bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrolyte Effects on Chemical Stability of NaO2 in Na-O2 Batteries
Authors: Ortiz-Vitoriano Nagore , Amanchukwu Chibueze V , Kwabi David , Hammond Paula T , Shao-Horn Yang
Published: February 2020

Rechargeable metal-air (oxygen) batteries are receiving intense interest as possible alternatives to lithium-ion batteries, because of their potential to provide higher gravimetric energies. 1 While much attention has been focused on aprotic Li-O2 batteries, substantial challenges must be addressed before widespread commercialization is possible. Recently, a metal-air battery in which lithium has been replaced by sodium has received increasing attention.2 Although Na-O2 batteries present lower gravimetric energies on a cell basis (1605 or 1108 Wh/kg based on Na2O2 or NaO2 discharge products, respectively)2 than Li-O2 batteries (3505 Wh/kgLi2O2),1 much lower charge overpotentials (~100 mV) than those in typical Li-O2 batteries (~1000 mV) have been reported based on reversible sodium superoxide (NaO2) formation . Unlike Li-O2 batteries, for which Li2O2 is the only discharge product, NaO2, sodium peroxide (Na2O2) and sodium peroxide dihydrate (Na2O2 · 2H2O), or a mixture, have been identified in Na-O2 batteries in ether-based electrolytes using a range of carbon electrode types.

Here we show NaO2 as the only discharge product from Na-O2 cells with carbon nanotubes in 1,2-dimethoxyethane from X-ray diffraction and Raman spectroscopy. Immediately after discharge, no Na2O2 · 2H2O was found in the discharged electrode, even with H2O present in the electrolyte up to 6000 ppm. However, Na2O2 · 2H2O was detected upon ageing the electrode in the electrolyte, and the amount of Na2O2 · 2H2O was seen to increase with increasing rest time in the electrolyte. More importantly, the reversibility of the reaction is affected by the presence of Na2O2 · 2H2O, where the cells charged immediately after discharge show a reversible reaction , and the cells charged after resting the electrode in the electrolyte for several hours  showed poor reversibility.

The rechargeable nature of a Na-O2 cell is governed by the formation and oxidation of NaO2, making it vital to study the stability of the electrolyte in the presence of NaO2, and also to prevent the formation of irreversible side products.

 

REFERENCES

1. D. G. Kwabi, N. Ortiz-Vitoriano, S. A. Freunberger, Y. Chen, N. Imanishi, P. G. Bruce, Y. Shao-Horn, MRS Bull.
39, 443 (2014).

2. P. Hartmann, C. L. Bender, M. Vračar, A. K. Dürr, A. Garsuch, J. Janek, P. Adelhelm, Nat. Mater.
12, 228 (2013).
Homogeneous and Stable Lithium Electrodeposition through a Thin Single-Ion Conducting Layer for High Cycling Stability of Lithium Metal Secondary Batteries
Authors: Song Jongchan , Lee Hongkyung , Park Jung-Ki , Kim Hee-Tak
Published: February 2020

The inhomogneous and unstable Li electrodeposition of lithium metal electrode has been a major impedment to the realization of rechargeable lithium metal batteries. Although polymeric single ion conductors can provide more homogeneous Li electrodeposition owing to their high Li ion transference number, their poor ionic conductivities prevent practical battery design. In the paper, we sugest a hybrid electrolyte based on a few micron-thick single-ion conducting layer laminated on Li metal electrode and a liquid electrolyte, which allows rapid Li+ transport and stable Li electrodeposition. The introduction of the single-ion conducting layer increases Li+ transference number from 0.451 to 0.855, consequently suppressig the generation of Li dendrite as demonstrated by SEM and impedance analysis. Reducing the thickness of the single-ion conducting layer down to a few microns and incorporating the bi-ionic liquid electrolyte permit room-temperature operation at high current densities.  The Li/Li symmetric cell with the hybrid approach operate at a high current density of 10 mA cm-2 for more than 2000 h, which corresponds to more than five-fold enhancement compared to bare Li metal electrode, and the protyopye Li/LiCoO2 battery offers cycling stability more than 350 cycles at 0.2 C, demonstraing the practical applicability of this approach.
Electrolyte Solutions for Magnesium Rechageable Batteries
Authors: Abe Takeshi , Inoo Akane , Miyazaki Kohei , Fukutsuka Tomokazu
Published: February 2020

Innovative rechargeable batteries beyond the present lithium-ion batteries (LIB) have been investigated for the environmental and energy problem. Among the batteries, rechargeable magnesium metal batteries have attracted much attention, since magnesium metal as negative electrode can bring high volumetric energy density owing to the two-electron reaction, low electrode potential, and lightness. Until now, the electrolyte solutions used for rechargeable magnesium metal batteries are tetrahydrofuran (THF) or 1,2-dimethoxyethane (DME) based electrolyte solutions. The boiling points of THF and DME are 66°C and 85°C, respectively. In order to eliminate the water in the rechargeable magnesium metal battery system, at least 100°C is required for the working temperature. However, THF or DME based electrolyte solutions do not meet the criteria. Therefore, we focused on the triethylene glycol dimethyl ether (triglyme) as a solvent since the boiling point of triglyme is 216°C. As a magnesium salt, magnesium(II) bis(trifluoromethane sulfonyl) amide (Mg(TFSA)2) was selected since TFSA- is a large anion and Mg(TFSA)2 is expected to be highly dissociative in triglyme. By using this system, we can increase the working temperature more than 100°C for rechargeable magnesium metal battery system[1]. As a proof-of-concept, Orikasa et al. reported that a rechargeable magnesium metal battery composed of insertion-type MgFeSiO4 positive electrode can work at 100°C in triglyme-based electrolyte solution [2]. This result indicated that we can fabricate high safety rechargeable magnesium metal batteries at ambient temperature by selecting appropriate positive electrodes. Then, the electrochemical behavior of electrolyte solution at ambient temperature is also important. In this conference, we reports the reversible magnesium metal deposition and dissolution in the Mg(TFSA)2/glymes solution at ambient temperature. In addition, we report new magnesium salt in triglyme.

