bluuebunny/crossref_metadata_2025 huggingface dataset:
(Invited) 2D Materials: Crystal Growth for Future Device Structures
Authors: Colombo Luigi
Published: February 2020

The isolation of graphene, now over decade ago, has given rise to the revitalization of an old full set of materials, two-dimensional materials (2DM), which have exceptional electrical, chemical and physical properties. Some of the materials under investigation in addition to graphene are hexagonal boron nitride (h-BN), semiconducting, metallic, and superconducting transition metal dichalcogenides (TMD) with a general chemical formula, MX2 where M is for example equal to Mo, W, Ta, Nb, Zr, Ti, and X = S, Se and Te, and others. While graphene is a material with many exceptional properties and h-BN is an excellent 2D insulator, TMD materials provide what neither graphene nor h-BN can, bandgap engineering that, in principle, can be used to create new devices that cannot be fabricated with h-BN and graphene alone. There is hope that 2DM can be integrated to fabricate numerous device types for many applications ranging from inkjet-printed circuits, photonic applications, flexible electronics, and high-performance electronics. However, to fully realize the benefits of these materials, the community will have to work together to define new device structures, device integration schemes, and materials growth processes to help the semiconductor industry make progress toward meeting the original goals of the International Technology Roadmap for Semiconductors (ITRS).

A number of deposition/growth techniques have been used to prepare large area graphene, such as growth on SiC through the evaporation of Si at high temperatures, precipitation of carbon from metals, and catalytic chemical vapor deposition on Cu and Pt. Direct growth of good quality graphene on dielectrics/semiconductors other than SiC with reasonable properties has also been reported recently on Ge. Due to its insulating nature and compatibility with graphene and TMDs, the preparation of large area h-BN is also being developed on both metals and dielectrics. Transition metal dichalcogenides, however, present altogether different opportunities and difficulties in the preparation of low defect density large area single crystals. Vapor transport, chemical vapor deposition (CVD), and molecular beam epitaxy (MBE) are being developed to produce these materials for initial studies of materials physics and device fabrication. In addition, there is some effort in performing simulations to guide growth for both CVD and MBE growers. Therefore, there is an opportunity here to have the crystal growers and the modeling community collaborate to develop high quality materials and processes.

A number of devices structures are currently under evaluation to take advantage of the basic properties of graphene, bi-layer graphene, h-BN and TMDs. Some of the devices are based on tunneling phenomena while others are based on excitonic phenomena. In this presentation I will present the state of the art results of graphene, h-BN, and a few TMD materials and their prospects for future electronic device applications.
Controlling the Performance of Si(111) Photoelectrodes Via Molecular Modification
Authors: Boucher Dylan G , Rose Michael J
Published: February 2020

The small bandgap of silicon (1.1 eV) and large photocurrent density (~25 mA/cm2 under 100 mW/cm2 illumination) renders silicon a nearly ideal photocathode material for photoelectrochemical hydrogen evolution (PEC-HER). However, freshly etched silicon surfaces are highly unstable and readily react with both air and solution to form chemically amorphous junctions that limit device efficacy. Stabilization of Si(111) by alkylation has resulted in well-passivated and chemically well-defined surfaces. However, the resulting interfacial dipole of the alkylated surface shifts the onset of HER to negative efficiencies, even on Pt decorated surfaces. To this end, our group has explored low-temperature, solution-based processing methods to significantly alter the interfacial properties of Si(111). Chemically derivatized surfaces (R: phenyl, para-nitrophenyl, 9-anthracene, and 9-nitroanthracene) have been achieved using organolithium precursors. Mott-Schottky analysis revealed energetic shifts in the band-edge positions of the surfaces with unity correlation to potential shifts in PEC-HER onset of pSi-R|TiO2|Pt devices. This indicates quantitative control of the photovoltage through molecular design, even in the presence of MOx and catalyst overlayers. In addition, we find that the molecular identity of the modifier plays a significant role in determining rates of surface recombination; we explore this relationship with two model systems: para-nitrophenyl (low surface recombination) and meta-dinitrophenyl (high surface recombination) and develop design principles for retaining high-quality surfaces with low rates of recombination. Finally, we utilized kinetically simple systems (one-electron, outer sphere redox couple) to demonstrate the effects of surface recombination, TiO2 deposition, and Pt deposition on the current-voltage character of these surfaces in the absence of a catalytic overpotential. Without the convoluted kinetics of catalysis, we develop a detailed view of the interfacial energetics of the chemically complex pSi-R|TiO2|Pt junctions.
(Invited) A New Perspective for Photoelectrochemical Water Splitting: Black TiO2 Nanotubes
Authors: Schmuki Patrik , Liu Ning , Altomare Marco
Published: February 2020

In the past decade, anodically formed self-organized TiO2 nanotube layers attracted considerable scientific interest (see e.g. reviews [1,2]). This is mainly due to the fact that these layers combine the defined nanotubular geometry with a broad range of functional features inherent to TiO2. Particularly in photoelectrochemical water splitting nanotubular layers have shown remarkable results. The presentation will first give an overview of the most promising features of classic nanotube layers for photocatalysis and then focus on TiO2 nanotube (NT) arrays, converted by a high pressure H2 treatment to anatase-like "black titania"[3,4]. This modification shows a remarkable high open-circuit photocatalytic hydrogen production rate without the presence of any cocatalyst. Other attempts to form black titania, namely using classic reduction treatments (e.g., atmospheric pressure H2/Ar annealing) do not show this surprising effect. We will discuss the main difference caused by the high H2 pressure annealing and the origin of this outstanding activity of black titania.

