Synthesis of Homogeneous Distributed Nanoparticles Using a Fluidized Bed Reactor
Authors: Hornberger Elisabeth , Schmies Henrike , Strasser Peter 
Published: February 2020

The proton exchange membrane fuel cell (PEMFC) has drawn attention as a promising alternative to fossil fuel-based combustion engines. The requirements for next-level PEMFC electrocatalysts are severe: high mass activity accompanied by high performance stability for low loaded Pt-based electrodes.[1] This results in a challenging target for the catalyst design on the cathode site. Further reduction of the Pt loading is limited due to additional voltage losses originating from O2 mass transport resistances.[2] The objective of future catalyst design thus needs to include well-ordered Pt distribution and improved Pt particle-ionomer interaction. Orfanidi et al. showed that a more homogeneous ionomer coverage over the entire catalysts’ surface could be achieved when its carbon surface is enriched with N-functionalities, which was deduced to the ionic interaction between negatively charged ionomer groups (-SO3H) and positively charged surface groups of the modified carbon (-NHx).[3]

In prior work, we showed an enhanced stability for a Pt nanoparticle catalyst based on N-modified carbon with respect to unmodified carbon.[4] However, the furnace-based Pt deposition used in that work can be further improved to yield a better Pt nanoparticle distribution. In the present study, the carbons were functionalized by introducing N-functionalities to pre-oxidized carbons via ammonolysis at elevated temperatures in pure NH3 atmosphere. Afterwards, we performed the Pt deposition in a homemade fluidized bed reactor, where the N-modified carbons were first impregnated with hexachloroplatinic acid and in a second step reduced in hydrogen atmosphere. The fluidized bed reactor enables a homogenous reduction of the metal precursor, which results in a more well-defined and narrow particle size distribution of the Pt nanoparticles. The synthesized catalysts were investigated as cathode material for the application in fuel cell devices in an accelerated stress test mimicking fuel cell operating conditions. Based on rotating disk electrode experiments reasonable mass activities and high electrochemical stabilities were found. The catalyst based on N-modified carbon showed enhanced electrochemical stability compared to a catalyst based on an unmodified carbon. Complementary in situ X-ray measurements revealed the morphologic stability of the Pt nanoparticles. Therefore, the applied chemical tailoring of carbon supports indicates a strategy toward an improved stability for PEMFC applications.
References:
[1] Retrieved from https://www.energy.gov/eere/fuelcells/doe-technical-targets-polymer-electrolyte-membrane-fuel-cell-components (2019, April 09).
[2] A. Kongkanand, M. F. Mathias, J. Phys. Chem. Lett.
2016, 7, 1127−1137.
[3] A. Orfanidi, P. Madkikar, H. A. El-Sayed, G.S. Harzer, T. Kratky, H. A. Gasteiger, J. Electrochem. Soc.
2017, 164, F418-F26.
[4] H. Schmies, E. Hornberger, B. Anke, T. Jurzinsky, H. N. Nong, F. Dionigi, S. Kühl, J. Drnec, M. Lerch, C. Cremers, P. Strasser, Chem. Mater.
2018, 30, 7287−7295.