodes.

Copied on the conserved functional features of the active site of hydrogenases, the nickel catalysts created by Dubois et al. [4] show impressive turn-over frequency with no over-potential for hydrogen oxidation in solution. Not only has the central part of the catalyst, but also its outer sphere has a strong influence on the catalytic capacity [5]. Modification of the outer sphere can be used to immobilize the catalyst on a conductive matrix [6-8], a requirement for further implementation in PEMFC [9].

However, long-term ion transport in PEMFC requires creating ion conductive paths in the electrode, a task usually fulfilled by the addition of an ionomer. Although it allowed us to assemble the first fully Pt-free PEMFC [3], the current density was very low when the anode contained ionomer. The electrode microstructure thus appears critical to the system performance, yet it was hardly studied. Here, we show that molecular engineering coupled with three-dimensional structuring of the carbon electrode plays a major role on the catalytic activity through enhancement of catalyst grafting and substrate/product diffusion inside the electrode.

In particular, we compare the effect of Nafion ionomer addition on the performances of bioinspired catalytic layers produced via three distinct surface chemistries [6-9]. We use microscopy as well as small angle neutron scattering techniques to characterize the self-assembly of the ionomer [10] with the carbon nanotubes/catalyst composite. A strong correlation appears between current drop in the presence of ionomer, and absence of ionomer hydrophilic/hydrophobic nanostructure. We propose a model that describes how the surface charge of the functionalized nanotubes drives the structuration of the Nafion ionomer and impacts diffusion of protons and gas to and from catalytic centres.

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