Investigation of Supported Pd-Based Electrocatalysts for the Oxygen Reduction Reaction: Performance, Durability and Methanol Tolerance

Tuning the Structure of Pd@Ni-Co Nanowires and Their Electrochemical Properties

One-dimensional transition metal materials are promising supports for precious metals used in energy production processes. Due to their electrochemical properties, 3d-group metals (such as Ni, Co, and Fe) can actively interact with catalysts by a strong metal-support interaction. This study shows that changing the Ni:Co ratio makes it possible to modulate the structure of the catalyst supports, which, in turn, provides a tool for designing their electrical and electrochemical properties. For example, Ni1-Co9 shows the highest electrical conductivity (5.8-10-4 S/cm) among all of the materials examined. On the contrary, the Pd@Ni7-Co3 system presents the highest mass activity (>2000 mA mg-1) at 0.7 V, exceeding by several times that of commercial Pt/C (>300 mA mg-1) at the same potential. Our study opens the gateway for applications of bimetallic transition metal nanowires in catalytic conversion and energy production processes.

Computational Insights and Design of Promising Ultrathin PdM Bimetallenes for Oxygen Reduction Electrocatalysis

Bimetallic Palladium-based catalysts as an alternative of Pt-free electrocatalysts play a vital role in electrocatalysis. The doping of transition metal (M) into the ultrathin Pd nanosheets is new promising strategy to regulate the reactivity and durability of surface Pd sites. In this work, an in-depth investigation of the origin oxygen reduction reaction (ORR) activity and stability over 2D ultrathin PdM bimetallenes is presented. The M doping can greatly modify the reactivity of the Pd site by changing the local Fermi softness (S_F(r) ). All PdM bimetallenes follow the dissociative 4e^- pathway, and a thorough screening identified several promising alternatives (PdTa, PdHf, PdZr, and PdNb) with much lower ORR overpotential than the pure Pd(111) metallene. The Pd-O bond length and the Fermi softness of surface Pd atoms are effective descriptors of the adsorption of O* key intermediates. The hetero-metal induced ligand effect plays the key role for the activity improvement, which modifies the electronic properties and surface reactivity of Pd by the Pd-M orbital hybridization and result in the decline of bonding filling between Pd 3d_z ² and O* 2p orbital. The computational insight provides useful guideline for future experimental realizations of bimetallic Pd-based nanoalloys in ORR and other electrocatalytic reactions.

Uniform Distribution of Pd on GO-C Catalysts for Enhancing the Performance of Air Cathode Microbial Fuel Cell

Metal, as a high-performance electrode catalyst, is a research hotspot in the construction of a high-performance microbial fuel cell (MFC). However, metal catalyst nanoparticles and their dispersed carriers are prone to aggregation, producing catalytic electrodes with inferior qualities. In this study, Pd is uniformly dispersed on the graphene framework supported by carbon black to form nanocomposite catalysts (Pd/GO-C catalysts). The effect of the palladium loading amount in the catalyst on the catalytic performance of the air cathode was further studied. The optimized metal loading afforded a reduced resistance and improved accessibility of Pd particles for the ORR. The maximum current output of the 0.250 Pd (mg/cm2) MFC was 1645 mA/m2, which is 4.2-fold higher than that of the carbon paper cathode. Overall, our findings provide a novel protocol for the preparation of high-efficient ORR catalyst for MFCs.

Synthesis of Highly Active Pd@Cu-Pt/C Methanol Oxidation Electrocatalysts via Continuous, Co-Electroless Deposition

Controlled deposition of metals is essential for the creation of bimetallic catalysts having predictable composition and character. Continuous co-electroless deposition (co-ED) permits the creation of bimetallic catalysts with predictive control over composition. This method was applied to create a suite of Cu–Pt mixed-metal shell catalysts for use in methanol electrooxidation in direct methanol fuel cell applications (DMFCs). Enhanced performance of Cu–Pt compositions over Pt alone was predicted by existing computational studies in the literature. Experimental evidence from this study supports the bifunctional catalyst explanation for enhanced activity and confirms the optimum Cu:Pt ratio as Cu₃Pt for this methanol electrooxidation. This ability to control the composition of a bimetallic shell can be extended to other systems where the ratio of two metals is critical for catalytic performance.