[1] T. Fukutsuka, K. Asaka, A. Inoo, R. Yasui, K. Miyazaki, T. Abe, K. Nishio, and Y. Uchimoto, Chemistry Letters, 11, 1788-1790 (2014).

[2] Y. Orikasa, T. Masese, Y. Koyama, T. Mori, M. Hattori, K. Yamamoto, T. Okado, Z. D. Huang, T. Minato, C. Tassel, J. Kim, Y. Kobayashi, T. Abe, H. Kageyama, and Y. Uchimoto, Sci. Rep. 2014, doi: 10.1038/srep05622.
The Mg-V2O5 Intercalation Phase Diagram First Principles
Authors: Gautam Gopalakrishnan Sai , Canepa Pieremanuele , Abdellahi Aziz , Urban Alexander , Malik Rahul , Ceder Gerbrand
Published: February 2020

Multi-valent (MV) ion intercalation batteries that replace the Li+ with a MV cation such as Mg2+, provide a realistic and compelling approach to meet the high energy density demanded by the next generation of electronics and vehicles. One of the challenges in achieving high energy density MV-ion systems is to develop a suitable cathode with a high enough voltage and diffusivity of the MV cation. Mg intercalation into V2O5 is one of the very few that has been shown to function reversibly at reasonable efficiency. In this study, we gain insight into the thermodynamics of Mg insertion into V2O5 from first-principles calculations.  We have calculated the 0 K phase diagram, the equilibrium voltage curves and the migration barriers for Mg diffusion in both the α and δ-V2O5 polymorphs. Our calculations indicate that Mg in V2O5 is a “phase separating” system at room temperature and benchmarking our calculations with experiments leads to the conclusion that Mg cycles on a metastable α-V2O5 host. We believe that this study can be further used to improve the performance of V2O5 as a cathode material for Mg-batteries.
Composite of Two-Dimensional Titanium Carbonitride Mxene and Nano-Sulfur As Cathode for Li-S Batteries
Authors: Naguib Michael , Wang Hui , Liang Chengdu , Nanda Jagjit
Published: February 2020

In 2011, a new family of two-dimensional, 2D, transition metal carbides and carbonitrides called MXenes were produced for the first time by etching atomically thin metal layers from ternary layered materials called MAX phases. The latter have a composition of Mn+1AXn, where M stands for early transition metal, A is a metal that belongs to groups 13 and 14 in the periodic table, X is carbon/nitrogen, and n = 1, 2, or 3. So far, nine different MXenes were reported MXenes were reported, viz., Ti3C2, Ti2C, Nb2C, V2C, (Ti0.5,Nb0.5)2C, (V0.5,Cr0.5)3C2, Ti3CN, Ta4C3, and Nb4C3. In addition, many more are predicted to be stable. Since the synthesize process of MXenes is carried in F-contained aqueous systems, the surfaces of the as synthesized material are terminated with a mixture of moieties including O, OH, and F. Bare MXenes are predicted to be metallic conductors while terminated ones are predicted to be semiconductors with relatively small band gaps. Titanium carbonitride is predicted to be an exception since it is predicted to metallic even after termination.   

In this work we used a simple mixing approach to prepare Ti3CN/nano S composite with a high sulfur content of ~75 wt.%, and high mass loading. Analysis for their performance in Li-S batteries and the effect of mass loading in addition to the composite material characterization will be presented.
Rational Design of Cathode Structure in Li-S Batteries
Authors: Chen Liwei
Published: February 2020

As one of the most intensely investigated technologies in the electrochemical energy storage beyong Lithium-ion, lithium-sulfur (Li-S) batteries have observed rapid improvements in their properties in recent years. This presentation reports recent progresses in the understanding and development of Li-S cathode design.

One of the technological bottlenecks in Li-S batteries is the insulating nature of elementary sulfur and the discharging products Li2S2 and Li2S. The slow charge transfer process in the cathode structure thus imposes restrictions on the degree of sulfur utilization, initial capacity and the rate performance.The nano-size effect in Li-S cathodic charge transfer has been thoroughly investigated. S nanoparticles with diameters below 50 nm were fabricated via a membrane-assisted precipitation method. With conductive PEDOT coating, these S nanoparticles show high specific capacity and improved cycle stability. The study indicated that nanosized sulfur required shorter charge diffusion distance and thus showed higher capacity (Sci.Rep.2013,3,1910). Further understanding of the nano-size effect needs better size-controlled S materials, which were obtained using the sulfur-amine chemistry recently developed (Chem. Commun., 2014,50,1202). A series of mono-dispersed S nanoparticles with diameters ranging from 5 nm to 150 nm were employed to investigate the size-dependent charge transfer process and the influence on device performance. Experimental results revealed much improved specific capacity, rate performance and cycle stability upon the reduction of S nanoparticle size. Importantly, the 5 nm S particles showed an initial capacity of 1670 mAh/g, which is the theoretical limit of sulfur (Nano Lett. 2015, 15, 798). This work demonstrates that the high theoretical performance can indeed be realized. The understanding of S cathode charge transfer will help future cathodic design in the Li-S technology.

