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

One-Pot Synthesis of Pd Nanoparticles Supported on Carbide-Derived Carbon for Oxygen Reduction Reaction

We explored two methods for synthesizing Pd nanoparticles using three different carbide-derived carbon (CDC) support materials, one of which was nitrogen-doped. These materials were studied for oxygen reduction reaction (ORR) in 0.1 M KOH solution, and the resulting CDC/Pd catalysts were characterized using TEM, XRD, and XPS. The citrate method and the polyol method using polyvinylpyrrolidone (PVP) as a capping agent were employed to elucidate the impact of the support material on the final catalyst. The N-doping of the CDC material resulted in smaller Pd nanoparticles, but only in the case of the citrate method. This suggests that the influence of support is weaker when using the polyol method. The citrate method with CDC1, which is predominantly microporous, led to a higher degree of agglomeration and formation of larger particles in comparison to supports, which possessed a higher degree of mesoporosity. We achieved smaller Pd particle sizes using citrate and NaBH4 compared to the ethylene glycol PVP method. Pd deposited on CDC2 and CDC3 supports showed similar specific activity (SA), suggesting that the N-doping did not significantly influence the ORR process. The highest SA value was observed for CDC1/Pd_Cit, which could be attributed to the formation of larger Pd particles and agglomerates.

Bimetallic Pd-Co Nanoparticles Supported on Nitrogen-Doped Reduced Graphene Oxide as Efficient Electrocatalysts for Formic Acid Electrooxidation

In this work, bimetallic PdxCoy nanoparticles supported on nitrogen-doped reduced graphene oxide catalysts were synthesized and tested for formic acid oxidation as potentially efficient and durable electrocatalysts. Graphene oxide was nitrogen doped through hydrothermal chemical reduction with urea as a nitrogen source. The PdxCoy nanoparticles were deposited on the nitrogen-doped graphene oxide support using the impregnation-reduction method with sodium borohydride as a reducing agent and sodium citrate dihydrate as a stabilizing agent. The structural features, such as phases, composition, oxidation states, and particle sizes, of the nanoparticles were characterized using X-ray diffraction, transmission electron microscopy, scanning electron microscopy–energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The Pd nanoparticle sizes in Pd1Co1/N-rGO, Pd/N-rGO, and Pd1Co1/CNT were 3.5, 12.51, and 4.62 nm, respectively. The electrochemical performance of the catalysts was determined by CO stripping, cyclic voltammetry, and chronoamperometry. Pd1Co1/N-rGO showed the highest mass activity of 4833.12 mA–1 mg Pd, which was twice that of Pd1Co1/CNT. Moreover, Pd1Co1/N-rGO showed a steady-state current density of 700 mA–1 mg Pd after 5000 s in chronoamperometry carried out at +0.35 V. Apart from the well-known bifunctional effect of Co, nitrogen-doped graphene contributed to the performance enhancement of the Pd1Co1/N-rGO catalyst.

Strained Ultralong Silver Nanowires for Enhanced Electrocatalytic Oxygen Reduction Reaction in Alkaline Medium

Many noble metals are efficient catalysts for oxygen reduction reaction (ORR), including silver (Ag). Among all these noble metals, Ag is the most affordable because of its relative abundance. Surface energy has been proven to play a crucial role in the catalytic process, and straining is an effective operation to raise the surface energy over electrocatalysts. In this work, sonication was utilized to induce strain in Ag nanowires (NWs) through lattice deformation. A 0.18 J/m² improvement of the surface energy around the stacking faults area has been calculated via density functional theory. The diffusion-limiting current density was evaluated and increases by >20% (from -4.98 to -6.00 mA/cm²) after sonication straining. Meanwhile, the onset potential remains almost constant (i.e., 0.95 V vs RHE). The results show that induction of strain has a strong impact on the diffusion-limiting current density and significantly improves the ORR catalytic performance of Ag NWs.

