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An anion-exchange electrolyte membrane, QPAF(C6)-4, polymerized with hydrophobic 1,4'-bis(3-chlorophenyl)perfluorohexane and hydrophilic (6,6'-(2,7-dichloro-9

The distribution and the chemical states of water in the electrolyte membrane of proton-exchange membrane fuel cells (PEMFCs) directly affect the performance and stability during cell operation. Co...

The three-dimensional structures of polymer membranes are different at surfaces and inside bulks, and thus, in general, physical/chemical properties are also different. Morphologies and properties of membrane surfaces are now visualized by current-sensing atomic force microscopy. The increase in performances of a single cell is discussed based on the three-dimensional structures of the polymer membrane, anion-exchange membrane as an example, used for fuel cells. Other reports on Nafion®, proton-exchange membrane, are also introduced to show the importance of this microscopic method.

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...

On the performance and stability of proton exchange membrane fuel cells (PEMFCs), the water distribution inside the membrane has a direct influence. In this study, coherent anti-Stokes Raman scatte...

The crystal structure of platinum–cobalt (PtCo) nanoparticles with stabilized Pt-skin layers (PtxAL–PtCo) has been found, for the first time, to change dramatically by conventional potential cycles...

Novel anion conductive aromatic copolymers containing hexafluoroisopropylidene groups as the hydrophobic component and fluorenyl groups substituted with pendant hexyltrimethylammonium groups as the...

The electro-oxidation of CO on Pt surface is not only fundamentally important in electrochemistry, but also practically important in residential fuel cells for avoiding the poisoning of Pt catalysts by CO. We carried out cyclic voltammetry on Pt(111), (110), (100), (10 10 9), (10 9 8), (10 2 1), (432), and (431) single-crystal surfaces using a three compartment cell to understand the activity and durability towards the electro-oxidation of CO saturated in 0.1 M HClO4. During the potential cycles between 0.07 and 0.95 V vs. the reversible hydrogen electrode, the current for the electro-oxidation of CO at potentials lower than 0.5 V disappeared, accompanied by surface reconstruction. Among the electrodes, the Pt(100) electrode showed the lowest onset potential of 0.29 V, but the activity abruptly disappeared after one potential cycle, the active sites were extremely unstable. In order to investigate the processes of the deactivation, potential-step measurements were also conducted on Pt(111) in a CO-saturated solution. Repeated cycles of the formations of Pt oxides at a high potential and Pt carbonyl species at a low potential on the surface were proposed as the deactivation process.

Anion exchange membrane fuel cells (AEMFCs) are vulnerable to water management problems, since water is produced at the anode and consumed at the cathode. Previously we found severe voltage losses when increasing the current density in an AEMFC with a commercial Fe–N–Cp cathode catalyst and an in-house-developed anion exchange ionomer (quaternized poly(arylene perfluoroalkylene), i.e., QPAF-4, developed in this laboratory (Figure 1)1,2. In the present work,3 we have clearly identified the problem as being related to water management and developed two approaches to alleviating the problem: i) by use of a thin hydrophilized membrane, the diffusivity of water at the surface was improved, and the severe I–V hysteresis was suppressed, despite the cell using an Fe–N–Cp cathode catalyst with high-water absorption (Figs. 2a and 3a); ii) the voltage loss was also alleviated by the use of a recently developed Fe–N–Cc catalyst with higher hydrophobicity, which decreased the absorption of back-diffusing water into the catalyst layer and increased the amount of water supplied to the reaction sites. These improvements have demonstrated that water transport is the main limitation for the previously reported hysteresis and provide strategies to achieve higher performance AEMFCs through proper water management and formation of water transport pathways (Figs. 2b and 3b). Acknowledgements This project was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) Japan through funds for the “Advanced Research Program for Energy and Environmental Technologies,” by the Japan Society for the Promotion of Science (JSPS) and the the Swiss National Science Foundation (SNSF) under the Joint Research Projects (JRPs) program, and by the Japan Science and Technology (JST) through the Strategic International Collaborative Research Program (SICORP). We are grateful to Pajarito Powder supplying the Fe–N-Cp catalyst. Also, we are deeply grateful to Dr. Weilin Xu and Dr. Ping Song of Changchun Institute of Applied Chemistry and Dr. Shuanjin Wang of Sun Yat-sen University for supplying the Fe–N-Cc catalyst, and to Takahata Precision Co. Ltd. for kindly providing the QPAF-4 membrane and ionomer. References H. Ono, T. Kimura, A. Takano, K. Asazawa, J. Miyake, J. Inukai, K. Miyatake, Robust anion conductive polymers containing perfluoroalkylene and pendant ammonium groups for high performance fuel cells, J. Mater. Chem. A 5 (2017) 24804–24812, https://doi.org/10.1039/c7ta09409d. K. Otsuji, N. Yokota, D. A. Tryk, K. Kakinuma, K. Miyatake, M. Uchida, Performance hysteresis phenomena of anion exchange membrane fuel cells using an Fe–N–C cathode catalyst and an in-house developed polymer electrolyte, J. Power Sources, 487 (2021) 229407, https://doi.org/10.1016/j.jpowsour.2020.229407 K. Otsuji, Y. Shirase, T. Asakawa, N. Yokota, K. Nagase, W. Xu, P. Song, S. Wang, D. A. Tryk, K. Kakinuma, J. Inukai, K. Miyatake, M. Uchida, J. Power Sources, 522 (2022) 230997, https://doi.org/10.1016/j.jpowsour.2022.230997 Figure 1

