Kinetics and electrocatalytic activity of IrCo/C catalysts for oxygen reduction reaction in PEMFC

The purpose of this study is to develop a novel binary Iridium-Cobalt/C catalyst as a suitable substitute for platinum/C applied in proton exchange membrane fuel cells (PEMFCs). The carbon-supported IrCo catalysts were successfully synthesized using IrCl₃ and C₄H₆CoO₄ as the Ir and Co precursors respectively, in ethylene glycol (EG) refluxing at 120 °C. The nanostructured catalysts were characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscope (TEM). Homogeneous catalyst particles supported on carbon showed a size of proximately 2 nm. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were conducted for the characterization of the catalyst performances. With a cathodic loading of 0.4 mg_Ir cm⁻², 20%Ir-30%Co/C achieved a maximum power density of 501.6 mW cm⁻² at 0.418 V, with a 50 cm² H₂/O₂ single cell. Although such a performance is about 26% lower than commercial Pt/C catalyst, it is still helpful in terms of Pt replacement and cost reduction.     

Nanocrystaline tungsten carbide supported Au–Pd electrocatalyst for oxygen reduction

Au–Pd nanobimetallic particles supported on nanocrystaline tungsten carbide as electrocatalysts for oxygen reduction were prepared by an intermittent microwave heating (IMH) method. XRD measurement revealed that AuPd alloy formed during the IMH process. We showed these novel electrocatalysts could offer the activities that surpass that of the state-of-the-art Pt-based electrocatalysts for oxygen reduction reaction. The AuPd–WC/C electrode showed an over 70 mV shift towards more positive potentials compared to Pt/C electrode for ORR. The advantage seemed to come from the novel support of tungsten carbide which itself has the catalytic activity to enhance the catalytic activity of the metal electrocatalysts.     

Characterization of Pt supported on commercial fluorinated carbon as cathode catalysts for Polymer Electrolyte Membrane Fuel Cell

Pt nanoparticles supported on carbon monofluoride (CFx), synthesized from H₂PtCl₆ using NaHB₄ as a reducing agent has been investigated as a cathode electrocatalyst in fuel cells. Surface characterization, performed by transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD), shows a homogeneous distribution and high dispersion of metal particles. Kinetic parameters for the electrocatalyst are also obtained from the steady state measurements using a rotating disk electrode (RDE) in 0.5 M H₂SO₄ solution. Analysis by Koutecky–Levich equation indicates an overall 4 e^? oxygen reduction reaction (ORR). Evaluation of the catalyst in single cell membrane electrode assemblies (MEAs) for proton exchange membrane based Direct Methanol Fuel Cell (DMFC) and H₂ Fuel Cell at different temperatures and flows of O₂ and Air are shown and compared against commercial Pt/C as the cathode electrocatalyst. Evaluation of Pt/CFx in H₂ fed fuel cells shows a comparable performance against a commercial catalyst having a higher platinum loading. However, in direct methanol fuel cell cathodes, an improved performance is observed at low O₂ and air flows showing up to 60–70% increase in the peak power density at very low flows (60 mL min^?1).     

Pt/C doped by MoOx as the electrocatalyst for oxygen reduction and methanol oxidation

The oxidation of methanol and reduction of oxygen were studied on MoO_x–Pt/C nano-catalysts prepared by the polyole method combined by MoO_x post-deposition. The catalysts were characterized by TEM and EDX. The presented composition of the electrode is very similar to the nominal ones and post-deposited MoO_x species block only a small fraction of the active Pt particle surface area. MoO_x deposition on the carbon support can be ruled out from the EDX results and the low mobility of these oxides at corresponding conditions. The electrode catalytic activity in the electrooxidation of methanol and the reduction of oxygen was studied by steady-state voltammetry and cyclic voltammetry. MoO_x–Pt/C catalyst exhibits higher catalytic activity than Pt/C for the oxygen reduction. The catalytic effect in oxidation of methanol is achieved only under potentiodynamic conditions, when poisoning species have no enough time to develop fully.     

