Conventional Polymer Electrolyte Research Articles

Rechargeable-Liquid Fuel Cells with a Novel Recharging Method: Exploratory Results with IPA and VII/VIII Solutions

Rechargeable-liquid fuel cells (RLFCs) have the potential to be utilized in a wide variety of applications, if the performance of RLFCs can be substantially improved. Two of the major historical barriers have been the relatively poor performance of RLFCs compared to conventional polymer electrolyte fuel cells operating on hydrogen and the irreversibility of the oxygen electrode. This work demonstrates a new recharging concept and improved liquid-electrode designs, which help address both of these issues. These two concepts are demonstrated using two different types of rechargeable liquids: 1) an alcohol (isopropanol) that is reversibility dehydrogenated to the corresponding ketone (acetone), and 2) a flow-battery vanadium electrolyte (VII/VIII dissolved in H2SO4). The liquid electrodes utilize interdigitated flow fields, which enable relatively good power densities for a RLFC during discharge with air: 0.14 W/cm2 at 0.6 V with 2 M IPA and 0.42 W/cm2 at 0.7 V with 1.5 M VII/VIII. It is also shown that the liquids can be efficiently recharged using the same cell that was used for the discharge, by supplying hydrogen to the gas electrode to generate protons via the hydrogen-oxidation reaction instead of splitting water, which requires a different catalyst or a bidirectional catalyst.

Polymer electrolytes based on a homogeneous poly(ethylene glycol) network and their application to polymer actuators

Polymer electrolytes consisting of a homogeneous poly(ethylene glycol) (PEG) network and Li[NTf2] ([NTf2]: bis(trifluoromethanesulfonyl)amide) were prepared by an addition reaction between two tetra-arm PEGs (tetra-PEG; Mn = 10,000) with a maleimide group and an amino group at the polymer ends. Their properties were compared with those containing [C2mim][NTf2] ([C2mim]: 1-ethyl-3-methylimidazolium). Michael addition reaction between the maleimide and amino groups yielded polymer electrolyte membranes with homogeneous network structures. The glass transition temperature (Tg) of the polymer electrolytes containing Li[NTf2] was observed to be lower than those of conventional polymer electrolytes using PEG. The low Tg values contributed to relatively high ionic conductivity (6.3 × 10−5 S cm−1 at 30 °C with [Li]/[O] = 0.10) compared to that of conventional polymer electrolytes. Moreover, superior mechanical properties including an elastic modulus of 171 kPa and fracture energy of 1890 kJ m−3 were achieved due to the homogeneous network structure. Furthermore, the electroactive actuation behaviors of polymer actuators using polymer electrolytes containing either Li[NTf2] or [C2mim][NTf2] were explored. The Li[NTf2]-based polymer actuator was observed to exhibit larger displacement with slower response compared to the [C2mim][NTf2]-based actuator. The bending directions were opposite in these two actuators. These behaviors were well described by a model that considers the ionic volume and ionic transference number of the polymer electrolytes.

Electrochemical Characterization of Single Lithium-Ion Conducting Polymer Electrolytes Based on sp3 Boron and Poly(ethylene glycol) Bridges

A novel series of single lithium-ion conducting polymer electrolytes (SLICPE) based on sp3 boron and poly(ethylene glycol) (PEG) bridges is presented, in the context of the development of a new generation of batteries, with the aim to overcome the problems related to concentration overpotential and low ion transport numbers in conventional solid polymer electrolytes (SPE). The phase separation generated by the physical mixture of SPE with plasticizers such as poly(ethylene oxide) is still a serious problem. In this work, the use of PEG with different chain lengths, for the polycondensation reaction with LiB(OCH3)4, to synthesize SLICPE allows preventing phase separation while tuning the predominant conduction mechanism, and thus the electrical properties, especially the lithium-ion transference number. The ionic transport is promoted by chain mobility as the chain length is increased. SLICPE with the best ionic conductivity values (4.95 ± 0.05) × 10-6 S cm-1 was the one synthesized from poly(ethylene glycol) with an average MN of 400 (BEG8), having an O/Li+ ratio of 20. The lithium transference number ( tLi+) and electrochemical stability window of SLICPE membranes at 25 °C decreased as the PEG bridge length between sp3 boron atoms increased from 0.97 to 0.88 and 5.4 to 4.2 V vs Li0/Li+, respectively, for SLICPE synthesized from PEG with an average MN of 50-400 (BEG1 to BEG8).

