bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanistic Understanding of the Role of Ionomer in the PEM Fuel Cell Catalyst Layer
Authors: Goswami Navneet , Mistry Aashutosh N , Verma Ankit , Grunewald Jonathan B , Fuller Thomas F. , Mukherjee Partha P.
Published: February 2020

With an attempt to design fuel cell catalyst layers with low Pt loading capable of delivering suitable performance and life expectancy, interfacial interactions between the different phases need to be improved. In this aspect, modulating the distribution of ionomers in the cathode catalyst layer is an interesting direction that is worth exploring as it can provide a highly connected proton conducting network in addition to reducing transport limitations.

In the present work, microstructural realisations are achieved through tomography data based on focused ion beam scanning electron microscopy (FIB-SEM) to obtain the Pt/C phase and an interfacial energy based erosion model to generate the ionomer phase. These microstructural features are then deployed to a catalyst layer based transport model to address the interplay of geometrical stochasticity and operating conditions such as relative humidity. The microstructural heterogeneity is thus investigated to unravel the structure-transport-property interactions of Pt-C and ionomer phases to provide a comprehensive understanding of the electrochemical performance.
Cathode Catalyst Layer Design with Graded Porous Structure for Proton Exchange Membrane Fuel Cells
Authors: Garsany Yannick , Atkinson Robert W. , Bethune Keith , St-Pierre Jean , Gould Benjamin D. , Swider-Lyons Karen
Published: February 2020

Cathode catalyst layer (CL) design is important for improved performance, durability and stability of proton exchange membrane fuel cells (PEMFCs). An effective cathode CL must serve multiple functions simultaneously: electron and H+ conduction, O2 or H2 supply, and effective water management. The reaction in the CL requires three-phase boundaries (or interfaces) among ionomer (for proton transfer), platinum (for catalysis), and carbon (for electron transfer), as well as voids (reactant diffusion); an optimized cathode CL structure should have micropores and mesopores to balance water egress and O2 diffusion.

Our previous research showed that increasing microporosity in the cathode CL increases the power production of the PEMFCs [1]. Modeling indicates that high CL microporosity enhances water evaporation, which reduces mass transport resistances to enable higher power production [2]. Meanwhile, low microporosity encourages liquid water retention.

It is known that catalyst support type has a profound effect on electrode porosity, influencing both micro- and mesoporosity [3, 4]. By constructing electrodes with layers of different carbon supported catalyst types, we demonstrate that we can create porosity gradients in the CLs that vary the electrode water management properties and the power of the PEMFC. Two different types of commercial platinum/carbon electrocatalysts are used in dual cathode CLs – 40 wt. % Pt on Vulcan carbon (Pt/VC) having low micropores and low porosity and 40 wt. % Pt on Ketjen black (Pt/KB) having large micropores and high porosity. CLs are created with both homogeneous porosity in the Z direction (Figure 1a), and compared to ones with non-uniform porosity across the Z-direction (Figure 1b).

The microstructure of these cathode CLs is characterized using N2-sorption porosimetry. The CLs are assembled into PEMFCs and probed with polarization curves (at 100, 50 and 25% RH), cyclic voltammetry, O2 gain, electrochemical impedance spectroscopy, and limiting current measurements to quantify the sources of mass transport resistance.

The results indicate that the regions with low microporosity encourage liquid water retention, while high microporosity encourages water evaporation. These countering actions are balanced for high CL hydration and gas diffusion, and better PEMFC performance.

References

[1] Y. Garsany, R.W. Atkinson, M.B. Sassin, R.M.E. Hjelm, B.D. Gould, K.E. Swider-Lyons, J. Electrochem. Soc., 165 (2018) F381-F391.

[2] M. Eikerling, J. Electrochem. Soc., 153 (2006) E58-E70.

[3] T. Soboleva, K. Malek, Z. Xie, T. Navessin, S. Holdcroft, ACS Appl. Mater. Interfaces, 3 (2011) 1827-1837.

[4] Y.-C. Park, H. Tokiwa, K. Kakinuma, M. Watanabe, M. Uchida, Journal of Power Sources, 315 (2016) 179-191.

Figure 1
Low Cost Gas Diffusion Layer Materials and Treatments for Durable High-Performance PEM Fuel Cells
Authors: Leonard Daniel Philip , Borup Rod L.
Published: February 2020

The gas diffusion layer (GDL) is a critical component of proton exchange membrane fuel cells (PEMFC). It serves as the primary mediator in electrical and thermal conductivity as well as gas and water transport within the cell. Currently, the production of GDLs is dependent on the use of high-cost materials and manufacturing methods. The most common base material for GDLs is polyacrylonitrile (PAN). PAN, in addition to its high base cost ($15 - 20/kg), requires high graphitization temperatures (> 1700°C) in order attain sufficient conductivity and post-carbonization treatment with Teflon to increase the hydrophobicity for improved water transport. The high strength of PAN based GDLs has been required to prevent fiber intrusion into the channel of the flow field. Some state-of-the-art flow fields no longer utilize the land/channel design, and as such may not require the use of high strength materials like PAN in order to be functional. This presents the opportunity to create novel GDLs with equivalent or superior performance at lower cost.

We are investigating methods to significantly reduce the cost of GDL production while maintaining durability and performance characteristics necessary to PEM fuel cells. First, we are developing the use of inexpensive, natural fiber materials, such as cotton, jute, and bamboo. Second, we are developing methods to reduce the carbonization temperatures and eliminate manufacturing steps, such as the need for a microporous layer.

