San Ping Jiang

A comparison of O2 reduction reactions on porous (La,Sr)MnO3 and (La,Sr)(Co,Fe)O3 electrodes

O2 reduction reactions were investigated on (La,Sr)MnO3 (LSM) and (La,Sr)(Co,Fe)O3 (LSCF) electrodes at temperatures from 700 to 900 °C in air with and without the presence of gaseous Cr species. Gaseous Cr species were introduced to the system by using chromia-forming alloy interconnect in contact with the electrode. For O2 reduction on the LSM electrode, three reaction steps have been identified: surface dissociative adsorption and diffusion, charge transfer and oxygen ion migration into the zirconia electrolyte phase. The reaction is controlled by the dissociative adsorption and diffusion at LSM electrode surface at low temperatures and the oxygen ion migration/diffusion into zirconia electrolyte at high temperatures. By comparing the electrode behavior of LSM and LSCF in the absence and presence of chromia-forming alloy under identical experimental conditions, reaction steps on LSCF electrodes have also been identified. The results indicate that both surface and bulk diffusion processes play important roles in the overall reaction kinetics for the O2 reduction on LSCF electrodes.

PtRu nanoparticles supported on 1-aminopyrene-functionalized multiwalled carbon nanotubes and their electrocatalytic activity for methanol oxidation.

A new synthesis method for the preparation of high-performance PtRu electrocatalysts on multiwalled carbon nanotubes (MWCNTs) is reported. In this method, bimetallic PtRu electrocatalysts are deposited onto 1-aminopyrene (1-AP)-functionalized MWCNTs by a microwave-assisted polyol process. The noncovalent functionalization of MWCNTs by 1-AP is simple and can be carried out at room temperature without the use of expensive chemicals or corrosive acids, thus preserving the integrity and the electronic structure of MWCNTs. PtRu electrocatalysts on 1-AP-functionalized MWCNTs show much better distribution with no formation of aggregates, higher electrochemically active surface area, and higher electrocatalytic activity for the electrooxidation of methanol in direct methanol fuel cells as compared to that on conventional acid-treated MWCNTs and carbon black supported PtRu electrocatalysts. PtRu electrocatalysts on 1-AP-functionalized MWCNTs also show significantly enhanced stability.

An electrode kinetics study of H2 oxidation on Ni/Y2O3–ZrO2 cermet electrode of the solid oxide fuel cell

Hydrogen oxidation on Ni/3 mol% Y2O3–ZrO2 (Ni/Y-TZP) cermet electrodes has been studied in moist H2 environments. The impedance behaviour is characterised by two clearly separated arcs in the frequency domain, indicating that the overall electrode reaction is controlled by at least two rate limiting processes in series. The electrode process associated with the low frequency arc is mainly dependent on the H2 concentration. Moreover, the electrode conductivity of the low frequency arc (σL) is independent of temperature and the polarisation potential. Based on these observations, the electrode process associated with the low frequency arc has been attributed to hydrogen dissociative adsorption/diffusion on the surface of Ni particles. The electrode process of the high frequency arc is also affected by the H2 concentration and by oxygen partial pressure (PO2), however in contrast to the low frequency arc, the electrode conductivity associated with the high frequency electrode process (σH) showed a strong temperature dependence with an activation energy of ∼162 kJ mol−1 and it increases with increasing polarisation potential. This arc has been attributed to hydrogen transfer from the Ni electrode surface to the Y-TZP electrolyte surface, followed by a charge transfer process on the electrolyte surface near the interface region. The results of this study clearly demonstrate that the Y-TZP in the Ni cermet electrodes has little effect on the reaction mechanism, but plays an important role in modifying the electrode microstructure and kinetics of the fuel oxidation reaction.