REFERENCES

Roy, S. Berger, P. Schmuki, Angew. Chem. Int. Ed. (2011), 2904

Lee, A. Mazare, P. Schmuki, CHEMICAL REVIEWS, 114 (2014) 9385

Liu, P. Schmuki et.al. Nano Letters, 14 (2014) 3309

Liu, P. Schmuki et.al. 53 (2014) 14201
Quantum-Enhanced Hydrogen Evolution Reactions in a Monolayer Crystal
Authors: Yan Mengyu , Brischetto Martin , McCune Curt , Yang Jihui
Published: February 2020

We have demonstrated the Fermi-level dependent catalysis performance in hydrogen evolution reactions 1-2. To go one step further, there is still plenty of room for catalysts to improve their performance if we can modulate the electronic structure without altering the chemical composition. Two-dimensional materials, which have shown unusual layer-dependent electronic structures and the corresponding quantum effects, are an excellent choice to verify this hypothesis. Here, 2H-MoS2 is chosen as an example due to its high activity for hydrogen evolution reactions. It is well known that the in-plane bandgap of monolayer MoS2 (~1.85 eV) is larger than that of the multilayer (~1.30 eV), resulting in inferior conductivity and presumably poorer electrocatalysis activity. Surprisingly, we reveal here that the monolayer 2H-MoS2 electrocatalyst possesses a small overpotential of 30 mV at 70 mA/cm2 and a Tafel slope of 34 mV/dec, considerably superior to that of the multilayer 2H-MoS2 (550 mV at 70 mA/cm2; Tafel slope: 103 mV/dec). Our studies suggest a relationship among the electronic structures, material thickness, and the Helmholtz layer. This work highlights opportunities for developing superior catalysts based on the unusual electronic structures of atomically thin materials.

Reference:

Mengyu Yan; Xuelei Pan; Peiyao Wang; Fei Chen; Liang He; Gengping Jiang; Junhui Wang; Jefferson Z Liu; Xu Xu; Xiaobin Liao, Field-effect tuned adsorption dynamics of VSe2 nanosheets for enhanced hydrogen evolution reaction. Nano Letters
2017,
17 (7), 4109-4115.

Junhui Wang; Mengyu Yan; Kangning Zhao; Xiaobin Liao; Peiyao Wang; Xuelei Pan; Wei Yang; Liqiang Mai, Field Effect Enhanced Hydrogen Evolution Reaction of MoS2 Nanosheets. Advanced Materials
2017,
29 (7).
Semiconductor Nanostructure Tailoring Via Spontaneous Interface Shaping for Light-Collection Maximization
Authors: Carim Azhar I , Batara Nicolas A. , Premkumar Anjali , Atwater Harry A , Lewis Nathan S
Published: February 2020

Electrodeposition of Se-Te alloy films under illumination spontaneously generates nanopatterned films with significant periodic order. The feature sizes, periodicities, anisotropies, and orientations of the nanoscale pattern can be tailored by manipulating the input optical excitation. Isotropic morphologies consisting of ordered arrays of nanopores were generated using unpolarized illumination whereas linearly polarized light resulted in highly-anisotropic nanoridge/trough morphologies with the in-plane orientation of the patterns controlled by the direction of the light polarization. The pattern periodicity was encoded by the illumination spectral profile. A single periodicity in single spatial direction was only generated even with the use of broadband and multimodal spectral profiles and multiple polarization inputs and the periodicity was found to be sensitive to all investigated tuning of such profiles. Structures with nonequal periodicities in the two orthogonal in-plane directions could also be generated and both periodicities could be independently controlled. Modeling of the growth using a combination of full-wave electromagnetic simulations of light absorption and scattering coupled with Monte Carlo simulations of mass addition successfully reproduced the experimentally observed morphologies and indicated that morphology development was directed by evolution of the growth interface to maximize anisotropic light collection.
Emergent Growth of Semiconductor Nanopatterns Via Directed Illumination
Authors: Meier Madeline Claire , Carim Azhar I , Yalamanchili Sisir , Thompson Jonathan R , Atwater Harry A , Lewis Nathan S
Published: February 2020