Figure. SEM images of commercial (A) (without microporous layer), cotton (B), and jute (C) based GDLs. The cotton and jute GDLs had been carbonized at 1100°C in N2 atmosphere.

Figure 1
Increase in Oxygen Transport Resistance Caused By Degradation in the Porous Layers in a Proton Exchange Membrane Fuel Cell
Authors: Zhang Xu , Liu Hongtan , Zhang Xuyang , Alrashidi Abdullah
Published: February 2020

The increase in mass transport loss accounts for important performance loss when various degradation mechanisms occur in the proton exchange membrane fuel cell (PEMFC)[1-5]. The deterioration of mass transport could be ascribed to two aspects: the increase of liquid water saturation, due to the worse water transport, and increased oxygen diffusion resistance in the pore space. The loss of hydrophobicity has been identified in catalyst layer (CL) [1, 6], gas diffusion layer (GDL) [7, 8], and micro porous layer (MPL) [9, 10], which leads to worse water management. Microscopic characterizations also confirm severe damages to the microstructure, which may hinder the oxygen diffusion in the multiscale porous components[4, 11]. In this study, the oxygen transport resistance at the cathode side of a PEMFC is measured to unravel the increase in oxygen transport resistance associated with microstructure damage. Limiting current measurements are conducted under different dry oxygen fraction conditions before and after an accelerated stress testing (AST). Both GDLs with and without MPL are employed at the cathode side. The results show the presence of MPL increase the total oxygen transport resistance due to the additional layer and its lower porosity. After the aging testing, the oxygen transport resistances in both cells increases significantly. These agree with the severe pore microstructure damage caused by aging testing. The cell with MPL has even larger increases. Further decouple of the total resistance indicates that the cell with MPL has smaller increases in molecular diffusion resistance, but presents significantly greater increases in the other resistance. This implies that pore structure damage occurs in MPL itself and results in a severe increase in Knudsen diffusion resistance.

Fig.1 (a) Oxygen transport resistance under different pressure; (b) Decouple of oxygen transport resistance without and with MPL at the cathode.

Figure 1
Optical Properties and Swelling of Thin Film Perfluorinated Sulfonic-Acid Ionomer
Authors: Bayani Ahangar Shahab , Bellur Kishan , Medici Ezequiel , Tajiri Kazuya , Allen Jeffrey S , Choi Chang Kyoung
Published: February 2020

Perfluorinated sulfonic-acid (PFSA) ionomers are members of a class of ion-conductive polymers that are commonly used as a proton-exchange membrane or catalyst layers in polymer-electrolyte fuel cells. Numerous studies have been undertaken to understand how the structure of PFSA ionomers with thickness of 1 to 200 µm and their water uptake affect the material properties and the performance of fuel cells. Thin film confinement affects the hydration behavior of thin film ionomers; the deviation from transport properties are still unclear. Advanced tools have been used to study water uptake of sub-micron PSFA ionomers at different humidity levels; however, their temporal resolution are of the order of 1 minute. Label free, high-speed Surface Plasmon Resonance imaging (SPRi) is proposed to provide a 3D map of a thin film (<300 nm) at high thickness resolution (~ 0.1 nm), high lateral resolution (~ 4 μm), and high temporal resolution (up to 3000 frames per second). Nafion films with 28-250 nm and 3M films with 7-150 nm are used for a systematic study of the swelling and diffusion transport phenomena of water in thin film ionomers deposited on a 50 nm gold film on a glass substrate. SPRi with angular modulation (AM) and intensity modulation (IM) is employed to determine the swelling ratios of the thin film PSFA’s at different humidity levels. SPRi with IM is used to study the kinetics of diffusion transport of water in the thin film ionomers. The preliminary results show that the water uptake in the thin film ionomers locally decreases the refractive index of the material, whose changes are tracked by SPRi. This can provide the diffusion map during water transports through the thin film ionomer.
A High-Power Non-Enzymatic Glucose Biofuel Cell Based on a Nano/Micro Hybrid-Structured Gold Anode
Authors: Chu Tien-Fu , Wang Gou-Jen
Published: February 2020

Biofuel cell (BFC) that has attracted considerable attentions recently is a green energy generator that can generate electricity directly from the oxidation of biological organic matter. Glucose biofuel cells (GBFCs) that uses glucose as fuel has emerging need due to the rapid growth of wearable or implantable medical devices. In general, enzyme based GBFCs that use enzymes as the catalyst are still the mainstream of current development due to the excellent selectivity and reaction rate of specific enzymes. However, the enzyme based GBFCs suffer from the disadvantages of prone to poisoning, limited enzyme stability and cumbersome enzyme immobilization procedures. Therefore, non-enzymatic technologies that use nanostructures of noble metals such platinum, gold, and silver as the catalyst has become a new development trend.

In this study, a high-power non-enzymatic glucose biofuel cells based on a nano/micro hybrid-structured gold anode is proposed. In this novel electrode, a uniformly distributed micro hemisphere array of polycarbonate was fabricated by hot embossing. Then a self-assembled monolayer of gold nanoparticle was formed on the micro hemisphere array as the anode. The cathode was composed of a graphene film coated glassy carbon electrode. The anode and cathode are separated by a proton exchange membrane (nafion) to complete the battery assembly. Experimental results demonstrated that the proposed non-enzymatic GBFC has a high power density of 9.459 mWcm-2, a high current density of 8 mAcm-2, and an open circuit voltage (Voc) of 0.7128 V at room temperature. In addition, the proposed GBFC can be easily produced on a large scale, the cost of fabrication is low, repeatability is high, and is easy to preserve on a long-term basis. Therefore, it is highly feasible to be commercialized and used in practical applications.

