Performance and stability characteristics of MEAs with carbon-supported Pt and Pt1Ni1 nanoparticles as cathode catalysts in PEM fuel cell

Polarization curves of membrane electrode assemblies (MEAs) containing carbon-supported platinum (Pt/C) and platinum-nickel alloy (Pt₁Ni₁/C) as cathode catalysts were obtained for durability test as a function of time over 1100 h at constant current. Charge transfer resistance was measured using electrochemical impedance spectroscopy and postmortem analysis such as X-ray diffraction and high-resolution transmission electron microscopy was conducted in order to elucidate the degradation factors of each MEA. Our results demonstrate that the reduced performance of MEAs containing Pt₁Ni₁/C as a cathode catalyst was due to decreased oxygen reduction reaction caused by the corrosion of Ni, whereas that of MEAs containing Pt/C was because of reduced electrochemical surface area induced by increased Pt particle size.     

Comparative study between platinum supported on carbon and non-noble metal cathode catalyst in alkaline direct ethanol fuel cell (ADEFC)

In this study, we reported a comparison study between the performance of the commercial non-noble metal cathode electrode (made by Hypermec™ K14 catalyst - provided by Acta S.p.A.) and cathode electrode containing 10 wt% Pt/C, in the alkaline direct ethanol fuel cell (ADEFC) under different conditions.Further electrochemical investigations have been done by RDE and driven mode cell to compare the intrinsic activity and selectivity of 10 wt% Pt/C and Hypermec™ K14 transition metals cathode catalysts. It is worthwhile to point out that Hypermec™ K14 cathode catalyst shows a remarkable selectivity to oxygen reduction and it has a superior intrinsic activity in oxygen reduction reaction (ORR) especially in terms of volumetric current density (A/cm³). Test results of active DEFC made by non-noble cathode catalyst showed superior performance compared to the cell made by 10 wt% Pt/C cathode catalysts in terms of power density and OCV at 60 °C and ambient pressure. This result is related to the higher ORR kinetic of non-noble metal cathode catalyst in alkaline media.     

Development of nanophase CeO2-Pt/C cathode catalyst for direct methanol fuel cell

Incorporation of nanophase ceria (CeO₂) into the cathode catalyst Pt/C increased the local oxygen concentration in an air atmosphere, leading to enhanced single-cell performance of direct methanol fuel cell (DMFC). Ceria doped catalysts were effective at low oxygen partial pressure (≤0.6 atm) conditions and 1 wt.% CeO₂ doped Pt/C exhibited the highest performance. The effect of ceria was more prominent with air as the cathode reactant and the ceria acted as a mere impurity in a pure oxygen atmosphere, decreasing the DMFC performance. Impedance spectra showed a decrease in polarization resistance with the ceria addition to the cathode catalyst in low-potential regions confirming the facile mass transfer of the reactant oxygen molecules to catalytic sites. Transmission electron microscopy (TEM) pictures showed a uniform distribution of CeO₂ around platinum sites.     

Problems of corrosion and other electrochemical side processes in lithium chemical power sources with non-aqueous electrolytes

The following electrochemical side processes were studied: (i) electrochemical corrosion processes in a short-circuited couple of active cathode material (FeS₂)-current-collector material (ii) electrochemical and chemical decomposition of non-aqueous electrolytes proceeding in parallel with the base electrochemical reaction in power sources with a working discharge voltage of 1.5 V. The dynamics and direction of corrosion processes in the couple of FeS₂-current collector depend on the potential difference between the active cathode substance and the current-collector material and on the overvoltage value of conjugated electrochemical processes. In the case of a starting unreduced cathode, the reduction process takes place on pyrite and the oxidation process occurs on the current collector. After a partial cathode reduction the process direction changes. The rate of decomposition of the electrolyte in the potential range of 1.5 V is determined by its composition, the conditions of its, preparation and purification, and the cathode material used as catalyst in the process of the decomposition of the electrolyte.     