A uniform, incoherent beam of light can exert control over the morphology and growth direction of an evolving, inorganic semiconductor film in three-dimensional space. This phenomenon is an emergent process that has been observed in the light-directed deposition of semiconducting chalcogen alloys and results in the generation of highly anisotropic semiconductor nanostructures. Such morphology evolution is similar to natural phototroposism exhibited by many photosynthetic plants, including palm trees and sunflowers, wherein the physical extension of the biological system proceeds preferentially towards the time-averaged position of the sun while being influenced by competition from neighboring organisms. In analogy, during semiconductor deposition, the orientation and growth direction of the structures with respect to the substrate can be defined by controlling the angle of incidence of the light; mass is spontaneously added most preferentially to the localized regions capturing the maximal light flux. Morphological complexity can be generated by combining the use of inclined illumination with a series of defined polarization and wavelength inputs which allowed in-plane and out-of-plane orientation, spacing, and periodicity. Computer simulation of the growth using a probabilistic model based on light absorption successfully reproduced the empirically observed morphologies. The underlying optical basis was further investigated using a simplified light scattering model that corroborated the magnitude and direction of the empirically-observed phototropic responses.
(Invited) Enhancing the Performance of Polymer Thin-Film Transistors
Authors: Dodabalapur Ananth , Wang Xiao , Liang Kelly
Published: February 2020

One of the important challenges in the field of thin-film transistors is to improve designs that result in performance speeds. With polymer semiconductors, non-quasistatic circuits such as rectifiers in which the maximum frequency depends on the carrier transit time, have been demonstrated to work to a few 10’s of MHz. The challenge is to realize clocked sequential circuits that operate as speeds much larger than the few 10’s of kHz that have been demonstrated so far. This talk will describe several approaches to increase the performance characteristics of polymer thin-film transistors. The metrics that we seek to enhance include mobility, on/off current ratio, and short channel length performance. Using a combination of experimental data and theoretical calculations and simulations, we describe device architectures and fabrication methods that improve performance. This includes nanostructured geometries, gate dielectric engineering, and new materials combinations. We present a detailed theoretical model of charge transport in such polymers that provides considerable insight into the operation of such TFTs. There have been many reports of thin-film transistors based on semiconductors with mobilities in the range 1-20 cm2/(Vs) in the past few decades. The mobilities and mean free paths are simply too low for the application of conventional semiconductor transport theories based on solutions to the Boltzmann transport equation (BTE). There have been some material-specific treatments that have tried to circumvent the basic difficulty of the mean free path being less than, or at best comparable to, the lattice constant or the minimum transport lengths required for extended state transport. We will present a very general solution based on the statistical nature of charge transport and introduction of a factor related to the probability of mean free path exceeding the minimum transport length. We are then able to apply the BTE with appropriate scattering mechanisms and obtain very good results that agree well with experiment. This approach is very well suited to thin-film transistors based on polymer and organic semiconductors with room temperature mobilities in the range 1-20 cm2/(Vs).
(Electronics and Photonics Division Award) New Directions in GaN Electronic and Photonic Devices Enabled By Electrochemical Processes
Authors: Han Jung
Published: February 2020

In the past 30 years, GaN has emerged as one of the most important semiconductors that has made real impact to our society. In this talk I will share my perspective regarding the key breakthroughs, major applications, and under-explored opportunities associated with this material system. Especially I will focus on how electrochemistry can help to circumvent impasses and creates new dimensions in the exploration of electronic and photonic devices using GaN. Examples will be given in photonics and flexible electronics.

The research is supported by US Department of Energy, National Science Foundation, and DARPA.
New Avenues Towards Designing Transition Metal Phosphides for Efficient Electrocatalysis
Authors: Das Debanjan , Nanda Karuna Kar
Published: February 2020

Transition metal phosphides (TMPs) have emerged as an intriguing class of efficient electrocatalysts finding widespread application in hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) etc. Unfortunately, however, currently adopted wet chemistry and solid-gas synthesis techniques rely on the use of surfactants/ toxic organic solvents or toxic gas which restricts facile fabrication of TMPs. Moreover, hybridizing TMPs with suitable carbon support (graphene, carbon nanotubes etc.) which favorably optimizes the electrochemical activity of TMPs is often achieved via post-synthesis modification further complicating the synthesis procedure.

In an attempt to overcome these limitations, we have recently developed a solid state, phosphine free method to fabricate Co2P nanoparticles encapsulated N,P co-doped CNTs. These as prepared catalysts were applied as a bi-functional electrocatalyst for HER and OER. In order to increase the surface area and loading of Co2P nanoparticles, we next designed Co2P encapsulated CNTs grafted N,P co-doped hollow spheres via MOF-templated strategy with superior catalytic activity compared to Co2P@CNTs.

We then introduced sub-2nm IrP2 nanoparticles encapsulated bilayered N, P co-doped graphene as a novel electrocatalyst which exhibits better HER activity than benchmark Pt/C in acid, neutral as well as the alkaline medium. Interestingly, careful tuning of the reaction parameters also enables the synthesis of phase pure Ir2P. Finally, a MOF-templated strategy was adopted to fabricate N, P co-doped hollow carbon nanocages grafted with ~ 2nm Ru2P nanoparticles which exhibited superior bifunctional HER and ORR activity in alkaline medium.

