Various states of water species in an anion exchange membrane characterized by Raman spectroscopy under controlled temperature and humidity.

Anion exchange membrane fuel cells (AEMFCs) hold the key to future mass commercialisation of fuel cell technology, even though currently, AEMFCs perform less optimally than proton exchange membrane fuel cells (PEMFCs). Unlike PEMFCs, AEMFCs have demonstrated the capability to operate independently of Pt group metal-based catalysts. Water characterization inside the membrane is one factor that significantly influences the performance of AEMFCs. In this paper, different water species inside an anion exchange membrane (AEM), QPAF-4, developed at the University of Yamanashi, were studied for the first time using micro-Raman spectroscopy. Spectra of pure water, alkaline solutions, and calculations based on density functional theory were used to identify the water species in the AEM. The OH stretching band was deconvoluted into nine unique Gaussian bands. All the hydrogen-bonded OH species increased steadily with increasing humidity, while the CH and non-H-bonded OH remained relatively constant. These results confirm the viability of micro-Raman spectroscopy in studying the various water-related species in AEMs. The availability of this technique is an essential prerequisite in improving the ionic conductivity and effectively solving the persisting durability challenge facing AEMFCs, thus hastening the possibility of mass commercialisation of fuel cells.

Sublayered Thin Films of Hydrated Anion Exchange Ionomer for Fuel Cells Formed on SiO2 and Pt Substrates Analyzed by Neutron Reflectometry under Controlled Temperature and Humidity Conditions.

Anion-conductive ionomers are used for electrolyte membranes in membrane-electrode assemblies and for binders in catalyst layers in anion exchange membrane fuel cells (AEMFCs). The conformations of these ionomers as well as their water distribution are important for designing new efficient/durable anion-conductive ionomers for AEMFCs. For a deeper understanding of the distribution of deuterium oxide (D2O) as a function of depth, neutron reflectometry (NR) was carried out on thin films of an anion exchange ionomer, BAF-QAF, with a thickness of approximately 60 nm formed on a thermally formed SiO2 film on Si(100) and on a 20 nm Pt layer deposited on the SiO2 film at a temperature of 60 °C and relative humidities of 0, 50, 70, and 90%. Clear NR modulation was obtained under each condition. The NR data were fit very well with a three-sublayered model parallel to the substrate with different densities of BAF-QAF and D2O. The influence of the SiO2 and Pt substrates was observed not only at the BAF-QAF/substrate interface but also on the entire thin film. The D2O absorption/desorption behavior in each sublayer differed in the BAF-QAF films cast on SiO2 and Pt. The BAF-QAF/SiO2 interface was rather hydrophilic, while the BAF-QAF/Pt interface was very hydrophobic.

Discerning the Redox-Dependent Electronic and Interfacial Structures in Electroactive Self-Assembled Monolayers.

We explore the redox-dependent electronic and structural changes of ferrocene-terminated self-assembled monolayers (Fc SAMs) immersed in aqueous solution. By exploiting X-ray and ultraviolet photoelectron spectroscopy combined with an electrochemical cell (EC-XPS/UPS), we can electrochemically control the Fc SAMs and spectroscopically probe the induced changes with the ferrocene/ferrocenium (Fc/Fc+) redox center (Fe oxidation state), formation of 1:1 Fc+-ClO4- ion pairs, molecular orientation, and monolayer thickness. We further find the insignificant involvement of interfacial water in the Fc SAMs irrespective of redox state. Electrolyte dependencies could be identified with 0.1 M NaClO4 and HClO4 when probing partially oxidized Fc/Fc+ SAMs. Corroborating the occurrence of electrochemically induced oxidation, EC-UPS shows that oxidation to Fc+ is accompanied by a shift of the highest occupied molecular orbital toward higher binding energy. The oxidation to Fc+ is also met with an increase in work function ascribed to the induced negative interfacial dipole caused by the presence of Fc+-ClO4- ion pairs along with a contribution from the reorientation of the Fc+ SAMs. The reversibility of our observations is confirmed upon conversion from Fc+ back to the neutral Fc. The approach shown here is beneficial for a broad range of redox-responsive systems to aid in the elucidation of structure-function relationships.

