bluuebunny/crossref_metadata_2025_split huggingface dataset:
Using the Complementarity of Electrochemistry and Photoelectron Transfer to Probe and Develop the Chemistry of Radical Cations
Authors: Moeller Kevin D , Gonzalez Luisalberto , Feng Ruozhu , Graaf Matthew D. 
Published: February 2020

Through the years, we have utilized electrochemistry as a method to explore radical cation intermediates and the oxidative cyclization reactions they trigger. On the mechanistic side of things, we have uncovered a set of empirical rules that govern radical cation reactivity and allow for the design of new reactions. On the synthetic side of things, we have used the ensuing reactions to make heterocycles, fused-, spirocyclic-, and bridged molecules, and highly hindered tetrasubstituted carbons.
Recently, we have found that these initial mechanistic models have to be adjusted to account for not only the initial cyclization reaction, but also the intermediates generated downstream of that step. In particular, the oxidation of a second electron from the molecule (k2
in Scheme 1) frequently plays a key role in the synthetic success of a transformation.
While this observation was clear for cases for slower trapping reactions that might have a reversible first step, it was assumed that our very best trapping reactions that were known to be kinetically fast had no such issues. This assumption has also proven problematic.
Photoelectron transfer reactions that are ideally suited for conducting one-electron oxidation based processes are ideal tools for probing cyclizations that require the loss of a second electron (Scheme 2). Note that all three substrates shown underwent a two electron oxidative cyclization nicely at an anode to give the cyclic products shown. The substrate with the alcohol trapping group also worked well providing a redox neutral complementary product to the electrolysis reaction. The substrate with the sulfonamide trapping group led to no reaction. This was not a surprise since the cyclization with the sulfonamide trapping group is slower than the alcohol trapping reaction. The reaction is dominated by a back electron transfer from the dithioketene acetal derived radical cation to the catalyst. The reaction with the enol ether trapping group led to polymer that showed evidence for consumption of both olefins in the starting material. This was a surprise since the cyclization is known to be very fast (much faster than the alcohol trapping of the radical cation). The reaction clearly required the loss of a second electron to form the desired cyclized product. This was a conclusion that could only be reached by comparing the photochemical and electrochemical reactions.
In the talk to be presented, the historical context and future synthetic ramifications of this and other mechanistic studies that define the nature of the reactive intermediate needed for the initial cyclization will be discussed.

Figure 1
Electrochemical Oxidation/Modification of Lignin Mediated By Aminoxyl Radicals
Authors: Rafiee Mohammad , Stahl Shannon S. 
Published: February 2020

Lignin is an abundant biopolymer with an aromatic backbone and offers a bio-based source of aromatic commodity chemicals upon depolymerization.[i] Oxidation methods are widely used for the deconstruction of lignin; successful examples include oxidative delignification in the pulp and paper industry and breakdown of lignin by oxidative fungal enzymes. The hydroxyl functional groups of lignin are particularly susceptible to oxidation. Building on these concepts, our group recently began exploring the controlled oxidation of lignin hydroxyl groups as a means to facilitate lignin depolymerization.[ii] We previously reported an aerobic oxidation method that employs aminoxyl radicals, such as TEMPO, as the catalyst. Acid treatment of the resulting oxidized lignin lead to depolymerization, affording up to 60 wt% yield of low-molecular-weight aromatic compounds.[iii]
Aminoxyl radicals undergo facile electron transfer at the electrode surface to generate an oxoammonium species that is reactive towards hydroxyl oxidation (Figure 1a). This facile electron transfer provides a foundation for selective oxidation of hydroxyl groups under electrochemical conditions.[iv] This talk will highlight some of our advances in the use of aminoxyl radicals as mediators for chemoselective oxidation of hydroxyl groups in the structure of lignin. The beta-O-4 linkage (Figure 1b) is the most prevalent linkage in the structure of lignin that includes hydroxyl functional groups and was used as the model for this study.
Nitroxyl-mediated electrochemical oxidation methods enable selective and quantitative conversion of the primary hydroxyl group of lignin and beta-O-4 model compounds to the corresponding carboxylate under mildly basic conditions. The modified lignin has a high loading of carboxylic acids, which enhance its water solubility relative to native lignin. The sodium salt of the modified lignin behaves as a polyacid polyelectrolyte in aqueous solution. Acid treatment of the oxidized lignin model affords approximately 30 wt% of monomeric aromatic compounds.
[i]. Upton, B. M.; Kasko, A. M. Chem. Rev., 2016, 116, 2275.
[ii]. Rahimi, A.; Azarpira, A.; Kim, H.; Ralph, J.; Stahl, S. S. J. Am. Chem. Soc.
2013, 135, 6415.
[iii]. Rahimi, A.; Ulbrich, A.; Coon, J. J.; Stahl S. S. Nature
2014, 515, 249.
[iv]. Rafiee, M; Miles, K. C.; Stahl, S. S. J. Am. Chem. Soc.
2015, 137, 14751.