Organically tailorable monolayers of ferrocene-terminated alkanethiol self-assembled monolayers (Fc SAM, Fig. 1a) on Au surfaces are attractive model systems to elucidate the molecular-level physicochemical properties of solid-liquid electrochemical (EC) interfaces [1]. In particular, Fc SAM has been shown to be an attractive spectroscopic molecular probe for photoelectron spectroscopy because the Fe heteroatom is at a fixed position above the electrode surface. Thus, this allows photoelectron spectroscopy to ascertain the electronic and structural properties at a fixed position above the electrode surface [2-3]. The ability to obtain molecular-level structure-function-property relationships with Fc SAM has relevance to bio(chemical) sensors, micro-actuators, and molecular electronics. Here, we use X-ray and ultraviolet photoelectron spectroscopy combined with an electrochemical cell (EC-XPS/UPS) to electrochemically control the Fc SAM and spectroscopically probe the corresponding electronic and structural changes [2-3]. To perform our experiments, electrochemical (EC) measurements are performed in a separate and dedicated EC chamber in a solution environment under Ar atmosphere, followed by evacuation and finally transfer to the XPS/UPS analysis chamber [2-3]. The EC chamber contains a retractable PTFE (polytetrafluorethylene) cell and utilizes a hanging meniscus setup. I will discuss the merits and challenges of EC-XPS/UPS methodology in the context of other approaches. Using EC-XPS/UPS, we are able to discriminate anion dependencies (Fig. 1b-d) with respect to the Fe oxidation state, monolayer thickness, and changes to the valence structure regarding the highest occupied molecular orbital (HOMO. Lastly, we are able to spectroscopically probe the changes relating to the interfacial potential distribution across the electrode-monolayer-electrolyte interface or so-called “potential drop” in the form of systematic potential-induced binding energy shifts in the XPS/UPS spectra. Further details will be provided at the presentation. Fig. 1 (a) Ferrocene-terminated alkanethiol SAM on Au. Schematic of electrode-monolayer-electrolyte interface and interfacial potential distribution. (b) Cyclic voltammograms at 50 mV s-1 in 0.1 M NaTFSI or NaPF6 (c-d) Fe 2p3/2 spectra of Fc SAM, pristine and after anodic/cathodic polarization at 0.625 and 0 V, respectively. References [1] K. Uosaki, Electrochemistry 12, 1105-1113 (1999). [2] R. A. Wong, Y. Yokota, M. Wakisaka, J. Inukai, Y. Kim, J. Am. Chem. Soc. 140, 13672-13679 (2018). [3] R. A. Wong, Y. Yokota, M. Wakisaka, J. Inukai, Y. Kim, Nat. Commun. 11, 1-9 (2020). Figure 1

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., Conveying electrochemistry in terms of the electrode/electrolyte interfacial properties remains challenging. Here, the authors employ a surface-bound molecular probe and photoelectron spectroscopy to peer into the anion-dictated and potential-induced interfacial electronic and structural properties.

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 (D

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–Pt...

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 functio...

We report synthesis and properties of a novel series of ammonium-functionalized perfluoroalkylene/aromatic copolymers. In particular, the effect of number and position of ammonium groups on the properties of the copolymer thin membranes was investigated. Fluorenylidene biphenylene groups were used as a scaffold for the ammonium groups. By controlling the chloromethylation reaction conditions, the ammonium groups could be introduced up to 4.0 per fluorenylidene biphenylene unit, resulting in the ion exchange capacity (IEC) of the resulting copolymers ranging from 1.0 to 1.8 meq g−1. The copolymers provided bendable and transparent membranes by solution casting. The membranes exhibited phase-separated morphology based on the hydrophilic/hydrophobic differences in the copolymer components. A copolymer membrane with IEC = 1.0 meq g−1 showed high hydroxide ion conductivity (ca. 70 mS cm−1) at 80 °C in water and good alkaline stability in 1 M KOH aq. over 1000 h at 60 °C. The membrane showed only minor degradat...

We have developed an operando analytical system designed for the simultaneous measurements of the electrochemical reaction rate and the hard X-ray photoemission spectrum. In this system, a solution containing reactants was continuously supplied in a Hagen-Poiseuille flow to a specimen, such as catalyst, under the control of temperature, pressure, concentration, and solution flow rate, so that the electrochemical reaction rate is able to be accurately measured or controlled by electrochemical current generated by the reaction. Two different environments, solution and ultra-high vacuum, were connected with an ultra-thin Au/SixNy window with a thickness of 20 nm to make the operando hard X-ray photoemission spectroscopy possible during the steady-state electrochemical reaction. In this paper, we report this analytical system in detail.