In conclusion, the versatile strategy developed here can be extended to fabricate a variety of TMPs with widely varied applications.
One-Step Electrodeposition Synthesized High Performance Quarternary Ni-Co-S-P for Efficient Water-Splitting
Authors: Tian Yumeng , Yu Jing , Lin Zaiwen , Wang Jun
Published: February 2020

Environmental problems are getting worse and there is a growing demand for developing clean energy to replace decreasing fossil fuels. Hydrogen is one of the most promising alternatives to fossil energy. Electrochemical water splitting is a simple, efficient and environmentally friendly technology of hydrogen production. It has attracted more attention and research because it can transform electrical energy into chemical energy, which is more convenient for transportation and storage. However, overpotential often exists in practical application due to the influence of uphill reaction. The use of electrocatalysts can reduce the overpotential. For decades, people have made great efforts in lowering the driving overpotential and improving catalytic activity of low-cost electrocatalysts. There is a need for synergy in the following aspects: 1. Increasing the intrinsic activity and density of the catalytic sites. Optimizing the adsorption Gibbs free energy of reaction adsorption and constructing low dimensional morphology to increase the specific surface area of the catalysts. 2. Increase the carrier concentration and diffusion rate by combining with carbon materials or by doping impurities into intrinsic semiconductors. 3. Multicomponent synergism. For example, in nickel-cobalt alloyed, Ni sites is more favorable to desorption of hydroxide in alkaline condition, thus improving TOFs of catalyst. Meanwhile, Co sites offer a faster kinetics the Heyrovsky and Tafel steps, which is more favorable to the rapid detach of as-generated hydrogen. 4. Improving mass transfer performance is helpful to the rapid detachment of generating bubbles and the acceleration of water splitting. Transition metal phosphides (TMPs) have been extensively studied in the past decade because phosphide-based materials have faster kinetics than common oxides, sulfides, and hydroxides. Meanwhile, transition metal sulfides (TMSs), with relatively lower intrinsic electrical resistivity, which could facilitate charge transfer during the electrocatalytic HER/OER process, have attracted considerable interest. Based on the above design principles, we prepared a superaerophobic Ni-Co-S-P by one-step electrodeposition. The obtained catalysts have extremely low overpotential and favorable kinetic properties. This study provides a new approach for the development of non-noble metals catalyst toward electrochemical water splitting.
Building Blocks for Symmetric Multielectron Nonaqueous Redox Flow Batteries
Authors: Porath Anthony , Mann Shane C. , Dumitrascu Adina , Henton Daniel R. , Gillmore Jason G. , Guarr Thomas F.
Published: February 2020

A series of redox active species were synthesized and assessed for their potential use in nonaqueous redox flow batteries (RFBs). The building blocks for these compounds consist of electron donor groups D and electron acceptor groups A, which are capable of undergoing highly reversible electrochemical oxidation and reduction, respectively. Different combinations of donors and acceptors, such as Dx-A­y, and Dx-Ay-Dx, give rise to a myriad of derivatives with distinct electrochemical properties. The donor/acceptor systems exist in at least three stable oxidation states during RFB operation: reduced (anolyte), neutral, and oxidized (catholyte) forms. The scope of this work includes organic heterocyclic donors attached to pyridinium, imide, and thiadiazole accepting groups. Polypyridinium compounds have also been investigated as a possible mulitelectron accepting platform.

Covalently linking the donor and acceptor groups eliminates crossover, and allows one to design an RFB that uses an inexpensive porous separator rather than an ion exchange membrane, reducing overall cost. In addition, multielectron redox processes serve to increase energy density and further reduce costs. The overall goal of this project is development of a durable, efficient, and inexpensive RFB system for large-scale grid storage applications.

The synthesis, characterization and preliminary electrochemical investigation of these molecules will be described.
Screening Membranes and Electrolytes for High Performance Nonaqueous Flow Cells
Authors: Liang Zhiming , Attanayake N. Harsha , Barton John Leonard , Brushett Fikile R. , Landon James , Odom Susan A.
Published: February 2020

Due to their wider electrochemical windows of solvents and higher cell voltages, non-aqueous electrolytes containing organic active materials are of interest for use in redox flow batteries (RFBs) for grid energy storage.1 The main challenges to achieving high performance in a nonaqueous RFBs include (i) improving the solubility and stability of the active organic molecules, (ii) preventing the negolyte and posolyte molecules from crossing over separators or membranes, and (iii) to do so without also limiting charging rates due to high cell resistance.2 Using a range of commercially available membranes and separators, we sought to improve the performance of full flow cells containing two materials developed in our laboratory: N-[2-(2-methoxyethoxy)ethyl]phenothiazine (MEEPT)3 and bis[2-(2-methoxyethoxy)ethyl]viologen bis(trifluoromethanesulfonyl)imide (B(MEE)Vol-TFSI2). We first screened electrolyte salts and solvents to optimize for a combination of electrochemical stability and solubility after which we measured the impedance of various membranes and separators with a subset of the electrolytes. Finally, we assembled flow cells with select electrolytes, comparing performance using a separator vs. with ion-selective membranes. Our results showed that the two active materials are stable in cycling and that cell performance was ultimately limited by membrane performance: Either the capacity was halved with a non-selective separator, or impedance rose upon fouling of ion-selective membranes. These results highlight the need for membrane development for nonaqueous RFBs containing small molecule active materials.
Modelling of a Light-Weight, Flexible, Air-Breathing PEMFC
Authors: Kiessling Aleksandr , Mayer Matthew , Besser Ronald S.
Published: February 2020

Flexible, light-weight and air breathing proton-exchange membrane fuel cells (F-PEMFC) are promising electricity generators for various future applications like drones and small portable devices [1, 2]. However, compared with “classic” fuel cells, such devices have worse performance in terms of power density on the basis of membrane area (W/cm²). Furthermore, reproducible fabrication is a challenge; promising prototypes were hand-fabricated using expensive equipment. For commercial success, F-PEMFCs must improve in terms of performance and manufacturability. However, this requires a good understanding of the respective device details down to the basic level.