References:

1. Hongwei Chen, Changhong Wang, Weiling Dong, Wei Lu, Zhaolong Du and Liwei Chen* “Mono-dispersed Sulfur Nanoparticles for Lithium-Sulfur Batteries with Theoretical Performance”
Nano Letters
, 15, 798-802 (2015)

2. Changhong Wang, Hongwei Chen, Weiling Dong, Jun Ge, Wei Lu*, Xiaodong Wu, Lin Guo and Liwei Chen*,”Sulfur-amine Chemistry Based Synthesis of Multi-walled Carbon Nanotube-Sulfur Composites for High Performance Li-S Batteries”,
Chem. Commun.
, 50, 1202 – 1204 (2014)

3. Hongwei Chen, Weiling Dong, Jun Ge, Changhong Wang, Xiaodong Wu, Wei Lu and Liwei Chen*, “Ultrafine sulfur nanoparticles in conducting polymer shell as cathode materials for high performance lithium/sulfur batteries”,
Scientific Reports
, DOI:10.1038/srep01910 (2013).
(Invited) High Power SiC Power Processing Unit Development
Authors: Scheidegger Robert , Santiago Walter , Bozak Karin E , Piñero Luis R , Birchenough Arthur
Published: February 2020

The NASA space flight program can be divided into manned and unmanned programs with each contributing and ultimately setting the stage for advanced space transportation systems, future space habitats, and spacecraft that can send humans deep into the solar system. Most spacecraft use solar arrays for the main source of power except for those that had relatively short flight duration, or for deep space probes that were designed for very long flight duration. As the vision for space exploration evolves; spacecraft, space stations, and space habitats power needs increase exponentially thus creating the need for more efficient and revolutionary power technologies. NASA is investing in the most cutting-edge power technologies to enable the human space exploration beyond the earth vicinity.

The NASA Human Exploration Framework Team (HEFT) recommended demonstrating Spacecraft buses with increasing power & decreasing specific mass to enable advanced electric and plasma propulsion spacecraft that will decrease trip times to Mars and beyond. Primary or auxiliary electric propulsion systems provide many potential benefits to the spacecraft developer when compared to chemical propulsion systems. The high specific impulse provided by electric thrusters substantially reduces the amount of propellant required to achieve a given mission. This mass savings can be leveraged in many ways including reduction of spacecraft mass, reduced launch vehicle requirements, increased on-orbit lifetime, or increased payload.

Recent advances in solar array technology that produce more efficient and lighter solar arrays have enabled the notion of high power spacecraft as large as 300kW to MW class.  These high power systems will require high power, higher voltage distribution and power converters all requiring high voltage parts. 

In response to these technology needs, NASA GRC has been developing a solar electric power processing unit that would operate from a nominal 300VDC spacecraft bus.  The unit utilizes 1200V commercial silicon carbide power MOSFETs and high voltage SiC schottky diodes.  This developments main objective is to demonstrate the capability of high power, high voltage, and power processing for a 2000-second specific impulse Hall Effect Thruster.  To date this development has demonstrated the mass and efficiency gains which using silicon carbide at high voltage provides illustrating the need for radiation hardened space rated components.
(Invited) Radiation-Induced Defect Mechanisms in GaN HEMTs
Authors: Koehler Andrew D. , Anderson Travis J , Specht Petra , Weaver Brad D , Greenlee Jordan D , Tadjer Marko J , Shahin David I , Hobart Karl D , Kub Francis J
Published: February 2020

Gallium nitride (GaN) high electron mobility transistors (HEMTs) have been recognized as a key enabling technology for power amplifiers and high power converters for next-generation military and civilian systems.  The superiority of GaN over Si benefits applications such as wireless communications, and electronic warfare attack, protection, and support systems.  Also, GaN HEMTs also are excellent candidates for applications in radiation intensive environments such as counter weapons of mass destruction system survivability and space systems.  The objective is to investigate radiation-induced defects on GaN HEMTs, leading to understanding about the defect formation mechanisms and their impact on performance and reliability of GaN HEMTs.

AlGaN/GaN HEMT epitaxial layers were grown on Si substrates by metal organic chemical vapor deposition (MOCVD).  The GaN buffer layer is >1 μm and the AlxGa1-xN (x=27%) barrier layer is 17 nm thick.  HEMTs were exposed to 2 MeV protons at room temperature in a tandem Van de Graaff accelerator iteratively, up to a fluence of 6 x 1014 H+/cm2.  I-V, C-V, and Hall measurements were performed to characterize the radiation-induced changes in HEMT performance, and high-resolution transmission electron microscopy (HRTEM) is used to identify radiation-induced defect mechanisms.

Bombardment of 2 MeV protons causes a uniform distribution of vacancies, interstitials, and Frenkel pairs, with minimal larger damage cascades.  The stopping range of 2 MeV is much larger than the epitaxial GaN thickness. The AlGaN/GaN interface of an as-fabricated HEMT has an abrupt junction between the AlGaN and GaN.  However, after irradiation, atomic recoil of displaced atoms results in intermixing of the AlGaN and GaN layers.  The irradiated HEMT has an increased surface roughness.  Since the AlGaN/GaN interface is where the two-dimensional electron gas (2DEG) forms, the surface roughness impacts the 2DEG mobility.  The Hall mobility measured on Van der Pauw structures begins to degrade at a fluence of 5 x 1013 H+/cm2.  This mobility degradation is attributed to the increase in surface roughness between the AlGaN and GaN layers.  In addition, TEM analysis revealed where certain threading dislocations exist, that after proton irradiation, Ni from the Schottky Ni gate diffuses down into the dislocation.  This impacts the HEMT performance by changing leakage paths, but also, voids were observed in the Ni gate, where the nickel had diffused into the semiconductor at the gate edges.