Figure 1
Phosphonic Acid-Functionalized Polyaromatic Electrolytes for High Temperature Proton Exchange Membrane Fuel Cells
Authors: Park Eun Joo , Lee Albert S , Kim Yu Seung
Published: February 2020

Proton exchange membrane fuel cells (PEMFCs) are one of the most promising environmentally friendly energy conversion devices. The high temperature PEMFC system requires simplified heat and water management, which would provide substantial cost benefits and high potential for commercialization. Phosphonated polyaromatic electrolytes are promising candidates as proton conductors at high temperatures due to their ability to conduct protons under the full range of humidity levels and their covalently bound phosphonic acid structure, which eliminates the risk of leaching of the acid group at high humidity conditions. Despite their synthesis of research for more than decades, only few successful examples of phosphonated polymers in the PEMFCs have been reported.

Here we present the new synthesis of phosphonic acid-functionalized polyaromatic electrolytes as electrode ionomers of high temperature PEMFCs. The polyaromatic electrolytes were synthesized by acid-catalyzed Friedel-Crafts polycondensation and followed by the SN
Ar Michaelis-Arbuzov reaction of aryl pentafluorides. The electrolytes with the ion exchange capacity ranged 1.9-2.2 meq/g have good solubility in polar aprotic solvents, which eases the fabrication of materials as electrode binders. We could obtain the peak power density up to 700 mW/cm2 at current density of 2.1 A/cm2 at 200 °C with the membrane electrode assembly using the phosphonic acid functionalized ionomer. We will compare the fuel cell performance to that of ion-pair coordinated electrode binders1 and discuss the high applicability of the phosphonated polymer electrolytes in high temperature systems.

Reference

K.-S. Lee, J.S. Spendelow, Y.K. Choe, C. Fujimoto, Y. S. Kim, Nature Energy, 2016, 1, 16120-16124.
High Temperature Polymer Electrolyte Membrane Achieved By Grafting Poly(1-vinylimidazole) on Polysulfone for Fuel Cell Applications
Authors: Bai Huijuan , Wang Haining , Zhang Jin , Zhang Jujia , Lu Shanfu , Xiang Yan
Published: February 2020

High temperature polymer electrolyte membranes with phosphoric acid doping are crucial materials of high temperature fuel cells. However, the evolution of the type membrane is suffering from the trade-off between proton conductivity and mechanical strength, because high proton conductivity requires high phosphoric acid doping level, which will reduce the mechanical properties due to the plasticization of phosphoric acid on polymer backbone. Here, a new strategy is employed to respond to the unresolved challenges by grafting poly(1-vinylimidazole) as phosphoric acid doping sites on polysulfone backbone via atom transfer radical polymerization. Then poly(1-vinylimidazole) grafted polysulfone membrane with different length of side chains is obtained and soaked in phosphoric acid to work as high temperature polymer electrolyte membrane. The tapping-mode AFM images and 1-D SAXS result of PA doped membrane suggest the formation of micro-phase separated structures, and the ionomer peaks shift from 1.34 nm-1 to 1.19 nm-1 with increasing the length of side chain, corresponding to d-spacing of 4.67 nm and 5.28 nm, respectively. Therefore, the prepared phosphoric acid doped membranes possess excellent proton conductivity of 127 mS cm-1 under 160 °C due to the structure, while mechanical properties are retained due to the reduction of plasticizing effect caused by the separation of phosphoric acid adsorption sites and polymer backbone, presenting an outstanding tensile strength of 7.94 MPa. Meanwhile, the impressive fuel cell performance with the membranes is gained, reaching a maximum peak power density of 559 mW cm-2 under 160 oC. More importantly, the work provides a new sight to solve the trade-off between proton conductivity and mechanical strength for phosphoric acid doped high temperature polymer electrolyte membranes.

Figure 1
Anomalous Appearance of a Distinct Glass Transition in Perfluoroimide Acid Ionomers
Authors: Orsino Christina Marie , Lindell Matthew , Yandrasits Michael , Hamrock Steven , Moore Robert B
Published: February 2020

Perfluorosulfonic acid ionomers (PFSAs) are the benchmark materials used as proton exchange membranes in fuel cells due to their exceptional proton conductivity and chemical and mechanical stability. Typical PFSAs consist of a PTFE backbone with perfluoroether sidechains bearing pendant sulfonic acid groups. PFSAs of different ion contents and sidechain compositions have been developed in an effort to optimize functionality while maintaining mechanical stability. One such ionomer is 3M’s perfluoroimide acid (PFIA) containing a sulfonimide acid group in addition to the sulfonic acid on each sidechain to increase proton conductivity while preserving TFE backbone segments to maintain sufficient crystallinity. In this study, we investigate the effect of this additional acidic site on the thermomechanical properties and morphology of PFIA compared to the single acid PFSA. Utilizing alkylammonium counterions we are able to systematically manipulate the strength of the physically crosslinked network in the perfluorinated ionomers to probe the origins of thermomechanical transitions. The addition of the sulfonimide in PFIA leads to interesting differences in the morphology and thermomechanical properties including the appearance of an anomalous glass transition that is not observed in single acid PFSAs. Techniques including dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and variable temperature small angle x-ray scattering (SAXS) are utilized to probe the anomalous glass transition and assign it to the true glass transition temperature for PFIA.