Effects of Sulfate on the Oxygen Reduction Reaction Activity on Stabilized Pt Skin/PtCo Alloy Catalysts from 30 to 80 °C.

The effects of the concentration of H2SO4 ([H2SO4]), which is the major decomposition product of polymer electrolyte membranes during the operation of fuel cells, on the performance of stabilized Pt skin/PtCo alloy nanocatalysts supported on high-surface-area carbon (PtxAL-PtCo/C) were investigated. Kinetically controlled activities for the oxygen reduction reaction (ORR) and the H2O2 yields ( P(H2O2)) on the PtxAL-PtCo/C were examined based on hydrodynamic voltammograms in O2-saturated 0.1 M HClO4 + X M H2SO4 ( X = 0 to 5 × 10-2) by use of the channel flow double electrode method at temperatures between 30 and 80 °C. At X ≤ 10-6 (1 µM) and all temperatures examined, the apparent ORR rate constants kapp@0.85 V (per unit electrochemically active surface area) on PtxAL-PtCo/C at 0.85 V vs the reversible hydrogen electrode (RHE) were nearly identical with those in sulfate-free 0.1 M HClO4 and were at least twice as high as those on a commercial Pt/C catalyst (c-Pt/C). The values of kapp@0.85 V on both PtxAL-PtCo/C and c-Pt/C decreased linearly with log[H2SO4] in the concentration range 10-6 < X ≤ 5 × 10-2. The detrimental effect by H2SO4 was less pronounced on PtxAL-PtCo/C than on c-Pt/C at high temperatures; the kapp@0.85 V value at X = 5 × 10-2 on the former at 80 °C was maintained as high as 87%, whereas that of the latter was 66% (34% loss). The values of peroxide production percentage P(H2O2) on PtxAL-PtCo/C at 80 °C were nearly constant (ca. 0.22% at 0.76 V vs RHE) up to X = 5 × 10-2. These superior characteristics are ascribed to weakened adsorption of sulfate on the Pt skin surface, supported by DFT calculations, which provides the great advantage of robustness in the presence of impurities, maintaining active sites for the ORR during the PEFC operation.

Effect of Surface Ion Conductivity of Anion Exchange Membranes on Fuel Cell Performance.

Anion conductivity at the surfaces of two anion-exchange membranes (AEMs), quaternized ammonium poly(arylene ether) multiblock copolymer (QPE-bl-3) and quaternized ammonium poly(arylene perfluoro-alkylene) copolymer (QPAF-1), synthesized by our group was investigated using current-sensing atomic force microscopy under purified air at various relative humidities. The anion-conducting spots were distributed inhomogeneously on the surface of QPE-bl-3, and the total areas of the anion-conducting spots and the current at each spot increased with humidity. The anion-conductive areas on QPAF-1 were found on the entire surface even at a low humidity. Distribution of the anion-conducting spots on the membrane was found to directly affect the performance of an AEM fuel cell.

Coherent anti-Stokes Raman scattering spectroscopy system for observation of water molecules in anion exchange membrane.

Anion exchange membrane fuel cells (AEMFCs) provide one of the most feasible remedies to fuel cells' dependency on the dwindling Pt group catalysts. Nevertheless, AEMFCs still suffer reduced durability, which requires an in-depth understanding of their membranes. The low thermal endurance of the anion exchange membranes (AEMs) usually limits the direct application of powerful techniques, such as Raman spectroscopy. We sought to establish a system for coherent anti-Stokes Raman scattering (CARS) spectroscopy capable of taking measurements inside an AEM rapidly and accurately without photodamage. A 785 nm CARS system was newly developed to study the water species in an AEM (QPAF-4) located vertically in a fuel cell. From the results of water measurement in a QPAF-4 membrane, the OH-related region was deconvoluted into nine Gaussian peaks: Five H-bonded OH peaks, non-H-bonded OH, OH-, and two CH peaks. The H-bonded species increased with increasing relative humidity, but the other species remained constant. These results open unlimited possibilities for studying and comparing different AEMFCs, enabling more rapid technology optimization.

Direct STM elucidation of the effects of atomic-level structure on Pt(111) electrodes for dissolved CO oxidation.