Toward Overcoming the Challenges in the Comparison of Different Pd Nanocatalysts: Case Study of the Ethanol Oxidation Reaction

Precious metal nanoparticles, in particular palladium nanomaterials, show excellent catalytic properties and are key in the development of energy systems. For instance, ethanol fuel cells are promising devices for sustainable energy conversion, where Pd-based catalysts are key catalysts for the related ethanol oxidation reaction (EOR). Pd is a limited resource; thus, a remaining challenge is the development of efficient and stable Pd-based catalysts. This calls for a deeper understanding of the Pd properties at the nanoscale. This knowledge can be gained in comparative studies of different Pd nanomaterials. However, such studies remain challenging to perform and interpret due to the lack of cross-studies using the same Pd nanomaterials as a reference. Here, as-prepared sub 3 nm diameter surfactant-free Pd nanoparticles supported on carbon are obtained by a simple approach. The as-prepared catalysts with Pd loading 10 and 30 wt % show higher activity and stability compared to commercially available counterparts for the EOR. Upon electrochemical testing, a significant size increase and loss of electrochemical active surface are observed for the as-prepared catalysts, whereas the commercial samples show an increase in the electrochemically active surface area and moderate size increase. This study shines light on the challenging comparison of different catalysts across the literature. Further advancement in Pd (electro)catalyst design will gain from including self-prepared catalysts. The simple synthesis detailed easily leads to suitable nanoparticles to be used as a reference for more systematic comparative studies of Pd catalysts across the literature.

Electrochemical Studies of Pd-Based Anode Catalysts in Alkaline Medium for Direct Glycerol Fuel Cells

This study investigates the most effective electrocatalyst for glycerol oxidation reaction (GOR) in alkaline medium for five synthesized electrocatalysts, Pd, PdNi, PdNiO, PdMn3O4 and PdMn3O4NiO, supported on multi-walled carbon nanotubes (MWCNTs) prepared using the polyol method. The particle size and crystalline size of the electrocatalysts were determined using HR-TEM and XRD techniques, respectively, while EDS was used to determine the elemental composition. XRD showed crystalline sizes ranging from 3.4 to 10.1 nm, while HR-TEM revealed particle sizes within the range of 3.4 and 7.2 nm. The electroactivity, electron kinetics and stability of the electrocatalysts towards glycerol in alkaline medium was evaluated using linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA), respectively, while the electroactive surface area (ECSA) of the electrocatalysts was determined using cyclic voltammetry (CV). The metal oxide-based Pd electrocatalysts PdNiO and PdMn3O4 were the most electrochemically active, while the addition of the second metal oxide to the Pd electrocatalyst PdMn3O4NiO did not show any improvement. This was associated with this electrocatalyst having the highest particle and crystalline sizes.

Bismuth as Smart Material and Its Application in the Ninth Principle of Sustainable Chemistry

This paper reports an overview of Green Chemistry and the concept of its twelve principles. This study focusses on the ninth principle of Green Chemistry, that is, catalysis. A report on catalysis, in line with its definition, background, classification, properties, and applications, is provided. The study also entails a green element called bismuth. Bismuth’s low toxicity and low cost have made researchers focus on its wide applications in catalysis. It exhibits smartness in all the catalytic activities with the highest catalytic performance among other metals.

High Active PdSn Binary Alloyed Catalysts Supported on B and N Codoped Graphene for Formic Acid Electro-Oxidation

A series of PdSn binary catalysts with varied molar ratios of Pd to Sn are synthesized on B and N dual-doped graphene supporting materials. The catalysts are characterized by X-ray diffraction (XRD) and Transmission electron microscopy (TEM). Formic acid electro-oxidation reaction is performed on these catalysts, and the results reveal that the optimal proportion of Pd:Sn is 3:1. X-ray photoelectron spectroscopy (XPS) measurements show that when compared with 3Pd1Sn/graphene, B and N co-doping into the graphene sheet can tune the electronic structure of graphene, favoring the formation of small-sized metallic nanoparticles with good dispersion. On the other hand, when compared with the monometallic counterparts, the incorporation of Sn can generate oxygenated species that help to remove the intermediates, exposing more active Pd sites. Moreover, the electrochemical tests illustrate that 3Pd1Sn/BN-G catalyst with a moderate amount of Sn exhibits the best catalytic activity and stability on formic acid electro-oxidation, owing to the synergistic effect of the Sn doping and the B, N co-doping graphene substrate.

