Tailoring Chemical Composition To Achieve Enhanced Methanol Oxidation Reaction and Methanol-Tolerant Oxygen Reduction Reaction Performance in Palladium-Based Nanowire Systems

In this article, we address two key challenges in the development of electrocatalysts for direct methanol fuel cells by rationally tailoring the morphology and chemical composition of Pd-based nanowires (NWs) for enhanced performance. First, we have examined the morphology and composition-dependent performance of Pt_1–<i>x</i>Pd_<i>x</i> NWs toward the methanol oxidation reaction (MOR). Elemental Pt NWs were found to possess a significant morphology-dependent enhancement of nearly 3-fold in terms of peak MOR-specific activity over that of commercial Pt NP/C. In addition, tailoring the chemical composition in Pt_1–<i>x</i>Pd_<i>x</i> NWs can lead to measurable increases in MOR kinetics, which can be attributed to improved oxidation of formic acid and, potentially, increased selectivity for a direct, CO-free pathway. Second, we have explored the stability of ORR performance in the presence of measurable concentrations of methanol as a function of chemical composition in Pt_1–<i>x</i>Pd_<i>x</i> NWs and Pt-free Pd₉Au NWs. In the context of the Pt_1–<i>x</i>Pd_<i>x</i> NWs, a distinctive volcano-type dependence has been noted with respect to chemical composition, and on the basis of the MOR activities and methanol tolerant ORR behavior, Pt₇Pd₃ NWs have been highlighted as an optimal catalyst architecture. We have also analyzed the methanol tolerance in Pd₉Au NWs, which represents a highly active, durable Pt-free alternative to traditional Pt-based nanostructured catalysts. Herein, we have demonstrated that Pd₉Au NWs (0.42 mA/cm²) with no effective Pt content can outperform Pt-based nanostructures, such as Pt NWs (0.32 mA/cm²) and nanoparticulate Pt NP/C (0.24 mA/cm²) in the presence of 4 mM methanol/0.1 M HClO₄

Size- and Composition-Dependent Enhancement of Electrocatalytic Oxygen Reduction Performance in Ultrathin Palladium–Gold (Pd_1–<i>x</i>Au_<i>x</i>) Nanowires

In this report, we examine the composition- and size-dependent performance in hierarchical Pd_1–<i>x</i>Au_<i>x</i> nanowires (NWs) encapsulated with a conformal Pt monolayer shell (Pt∼Pd_1–<i>x</i>Au_<i>x</i>). The ultrathin Pd_1–<i>x</i>Au_<i>x</i> NWs are prepared by a solution-based method wherein the chemical composition can be readily and predictably controlled. Importantly, as-prepared Pd₉Au NWs maintain significantly enhanced oxygen reduction reaction (ORR) activity (0.40 mA/cm²), as compared with elemental Pd NW/C (0.12 mA/cm²) and Pt nanoparticles (NP)/C (0.20 mA/cm²), respectively. After the deposition of a Pt monolayer, a volcano-type composition dependence is observed in the ORR activity of the Pt∼Pd_1–<i>x</i>Au_<i>x</i> NWs as the Au content is increased from 0 to 30% with the activity of the Pt∼Pd₉Au NWs (0.98 mA/cm², 2.54 A/mg_Pt), representing the optimum performance. We note that the platinum group metal activity of the ultrathin 2 nm NWs (0.64 A/mg) is significantly enhanced as compared with that of analogous 50 nm NWs (0.16 A/mg) and commercial Pt NP/C (0.1–0.2 A/mg), thereby highlighting a distinctive size-dependent enhancement in NW performance

Probing Ultrathin One-Dimensional Pd–Ni Nanostructures As Oxygen Reduction Reaction Catalysts

