The inhibition of electrochemical carbon corrosion in polymer electrolyte membrane fuel cells using iridium nanodendrites

The addition of Ir-based water electrolysis catalysts to the catalyst layer in polymer electrolyte membrane fuel cells was examined as a promising approach for preventing electrochemical carbon corrosion under severely corrosive conditions. Electrochemical carbon corrosion of membrane electrode assemblies containing different amounts of IrO₂ or shape-controlled Ir dendrite catalysts were characterized using on-line mass spectrometry. In particular, Ir dendrite catalysts possess high activity toward oxygen evolution reactions when compared to IrO₂. As a result, Ir dendrites provided a very effective method of removing water from the catalyst layer. Therefore, the addition of 1 wt% Ir dendrite (0.008 mg cm⁻²) to the catalyst layer of the cathode decreased electrochemical carbon corrosion by 84% at 1.6 V_NHE compared with a conventional membrane electrode assembly in the absence of water electrolysis catalysts.     

Stable support based on highly graphitic carbon xerogel for proton exchange membrane fuel cells

Highly graphitic carbon xerogel (GCX) is prepared by the modified sol-gel polymerization process using cobalt nitrate as the catalyst, followed by high temperature treatment at 1800 °C. The as-prepared GCX is explored as a stable support for Pt in proton exchange membrane fuel cells. The results of N₂ sorption measurement and X-ray diffraction analysis (XRD) reveal that GCX has a better mesoporous structure and a preferably higher degree of graphitization, compared with the commercial XC-72 carbon black. The transmission electron microscopy (TEM) image indicates that Pt nanoparticles are well dispersed on GCX and exhibit relatively narrow size distribution. Accelerated aging test (AAT) based on potential cycling is used to investigate the durability of the as-prepared Pt/GCX in comparison with the commercial Pt/C. Electrochemical analysis demonstrates that the catalyst with GCX as a support exhibits an alleviated degradation rate of electrochemical active surface area (39% for Pt/GCX and 53% for Pt/C). The results of single cell durability tests indicate that the voltage loss of Pt/GCX at 100 mA cm⁻² is about 50% lower than that of Pt/C. GCX is expected to be a corrosion resistant electrocatalyst support.     

Platinum-sputtered electrode based on blend of carbon nanotubes and carbon black for polymer electrolyte fuel cell

A novel Pt-sputtered electrode based on a blend layer of carbon black (CB) and carbon nanotubes (CNTs) is developed for polymer electrolyte fuel cells. The Pt is sputtered on the surface of the blend to form a catalyst layer. The CNTs generate a pore in the blend layer, and the CB provides a high surface roughness for the blend layer. At a CNT content of 50 wt.%, the maximum value (20.6 m² g⁻¹) for the electrochemical area of the Pt is obtained, which indicates that the surface area of the blend layer exposed for Pt deposition is the largest. The power density of a membrane-electrode assembly (MEA) employing the Pt-sputtered electrodes shows a linear increase with electrochemical area. The mass activity of the optimized Pt-sputtered electrode with a Pt loading of 0.05 mg cm⁻² is 8.1 times that of an electrode with a Pt loading of 0.5 mg cm⁻² prepared using a conventional screen-printing technique. Excellent mass transfer is obtained with the Pt-sputtered electrode.     

Performance increase of microfluidic formic acid fuel cell using Pd/MWCNTs as catalyst

This paper shows that the combination of an O₂ saturated acidic fluid setup (O₂-setup) and a composite of Pd nanoparticles supported on multiwalled-carbon nanotubes (Pd/MWCNTs) as anode catalyst material, results in the improvement of microfluidic fuel cell performance. Microfluidic fuel cells were constructed and evaluated at low HCOOH concentrations (0.1 and 0.5 M) using Pd/V XC-72 and Pd/MWCNTs as anode and Pt/V XC-72 as cathode electrode materials, respectively. The results show a higher power density (2.9 mW cm⁻²) for this cell when compared to the value reported in the literature that considers a commercial Pd/V XC-72 and 3.3 mW cm⁻² using a Pd/MWCNTs with a 50% less Pd loading than that commercial Pd/V XC-72.     