Figure 1
Phosphoric Acid Loss Mechanism of Acid-Base and Ion-Pair Coordinated Proton Exchange Membranes
Authors: Kim Yu Seung , Lee Albert S , Choe Yoong-Kee , Matanovic Ivana
Published: February 2020

Acid retention of phosphoric acid-doped proton exchange membranes (PEMs) is one of the most critical factors that determine the performance and durability of high-temperature PEM fuel cells. However, the mechanism of phosphoric acid loss in the PEMs in the presence of water has not been discussed in terms of the energetics of phosphoric acid. In this study, we investigate the interactions of the phosphoric acid cluster by 31P NMR experiments and density functional theory calculations to propose new phosphoric acid loss mechanism of the phosphoric acid-polybenzimidazole PEM and phosphate-quaternary ammonium ion-pair PEM.

The small molecule study suggests that the removal of the phosphoric acid from the phosphoric acid cluster does not occur due to the stronger interaction of phosphoric acid and water. On the contrary, adding water molecules to the PA cluster increases the cluster interaction energy by creating hydrogen bonding. In the phosphoric acid incorporated PEMs, we observed a consistent result that adding water increases the PA cluster volume rather than losing phosphoric acid. However, with excessive water activity, the polymer loses phosphoric acid due to the incapability to hold the water and phosphoric acid beyond a certain level.1 In the phosphoric acid loss process, the equilibrium phosphoric acid composition in the cluster plays a critical role.

In the latter part of this presentation, we explain how the acid-base interaction and ion-pair interaction impact the equilibrium phosphoric acid composition the phosphoric acid cluster. We also explain how the equilibrium phosphoric acid composition affects the proton conductivity at different RH level. Finally, we demonstrate the high-temperature membrane fuel cell durability in the presence of water to prove the newly proposed acid loss mechanism. This presentation provides critical insights to design high-temperature proton exchange membranes.

Reference

1. A. S. Lee, Y. -K. Choe, I. Matanovic, Y. S. Kim, The energetic of phosphoric acid interactions reveals a new acid loss mechanism, J. Mater. Chem. A. DOI: 10.1039/C9TA01756A (2019).
Self-Supported Nanocolumnar Platinum Thin Film Catalysts for Oxygen Reduction Reaction
Authors: Ergul Busra , Begum Mahbuba , Yang Zhiwei , Perry Mike L. , Macauley Natalia , More Karren L. , Karabacak Tansel
Published: February 2020

Nanocolumnar platinum thin films (Pt-TFs) were produced by high pressure sputtering (HIPS) and investigated as oxygen reduction reaction electrocatalysts for polymer electrolyte membrane fuel cells. Conventional high-density Pt-TF by low pressure sputtering was also studied for comparison. Pt-TFs were deposited on a microporous layer (MPL)-like surface composed of carbon particles in order to mimic the catalyst coated gas diffusion layer in a membrane electrode assembly (MEA). Scanning electron microscopy and transmission electron microscopy images revealed that HIPS Pt-TFs developed a cauliflower-like columnar microstructures, which originated from a shadowing effect. At high working gas pressure conditions of HIPS, sputtered Pt atoms go through several gas phase collisions, lose their directionality, and can arrive at the substrate surface at oblique angles that results in the formation of columnar islands that shadow the gaps among them from incident flux of atoms. This shadowing effect is believed to be enhanced on the rough surface of MPL-like carbon particles that leads to the formation of nano-cauliflower structures. Electrochemical characterization of the samples was performed by cyclic voltammetry and rotating disk electrode measurements on Pt-TF/MPL-like-layer/glassy-carbon-insert samples in aqueous perchloric acid electrolyte. With this approach, we also aimed to relate the catalyst performance obtained by benchtop tests directly to MEA results. Electrochemically active surface area (ECSA) of PT-TFs were found to be ranging from 10 m2/g to 19 m2/g with increasing sputter pressure, which was determined from the charge for hydrogen adsorption and desorption in the cyclic voltammograms. In addition, carbon monoxide (CO)-stripping of Pt-TF coated gas diffusion electrodes were performed in a fuel cell station and similar ECSA values were recorded (ranging from 11 to 15 m2/g). This indicated that successful mimicking of a gas diffusion electrode in an MEA can be achieved by utilizing an MPL-like substrate surface for benchtop tests in aqueous electrolyte environment as opposed to directly depositing Pt-TFs on glassy carbon inserts. Specific activity (SA) values for conventional high-density and nanocolumnar films were similar that was approximately ~600 μA/cm2, which is believed to correspond to grain sizes of >5 nm as obtained from TEM and XRD analysis. On the other hand, mass-specific (MA) activity values increased from 0.06 A/mgPt for conventional Pt-TF to 0.13 A/mgPt for HIPS Pt-TFs. Higher MA agrees with the columnar microstructure of HIPS thin films that increases the surface to volume ratio of the Pt layer that provides a better catalyst utilization compared to conventional Pt-TFs.
Ionomer Self-Assembly during Drying Process of Alcohol/Water Solutions Using Coarse-Grained Molecular Dynamics Simulations
Authors: Mabuchi Takuya , Tokumasu Takashi
Published: February 2020