Figure 1
The Effect of Carbon-Based Substrates onto Non-Precious and Precious Electrocatalytic Centers
Authors: Sreekuttan M. Unni , Campos-Roldan Carlos A. , Mora-Hernandez Juan M. , Luo Yun , Estudillo-Wong Luis A. , Alonso-Vante Nicolas
Published: February 2020

Novel catalysts based on precious and non-precious catalytic centers, as well as support materials are currently under investigation all over the world. The improvement of oxygen reduction reaction (ORR) on Pt-based catalysts is strongly related to its dispersion and interaction with the substrates. [1],[2] The tolerance to small organics is also influenced by the electronic modification of the catalytic center itself or induced by the substrate.[3] A variety of carbon-based substrates such as carbon nanotubes (CNT), graphite (HOPG), graphene, carbon nanohorns (CNH),[4] various graphitic domains of MWCNT, and non-carbonaceous based materials, such as metal oxides (Titania, doped-Titania, Yttria, Ceria, etc.) [5],[6]can be used as both catalyst and catalyst support materials.

On the way to further explore the substrate effect on non-precious electrocatalyst, CoSe2was supported on Carbon, CNH and nitrogen doped nanohorn (NCNH). The activity of non-Pt catalysts was found to be strongly dependent on the surface composition of substrate materials. The improved interaction of modified substrate and the metal centers (either precious or non-precious) modify the active reaction centers of the electrocatalyst, which in turn boost the catalytic activity towards molecular oxygen reduction with minimum overpotential. The analysis indicates that the improved activity measured in half-cells was validated in microfluidic fuel cells.

Acknowledgements

Authors acknowledge the partial support of the European Union’s Seventh Framework Programme (FP7/2007-2013) for the Fuel Cell and Hydrogen Joint Technology Initiative under grant agreement nr 303492 CathCat. SM U acknowledges Raman-Charpak fellowship, JMMH, J.M.M.-H., C.A. C.-R., and L.A. E-W acknowledge financial support from CONACYT-Mexico Nr.: MX-351743; MX-561206; MX-234729.

 

[1] Y. Luo, N. Alonso-Vante, Electrochim. Acta, doi: 10.1016/j.electacta.2015.04.098.

[2] J. Ma, A. Habrioux, Yun Luo, G. Ramos-Sanchez, L. Calvillo, G. Granozzi, P. B. Balbuena, N. Alonso-Vante, J Mater. Chem. A, Submitted; J. Ma, A. S. Gago, W. Vogel, N. Alonso-Vante, Chemcatchem, 2013, 5, 701-705.

[3] J. Ma, A.S. Gago, W. Vogel, N. Alonso-Vante, Tailoring and Tuning the Tolerance of a Pt Chalcogenide Cathode Electrocatalyst to Methanol, ChemCatChem, 5 (2013) 701-705.

[4] Sreekuttan M. Unni, J. M. Mora-Hernandez, Sreekumar K., N. Alonso-Vante, ChemElectroChem, Submitted.

[5] C.Liu , Y.Wei and K.Wang, Chem.Comm., 2010, 46, 2483-2485.

[6] Y. Luo, A. Habrioux, L. Calvillo, G. Granozzi, N. Alonso-Vante, ChemCatChem, doi: 10.1002/cctc.201500130.
Helox Study of Transport Limitations in Non-PGM Cathodes for PEM Fuel Cells
Authors: Pavlicek Ryan , Leonard Nathaniel , Calabrese Barton Scott , Mukerjee Sanjeev
Published: February 2020

A study of mass transfer limitations in a non-PGM cathode MEA were undertaken by studying the limiting current as a function of flow rate with varying oxygen carrier gases. Previous reports have indicated that the mass transfer coefficient for oxygen in the gas phase and electrolyte phase can be elucidated by studying the limiting current behavior in the presence of carrier gases of varying molecular weight. [1] We apply this approach to a non-PGM cathode using helium, nitrogen, and argon as carrier gases. Under conditions of low fractional O2 conversion, mass transfer limitations were found to exist mainly in the catalyst layer and flow field, not in the gas diffusion layer. Combined with mercury porosimetry studies to elucidate the macropore size distribution, this data is modeled using a previously-develop code [2] to provide an estimate of the gas-phase mass transfer coefficient, and by extension the fraction of hydrophobic, gas-phase pores in the catalyst layer. 1. T. V. Reshetenko and J. St-Pierre, J. Electrochem. Soc., 161, F1089 (2014). doi:10.1149/2.1021410jes. 2. N. D. Leonard, K. Artyushkova, B. Halevi, A. Serov, P. Atanassov, and S. Calabrese Barton, J. Electrochem. Soc., submitted (2014).

Figure 1
PEM Electrolysis: Ready for Impact
Authors: Mittelsteadt Cortney K
Published: February 2020

The need for energy storage has never been so pressing; due to their increasing proliferation and the intermittent nature of wind and solar power sources, many regional grids can no longer accept more of these sources.  Electrolysis of water using polymer electrolyte membranes (PEMs) has long held the promise of large-scale energy storage, needing only the simplest of reactants, water.  More importantly it is perfectly suited for storing excess energy from these sources as they can handle tremendously high ramp rates as well as many start-stop cycles.  The adoption of PEM electrolyzers has been impeded due to the headwinds of high capital cost, (including expensive high precious metal content), poor efficiency, and the empty promises made by the hydrogen economy from many years.  Numerous incremental improvements, and increasing economies of scale over the past 10 years have led to very large decreases in the cost of PEM electrolysis.  The complexity of the electrolyzer stack and precious metal content have been decreased, while simultaneously increasing lifetime.  Increasing operating temperature has had the largest impact on efficiency and output, while improved membrane supports have allowed for higher differential pressures; reducing the need and cost of ancillary systems (see Figure 1).  These advances have had the combined effect of reducing the cost of hydrogen from PEM electrolysis by nearly an order of magnitude, allowing it to approach the cost of pumped hydro, the standard in large-scale energy storage.  Subsequently, whereas only a few years ago there were no commercial PEM electrolyzer stacks in the 1 MW range, today no fewer than five companies are at or near launch of MW electrolyzer systems.