We sought to establish a new standard for direct comparison of electrocatalytic activity with surface structure using in situ scanning tunneling microscopy (STM) by examining the electrooxidation of CO in a CO-saturated solution on Pt(111) electrodes with steps, with combined electrochemical measurements, in situ STM, and density functional theory (DFT). On pristine Pt(111) surfaces with initially disordered (111) steps, CO oxidation commences at least 0.5 V lower than that for the main oxidation peak at ca. 0.8-1.0 V vs the reversible hydrogen electrode in aqueous perchloric acid solution. As the potential was cycled between 0.07 and 0.95 V, the CO oxidation activity gradually decreased until only the main oxidation peak remained. In situ STM showed that the steps became perfectly straight. A plausible reason for the preference for (111) steps in the presence of CO is suggested by DFT calculations. In contrast, on a pristine Pt(111) surface with rather straight (100) steps, the low-potential CO oxidation activity was less than that for the pristine, uncycled (111) steps. As the potential was cycled, the activity also decreased greatly. Interestingly, after cycling, in situ STM showed that (111) microsteps were introduced at the (100) steps. Thus, potential cycling in the presence of dissolved CO highly favors formation of (111) steps. The CO oxidation activity in the low-potential region decreased in the following order: disordered (111) steps > straight (100) steps > (100) steps with local (111) microsteps ≈ straight (111) steps.

Operando Monitoring of Oxygen Drop in Catalyst Layers Accompanied by a Water Increase in Running Proton Exchange Membrane Fuel Cells.

Accurate understanding of internal phenomena and their feedback is intrinsically important for improving the performance and durability of proton exchange membrane fuel cells. The oxygen partial pressure (p(O2)) at 10 µm from the cathode catalyst layer (CL) in the gas diffusion layer was measured by using an optical fiber with an oxygen-sensitive dye applied to the apex, when the current density was abruptly increased. p(O2) decreased with increasing current density at constant air utilization. This decrease in oxygen partial pressure is attributed to the increased amount of water at the CL at the cathode due to the oxygen reduction reaction and electro-osmotic drag, as previously proposed. A shortage of oxygen in the CL for 1 s was also detected by using this p(O2) monitoring. These results are consistent with the previous results obtained by operando time-resolved measurements of the water distribution in the electrolyte membranes of a running fuel cell.

Potential-induced phase transition of low-index Au single crystal surfaces in propylene carbonate solution.

In situ scanning tunneling microscopy (STM) was employed to examine the surface structures of Au(111), Au(100), and Au(110) single crystals in propylene carbonate (PC) containing tetrabutylammonium perchlorate (TBAP). All three electrodes exhibited potential-induced phase transition between the reconstructed and unreconstructed (1 × 1) structures at negative and positive potentials, respectively. The potential-induced phase transition of the Au electrode surfaces is attributed to the interaction of the TBA cation and the perchlorate anion at the electrode surface, which is similar to that which takes place in aqueous solutions. In addition to static atomic structures, dynamic processes of both the reconstruction and the lifting of the reconstruction were investigated by means of in situ STM. The lifting of reconstructed Au(111)-(√3 × 22) on Au(111) to the (1 × 1) structure is completed within 1 min at a positive potential. The diffusion of Au atoms on the Au(100) plane in the PC solution proceeds more rapidly than that in the aqueous solution, suggesting that the PC solvent plays an important role in accelerating the diffusion of Au atoms.

Properties and Morphologies of Anion-Exchange Membranes with Different Lengths of Fluorinated Hydrophobic Chains.

An anion-exchange electrolyte membrane, QPAF(C6)-4, polymerized with hydrophobic 1,4'-bis(3-chlorophenyl)perfluorohexane and hydrophilic (6,6'-(2,7-dichloro-9H-fluorene-9.9-diyl)bis(N,N-dimethylhexan-1-amine) is physically flexible and chemically stable. The drawbacks are relatively large water swelling and lower OH- conductivity at higher water uptakes, which are considered to be due to the entanglement of the flexible hydrophobic structure of the membrane. In this study, a QPAF(C4)-4 membrane was newly synthesized with shortened hydrophobic fluoroalkyl chains. Unexpectedly, QPAF(C4)-4 showed a higher water uptake and a lower bulk/surface conductivity than QPAF(C6)-4 possibly due to the decrease in hydrophobicity with a smaller number of fluorine atoms. The thermal stability of QPAF(C4)-4 was higher than that of QAPF(C6)-4, possibly due to the rigidity of the QAPF(C4)-4 structure. A higher mechanical strength of QAPF(C6)-4 than that of QPAF(C4)-4 could be explained by the larger interactions between molecules, as shown in the ultraviolet-visible spectrum. The interactions of molecules were understood in more detail with density functional theory calculations. Both the chemical structures of the polymers and the arrangements of the polymers in the membranes were found to influence the membrane properties.