(Keynote) Innovative Polymer Electrolytes for Secondary Lithium and Magnesium Batteries

The development of new electrolytes is a crucial target to devise secondary batteries running on alkaline- and alkaline-earth elements characterized by a high specific energy and power and an extensive cyclability, able to provide power for a wide range of applications ranging from portable electronic devices to electric vehicles. The electrolytes must satisfy very demanding requirements, including: (a) easy migration of the alkaline- and alkaline-earth cations between the electrodes of the battery; (b) high compatibility with all the other functional materials used in the assembly of the device; (c) wide potential window and excellent chemical stability. This report summarizes the preparation and characterization of innovative families of electrolytes meant to address these issues and opens the way for future research trends in this field. The investigated materials comprise: (i) single-ion-conducting nanocomposite polymer electrolytes; (ii) solid-state single-ion-conducting polymers; and (iii) conventional polymer electrolytes. In detail, the first approach comprises the study of nanocomposite polymer electrolytes (nCPEs) consisting of PEG400 and innovative lithiated fluorinated nanofillers (e.g., titanium or iron oxide) whose surface anion groups are neutralized with Li+ cations. These materials are single-ion conductors, and at room temperature they exhibit a Li+ conductivity higher than 10-5 S/cm. The second approach includes the investigation of Poly(vinyl alcohol)-based solid polymer electrolytes which are obtained by direct lithiation of partially hydrolyzed poly(vinyl alcohol), forming a lithium-poly(vinyl alkoxide) macromolecular salt. In this case it is possible to demonstrate a conductivity higher than 10-5 S/cm at RT upon plasticization with EMImTFSI ionic liquid. Finally, Broadband Electrical Spectroscopy (BES) studies of classic polymer electrolytes and innovative Magnesium salts for secondary Mg batteries will be presented. This technique will allow for the detailed elucidation of the interplay taking place between structure and conductivity of in these systems, thus tailoring the design of future polymer electrolytes. The chemical composition of the studied materials is analyzed by ICP-AES and microanalysis. The thermal properties are investigated by HR-TG and DSC measurements. The structure and the interactions in the materials are studied by vibrational spectroscopies (FT-MIR and –FIR). The electric response is elucidated by Broadband Electrical Spectroscopy (BES). Results allow to propose a conduction mechanism and to define the interplay existing between structural, thermal transitions and electrochemical properties of proposed innovative electrolytes. Acknowledgement This project has received funding from the BIRD 2016 program of UNIPD.

Experimental Study on Ionic Conductivity of Carbon Support within Polymer Electrolyte Fuel Cell Catalyst Layers

Conventional polymer electrolyte fuel cells (PEFCs) use carbon supported Platinum (Pt/C) as the catalyst. With decades of efforts, researchers have brought down the loading of Pt below 0.1 mg/cm2 on the anode and 0.4 mg/cm2 on the cathode. However, the high cost of Pt based catalyst still serves as a big barrier for PEFCs’ fully commercialization. Within the catalyst layer, polymer electrolyte ionomer (e.g. Nafion) is usually used as the proton conducting media. During the catalyst layer fabrication process, Pt/C particles are present as aggregates. Due to size exclusion, the polymer electrolyte mainly coats the large mesopores of the Pt/C aggregates, i.e. ionomer covers the secondary pores. In high surface area carbon supports, a significant amount of Pt exists within the smaller mesopores and micropores of the support, which is inaccessible to the ionomer. Although it is known that protons are able to reach Pt within the aggregates when those pores are filled with liquid water, the magnitude and mechanism of proton transport is largely unknown. In past studies, the proton conductivity of continuous Pt surfaces have been studied (1), with hypothesis proposing that proton can transport through Pt surface by surface diffusion or through electric double layer (EDL) at the metal/water interface. It has also been reported that proton conductivity is severely hindered over extended glassy carbon surfaces (2). However, the widely used Pt/C catalyst doesn’t have continuous metal surface, and unlike glassy carbon, the carbon support used in the catalyst layer usually has a mixture of functional groups on the surface (e.g., carboxyl and sulfur species), which will alter the surface ionic conductivity through deprotonation. In this work, we measure the conductivity in ionomer free carbon (Vulcan XC72R) and ionomer free Pt/C (10 wt.% HiSPEC 2000) electrodes to better understand the conduction properties of ionomer-free carbon support surfaces. Our experiments evaluate the effects of both electrode potential and the relative humidity (RH) on the conductivity. Electrochemical impedance spectroscopy (EIS) and transmission line modeling show the proton conductivity of the ionomer-free electrode decreases as electrodes’ potential increase (see Figure 1). The potential dependence of the ionic conductivity is supported by a cyclic voltammetry (CV) analysis. Herein, we present several aspects related with the proton conductivity within the ionomer-free carbon domains of the catalyst layer and their effects on fuel cell performances. Acknowledgement This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271. References E. L. Thompson and D. Baker, ECS Trans., 41, 709 (2011).U. Paulus, Z. Veziridis, B. Schnyder, M. Kuhnke, G. Scherer and A. Wokaun, J. Electroanalytical Chemistry, 541, 77 (2003). Figure 1