For this work, a COMSOL Multiphysics simulation of a F-PEMFC prototype [3] is built. The simulation will be fed with accurate catalytic and physical parameters and used for an interactive process of (A) describing the F-PEMFC’s operating characteristics in detail, (B) finding approaches for performance optimization and (C) comparing experimental with simulation results. Therefore, polarization curves are created, and reactant and current generation distributions are examined.

Although collection of accurate catalytic and physical parameters is ongoing, preliminary results with comparable data [4] showed that it is likely that the F-PEMFC design is limited by oxygen mass transport. Consequently, we will present results of various changes in device configuration which might enhance cell performance, for example, in the cathode’s catalyst layer morphology geometry.

References

[1] S. R. Mahmoodi and R. S. Besser, “Fabrication and Characterization of a Thin, Double-sided Air Breathing Micro Fuel Cell,” Fuel Cells, vol. 18, no. 4, pp. 499–508, 2018.

[2] F. Ning et al., “Flexible and Lightweight Fuel Cell with High Specific Power Density,” (eng), ACS nano, vol. 11, no. 6, pp. 5982–5991, 2017.

[3] R. S. Besser and S. R. Mahmoodi, “FLEXIBLE PLANAR DOUBLE SIDED AIR BREATHING MICROSCALE FUEL CELL,” US 2018 / 0138516 A1 15/814,208, May 17, 2018.

[4] C. Song et al., “PEM fuel cell reaction kinetics in the temperature range of 23–120°C,” Electrochimica Acta, vol. 52, no. 7, pp. 2552–2561, 2007.

Figure 1
Electron Microscopy Study of Degradation Mechanisms in Platinum Group Metal-Free Catalysts
Authors: Cullen David A. , More Karren L. , Osmieri Luigi , Neyerlin K.C.
Published: February 2020

Commercial low-temperature polymer electrolyte membrane fuel cells (PEMFCs) rely on expensive platinum-based catalysts to drive the oxygen reduction reaction at the cathode. In order to overcome cost barriers limiting market penetration in both transportation and stationary applications, significant efforts are being made to develop suitable platinum group metal-free (PGM-free) catalysts. The latest classes of PGM-free catalysts show very promising mass activity, but the poor durability of these carbon-based materials is still a significant challenge that must to be addressed. In this effort, advanced analytical electron microscopy methods were employed to study the degradation pathways of PGM-free catalysts which were submitted to accelerated stress tests. Catalysts provided by four different suppliers were tested concurrently in a specially designed segmented fuel cell. Polarization curves were recorded both before and after a durability stress test which involved 10k square wave cycles between 0.95 – 0.60 V in air. Analytical electron microscopy performed across a wide length scale (sub-nanometer to micron) will be presented which links the observed performance losses with changes to the atomically dispersed active sites, carbon support, and ionomer distribution. The advantages and disadvantages of different catalyst precursors (metal organic frameworks, cyanamide/polyaniline, etc.) will also be discussed from a durability standpoint. This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the Electrocatalysis Consortium (ElectroCat). Microscopy was performed as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility
CVD Graphene Modified with Au Nanoparticles As Electrocatalysts for Sodium Borohydride and Hydrazine Oxidation
Authors: Tamašauskaitė-Tamašiūnaitė Loreta , Balčiūnaitė Aldona , Zabielaitė Aušrinė , Vaičiūnienė Jūratė , Norkus Eugenijus
Published: February 2020

In the present study, the commercially available CVD graphene-Ni/SiO2/Si (Graphene-Supermarket) template was used as a substrate for the deposition of gold nanoparticles (AuNPs). The immersion of the CVD graphene into the 1 mM HAuCl4 solution at a temperature of 25 oC for 1 min results in the deposition of AuNPs on the surface of the CVD graphene-Ni/SiO2/Si. The surface morphology and structure as well as composition of the prepared catalysts were characterized by Scanning Electron Microscopy (SEM) and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The electrocatalytic activity of the AuNPs modified CVD graphene-Ni/SiO2/Si catalyst was evaluated for the oxidation of sodium borohydride and hydrazine using cyclic voltammetry and chrono-techniques.

It was found that the CVD graphene-Ni/SiO2/Si modified with AuNPs exhibits a significantly higher electrocatalytic efficiency for sodium borohydride and hydrazine oxidation as compared with that of pure CVD graphene-Ni/SiO2/Si electrode, and seems to be a promising anode material for direct borohydride and hydrazine fuel cells.
Visualization of Liquid Water in a Metal Foam Flow-Field Based Polymer Electrolyte Fuel Cell Via Neutron Radiography
Authors: WU Yunsong , Brett Daniel J.L. , Shearing Paul R.
Published: February 2020

Polymer electrolyte fuel cells (PEFCs) have shown significant advances in terms of performance, efficiency and durability for a wide range of applications. Flow-fields are crucial components that affect the water management and performance of PEFCs. The existence of gas channel and land configurations in the conventional flow-field designs render gas and water distribution highly non-uniform. Such issue can lead to a series of events detrimental to PEFC performance and longevity [1].