Deposition of Chromium Species at Sr‐Doped LaMnO3 Electrodes in Solid Oxide Fuel Cells. I. Mechanism and Kinetics

Deposition processes of chromium (Cr) species for the system chromia-forming alloy metallic interconnect/Sr-doped LaMnO 3 (LSM) electrode/3 mol % Y 2 O 3 -ZrO 2 (TZ3Y) electrolyte have been investigated under various conditions at 900°C. Deposition of Cr species preferentially occurred on the zirconia electrolyte surface under both cathodic (O 2 reduction) and anodic (O 2 oxidation) polarization, and formed deposit rings at the edge of the LSM electrode. The deposit ring was ca. 60 μm wide after cathodic polarization for 50 h and ca. 89 μm for 129 h. Cr was not detected on the surface of the LSM electrode or inside the electrode body under the conditions studied. In the absence of polarization potentials, Cr deposits were also found on the zirconia electrolyte surface heated at 1100°C but not at 900°C. In all cases, energy despersive spectrometry analysis indicated that deposits mainly consisted of Cr and/or Cr-Mn, indicating the formation of Cr 2 O 3 and (Cr,Mn) 3 O 4 -type spinel phases. The results clearly demonstrated that the deposition of Cr species at the LSM electrode/TZ3Y electrolyte is not dominated by electrochemical reduction of high valent Cr vapor species to Cr 2 O 3 in competition with O 2 reduction. The driving force for the Cr deposition processes is most likely related to the Mn species, in particular the Mn 2+ ions, generated under polarization potentials or at high temperatures. The proposed deposition mechanism is consistent with the electrochemical behavior of LSM electrodes in the presence of chromia-forming alloy interconnect.

Layer-by-layer self-assembly in the development of electrochemical energy conversion and storage devices from fuel cells to supercapacitors

As one of the most effective synthesis tools, layer-by-layer (LbL) self-assembly technology can provide a strong non-covalent integration and accurate assembly between homo- or hetero-phase compounds or oppositely charged polyelectrolytes, resulting in highly-ordered nanoscale structures or patterns with excellent functionalities and activities. It has been widely used in the developments of novel materials and nanostructures or patterns from nanotechnologies to medical fields. However, the application of LbL self-assembly in the development of highly efficient electrocatalysts, specific functionalized membranes for proton exchange membrane fuel cells (PEMFCs) and electrode materials for supercapacitors is a relatively new phenomenon. In this review, the application of LbL self-assembly in the development and synthesis of key materials of PEMFCs including polyelectrolyte multilayered proton-exchange membranes, methanol-blocking Nafion membranes, highly uniform and efficient Pt-based electrocatalysts, self-assembled polyelectrolyte functionalized carbon nanotubes (CNTs) and graphenes will be reviewed. The application of LbL self-assembly for the development of multilayer nanostructured materials for use in electrochemical supercapacitors will also be reviewed and discussed (250 references).

Deposition of Cr Species at ( La , Sr ) ( Co , Fe ) O3 Cathodes of Solid Oxide Fuel Cells

Deposition process of Cr species at the (La,Sr)(Co,Fe)O 3 (LSCF) electrode and Gd 0 . 2 Ce 0 . 8 O 2 (GDC) electrolyte system is investigated under the O 2 reduction conditions in the presence of a Fe-Cr alloy interconnect for solid oxide fuel cells. Deposition of Cr species preferentially occurs on the surface of the LSCF electrode with and without the cathodic polarization at 900°C, forming SrCrO 4 and Cr 2 O 3 phase. At the initial stage of the reaction, Cr deposition was not detected inside the LSCF electrode or at the LSCF electrode/GDC electrolyte interface. Deposition of Cr species on the LSCF electrode surface under the rib of Fe-Cr alloy interconnect is substantial in comparison to that under the channel of the interconnect. The results demonstrate clearly that the deposition of Cr species at the LSCF electrode is essentially a chemical reaction and is kinetically controlled by nucleation reaction between the gaseous Cr species and SrO-enriched/segregated on the LSCF electrode surface.