Figure 1.  Total cost of hydrogen as a function of plant size and operating current

Figure 1
Gas Permeation Study in Thin and Ultra-Thin Ionomer Films
Authors: Weber Adam Z , Tesfaye Meron , McCloskey Bryan D
Published: February 2020

Significant advances in the performance of polymer-electrolyte fuel cells (PEFCs) can be attributed to perfluorinated sulfonic-acid (PFSA) ionomers like Nafion and 3M-ionomer. They serve both as electrolyte membranes and as conductive binders in electrode structures. In electrode structures, ionomers are major contributors to unexplained large mass-transfer over potentials in PEFCs [1, 2]. Thin (< 1 µm) and ultra-thin (< 0.025 µm) ionomer films in the catalyst layer serve as ionic charge carriers to catalytic sites. Low resistance to gas transport for increased supply of reactants to catalytic sites is required for optimal performance. However, increase in transport resistance of thin-film ionomers is observed indicating deviation from bulk (> 1 µm thickness) ionomer properties [2]. This resistance is further exasperated with decrease in platinum loading, which is an essential consideration for decrease in cost of PEFCs [3]. Much has been done to shed light on the aberration of ionomer thin-film from bulk in other properties such as proton conductivity, morphology and water uptake. This has allowed many to attribute the increase in local mass transfer resistance at the catalyst layer to the decrease in permeability of thin-film ionomers as compared to that of bulk. However, direct measurement of thin-film ionomer membranes has not been done.

Substantial amount of work has been done to study gas transport and permeation in bulk ionomer films [4]. However, much remains to be explored in thin and ultra-thin ionomer films. In this talk, we present data for gas permeation in thin and ultra-thin PFSA films. Thin ionomer films are supported on well studied, highly permeable rubbery poly(dimethylsiloxane) (PDMS). O2, N2 and H2 permeability of the ionomer film is measured using a constant-volume, variable-pressure permeation system. Experimental results and variations of permeability of thin films from bulk films as a function of temperature and thickness will be presented. In addition, to tackle the complex and not fully understood effect of water in gas transport, gas permeability as a function of humidity will also be presented.

 Acknowledgement

We would like to thank Norman Su for providing assistance in permeation system assembly and operation. This work made use of facilities at the Biomolecular Nanotechnology Center at University of California, Berkeley and the Molecular Foundry at Lawrence Berkeley National Laboratory. This work was funded in part by the University of California Chancellor's Graduate Fellowship and the Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231.

References

[1]Nonoyama, N. et al (2011). Journal of Electrochemical Society. 158(4)

[2]Gresler, T. et al (2012). ). Journal of Electrochemical Society. 159(12)

[3]Weber, A. et al (2012). Journal of Materials Chemistry A. 2,17207

[4] Kocha, S. et al (2006).AIChE Journal. 52(5)
Thermo-Stable Non-PFSA Cation-Conductive Fluoropolymers for Medium-Temperature PEMFCs
Authors: Xu Shaoyi , Jiang Rong , Gao Yong
Published: February 2020

Medium-temperature PEMFCs operated at a temperature between 100 and 180 oC can be used as an alternative to phosphoric acid fuel cells—avoiding the leach and loss of phosphoric acid electrolytes from the cells during fuel cell operations.  At an elevated temperature, Pt catalysts are less sensitive to carbon monoxide poisoning—allowing the use of low-quality hydrogen fuels produced from reforming processes.  However, the thermal stability of perfluorosulfonic acid (PFSA, e.g., NafionTM) membranes and PFSA catalyst binders is the bottleneck problem for raising the operation temperature of PEMFCs above 100 oC.  It was reported that NafionTM membranes can only have a lifespan of several hundred hours under humidified conditions at 130 oC.

Recently, we designed, synthesized and tested a group of non-PFSA fluoropolymers as cation-conductive electrolytes via a novel C-C coupling reaction.  Heteroaromatic groups can be covalently attached to the side chains of a PFSA—with no acid or base labile sulfonyl groups.  The protonated form of such fluoropolymers can transport protons and can be fabricated into thin films.  An example of non-PFSA polymers examined in our laboratories is shown in the enclosed figure.  Many of non-PFSA fluoropolymer membranes showed ion conductivities, mechanic strengths, and electrochemical properties, compatible to NafionTM membranes, but with much improved thermal stabilities.  Some preliminary test results of a prototype membrane are shown in table 1:

Table 1.  Comparison of our prototype membrane vs. commercial N-117 membrane.

 
Thickness (µm)
In-Plane Conductivitya

(in de-ionized water, 25 oC)
IEC
(meq/g)
Water uptake

(%,25 oC)

N-117
183
77 mS/cm
0.9
17

Our Prototype
193
71 mS/cm
0.7
18

*Four-electrode AC impedance measurements.

Most remarkably, our prototype membrane showed much improved thermal stability against N-117.  Within three hours, a commercial N-117 membrane quickly lost its ion conductivity in a heated water bath of 150 oC in a pressure reactor, while our prototype membrane showed no sign of the reduction of its conductivity after five months.

Figure 1
Bing Energy Reduced Pt Membrane Electrode Assemblies
Authors: Zheng Jim P. , Zhu Wei
Published: February 2020

We have explored and demonstrated a novel nano-engineered electrode technology to meet the DOE’s targets for platinum group metal utilization and electrode durability for proton exchange membrane fuel cells. Our new electrode technology has made three major technical improvements of (1) the intrinsic electrocatalytic activity and stability of the catalystic electrodes; (2) the utilization of these materials using nano-structured and functionalized supports; and (3) the stability of the electrocatalyst/support system by strengthening the electrocatalyst-support interaction and by using corrosion resistant supports.