Electrochemical catalytic activities of nanoporous palladium rods for methanol electro-oxidation

A novel electrocatalyst, nanoporous palladium (npPd) rods can be facilely fabricated by dealloying a binary Al₈₀Pd₂₀ alloy in a 5 wt.% HCl aqueous solution under free corrosion conditions. The microstructure of these nanoporous palladium rods has been characterized using scanning electron microscopy and transmission electron microscopy. The results show that each Pd rod is several microns in length and several hundred nanometers in diameter. Moreover, all the rods exhibit a typical three-dimensional bicontinuous interpenetrating ligament-channel structure with length scale of 15-20 nm. The electrochemical experiments demonstrate that these peculiar nanoporous palladium rods (mixed with Vulcan XC-72 carbon powders to form a npPd/C catalyst) reveal a superior electrocatalytic performance toward methanol oxidation in the alkaline media. In addition, the electrocatalytic activity obviously depends on the metal loading on the electrode and will reach to the highest level (223.52 mA mg⁻¹) when applying 0.4 mg cm⁻² metal loading on the electrode. Moreover, a competing adsorption mechanism should exist when performing methanol oxidation on the surface of npPd rods, and the electro-oxidation reaction is a diffusion-controlled electrochemical process. Due to the advantages of simplicity and high efficiency in the mass production, the npPd rods can act as a promising candidate for the anode catalyst for direct methanol fuel cells (DMFCs).     

Electrochemical double layer capacitor and lithium-ion capacitor based on carbon black

In this paper we report the physical investigation and the electrochemical performance of the carbon black SC3 from Cabot Corporation. The SC3 carbon black was investigated in terms of BET surface area, pore size distribution, resistivity and morphology. Composite electrodes containing SC3 as active material were prepared and used for the realization of electrochemical double layer capacitor (EDLC) and lithium-ion capacitor (LIC). In EDLC, at 5 mA cm⁻² charge-discharge currents, the carbon black displays a specific capacity of 40 mAh g⁻¹ and a specific capacitance of 115 F g⁻¹. It also displays a very good cycling stability for over 50,000 cycles and excellent performance retention at currents up to 50 mA cm⁻². The performance retention at high currents outstandingly differentiates this carbon black from a few commercially available EDLC-grade activated carbons. Because of the high specific capacity of SC3, the carbon black electrodes were also used in combination with LiFePO₄ electrodes in LIC. The results of this study indicate that SC3 carbon black is an interesting carbonaceous candidate for the realization of LIC.     

V2O3 modified LiFePO4/C composite with improved electrochemical performance

In addition to lattice doping and carbon-coating, surface modification with other metal oxides can also improve the electrochemical performance of LiFePO₄ powders. In this work, highly conductive vanadium oxide (V₂O₃) is in situ produced during the synthesis of carbon-coated LiFePO₄ (LiFePO₄/C) powders by a solid state reaction process and acts as a surface modifier. The structures and compositions of LiFePO₄/C samples containing 0-10 mol% vanadium are analyzed by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. Their electrochemical properties are also characterized with galvanostatic cell cycling and cyclic voltammetry. It is found that vanadium is present in the form of V₂O₃ that is incorporated in the carbon phase. The vanadium-modified LiFePO₄/C samples show improved rate capability and low-temperature performance. Their apparent lithium diffusion coefficient is in the range of 10⁻¹² to 10⁻¹⁰ cm² s⁻¹ depending on the vanadium content. Among the investigated samples, the one with 5 mol% vanadium exhibits the best electrochemical performance.     

Pt/carbon nanofibers electrocatalysts for fuel cells: Effect of the support oxidizing treatment

Different Pt-based electrocatalysts supported on carbon nanofibers and carbon black (Vulcan XC-72R) have been prepared using a polymer-mediated synthesis. The electrocatalysts have been characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and cyclic voltammetry. The effect of carbon nanofibers treatment with HNO₃ solution on Pt particle size and electroactive area has been analyzed. Highly dispersed Pt with homogeneous particle size and an electroactive area around of 100 m² g⁻¹ is obtained in raw carbon nanofibers. The oxidizing treatment of the carbon nanofibers produces agglomeration of the platinum nanoparticles and an electroactive area of 53 m² g⁻¹. Durability studies indicate a decrease of 14% in the electroactive area after 90 h at 1.2 V in 0.5 M H₂SO₄ for platinum supported on raw carbon nanofibers and Vulcan XC-72R. The electrocatalyst supported on oxidized carbon nanofibers are stable under similar conditions.     