Degradation mechanism of electrocatalyst during long-term operation of PEMFC

Long-term operation of a polymer electrolyte membrane fuel cell (PEMFC) was carried out in constant-current (CC) and open-circuit-voltage (OCV) modes. The main factors causing electrocatalyst deactivation were found to be Pt sintering and dissolution. In Pt sintering, growth in particle size occurred mostly during the initial stage of operation (40 h). Pt dissolution occurred mostly at the cathode, rather than the anode, due to chemical oxidation of Pt to PtO by residual oxygen present in the cathode layer, resulting in a gradual decrease in cell performance during long-term operation. After the dissolution of PtO in water, Pt²⁺ was formed, which migrated from the cathode to the membrane phase, and was re-deposited as Pt crystal upon reduction by crossover hydrogen, as was confirmed by transmission electron microscopy (TEM) after long-term operation. Under normal operating conditions, there exists a balance at the cathode between chemical oxidation by oxygen and electrochemical reduction by input electrons. Therefore, Pt dissolution at the cathode is accelerated by an imbalance of these reactions under OCV conditions or by a high O₂ concentration in the feed.     

Electrochemical contribution of silver current collector to oxygen reduction reaction over Ba0.5Sr0.5Co0.8Fe0.2O3−δ electrode on oxygen-ionic conducting electrolyte

Nowadays, there is a doubt about the electrochemical contribution of silver current collector on the oxygen reduction reaction (ORR) over oxide electrodes in SOFCs since many reports have demonstrated that the modification of porous oxide electrodes with nano-size silver can obviously improve the electrocatalytic activity for ORR. In this study, the electrochemical contribution of silver current collector to the performance of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) electrode on Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte for ORR was specifically investigated. The active layer of BSCF electrode was found to be around 25 μm by using both silver and gold current collectors. Much better performance was demonstrated by using silver current collector, both from symmetric cell and single cell tests. However, EIS of silver on SDC electrolyte demonstrated the silver alone as electrode actually had poor performance for ORR. In addition, SEM-EDX confirmed that there was no silver diffused from the current collector layer to modify the porous BSCF electrode. Interestingly, the activation energy for oxygen reduction over BSCF electrode was reduced by applying silver current collector. We then proposed a mechanism to explain the improved electrochemical performance of BSCF electrode by considering the high activity of silver for oxygen surface diffusion.     

Comparative study of Pd and PdO as cathodes for oxygen reduction reaction in intermediate temperature solid oxide fuel cells

Metallic Pd and PdO electrodes were prepared by using Pd and PdCl₂ slurries, respectively, and their electrochemical performance as a cathode for oxygen reduction reaction in intermediate temperature solid oxide fuel cells was evaluated by electrochemical impedance spectroscopy (EIS) and direct current polarization (DC polarization). The electrochemical activity of metallic Pd was much higher than that of PdO for the reaction of oxygen reduction; below the decomposition temperature, a thin layer of PdO formed on the surface of metallic Pd electrode, which increased its polarization resistance. The decomposition temperature of PdO decreased from 810 to 750 °C as oxygen partial pressure decreased from 20 to 5 kPa, and was further lowered under the influence of the applied current during DC polarization test. The charge transfer resistance of PdO increased by decreasing oxygen partial pressure, while that of metallic Pd was less sensitive to it.     

Electrophoretic deposition of carbon-supported octahedral Pt–Ni alloy nanoparticle catalysts for cathode in polymer electrolyte membrane fuel cells