Effect of contact between electrode and current collector on the performance of solid oxide fuel cells

Abstract The effect of contact area between electrode and current collector (i.e., the interconnect) on the performance of anode-supported solid oxide fuel cells (SOFC) has been investigated using current collector with various contact area on the (Pr,Sr)MnO 3 (PSM) cathode side. The cell resistance decreased significantly with the increase in the contact area between the PSM cathode and the current collector. When the contact area of the current collector increased from 4.6% to 27.2%, the cell resistance decreased from 1.43 to 0.19 Ω cm 2 at 800 °C, a reduction of more than 80%. Furthermore, the polarization losses of the cell were also significantly reduced with the increase in the contact area of the current collector. The results indicate that there is close correlation between the contact area of the current collector and the cell performance. This shows that the constriction effect as frequently observed in solid electrolyte cells not only occurs at the electrode/electrolyte interface but also at the interface of the electrode/current collector. A hypothesis on the effect of the discrete contact between current collector and electrode on the current distribution in the electrolyte cell has been proposed.

Fabrication and Performance of GDC-Impregnated ( La , Sr ) MnO3 Cathodes for Intermediate Temperature Solid Oxide Fuel Cells

High-performance (La,Sr)MnO 3 (LSM) cathodes for intermediate temperature solid oxide fuel cells were fabricated by the ion impregnation of oxygen ion conducting Gd-doped CeO 2 (GDC) oxide. The microstructure of the GDC-impregnated LSM electrode was characterized by the uniform dispersion of very fine GDC particles in the LSM porous framework. The particle size of the impregnated GDC phase was in the range of 100-200 nm. Uniform distribution of the nanosized ionic conducting GDC phase significantly enhanced the electrode activities for the O 2 reduction reaction. In the case of the 5.8 mg cm - 2 GDC-impregnated LSM electrode, electrode polarization resistance was 0.21 Ω cm 2 at 700°C, 56 times lower than that of the pure LSM at the same temperature. GDC-impregnated LSM electrode showed lower activation energy and low reaction order with respect to the partial pressure of oxygen, as compared to that on the pure LSM electrodes. The results indicate that GDC impregnation has significant electrocatalytic effect on the O 2 reduction reactions on the LSM electrodes.

Deposition of Chromium Species at Sr‐Doped LaMnO3 Electrodes in Solid Oxide Fuel Cells II. Effect on O 2 Reduction Reaction

The O 2 reduction reactions on Sr-doped LaMnO 3 (LSM) electrodes in solid-oxide fuel cells in the absence and presence of a chromia-forming alloy metallic interconnect have been investigated. Similar to those in the absence of the chromia-forming alloy, the O 2 reduction reactions on LSM electrodes in the presence of chromia-forming alloys are limited by two electrode processes, the diffusion of oxygen species on the LSM surface and the migration of oxygen ions into zirconia electrolyte. In the presence of chromia-forming alloy, the dissociative adsorption and diffusion of oxygen on the LSM electrode surface are inhibited by the gaseous Cr species (e.g., CrO 3 ). and the migration processes of oxygen ions into zirconia electrolyte are inhibited by the solid Cr species [e.g., Cr 2 O 3 /(Cr, Mn) 3 O 4 ] deposited on the electrolyte surface, The inhibiting effect of the solid Cr species becomes increasingly dominant on the O 2 reduction with the polarization time. The mechanism and kinetics of the O 2 reduction on the LSM electrodes in the absence and presence of chromia-forming alloy are discussed.

HPW/MCM‐41 Phosphotungstic Acid/Mesoporous Silica Composites as Novel Proton‐Exchange Membranes for Elevated‐Temperature Fuel Cells

