(Invited) Semi-Automated System for Electrodeposition of Pt Monolayer Shell on Refractory Metal Core Fuel Cell Electrocatalysts Directly on Gas Diffusion Layer

According to the US Department of Energy (DOE), reducing the cost and improving the durability of the polymer electrolyte membrane fuel cells (PEMFCs) are the main challenges that hinder their commercialization [1]. The largest portion of the cost of the fuel cell stacks (49%) is attributed to the large amount of Pt on the cathode side, because of the sluggish kinetics of the oxygen reduction reaction (ORR) [1, 2]. Reduction of the amount of the Pt contained in the PEMFCs stacks is required for large scale automotive applications, both for reasons of cost and Pt supply limitations. In order to challenge the researchers all over the world, and to boost the implementation of the PEMFCs in transportation, DOE has set demanding technical targets for development of fuel cell electrocatalysts for transportation applications [1].

It has recently been demonstrated that the mass and specific activities, and as well as the durability of the Pt monolayer (ML) type electrocatalysts exceed the DOE targets for 2020, which renders them as the most advanced type of fuel cell electrocatalysts [3, 4]. In this class of electrocatalysts, a surface limited redox replacement (SLRR) strategy is used in order to deposit a Pt ML shell on another transition metal (or alloy) nanoparticle core [3]. A pre-deposited Cu ML on respective nanoparticles at underpotentials (UPD) is galvanically displaced by Pt in a consecutive step, upon immersion of the sample in Pt²⁺ solution. Thus, a full Pt utilization is achieved in the Pt ML core-shell electrocatalysts, and their excellent activity and performance stability has been demonstrated in either rotating disk electrode (RDE) or MEA studies [3-5]. Although, those catalysts showed outstanding performance, their reproducible synthesis in large scale remained unresolved.

In this contribution we are reporting on novel fast and facile methodology for fabrication of PEMFC cathodes with ultra-low PGM loading. We have designed and developed a semi-automated system for electrodeposition of Pt ML core-shell fuel cell electrocatalysts, utilizing the SLRR strategy, directly on gas diffusion layer (GDL). The system is software controlled and allows fabrication of highly active and durable FC electrodes with geometric area of up to 500 cm² in a couple of hours, starting from raw materials (chemical salts, and commercial GDLs).

The system is consisting of: (i) electrochemical cell that allows fabrication of large electrodes with industrial relevance; (ii) pneumatic control unit with respective electronic control that governs the exchange of different electrolyte solutions from the de-aerated stock solutions to the cell in a particular order; (iii) electronic data acquisition board from National Instruments that allows the LabView software to control step by step all the processes; and (iv) BioLogic potentiostat-galvanostat that controls the refractory alloy electrodeposition, Pd displacement, and Cu UPD and Pt ML deposition processes.

The electrodeposition of Pt_ML/Pd/W(Mo)Ni catalysts directly on the GDL is realized in three sequential steps. In the first step, refractory WNi or MoNi alloy nanoparticles are co-electrodeposited directly on functionalized GDL by applying a constant current or pulse deposition protocol, from solution containing Na₂WO₄ or Na₂MoO₄, NiSO₄, and sodium citrate. In the consecutive step, the solution in the cell is replaced by PdCl₂ solution, and Ni is partially displaced by Pd. As a result, WNi (MoNi) core nanoparticles are covered with thin (2-3 monolayers) Pd shells. In the third step, Pt ML is deposited on Pd/WNi (Pd/MoNi) nanostructures in “one-cell configuration” [6] from solution containing both Pt²⁺ and Cu²⁺ ions. At the end of the deposition process, the electrode is rinsed with nanopure water, and after drying is ready for assembly. Thus, highly active, inexpensive and durable Pt_ML/Pd/WNi (Pt_ML/Pd/MoNi) electrocatalysts with ultra-low PGM loading are directly electrodeposited on up to 500 cm² GDL. The features of the system will be discussed along with the electrodes characterization and MEAs performance at real fuel cell operating conditions.

Acknowledgment

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

 References

1.  http://energy.gov/eere/fuelcells/downloads/fuel-cell-technologies-office-multi-year-research-development-and-22.
2. H. A. Gasteiger, S. S. Kocha, B. Sompalli and F. T. Wagner, Applied Catalysis B-Environmental, 56, 9 (2005).
3. R. R. Adzic, Electrocatalysis, 3, 163 (2012).
4. K. Sasaki, H. Naohara, Y. Choi, Y. Cai, W.-F. Chen, P. Liu and R. R. Adzic, Nature Communications, 3, 1115 (2012).
5. R. Adzic, S. Bliznakov, and M. Vukmirovic. US Patent: Core-Shell Fuel Cell Electrodes,US2015/0017565 A1, Jan. 15, 2015.
6. M. Fayette, Y. Liu, D. Bertrand, J. Nutariya, N. Vasiljevic and N. Dimitrov, Langmuir, 27, 5650 (2011).