The cathode catalyst layer (CL) is one of the principal components for proton exchange membrane fuel cells. CL is typically prepared by drying a catalyst ink consisting of Pt-C (platinum on a carbon powder support), ionomer, and solvents (water/alcohol mixtures). The structure and stability of ionomer aggregates in solutions are the key to the physical properties of PEMs, including water distribution, conductivity, and mechanical response of the membrane. Thus, understanding the ionomer aggregation behaviors in solutions is of great importance to gain a deep insight into fundamental morphological characteristics of ionomer membranes. The characterization of the ionomer aggregate structures in solutions has also been extensively explored in detail using experimental measurements. In this study, self-assembly of ionomers in a mixture of 1-propanol (NPA) and water have been investigated using coarse-grained molecular dynamics simulations. The dependence of NPA content on the ionomer structures was studied by systematically changing the NPA content in the system. The self-assembly behavior of ionomers into cylindrical bundle-like aggregates was observed and the aggregate size was found to be dependent on the NPA content in solutions. The effects of drying process on the ionomer aggregations will also be discussed in the presentation using the dimensionless Peclet number, which is used to describe the importance of evaporation to diffusion in a film of initial thickness.

Figure 1
Pentavalent Metal Doped TiO2 As Corrosion-Resistant Electrocatalyst Supports in Polymer Electrolyte Membrane Fuel Cells
Authors: Kodali Mounika , Asset Tristan , Herrera Sergio , He Cheng , Sankarasubramanian Shrihari , Matanovic Ivana , Ramani Vijay , Atanassov Plamen
Published: February 2020

Although being an extremely promising technology, proton exchange membrane fuel cells (PEMFCs) commercialization is still hindered by several challenges, such as the cathode (where the oxygen reduction reaction – ORR – occurs) catalyst activity and stability. Pt nanoparticles homogeneously distributed over carbon-based supports are state of the art catalyst materials at the cathode. However, carbon supports are particularly susceptible to electrochemical corrosion at high potentials. Hence, support materials with an higher stability are needed [1]. Pt supported on TiO2 shows an improved corrosion resistance for ORR compared to carbon supported catalysts [2]. However, the low intrinsic conductivity of TiO2, as well as the high cost of Pt, require further materials optimization before these technologies can hit the market. Here, we present our results on the modification of the electronic structure of TiO2 by doping with pentavalent metals such as Ta, Nb, W, and Mo to improve the conductivity of the support, as well as stabilize Pt on the support surface.

DFT calculations were performed to understand the band structure of doped M:TiO2 (4 at%, M= Ta, Nb, W, Mo) which shows that these materials will be n-type semiconductors, characteristic of defective or nonstoichiometric oxides [3]. The density of d-states for the Pt supported on M:TiO2 was calculated and compared to that of unsupported Pt as shown in Figure 1, evidencing a shift in the d-band center towards lower values (i.e. for M = Ta (-2.34 eV) < Nb (-2.32 eV) < W (-2.30 eV) < Mo (-2.28 eV)). Shift in the d-band center to lower values reduces the strength by which Pt supported on M:TiO2 binds oxygen, hence increasing its reactivity toward the ORR. Based on these results, we predict Ta:TiO2 will not only possess improved conductivity, but also facilitate ORR reduction similarly to Pt alloyed with Co and Ni [4]. Then, we applied those principles to synthesize, and characterize for the ORR, M:TiO2 supported Pt electrocatalysts.

References:

[1] X. Yu and S. Ye, J. Power Sources., 172(1), 145–154, (2007).

[2] S. Bagheri, N. Muhd Julkapli, and S. Bee Abd Hamid, Sci. World J., 2014, 1–21, (2014).

[3] C. He, S. Sankarasubramanian, I. Matanovic, P. Atanassov, and V. Ramani, ChemSusChem., (2019).

[4] V. Stamenkovic et al., Angew. Chem. Int. Ed., 45(18), 2897–2901, (2006).

Figure 1
Development of Fe/N-Doped Carbon Nanotubes As a Stable Non-Precious Electrocatalyst for Oxygen Reduction Reaction
Authors: Xia Dongsheng , Yang Xin , Kang Feiyu , Li Jia , Gan Lin
Published: February 2020

Heteroatom-doped carbon materials, particularly carbons co-doped with single atomic transition metals (e.g. Fe) and nitrogen, have emerged as one of the most competitive non-precious electrocatalysts for the cathodic oxygen reduction reaction (ORR) in proton-exchange-membrane fuel cells to replace precious platinum (Pt) catalyst. Most carbon-based non-precious electrocatalysts, however, were built on conventional carbon materials such as activated carbon and more recently metal-organic-framework-derived carbon, both of which feature essential drawbacks of carbon corrosion and low electron conductivity due to their low extent of graphitization.1-2

We report here the direct synthesis of well-defined CNTs co-doped with a high density of Fe-Nx active sites (denoted as Fe-N/CNT) through a solid-phase catalytic growth approach, where Fe precursor catalyzed the growth of CNTs from melamine sponge precursor under NH3 atmosphere and simultaneously doped in-situ into the CNT walls with nitrogen. The atomic structure and electrocatalytic properties of the Fe-N/CNT electrocatalysts were characterized by aberration-corrected scanning transmission electron microcopy, Mößbauer spectroscopy, electrochemical analysis and density functional theory calculations. We further design and synthesis a hierarchical Fe-N/CNT electrocatalyst where primary Fe-N/CNTs are enwrapped with secondary Fe-N/CNTs in order to increase the proportion of the Fe-N/CNT active site. The optimized Fe-N/CNT catalyst featured with a high degree of graphitization and high density of Fe-Nx catalytic sites, demonstrating both high ORR activity and remarkable stability in both half-cell test and a single hydrogen fuel cell membrane-electrode-assembly test.

