Growth of novel
Pt 3D
networks on tungsten suboxide electrodes and their effect on the performance of fuel cells
Authors: Hengge Katharina , Heinzl Christoph , Perchthaler Markus , Welsch Marina , Scheu Christina 
Published: December 2016

Green energy gains in importance in the spotlight of current research activities. Solar, wind and water are promising energy sources and they are already in use in different ways. Their practical disadvantage – their dependence on daytime or seasonal conditions – requires an intermediate storage of the produced energy. A convenient carrier could be hydrogen (H
2
). Fuel cells (FCs) can then be used to convert the chemically bound energy in H
2
into electrical power as well as heat. One of the most simple redox reactions, the oxidation of H
2
and reduction of oxygen (O
2
) followed by the formation of water (H
2
O), is the basis of this promising route of energy conversion and electricity production. In our research work we develop and investigate novel electrode materials for improved and long‐term stable FC performance. Tungsten suboxide (WO
3‐x
) is chosen as support material due to its ability to minimize poisoning of the platinum (Pt) catalyst. For most efficient hydrogen spill over and electron transport, the interface between the catalyst and the support material is of great interest. In addition, the distribution and shape of the catalyst on its support are important. To address these questions we used various electron microscopy techniques. Via scanning electron microscopy (SEM) we obtained overview images of the Pt catalyst particles and their distribution on the WO
3‐x
support. Detailed analysis regarding growth and architecture of the Pt catalyst was performed with the help of a transmission electron microscope (TEM) equipped with a monochromator and C
s
‐corrector for the condensor system (Titan Themis 60 – 300 kV). The preparation of electron transparent samples was performed by site specific focused ion beam (FIB) sectioning.

Pt is deposited on the WO
3‐x
support by a wet chemical approach and the different growth stages were investigated after varied deposition times. SEM images revealed octahedral and truncated octahedral shaped 3D Pt catalyst morphologies on the WO
3‐x
support with lateral dimensions of 2 – 4 µm. Depending on the deposition time of the Pt precursor (4 min, 20 min, 60 min) their appearance differs (Fig. 1). TEM and STEM images revealed highly porous 3D networks formed by Pt nanorods with a width of a few nanometers (Fig. 2a). These networks develop as a result of the reduction of the catalyst precursor in H
2
atmosphere. High resolution TEM (HRTEM), HRSTEM and electron diffraction experiments revealed that the Pt rods have different orientations. The nanorods are connected to each other as visualized by TEM tomography (Fig. 2b). The formation of the Pt catalyst 3D network takes place in a 2‐step procedure: firstly octahedral or truncated octahedral shaped compact crystals of the precursor are formed on the WO
3‐x
support. These crystals are then reduced to metallic Pt with side products evaporating in the second step in the presence of H
2
. The corresponding redox reaction starts at the outside of the bulk crystal and penetrates to its inside while reducing the precursor step by step. This is shown in Fig. 3a where the outer area of the displayed 3D catalyst is formed by a network of Pt nanorods while its interior is still compact. Energy dispersive X‐Ray (EDX) analysis and STEM reveal that small Pt particles are embedded in a reduced form of the amorphous precursor matrix (Fig. 3b). The long‐term stability and performance continuity of the Pt/WO
3‐x
system as an anode of high‐temperature polymer‐electrolyte‐membrane FCs was demonstrated in accelerated and continuous FC operation times of up to 2000 h. Via SEM and different TEM based techniques the improved degradation behavior compared to standard carbon support material was shown.
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