Structure of carbon-supported Pt-Ru nanoparticles and their electrocatalytic behavior for hydrogen oxidation reaction

The electrochemical activity towards hydrogen oxidation reaction (HOR) of a high performance carbon-supported Pt-Ru electrocatalyst (HP 20 wt.% 1:1 Pt-Ru alloy on Vulcan XC-72 carbon black) has been studied using the thin-film rotating disk electrode (RDE) technique. The physical properties of the Pt-Ru nanoparticles in the electrocatalyst were previously determined by transmission electron microscopy (TEM), high resolution TEM, fast Fourier transform (FFT), electron diffraction and X-ray diffraction (XRD). The corresponding compositional and size-shape analyses indicated that nanoparticles generally presented a 3D cubo-octahedral morphology with about 26 at.% Ru in the lattice positions of the face-centred cubic structure of Pt. The kinetics for HOR was studied in a hydrogen-saturated 0.5 M H₂SO₄ solution using thin-film electrodes prepared by depositing an ink of the electrocatalyst with different Nafion contents in a one-step process on a glassy carbon electrode. A maximum electrochemically active surface area (ECSA) of 119 m² g Pt⁻¹ was found for an optimum Nafion composition of the film of about 35 wt.%. The kinetic current density in the absence of mass transfer effects was 21 mA cm⁻². A Tafel slope of 26 mV dec⁻¹, independent of the rotation rate and Nafion content, was always obtained, evidencing that HOR behaves reversibly. The exchange current density referred to the ECSA of the Pt-Ru nanoparticles was 0.17 mA cm⁻², a similar value to that previously found for analogous inks containing pure Pt nanoparticles.     

Preparation and properties of carbon nanotube-reinforced vinyl ester/nanocomposite bipolar plates for polymer electrolyte membrane fuel cells

Novel multiwalled carbon nanotubes (MWNTs) were prepared using poly(oxypropylene)-backboned diamines of molecular weights M_w 400 and 2000 to disperse acid-treated MWNTs, improving the performance of composite bipolar plates in polymer electrolyte membrane fuel cells. A lightweight polymer composite bipolar plate that contained vinyl ester resin, graphite powder and MWNTs was fabricated using a bulk molding compound (BMC) process. Results demonstrate that the qualitative dispersion of MWNTs crucially determined the resultant bulk electrical conductivity, the mechanical properties and the physical properties of bipolar plates. The flexural strength of the composite bipolar plate with 1 phr of MWNTs was approximately 48% higher than that of the original composite bipolar plate. The coefficient of thermal expansion of the composite bipolar plate was reduced from 37.00 to 20.40 μm m⁻¹ °C⁻¹ by adding 1 phr of MWNTs, suggesting that the composite bipolar plate has excellent thermal stability. The porosity of the composite bipolar plate was also evaluated. Additionally, the bulk electrical conductivity of the composite bipolar plate with different MWNTs types and contents exceeds 100 S cm⁻¹. The results of the polarization curves confirm that the addition of MWNTs leads to a significant improvement on the single cell performance.     

Performance of supported Au–Co alloy as the anode catalyst of direct borohydride-hydrogen peroxide fuel cell

Au–Co alloys supported on Vulcan XC-72R carbon were prepared by the reverse microemulsion method and used as the anode electrocatalyst for direct borohydride-hydrogen peroxide fuel cell (DBHFC). The physical and electrochemical properties were investigated by energy dispersive X-ray (EDX), X-ray diffraction (XRD), cyclic voltammetry, chronamperometry and chronopotentiometry. The results show that supported Au–Co alloys catalysts have higher catalytic activity for the direct oxidation of BH₄⁻ than pure nanosized Au catalyst, especially the Au₄₅Co₅₅/C catalyst presents the highest catalytic activity among all as-prepared Au–Co alloys, and the DBHFC using the Au₄₅Co₅₅/C as anode electrocatalyst shows as high as 66.5 mW cm⁻² power density at a discharge current density of 85 mA cm⁻² at 25 °C.     