Reference:

[1] Lefevre M, Proietti E., Jaouen F., Dodelet J. Iron-based catalysts with improved oxygen reduction activity in polymer electrolyte fuel cells. Science, 324, 71-74 (2009).

[2] Chen, YJ. Ji SF., Wang D.S., Li Y.D., Angew Chem Int Ed, 56, 6937-6941 (2017)

[3] Xia D.S., Wang R.Z., Wei Y.P., Gan L., Kang F.Y. Melamine-sponge-derived non-precious fuel cell electrocatalyst with hierarchical pores and tunable nitrogen chemical states for exceptional oxygen reduction reaction activity. Materials Today Energy, 9, 271-278 (2018)
Contamination of Metal Impurities during Hydrogen Evolution/Oxidation Reaction
Authors: Weber Daniel Jochen , Dosche Carsten , Oezaslan Mehtap
Published: February 2020

One of the most promising fuels in the near future will be hydrogen which should produce only by water

electrolysis.1 Although the kinetics of the hydrogen oxidation/evolution reaction (HOR/HER) on platinum

are extremely fast in acidic media, its catalytic performance tremendously drops by two order of magnitudes

in alkaline media.2

To obtain kinetic parameters of electrochemical reactions, e.g. HOR/HER or oxygen reduction reaction

(ORR), rotating disc electrode (RDE) experiments are the main tool in electrochemistry nowadays.

Although the RDE experiments can be performed very easily, fast and cost-efficiently in the labs, reliable

and reproducible evaluation of catalytic performances of materials remain still a critical challenge. In

particular, various sources of impurities need to take into account and to be eliminated prior to

electrochemical measurements. Not till then, structure-activity relationship of electrochemical reactions can

successfully be uncovered. In this study, we discuss possible contamination on polycrystalline platinum

during HER/HOR in various alkaline electrolyte solutions (KOH, NaOH and LiOH). We evidently show

that the observed contamination is not caused by the used hydrogen (quality of 99.999% vs. 99.9999%) or

by glass corrosion which can be avoided by frequently exchange of electrolyte solution. More importantly,

we point out that trace metals stemmed from commercially available used electrolyte salts quickly deposit

on a pure platinum surface during the initial HOR/HER polarization measurements and dramatically

decrease the electrochemically active Pt surface area (HUPD). In addition, we determined the losses of

HUPD over a broad temperature range in KOH, NaOH and LiOH. Interestingly, by chronoamperometric

investigations on a platinum foil followed by surface-sensitive X-ray photoelectron spectroscopy (XPS)

analysis we could detect qualitatively metal impurities originated from the electrolyte solution. We provide

a chronoamperometric method for further purification of electrolyte solution to reduce the contamination

and thus the HUPD losses after the HOR/HER. In this talk, we show critical approaches and solutions

which enable reliable and reproducible HOR/HER activity measurements on platinum in alkaline media.

References

1. M. T. M. Koper, Nature chemistry, 5, 255 EP - (2013).

2. J. Durst, A. Siebel, C. Simon, F. Hasché, J. Herranz and H. A. Gasteiger, Energy Environ. Sci.,

7(7), 2255–2260 (2014).
Electrodeposition of Graded Metal Coatings on Spherical Mandrels
Authors: Peters Vanessa N. , Horwood Corie , Bhandarkar Neal , Stadermann Michael , Bunn Thomas
Published: February 2020

Inertial confinement fusion energy research at the National Ignition Facility (NIF) requires the fabrication of hollow metal spherical capsules for the laser-driven fusion reaction. These sub-millimeter diameter capsules must have perfect coating uniformity and a specific gradient density, achieved by plating two metal elements with graded composition. We are investigating electrodeposition to produce hollow metal spheres as opposed to other deposition techniques (i.e. physical vapor deposition, film casting, etc.), because electrodeposition coats faster, more smoothly, with good thickness uniformity, and better grain size control.

Electroplating metals on spheres presents its own specific challenges; the spherical samples must make contact with the electrified cathode, but must also remain in motion during plating to ensure an even coating. Additionally, electroplating on hollow spheres also involves the challenge of keeping the sphere submerged in the electrolyte. These issues were mitigated by enclosing the mandrel in an electrified cage which the electrolyte is flowed through, resulting in intermittent contact of the mandrel with the cathode and continuous flow of fresh electrolyte to the mandrel surface while plating.

We have developed several approaches for electroplating alloys[1] and density gradients on metal spherical mandrels. We will present results from a parametric study that improved coating uniformity, obtained desired thickness, and achieved the required density gradients during electrodeposition.