As an alternative, new type of flow-field such as metal foam has been introduced. In-depth understanding of water management in metal foam flow-field based PEFC is indispensable for the optimisation of performance and durability. Here, liquid water formation and transport across the 25 cm2 metal foam flow-field based PEFC is evaluated using neutron radiography. A nickel foam was put on cathode aluminium plate and a silicone gasket was then placed around the foam to seal in gases. A vertical single-channel serpentine was used for the anode aluminium flow-field, as shown in Fig. 1 (a). All experiments were conducted at the cold neutron radiography (CONRAD) beamline facility at Helmholtz-Zentrum Berlin (HZB). The setup for neutron imaging has been described in [2].

The result showed the enhancement of mass transport and cell performance in the metal foam flow-field compared to the conventional triple-serpentine design. Correlation of performance to neutron radiography reveals that the performance deviation in the mass transport region is likely due to flooding issues. The metal foam flow-field based PEFC exhibits less flooding and better uniformity in the local water distribution compared with that of serpentine flow-field design (Fig. 1 (b)).

[1] Wu, Y., et al. "Effect of serpentine flow-field design on the water management of polymer electrolyte fuel cells: An in-operando neutron radiography study." Journal of Power Sources 399 (2018): 254-263.

[2] Wu, Y., et al. "Effect of compression on the water management of polymer electrolyte fuel cells: An in-operando neutron radiography study." Journal of Power Sources 412 (2019): 597-605.

Figure 1
Impact of Surface Functionalization of Pt/Graphitized Carbon on Catalyst Layer Structure and PEMFC Performance
Authors: Taning Ahmad Zulfikri , Woo Seunghee , Park Seok-Hee , Yim Sung-Dae
Published: February 2020

The understanding of catalyst layers (CL) structure is of great importance to reduce Pt loading in a MEA of PEM fuel cell. Recent studies have shown that the CL structure is largely dependent on ionomer microstructure, affecting Pt utilization by controlling reactant transport [1]. The ionomer microstructure such as the distribution of ionomer thin film in the CL can be controlled by introducing functional groups on the carbon support to tune the interaction between PFSA ionomers and carbon supports, which was demonstrated by MD calculation [2] as well as experimental MEA study [3].

We have tried to introduce the surface functional groups to the Pt loaded highly graphitized carbon supports using non-covalent approaches [4] to resolve the non-uniform ionomer distribution issue, particularly in highly graphitized carbon-based catalysts. The pyrene aromatic compounds with different functional groups to form different surface charge were applied to the Pt/C commercial catalyst, and the surface modified catalysts were fabricated to cathode CL of MEAs through decal process. The MEA performance evaluated in a single cell showed that the Pt/C catalyst modified with amine groups performs better than the catalyst modified with other functional groups and non-functionalized reference catalyst. The reason of better MEA performance of the surface modified Pt/C catalyst will be discussed in terms of better ionomer distribution in the CL. In addition, the issues affecting the MEA performance such as the Pt poisoning by the functional groups during the non-covalent functionalization of Pt/C will also be presented.

[1] F. C. Cetinbas, R. K. Ahluwalia, N. Kariuki, V. D. Andrade, D. Fongalland, L. Smith, J. Sharman, P. Ferreira, S. Rasouli, D. J. Myers, J. Power Sources 344 (2017) 62.

[2] T. Mashio, A. Ohma, T. Tokumasu, Electrochim. Acta 202 (2016) 14.

[3] A. Orfanidi, P. Madkikar, H. A. El-Sayed, G. S. Harzer, T. Kratky, H. A. Gasteiger, J. Electrochem. Soc. 164 (2017) F418.

[4] H.-S. Oh, H. Kim, Adv. Funct. Mater 21 (2011) 3954.
A Microfluidic Artificial Photosynthesis Cell for CO2 Reduction on TiO2-CuO Photoelectrodes
Authors: Kalamaras Evangelos , Belekoukia Meltiani , Xuan Jin , Andresen John , Maroto-Valer Mercedes
Published: February 2020

Utilization of solar energy for photo-electrosynthesis of liquid fuels from water and CO2 is a very promising pathway to reduce anthropogenic CO2 emissions and tackle energy shortage problems. Over the last decades, great efforts have been done to develop efficient semiconducting photocatalysts that promote photoelectrochemical (PEC) CO2 reduction. On the other hand, reactor design of PEC cells has not gained much attention even though it can significantly affect the efficiency of such devices. In this study, TiO2-CuO were deposited on fluorine-doped tin oxide (FTO) substrates and were used as photoelectrodes in a continuous flow PEC microfluidic device under simulated solar light illumination (AM 1.5G) in the presence of CO2-saturated aqueous NaHCO3 electrolyte solution. The optimal photoelectrode exhibited a photocurrent density of 0.82 mA/cm2 at 0.3 V vs RHE while a solar-to-fuel (STF) efficiency of 0.13% was observed in microfluidic PEC cell.