Highly Active Pt‒M (M=Co, Fe) Alloy Catalysts for the Hydrogen Evolution Reaction in Polymer Electrolyte Water Electrolysis

Conventional polymer electrolyte water electrolysis (PEWE) requires a large amount of noble metal catalysts such as Pt and/or Ir. Platinum (ranging from 0.5 to 2 mgPt cm−2) has been used as the cathode catalyst for the hydrogen evolution reaction (HER).1, 2 Development of highly active and durable catalysts for the PEWE is essential for the efficient production of hydrogen with reduced amounts of noble metals.3 While some Pt alloys have been examined for the HER in the PEWE, the durability was not sufficient due to dealloying.2 Recently, we have found that PtxAL–PtM (M=Fe, Co, Ni)/C (with stabilized Pt skin of 1 to 2 atomic layers, PtxAL) nanoparticles on carbon supports exhibited superlative activity for the hydrogen oxidation reaction (HOR) in H2-saturated 0.1 M HClO4 at 70 and 90ºC.4 In this presentation, we demonstrate that the PtxAL–PtM and PtM/C catalysts also exhibited higher activity for the HER, compared with that of Pt. Pt3Co/C catalyst was prepared by the nanocapsule method,5 followed by heat-treatment in 5% H2 (denoted as Pt3CoHT/C). The PtxAL–PtM/C catalysts were prepared in the same manner as that described previously.6, 7 Two commercial catalysts, c-Pt/C (d TEM =2.6±0.9 nm) and c-Pt3Co/C (d TEM =5.1±1.8 nm), were used for comparison. To measure the HER activities of the catalysts in H2-saturated 0.1 M HClO4 solution, the channel flow electrode (CFE) cell4 was used to minimize the ohmic loss by H2 bubble evolution. Each catalyst was uniformly dispersed on an Au substrate as the working electrode in the CFE cell with a constant loading of carbon support of 11 µg cm−2, which corresponds to approximately two monolayers in height of the carbon particles. Nafion film was coated on the catalyst layer with an average thickness of 0.10 µm. It was found for the Pt3CoHT/C by TEM, XRD and ICP that Pt3Co nanoparticles with narrow size-distribution (d TEM =2.9±0.5 nm) were uniformly dispersed on the carbon support. The values of electrochemically active area (ECA) of all catalysts examined accorded well with those estimated from d TEM with the assumption of spherical particles. Figure 1(a) shows iR-free polarization curves for our Pt3CoHT/C and commercial catalysts at 80ºC in H2-saturated 0.1 M HClO4 with the mean flow rate U m = 111 cm s−1. Apparent area-specific activities j S for the HER at −20 mV vs. reversible hydrogen electrode (RHE) are summarized in Fig. 1(b). The value of j S on the Pt3CoHT/C was about 1.6 times higher than that of c-Pt3Co/C and 3 times higher than that of c-Pt/C. The value of apparent mass activity MA at −20 mV on the Pt3CoHT/C was as high as 4.3 A mgPt −1 (see Fig. 1(a)), indicating the possibility of reduction of the Pt cathode loading to a level as low as 0.23 mgPt cm−2 for operation at 1 A cm−2 with an overpotential of −20 mV. Evaluation of performance and durability of PEWE cells with these cathode catalysts is now in progress. This work was supported by “Fundamental Research on Highly Efficient Polymer Electrolyte Water Electrolyzers with Low Noble Metal Electrocatalysts” from Grant-in-Aid No. 17H01229 for Scientific Research (A) from the Ministry of Education, Science, Sports and Culture, Japan. References M. Carmo, D. L. Fritz, J. Mergel, and D. Stolten, Int. J. Hydrogen Energy, 38, 4901 (2013).M. K. Debe, S. M. Hendricks, G. D. Vernstrom, M. Meyers, M. Brostrom, M. Stephens, Q. Chan, J. Willey, M. Hamden, C. K. Mittelsteadt, C. B. Capuano, K. E. Ayers, and E. B. Anderson, J. Electrochem. Soc., 159, K165 (2012).H. Ohno, S. Nohara, K. Kakinuma, M. Uchida, A. Miyake, S. Deki, and H. Uchida, J. Electrochem. Soc., 164, F944 (2017).G. Y. Shi, H. Yano, D. A. Tryk, A. Iiyama, and H. Uchida, ACS Catal., 7, 267 (2017).H. Yano, J-M. Song, H. Uchida, and M. Watanabe, J. Phys. Chem. C, 112, 8372 (2008).M. Watanabe, H. Yano, D. A. Tryk, and H. Uchida, J. Electrochem. Soc., 163, F455 (2016).M. Chiwata, H. Yano, S. Ogawa, M. Watanabe, A. Iiyama, and H. Uchida, Electrochemistry, 84, 133 (2016). Figure 1