[1] C. Horwood, M. Stadermann, T.L. Bunn, Metal Alloy ICF Capsules Created by Electrodeposition, Fusion Sci. Technol. 73 (2018) 335–343. doi:10.1080/15361055.2017.1387458.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. IM Release# LLNL-ABS-748349
Noble Metal Decorated Anodic TiO2 Nanotubes: Excellent (Electro)Catalyst
Authors: Krbal Milos , Sopha Hanna , Zazpe Raul , Prikryl Jan , Ng Siowwoon , Macak Jan M.
Published: February 2020

The synthesis of highly-ordered nanostructures of valve metal oxides has recently attracted huge scientific and technological interest motivated by their possible use in many applications. The most established member of this group of materials is nanoporous Al2O3, which has been prepared two decades ago by anodic oxidation of Al into perfectly ordered, honeycomb-like porous structures employing suitable electrochemical conditions [1]. Owing to the flexibility of the pore diameter/length and the relative ease of the Al2O3 dissolution, its porous membranes have been since than widely used as template material of choice for a range of materials [2-4].

Self-organized TiO2 nanotube layers have received the highest attention after Al2O3 motivated by its range of applications, including photocatalysis, water splitting, solar cells and biomedical uses. Very significant research efforts have led to reproducible synthesis of self-organized TiO2 nanotube layers by means of anodic oxidation, during which the starting Ti substrate is converted into a highly-ordered nanotubular layer by anodization in suitable electrolytes [5-8]. Many new applications can be derived upon modification of the nanotubes by decoration, filling or coating by secondary materials [9].

Atomic layer deposition (ALD) has been shown to be an effective technique to coat TiO2 nanotube layers homogenously with secondary materials or to decorate them with noble metals [9]. The resulting structures offer many advantages and opportunities in various applications. TiO2 nanotubes in particular appeal as highly ordered electron-conductive supports for noble metal particles.

This presentation will focus in detail on the decoration of the nanotube layers of different aspect ratios by noble metals using ALD. We will show and discuss our recent catalytic results on water splitting [10] and electrocatalytic results on the oxidation of methanol [11].

References:

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Stat. Sol. (RRL) 1 (2007) 181.

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Electrolytes Based on Nanoscale Organic-Inorganic Hybrid Materials (NOHMs)
Authors: Cantillo Nelly M. , Bruce Maria , Hamilton Sara Triana , Foister Shane , Park Ah-Hyung , Zawodzinski Thomas A.
Published: February 2020

The potential use of nanoparticle organic hybrid materials (NOHM) as electrolytes in redox flow batteries is being explored. NOHMs are composite particles formed by polymers grafted (either ionically or covalently) onto surface-modified inorganic nanoparticles. Such structure has great potential to capture and carry nonpolar electroactive species.1

In this work, the protonation behavior, and the effect NOHM structures have on metal cation uptake is evaluated. Both NOHMs considered in this analysis comprise silica cores ionically grafted with polymer tails. The polymer tails correspond to polyethylimine for the NOHM-I-PEI, and to polyetheramine with a sulfonic acid linker for the NOHM-I-HPE.

Cyclic voltammetry was used to investigate the interaction of the copper species as the concentration of the NOHM of interest increases (Figure 1). NOHM-I-PEI showed inhibition of the oxidation and reduction peaks as the NOHM concentration increased, suggesting the formation of the cuprammonium complex. The formation of this complex was also explored via acid-base titration and UV-Vis.

On the other hand, the dynamics of neat and NOHM-based fluids was explored via NMR. DOSY was used to explore the diffusion coefficients of the pure polymers and the NOHMs.

References

1. Y. Park, C. Petit, P. Han, and A. H. Alissa Park, RSC Adv., 4, 8723–8726 (2014).

Figure 1
Heat-Treated Transition Metal Hexacyanometallates As Electrocatalysts for the Oxygen Reduction Reaction
Authors: Zakrzewska Barbara , Miecznikowski Krzysztof , Dembinska Beata , Jablonska Anna , Zoladek Sylwia , Rutkowska Iwona Agnieszka , Negro Enrico , Kulesza Pawel J. , Di Noto Vito
Published: February 2020

In the recent research years is observed huge interest in design of new functional nanomaterials for developing new electrocatalysts for oxygen reduction reaction (ORR) in acid and alkaline media. The research has been focused on developing carbon nanostructures materials with the different transition metals (e.g. Co, Ni, Ag, Au, Cu, Mn) analogue of polynuclear Prussian Blue for fuel cell applications. Electrocatalysts based on graphene and graphene-oxide (GO) are more homogeneous and possess properties such as excellent conductivity, good chemical stability and can be functionalized in a controlled manner. Following the heat-treatment step at higher temperatures, some thermal decomposition of the cyanometallate network occurs and, consequently, metallic sites are generated. Their formation and distribution are facilitated by the voltammetric potential cycling in acid and alkaline electrolytes. The most promising electrocatalytic results with respect to the reduction of oxygen (the highest currents and the most positive electroreduction potentials) have been obtained when carbon nanostructures materials are combined with transition metal hexacyanometallates. An electrocatalytic system, that utilizes transition metal hexacyanometallates nanoparticles modified carbon nanostructures materials (e.g. reduced graphene oxide, carbon nanotubes), is developed and characterized here using transmission electron microscopy and such electrochemical diagnostic techniques as cyclic volammetry and rotating ring-disk voltammetry in a 0.5 M H2SO4 electrolyte and in a 0.1 M KOH electrolyte and upon introduction (as cathode) to the low-temperature hydrogen-oxygen fuel cell. Comparative measurements have been performed against the model noble metal (Vulcan-supported platinum nanoparticles) catalyst.
Electrocatalytic Activities of Atomically Controlled Metal Nanoclusters for Clean Energy Conversion
Authors: Choi Woojun , Kwak Kyuju , Kim Minseok , Lee Dongil
Published: February 2020