A double-layered buckypaper (DLBP) was prepared by filtering the carbon nanotube (CNT)/carbon nanofiber (CNF) mixture and the CNF suspension sequentially under full vacuum. CNTs of ~10 nm in diameter and 10 mm in length and CNFs of 100-200 nm in diameter and 30-100 μm long. Platinum (Pt) nanoparticles were deposited onto the buckypaper using a pulse electrodeposition technique.

As shown in Fig. 1(a), large-sized CNFs entangled randomly forming a highly porous sublayer with a porosity of 90.8% and an average pore size of 85 nm, while fewer and smaller pores were formed in the CNT/CNF sublayer by adding 25 wt.% smaller CNTs. After depositing Pt on the buckypaper by electrochemical deposition, an energy-dispersive X-ray spectrometer analysis showed an obvious gradient distribution of Pt (Fig. 1(b)); over 70% Pt was distributed in the 7 mm-thick CNT/CNF sublayer. A large amount of Pt deposited on the surface of the CNT/CNF sublayer (Fig. 1(c)), while only a small amount deposited on the surface of the CNF sublayer (Fig. 1(d)). This could provide an indication of how Pt was distributed inside the buckypaper since the Pt distribution was quite uniform within each sublayer. The pictures of surface morphology showed the Pt preferred to grow on the surface of CNT instead of CNF. Although, the mechanism of selective deposition of Pt on CNT/CNF buckypaper is not fully understood, the most possible reasons we suppose are: (1) CNT has much higher surface area and more surface defects resulting in more anchor sites for Pt nucleation; and (2) a higher deposition current on CNTs because of their higher conductivity.

By applying this double-layered buckypaper as a cathode catalyst layer, the MEA exhibited excellent power performance with a relative low Pt loading. The rated powers of 1.5 W/cm2 and 0.8 W/cm2 at 0.65 V were achieved with a cathode Pt loading of 0.2 mgPt/cm2 (Fig. 2) for oxygen and air, respectively. The Pt utilization was better than the current state-of-the-art value (>0.4 mgPt/cm2) achieved by the conventional Pt/C catalys at a comparable cell (0.8–1.0W/cm2) output. The CNT/CNF buckypaper-based Pt catalyst has also shown good durability under an accelerated degradation test in a mimic cathode environment. This was due to the high corrosion resistance of CNF and its high graphitization degree. Hence, we evaluated the durability of catalyst support for Pt/DLBP in a MEA per US DOE’s suggested protocol.

Figure 1
A Urine/Cr(VI) Fuel Cell: Electrical Power from Processing Heavy Metal and Human Excretion
Authors: Xu Wei , Fan Zheng , Wu Xingxing , Wu Zucheng
Published: February 2020

Hexavalent chromium (CrO4
2-, Cr2O7
2-, CrO3, CrF6, etc.) is seriously hazardous substance inducing various point mutations in DNA and oxidative changes in proteins due to its strong oxidational properties[1]. Notably, Cr(VI) is strongly oxidative which can oxidize most organics such as urine in wastewater. Thus, it is possible to develop a fuel cell by using hexavalent chromium and urine as fuels according to the following reaction mechanism:

Anode reaction:     CO(NH2)2+6OH-→N2+CO2+5H2O+6e     E0= -0.746 V vs. SHE

Cathode reaction:  Cr2O7
2-+14H++6e→2Cr3++7H2O      E0= +1.33 V vs. SHE

Overall reaction:    CO(NH2)2+Cr2O7
2-+8H+→N2+CO2+2Cr3++6H2O     E0= +2.076 V

This work shows a strategy of reducing Cr(VI) by human urine with self-generation electricity via a urine/Cr(VI) fuel cell (UCrFC), in which urine functions as fuel and Cr(VI) severs as electrons acceptor. Urine is electro-oxidized on carbon supported nano-Ni catalyst at anode and Cr(VI) is electro-reduced at catalyst-free cathode in acid medium[2,3]. An ion selective separator, consisting of an anion exchange membrane (AEM) and a cation exchange membrane (CEM), AEM||KClaq||CEM separator, is introduced to improve the cell performance by hindering the crossover of dichromate and ammonium.

A maximum power density of 3400 mW m
-2
(159 times to microbial fuel cells[4]) was achieved with an OCV of 1.3 V (twice of that in microbial fuel cells[4]) in AEM||KClaq||CEM-UCrFC, when using 50 mg L-1 Cr(VI) in 0.25 M H2SO4 as catholyte and neat urine as anolyte. The apparent first-order kinetic constants of Cr(VI) reduction were -0.224, -0.145 and -0.118 h-1, respectively, when initial concentration of Cr(VI) were 13, 22 and 50 mg L-1. Total organic carbon and total nitrogen removal from urine were observed in 71 h in the anode chamber, with removal efficiency of 79.2% and 78.4%, respectively. At the same time, ~93% of Cr(VI) was removed in the cathode chamber. A high cathodic coulombic efficiency of more than 98% was achieved. One liter of fresh urine could reduce about 28 g of Cr(VI) via UCrFC. This fuel cell provides an alternative technology for using waste (human urine) to treat another waste (hexavalent chromium), and is potentially applicable to other waste/waste system.