Methanol-Tolerant M–N–C Catalysts for Oxygen Reduction Reactions in Acidic Media and Their Application in Direct Methanol Fuel Cells

Direct methanol fuel cells (DMFCs) are emerging technologies for the electrochemical conversion of the chemical energy of a fuel (methanol) directly into electrical energy, with a low environmental impact and high efficiency. Yet, before this technology can reach a large-scale diffusion, specific issues must be solved, in particular, the high cost of the cell components. In a direct methanol fuel cell system, high capital costs are mainly derived from the use of noble metal catalysts; therefore, the development of low-cost electro-catalysts, satisfying the target requirements of high performance and durability, represents an important challenge. The research is currently addressed to the development of metal–nitrogen–carbon (M–N–C) materials as cheap and sustainable catalysts for the oxygen reduction reaction (ORR) in an acid environment, for application in polymer electrolyte fuel cells fueled by hydrogen or alcohol. In particular, this mini-review summarizes the recent advancements achieved in DMFCs using M–N–C catalysts. The presented analysis is restricted to M–N–C catalysts mounted at the cathode of a DMFC or investigated in rotating disk electrode (RDE) configuration for the ORR in the presence of methanol in order to study alcohol tolerance. The main synthetic routes and characteristics of the catalysts are also presented.

Application of Low-Cost Me-N-C (Me = Fe or Co) Electrocatalysts Derived from EDTA in Direct Methanol Fuel Cells (DMFCs)

Co-N-C and Fe-N-C electrocatalysts have been prepared by mixing Fe or Co precursors, ethylene diamine tetra acetic acid (EDTA) as a nitrogen source, and an oxidized carbon. These materials were thermally treated at 800 °C or 1000 °C under nitrogen flow to produce four samples, named CoNC8, CoNC10, FeNC8, and FeNC10. They have been physicochemically characterized by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Direct methanol fuel cell (DMFC) analyses have been carried out to investigate the performance of the nonprecious cathode catalysts, using a low amount of Pt (0.7 mg/cm²) at the anode side. It appears that FeNC8 is the best performing low-cost cathode catalyst in terms of higher oxygen reduction reaction activity and methanol tolerance.

Carbon-Supported Pd and PdFe Alloy Catalysts for Direct Methanol Fuel Cell Cathodes

Direct methanol fuel cells (DMFCs) are electrochemical devices that efficiently produce electricity and are characterized by a large flexibility for portable applications and high energy density. Methanol crossover is one of the main obstacles for DMFC commercialization, forcing the search for highly electro-active and methanol tolerant cathodes. In the present work, carbon-supported Pd and PdFe catalysts were synthesized using a sodium borohydride reduction method and physico-chemically characterized using transmission electron microscopy (TEM) and X-ray techniques such as photoelectron spectroscopy (XPS), diffraction (XRD) and energy dispersive spectroscopy (EDX). The catalysts were investigated as DMFC cathodes operating at different methanol concentrations (up to 10 M) and temperatures (60 °C and 90 °C). The cell based on PdFe/C cathode presented the best performance, achieving a maximum power density of 37.5 mW·cm^-2 at 90 °C with 10 M methanol, higher than supported Pd and Pt commercial catalysts, demonstrating that Fe addition yields structural changes to Pd crystal lattice that reduce the crossover effects in DMFC operation.

Synthesis of 2D Nitrogen-Doped Mesoporous Carbon Catalyst for Oxygen Reduction Reaction

2D nitrogen-doped mesoporous carbon (NMC) is synthesized by using a mesoporous silica film as hard template, which is then investigated as a non-precious metal catalyst for the oxygen reduction reaction (ORR). The effect of the synthesis conditions on the silica template and carbon is extensively investigated. In this work, we employ dual templates—viz. graphene oxide and triblock copolymer F127—to control the textural features of a 2D silica film. The silica is then used as a template to direct the synthesis of a 2D nitrogen-doped mesoporous carbon. The resultant nitrogen-doped mesoporous carbon is characterized by transmission electron microscopy (TEM), nitrogen ad/desorption isotherms, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and rotating disk electrode measurements (RDE). The electrochemical test reveals that the obtained 2D-film carbon catalyst yields a highly electrochemically active surface area and superior electrocatalytic activity for the ORR compared to the 3D-particle. The superior activity can be firstly attributed to the difference in the specific surface area of the two catalysts. More importantly, the 2D-film morphology makes more active sites accessible to the reactive species, resulting in a much higher utilization efficiency and consequently better activity. Finally, it is noted that all the carbon catalysts exhibit a higher ORR activity than a commercial Pt catalyst, and are promising for use in fuel cells.