Figure 1
Inside the Reaction Layer: Investigation of Electrochemical Reactions By Coupling Electrochemistry and Confocal Fluorescence Microscopy
Authors: Doneux Thomas , de Poulpiquet Anne , Arredondo Imelda Bonifas , Lefrançois Pauline , Vajrala Venkata Suresh Reddy , Goudeau Bertrand , Garrigue Patrick , Sojic Neso , Arbault Stéphane , Bouffier Laurent 
Published: February 2020

Electrochemical reactivity studies are usually performed by conventional electrochemical measurements carried out with a single electrode of either macroscopic (∼millimeter) or microscopic (∼micrometer) dimensions. Such kind of measurements provides information which can be time resolved down to very short time scales, but which is typically averaged over the entire electrode area and is thus insensitive to spatial heterogeneities in the x-y plane of the electrode. The measured current also gives access to the transport properties of the involved redox-active species, but this additional information corresponding to the z-direction axial to the electrode is rather indirect. Coupling electrochemistry with confocal fluorescence microscopy (EC-fluorescence) is an expedient way to obtain 3-dimensional spatial information on electrochemical reactions, in-situ and in real time. The optical sectioning enabled by confocal microscopy is particularly useful to map individual chemical species in the diffusion layer, especially for unstable electrogenerated species that further react homogeneously.
The present contribution focuses on the properties of the electroactive, fluorogenic molecule resorufin (see Figure), to illustrate the many potentialities of EC-fluorescence. The highly fluorescent resorufin can be generated electrochemically by the irreversible reduction of its N-oxide parent compound resazurin which is slightly fluorescent. It can also be reduced reversibly to the non-fluorescent dihydroresorufin, whose acetyl derivative is actually the popular Amplex® Red that can be oxidized to generate resorufin. This exhaustive electrochemistry covers several electrochemical reaction mechanisms ranging from simple electron transfers to coupled electrochemical-chemical steps, with different degrees of reversibility. A comproportionation scheme is also evidenced for the first time. Each of these reactions is associated with specific characteristics of the reaction-diffusion layer that can be probed with an appropriate spatial resolution by EC-fluorescence.

Figure 1
Interfacial Electrochemistry of Chemically Modified Electrode Materials, Relevant for Energy Conversion and Storage Systems
Authors: Strmcnik Dusan , Genorio Bostjan , Danilovic Nemanja , Zorko Milena , Martins Pedro F. B. D. , Lopes Pietro Papa , Stamenkovic Vojislav , Markovic Nenad M 
Published: February 2020

Electrochemical reactions take place at the electrochemical interface, the region in which the properties change from those in the bulk of the electronic conductor (electrode) to those in the bulk of the ionic conductor (electrolyte). Not surprisingly, then, efficiency of energy conversion and energy storage processes are almost entirely determined by the richness of interfacial processes that control the rate of electron transfer and various types of interactions between the substrate and reactants, intermediates, and products of the reaction. The “interfacial bridge” between two bulk properties is built either by the formation of unique (near)surface atom arrangements (e.g. surface relaxation or reconstruction) and/or by significant differences in electrode composition close to the surface (e.g. segregation profile) and/or via substrate-adsorbates covalent and non-covalent forces and/or ordering of solvent and/or electrolyte molecules observed in the proximity of the surface (up to 10 nm). All of these processes are rather complex, requiring special arsenal of tools that are capable of resolving interfacial properties at atomic and molecular levels.
Despite establishing the structure-function relationships, i.e. correlations between the morphology, composition of the electrode/electrolyte and the physico-chemical properties of the interface such as activity, stability and selectivity, we are still not in a position to provide the fundamental knowledge that is required to control the interfacial processes.
This presentation will give an overview of our recent efforts in predetermining the properties of the electrochemical interface by influencing the delicate interplay between the covalent and non-covalent interactions at or near the electrode surface. Several examples of chemical modification of the interfaces are going to be presented, including the variation in oxygen reduction reaction (ORR) in alkaline solution with the nature of non-covalently adsorbed cation in the double layer, the ORR activity enhancement of PA fuel cell catalyst and hydrogen oxidation reaction selectivity enhancement on PEM fuel cell catalysts. Finally, we will discuss the control of interfacial electrochemical processes through the application of mono- and multi-layer graphene architectures.
(Invited) Effect of the Carbon Matrix Surface Area and Thermal Treatment on the Activity and Durability of Fe-N-C Oxygen Reduction Catalysts
Authors: Lima Fabio H. B. , Galiote Nelson A. 
Published: February 2020