Impact of Ionomer Dispersion on PEM Fuel Cell Performance

Conventional polymer electrolyte membrane (PEM) fuel cell electrodes using aqueous ionomer dispersion suffer from high manufacturing costs and rapid performance degradation. We have investigated an innovative method of designing fuel cell electrodes to reduce manufacturing costs and extend the lifetime. Our approach to obtaining good fuel cell performance is to; 1) understand ionomer particle morphology in dispersion; 2) investigate the electrode morphology, catalyst, and ionomer binder distributions in the electrodes; 3) evaluate the electrode performance using various electrodes prepared from different dispersing agents. First, laser diffraction particle size analysis was used to understand the various solvents’ impacts on catalyst ink structure. It can be seen that nPA/H2O and ethylene glycol (EG) solvent systems provide better ink structure, implied by much smaller agglomeration sizes. IPA/H2O and pentanediol based solvent systems exhibit large agglomerations in the ink, which may account for their poor quality. The laser diffraction particle size will be correlated with electrode structure and fuel cell performance later. Low magnification transmission electron microscopy element mapping was then used to characterize the ionomer distribution in these cathode layers. The best ionomer distribution was found in the EG based sample, with ionomer aggregates <50 nm. The porosity and pore size distribution of the electrode layers were also obtained from transmission electron microscopy image analysis. The EG-based electrode shows the lowest porosity (i.e., 13%). It also contains the smallest pore sizes and the highest number of pores. The effect of the dispersing agents on initial fuel cell performance was investigated. In this experiment, four ionomer dispersions were used and compared with nPA/H2O and IPA/H2O. All MEAs had a Pt loading of ~0.20 mgPt/cm2. The IPA/H2O baseline shows very low performance especially in the mass transport region, which is probably attributed to its poor coating quality. The ionomer dispersions have shown significant influence on the fuel cell performance, with a general ranking as: nPA/H2O > ethylene glycol > butanediol > pentanediol). The ethylene glycol based sample displays the best performance of all the non-aqueous samples, and its performance is very comparable to the nPA/H2O baseline.

Globally Suppressed Dynamics in Ion-Doped Polymers.

We investigate how ion-polymer complexation suppresses molecular motion in conventional polymer electrolytes using molecular dynamics (MD) simulations of lithium hexafluorophosphate in poly(ethylene oxide) and a modified Rouse model. The employed model utilizes an inhomogeneous friction distribution to describe ion-polymer interactions and provides an effective way to examine how ion-polymer interactions affect polymer motion. By characterizing the subdiffusive Li+ transport and polymer relaxation times at several salt concentrations, we observe that increases in local friction due to ion-polymer complexation are significantly smaller than previously assumed. We find that a Rouse-based model that only considers local increases in friction cannot simultaneously capture the magnitude of increased polymer relaxation times and the apparent power-law exponent for Li+ subdiffusion observed in MD simulations. This incompatibility is reconciled by augmenting the modified Rouse model with a term that increases the global friction with the square of the salt concentration; this significantly improves the agreement between the model and MD, indicating the importance of ion-ion interactions and distributions on ion/polymer mobility.

Bactericidal and Sporicidal Activities against Pathogenic Bacteria of Direct Flow Electrolyzed Water

The purpose of this investigation was to examine the bactericidal activity of long lifetime ozone water (LLO water) against pathogenic bacteria of medical, veterinary, and public health interest. An electrolytic system using Pt mesh electrodes combined with a conventional polymer electrolyte, the Nafion 117 membrane, produced LLO water from tap water with high efficiency. LLO water killed 7.144 ± 0.0507 log10 CFU of Escherichia coli O157:H7, 6.979 ± 0.045 log10 CFU of methicillin-resistant Staphylococcus aureus, 6.503 ± 0.3681 log 10 CFU of Pseudomonas aeruginosa and 6.579 ± 0.123 log 10 CFU of Legionella pneumophila within one minute. In contrast, endospores of Bacillus anthracis were killed within 20 min. Results of observation of the bactericidal activity of LLO by using an electron microscope, it was revealed that LLO water was shown the bactericidal action by swelling and destroying on the cell wall of the bacterial cells. Our studies demonstrated that LLO water has the potential to be widely applicable in food hygiene, medicine and veterinary medicine as a sterilizing agent. This is the first study to demonstrate the bactericidal activity and characteristics of LLO water.