References:

1. A. D. Dayan, A. J. Paine, Hum. Exp. Toxicol.
2001, 20, 439-451.

2. W. Xu, H. Zhang, G. Li & Z. Wu, Scientific Reports, 2014, 4: 5863.

3. B. Yu, H. Zhang, W. Xu, G. Li & Z. Wu, Scientific Reports, 2014, 4: 5860..

4. L. P. Huang, X. L. Chai, S. A. Cheng, G. H. Chen, Chem. Eng. J.
2011, 166, 652-661.
Rotating Disk Slurry Electrochemically Modified Platinum Carbon Nano-Onion Anode Material for Polymer Electrolyte Membrane Fuel Cells
Authors: Guzman-Blas Rolando , Suleiman Amal , Echegoyen Luis , Jose-Yacaman Miguel , Cabrera Carlos R
Published: February 2020

Platinum modified carbon nano-onion anode materials (Pt-CNO) with 18.5% metal loading were prepared by the Rotating Disk Slurry Electrode (RoDSE) technique for Polymer Electrolyte Membrane Fuel Cell applications. The Pt-CNO was characterized and tested in direct ethanol, methanol, and hydrogen polymer electrolyte membrane fuel cells at 55°C. Platinum particles showed an average size of (9±1) nm according to XRD data and applying the Scherrer equation. However, high-resolution transmission electron microscopy images showed well-dispersed Pt nanoparticles, clusters, and atoms on the carbon based support material. The 5 cm2 fuel cell studies were done with commercial Pt black and Pt-CNO catalysts anode electrode loadings of 4 mg/cm2. Pt-CNO exhibited enhanced maximum power density compared to Pt black, when normalized by the Pt anode loading, i.e. 20mg and 3.7mg for Pt black and Pt-CNO, respectively. For ethanol, methanol, and hydrogen the anode Pt mass normalized power densities showed increments of 32%, 66%, and 353%, respectively.
Proton Conductivity and Gas Barrier Properties of Graphene Oxide for PEMFC Membranes
Authors: Bayer Thomas , Selyanchyn Roman , Fujikawa Shigenori , Sasaki Kazunari , Lyth Stephen Matthew
Published: February 2020

Treatment of graphite by the modified Hummers method results in exfoliation of the graphene layers and the introduction of oxygen-containing surface functional groups.1 The resulting graphene oxide offers interesting possibilities for a wide range of applications in areas as diverse as e.g. humidity sensing,2 molecular sieving,3 gas barriers,4 dielectrics,5 and fuel cells.6,7

Polymer electrolyte membrane fuel cells (PEMFCs) are seen as a sustainable energy source for the future. So far the most commonly used membrane in PEMFCs is Nafion, due to its high mechanical and chemical stability as well as its high proton conductivity. However Nafion is expensive, has limited performance at temperatures > 100°C,8 and has only moderate gas barrier properties (leading to fuel crossover in very thin membranes). Graphene oxide membrane fuel cells (GOMFCs) have displayed reasonable power densities at room temperature,7and with some improvements could act as an electrolyte in low temperature fuel cells.

Here, the proton conductivity, fuel cell performance, fuel crossover of graphene oxide paper with different oxygen content, surface morphology, and surface functional groups are investigated by impedance spectroscopy, membrane electrode assembly testing, and gas permeation measurements. The manufacturing techniques for production of graphene oxide membranes and the optimal thickness of membranes are also discussed.

References:

1.           Zhu, Y. et al., Adv. Mater.
22,3906–3924 (2010).

2.           Borini, S. et al., ACS Nano
7,11166–11173 (2013).

3.           Joshi, R. K. et al., Science
343,752–4 (2014).

4.           Yoo, B. M. et al., Polym. Sci.
131,n/a–n/a (2014).

5.           Wang, D.-W. et al., J. Mater. Chem.
22,21085 (2012).

6.           Tateishi, H. et al., J. Electrochem. Soc.
160,F1175–F1178 (2013).

7.           Bayer, T. et l. , J. Power Sources
272,239–247 (2014).

8.           Yee, R. S. L. et al., Chem. Eng. Res. Des.
90,950–959 (2012).

Figure 1
Water Uptake in Novel, Water-Stable, Heteropoly Acid Films
Authors: Motz Andrew R , Horan James L , Kuo Mei-Chen , Herring Andrew M
Published: February 2020

Operating a fuel cell at high temperature and low humidity can reduce catalyst and heat and water management cost in addition to improved tolerance to fuel impurities [1]. Traditional perfluorinated sulfonic acid proton exchange membranes do not operate well under these conditions, due to a need for high hydration to facilitate proton transport. Heteropoly acids (HPA)s have demonstrated promise for high temperature, low humidity fuel cell operation, but often are not immobilized, resulting in premature fuel cell performance loss[2]. In order to overcome this, a novel, chemically and thermally robust chemistry has been explored. Phenyl phosphonic acid side chains are grafted onto a 3M polyvinyl fluoride-co-hexafluoropropylene, FC-2178. These phenyl phosphonic acid (PPA) side chains then react with lacunary silicotungstic acid, thus immobilizing the HPAs, via covalent bonds. Different levels of PPA attachment have been explored, giving different ion-exchange capacities.

31P and 1H NMR have been used to determine the side chain attachment and IEC of the PPA functionalized FC-2178. The immobilized tungsten content was measured using PIXE (Proton Induced X-ray Emission) elemental analysis. Knowledge of the IEC and tungsten content elucidates the contributions to IEC from both PPA and HPA. SAXS and AFM have been used to determine the membrane morphology. Samples with a continuous conducting phase have shown proton conductivities up to 20 mS/cm at 110°C and 50% RH.

References

[1] Chandan, A.; Hattenberger, M.; El-kharouf, A.; Du, S.; Dhir, A.; Self, V.; Pollet, B. G.; Ingram, A.; Bujalski, W., High temperature (HT) polymer electrolyte membrane fuel cells (PEMFC) – A review. Journal of Power Sources
2013, 231, 264-278.