Understanding the reaction distributions inside a polymer electrolyte fuel cell (PEFC) is essential for the higher performance and durability. We have developed a new see-through cell and visualized the distributions of oxygen partial pressure and current density inside a running PEFC at the temperature of 40 and 80 °C and the relative humidity of 53%. The oxygen utilization was changed from 0% to 80% by changing the current density. At higher oxygen utilizations, the current density was higher and therefore the water generation. Generated water droplets in the flow channel were also visualized, allowing for the simultaneous visualization of the distribution of the oxygen partial pressure, current density, and water droplets. By combining the observations of all three parameters, the reactions inside a membrane-electrode assembly were discussed.

In situ X-ray absorption spectroscopy has afforded detailed structural and electronic characterization of a newly developed stabilized Pt-skin/PtCo alloy nanoparticle catalyst for CO-tolerant H2 oxidation. The X-ray absorption near-edge structure (XANES) results show the significant effects of H and CO adsorption on white lines for pure Pt nanoparticles, which are correlated with the depletion of the density of states (DOS) just below the Fermi level (FL) calculated with density-functional theory (DFT). In contrast, for the Pt-skin/PtCo surface, there is little effect on the XANES for either H or CO adsorption. The lack of effect is consistent with the lack of variation of the DOS near the FL. The extended X-ray absorption fine structure (EXAFS) results for pure Pt nanoparticles have exhibited significant changes caused by H and CO adsorption. Such changes are suggested to be associated with the selected surface expansion of Pt atoms, as revealed from DFT-calculated radial distribution functions (RDFs). In contrast, for the Pt-skin/PtCo surface, there were negligible changes in the EXAFS, either with H or CO adsorption, which is consistent with the lack of changes in the DFT-calculated RDF. Notably, the rigidity of the Pt skin and the lower degree of overlap between Pt and CO orbitals, due to the narrowing of the d-band, are thought to be responsible for the weak adsorption.

A novel series of ammonium-containing copolymers (QPAF-4) were designed and synthesized as anion exchange membranes for alkaline fuel cell applications. The copolymers were prepared via a nickel promoted polycondensation reaction with high molecular weights (Mw = 72.7–276.4 kDa as precursors) and were composed of perfluoroalkylene and fluorenyl groups with pendant ammonium groups. The QPAF-4 membrane with optimized copolymer composition and ion exchange capacity exhibited high hydroxide ion conductivity (86.2 mS cm−1 in water at 80 °C) and excellent mechanical properties (large elongation at break = 269%). A severe alkaline stability test of the QPAF-4 membranes in 1 M KOH at 80 °C for 1000 h and the post-test analyses of the 1H NMR spectra, solubility, and mechanical properties revealed minor, or no, changes in the chemical structure and properties. Alkaline fuel cells using the QPAF-4 membrane were operated using hydrazine as a fuel and oxygen or air as oxidant to achieve the high maximum power density of 515 mW cm−2. The durability of the membrane was also confirmed in the operating fuel cell at a constant current density for longer than 1000 h.

The oxygen starvation was investigated in a single-serpentine proton exchange membrane fuel cell (PEMFC) at low oxygen concentrations of 3, 5, and 10% in air. At the voltage lower than 0.6 V, the cell performance abruptly decreased at all oxygen concentrations, but most significantly at 3%. To investigate the processes of the oxygen starvation, the oxygen partial pressure (p(O2)) on the gas diffusion layer (GDL) at low oxygen concentrations (Φ(O2)) of 3, 5, and 10% in air was visualized at the cathode in a running single-serpentine PEMFC by using an oxygen-sensitive dye coated on the GDL. p(O2) continuously lowered in the operating fuel cell along the gas flow channel. At Φ(O2) = 3 and 5% and at oxygen utilizations higher than 90%, p(O2) on the GDL became 0 kPa and susceptible to degradation. The evolution of hydrogen at the cathode was proposed, which might have chemically consumed oxygen near the outlet. Also in a stack, the influence of this hydrogen evolution prior to the hydrogen pumping must be considered.

Understanding of the structural and the chemical properties of Pt catalysts in polymer electrolyte fuel cells (PEFCs) is necessary to improve the activity for the durability of the catalysts. The small-angle X-ray scattering (SAXS) and the X-ray absorption spectroscopy (XAS) measurements are widely used to investigate these properties. Recently, we developed a measurement system of SAXS and fluorescence-yield (FY)XAS in the same observed area to investigate the structural and the chemical properties of low-concentrated catalysts. [1] As the results, the diameter size, distributions, and chemical state of catalysis were successfully characterized. Our measurement system also found to be easy to combine the electrochemical cell. In this study, we newly developed a channel flow cell to control the electrochemical conditions during SAXS and FYXAS measurement in the same observed area. All experiments were carried out at SPring-8 BL19B2. Electrochemical measurements were conducted using a newly developed channel flow cell. Using the flow system, the electrolyte solution with a controlled temperature was continuously supplied to the cell during the in-situ measurements. A commercial Pt/carbon black catalyst(TEC10E50E, Tanaka Kikinzoku Kogyo) was used as the sample in this experiment. In situ SAXS and FYXAS measurement was conducted while holding the potential at 0.4 V, 1.0 V, and 1.5 V vs. the reversible hydrogen electrode (RHE). For the XAS measurements, the Pt LⅢ edge was used. The ionization chamber and a single-element Ge solid state detector were used to obtain the FYXAS spectra. For the SAXS measurements, the X-ray energy was set to 11.5 keV and 11.55 keV, which are below and near to the Pt LIII absorption edge, respectively. The scattered X-ray was detected using an area detector (PILATUS 2M). As the result, we could estimate the diameter size, distributions, and chemical state of Pt catalysis under controlling the electrochemical conditions. We will discuss the degradation mechanism of the Pt nanoparticle catalyst from both SAXS and FYXAS data. Takeshi Watanabe, et al., ECS 236th I01D-1593, (2019) Atlanta, USA.