Performance comparison of protonic and sodium phosphomolybdovanadate polyoxoanion catholytes within a chemically regenerative redox cathode polymer electrolyte fuel cell

The direct reduction of oxygen in conventional polymer electrolyte fuel cells (PEFCs) is seen by many researchers as a key challenge in PEFC development. Chemically regenerative redox cathode (CRRC) polymer electrolyte fuel cells offer an alternative approach via the indirect reduction of oxygen, improving durability and reducing cost. These systems substitute gaseous oxygen for a liquid catalyst that is reduced at the cathode then oxidised in a regeneration vessel via air bubbling. A key component of a CRRC system is the liquid catalyst or catholyte. To date, phosphomolybdovanadium polyoxometalates with empirical formula H3+nPVnMo12-nO40 have shown the most promise for CRRC PEFC systems. In this work, four catholyte formulations are studied and compared against each other. The catholytes vary in vanadium content, pH and counter ion, with empirical formulas H6PV3Mo9O40, H7PV4Mo8O40, Na3H3PV3Mo9O40 and Na4H3PV4Mo8O40. Thermodynamic properties, cell performance and regeneration rates are measured, generating new insights into how formulation chemistry affects the components of a CRRC system. The results include the best CRRC PEFC performance reported to date, with noticeable advantages over conventional PEFCs. The optimum catholyte formulation is then determined via steady state tests, the results of which will guide further optimization of the catholyte formulation.

Composite-supported Pt catalyst and electrosprayed cathode catalyst layer for polymer electrolyte membrane fuel cell

SUMMARY A major limitation of the conventional polymer electrolyte membrane fuel cell (PEMFC) catalysts is the fast oxidative degradation of their carbon black supports. Complete replacement of carbon black is difficult because of its low-cost and high electrical conductivity. Reported here are the development and optimization of composite-supported Pt catalysts and the electrosprayed cathode catalyst layer with these catalysts for PEMFC. These catalysts are supported by a composite of carbon black (Vulcan XC-72R) and the electrochemically much more stable carbon-embedded niobium-doped titanium dioxide nanofibers (C/Nb0.1Ti0.9O2). Four different catalyst supports with 20 wt.% Pt were prepared by air spraying and electrospraying to compare their activity and stability. Vulcan XC-72R and C/Nb0.1Ti0.9O2 were tested as pristine support materials for comparison as well as 1:3 and 3:1 mixtures by weight of the two pristine support materials (composite supports). The amount of Nafion in the catalyst ink was optimized for each catalyst layer by a volumetric method. An increase in carbon black content of the support layer from 0% to 100% increases the performance of these catalysts in H2/air PEMFCs but also increases the loss of oxygen reduction reaction mass activity. The best balance between PEMFC performance and durability was obtained for the Pt catalyst with 25% carbon black in the support layer, while the highest initial oxygen reduction reaction mass activity was obtained for the catalyst with 75% carbon black content. Copyright © 2017 John Wiley & Sons, Ltd.

Ion conducting nano-composite polymer electrolytes: synthesis and ion transport characterization

Synthesis and ion transport characterization of a new K+-ion conducting nano-composite polymer electrolytes (NCPEs): (1−x) [70PEO:30KBr] + x SiO2, where 0 < x < 20 wt%, are reported. The present NCPEs have been cast using a novel hot-press technique in place of the traditional solution cast method. The conventional solid polymer electrolyte (SPE) composition: (70PEO:30KBr), identified as the highest conducting composition at room temperature, has been used as first-phase host matrix and nano-size (~8 nm) particles of SiO2 as second-phase dispersoid. As a consequence of dispersal of SiO2 in SPE host, two orders of conductivity enhancement have been observed in NCPE composition: [95(70PEO:30KBr) + 5SiO2] and this has been referred to as optimum conducting composition (OCC). The polymer-salt/nano-filler SiO2 complexation and thermal properties characterization were done with the help of XRD, FTIR, SEM, DSC and TGA studies. The ion transport behavior in NCPEs have been discussed on the basis of experimental measurements on some basic ionic parameters, viz. conductivity (σ), ionic mobility (μ), mobile ion concentration (n), ionic transference number (tion), etc. The temperature-dependent conductivity studies of NCPE OCC have been done and activation energy (Ea) value was determined using log σ−1/T Arrhenius plot.

Cost Analysis of Direct Methanol Fuel Cell Stacks for Mass Production

Fuel cells are very promising technologies for efficient electrical energy generation. The development of enhanced system components and new engineering solutions is fundamental for the large-scale deployment of these devices. Besides automotive and stationary applications, fuel cells can be widely used as auxiliary power units (APUs). The concept of a direct methanol fuel cell (DMFC) is based on the direct feed of a methanol solution to the fuel cell anode, thus simplifying safety, delivery, and fuel distribution issues typical of conventional hydrogen-fed polymer electrolyte fuel cells (PEMFCs). In order to evaluate the feasibility of concrete application of DMFC devices, a cost analysis study was carried out in the present work. A 200 W-prototype developed in the framework of a European Project (DURAMET) was selected as the model system. The DMFC stack had a modular structure allowing for a detailed evaluation of cost characteristics related to the specific components. A scale-down approach, focusing on the model device and projected to a mass production, was used. The data used in this analysis were obtained both from research laboratories and industry suppliers specialising in the manufacturing/production of specific stack components. This study demonstrates that mass production can give a concrete perspective for the large-scale diffusion of DMFCs as APUs. The results show that the cost derived for the DMFC stack is relatively close to that of competing technologies and that the introduction of innovative approaches can result in further cost savings.