[2] Herring, A. M., Inorganic–Polymer Composite Membranes for Proton Exchange Membrane Fuel Cells. Journal of Macromolecular Science, Part C: Polymer Reviews
2006, 46, 245-296.
Synthesis of Pt-Cu Bimetallic Alloys and Their Electrocatalytic Activities for Oxygen Reduction Reaction
Authors: Fu Shaofang , Zhu Chengzhou , Du Dan , Lin Yuehe
Published: February 2020

One major limitation for the commercialization of proton exchange membrane fuel cells (PEMFCs) is the high cost of oxygen reduction reaction (ORR) catalysts. The predominant catalysts are Pt nanoparticles (NPs) supported on carbon materials. However, there materials present low activity and durability. In this case, unsupported catalysts with high surface area and porosity are promising candidates to improve the activity and durability. The unsupported catalysts would not only eliminate the activity lost caused by erosion of supports but also allow for a thinner electrode catalyst layer and for improved mass transport and Pt utilization. To this end, the three dimensional (3D) nanofoams with PtCu@Pt core-shell structures were synthesized with a facile procedure and etched by nitric acid. After appropriate acid treatment, the catalysts achieve a factor of 3 enhancement in mass activity and a factor of 6 enhancement in specific activity, respectively, compared with commercial Pt/C catalysts. The enhancements are attributed to the high surface area and the special structure. The unsupported PtCu@Pt catalysts are promising materials for ORR.
Effects of Contact Angle Variation of the Gas Flow Channel Walls on Liquid Water Removal in PEMFC
Authors: Lee Geunchan , Song Myeongho , Kim Kyoungyoun
Published: February 2020

Three-dimensional two-phase flow simulations have been performed by employing the volume of fluid (VOF) method to examine the effects of contact angle of gas flow channel walls on the droplet dynamics in the serpentine flow channel of PEMFC. Liquid water is injected from four adjacent pores on the bottom gas diffusion layer (GDL) surface. The droplets grow and they are detached and finally discharged due to gas flow in the channel. The influences of the channel wall contact angle on liquid water dynamics have been examined quantitatively for liquid water saturation and its coverage on the GDL surface. The present numerical results reveal that spatial variation of channel wall hydrophobicity can effectively control the liquid water removal in the gas flow channel. The spatiotemporal evolutions of liquid water droplets in the gas flow channels show that the hybrid case is most effective since the hydrophobic wall at the turning part reduces the water saturation in the channel and the hydrophilic wall at the straight part prevents the liquid water from covering the GDL surface.
Relating Direct Methanol Fuel Cell Performance to Measurements in a Liquid Half Cell
Authors: Pedersen Christoffer Mølleskov , Tynelius Oskar , Lund-Olesen Torsten , Hales Jan Harry , Christensen Leif Højslet , Stephens Ifan E. L. , Chorkendorff Ib
Published: February 2020

Direct methanol fuel cells (DMFC) could act as a replacement for batteries in low power electronics. For instance, micro—DMFC’s could be used to power hearing instruments[1]. The power output of a DMFC is limited by the sluggish kinetics of both the methanol oxidation reaction (MOR) on the anode and the oxygen reduction reaction (ORR) on the cathode. Thus far, to achieve high power densities with a single cell, the catalyst loadings have been increased much as possible (20 mg/cm2 PtRu/C on anode and 4 mg/cm2 Pt/C on cathode). More active catalysts would yield higher power densities which in turn would allow further miniaturization or powering more advanced and more power hungry devices.

The activity of fuel cell catalysts is often probed in the form of thin films in liquid half cells. However, it is challenging to mimic the conditions in an actual DMFC. On the other hand, it can also be problematic to extract the catalyst activity from a fuel cell measurement.

In this work, we attempt to narrow the gap between fuel cell testing and liquid half-cell measurements. First, by placing a custom reference electrode within the fuel cell, we can determine the potential at the anode and cathode under in-operando conditions.  This in turn, allows us to directly correlate our measurements to those performed in a liquid half-cell at the same potential. For our half-cell measurements, we have tested different catalysts (Pt/C, PtRu/C) for the methanol oxidation reaction (MOR) and the oxygen reduction reaction (ORR) in the presence of methanol.  By comparing the two measurements, we make recommendations for performing liquid half-cell measurements under realistic conditions.

[1]      J.H. Hales, C. Kallesøe, T. Lund-Olesen, A.-C. Johansson, H.C. Fanøe, Y. Yu, et al., Micro fuel cells power the hearing aids of the future, Fuel Cells Bull. 2012 (2012) 12–16. doi:10.1016/S1464-2859(12)70367-X.
Electrocatalysis in Water Electrolysis
Authors: Gottesfeld Shimshon
Published: February 2020

In a recent publication, generation of active surface sites by the overpotential was described as a common pattern in fuel cell electrocatalytic processes [1]. This paper discusses the applicability of the same pattern to the electrocatalytic processes in the water electrolyzer .  

A wide variety of electrocatalytic processes taking place at either functionalized surfaces , oxide surfaces ,or “metal”  surfaces,  are all , in fact,  surface redox mediated . This means  that the electrode potential defines the steady state population of active sites ,suggesting that the role of the potential  in active site generation must  be incorporated in the pre-exponential factor of the overall rate expression.  In fact, the   potential driven active site generation serves to “ignite” the faradaic process.  This explains why the E0 
redoc
 
system  of M/MOx , or  MOx+n/MOx+n+m  surface couples , must be as close as possible to  E0
cell process , to enable high conversion efficiency as well as higher rates at low overpotentials.  

It will be shown here, that, consideration of this general pattern/mechanism  of electrocatalytic processes,  facilitates understanding of the relative activities of various OER electrocatalysts ,in either acid or alkaline environments. The Pourbaix diagram, provides good insight into the expected relative activities in these, surface redox mediated  electrocatalytic processes, together with the other important information regarding expected relative stabilities in acid and in alkali.

[1] Shimshon Gottesfeld , ECS Trans. 2014 61(31): 1-13