Carbon nanotubes supported Pt–Au catalysts for methanol-tolerant oxygen reduction reaction: A comparison between Pt/Au and PtAu nanoparticles

Two kinds of Pt–Au/CNT catalysts with different structures are prepared by synthesizing the formerly separated Pt/Au (Au separating Pt) in separate solutions, and PtAu nanoparticles in mixed solution using the borohydride reduction method with trisodium citrate as the stabilizing agent, and then depositing the metal colloid nanoparticles on the carbon nanotubes supporting material. The structural information and particle size are characterized by UV–vis absorption spectra, transmission electron microscopy (TEM), and X-ray diffraction (XRD). The results confirm the formation of Pt–Au nanoparticles with PtAu and separated Pt/Au structures. The catalytic activities for methanol oxidation reaction and oxygen reduction reaction are examined by electrochemical measurements. Compared with the Pt/CNT catalyst, the two Pt–Au/CNT catalysts show a lower overpotential for the oxygen reduction reaction in the presence of methanol, indicating a higher methanol tolerance on Au-modified Pt nanoparticles. Particularly, the Pt/Au/CNT catalyst with Au separating Pt nanoparticles structure exhibits the significantly higher methanol tolerance than PtAu/CNT catalyst. The enhanced methanol tolerance may be attributed to less methanol adsorption on Pt surface due to the effect of Au nanoparticles.     

A study on novel pulse preparation and electrocatalytic activities of Pt/C-Nafion electrodes for proton exchange membrane fuel cell

To aim at reducing the platinum loading and increasing the utilization of platinum in PEMFC electrode, a new pulse electrodeposition technique for preparing proton exchange membrane fuel cell (PEMFC) electrodes has been developed in this paper. This method combines coating Pt seeds on the C-Nafion substrate and introducing polyethylene glycol (PEG) into the deposition solution. SEM images of the samples show that Pt seeds and PEG take an important role in the morphology of the Pt deposit. The surface area and average particle size of Pt were determined by charge integration under the hydrogen desorption peaks of cyclic voltammetry. The electrocatalytic activities of these electrodes towards oxygen reduction reaction (ORR) were investigated by using rotating disc electrode (RDE). The Pt catalyst which was prepared by Pt seeds and PEG, its active surface area and electrocatalytic activity towards ORR were improved remarkably. And the optimized electrode displayed higher catalytic activity than a conventional electrode made from commercial Pt/C catalyst. The possible reasons for the effects of Pt seeds and PEG on the higher catalytic activity of prepared Pt catalysts have been preliminarily discussed.     

Evaluation of the catalytic activity of Pd-Ag alloys on ethanol oxidation and oxygen reduction reactions in alkaline medium

Pd-Ag alloys containing different amounts of Ag (8, 21 and 34 at.%) were prepared in order to evaluate their catalytic activity towards the ethanol oxidation (EOR) and oxygen reduction (ORR) reactions. A sequential electroless deposition of Ag and Pd on a stainless steel disc, followed by annealing at 650 °C under Ar stream, was used as the alloy electrode deposition process.From half-cell measurements in a 1.0 M NaOH electrolyte at ≅20 °C, it was found that alloying Pd with Ag leads to an increases of the ORR and EOR kinetics, relative to Pd. Among the alloys under study, the 21 at.% Ag content alloy presents the highest catalytic activity for the EOR and the lowest Ag content alloy (8 at.% Ag) shows the highest ORR activity. Moreover, it was found that the selectivity of Pd-Ag alloys towards ORR is sustained when ethanol is present in the electrolyte.     