On the performance and stability of proton exchange membrane fuel cells (PEMFCs), the distribution of water inside the membrane during the power generation has a direct, profound influence, especially for the automotive applications. In this study, coherent anti-Stokes Raman scattering (CARS) spectroscopy was applied to investigating the distribution of water and its chemical states inside the membrane with high spatial (10 μm φ (area) ×1 μm (depth)) and time (0.5 s) resolutions during the power generation. Figure 1 shows a schematic illustration of the measurement system. Two laser lights, pump light (ω 1: 532 nm) and Stokes light (ω 2: 550-700 nm), were set coaxial and irradiated to the membrane inside the single fuel cell having a glass window in the middle on the cathode-side endplate1. The CARS light emitted was separated by filters and introduced into a spectrometer. Five measurement locations were set from the membrane surface (0 μm) of the cathode side through the membrane to that of the anode side separated by 5μm. Each spectrum was recorded every 0.5 s at each location during the current-density jump from 0.1 to 1.0 A cm−2. The number of water molecules per sulfonic acid group,λ, was quantified using spectral information from a calibration curve created in advance2. Figure 2 shows the number of water molecules per sulfonic acid group, λ, at five locations of the membrane with the ohmic resistance and the cell voltage during the current-density jump. Upon the jump, the cell voltage dropped abruptly but increased slowly and reached a steady state, whereas the ohmic resistance of the cell decreased abruptly and increased immediately to reach a steady state after the current density jump. Upon the jump, the λ value at 0 μm overshot with a peak at 4.5 s and reached a steady state at 7.0 s. At other locations, a gradual increase in λ was observed corresponding to the electro-osmosis drag and the back diffusion of water. The results of the analysis under conditions other than those in Fig. 2 will also be reported. Nishiyama et al. J. Phys. Chem. C 2020, 124, 18, 9703–9711 Hara et al. Electrochim. Acta 2012, 82, 277-283 Figure 1

One of the major goals of surface science researches is to clarify atomic scale characteristics of the structures of electrode surfaces during reaction processes, since many electrochemical reactions on electrode surfaces are structure-sensitive [1]. Correlational studies between surface structure and catalytic activity typically employ two approaches, the researches based on (1) single-crystal surfaces with steps and (2) nanoparticles supported on electrodes. The electrocatalytic properties of nanoparticles, especially Pt, have been studied to gain fundamental knowledge on the behavior of electrocatalysts. The development of Pt nanoparticles with appropriate activities, which is tailored as a function of parameters such as the structure of catalyst surfaces, the particle size and shape of the catalyst, and the nature of the substrate, is important for technological application of the electrocatalysts. We previously reported the electrodeposition of Pt from spontaneously adsorbed Pt complexes with various halide ligands on Au(111) [2]. However, the effect of substrate structures on adlayer formation and deposition of spontaneously adsorbed Pt complexes is still obscure, and a systematic investigation of the interplay of deposition conditions, Pt adlayer morphology, and electrocatalytic behavior of electro-reactions such as oxygen reduction reaction (ORR) is required. In this study, we observed the structures and electrochemical deposition of Pt complex adlayers, PtCl4 2- and PtBr4 2- on Au(100), (111), and (110) surfaces, by using electrochemical scanning tunneling microscopy (EC-STM) in 0.1 M HClO4 solution, and evaluated the catalytic activities for ORR on the Pt nanoparticles-modified Au electrodes [3]. Single-crystal Au electrodes were prepared by the method of Clavilier et al. as substrates. Adlayers of K2PtCl4 were prepared by immersing the Au electrodes for 1 minute into a 100 μM K2PtCl4 + 0.1 M HClO4 solution. Subsequently, the sample was washed with water, and then immersed in 0.1 M HClO4, following the previously report [2]. Electrochemical measurements were carried out using an HAB-151A potentiostat (Hokuto Denko) and EC-STM images were obtained with a Nanoscope E (Digital Instruments [currently Bruker Co.]). The STM tip was made from a Pt/Ir wire, and the tips were then coated with a transparent nail polish to minimize the faradic current. Figure 1 shows the cyclic voltammograms for the deposition of the K2PtCl4 adlayer in 0.1 M HClO4 on Au(100). The potential was first swept negatively from 0.95 V, and a large reduction peak due to the reduction of adsorbed PtCl4 2- adlayer and deposition of Pt nanoparticles was observed at 0.72 V only in the first scan. The result reveals the formation of a stable PtCl4 2- monolayer on Au(100), deposition of Pt(0) from the Pt complexes, and no remaining Pt complexes in the electrolyte solution. The estimated coverage of PtCl4 2- on Au(100) calculated from the charge density of a reduction peak, ca. 46 μC cm-2, is 0.12. After Pt deposition, a broad redox peak corresponding to hydrogen adsorption and desorption on Pt appeared around 0.10 ~ 0.30 V, which was not observed on bare Au electrodes. The high-resolution STM image in Fig. 2(a) shows the adlayer structure of PtCl4 2- as bright pinwheel-like spots on the Au(100) electrodes observed by EC-STM at 0.95 V. The structure of the ordered PtCl4 2- adlayer exhibited a square arrangement with a nearest-neighbor Pt complexes distance of 0.63 nm, which corresponds to √5 times the Au-Au distance. Our result suggested that the PtCl4 2- adlayer on Au(100) formed a (√5 × √5)R26.7° structure, whose model with a unit cell is depicted in Fig. 2(b). The coverage of PtCl4 2- calculated from the model is 0.20, and the coverage is higher than that calculated from CV measurements because of the co-adsorption of chloride anions originated from the coordinated chloride of the PtCl4 2- complexes. Then, we stepped the electrode potential to 0.40 V and observed the Pt nanoparticles deposited on Au(100) formed from PtCl4 2 adlayer. Figure 3 shows an STM image of the Pt deposited Au(100) surface, where island-like spots with 0.2 ~ 0.5 nm in height appeared and were distributed randomly on terraces and step edges. Pit-like defects were also observed on the terrace because of surface etching by chloride ion derived from the PtCl4 2- complex. In our talk, we will introduce the results of ORR on the Pt nanoparticles-deposited Au(100) surface and compare to that on Pt deposited Au(111) and Au(110) surfaces. References [1] K. Itaya, Prog. Surf. Sci. 58 (1998) 121. [2] Y. Nagahara, M. Hara, S. Yoshimoto, J. Inukai, S.-L. Yau, K. Itaya, J. Phys. Chem. B 108 (2004) 3224. [3] M. Hara, K. Suto, J. Inukai, S. Yoshimoto, K. Itaya, J. Electroanal. Chem., in press, https://doi.org/10.1016/j.jelechem.2020.114019. Figure 1