Figure 1
(Invited) Multiple Heterojunction in Single Titanium Dioxide Nanomaterials for Photocatalysis and Photoelectrochemical Cells
Authors: Park Jong Hyeok
Published: February 2020

Despite a longstanding controversy surrounding TiO2 materials, TiO2 polymorphs with heterojunctions composed of anatase and rutile outperform individual polymorphs because of the type-II energetic band alignment at the heterojunction interface. Improvement in photocatalysis has also been achieved via black TiO2 with a thin disorder layer surrounding ordered TiO2. However, localization of this disorder layer in a conventional single TiO2 nanoparticle with the heterojunction composed of anatase and rutile has remained a big challenge. Here we report the selective positioning of a disorder layer of controlled thicknesses between the anatase and rutile phases by a conceptually different synthetic route to access highly efficient novel metal-free photocatalysis for H2 production. The presence of localized disorder layer within a single TiO2 nanoparticle was confirmed for the first time by high-resolution transmission electron microscopy (TEM) with electron energy-loss spectroscopy (EELS) and inline electron holography. Multiple heterojunctions in single TiO2 nanoparticles composed of crystalline anatase/disordered rutile/ordered rutile layers give the nanoparticles superior electron/hole separation efficiency and novel metal-free surface reactivity, which concomitantly yields an H2 production rate that is ~11 times higher than that of Pt decorated conventional anatase and rutile single heterojunction TiO2 systems.
Electrodeposition of Silver Metavanadate (AgVO3) As a Visible Light-Active P-Type Semiconductor
Authors: Vali Abbas , Firouzan Farinaz , Rajeshwar Krishnan
Published: February 2020

The silver-based ternary oxides are promising candidates to use as a visible light active semiconductor because their band gaps are decreased due to the presence of the Ag 4d with O 2p orbitals in the valence band. Silver metavanadate (AgVO3), one of myriad silver-based ternary oxides, is a visible light active semiconductor because of its small band gap (2.5 eV).1

This paper describes the electrodeposition and characterization of electrodeposition of AgVO3. Electrodeposition is a fast and low temperature method which can be used to make thin films of a given semiconductor. In addition, the optical parameters (e.g., photocurrent, flat band potential) of the thin film semiconductors prepared by electrodeposition can be measured easily without any more processing steps.

A two-step electrodeposition strategy was demonstrated for ternary oxides2 and was applied in this study, to prepare AgVO3 thin films on FTO substrate. In the first step, a silver thin film was deposited cathodically from silver nitrate solution. In the second step, the silver thin film was stripped anodically in ammonium metavanadate solution to generate Ag+ and subsequent in situ precipitation with VO3
- to yield AgVO3 thin film on the substrate. The electrochemical crystal micro balance (EQCM) analysis confirmed one electron transfer in both steps.

The XRD pattern of the as-prepared sample confirmed that the as-prepared thin film was in α-AgVO3 form. Energy-dispersive X-ray analyses of the as-prepared sample gave the composition: Ag0.94±0.02V1.02±0.01O2.90±0.07 for the as-prepared thin film. β-AgVO3 was prepared by thermally annealing the as-prepared sample at 300 oC for 30 min. The energy band gaps of α-AgVO3 and β-AgVO3 thin films were measured by diffuse reflectance spectroscopy (DRS) and found to be 2.46 and 2.12 eV respectively. Atmospheric photoemission spectroscopy (APS) showed the valence band positions of α-AgVO3 and β-AgVO3 to be -5.65 and -5.41 eV respectively. By combining the DRS and APS results, the calculated conduction band edge positions of α-AgVO3 and β-AgVO3 were -3.19 and -3.29 eV respectively. These results will be discussed in the context of the application of these compounds for photoelectrochemical reduction processes such as hydrogen evolution or CO2 reduction.

References

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H. Antonia, N. R. Tacconi, W. Chanmanee, H., N. Myung, and K. Rajeshwar, Electrochem. Solid-State Lett., 2010, 13, D29-D32.
(Invited) Red TiO2 and Ferroelectric Photocatalysts
Authors: Liu Gang
Published: February 2020

The strong band-to-band absorption of photocatalysts spanning the whole visible light region (400-700 nm) is critically important for solar-driven photocatalysis. Although it is actively and widely used as photocatalyst for various reactions in the past four decades, TiO2 has a very poor ability to capture the whole-spectrum visible light. Here, by controlling the spatially homogeneous distribution of boron and nitrogen heteroatoms in anatase TiO2 microspheres with a predominance of high-energy {001} facets, a strong visible light absorption spectrum with a sharp edge beyond 680 nm is achieved. The red TiO2 with the homogeneous doping of boron and nitrogen obtained shows no increase in defects like Ti3+ that are commonly observed in doped TiO2. More importantly, it has the ability to induce photocatalytic water oxidation to produce oxygen under the irradiation of visible light beyond 550 nm and also photocatalytic reducing water to produce hydrogen under visible light.