Fabrication of a bimetallic Cu/Pt particle-modified carbon nanotube paste electrode and its use for the electrocatalytic oxidation of methanol

In this work, carbon nanotube paste electrode (CNTPE) was used as a substrate for deposition of bimetallic Cu/Pt particles. At first, a Cu film was prepared by electrochemical reduction of Cu ions onto the CNTPE in 0.1 M H₂SO₄ solution. Cu/Pt catalysts were prepared by partial galvanic replacement of Cu with Pt by simply immersion of the Cu-coated CNTPE in 2.0 mM H₂PtCl₆ solution. The nature and surface morphology of the bare CNTPE and fabricated Cu/Pt species were characterized by scanning electron microscopy and energy dispersive X-ray spectrometry. The Cu/Pt-modified CNTPE exhibits remarkable electrocatalytic activity towards methanol oxidation. It has been shown that carbon nanotubes improve the electrocatalytic activity of the catalysts towards oxidation. Then, the influence of various parameters such as Cu source concentration, electrodeposition time, replacement time, and methanol concentration on its oxidation as well as long-term stability of the modified electrode have been investigated by electrochemical methods.     

Readily fabricated NiCo alloy-metal oxide-carbon black hybrid catalysts for the oxygen reduction reactions in the alkaline media

Highly efficient, cost-effective and environmental-friendly electrocatalysts play a crucial role in oxygen reduction reaction (ORR) for fuel cells and metal-air batteries. Herein, a series of hybrids comprising of NiCo alloy, metal oxides and carbon black were readily prepared by a one-pot pyrolysis approach and employed as efficient ORR electrocatalysts in the alkaline media. Different amounts of Ketjen Black EC 300J (EC) with a large mesoporous area and exceptional electrical conductivity were directly added to synthesize the hybrids. Among the hybrids tested, the NC-MMO-EC-3 (where NC stands for NiCo alloy and MMO for mixed metal oxides) with an appropriate amount of EC displayed the best ORR electrochemical activity. The enhanced activity of the NC-MMO-EC-3 could be attributed to the conductivity improved by EC, the high dispersion of MMO and NC on EC support, and the beneficial interaction among those three components.     

Construction of carboxyl functional groups and their enhancement effect for methanol electrocatalytic oxidation reaction

In this paper, we investigated the effect of ozone oxidation on properties of commercial carbon black supported platinum (Pt) nanoparticles for the methanol electro-oxidation reaction. The results indicated that the oxygenated functional groups could be introduced on the carbon black evenly with the increase of processing time. Apparently, mainly introduced oxygenated functional group is carboxyl. Platinum nanoparticles could be uniformly immobilized on the surfaces of carbon black treated with ozone, which has significant high electro-catalytic activity and stability for methanol electrooxidation. This phenomenon is attributed to the fact that oxygen-containing groups (mainly for carboxyl functional groups) produced by ozone oxidation are good for improving the dispersion and strengthening the interaction between support and platinum nanoparticles. The ozone oxidation conditions had significant effects on the defects properties of carbon black which showed a positive correlation between the defect levels and methanol electro-oxidation performances. This paper also fully demonstrated the positive relationship between carboxyl functional groups and the performance of methanol electrocatalytic oxidation.     

Simple preparation of Pd–Pt nanoalloy catalysts for methanol-tolerant oxygen reduction

Carbon-supported Pd–Pt bimetallic nanoparticles of different atomic ratios (Pd–Pt/C) have been prepared by a simple procedure involving the complexing of Pd and Pt species with sodium citrate followed by ethylene glycol reduction. As-prepared Pd–Pt alloy nanoparticles evidence a single-phase fcc disordered structure, and the degree of alloying is found to increase with Pd content. Both X-ray diffraction and transmission electron microscopy characterizations indicate that all the Pd–Pt/C catalysts possess a similar mean particle size of ca. 2.8 nm. The highest mass and specific activity of the oxygen reduction reaction (ORR) using the Pd–Pt/C catalysts are found with a Pd:Pt atomic ratio of 1:2. Moreover, all Pd–Pt alloy catalysts exhibit significantly enhanced methanol tolerance during the ORR than the Pt/C catalyst, ensuring a higher ORR performance while diminishing Pt utilization.     