In recent years, electrochemical capacitors, also called supercapacitors, have attracted attention because of their high power density and long cycle life compared with secondary batteries. Manganese oxide having a high theoretical capacity has been actively studied as a typical electrode material for redox capacitors. Zuo et al. reported that an aqueous asymmetric capacitor combining a divalent Mn-Ni oxide solid solution (Mn-Ni-O) electrode and an activated carbon electrode exhibited an operating voltage up to 2.4 V.1 However, in the paper, essential parameters such as specific capacitance and energy density based on mass of electrode materials are not shown, and it is also unclear whether the synthesis condition and microstructure of Mn-Ni-O are optimal. For further improvement of supercapacitive performance of the Mn-based oxide electrodes, these issues need to be clarified. In this study, the synthesis conditions of Mn-Ni-O were examined, and the potential window was successfully expanded to an upper limit of 1.5 V vs. Ag/AgCl in LiCl aqueous solution. This led to achievement of the cell voltage of 2.5 V in an asymmetric capacitor with an activated carbon (AC) electrode. Moreover, relationship between the oxidation number and potential of divalent Mn-Ni-O electrode was estimated and discussed to clarify the charge-discharge mechanism. Mn-Ni-O was synthesized by hydrothermal method and subsequent heat treatment similar to those described in the literature.1 Carbon was introduced to the active material to improve the conductivity. These were deposited on a Ti mesh substrate to prepare an Mn-Ni-O/C electrode. The synthesis conditions such as temperature and concentration of constituents in precursor solution were optimized. The resultant electrodes were characterized by SEM, EDX, XRD, and ICP. The electrochemical characteristics were evaluated by cyclic voltammetry (CV) in a 1 M LiCl aqueous solution at room temperature, using an AC electrode as a counter electrode and an Ag/AgCl/saturated KCl electrode as a reference electrode. An aqueous asymmetric capacitor was assembled using an Mn-Ni-O/C electrode as the cathode and an AC electrode as the anode. Its capacitive performance was investigated by galvanostatic charge-discharge (GCD) tests. In order to obtain an insight for charge-discharge mechanism of the electrode, average oxidation numbers of Mn were estimated by XPS after holding the electrode at various potentials of -0.2, 0, 0.5, 1.0, and 1.5 V. For an optimized Mn-Ni-O/C electrode, it was found that oxide solid solution with composition of Mn0.75Ni0.25O was deposited relatively uniformly on the Ti mesh substrate. Fig. 1 shows CV curves of the Mn-Ni-O/C electrodes. It was found that the optimized electrode had a potential window up to 1.5 V vs. Ag/AgCl and a specific capacitance of 363 F g-1 (based on mass of Mn-Ni-O). This electrode was synthesized with decreasing the conventional concentration of NH4F in precursor solution from 0.2 M as described in the literature1 to 0.1 M, resulting in significant improvement in the specific capacitance. This can be ascribed to an increase in the utilization ratio of the active material by forming further uniform, dense and thin layer on the Ti mesh substrate. As shown in Fig. 2, an aqueous asymmetric capacitor cell with the optimized Mn-Ni-O/C and AC electrodes exhibited an upper limit voltage of 2.5 V in GCD tests. The energy density and average power density (based on total mass of Mn-Ni-O and AC) can be calculated as 23.3 Wh kg-1 and 12.7 kW kg-1, respectively. The average oxidation numbers of Mn at different electrode potentials were shown in Fig. 3. It was indicated for the first time that the oxidation number of Mn in the electrode changed nearly from divalent to tetravalent during charge and discharge. Considering such a large available range of the oxidation number compared with conventional MnO2-based electrodes, divalent MnO-based electrode materials can be very important candidates for further increasing the capacitance and energy density of supercapacitors. Acknowledgement This work was supported by JSPS Grants-in Aid for Scientific Research (C) JP19K05596. References 1) W. Zuo et al., Adv. Mater., 29, 1703463 (2017). Figure 1