Preparation and activity of carbon-supported porous platinum as electrocatalyst for methanol oxidation

In this paper, we reported a novel electrocatalyst, Vulcan XC-72-supported porous platinum nano-particles (Pt_p/C) for methanol oxidation. In the preparation of Pt_p/C, platinum precursor was first adsorbed on carbon and then reduced by l-ascorbic acid in ethylene glycol solution. The structure and morphology of Pt_p/C and its activity toward methanol oxidation were characterized by transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) measurement, X-ray diffraction (XRD), energy-dispersion spectrometer (EDS), cyclic voltammetry (CV), and chronoamperometry (CA), with a comparison of the electrocatalyst prepared with sodium borohydride as reducer (Pt_s/C). It is found that both electrocatalysts have similar particle size but have different surface morphology of platinum and thus exhibit different electrocatalytic activity toward methanol oxidation. The platinum particle size of both electrocatalysts is 3–5 nm, but the corresponding BET surface areas are different significantly, 131.6 m² g⁻¹ and 87.7 m² g⁻¹ for Pt_p/C and Pt_s/C, respectively, indicative of the porous structure of platinum particles in Pt_p/C. The peak current for methanol oxidation on CV is 167 mA mg⁻¹ and 44 mA mg⁻¹ for Pt_p/C and Pt_s/C, respectively, indicative of the high electrocataytic activity of Pt_p/C toward methanol oxidation. The result from CA shows that Pt_p/C has good stability as the electrocatalyst for methanol oxidation.     

Development of high performance carbon composite catalyst for oxygen reduction reaction in PEM Proton Exchange Membrane fuel cells

Highly active and stable carbon composite catalysts for oxygen reduction in PEM fuel cells were developed through the high-temperature pyrolysis of Co–Fe–N chelate complex, followed by the chemical post-treatment. A metal-free carbon catalyst was used as the support. The carbon composite catalyst showed an onset potential for oxygen reduction as high as 0.87 V (NHE) in H₂SO₄ solution, and generated less than 1% H₂O₂. The PEM fuel cell exhibited a current density as high as 0.27 A cm⁻² at 0.6 V and 2.3 A cm⁻² at 0.2 V for a catalyst loading of 6.0 mg cm⁻². No significant performance degradation was observed over 480 h of continuous fuel cell operation with 2 mg cm⁻² catalyst under a load of 200 mA cm⁻² as evidenced by a resulting cell voltage of 0.32 V with a voltage decay rate of 80 μV h⁻¹. Materials characterization studies indicated that the metal–nitrogen chelate complexes decompose at high pyrolysis temperatures above 800 °C, resulting in the formation of the metallic species. During the pyrolysis, the transition metals facilitate the incorporation of pyridinic and graphitic nitrogen groups into the carbon matrix, and the carbon surface doped with nitrogen groups is catalytically active for oxygen reduction.     

Textural and electrochemical characterization of porous carbon nanofibers as electrodes for supercapacitors

Porous carbon nanofibers (CNFs) enriched with the graphitic structure were synthesized by thermal decomposition from a mixture containing polyethylene glycol and nickel chloride (catalyst). The textural and electrochemical properties of porous CNFs were systematically compared with those of commercially available multi-walled carbon nanotubes (MWCNTs). The high ratio of mesopores and large amount of open edges of porous CNFs with a higher specific surface area, very different from that of MWCNTs, are favorable for the penetration of electrolytes meanwhile the graphene layers of porous CNFs serve as a good electronic conductive medium of electrons. The electrochemical properties of porous CNFs and MWCNTs were characterized for the application of supercapacitors using cyclic voltammetry, galvanostatic charge–discharge method, and electrochemical impedance spectroscopic analyses. The porous CNFs show better capacitive performances (C_S = 98.4 F g⁻¹ at 25 mV s⁻¹ and an onset frequency of behaving as a capacitor at 1.31 kHz) than that of MWCNTs (C_S = 17.8 F g⁻¹ and an onset frequency at 1.01 kHz). This work demonstrates the promising capacitive properties of porous CNFs for the application of electrochemical supercapacitors.     