The effect of heteroatoms doping for the Pt supported graphene hollow spheres on electrocatalytic properties towards oxygen reduction and hydrogen evolution reaction

Noble metal Pt is the acknowledged efficient catalyst for oxygen reduction (ORR) and hydrogen evolution reaction (HER) in commercial applications. However, due to its high price and limited reserves, its large-scale application is limited. In order to overcome this defect, the loaded Pt nanoparticles (NPs) should be small and dispersed efficiently through the design of electrode materials, so as to improve the utilization efficiency of Pt. In addition, the introduction of non-noble metal active sites can reduce the consumption of Pt efficiently. In this work, hollow graphene spheres are used as the carrier and the heteroatoms (N, Fe and Co) are introduced. The results show that the introduction of Fe and Co can form very effective heteroatom active sites (carbon encapsulated Fe/Co metals and FeCo alloy, and/or metal nitrides Fe/Co-N_x-C) in the substrate material, which improve the catalytic activity of the electrode material effectively and the utilization efficiency of Pt. In addition, the generation of Fe/Co-N_x-C active sites and the loading of Pt are also closely related to the doped N atoms. The onset potential, limiting current density (J_L), half-wave potential (E_1/2) and Tafel slope of sample FeCo-N_xHGSs/Pt (10 wt%) can exceed or comparable to those of commercial catalysts Pt/C (20 wt%) towards ORR both in acid and alkaline electrolyte. Moreover, the values of η₁₀₀ and the Tafel slope for FeCo-N_xHGSs/Pt towards HER can also exceed the commercial catalysts Pt/C (20 wt%) in acid and alkaline electrolytes. The purpose of reducing the usage amount of precious metals without reducing the catalytic performance is realized. The relationship between the ORR and HER performance of the resultant electrode catalyst and the doped heteroatoms, such as nitrogen (N), iron (Fe) and cobalt (Co) atoms, was studied and discussed in detail.     

PtFe alloy catalyst supported on porous carbon nanofiber with high activity and durability for oxygen reduction reaction

To reduce the high cost of oxygen reduction reaction (ORR) catalyst and improve the performance of the proton exchange membrane fuel cell (PEMFC), low-Pt or non-Pt catalysts have been studied in recent years. In this paper, PtFe alloy nanoparticles are loaded on porous carbon nanofiber (PCNF) via one-step modified glycol reduction method by adjusting solution pH. On the surface of PCNF, PtFe alloy nanoparticle can be uniformly dispersed with a narrow particle size distribution. The catalyst Pt_4.8Fe/PCNF prepared in pH = 7 solution with PCNF as carbon support exhibits better ORR performance, which shows even 18 mV higher onset potential than that of commercial catalyst Pt/C (Johnson Matthey, JM20). Moreover, comparable durability is also obtained through accelerated durability test (ADT) test after 2000 cycles. The excellent performance of Pt_4.8Fe/PCNF catalyst may attribute to the structural and electronic effects of transition metal in the PtFe alloy. The rough surface and porous structure of PCNF is also supposed to be beneficial for performance improvement.     

Preparation of Pt/NiO-C electrocatalyst and heat-treatment effect on its electrocatalytic performance for methanol oxidation

Pt catalyst supported on Vulcan XC-72R containing 5 wt% NiO (Pt/NiO–C) showed larger electrochemical active surface area and higher electrochemical activity for methanol oxidation than Pt catalyst supported on Vulcan XC-72R using polyol method without NiO addition. Prepared Pt/NiO–C electrocatalyst was heat-treated at four temperatures (200, 400, 600, and 800 °C) in flowing N₂. X-ray diffraction and temperature-programmed desorption results indicated that NiO was reduced to Ni in inert N₂ during heat-treatments at temperatures above or equal to 400 °C, while oxygen from NiO reacted with carbon support due to the catalytic effect of Pt. The reduced Ni formed an alloy with Pt, which, according to the X-ray photoelectron spectroscopy data, resulted in a shift to a lower binding energy of Pt 4f electrons. The Pt/NiO–C electrocatalyst heat-treated at 400 °C showed the best activity in methanol oxidation due to the change in Pt electronic structure by Ni and the minimal aggregation of Pt particles.     