The polymer electrolyte fuel cells (PEFCs) have attracted attentions as energy-conversion devices of high power density with no emission of carbon dioxide. For the efficient and stable power generation of a PEFC, the water management inside the membrane-electrode assembly (MEA) is very important. A proper water management can adequately moisten the electrolyte membrane and improve the performance of a PEFC, while an excessive water can block the flow channels and the pores in gas diffusion layers (GDLs), as well as the catalyst layers; the mass transports are inhibited from the flow channels to the Pt catalysts resulting in a poor power generation. In case of large liquid water accumulations, the performance of the power generation is temporary decreased.1 Neutron imaging techniques have been used for understanding the distribution of liquid water inside PEFCs, and water droplets within the pores of the GDL and/or on the flow-channel walls have been mainly reported.2–4 However, the distribution of smaller amounts of water contained in catalyst layers or an electrolyte membrane has not been reported, which is very important because of the large effect on the mass transport inside the MEA. The visualization of water in an MEA was carried out using a pulse neutron beam at BL22 (RADEN) of MLF, J-PARC, Japan 5. The neutrons with the wavelength of 5-13.5 Å was separated and used to increase the attenuation of the neutron beam by water. A PEFC with 10 straight channels, 30 mm in length and 1 mm in width, was used. The endplates and current collectors were made of aluminum alloy (A7075) for the neutron penetration. The transmitted neutrons were converted to photons by a scintillator (ZnS(Li)) for imaging with a CCD camera. The neutron beam size was 102.4 mm ×102.4 mm (2048×2048 pixels), while a plate containing boron carbide (B4C) powders was used for a window of 50 mm × 50 mm to reduce the unpreferable radio activation of the cell. The cell temperature was kept at 80 oC. To obtain the background image, a dry N2 gas was supplied both to the anode and the cathode. Figures 1(a) and 1(b) show the background image under the dry condition and the image of water in the MEA at 100% RH (obtained by the division by the background image using an “ImageJ” software), respectively. The values at pixels aligned in the perpendicular direction in the area surrounded by yellow solid lines in Fig. 1(b) were integrated to be shown in Fig. 1(c). The neutron transmittance at the MEA shown by a red rectangle in Fig. 1(b) was lower than that in the other area. In this way, water in the MEA was detected by neutron imaging for the first time. The imaging of water during the power generation is now in progress, along with the qualification of water. Reference Hirakata, S. et al. Investigation of the effect of pore diameter of gas diffusion layers on cold start behavior and cell performance of polymer electrolyte membrane fuel cells. Electrochim. Acta 108, 304–312 (2013). Kramer, D. et al. In situ diagnostic of two-phase flow phenomena in polymer electrolyte fuel cells by neutron imaging: Part A. Experimental, data treatment, and quantification. Electrochim. Acta 50, 2603–2614 (2005). Turhan, A. et al. Quantification of liquid water accumulation and distribution in a polymer electrolyte fuel cell using neutron imaging. J. Power Sources 160, 1195–1203 (2006). Park, J. et al. Neutron imaging investigation of liquid water distribution in and the performance of a PEM fuel cell. Int. J. Hydrogen Energy 33, 3373–3384 (2008). Shinohara, T. et al. The energy-resolved neutron imaging system , RADEN. Rev. Sci. Instrum. 91, 043302 (2020). Acknowledgment This study was supported by Japan Science and Technology Agency (JST) and “SPer-FC Project” of New Energy and Industrial Technology Development Organization (NEDO). The neutron imaging was performed at BL22 (RADEN) of J-PARC (Proposal No. 2019A0217). Figure 1

