Proton exchange membrane fuel cell (PEMFC) has gained tremendous popularity in the scientific community as a viable candidate for sustainable energy production. PEMFC technology with high power density, high energy conversion efficiency and near-zero emission, is highly promising for niche applications such as portable power sources and electric vehicles 1, 2. However, the large scale applications of PEMFCs are impeded by the sluggish oxygen reduction reaction (ORR) kinetics at the cathode which amounts to overpotential loss of > 300 mV and leads to lower cell performance 3. Pt supported on carbon black (Pt/C) is the most commonly used cathode catalyst for PEMFC. However, Pt/C suffers from major disadvantages such as low ORR activity, poor corrosion resistance of the carbonaceous support material under dynamic fuel cell operating conditions and high cost resulting from the high Pt content in it 4. A practical approach towards realization of a more active, durable and cost-effective cathode catalyst calls for equal attention towards both the catalytic active metal component and the support material.
Alloying Pt with a second transition metal (Pt-M; M= transition metal) is one of the effective strategies that can tune the intrinsic catalytic activity of Pt along with a reduction in the Pt content 5, 6. Incorporation of a suitable alloying element (M) favorably modifies the Pt-Pt geometric (interatomic) distance 7, 8 and electronic vacancy (d band) parameter of the Pt metal 9, 10, which improves ORR kinetics and stability of the catalyst 11. Among the transition metals, Pd is a suitable candidate to alloy with Pt due to its similar crystal structure, lattice constant and valence electronic configuration 12, 13. Furthermore, Pd exhibits good stability under acidic conditions comparable to that of Pt 14; and improved corrosion resistance of Pt based electrocatalysts have been reported when alloyed with Pd 15, 16. The synergistic interactions between Pt and Pd enhances the ORR activity and durability characteristics of the resulting alloyed electrocatalysts and consequently improves the fuel cell performance 17.
Dispersing metal nanoparticles (NPs) on suitable support materials is another useful and widely employed approach to decrease the Pt metal loading as well as to improve the catalytic activity and durability through metal-support interactions 18-20. The conventional carbon black (CB) support suffers from serious drawbacks such as mass transport limitation due to the presence of dense micro-pores, lower Pt utilization owing to entrapment of catalyst NPs in the micro-pores and electrochemical oxidation under PEMFC working condition 21. Compared to CB, nanostructured multi-walled carbon nanotubes (MWCNTs) possess a wide array of remarkable properties, particularly high electrical conductivity, high mechanical strength, large catalytic surface area, excellent corrosion resistance and high stability, making them an ideal candidate as support materials for Pt based catalysts 22, 23. Thus, for application as electrocatalyst in PEMFCs, the materials Pt-Pd alloy and MWCNTs have tremendous potential individually as catalytically active component and support, respectively. A functional hybrid of the two components (Pt-Pd/MWCNT) is anticipated to synergistically combine the advantageous properties of both to result in an advanced electrocatalyst material with enhanced ORR activity and durability.
Pt-Pd alloy NPs supported on MWCNT (Pt-Pd/MWCNT) electrocatalysts have been explored as cathode catalysts in PEMFCs and have demonstrated improved ORR activity and durability 24-27. Most of these studies focused on synthesis of catalysts and influence of preparation conditions on catalyst performance. Lee et. al prepared Pt-Pd NPs supported on MWCNT via self-regulated reduction of sodium n-dodecyl sulphate and the ORR activity of the resulting catalyst was evaluated through rotating ring disk electrode (RRDE) experiment with H2SO4 as the acidic electrolyte 24. Results showed that ORR activity of Pt-Pd/MWCNT catalyst proceeded through the 4-electron pathway suggesting its suitability for application in PEMFCs. Golikand et. al studied the influence of KOH (stabilizing agent) on the synthesis of Pt-Pd NPs supported on MWCNT 25. Smaller Pt-Pd NPs were obtained with this synthesis route employing KOH and the electrochemical measurements through rotating disk electrode (RDE) technique demonstrated the prevalence of 4-electron ORR pathway.
In another study by Golikand et. al, detailed durability investigation of Pt-Pd/MWCNT catalyst was carried out and the performance was compared with that of Pt/MWCNT catalyst 27. The catalysts were coated on real fuel cell gas diffusion electrodes to evaluate durability characteristics. However, the electrochemical evaluation was done in a three-electrode configuration with acidic electrolyte. It was found that Pd hinders the dissolution of Pt NPs from the MWCNT surface imparting higher durability to Pt-Pd/MWCNT catalyst. Zapata-Fernández et. al prepared Pt-Pd/MWCNT catalyst through galvanic displacement method without usage of any additional surfactant, additive or post-treatment process 26. The catalytic activity of the Pt-Pd/MWCNT catalyst was evaluated using RDE and RRDE experiments and the electrochemical results demonstrated improved ORR activity with favorable 4-electron process. It is worth mentioning that the performance evaluation of the Pt-Pd/MWCNT catalysts reported so far have been mostly carried out in three cell configuration with acidic liquid electrolyte, which does not represent the true situation in an actual fuel cell. It is not certain that the high performance obtained with half cell evaluation can be achieved with a real fuel cell, since the operating conditions are totally different in both cases 28. Hence, it is imperative to validate the half cell results with full cell evaluation of the catalysts in an actual PEMFC under operating conditions. In this context, a detailed electrochemical evaluation of Pt-Pd/MWCNT catalyst comprising of both half cell and full cell measurements is crucial to establish their suitability in PEMFCs.