Synthesis and characterization of Pt nanoparticles on sulfur-modified carbon nanotubes for methanol oxidation

Pt nanoparticles supported on carbon nanotubes (Pt/CNTs) have been synthesized from sulfur-modified CNTs impregnated with H₂PtCl₆ as Pt precursor. The dispersion and size of Pt nanoparticles in the synthesized Pt/CNT nanocomposites are remarkably affected by the amount of sulfur modifier (S/CNT ratio). The results of X-ray diffraction and transmission electron microscopy indicate that an S/CNT ratio of 0.3 affords well dispersed Pt nanoparticles on CNTs with an average particle size of less than 3 nm and a narrow size distribution. Among different catalysts, the Pt/CNT nanocomposite synthesized at S/CNT ratio of 0.3 showed highest electrochemically active surface area (88.4 m² g⁻¹) and highest catalytic activity for methanol oxidation reaction. The mass-normalized methanol oxidation peak current observed for this catalyst (862.8 A g⁻¹) was ∼ 6.5 folds of that for Pt deposited on pristine CNTs (133.2 A g⁻¹) and ∼ 2.3 folds of a commercial Pt/C (381.2 A g⁻¹). The results clearly demonstrate the effectiveness of a relatively simple route for preparation of sulfur-modified CNTs as a precursor for the synthesis of Pt/CNTs, without the need for tedious pretreatment procedures to modify CNTs or complex equipments to achieve high dispersion of Pt nanoparticles on the support.     

Melamine-assisted synthesis of paper mill sludge-based carbon nanotube/nanoporous carbon nanocomposite for enhanced electrocatalytic oxygen reduction activity

Developing efficient and cheap electrocatalysts as substitutes for commercial Pt/C in the oxygen reduction reaction(ORR)is extremely necessary. Herein, paper mill sludge (PMS) was utilized to produce iron, nitrogen and sulfur co-doped carbon nanotube/nanoporous carbon nanocomposite (PMS-CNT/C) by pyrolysis. PMS-CNT/C-b, one of as-prepared PMS-CNT/C exhibited excellent oxygen reduction reaction activity with an onset potential of 0.99 V vs. RHE and half-wave potential of 0.77 V vs. RHE, which was similar to the commercial Pt/C catalyst (onset potential of 0.99 V vs. RHE and half-wave potential of 0.76 V vs. RHE). It had longer-term stability and higher methanol tolerance in alkaline medium than Pt/C. Moreover, the new catalyst also exhibited excellent catalytic performance in neutral solution. The energy output of microbial fuel cells loaded with PMS-CNT/C-b catalyst was also higher than that of commercial Pt/C under neutral condition. The excellent ORR performance of PMS-CNT/C-b was due to the carbon nanotube/nanoporous structure and the synergistic effect of abundant N groups, iron nitrides and thiophene-S. The formation of CNTs in the carbon nanotube/nanoporous carbon nanocomposite was mainly attributed to melamine, which was added into PMS and was at first just considered as a nitrogen source to develop N-doped PMS-based catalysis in this work. The synthesis of paper mill sludge-based carbon nanotube/nanoporous nanocomposite and its excellent ORR activity will make the new catalyst a promising cathodic electrocatalyst alternative for fuel cells.     

Highly stable and methanol tolerant oxygen reduction reaction electrocatalyst Co/CoO/SnO@N-C nanocubes by one-step introduction of functional components

To develop high-performance non-precious metal-based electrocatalysts for oxygen reduction reaction (ORR) is an urgent demand. We herein report an ingenious strategy to develop highly selective and competitive precious metal-free ORR electrocatalyst Co/CoO/SnO encapsulated in N-rich mesoporous carbon (CCS@NPC) nanocubes, via a simple one-step introduction of all functional components. Atomically mixed composites with significantly enhanced catalytic activity were obtained by adopting amorphous mesoporous ternary metal oxides CoSnO₃ nanocubes as precursor and template. The synergistic effect of the triphase nanohybrids (Co, CoO & SnO) anchored in nitrogen-doped mesoporous carbon nanocubes promoted the catalytic reactions, owning to the anisotropic morphology, great heterojunction interfaces, and structural stability. The large-scale prepared CCS@NPC exhibited excellent electrocatalytic activity toward ORR with an admirable onset potential (1.01 V) and diffusion-limited current density (5.88 mA cm^?2). The CCS@NPC showed stronger stability (20 h) and higher methanol tolerance with the concentration up to 8.0 M, compared to that of commercial Pt/C.     

