A Life-Cycle of Ni in Proton Exchange Membrane Fuel Cells
Authors: Ronovský Michal , Myllymäki Mila , Polani Shlomi , Dunseath Olivia , Kúš Peter , Pan Lujin , Klingenhof Malte , Götz Daniel , Kubát Jan , Hrbek Tomas , Fusek Lukáš , Dionigi Fabio , Mirolo Marta , Martens Isaac , Chattot Raphaël , de Gutierrez Martha Briceno , Brooke Emily , Sharman Jonathan , Strasser Peter , M. Bonastre Alejandro , Drnec Jakub 
Published: September 2023

The usage of Proton Exchange Membrane Fuel Cells (PEMFCs) in the automotive industry is currently limited by the price, performance, and durability of a platinum catalyst. Alloying with nickel provides lower cost and enhances activity. However, the membrane electrode assembly (MEA) performance is, in practice, much lower than expected from liquid laboratory experiments on the catalyst layer. One of the identified issues is Ni leaching from nanoparticles (NPs) and subsequent Ni poisoning of the Nafion membrane.

Here, we use Wide-Angle X-ray Scattering (WAXS) and X-ray Absorption Near-Edge Structure (XANES) to follow Ni dissolution from the catalyst layer and its movement in the MEA. We shine (synchrotron) light on the full life cycle of Ni, starting from (i) characterization of the catalyst powder, followed by (ii) the changes in catalyst composition during the ink-making process and (iii) membrane coating and finishing with (iv) NP characterization and Ni tracking during the operation of MEAs.

Proper incorporation of PtNi catalyst requires modification of all the aforementioned steps that are otherwise well optimized for pure Pt catalyst. It is even more critical for shape-controlled octahedra (oh) PtNi NPs as their activity is closely related to their structure [1]. Highly active oh-PtNi NPs are usually made from precursors such as Nickel(II) bis(acetylacetonate). Using EDX, we find precursor residues in catalyst powders that dissolve upon further processing and add to membrane poisoning. We conclude that we need to develop a cleaning protocol that would remove all Ni residue while retaining the nanoparticle shape.

During ink-making, high ionomer concentrations and elevated temperatures promote Ni dissolution from the catalyst, which can, in turn, poison the membrane even before the MEA is put in use. With the WAXS technique, we track the dissolution during ink-making and MEA operation by following changes in lattice parameter, showing the dynamics and the extent of Ni dissolution in each step of aging.

Furthermore, we use angle-resolved XANES to track the movement of dissolved Ni. We show that Ni ions are getting reduced back to metallic form within the MEA, likely due to hydrogen crossover. The presence of such a metal band in the membrane blocks proton conductivity and decreases performance [2]. That is why it is crucial to set manufacturing and operational boundaries to prevent dissolution.

For this reason, we follow WAXS total scattering intensity during oxidation and reduction cycles to understand the Ni dissolution dynamics during operation. We find that limiting both upper and lower potential cycling limits greatly reduces the redox extent and subsequent dissolution. It is, therefore, possible to find and understand the trade-off between high power density and dissolution in operational cells.

Even though this work looks at the Ni life cycle, the presented techniques and conclusions are transferable to all multimetallic high-performance PEMFC catalysts.

References:

[1] Shlomi Polani et al. ACS Appl. Mater. Interfaces 2022, 14, 26, 29690–29702

[2] Wu Bi et al. Electrochem. Solid-State Lett. 2007 10 B101

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