The sluggish oxygen reduction reaction (ORR) and the high platinum loading in the cathode are important obstacles that should be overcome for the development of H2/air fuel cells. Thus, studies focused on highly active and cost-effective electrocatalysts have gained attention. Electrocatalysts formed by earth-abundant elements such as those based on iron, nitrogen and carbon (Fe-N-C) have gained considerable attention in research because they allow widespread commercialization of the fuel cells. Previous achievements regarding Fe-N-C oxygen reduction electrocatalysts have been described elsewhere and showed a significant effect of the carbon matrix surface area on the electrocatalytic activity and stability [1,2]. In the present paper, we will focus on the results of the consequence on activity and durability of the thermal treatments on Fe-N-C. The electrocatalyst were synthesized via Fe complexation with phenanthroline, followed by impregnation on carbon powder with different surface areas (Vulcan XC-72R and Black Pearls 2000). The thermal treatments were conducted at different temperatures and on Ar or NH3 atmosphere. The discussion will be focused on the correlation between the results encountered in the characterization by EXAFS, XRD and HR-TEM with those obtained during polarization experiments for the ORR using rotating disk electrode and during fuel cell tests. A strong effect of the thermal treatment temperature and of the surface area of the carbon powder was observed on the activity and durability, mainly for the fuel cell tests with H2/air. We believe that the results from this study may contribute to further understand the parameters that govern the activity and durability of Fe-N-C ORR catalysts.
[1] G.Wu, K.L. More, P. Xu, H-L. Wang, M. Ferrandon, A.J. Kropf, D.J. Myers, S. Ma, C.M. Johnston, P. Zelenay, Chem. Commun., 49 (2013) 3291 – 3293.
[2] L. Yang, N. Larouche, R. Chenitz, G. Zhang, M. Lefevre, J-P. Dodelet, Electrochim. Acta 159 (2015) 184 – 197.
Acknowledgements
The authors gratefully acknowledge financial support from FAPESP (2013/16930-7, and 2016/13323-0) and CNPq.
(Invited) Active Carbon Supports for Pt Cathode Catalysts in PEM Fuel Cells
Authors: Chen Mengjie , Wang Xiaoxia , Wu Gang 
Published: February 2020

Polymer electrolyte membrane fuel cells (PEMFCs) have been widely considered one of environmentally friendly and highly efficient electric power sources for a wide variety of power applications ranging from portable and stationary power supplies to transportation. However, some of the most critical barriers to their commercialization are the intrinsic high cost and insufficient long-term performance durability. Among others, the spherical carbon black particle support used for Pt catalyst in MEAs have a number of intrinsic structural disadvantage. It is partially responsible for current insufficient performance and durability of MEAs. They include (1) inefficient O2 transport due to significant tortuosity in carbon black-based cathode, (2) non-uniform distribution of ionomer due to significant microporosity of carbon blacks, (3) catastrophic corrosion during start-up/shut-down events due to low corrosion resistance, (4) low electrical conductivity, and (5) weak binding sites with Pt nanoparticles.
We has developed a new type of high-surface areas (up to 850 m2/g) and large size nitrogen-diped graphene tubes (NGT) in a highly controlled manner and at high yields. The diameters of these graphene tubes can be tuned from 50 to 500 nm, which is much larger than conventiona carbon nanotubes (20 nm). It should be noted that the graphene tube is low cost using inexpensive dicyandiamide as precursors ($1-2/kg) and the synthesis method is easily scaled up. Apart from the obvious advantage of high electronic conductivity and enhanced corrosion resistance, the large tube size provided better Pt nanoparticle dispersion and more facil mass transfer including O2 and H2O. Importantly, the NGT exhibited oxygen reduction reaction (ORR) activity in acidic media comparable with state of the art PGM-free catalsyts. Thus, the novel ORR active nitrogen-doped graphene tubes are able to not only offer remarkable support effects by geometrically and electronically modifying the deposited Pt nanoparticles, but also provide a large amount of complementary nonprecious ORR active sites, thereby significantly improving the overall activity and durability of the Pt catalysts. Thus, relative to conventional carbon blacks, the highly graphitized and active N-GT is an ideal support for low-PGM catalysts for high-performance MEAs with enhanced mass transport and durability along with ideal interfaces with ionomer.
(Invited) The Interplay of Oxygen Electrochemistry, Electrocatalysis, and Anionic Redox Processes in the Development of Electrochemical Energy Storage Technology for Vehicular Applications
Authors: Faguy Peter William 
Published: February 2020