Electrochemical activity and stability of dealloyed Pt–Cu and Pt–Cu–Co electrocatalysts for the oxygen reduction reaction (ORR)

A comparative study of the electrochemical stability of Pt₂₅Cu₇₅ and Pt₂₀Cu₂₀Co₆₀ alloy nanoparticle electrocatalysts in liquid electrolyte half-cell environment was conducted. The aforementioned catalysts were shown to possess improved resistance to electrochemical surface area (ECSA) loss during voltage cycling relative to commercially available pure Pt electrocatalysts. The difference in ECSA loss was attributed to their initial mean particle size, which varied depending on the temperature at which the alloy catalysts were prepared (e.g. 600, 800 and 950 °C). Higher preparation temperatures resulted in larger particles and lead to lower ECSA loss. Liquid electrolyte environment short-term durability testing (5000 voltages cycles) revealed the addition of cobalt to be beneficial as ternary compositions exhibited stability advantages over binary catalysts.     

Nitrogen and sulfur doped ZnAl layered double hydroxide/reduced graphene oxide as an efficient nanoelectrocatalyst for oxygen reduction reactions

In this work, to synthesize an efficient and low-cost electrocatalysts for Oxygen Reduction Reaction (ORR), the combination of N,S-rGO and ZnAl-LDH with several concentrations is studied for the first time. For this purpose, six electrocatalysts including Graphene Oxide (GO), functionalized reduced graphene oxide with nitrogen and sulfur atoms (N,S–rGO), Zinc–Aluminum layered double hydroxides (ZnAl-LDH), and ZnAl-LDH/N,S–rGO hybrids in three weight ratios of 1:1, 1:3, and 1:5 (the weight ratio of N,S-rGO is 1) are synthesized by the hydrothermal method. The physical properties, morphology, and structure of the synthesized electrocatalysts are determined by using X-Ray Diffraction (XRD) analysis, Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive X-Ray Analysis (EDX), the Fourier Transform Infrared Spectroscopy (FTIR) and Raman analysis. Electrochemical measurements are implemented by using Cyclic Voltammetry (CV), Linear Scanning Voltammetry (LSV), and chronoamperometric. Also, the electron transfer number is calculated by K-L plot. The obtained results for all samples are compared with the %20 Pt/C commercial catalyst. Based on the results of the physical tests, in addition to the uniform distribution and the correct deposition of the synthesized electrocatalysts, the particle size also reached the nanometer range. According to the electrochemical results, among the synthesized electrocatalysts, the ZnAl-LDH/N,S–rGO with 1:1 wt ratio has the best electrochemical activity. This result indicates a well synergistic and interaction effect between N,S–rGO and ZnAl-LDH for the ORR. The onset potential is obtained to be −0.01 V vs Ag/AgCl. The average of electron transfer number by this electrocatalyst is 3.60, which indicates that it is close to the 4e pathway for the ORR. The electrocatalytic stability was favorable in the alkaline medium. It can be concluded that the Layered Double Hydroxides (LDHs) improve the electrical conductivity, the electrocatalytic activity, the active surface area, and the stability for the oxygen reduction reaction after the combination with carbon bases. To be clear, the combination of N,S-rGO and ZnAl-LDH with several concentrations has been investigated for the first time on the ORR applications. The sensitivity analysis is implemented to determine the optimal concentration. This study proposes a new approach for using N, S-rGO composite to improve the low electron conductivity of LDHs.