Proton-exchange membrane and direct methanol fuel cells (PEMFCs and DMFCs) have attracted much attention as clean energy sources for various applications, such as electric vehicles, portable electronics, and domestic power generation, because of their high power density, high efficiency, and low greenhouse gas emission. Especially, the operation of PEMFCs and DMFCs at temperatures above 100 8C is considered to have many advantages, such as the elimination of CO poisoning of the platinum electrocatalyst, faster electrode reaction kinetics, simplified water and heat management, higher energy efficiency, and reduced usage of precious Pt and Pt alloy catalyst. However, the state-of-the-art proton-exchange membranes (PEMs) based on perfluorosulfonic acid (PFSA), such as Nafion, are unstable at elevated temperatures ( 100 8C) and proton conductivity decreases significantly due to the loss of water from the membrane under conditions of high temperatures or low humidities. Therefore, development of PEMs with high proton conductivity and stability at elevated temperatures is a major challenge. Great efforts have been dedicated to developing PEMs for operation at elevated temperatures based on mesoporous or nanoporous inorganic materials. Mesoporous inorganic materials have a pore size range of 2–50 nm and are characterized by high specific surface area, nanometer-sized channels or frameworks with an ordered or disordered interconnected internal structure, and high structural stability, which make feasible potential applications as proton-exchange membranes operating at elevated temperatures. Lu and co-workers reported sol–gel-derived mesostructured zirconium phosphates with proton conductivities of about 10 –10 6 S cm . Colomer et al. synthesized nanoporous anatase thin films with conductivity values from 10 5 to 10 3 S cm 1 in the range of 33%–81% relative humidity (RH) at room temperature. Li and Nogami prepared proton-conducting mesoporous silica films with conductivity ranging from 10 6 to 10 4 S cm 1 under 40%–90% humidity. However, the proton conductivity of the pure mesoporous materials depends significantly on their textural characteristics. For instance, Colomer et al. also reported a proton conductivity of 2.0 10 2 S cm 1 on mesoporous acid-free silica xerogels and 3.78 10 2 S cm 1 on a nanoporous anatase thin film at 80 8C and 81% RH. Yamada et al. reported a TiO2-P2O5 mesoporous nanocomposite with a proton conductivity value of 2 10 2 S cm 1 at 160 8C. Halla et al. synthesized meso-SiO2-C12EO10OH-CF3SO3H as a new protonconducting electrolyte and reported a conductivity of 1 10 3 S cm 1 at room temperature and 90% RH. Although the conductivity values of mesoporous acid-free silica xerogels, meso-SiO2-C12EO10OHCF3SO3H or anatase thin films are adequate for fuel cell applications, their performance as an electrolyte in a PEMFC has not been evaluated yet. Yamada and Honma synthesized a H3PW12O40 (abbreviated as HPW) and polystyrene sulfonic acid (PSS) composite by self-assembly of –SO3H onto the HPW surface, achieving a proton conductivity of 1 10 2 S cm 1 at 180 8C. Nevertheless, the power density of the cell based on a PSS with 10wt% HPW composite membrane is very low, ca. 3mW cm 2 at 160 8C in H2/O2 with no external humidity. Uma and Nogami synthesized an inorganic glass composite membrane consisting of a mixture of phosphotungstic acid (HPW) and phosphomolybdic acid (HPM), and reported very high conductivity values, 1.014 S cm 1 at 30 8C and 85% RH for a mesoporous-structured HPW/HPM-P2O5-SiO2 glass, [24] and 1.01 10 1 S cm 1 at 85 8C under 85% RH for a mesoporousstructured HPW-P2O5-SiO2 glass. [17] The cell performance based on these inorganic PEMs was 35–42mW cm 2 in H2/O2 at ca. 30 8C under 30% RH. However, there is little information on the performance of HPWand HPM-incorporated P2O5-SiO2 glass electrolyte cells at elevated high temperatures or in methanol fuels. Here, we present a novel inorganic PEM based on highly ordered mesoporous MCM-41 silica with assembled HPW nanoparticles by the vacuum-assisted impregnation method (VIM). The proton conductivity of the HPW/MCM-41 mesoporous silica inorganic PEM is 0.018 and 0.045 S cm 1 at 25 and 150 8C, respectively. Most significantly, the PEMFCs based on the HPW/MCM-41 mesoporous-silica membrane showed a very impressive performance, achieving a maximum power density of 95mWcm 2 in H2/O2 at 100 8C and 100% RH, and 90mWcm 2 in methanol/O2 at 150 8C and 0.67% RH of the cathode. Highly orderedmesoporous silica MCM-41 can be synthesized according to the procedure given in the literature. Thus, the key issue is to anchor and assemble HPW into the mesopores or channels of MCM-41 host. We have derived a VIM to assemble HPW molecules into the mesoporous silica. In this process, the impurities or trapped air inside the mesopores are removed under vacuum, and the vacuum-treated mesoporous silica