Integrating a semiconducting light absorber with an appropriate co-catalyst appears almost indispensable for photocatalytic solar fuel generation. Although ferroelectric materials with spontaneous electrical polarization are considered promising light absorbers with the ability to induce oppositely directed transport of photogenerated electrons and holes in the bulk, their applications are intrinsically restricted by the large Schottky barrier at the interface of the ferroelectric material and the co-catalyst, which has a larger work function. Here, we demonstrate that, by selective chemical epitaxial growth of anatase TiO2 islands on the positively poled (00-1) facet of PbTiO3 single-crystal particles to form an atomically smooth interface with a small potential difference, the material shows significantly improved photocatalytic hydrogen and oxygen generation under both UV-visible and visible light, while the island-free PbTiO3 is inactive in visible light.
(Invited) Mixed-Metal-Oxide Based Electrocatalysts for Efficient Reduction of Carbon Dioxide
Authors: Rutkowska Iwona Agnieszka , Kulesza Pawel J.
Published: February 2020

Of particular interest to the preparation of advanced catalytic materials is efficient utilization of catalytic sites (metal and metal oxide nanostructures, their stabilization and intentional activation, as well as organization into two-dimensional arrays, ultra-thin films or three-dimensional networks (e.g. through sequential deposition) on electrode surfaces. They can form nanosized materials with well-defined composition, structure and thickness that exhibit desirable electrocatalytic properties (e.g. toward reduction of CO2). We explore here the ability of polynuclear inorganic metal oxo systems to stabilize and functionalize metal (e.g. copper) nanostructures. Here certain nanostructured metal oxides of zirconium, titanium, zinc or tungsten have been demonstrated to influence supported metal centers in ways other than simple dispersion over electrode area. Evidence is presented that the support can modify activity (presumably electronic nature) of catalytic metal nanoparticles (e.g. Cu, Ag, Au, Pd, Ru) thus affecting their chemisorptive and catalytic properties. Metal oxide nanospecies can generate –OH groups at low potentials that induce proton mobility at the electrochemical interface.

Our research interests concern development of systems for the electrocatalytic reduction of carbon dioxide not only in neutral but also in acid media. For example, nanosized Cu or Ru naoparticles immobilized within ultra-thin films of tungsten oxide or zirconia have been considered. Selectivity of the catalytic systems largely depends on the activing adsorptive (CO2) phenomena and the affinity of catalytic centers to the adsorbed carbon monoxide (CO) type intermediates leading to their protonation or hydrogenation. Reduction of carbon dioxide begins now at less negative potentials and is accompanied by significant enhancement of the CO2-reduction current densities relative to the competitive hydrogen evolution.
(Keynote) Electrochemical and Microstructural Studies of Degradation Mechanisms in Solid Oxide Cells
Authors: Barnett Scott A
Published: February 2020

Solid oxide cells (SOCs) have important applications for fuel production from renewable electricity and grid-scale electricity storage. However, in order to be economically viable, the cells should operate at high efficiency, in order to minimize the amount of electricity used, and should convert enough energy over the device lifetime to pay back capital cost. The latter issue makes long-term durability a key issue. This talk will review recent SOC electrode life tests where degradation is accelerated via elevated temperature or high current density, and microstructural evolution is characterized. High temperature degradation due to coarsening has been observed in Ni-YSZ fuel electrodes and in infiltrated oxygen electrodes; the observed coarsening is used in the transmission line model and found to quantitatively explain increases in electrode polarization resistance over time. The models are extrapolated to make long-term performance stability predictions. The effects of high current density (and associated high overpotential) on electrode evolution is also discussed. For both oxygen and fuel electrodes, significant microstructural and electrochemical degradation is observed in electrolysis mode for overpotential ≳ 0.2 V. A model for the degradation mechanisms is discussed.
(Invited) High Conductivity, Stable Anion Conducting Membranes Based on Poly(norbornene)
Authors: Kohl Paul A , Huang Garrett , Mandal Mrinmay
Published: February 2020

Alkaline exchange membranes (AEM) fuel cells and electrolyzers are potentially cost-saving alternatives to traditional proton exchange membrane (PEM) devices. Recently, anion conducting polymer membranes have achieved ionic conductivity and chemical stability comparable to Nafion, the current state of the art in PEM technology. Phase segregation is an effective way to achieving high hydroxide mobility. An all-carbon backbone with light cross-linking and a long, alkyl tether for the cation head-group are effective means of achieving chemical stability and limiting unwanted water hydration.

In this study, a series of tetrablock copolymers containing an all-hydrocarbon backbone and tethered quaternary ammonium groups were synthesized based on vinyl addition polymerization of norbornene. Several variations of block copolymer AEMs with different ion exchange capacities (IEC) were made by adjusting the ratio of hydrophilic to hydrophobic blocks. Correlations were found between the ionic conductivity and mechanical properties to the polymer structure, channel size, and degree of phase separation. Furthermore, it was found that light crosslinking of the membranes allows for high IEC materials to be used while maintaining acceptable water uptake. The crosslinked AEMs have demonstrated record-high hydroxide conductivities (198 mS/cm) for a chemically stable polymer. The chemical degradation at 80oC in 1 M KOH was <1% degradation in 1,200 hours. The AEMs were also used to construct MEAs for alkaline fuel cells.