Palladium Coated Copper Nanowires as a Hydrogen Oxidation Electrocatalyst in Base

Abstract

Palladium (Pd) nanotubes are synthesized by the spontaneous galvanic displacement of copper (Cu) nanowires, forming extended surface nanostructures highly active for the hydrogen oxidation reaction (HOR) in base. The synthesized catalysts produce specific activities in rotating disk electrode half-cells 20 times greater than Pd nanoparticles and about 80% higher than polycrystalline Pd. Although the surface area of the Pd nanotubes was low compared to conventional catalysts, partial galvanic displacement thrifted the noble metal layer and increased the Pd surface area. The use of Pd coated Cu nanowires resulted in a HOR mass exchange current density 7 times greater than the Pd nanoparticles. The activity of the Pd coated Cu nanowires further nears Pt/C, producing 95% of the mass activity. The commercial deployment of proton exchange membrane fuel cells (PEMFCs) is primarily limited by the cost of platinum (Pt) catalysts. Hydroxide exchange membrane fuel cells (HEMFCs) were recently developed as a potential alternative to PEMFCs as they can avoid the use of Pt group metal (PGM) catalysts, particularly at the cathode in the oxygen reduction reaction (ORR). [1][2][3][4][5] In both PEMFCs and HEMFCs, kinetic losses are primarily attributed to the cathode, where ORR on Pt requires an overpotential of approximately 0.3 V. In base, however, the hydrogen oxidation reaction (HOR) on Pt is two orders of magnitude slower than in acid.6,7 A reduction in HOR catalyst cost can significantly improve the commercial prospect of HEMFCs.Pd has previously been studied as a HOR catalyst and alternative to Pt in membrane electrode assemblies for PEMFCs and alkaline (liquid electrolyte) fuel cells. [8][9][10][11][12] Pd-based catalysts, including Pd-gold (Au) alloys, Pd-Pt alloys, and Pd modified polythiophene, have also been studied for catalyzing HORs in acid or gaseous environments. [13][14][15] Although fundamental studies of the kinetics of HOR on Pd in base are largely unavailable, recent rotating disk electrode (RDE) results provide insight, allowing for the design and electrochemical characterization of HOR catalysts in base. Shao-Horn et al. determined the HOR/hydrogen evolution reaction (HER) exchange current densities of carbon supported Pt (Pt/C) and bulk polycrystalline Pt (BPPt). Galvanic displacement has recently been used in the development of extended surface fuel cell catalysts. 17 Although most of the work into developing advanced geometry catalysts has focused on ORR in PEMFCs, Pt coated copper (Cu) nanowires (Pt/CuNWs) were recently studied as catalysts for HOR in base in RDE half-cells. 18Pt/CuNWs significantly exceeded the HOR mass and area exchange current densities of Pt/C, potentially benefitting from compressive strain and Cu surface impurities facilitating hydroxyl adsorption. 19,20 In this study, we galvanically displaced CuNWs with Pd, forming Pd coated CuNWs (Pd/CuNWs) and Cu templated Pd nanotubes (PdNTs (Cu)). These materials were investigated for HOR in base to: improve the HOR mass and area activiti...

Palladium-ceria nanocatalyst for hydrogen oxidation in alkaline media: Optimization of the Pd–CeO2 interface

“…0.6 mA cm -2 Pt for commercial Pt/C catalysts [19][20][21][22]. Significant promotion of the HOR electrocatalysis on Pd-based materials has been achieved with the use of Pd-coated Cu nanowires [23], Pd-decorated Ru nanotubes [24], as well as by doping with Ni [25,26], Ir [27], and CeO 2 [28,29]. Among them, Pd-CeO 2 catalyst has shown the highest anode performance so far reported for Ptfree-based AEMFCs [30].…”

Size-Dependent Hydrogen Oxidation and Evolution Activities on Supported Palladium Nanoparticles in Acid and Base

“…Pd and Pd-based catalysts have been studied as HOR catalysts in fuel cells, 9,10 and lower HOR/HER activities were observed on Pd/C than bulk Pd, [11][12][13] suggesting a particle size effect. However, a fundamental understanding of HOR/HER kinetics on Pd and a systematic study of the particle size effect are largely missing.…”

A novel Ir/CeO2–C nanoparticle electrocatalyst for the hydrogen oxidation reaction of alkaline anion exchange membrane fuel cells

“…This lower intrinsic activity under alkaline conditions has barricaded the use of remarkably low concentrations of Pt catalyst in anodes; this increases the cost and cancels out the merit that cathode requires non-noble metal catalysts such as Fe/N/C 4 and Co/N/C 5 . Fortunately, many non-platinum catalysts such as Pd, Ir, and Ru and alloys [6][7][8] or non-noble metals such as RANEY® Ni 9 , Ni/N-CNT, 10 and NiCoMo 11 are stable under alkaline conditions, which enlarge the scope of HOR catalysts and provide the tactics to solve this issue. There is no doubt that platinum group metals react approximately two orders of magnitude slower under basic conditions than under acidic conditions, 3 but the reason why this happens and a solution to make platinum group metals active are under debate.…”