Three-dimensional structures of polymer electrolyte membranes and binders under controlled temperature and humidity, as well as the concentrations of water in the electrolytes, are important for designing ionomers for fuel cells. A large body of work exists on bulk electrolyte membrane and catalyst layer, but less is known about the structure and behavior of ionomer on the nanometer scale especially at the interfaces. Neutron reflectometry (NR) is known as a powerful technique to study layered structures, but the investigations of polymer electrolyte thin films have been conducted mainly at room temperature, and thin films of anion exchange ionomers have rarely been examined. Here, under controlled temperature and humidity, we investigated the distributions of water by NR inside thin films of Nafion and BAF-QAF (Fig. 1), proton and anion exchange ionomers, respectively.1-3 Thin films were prepared by spin-coating dispersions of Nafion/alcohol and BAF-QAF/N,N-dimethylacetamide (DMAc) on SiO2/Si(100) and Pt/SiO2/Si(100). The Nafion and BAF-QAF specimen films were placed in an environment-controlled chamber continuously purged with N2 gas humidified with H2O for Nafion and D2O for BAF-QAF. NR measurements were carried out at the beamline BL16 of MLF, J-PARC, Japan. Figure 2 shows the NR profiles of Nafion (I) and BAF-QAF (II) films on SiO2, and Nafion (III) and BAF-QAF (IV) films on Pt. The symbols show the reflectivity data, whereas the solid lines the best-fit curves. Clear and different NR modulations were observed for Nafion and BAF-QAF films on different substrates. Figure 3 shows the structure models of Nafion on SiO2 and on Pt at 80 ° C and 80% RH, proposed from the fitting of NR curves (Fig. 2). On the SiO2 substrate, a 4-sublayered structure model was proposed for the Nafion film (Fig. 3(a)). The “Topmost Layer” of the interface between Nafion and humidified N2, 9.1 nm thick, contained only 0.20 g cm-3 of H2O. A large portion of the Nafion film was occupied by the "Bulk Layer" with 0.24 g cm-3 of H2O. The thickness of the “Intermediate Layer” was found to be 3.0 nm with an increasing H2O density of 0.44 g cm-3. The 1.2-nm thick “SiO2-attached Layer” at the interface between Nafion and SiO2 showed an extremely high H2O density of 0.97 g cm-3. On the Pt substrate, a 3-sublayered model was proposed to the Nafion film (Fig. 3(b)). The H2O density of the “Topmost Layer” was 0.22 g cm-3. A large portion of the Nafion film on Pt was also occupied by the “Bulk Layer” with a small H2O density, 0.16 g cm-3. The thickness of the "Pt-attached Layer" was less than 1 nm, and the density of water was 0.60 g cm-3, smaller than that on SiO2. Figure 4 shows the 3-sublayered structure models of the BAF-QAF films on SiO2 and on Pt at 60 ° C and 90% RH. The “Topmost Layer” of BAF-QAF on SiO2 contained the largest amount of water among the four specimens examined, with a D2O density of 0.44 g cm-3 (Fig. 4(a)). The thickness of the "SiO2-attached Layer" was 1.1 nm, and the D2O density was 0.26 g cm-3, similar to that of the “Bulk Layer”, 0.24 g cm-3. On Pt, the “Topmost Layer” was found to be rather hydrophobic with 0.15 g cm-3 (Fig. 4(b)). The "Pt-attached Layer", 4.3 nm, had a very small amount of water with a D2O density of 0.09 g cm-3. The water densities, thereby the nanostructures, at the interfaces were very different from those of bulks; the mass and energy transports through the membranes and binders in membrane-electrode assemblies should be much influences by the interfaces. Among all films, the interface between BAF-QAF and Pt was the most hydrophobic; the water density (0.09 g cm-3) was only one sixth of that of Nafion (0.60 g cm-3). In the AEMFCs, the nucleophilicity and basicity of the hydroxide ion increased with the decreasing hydration number of the hydroxide ion, resulting in the rapid chemical degradation of the ammonium groups.4 The hydrophobicity of the anion binder/Pt interface might be a cause of the lower performance and the durability of AEMFCs. (1) T. Kawamoto et al. Jpn. J. Appl. Phys. 2019, 58, SIID01. (2) T. Kawamoto et al. Electrochemistry 2019, 87, 270-275. (3) T. Kimura et al. Langmuir 2020, 36, 4955−4963. (4) D. R. Dekel et al. Chem. Mater. 2017, 29, 4425–4431. Figure 1