Dual Gas Diffusion H2/O2 Biofuel Cell in Direct Electron Transfer-Type System

Biofuel cells are energy conversion systems in which enzymes are utilized as biocatalysts. Thanks to the characteristics of the enzyme reaction, the power device is able to operate under very mild and safe conditions (e.g. neutral pH, room temperature, and atmospheric pressure). In addition, the active sites in enzymes consist of nonprecious metals or organic compounds. Although conventional polymer electrolyte H2/O2 fuel cells are known as clean and efficient energy conversion devices, they usually require Pt catalysts, which are expensive. By utilizing hydrogenases and multicopper oxidases (MCOs) as catalysts for H2 oxidation and O2 reduction, respectively, a sustainable and environmentally friendly H2/O2 biofuel cell can be constructed. Bioelectrocatalysis, in which an enzyme reaction and an electrode reaction is coupled, is a key process for biofuel cells. The reaction is classified into two types: mediated electron transfer (MET) and direct electron transfer (DET). In DET-type systems, an enzyme can directly communicates with an electrode. This character makes it possible to construct a compact and simple biofuel cell and to minimize the thermodynamic loss in the electron transfer between an enzyme and an electrode. Combining a [NiFe]-hydrogenase-adsorbed bioanode and a MCO-adsorbed biocathode, we can construct a DET-type H2/O2 biofuel cell. The low solubility of the gaseous substrates (H2 and O2) frequently causes a power decline. To solve this critical problem, a gas diffusion electrode that can supply a gaseous substrate to an enzyme from the gas phase has been proposed. Moreover, the gas diffusion electrode can maintain high substrate concentrations near the active enzyme on the electrode. This system is also useful for overcoming the specific inactivation of [NiFe]-hydrogenases called “oxidative inactivation”, since the inactivation is thought to compete with H2 binding in the catalytic cycle. Furthermore, the gas diffusion electrode allows independent supply of gaseous substrates to the bioanode and the biocathode, thus avoiding the risk of explosion on mixing of H2 with and O2. In this research, we utilized O2-sensitive [NiFe]-hydrogenase from D. vulgaris Miyazaki F (DvMF) and bilirubin oxidase, one of the MCOs, from M. verrucaria (MvBOD) as biocatalysts for H2 oxidation and O2 reduction, respectively. We also constructed a novel gas diffusion bioelectrode with a sheet of waterproof carbon cloth as the electrode base and optimized the hydrophilicity/hydrophobicity of the electrode for both high gas permeability and high DET-type bioelectrocatalytic activity. The electrode exhibited a large catalytic current density of about 10 mA cm−2 in the steady state for both H2 oxidation and O2 reduction. DvMF-adsorbed bioanode and MvBOD-adsorbed biocathode were coupled to construct a dual gas diffusion H2/O2 biofuel cell in DET-type system. The dual gas diffusion system allowed the separate supply of gaseous substrates (H2 and O2) to the bioanode and biocathode, with consequent suppression of the oxidative inhibition of the hydrogenases. The open-circuit voltage of our H2/O2 biofuel cell is 1.14 V and is close to the standard motive force of the ideal H2/O2 cell (1.23 V). The cell exhibited a maximum power density of 8.4 mW cm−2 at a cell voltage of 0.7 V under quiescent conditions and H2 and O2 atmospheric conditions for the bioanode or the biocathode, respectively. The performance of our biofuel cell is 5.6 times better than that of the best H2/O2 biofuel cell and 3.2 times better than that of DET-type biofuel cells with other fuels in the literatures. Figure 1

A New Membrane Electrode Assembly Structure with Novel Flow Fields for Polymer Electrolyte Fuel Cells