The advanced electrochemical catalytic activity for oxygen reduction reaction (ORR) based on the octahedral Pt–Ni alloyed catalyst has been demonstrated. However, a means of fabricating catalyst electrodes for use in PEMFCs that is cost-effective, scalable, and maintains the high activity of Pt–Ni_alloy/C has remained out of reach. Electrophoretic deposition (EPD) is a colloidal production process that has a history of successful deployment at the industrial scale. Here, we report on the facile preparation of an effective and active cathode consisting of Pt–Ni alloy loaded on the carbon cloth substrate using the electrophoretic deposition (EPD) technique, in which the optimum applied voltages and suspension pH are systematically investigated to obtain the highly porous Pt–Ni_alloy/C catalyst electrode. In a half cell test, the EPD-made Pt–Ni_alloy/C catalyst electrodes fabricated at 45 V and in a solution with a pH of 9.0 yields the best performances. On the other, as an active cathode, the EPD-made Pt–Ni_alloy/C electrodes deliver a superior performance in single cell test, with the maximum power density reaches 7.16 W/mg_Pt, ～28.1% higher than that of the spray-made Pt/C conventional electrode. The outperformance is attributed to the significantly higher porosity and surface roughness of the EPD-made electrode.     

Sputtered Pt loadings of membrane electrode assemblies in proton exchange membrane fuel cells

The fabrication of electrodes use in proton exchange membrane fuel cells (PEMFCs) by Pt sputter deposition has great potential to increase Pt utilization and reduce Pt loading without loss of cell performance. A radio frequency (RF) magnetron sputter deposition process (RF power = 100 W and argon pressure = 10^?3 Torr) was adopted to prepare Pt catalyst layers of PEMFC electrodes. The effects of cathode Pt and Nafion loadings on membrane electrode assembly (MEA)/cell performance were investigated using cell polarization, cyclic voltammetry, AC impedance, and microstructure analysis. Among the tested MEAs with various cathode Pt loadings (0.02–0.4 mg cm^?2), the one with 0.1 mg‐Pt cm^?2 (grain size = 3.90 nm, mainly Pt(111)) exhibited the best cell performance (320 and 285 mW cm^?2 at 0.44 and 0.60 V, respectively), which was similar to or better than those of some commercial nonsputtered/sputtered electrodes with the same or higher Pt loadings. The electrode Pt utilization efficiency increased as the Pt loading decreased. A Pt loading of greater than or lower than 0.1 mg cm^?2 yielded a lower electrode electrochemical active surface (EAS) area but a higher charge transfer and diffusion resistance. Nafion impregnation (0.1 to 0.3 mg cm^?2) into the sputtered Pt layer (Pt = 0.1 mg cm^?2) noticeably increased the EAS area, consistent with the decrease of the capacitance of the electrode double layer, but did not improve MEA/cell performance, mainly because of the increase in the kinetic and mass transfer resistances associated with oxygen reduction on the cathode. Copyright © 2011 John Wiley & Sons, Ltd.     

Performance and stability of Pd-Pt-Ni nanoalloy electrocatalysts in proton exchange membrane fuel cells

Pd-Pt-Ni nanoalloy catalysts have been synthesized by a polyol reduction method and characterized for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). The performance of the membrane-electrode assembly (MEA) fabricated with the Pd-Pt-Ni catalysts is found to increase continuously in the entire current density range with the operation time in the PEMFC until it becomes comparable to that of commercial Pt. The Pt-based mass activity of Pd-Pt-Ni exceeds that of commercial Pt by a factor of 2, and its long-term durability is comparable to that of commercial Pt within the 200 h of operation. Compositional characterizations by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) suggest a dealloyed active catalyst phase consisting of Pd-rich core and Pt-rich shell, formed by dissolution of Pd and Ni under the testing conditions. The surface catalytic activity of nanoparticles can be modified by the strain effect caused by lattice mismatch between the surface and core components. Transmission electron microscopy (TEM) observation of the MEA cross-section reveals that the Pd ions move into the Nafion membrane and even to the anode side and redeposit on reduction by hydrogen crossover. The deposition of Pd-rich PdPt particles mainly forms a band at the center of the membrane and along the cathode/membrane interface. On the other hand, the Ni ions ion-exchange with the protons in the Nafion membrane.     