Effect of Metal Substrate on Electrocatalytic Property of Palladium Nanowire Array for High Performance Ethanol Electro-Oxidation

In this research, a high performance, ionomer-free electrocatalyst based on vertically aligned palladium (Pd) nanowire array was developed as an anode electrode toward ethanol oxidation reaction (EOR) in an alkaline environment. Using a one-step electrodeposition method, the Pd nanowires with controlled length were obtained by varying the electrodeposition current density and the synthesis time. Scanning electron microcopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray powder diffraction (XRD) were employed to characterize the morphology, chemical composition, and crystal structure of the Pd nanowires. The length effects of the nanowires, in the range of 0.8-4.5 μm, and various metal substrates, such as Ag, Cu, Ni, and Ti, were investigated for their electrochemical activities. The results demonstrated that Ag was the most active substrate to facilitate the ethanol oxidation reaction of the Pd nanowire array (NWA) electrocatalyst, which could be related to its good electrical conductivity. The stability test of the Pd NWA/Ag over time for EOR was also carried out, and the catalytic activity was recovered after the electrode was replaced with a new ethanol solution. Electrochemical impedance spectroscopy (EIS) measurements were performed to provide insights in the electron transfer resistance between the electrode and analyte. Gas chromatography and UV-vis spectroscopy were employed to measure the concentration of chemical species, which helped elucidate the overall reaction mechanism on the electrode surfaces.

Deposition and morphological evolution of nanostructured palladium during potential cycling: a liquid-cell TEM study

In order to gain better control over the functionality of Pd nanostructures used in several CO₂-mitigating electrochemical energy conversion systems, it is imperative to underpin different nanoscale phenomena influencing their structural durability. Hitherto, such analyses have been carried out before/after an electrochemical treatment, but not during the entire process. Here, we demonstrate monitoring of morphological evolution in Pd nanostructures over the entire course of electrochemical treatment using a liquid-cell transmission electron microscope (TEM) set-up. Our findings reveal new insights into nanoparticle growth, dissolution, detachment, and aggregation that are relevant for the development of functional Pd nanomaterials.

Nickel Promoted Palladium Nanoparticles for Electrocatalysis of Carbohydrazide Oxidation Reaction

Carbohydrazide is a potential alternative to toxic hydrazine for fuel cell applications to overcome the challenges of storage and transportation of hydrogen. In this work, Ni-alloyed Pd nanoparticles (NPs) with varied Pd-Ni ratios supported on carbon black (PdNi_x /C) are prepared and their catalytic performance for the carbohydrazide electro-oxidation reaction is investigated. The catalytic performance of PdNi_x /C NPs is significantly improved in comparison to Pd/C NPs. The current density of PdNi_x /C NPs with optimized Pd-Ni atom ratio can reach 3.26 A mg^-1 _metal at a potential of 0.4 V (vs reversible hydrogen electrode), which is an increase of 2.4 times compared to that of Pd/C. The density functional theory calculation indicates the enhanced catalytic activity is caused by the change of adsorption energy of carbohydrazide molecules on the metal surface. It exhibits a volcano relationship between the adsorption energy and the catalytic current density of PdNi_x /C with varied Pd-Ni atom ratios.