Probing consequences of anion-dictated electrochemistry on the electrode/monolayer/electrolyte interfacial properties.

Altering electrochemical interfaces by using electrolyte effects or so-called "electrolyte engineering" provides a versatile means to modulate the electrochemical response. However, the long-standing challenge is going "beyond cyclic voltammetry" where electrolyte effects are interrogated from the standpoint of the interfacial properties of the electrode/electrolyte interface. Here, we employ ferrocene-terminated self-assembled monolayers as a molecular probe and investigate how the anion-dictated electrochemical responses are translated in terms of the electronic and structural properties of the electrode/monolayer/electrolyte interface. We utilise a photoelectron-based spectroelectrochemical approach that is capable of capturing "snapshots" into (1) anion dependencies of the ferrocene/ferrocenium (Fc/Fc+) redox process including ion-pairing with counter anions (Fc+-anion) caused by differences in Fc+-anion interactions and steric constraints, and (2) interfacial energetics concerning the electrostatic potential across the electrode/monolayer/electrolyte interface. Our work can be extended to provide electrolyte-related structure-property relationships in redox-active polymers and functionalised electrodes for pseudocapacitive energy storage.

In situ and real-time visualisation of oxygen distribution in DMFC using a porphyrin dye compound.

A luminescent porphyrin dye film has been coated onto a transparent separator on the cathode side of a direct methanol fuel cell (DMFC) to visualise clearly oxygen distribution under operating conditions by analysing emission from the dye.

Structurally Well-Defined Anion-Exchange Membranes Containing Perfluoroalkyl and Ammonium-Functionalized Fluorenyl Groups.

Novel anion-conductive polymers containing perfluoroalkyl and ammonium-functionalized fluorene groups were synthesized and characterized. The quaternized polymers synthesized using a dimethylaminated fluorene monomer had a well-defined chemical structure in which each fluorenyl group was substituted with two ammonium groups at specific positions. The resulting polymers had a high molecular weight (M n = 8.9-13.8 kDa, M w = 13.7-24.5 kDa) to provide bendable thin membranes with the ion-exchange capacity (IEC) ranging from 0.7 to 1.9 mequiv g-1 by solution casting. Both transmission electron microscopy images and small-angle X-ray scattering patterns suggested that the polymer membranes possessed a nanoscale phase-separated morphology based on the hydrophilic/hydrophobic differences in the polymer components. Unlike typical anion-exchange membranes found in the literature, hydroxide ion conductivity of the membranes did not increase with increasing IEC because of their high swelling capability in water. The membrane with IEC = 1.2 mequiv g-1 showed balanced properties of high hydroxide ion conductivity (81 mS cm-1 at 80 °C in water) and mechanical strength (>100% elongation and 14 MPa maximum stress at 80 °C, 60% relative humidity). The polymer main chains were stable in 4 M KOH for 1000 h, whereas the trimethylbenzyl-type ammonium groups degraded under the conditions to cause loss in the hydroxide ion conductivity. An H2/O2 fuel cell with the membrane with IEC = 1.2 mequiv g-1 exhibited a maximum power density of 242 mW cm-2 at 580 mA cm-2 current density.

Atomically Flat Pt Skin and Striking Enrichment of Co in Underlying Alloy at Pt3Co(111) Single Crystal with Unprecedented Activity for the Oxygen Reduction Reaction.

By the use of in situ scanning tunneling microscopy and surface X-ray scattering techniques, we have clarified the surface structure and the layer-by-layer compositions of a Pt skin/Pt3Co(111) single-crystal electrode, which exhibited extremely high activity for the oxygen reduction reaction. The topmost layer was found to be an atomically flat Pt skin with (1 × 1) structure. Cobalt was enriched in the second layer up to 98 atom %, whereas the Co content in the third and fourth layers was slightly smaller than that in the bulk. By X-ray photoelectron spectroscopy, the Co in the subsurface layers was found to be positively charged, which is consistent with an electronic modification of the Pt skin. The extremely high activity at the Pt skin/Pt3Co(111) single crystal is correlated with this specific surface structure.

