Catalytic Activity of Pt Monolayer on Electrodeposited W-Ni Nanoparticles for the Oxygen Reduction Reaction

Proton Exchange Membrane Fuel Cells (PEMFCs) are promising solution for ever growing concern about fossil fuel availability and related environmental effects. However, high Pt catalyst loading in the cathodes is required due to the sluggishness of the oxygen reduction reaction (ORR) and Pt dissolution under fuel cell operating conditions, causing high cost of PEMFCs. The concept of Pt monolayer (Pt_ML) electrocatalysts [1] offers a possible solution to these problems because it provides ultimate reduction in Pt loading and its complete utilization. We recently showed that Pt monolayer shell deposited on Pd or PdAu alloy core possesses higher activity and stability than pure Pt electrocatalysts [2]. Further reduction in nobel metal content can be achieved by using a less noble core that is stable under the oxidizing conditions of the ORR.

Low cost refractory metals, such as W and/or its alloys with other transition metals, are excellent candidates as a material core for Pt_MLshell electrocatalysts, since they possess a good corrosion resistance under the acidic operating conditions of PEMFCs. It is well known that W can only be electrodeposited from aqueous solutions by induced co-deposition with iron group metals [3]. The morphology, composition, and the properties of the deposits depend strongly from the solution composition, pH, temperature and current density. In addition, the microstructure and the morphology of electrodeposited WNi binary nanocrystalline alloys could be tailored by applying galvanostatic or pulse electrochemical protocols.

In this work we electrodeposited WNi alloys from citric bath solution at room temperature directly on functionalized carbon layer of gas diffusion layer (GDL) with geometric are of 25 cm². Both, constant current mode and pulse deposition strategies are used to control the microstructure and the composition of the deposits. As obtained WNi nanostructures contained up to 50 at.% W, and possessed composite nanocrystalline - amorphous microstructure and very fine particles size distribution of 2-3 nm. Figure 1 presents the chronopotenciometric curve of deposited WNi nanocrystalline alloys at constant current of 3 mA/cm². It is seen that the electrodeposition takes less than 7 minutes. The deposition is performed in a specially designed flow type cell that allows exchanging of the electrolytes. After rinsing the cell a Pd containing solution is introduced and part of the Ni atoms from the WNi alloys are galvanically displaced by Pd. Thus, we obtained Pd shell on WNi nanostructures. In consecutive steps we applied a well-established Cu UPD assisted protocol for Pt_ML deposition on Pd [4] and obtained Pt_ML/PdWNi/GDL electrocatalyst with total precious metal loading of 50 μg/cm². As prepared electrode is assembled with Nafion 211 membrane and a standard anode with Pt loading of 100 μg/cm², by hot pressing. The polarization curve measured on as fabricated MEA is presented in Figure 2. The mass activity of the catalyst of interest is 0.42 A/cm², which is in good agreement with the US DOE targets for 2017. Furthermore, the accelerated stability test at 80 ^oC, showed that this MEA possessed an excellent stability. 

Figure 1. Chronopotenciometric curve of electrodeposited WNi nanoparticles directly on GDL at content current of 3 mA/cm².

Figure 2. Polarization curve measured on MEA prepared from Pt_ML/Pd/WNi/GDL cathode, Nafion 211 membrane, and standard Pt anode with loading of 100 μg/cm².

Acknowledgment

Work at Brookhaven National Laboratory (BNL) is supported by US Department of Energy, Division of Chemical Sciences, Geosciences and Biosciences Division, under the Contract No. DE-AC02-98CH10886.

References

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