Effect of uniformity and surface morphology of Pt nanoparticles to enhance oxygen reduction reaction in polymer electrolyte membrane fuel cells

Platinum (Pt)-based electrocatalysts supported by reduced graphene oxide (rGO) is fabricated under microwave-assisted polyol method with various nucleation and growth conditions. The surface morphologies of the Pt nanoparticles (NPs) under various reaction conditions owing to different Pt NP sizes and inter-particle spacings are investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, cyclic and linear sweep voltammetry, and electrochemical impedance spectroscopy. The synthesized Pt/rGO catalyst under nucleation and growth times of 10 s and 50 s, respectively, exhibits excellent catalytic activity with increased electrochemical surface area, high density, good uniformity and surface morphology with a particle size and inter-particle spacing of 2.16 nm and 17.2 nm, respectively. These results elucidate the relationship between the Pt NP morphology distribution and oxygen reduction reaction of catalysts in polymer electrolyte membrane fuel cell systems. We also highlight the important role of size and inter-particle spacing on the Pt electrochemical catalystic performance.     

Cobalt/nitrogen-Co-doped nanoscale hierarchically porous composites derived from octahedral metal-organic framework for efficient oxygen reduction in microbial fuel cells

Microbial fuel cell, a promising energy conversion technology, plays a crucial role in the field of renewable and sustainable energy. In an air-cathode microbial fuel cell, the oxygen reduction reaction catalytic activity of cathode catalyst is a critical factor that determines the performance of microbial fuel cell. This work reports a facile route for the synthesis of Co/N incorporated carbonaceous electrocatalyst using a Zr-based metal-organic framework UiO66-NH₂ as a template. This electrocatalyst exhibits outstanding activity and stability toward four-electron mechanism. In the microbial fuel cell application, Co/UiO66-900 shows superb electrochemical performance with a stable output voltage of 395 mV and maximum power density of 299.62 mW/m², which is 95.8% of the power density achieved in microbial fuel cell catalyzed by Pt/C catalyst (312.59 mW/m²). Co/UiO66-900 possesses high-performance catalytic activity because of its 3D-structured micropores, nitrogen-coordinated cobalt species and the synergistic effects between carbon and metal ion center. These unique properties can facilitate the oxygen reduction reaction by exposing abundant efficient active sites and accelerating mass transfer at oxygen reduction reaction interfaces. This work suggests that Co/UiO66-900 catalyst with superb electrocatalytic ORR activity is a promising alternative which can replace the expensive Pt/C in air-cathode MFC.     

A novel non-platinum group electrocatalyst for PEM fuel cell application

Precious-metal catalysts (predominantly Pt or Pt-based alloys supported on carbon) have traditionally been used to catalyze the electrode reactions in polymer electrolyte membrane (PEM) fuel cells. However as PEM fuel systems begin to approach commercial reality, there is an impending need to replace Pt with a lower cost alternative. The present study investigates the performance of a carbon-supported tantalum oxide material as a potential oxygen reduction reaction (ORR) catalyst for use on the cathode side of the PEM fuel cell membrane electrode assembly. Although bulk tantalum oxide tends to exhibit poor electrochemical performance due to limited electrical conductivity, it displays a high oxygen reduction potential; one that is comparable to Pt. Analysis of the Pourbaix electrochemical equilibrium database also indicates that tantalum oxide (Ta₂O₅) is chemically stable under the pH and applied potential conditions to which the cathode catalyst is typically exposed during stack operation. Nanoscale tantalum oxide catalysts were fabricated using two approaches, by reactive oxidation sputtering and by direct chemical synthesis, each carried out on a carbon support material. Nanoscale tantalum oxide particles measuring approximately 6 nm in size that were sputtered onto carbon paper exhibited a mass-specific current density as high as one-third that of Pt when measured at 0.6 V vs. NHE. However, because of the two-dimensional nature of this particle-on-paper structure, which limits the overall length of the triple-phase boundary junctions where the oxide, carbon paper, and aqueous electrolyte meet, the corresponding area-specific current density was quite low. The second synthesis approach yielded a more extended, three-dimensional structure via chemical deposition of nanoscale tantalum oxide particles on carbon powder. These catalysts exhibited a high ORR onset potential, comparable to that of Pt, and displayed a significant improvement in the area-specific current density. Overall, the highest mass-specific current density of the carbon-powder supported catalyst was ˜9% of that of Pt.     

Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction

Spinel-type nickel cobaltite with numerous oxygen vacancies is successfully synthesized by hydrothermal and thermal reduction using hydrogen. The effects of oxygen vacancies on the electrochemical activity and stability for the oxygen reduction reaction are investigated. The prepared catalyst displays significantly enhanced oxygen reduction reaction (ORR) catalytic performance under alkaline conditions, which is comparable to that of commercial Pt/C. The oxygen-deficient NiCo₂O₄ exhibits a very high limiting current density of −5.44 mA cm⁻² with onset and half-wave potentials of 0.93 and 0.78 V versus the reversible hydrogen electrode (RHE), respectively. Additionally, it shows excellent durability and resistance to methanol. The enhanced ORR activity and stability of the catalyst can be ascribed to the synergistic effects of the relatively large electrochemical surface area, more exposed active sites, and good electrical conductivity derived from abundant oxygen vacancies.     

Preparation and electrocatalytic activity of palladium-platinum core-shell nanoalloys protected by a perfluorinated sulfonic acid ionomer

Palladium-platinum nanoalloys with a core-shell and nano-network structure were successfully synthesized by a hydrogen sacrificial protective method in an aqueous solution directly using a perfluorinated sulfonic acid (PFSA) ionomer as a protecting agent. The structure, local composition and electrocatalytic activity for the oxygen reduction reaction of the Pd/Pt/PFSA nanoalloys were investigated by transmission electron microscopy (TEM), aberration corrected scanning transmission electron microscopy (Cs-STEM), energy-dispersive X-ray spectrometry (EDS) and voltammetry. The core-shell structure was completed without contaminating reducing agents, organic solvents, useless protecting agents and a mediator. The Pd/Pt/PFSA core-shell nanoalloys realized a high electrochemical surface area and better electrocatalytic mass-activity for the oxygen reduction reaction than the Pt/PFSA nanoparticles.     

KOH-activated multi-walled carbon nanotubes as platinum supports for oxygen reduction reaction

In the present investigation, multi-walled carbon nanotubes (MWCNTs) thermally treated by KOH were adopted as the platinum supporting material for the oxygen reduction reaction electrocatalysts. FTIR and Raman spectra were used to investigate the surface state of MWCNTs treated by KOH at different temperatures (700, 800, and 900 °C) and showed MWCNTs can be successfully functionalized. The structural properties of KOH-activated MWCNTs supported Pt were determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM), and their electrochemical performance was evaluated by the aid of cyclic voltammetry (CV) and rotating disk electrode (RDE) voltammetry. According to the experimental findings of the present work, the surrface of MWCNTs can be successfully functionalized with oxygen-containing groups after activation by KOH, favoring the good dispersion of Pt nanoparticles with narrow size distribution. The as-prepared Pt catalysts supported on KOH treated MWCNTs at higher temperature, possess higher electrochemical surface area and exhibit desirable activity towards oxygen reduction reaction (ORR). More precisely, it has been found that the electrochemical active area of Pt/MWCNTs-900 is approximately two times higher than that of Pt/MWCNTs. It can be concluded that KOH activation is an effective way to decorate MWCNTs’ surface with oxygen-containing groups and bigger surface area, which makes them more suitable as electrocatalyst support materials.     

Transient and steady-state analysis of catalyst poisoning and mixed potential formation in direct methanol fuel cells

The present dynamic model is developed to investigate the coupled reaction mechanisms in a DMFC and therein associated voltage losses in the catalyst layers. The model describes a complete five-layer membrane electrode assembly (MEA), with gas diffusion layers, catalyst layers and membrane. The analysis of the performance losses are mainly focused on the electrochemical processes. The model accounts for the crossover of both, methanol from anode to cathode and oxygen from cathode to anode. The reactant crossover results in parasitic internal currents that are finally responsible for high overpotentials in both electrodes, so-called mixed potentials. A simplified and general reaction mechanism for the methanol oxidation reaction (MOR) was selected, that accounts for the coverage of active sites by intermediate species occurring during the MOR. The simulation of the anode potential relaxation after current interruption shows an undershoot behavior like it was measured in the experiment [1]. The model gives an explanation of this phenomenon by the transients of reactant crossover in combination with the change of CO and OH coverages on Pt and Ru, respectively.     