Electrochemical durability of gas diffusion layer under simulated proton exchange membrane fuel cell conditions

An effective ex-situ method for characterizing electrochemical durability of a gas diffusion layer (GDL) under simulated polymer electrolyte membrane fuel cell (PEMFC) conditions is reported in this article. Electrochemical oxidation of the GDLs are studied following potentiostatic treatments up to 96 h holding at potentials from 1.0 to 1.4 V (vs.SCE) in 0.5 mol L⁻¹ H₂SO₄. From the analysis of morphology, resistance, gas permeability and contact angle, the characteristics of the fresh GDL and the oxidized GDLs are compared. It is found that the maximum power densities of the fuel cells with the oxidized GDLs hold at 1.2 and 1.4 V (vs.SCE) for 96 h decreased 178 and 486 mW cm⁻², respectively. The electrochemical impedance spectra measured at 1500 mA cm⁻² are also presented and they reveal that the ohmic resistance, charge-transfer and mass-transfer resistances of the fuel cell changed significantly due to corrosion at high potential.     

A direct carbon fuel cell with (molten carbonate)/(doped ceria) composite electrolyte

A composite electrolyte containing a Li/Na carbonate eutectic and a doped ceria phase is employed in a direct carbon fuel cell (DCFC). A four-layer pellet cell, viz. cathode current collector (silver powder), cathode (lithiated NiO/electrolyte), electrolyte and anode current collector layers (silver powder), is fabricated by a co-pressing and sintering technique. Activated carbon powder is mixed with the composite electrolyte and is retained in the anode cavity above the anode current collector. The performance of the single cell with variation of cathode gas and temperature is examined. With a suitable CO₂/O₂ ratio of the cathode gas, an operating temperature of 700 °C, a power output of 100 mW cm⁻² at a current density of 200 mA cm⁻² is obtained. A mechanism of O²⁻ and CO₃²⁻ binary ionic conduction and the anode electrochemical process is discussed.     

An investigation into TiN-coated 316L stainless steel as a bipolar plate material for PEM fuel cells

In order to reduce the cost, weight and volume of the bipolar plates, considerable attention is being paid to developing metallic bipolar plates to replace the non-porous graphite bipolar plates that are in current use. However, metals are prone to corrosion in the proton exchange membrane (PEM) fuel cell environments, which decreases the ionic conductivity of the membrane and lowers the overall performance of the fuel cells. In this study, TiN was coated on SS316L using a physical vapor deposition (PVD) technology (plasma enhanced reactive evaporation) to increase the corrosion resistance of the base SS316L. X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical methods were used to characterize the TiN-coated SS316L. XRD showed that the TiN coating had a face-centered-cubic (fcc) structure. Potentiodynamic tests and electrochemical impedance tests showed that the corrosion resistance of SS316L was significantly increased in 0.5 M H₂SO₄ at 70 °C by coating with TiN. In order to investigate the suitability of these coated materials as cathodes and anodes in a PEMFC, potentiostatic tests were conducted under both simulated cathode and anode conditions. The simulated anode environment was −0.1 V versus SCE purged with H₂ and the simulated cathode environment was 0.6 V versus SCE purged with O₂. In the simulated anode conditions, the corrosion current of TiN-coated SS316L is −4 × 10⁻⁵ A cm⁻², which is lower than that of the uncoated SS316L (about −1 × 10⁻⁶ A cm⁻²). In the simulated cathode conditions, the corrosion current of TiN-coated SS316L is increased to 2.5 × 10⁻⁵ A cm⁻², which is higher than that of the uncoated SS316L (about 5 × 10⁻⁶ A cm⁻²). This is because pitting corrosion had taken place on the TiN-coated specimen.     