A conventional polymer electrolyte fuel cell (PEFC) incorporates a membrane electrode assembly (MEA) which is comprised of the anode catalyst layer-polymer electrolyte -cathode catalyst layer sandwiched between two gas diffusion layers (GDLs), and bipolar plates. Conventional GDL is typically comprised of highly porous carbon paper or cloth to help diffuse the reactant gases onto the electrode, which is coated with a thin micro-porous layer (MPL). At high current densities, mass transport limitations of fuel or oxidizer in the PEFC occur in porous structures of the GDL, particularly at the cathode, which result in a sharp drop in the output voltage. The key to minimizing mass-transport losses is effective water management in the cell. Liquid water in macro-pores of GDL decreases the fuel cell performance at high current density due to the lack of oxygen reaching the catalyst layer. A new MEA structure is recently introduced, where the carbon paper backing layer is eliminated and the entire gas diffusion layer consists of only the MPL [1]. We have further improved on this concept by directly depositing the MPL onto the CCM, resulting in an improvement in the interfacial contact between the MPL and the catalyst layer, as well as a simplified fabrication and assembly process. Spray deposition method was used for depositing this MPL onto a commercial CCM [2]. This concept was proven to work and perform better than a conventional cell with a micro-channel flow field used to provide the desired pathway for the reactant gases throughout the cell substituting for the macro porous carbon paper and the millimeter sized flow field. In this work, a porous foam is utilized as the flow field to distribute the reactant gases over the micro-porous layer instead of the micro-channel flow field. Various pore sizes of the foam, i.e. 60-100 PPI, are used to compare with the conventional micro channel flow field. Contact resistance, permeability, electrochemical impedance spectroscopy, and mechanical properties are used to further characterize the new MEA structure incorporating the porous foam flow field. This method can be an effective way of enabling very high power density operation by reducing the mass transport limitations, and provides an improvement in the interfacial contact between the MEA and the flow field as well as improved mechanical properties. [1] T. Kotaka, Y. Tabuchi, U. Pasaogullari, and C. Y. Wang, Electrochim. Acta, 146, 618 (2014). [2] J. Park, U. Pasaogullari, L. J. Bonville, ECS Transactions, 69, 1355-1362 (2015).

(Keynote) Enzymatic Biofuel Cell Based on Porous Carbon Materials

Enzymatic biofuel cells (EBFCs) are a type of fuel cells, which employ enzymes instead of conventional noble metal catalysts. The working principle is the same as in conventional polymer electrolyte membrane fuel cells, namely fuel is oxidized at the anode side and the electrons reach the cathode, where they combine with an oxygen to water. EBFCs are promising for sustainable green energy applications; however, they are still at an early stage of development, with many yet-to-be-resolved fundamental scientific and engineering problems. Two critical problems are short lifetime and poor power density, both of which are related to enzyme stability, electron transfer rate, and enzyme loading. To achieve the practical application of EBFCs, a promising approach is to use porous carbon materials. Strategies for the designing of hierarchically structured supports composed of mesoporous and macroporous are considered: the large surface area of mesoporous materials can increase the enzyme loading and electron transfer efficiency, and the macropores enable the efficient fuel transport. Our recent paper demonstrated that Magnesium oxide (MgO)-templated porous carbon forms are promising candidates as electrode materials for the elaboration of efficient bioelectrochemical devices. MgO-templated mesoporous carbon (MgOC, mean pore diameter 10, 20, 40, 80 nm) were used to increase the effective specific surface area for specific enzyme immobilization. MgOC particles were deposited on a current collector by an electrophoretic deposition method to generate micro meter-scale macropores to improve the mass transfer of glucose and electrolyte (buffer) ions. To create a glucose bioanode, the porous-carbon-modified electrode was further coated with a biocatalytic hydrogel composed of a conductive redox polymer, flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH), and a cross-linker. The electrode coated with FAD-GDH and a redox polymer showed a 33-fold increase in glucose oxidation current density (ca. 100 mA cm−2) compared to that of the flat electrode. MgOC-modified electrode can be applied for the biocathode using bilirubin oxidase (BOD) as an oxygen reduction reaction catalysts. Our recent wearable EBFC based on MgOC will be presented. FAD-GDH was used as an anodic catalyst. 1,4-naphtoquinone was modified on the MgOC mesopore surface. Bilirubin oxidase and ABTS mediator were modified on the MgOC surface. Aiming at the application of EBFCs to the wearable power devices, we modified the MgOC on the surface of flexible carbon cloth. The cell operation was initiated after addition of 0.4 mL of 1 M phosphate buffer solution containing various concentrations of glucose, ranging from 0.1 – 3 M. The maximum output power density per geometric surface area was 2 mW cm-2, which was limited by the cathode performance, especially O2 supply from the air. The operation time, i.e. cell capacity, was quantitatively determined by the mole number of applied glucose evaluated by galvanostatic experiments at 0.25 – 4 mA cm-2 of constant current density, indicating that the capacity was determined by the consumption of β-d-glucose. After consumption of the fuel, the cells were able to restart by adding the glucose solution. The operation time linearly depended on the mole number of glucose added. In this presentation, recent developments in EBFCs technology using porous carbon materials are highlighted. The essential properties of the resulting materials with respect to the EBFC application are also discussed. A combination of electron transfer technology and porous carbon material would be helpful in achieving a much higher and stable current output, thus contributing to a practical advance in the sustainable energy field.