1. Introduction Polymer electrolyte fuel cells are expected as next-generation energy devices because of their high energy conversion efficiency and no carbon dioxide emission. However, expensive noble metals such as platinum are used for the catalyst in the commercialized proton exchange membrane fuel cells. To solve this problem, anion exchange membrane fuel cells (AEMFCs) operable with non-noble metal catalysts have been attracting attentions. The low ionic conductivity and chemical instability of the anion exchange membranes (AEMs), however, are barriers to the practical use of AEMFCs. In our laboratory, we have been developing new AEMs with fluorinated hydrophobic groups, such as BAF-QAF1) and QPAF(C6)-42,3) (Fig. 1) to overcome the problems. QPAF(C6)-4 showed a high conductivity and a high mechanical strength but the thermal stability was insufficient. We have newly synthesized QPAF(C4)-4 with shorter perfluoroalkylene chains than those of QPAF(C6)-4 to tune the interpolymer interaction between the hydrophobic chains. The membrane structures and the properties of two membranes, QPAF(C6)-4 and QPAF(C4)-4, are compared. 2. Experimental QPAF(C6)-4 ionomers were synthesized, and the membranes were formed as previously reported.2,3) QPAF(C4)-4 ionomers were synthesized in the same manner as QPAF(C6)-42) except PAF(C4) monomers were used instead of PAF(C6) monomers. Transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) were used to understand the arrangements of the hydrophobic/hydrophilic clusters in membranes. To observe the surface morphology and visualize the current images on the film surfaces, current-sensing atomic force microscopy (CS-AFM) was used.4,5) The water uptake and conductivity were measured in water, and the dynamic mechanical analysis (DMA) was conducted. The storage and loss elastic modules were also measured at different temperatures. 3. Results and discussion By TEM, hydrophilic clusters with a diameter of ca. 2 nm were confirmed in both membranes in vacuum (Fig. 2). In situ SAXS measurements under humidified nitrogen gas revealed the existence of periodic structures with the distance more than two times larger than those obtained between the hydrophilic clusters observed by TEM. The rate of the increase in periodicity was larger for QPAF(C4)-4 when the relative humidity was increased. The QPAF(C4)-4 membrane showed a higher water uptake than that of QPAF(C6)-4, but the conductivity was lower. This decrease in conductivity for QPAF(C4)-4, caused by an excessive film swelling (Fig. 3) confirmed by SAXS, may be related with the lower hydrophobicity with the shorter perfluoroalkylene chains. In DMA curves, QPAF(C4)-4 film retained high storage and loss modules up to ca. 80 ˚C, 15 ˚C higher than those of QPAF(C6)-4 (Fig. 4). By the stress-strain measurements, QPAF(C4)-4 showed a lower strain and a higher maximum stress than QPAF(C6)-4 (Fig. 5). The mechanical improvements of QPAF(C4)-4 may be explained by the improved packing between benzene rings due to the shorter perfluoroalkylene chains in the hydrophobic structure. Other properties are discussed based on the chemical structures (Fig. 1) and the three-dimensional conformations of the membranes. References Kimura, A. Matsumoto, J. Inukai, and K. Miyatake, ACS Appl. Energy Mater., 3, 469 (2020). Ono, J. Miyake, S. Shimada, M. Uchida, and K. Miyatake J. Mater. Chem. A, 43, 21779 (2015). Miyake and K. Miyatake, Sustain. Energy Fuels, 3, 1916 (2019). Hara, T. Kimura, T. Nakamura, M. Shimada, H. Ono, S. Shimada, K. Miyatake, M. Uchida, J. Inukai, and M. Watanabe, Langmuir, 32, 9557 (2016). Kimura, R. Akiyama, K. Miyatake, and J. Inukai, J. Power Sources, 375, 397 (2018). Figure 1

We investigated the adlayer structures of Pt complexes, PtCl42− and PtBr42−, by using in situ scanning tunneling microscopy (STM) and the electrochemical behavior on the Pt nanoparticles deposited from the PtCl42− and PtBr42− precursors on Au(100), Au(111), and Au(110) surfaces in 0.1 M HClO4. The STM investigation revealed that the PtCl42− and the PtBr42− adlayers had very similar structures on Au(111) and Au(110) surfaces: the (√7 × √7)R19.1° structure on Au(111), and the c(2 × 4) structure on Au(110). On Au(100), PtCl42− formed a (√5 × √5)R26.7° structure, whereas PtBr42− was adsorbed randomly in a flat configuration. High-resolution STM images also revealed pinwheel features of the Pt complexes, resulting from the flat-lying adsorption configurations. The electrochemical reduction of the PtBr42− adlayer proceeded at a more negative potential than the PtCl42− adlayer to form metallic Pt nanoparticles. The average size of the Pt particles varied on the different Au surfaces. The cyclic voltammograms and the catalytic activities for the oxygen reduction reaction were different on the Pt complexes and the three Pt-modified Au(hkl) electrodes. The structures of deposited Pt nanoparticles depended on the substrate Au electrodes. The order of the ORR activity of Pt nanoparticles on Au substrates, Pt/Au(111) > Pt/Au(100) ≥ Pt/Au(110), corresponded to the relative ease of specific adsorption of anions on Pt single-crystal electrodes, Pt(111) > Pt(110) > Pt(100).

[Image: see text] 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/Pt(3)Co(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/Pt(3)Co(111) single crystal is correlated with this specific surface structure.

Visualization inside polymer electrolyte fuel cells (PEFCs) for elucidating the reaction distributions is expected to improve the performance, durability, and stability. An oxygen-sensitive film of a luminescent porphyrin was used to visualize the oxygen partial pressures in five straight gas-flow channels of a running PEFC with liquid-water blockages formed at the end of the channels. The blockage greatly lowered and unstabilized the cell voltage. The oxygen partial pressure decreased nearly to 0 kPa in the blocked channel. With a water blockage in a channel, the oxygen partial pressures in the adjacent channels were lowered due to an extra demand of oxygen consumption. When the number of the blocked channels increased, the oxygen partial pressure in the unblocked channels became much lowered. When the water blockages disappeared, the oxygen partial pressures quickly returned to the values before plugging. The influence of the cross flows of air through the gas diffusion layers in straight channels was much smaller than that in serpentine flow channels.