Imaging individual proton-conducting spots on sulfonated multiblock-copolymer membrane under controlled hydrogen atmosphere by current-sensing atomic force microscopy.

The proton-conductive spots on the membrane surface of sulfonated poly(arylene ether) multiblock copolymer were successfully imaged by current-sensing atomic force microscopy under hydrogen atmosphere at various temperatures and humidities. These spots should be connected to the proton-conductive paths inside the membrane. The average diameter of the spots was approximately 12 nm, consistent with the size of hydrophilic domains observed by transmission electron microscopy. The size of the proton-conducting spots was almost unchanged regardless of the temperature and humidity, whereas the number of the spots increased at higher humidity; the total area of the proton-conducting spots increased accordingly on the membrane surface. This increase in the conducting area at high humidity should be related to the bulk ionic conductivity measured by impedance spectroscopy.

Particle-size effect of nanoscale platinum catalysts in oxygen reduction reaction: an electrochemical and 195Pt EC-NMR study.

Oxygen reduction reaction (ORR) measurements and (195)Pt electrochemical nuclear magnetic resonance (EC-NMR) spectroscopy were combined to study a series of carbon-supported platinum nanoparticle electrocatalysts (Pt/CB) with average diameters in the range of roughly 1-5 nm. ORR rate constants and H(2)O(2) yields evaluated from hydrodynamic voltammograms did not show any particle size dependency. The apparent activation energy of 37 kJ mol(-1), obtained for the ORR rate constant, was identical to that obtained for bulk platinum electrodes. Pt/CB catalysts on Nafion produced only 0.7-1% of H(2)O(2), confirming that the direct four-electron reduction of O(2) to H(2)O is the predominant reaction. NMR spectral features showed characteristic size dependence, and the line shapes were reproduced by using the layer-deconvolution model. Namely, the variations in the NMR spectra with particle size can be explained as due to the combined effect of the layer-by-layer variation of the s-type and d-type local density of states. However, the surface peak position of (195)Pt NMR spectra and the spin-lattice relaxation time of surface platinum atoms showed practically no change with the particle size variation. We conclude that there is a negligible difference in the surface electronic properties of these Pt/CB catalysts due to size variations and therefore, the ORR activities are not affected by the differences in the particle size.

Adlayer of hydroquinone on Pt(111) in solution and in a vacuum studied by STM and LEED.

Hydroquinone (HQ) adlayers were formed on Pt(111) in HF solution and in a vacuum. By using scanning tunneling microscopy (STM) in solution, it was revealed that HQ formed an ordered structure on Pt(111) with a strong attractive interaction between two adjacent hydroxyl groups in neighboring HQ molecules. After the sample was transferred into a vacuum, low-energy electron diffraction (LEED) measurement was performed, which showed that the (2.56 x 2.56)R16 degrees incommensurate structure of the HQ adlayer was formed in solution. The HQ adlayer on Pt(111) was formed also by vapor deposition, and the identical (2.56 x 2.56)R16 degrees adlayer structure was found by LEED and STM in a vacuum.

Adlayers of C60-C60 and C60-C70 fullerene dimers formed on au(111) in benzene solutions studied by STM and LEED.

Scanning tunneling microscopy (STM) and low-energy electron diffraction were used to reveal the structures of ordered adlayers of [2+2]-type C60-C60 fullerene dimer (C120) and C60-C70 cross-dimer (C130) formed on Au(111) by immersingit in abenzene solution containing C120 or C130 molecules. High-resolution STM images clearly showed the packing arrangements and the electronic structures of C120 and C130 on the Au(111) surface in ultrahigh vacuum. The (2 square root3 x 4square root3)R30 degrees, (2square root3 x 5square root3)R30 degrees, and (7 x 7) structures were found for the C120 adlayer on the Au(111) surface, whereas C130 molecules were closely packed on the surface. Each C60 or C70 monomer cage was discerned in the STM image of a C130 molecule.