Thiolate stabilized metal nanoclusters containing a few to a few hundreds of atoms exhibit distinctive optical, electrochemical and catalytic properties. In various catalytic applications, metal nanoclusters show unique electrocatalytic activities for clean energy conversion such as hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and CO2 reduction reaction (CO2 RR). The catalytic activities and selectivity of metal nanoclusters can be modified by introducing foreign metal and surface modification. Here we present electronic structures and catalytic activities of atomically controlled Au25(SR)18 (SR = thiolate) and bimetallic nanoclusters. Au25(SC6H13)18–graphene composite led to four electron pathway for ORR revealed by rotating ring-disk electrode experiments. This result indicates that facile oxygen reduction to water occurred at this composite electrode. HER activities of metal nanoclusters were examined by constant potential electrolysis and the binding energies were calculated by density functional theory. HER turnover frequency (TOF) of Au25(SC6H13)18 (8.8 s-1 at -0.6 V vs. RHE) increased by doping of Pt (33.4 s-1). This enhanced HER activity upon Pt-doping is attributed to the facile H-binding on the Pt dopant. TOF of Pt-doped nanoclusters was further increased by surface modification using proton-relaying ligands. Pt-doped nanocluster showed higher HER mass activity than the benchmarking Pt/C nanoparticles. Furthermore, high OER activity of metal nanoclusters was further revealed in alkaline media.
A Charge-Transfer-Resistance Model for Proton Transmission through CVD Single-Layer Graphene in Proton-Exchange-Membrane Cells
Authors: Creager Stephen E , Bukola Saheed
Published: February 2020

High-rate proton transmission through single-layer graphene has been reported by several research groups but the mechanism by which it occurs is still not clear. Low activation energies for proton transmission have been experimentally obtained but are difficult to reconcile with computational modeling studies that predict high activation energies and low transmission rates for protons passing through pristine single-layer graphene at ambient temperatures. It seems likely that a reaction coordinate with an activated complex structure involving proton passage through graphene is involved, though the structural details of such a complex are still not clear. This lecture will review some of our recent experimental results on proton transmission rates through CVD single-layer graphene at variable temperature, with special attention to a kinetics model whereby experimentally-obtained proton-transfer resistances are interpreted as proton-transfer resistances that are linked to heterogeneous proton—transfer rate constants, in a manner that is formally similar to the Butler-Volmer model used to treat electron-transfer rate data in the low overpotential limit. The heterogeneous proton-transfer rate constants are then interpreted using a pre-equilibrium model for protons approaching graphene, coupled to a first-order rate constant for proton transmission through graphene from the pre-equilibrium complex. This first-order proton-transmission rate constant has fundamental significance and may be interpreted in terms of an activation energy and frequency factor for proton transmission. In applying such an analysis to our proton transmission rate data, we obtain a proton-transmission frequency factor that is within a factor of two of the vibrational frequency of the O-H bond in water, suggesting that O-H bond vibration is involved in the rate-determining step of proton transmission.
Mechanisms of Mobile Ion Hopping in Polymerized Ionic Liquids
Authors: Luo Xubo , Liu Hongjun , Paddison Stephen J.
Published: February 2020

Polymerized ionic liquids (polyILs) present a subclass of solid polymer electrolytes, where the free ions are mobile while the counter ions are grafted to the polymer chains 1. As a result of their unique chemical and physical properties, polyILs are a new class of materials with potential applications as the electrolytes for battery and fuel cells 2. It is believed that ionic conductivity is dependent on the chemical structures of both the polymer and counter ions 3, which enables us to tune the properties of polyILs.

To systematically study the effects of chemical structure and the choice of ions, molecular dynamics (MD) simulations where undertaken to provide atomic-level information of the ion hopping mechanisms. Four types of hopping were identified as: (1) intrachain; (2) interchain; (3) non-associating free ions; and (4) rattling. To find out the effects of different cationic groups and the length of alkyl tails, simulations were performed for poly(n-alkyl VIm) and poly(n-alkyl VATMA), with the counter ion of TFSI¯. Subsequently, the anion was substituted by Br¯ and BF4¯ for the poly(pentyl VIm) to investigate the effects of anions. Structural properties were analyzed from the results. The numbers of associating cations and chains to the anions were calculated, and the changing in the anion hopping types were found for the systems mentioned above. This quantitative analysis should guide the design of polyILs for electrochemical applications in the future.

(1) Stacy, E. W.; Gainaru, C. P.; Gobet, M.; Wojnarowska, Z.; Bocharova, V.; Greenbaum, S. G.; Sokolov, A. P. Fundamental Limitations of Ionic Conductivity in Polymerized Ionic Liquids. Macromolecules
2018, 51 (21), 8637–8645.

(2) Green, O.; Grubjesic, S.; Lee, S.; Firestone, M. A. The Design of Polymeric Ionic Liquids for the Preparation of Functional Materials. Polym. Rev.
2009, 49 (4), 339–360.

(3) Mecerreyes, D. Polymeric Ionic Liquids: Broadening the Properties and Applications of Polyelectrolytes. Prog. Polym. Sci.
2011, 36 (12), 1629–1648.

Figure 1