Palladium Nanoparticles Supported on 3D-Graphene Nanosheets for Oxygen Reduction Reactions in Alkaline Media
Authors: Kabir Sadia , Serov Alexey , Atanassov Plamen 
Published: February 2020

Fuel cells are considered to be one the most promising sustainable energy technologies for energy conversion and electric power generation. Due to their high electrical efficiency, low operating temperatures and zero tailpipe emissions, fuel cells have become ideal candidates for both transportation and residential applications. The electro-reduction of oxygen in the cathode compartment of fuel cells is an important reaction that determines the overall performance. As a result, understanding the oxygen reduction reaction (ORR) mechanistic pathways in terms of developing efficient, high performing and stable catalysts has been the main research interest over several decades.
With the recent developments in high-performance anion-exchange membranes, there has been increased interest towards studying ORR in alkaline media due to faster kinetics. Currently, Platinum/Carbon (Pt/C) based catalysts are regarded as the most active electrocatalysts for ORR. However, Pt/C catalysts are relatively expensive and lack long term stability. In addition, commercial amorphous carbon-supports such as Vulcan XC-72 utilized for Pt nanoparticles undergo severe carbon corrosion during aggressive fuel cell operating conditions, inevitably leading to Pt dissolution and loss in fuel cell efficiency. Therefore, the development alternative electrocatalysts with improved performance and durability has become a high research priority.
Recently, much more attention has been directed to the use of palladium (Pd) and Pd-based alloys deposited on graphitic supports such as carbon nanotubes and graphene oxide. However, the underlying relationship between catalytic activity, surface morphology and physiochemical properties of these heterogeneous materials are not well characterized. As a result, the knowledge related to understanding the influence of the supports on the performance of the electrocatalysts (e.g Pd) for ORR is still quite limited.
In the present study, we report the synthesis, characterization and electrochemical performance of Palladium nanoparticles deposited on porous three dimensional graphene nanosheets (Pd/3D-GNS) as efficient electrocatalysts for ORR in alkaline media. In order to understand the influence of the supports chemical properties on Pd nanoparticle dispersion, 3D-Graphene sheets were thermally and chemically reduced using 7 at.% H2 at 800C and hydrazine hydrate ( N2H4) respectively. The morphology of the 3D-GNS support was also modified by adopting the Sacrificial Support Method (SSM) [1] developed at UNM for controlled synthesis of highly ordered materials, where sacrificial silica templates are infused with the 3D-GNS support and etched out to leave a network of porous channels within the matrix. Pd nanoparticles were then deposited on the chemically and morphologically modified supports using a previously established Soft Alcohol Reduction method. [2] The physical-chemical and electrochemical properties of these as-prepared materials were investigated using various surface analysis and cyclic voltammetry techniques.
TEM particle micrographs are shown in Fig 1A, where Pd nanoparticles supported on thermally reduced 3D-GNS can be seen to exist in the form of spherical agglomerates with an average particle size of 4.3 nm. The RDE results in Fig. 1A shows significantly enhanced electrocatalytic activity of Pd/3D-GNS-H2_800C catalysts towards ORR, with an onset potential 0.93 V vs RHE , in addition to higher current densities compared to Pd/Vulcan and Pd/3D-GNS-N2H4. The RDE data obtained at different rotation speeds ( =600 to 2500 RPM) were analyzed using the Koutecky-Levich (K-L) equation the number of transferred electrons (n) from the K-L slope was found to be n=4, indicating the Pd/3D-GNS-H2_800 materials catalyze a 4-electron reduction of oxygen.
The remarkable activity of these catalysts can attributed to the uniform distribution of Pd nanoparticles within the porous 3D-Graphene matrix providing high surface area and efficient charge transfer ability, making these hybridized Pd/3D-Graphene engineered materials a promising candidate for electrocatalysis in sustainable energy fields.

References.

1. Serov, A., et al., Original Mechanochemical Synthesis of Non-Platinum Group Metals Oxygen Reduction Reaction Catalysts Assisted by Sacrificial Support Method. Electrochimica Acta, 2015. 179: p. 154-160.
2. Serov, A., et al., Palladium Supported on 3D Graphene as an Active Catalyst for Alcohols Electrooxidation. Journal of The Electrochemical Society, 2015. 162(12): p. F1305-F1309.

Figure 1