Electrochemically reduced Pt oxide thin film as a highly active electrocatalyst for direct ethanol alkaline fuel cell

The Pt oxide thin film and Pt thin film were prepared by reactive sputtering and the electrocatalytic activity of the ethanol oxidation reaction was investigated in a KOH solution for developing the alkaline direct ethanol fuel cells. After electrochemical reduction by passing a cathodic electric charge, the Pt oxide thin film showed 29 times larger ethanol oxidation current than the Pt thin film. This superior activity was caused by an increase in the electrochemical active surface area and the existence of residual oxygen, which was confirmed by cyclic voltammetry and XPS measurement. Due to the contribution of the residual oxygen, the rate-determining step of the ethanol oxidation reaction might change, because the Tafel slope of the Pt oxide thin film during the ethanol oxidation reaction was changed by electrochemical reduction. Despite the total Pt amount in the Pt oxide thin film being smaller than that in the Pt thin film, the Pt oxide thin film showed excellent ethanol oxidation activity. Therefore, the Pt oxide treated by electrochemical reduction may be a promising anode catalyst for the direct ethanol fuel cells.     

Stabilizing lithium plating-stripping reaction between a lithium phosphorus oxynitride glass electrolyte and copper thin film by platinum insertion

Lithium (Li) plating-stripping reaction properties at the lithium phosphorus oxynitride glass electrolyte (LiPON)/copper thin film (Cu) interface is improved by the insertion of nano-thickness platinum (Pt) layer at the interface. The LiPON films are formed on mirror-polished lithium-ion conductive solid electrolyte sheets, and current collector thin films of Li, Cu-Pt multi layer, and Cu are formed on the LiPON films. The plating-stripping reactions at the LiPON/current collector films interface are carried out by galvanostatic and potential sweep measurements. Galvanostatic measurements reveal that Pt layer insertion reduces the overvoltage of the reaction and improves its coulomb efficiency. Also, cyclic voltammetry measurement suggests formation of Li-Pt alloys at higher voltages than 0 V (vs. Li/Li⁺) during the lithium plating process. Scanning electron microscopy observation clarifies that platinum insertion moderate non-uniform lithium plating reaction. Most probably, Li-Pt alloys increase the reaction sites, resulting in both the stabilization of current collector and the reduction of the overvoltage of the lithium plating-stripping reaction upon cycling. The results shown here will be useful in improving the anode reaction of the “Li-free” all-solid-state lithium batteries.     

Nitrogen-doped multi-walled carbon nanocoils as catalyst support for oxygen reduction reaction in proton exchange membrane fuel cell

Multi-walled carbon nanocoils (MWNCs) are synthesized by chemical vapour deposition and nitrogen-doped MWNCs (N-MWNCs) are obtained by nitrogen plasma treatment. MWNCs and N-MWNCs are used as catalyst supports for platinum nanoparticles. Pt nanoparticles are dispersed over these support materials using the conventional chemical reduction technique and then used for the oxygen reduction reaction in proton-exchange membrane fuel cells. The morphology and structure of the MWNC-based powder samples are studied by means of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. Full cells are constructed with Pt-loaded MWNC/N-MWNC and the results are discussed. A maximum power density of 550 and 490 mW cm⁻² is obtained with Pt/N-MWNC and Pt/MWNC as the ORR catalyst, respectively. The improved performance of a fuel cell with a N-MWNC catalyst support can be attributed to the creation of pyrrolic nitrogen defects due to the nitrogen plasma treatment. These defects act as good anchoring sites for the deposition of Pt nanoparticles and to the increased electrical conductivity and improved carbon-catalyst binding.