Electrocatalytic Activities towards the Electrochemical Oxidation of Formic Acid and Oxygen Reduction Reactions over Bimetallic, Trimetallic and Core–Shell-Structured Pd-Based Materials

The structural design of nanosized electrocatalysts is extremely important for cathodic oxygen reduction reactions (ORR) and anodic oxidation reactions in small organic compounds in direct fuel cells. While Pt is still the most commonly used electrode material for ORR, the Pd electrocatalyst is a promising alternative to Pt, because it exhibits much higher electrocatalytic activity towards formic acid electrooxidation, and the electrocatalytic activity of ORR on the Pd electrode is the higher than that of all other precious metals, except for Pt. In addition, the mass activity of Pt in a core–shell structure for ORR can be improved significantly by using Pd and Pd-based materials as core materials. Herein, we review various nanoscale Pd-based bimetallic, trimetallic and core–shell electrocatalysts for formic acid oxidation and ORR of polymer electrolyte fuel cells (PEFCs). This review paper is separated into two major topics: the electrocatalytic activity towards formic acid oxidation over various Pd-based electrocatalysts, and the activity of ORR on Pd-based materials and Pd core–Pt shell structures.

Size effect studies in catalysis: a simple surfactant-free synthesis of sub 3 nm Pd nanocatalysts supported on carbon

Supported Pd nanoparticles are prepared under ambient conditions via a surfactant-free synthesis. Pd(NO₃)₂ is reduced in the presence of a carbon support in alkaline methanol to obtain sub 3 nm nanoparticles. The preparation method is relevant to the study of size effects in catalytic reactions like ethanol electro-oxidation.

A simple surfactant-free synthesis of sub 3 nm carbon-supported Pd nanocatalysts is introduced to study size effects in catalysis.

Metallic bionanocatalysts: potential applications as green catalysts and energy materials

Microbially generated or supported nanocatalysts have potential applications in green chemistry and environmental application. However, precious (and base) metals biorefined from wastes may be useful for making cheap, low-grade catalysts for clean energy production. The concept of bionanomaterials for energy applications is reviewed with respect to potential fuel cell applications, bio-catalytic upgrading of oils and manufacturing 'drop-in fuel' precursors. Cheap, effective biomaterials would facilitate progress towards dual development goals of sustainable consumption and production patterns and help to ensure access to affordable, reliable, sustainable and modern energy.

Turning the Halide Switch in the Synthesis of Au-Pd Alloy and Core-Shell Nanoicosahedra with Terraced Shells: Performance in Electrochemical and Plasmon-Enhanced Catalysis

Au-Pd nanocrystals are an intriguing system to study the integrated functions of localized surface plasmon resonance (LSPR) and heterogeneous catalysis. Gold is both durable and can harness incident light energy to enhance the catalytic activity of another metal, such as Pd, via the SPR effect in bimetallic nanocrystals. Despite the superior catalytic performance of icosahedral (IH) nanocrystals compared to alternate morphologies, the controlled synthesis of alloy and core-shell IH is still greatly challenged by the disparate reduction rates of metal precursors and lack of continuous epigrowth on multiply twinned boundaries of such surfaces. Herein, we demonstrate a one-step strategy for the controlled growth of monodisperse Au-Pd alloy and core-shell IH with terraced shells by turning an ionic switch between [Br(-)]/[Cl(-)] in the coreduction process. The core-shell IH nanocrystals contain AuPd alloy cores and ultrathin Pd shells (<2 nm). They not only display more than double the activity of the commercial Pd catalysts in ethanol electrooxidation attributed to monatomic step terraces but also show SPR-enhanced conversion of 4-nitrophenol. This strategy holds promise toward the development of alternate bimetallic IH nanocrystals for electrochemical and plasmon-enhanced catalysis.

Facile sonochemical synthesis of highly dispersed ultrafine Pd nanoparticle decorated carbon nano-onions with high metal loading and enhanced electrocatalytic activity

Highly dispersed, ultrafine Pd nanoparticle decorated carbon nano-onions (CNO) were prepared by a facile, one-step sonochemical method.