The importance of lithium batteries to both energy and environmental security and to economic development in the global marketplace has been well recognized by the United States Department of Energy. Beginning in 2002, the Vehicle Technologies Office within the Office of Energy Efficiency and Renewable Energy has directed and funded a significant R&D program across the national lab complex, academia, and industry specifically targeting lithium-based batteries. This family of electrochemical energy storage cell chemistries was seen as, and has been proven to be, foundational to the widespread electrification of the US vehicle fleet. Unlike previous major battery chemistries, for example lead acid and nickel metal hydride, viable lithium batteries have been developed with a chemically and structurally diverse set of active anode materials (AAMs): lithium metal, graphite, intermetallic alloy (Si- and Sn-containing) coupled to an even more diverse set of active cathode materials (ACMs): transition metal oxyanions, molecular sulfur, and molecular oxygen.
In the shorter term, ~< 10 years, transition metal oxyanions are and will continue to be the focus of intense R&D efforts. Two critical characteristics in need of improvement in these so-called next generation Li-ion cathodes are electrode specific capacity and electrode and electrolyte stability under high voltage operating conditions. The limit in capacity—in electrochemical energy stored—for lithium intercalating oxyanionic materials used in practical devices may well have already been reached. In the longer term, only with cell chemistries beyond lithium ion will batteries achieve the energy densities necessary for future applications such as autonomous electric vertical takeoff and landing (VTOL) aircraft. Next generation ACMs (NG-ACMs) will not provide the requisite specific capacity. On that horizon, lithium-air batteries have received a great deal of attention. Molecular oxygen-based cathodes are a critical class of beyond Li-ion ACMs (BLI-ACMs).
Considering this backdrop of technology development and the birth of a trillion dollar industry, there exists a pressing need for understanding the physicochemical processes at advanced electrodes involving oxygen electrochemistry, electrocatalytic oxygen reduction and evolution in BLI-ACMs and anionic redox processes in NG-ACMs. This interplay of oxygen electrochemistry and promising electrochemical energy storage devices will be the central theme of the presentation. What are the central issues in understanding structure and activity involving oxygen electrochemistry in energy storage materials. Examples of scientific progress from projects supported in national laboratories, academia, and industry will be presented and future directions of R&D discussed.
(Energy Technology Division Supramaniam Srinivasan Young Investigator Award Address) Enhanced Oxygen Electrocatalysis By Means of Electronic and Geometric Effects
Authors: Escudero-Escribano María 
Published: February 2020

