Preparation and Characterization of PdFe Nanoleaves as Electrocatalysts for Oxygen Reduction Reaction

Abstract

Novel PdFe-nanoleaves (NLs) have been prepared through a wet chemistry-based solution phase reduction synthesis route. High-resolution transmission electron microscopy (HR-TEM) and scanning transmission electron microscopy (S/TEM) coupled with high-spatial-resolution compositional analysis clearly show that this newly developed structure is assembled from Pd-rich nanowires (Pd-NWs) surrounded by Fe-rich sheets. The Pd-NWs have diameters in the range of 1.8-2.3 nm and large electrochemical surface areas of >50 m 2 /g. Their length (30-100 nm) and morphology can be tuned by altering the nanostructure synthesis conditions and the Fe amount in the NLs. With increasing Fe content, thinner and longer sheet-enveloped nanowires can be prepared. The side surfaces of PdNWs observed by HR-TEM are predominantly Pd(111) facets, while the tips and ends are Pd(110) and Pd(100) facets. By etching away the enveloping Fe-rich sheets using an organic acid, the Pd-rich NWs are exposed on the surfaces of the nanoleaves, and they demonstrate high reactivity toward electrocatalytic reduction of oxygen in a 0.1 M NaOH electrolyte, i.e., a factor of 3.0 increase in the specific activity and a factor of 2.7 increase in the mass activity, compared to a commercial Pt/C catalyst (at 0 V vs. Hg/HgO). The electrocatalytic activity enhancement can be attributed to the unique nanoleave structure, which provides more Pd(111) facets, a large surface area, and more resistance to Pd oxide formation. ' INTRODUCTIONWithout the Carnot cycle limitation, low-temperature proton exchange membrane fuel cells (PEMFCs) directly convert the chemical energy of fuels into electricity with high theoretical efficiency (i.e., 83% for H 2 fuel at a thermodynamic potential of 1.229 V under standard conditions) and zero emission; therefore, they are considered to be a promising sustainable power source. 1 However, the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode remains a great scientific challenge that hinders the high energy conversion efficiency of PEMFCs. 2 Platinum group metal (PGM)-based electrocatalysts are currently used for PEMFCs to reduce the large ORR overpotential. 2c-e Unfortunately, even on the most active Pt surface, the overpotential is >250 mV at open circuit voltage (OCV). The thermodynamic efficiency drops from 83% (1.229 V) to, i.e., 66% at an OCV value of 0.98 V under standard conditions. 1a,2 Since the exchange current density of the ORR is very small (10 -9 A/cm 2 at the Pt surface), 2e to get a usable output current, the operating voltage must be largely reduced to, i.e., 0.65 V, making the energy efficiency of PEMFCs decrease further to 44%.PGM-based multimetallic nanostructures with well-controlled size, shape, structure, chemical composition, and morphology can effectively enhance the catalytic reaction kinetics through modifying surface compositions, optimizing highly active crystalline facets, and tuning electronic structures. 3 Wet-chemical synthesis (WCS) approaches have emerged as one of the most promising me...

Recent Development of Pd-Based Electrocatalysts for Proton Exchange Membrane Fuel Cells

“…The thermodynamic efficiency droped from 83% (1.229 V) to 66% at an OCV value of 0.98 V under standard conditions. Since the exchange current density of the ORR is several orders of magnitude slower than that of the hydrogen oxidation reaction (HOR) (10 −9 A cm −2 vs. 10 −3 A cm −2 ), the operating voltage must be largely reduced to about 0.65 V at a reasonable current density, making the electronic efficiency of the PEMFCs only at 44% [194]. Pt is widely used as the catalyst for ORR due to its high activity and excellent chemical stability.…”

“…The surface specific activity of Pd nanorods (Pd-NRs) toward the ORR was found to be not only 10-fold higher than that of Pd nanoparticles (Pd-NPs), but also comparable to that of Pt at operating potentials of fuel cell cathodes [264]. Zhang et al [194] prepared PdFe-nanoleaves with Pd-rich nanowires surrounded by Fe-rich sheets. The structure demonstrated three times increased specific activity and 2.7 times increased mass activity compared witha commercial Pt/C catalyst.5.5.…”

PdNi Hollow Nanoparticles for Improved Electrocatalytic Oxygen Reduction in Alkaline Environments

“…7,[11][12][13][14] Pd alloyed with transition metals (Pd-TMs), such as Pd-Ni, [15][16] Pd-Cu, [17][18][19][20] 27 while the ORR performance was not improved, mainly because the larger lattice constant of silver would subject the Pd to a tensile strain effect, decreasing the electroactivity of Pd towards the ORR. 36 Pd-Co 37 and Pd-Cu 38 hollow nanostructures were also reported with large particle size (> 50 nm) but their ORR properties were not extensively studied due to the large particle size, which may decrease the active surface area.…”

1D Pd-Based Nanomaterials as Efficient Electrocatalysts for Fuel Cells

“…Among various Pd or Pd-based nanomaterials, one-dimensional (1D) nanostructured electrocatalysts, such as nanowires (NWs) [64][65][66][67][68][69][70][71][72], nanorods (NRs) [44,73], nanoleaves (NLs) [74], and nanotubes (NTs) [75][76][77][78][79][80], have attracted more and more attention in recent years due to their unique structures and high surface area. From a structural perspective, 1D Pd nanomaterials possess largely pristine surfaces with long segments of crystalline planes, as compared with their corresponding 0D morphologies.…”

“…In another report, Li and co-workers [74] reported the preparation and characterization of PdFe nanoleaves (NLs) for ORR. The novel nanoleaf-structured materials were synthesized through a wet chemical reduction method with the presence of oleylamine, and the composition of the final products can be tuned by changing the precursors of Pd(acac) 2 and Fe(CO) 5 .…”