Platinum nanoparticles supported on electrospinning-derived carbon fibrous mats by using formaldehyde vapor as reducer for methanol electrooxidation

The Pt nanoparticles have been well dispersed on electrospinning-derived carbon fibrous mats (CFMs) by using formaldehyde vapor as reducer to react with H₂PtCl₆·6H₂O adsorbed on the CFMs at 160 °C. The prepared electrodes of Pt-CFMs have been characterized by using scanning electron microscopy, transmission electron microscopy and X-ray diffraction spectroscopy, and the performance of the electrodes for methanol oxidation has been investigated by using cyclic voltammetry, chronoamperometry, quasi-steady state polarization and electrochemical impedance spectroscopy techniques. The results demonstrate that Pt-CFMs electrodes exhibit peak current density of 445 mA mg⁻¹ Pt, exchange current of 235.7 μA cm⁻², charge transfer resistance of 16.1 Ω cm² and better stability during the process of methanol oxidation, which are superior to the peak current density of 194 mA mg⁻¹ Pt, exchange current of 174.7 μA cm⁻² and charge transfer resistance of 39.4 Ω cm² obtained for commercial Pt/C supported on CFMs. It indicates that the novel process in which formaldehyde vapor is used as reducer to prepare Pt catalyst with high performance can be developed.     

Synthesis and characterization of carbon nanotubes supported platinum nanocatalyst for proton exchange membrane fuel cells

Multi-walled carbon nanotubes (MWCNTs) were used as catalyst support for depositing platinum nanoparticles by a wet chemistry route. MWCNTs were initially surface modified by citric acid to introduce functional groups which act as anchors for metallic clusters. A two-phase (water-toluene) method was used to transfer PtCl₆²⁻ from aqueous to organic phase and the subsequent sodium formate solution reduction step yielded Pt nanoparticles on MWCNTs. High-resolution TEM images showed that the platinum particles in the size range of 1-3 nm are homogeneously distributed on the surface of MWCNTs. The Pt/MWCNTs nanocatalyst was evaluated in the proton exchange membrane (PEM) single cell using H₂/O₂ at 80 °C with Nafion-212 electrolyte. The single PEM fuel cell exhibited a peak power density of about 1100 mW cm⁻² with a total catalyst loading of 0.6 mg Pt cm⁻² (anode: 0.2 mg Pt cm⁻² and cathode: 0.4 mg Pt cm⁻²). The durability of Pt/MWCNTs nanocatalyst was evaluated for 100 h at 80 °C at ambient pressure and the performance (current density at 0.4 V) remained stable throughout. The electrochemically active surface area (64 m² g⁻¹) as estimated by cyclic voltammetry (CV) was also similar before and after the durability test.     

Preparation and characterization of polypyrrole films for three-dimensional micro supercapacitor

As electro-active electrodes for supercapacitors, micro polypyrrole (PPy) films doping with ClO₄⁻ (PPyClO₄) and Cl⁻ (PPy_Cl) are prepared on Ni layers modified three-dimensional (3D) structures in Si substrates. The key process to fabricate the 3D structures is high-aspect-ratio deep reactive ion etching, which result in significant increase of available surface area. Homogeneous conformal Ni layers and PPy films are deposited on the 3D structures by electroless plating and electropolymerization, respectively. The supercapacitor properties of PPy films are investigated by using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge with three-electrode system in NaCl solution. It is shown that doping with ClO₄⁻ results in ideal supercapacitor behaviors with rectangle-like CV shapes at scan rates from 5 to 200 mV s⁻¹, linear galvanostatic charge/discharge curves at current loads from 0.5 to 2 mA and stable cyclic property. However, doping with Cl⁻ gives rise to non-ideal properties of supercapacitor. SEM of the PPyClO₄ shows that the surface of the PPyClO₄ electrode is smooth and the thickness of the PPyClO₄ film is about 2.5 μm. The geometric capacitance of PPyClO₄ is calculated as 0.030 F cm⁻² from CV at scan rate of 100 mV s⁻¹, 0.023 F cm⁻² from EIS and 0.027 F cm⁻² from galvanostatic discharge at 1 mA cm⁻² current density.