(Invited) Conventional and Innovative Electrocatalysts for PEM Water Electrolysis

In conventional polymer electrolyte membrane (PEM) water electrolysis cells, platinum particles are used at the cathode for the hydrogen evolution reaction (HER) and iridium oxide particles at the anode for the oxygen evolution reaction (OER). Although noble metal contents are small, the large scale deployment of PEM technology for power-to-gas applications is calling for cheaper materials. Molecular organo-metallic complexes containing transition metals as active centers offer some promising alternative. Examples with cobalt and iron clathrochelates used for the HER are reported in this communication.

High-Performance Flexible Solid-State Supercapacitor with an Extended Nanoregime Interface through in Situ Polymer Electrolyte Generation.

Here, we report an efficient strategy by which a significantly enhanced electrode-electrolyte interface in an electrode for supercapacitor application could be accomplished by allowing in situ polymer gel electrolyte generation inside the nanopores of the electrodes. This unique and highly efficient strategy could be conceived by judiciously maintaining ultraviolet-triggered polymerization of a monomer mixture in the presence of a high-surface-area porous carbon. The method is very simple and scalable, and a prototype, flexible solid-state supercapacitor could even be demonstrated in an encapsulation-free condition by using the commercial-grade electrodes (thickness = 150 μm, area = 12 cm(2), and mass loading = 7.3 mg/cm(2)). This prototype device shows a capacitance of 130 F/g at a substantially reduced internal resistance of 0.5 Ω and a high capacitance retention of 84% after 32000 cycles. The present system is found to be clearly outperforming a similar system derived by using the conventional polymer electrolyte (PVA-H3PO4 as the electrolyte), which could display a capacitance of only 95 F/g, and this value falls to nearly 50% in just 5000 cycles. The superior performance in the present case is credited primarily to the excellent interface formation of the in situ generated polymer electrolyte inside the nanopores of the electrode. Further, the interpenetrated nature of the polymer also helps the device to show a low electron spin resonance and power rate and, most importantly, excellent shelf-life in the unsealed flexible conditions. Because the nature of the electrode-electrolyte interface is the major performance-determining factor in the case of many electrochemical energy storage/conversion systems, along with the supercapacitors, the developed process can also find applications in preparing electrodes for the devices such as lithium-ion batteries, metal-air batteries, polymer electrolyte membrane fuel cells, etc.

Mass and Charge Transport in the Polymer-Ionic-Liquid System PEO-EMImI: From Ionic-Liquid-in-Polymer to Polymer-in-Ionic-Liquid Electrolytes.

Conventional polymer electrolytes based on inorganic salts are commonly characterized and utilized over a small salt-poor composition range because of phase transitions accompanied by loss of ion conductivity at high salt concentrations. By contrast, well-chosen polymer-ionic-liquid (IL) systems offer the possibility to vary the IL content from the IL-in-polymer to the polymer-in-IL domain. We have investigated the temperature-dependent ionic conductivity in PEOyEMImI systems consisting of poly(ethylene oxide) complexed with 1-ethyl-3-methylimidazolium iodide for y = EO/IL ratios ranging from 0.6 to 60 and compared diffusivity data with that arising from (1)H pulsed-field-gradient nuclear magnetic resonance for EMIm and (125)I radiotracer diffusion for iodine. Surprisingly, the diffusivity of cations and anions vary at most by 50% at fixed temperatures over the entire composition range. The much larger changes in the charge diffusivity Dσ relate to ion pairing exhibiting a minimum near the intermediate composition y = 10. Altogether, the results are relevant to application in dye-sensitized solar cells and show that a high ion density is crucial to enhance the iodine transport capacity.

All-solid state supercapacitors operating at 3.5 V by using ionic liquid based polymer electrolytes

All-solid state supercapacitors (SCs) using Ionic Liquid based Polymer Electrolyte (IL-b-PE) and activated carbon were assembled and characterized electrochemically. IL-b-PE consisted of a binary blend of a polymeric ionic liquid (PIL), poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (pDADMATFSI), and their corresponding ionic liquid (IL), N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR14TFSI) in a ratio of 40:60,w/w respectively. Assembling of supercapacitors simply consists of facing two impregnated electrodes without the need of additional separator. Several all-solid state SCs with impregnation ratios (IL-b-PE mass/active material mass) ranging from 5 to 18 were characterized. An all-solid state SC with impregnation ratio of 7 showed the best performance with specific capacitance (Cam) and real energy (Ereal) of 100 F g−1 and 32 Wh kg−1, respectively. After increasing temperatures to 60 °C, the performance of SCs further improved exhibiting Ereal values of 42 Wh kg−1 at 1 mA cm−2. These figures of merit are only slightly lower than those obtained for convectional SCs using pure PYR14TFS and much higher as compared with other solid SCs based on conventional polymer electrolytes. This is mainly due to the high electrochemical stability of this IL-b-PE that allows these solid SCs to operate at maximum voltages as high as 3.5 V for the first time.