An ambient, surfactant-based synthetic means was used to prepare ultrathin binary (<i>d</i> ∼ 2 nm) Pd–Ni nanowires, which were subsequently purified using a novel butylamine-based surfactant-exchange process coupled with an electrochemical CO adsorption and stripping treatment to expose active surface sites. We were able to systematically vary the chemical composition of as-prepared Pd–Ni nanowires from pure elemental Pd to Pd_0.50Ni_0.50 (atomic ratio), as verified using EDS analysis. The overall morphology of samples possessing >60 atom % Pd consisted of individual, discrete one-dimensional nanowires. The electrocatalytic performances of elemental Pd, Pd_0.90Ni_0.10, Pd_0.83Ni_0.17, and Pd_0.75Ni_0.25 nanowires in particular were examined. Our results highlight a “volcano”-type relationship between chemical composition and corresponding ORR activities with Pd_0.90Ni_0.10, yielding the highest activity (i.e., 1.96 mA/cm² at 0.8 V) among all nanowires tested. Moreover, the Pd_0.90Ni_0.10 sample exhibited outstanding methanol tolerance ability. In essence, there was only a relatively minimal 15% loss in the specific activity in the presence of 4 mM methanol, which was significantly better than analogous data on Pt nanoparticles and Pt nanowires. In addition, we also studied ultrathin, core–shell Pt∼Pd_0.90Ni_0.10 nanowires, which exhibited a specific activity of 0.62 mA/cm² and a corresponding mass activity of 1.44 A/mg_Pt at 0.9 V. Moreover, our as-prepared core–shell electrocatalysts maintained excellent electrochemical durability. We postulate that one-dimensional Pd–Ni nanostructures represent a particularly promising platform for designing ORR catalysts with high performance

Highly Enhanced Electrocatalytic Oxygen Reduction Performance Observed in Bimetallic Palladium-Based Nanowires Prepared under Ambient, Surfactantless Conditions

We have employed an ambient, template-based technique that is simple, efficient, and surfactantless to generate a series of bimetallic Pd_1–<i>x</i>Au_<i>x</i> and Pd_1–<i>x</i>Pt_<i>x</i> nanowires with control over composition and size. Our as-prepared nanowires maintain significantly enhanced activity toward oxygen reduction as compared with commercial Pt nanoparticles and other 1D nanostructures, as a result of their homogeneous alloyed structure. Specifically, Pd₉Au and Pd₄Pt nanowires possess oxygen reduction reaction (ORR) activities of 0.49 and 0.79 mA/cm², respectively, which are larger than the analogous value for commercial Pt nanoparticles (0.21 mA/cm²). In addition, core–shell Pt∼Pd₉Au nanowires have been prepared by electrodepositing a Pt monolayer shell and the corresponding specific, platinum mass, and platinum group metal mass activities were found to be 0.95 mA/cm², 2.08 A/mg_Pt, and 0.16 A/mg_PGM, respectively. The increased activity and catalytic performance is accompanied by improved durability toward ORR

Photocurrent Enhancement from Solid-State Triplet–Triplet Annihilation Upconversion of Low-Intensity, Low-Energy Photons

We present a red-to-blue upconversion system based on triplet–triplet annihilation in a solid-state film configuration that significantly enhances the photocurrent of a model solar cell device. The film is robust against oxygen quenching and can be readily tailored to existing solar cell architectures. The photovoltaic performance of upconversion-assisted dye-sensitized photoelectrochemical cells was measured under both high-power coherent laser and low-power incoherent light irradiation (light-emitting diode and simulated AM1.5G sunlight). By utilizing low-energy photons that would otherwise be wasted, the photocurrent is enhanced by as much as 35% under one-sun light intensity when a model solar cell device is coupled with a TTA film and a reflector

Photocurrent Enhancement from Solid-State Triplet–Triplet Annihilation Upconversion of Low-Intensity, Low-Energy Photons

We present a red-to-blue upconversion system based on triplet–triplet annihilation in a solid-state film configuration that significantly enhances the photocurrent of a model solar cell device. The film is robust against oxygen quenching and can be readily tailored to existing solar cell architectures. The photovoltaic performance of upconversion-assisted dye-sensitized photoelectrochemical cells was measured under both high-power coherent laser and low-power incoherent light irradiation (light-emitting diode and simulated AM1.5G sunlight). By utilizing low-energy photons that would otherwise be wasted, the photocurrent is enhanced by as much as 35% under one-sun light intensity when a model solar cell device is coupled with a TTA film and a reflector