The slow kinetics of the oxygen reduction and evolution reactions (ORR and OER) impede the widespread uptake of polymer electrolyte membrane (PEM) fuel cells and electrolysers. In order to improve the kinetics of the ORR and reduce the Pt loading at the fuel-cell cathode, we need to enhance the activity, stability and selectivity of the catalyst. This can be achieved by alteration of the electronic properties of the Pt surface atoms (electronic effects [1,2]) and/or modification of the geometric structure (atomic ensemble effects [3]).
First, I will address atomic ensemble effects in electrocatalysis. We used a self-ordered molecular pattern of cyanide on Pt(111) surfaces to study the role of atomic ensembles for the ORR [3]. Cyanide groups form an ordered structure that blocks all the three-fold hollow sites, effectively blocking the adsorption of spectator anions in the electrolyte. Nonetheless, the holes in this structure are sufficiently large to allow the adsorption of reaction oxygen molecules. As a consequence, cyanide-modified Pt(111) presents a 25-fold enhancement over Pt(111) [3].
Secondly, I will focus on electronic effects by alloying Pt with other metals. In order to reduce the Pt loading, researchers have intensively studied alloys of Pt with late transition metals such as Ni and Co. However, these compounds typically degrade under fuel-cell conditions, due to dealloying. In contrast, Pt-lanthanide alloys present a very negative alloying energy, which should increase their resistance to degradation. We have studied novel Pt-lanthanide and Pt-alkaline earth electrocatalysts for the ORR. Pt-lanthanide and Pt-alkaline earth alloys are amongst the most active polycrystalline Pt-based catalysts ever reported [1,2], presenting up to a 6-fold increase in ORR activity, relative to Pt. The active phase consists of a compressed Pt overlayer of few Pt layers formed onto the bulk alloys by acid leaching. Notably, the oxygen reduction activity versus the bulk lattice parameter follows a volcano relation [1]. We use the lanthanide contraction to control strain effects and tailor the activity, stability and reactivity of Pt alloys.
Finally, I will present new approaches to design and develop more efficient and stable OER electrocatalysts. We have explored the OER activity and stability of RuO2 nanoparticulate catalysts. Sub-monolayer amounts of IrOx on top of RuO2 thin films minimise Ru dissolution [4]. The strategy of tailoring the surface of the catalyst with sub-monolayer amounts of a stable oxide may allow us to tune the stability of active OER catalysts for PEM electrolysers.
References
[1] M. Escudero-Escribano et al., Science
2016, 352, 73.
[2] M. Escudero-Escribano et al., J. Am. Chem. Soc. 2012, 134, 16476.
[3] D. Strmcnik, M. Escudero-Escribano, K. Kodama, V. Stamenkovic, A. Cuesta, N.M. Markovic, Nature Chemistry
2010, 2, 880.
[4] M. Escudero-Escribano et al. J. Phys. Chem. B
2017, DOI: 10.1021/acs.jpcb.7b07047
(Invited) Kinetic Characterization of Ir, Pt, and Irpt-Alloy Nanocatalysts for Ammonia Oxidation Reaction
Authors: Wang Jia X. , Song Liang , Liang Zhixiu , Zhang Yu , Adzic Radoslav R. 
Published: February 2020

Direct ammonia fuel cells (DAFC) is an attractive alternative of hydrogen fuel cells because of lower cost in storage and distribution of liquid fuel than hydrogen gas. A drawback is the sluggish kinetics of ammonia oxidation reaction (AOR). Based on previous DAFC studies and measurements using rotating disk electrodes (RDEs), Ir, Pt, and IrPt-alloy nanoparticles are considered as the most active AOR catalysts. However, many questions remain open in correlating RDE results obtained at ambient temperature with the catalysts’ performance in DAFC at elevated temperatures. This is due to low sustainable AOR currents at sufficiently low potentials at ambient temperature and complicated temperature effect on AOR kinetics.
Here, we report an AOR study using gas diffusion electrode (GDE) in 1 M KOH solution at temperatures up to 60 oC. We kept ammonia concentration constant with time at temperatures higher than the boiling point of ammonia by saturating the solution with argon gas that bubbled through concentrated ammonia before entering the electrochemical cell. Three commercial nanocatalysts, Ir, Pt, and PtIr 1:1 alloy particles with 40 wt% metal on carbon support, were used to establish test protocol and to benchmark the platinum group metal (PGM) mass activity. The three catalysts exhibited distinct kinetic behaviors in terms of onset potential, maximum current, and activity enhancement factor with increasing temperature.
Clearly characterized AOR kinetic behavior can be explained using the reaction pathways proposed based on previous DFT calculations. We’ll discuss in detail why Ir has lower onset potential, but lower peak current than Pt does, why temperature enhancement factor is higher for Ir than Pt, what are the inactive intermediates and how catalysts deactivated due to increased coverage of less active AOR intermediates at potentials above and below 0.6 V. New insights can help developing advanced AOR catalysts and making effective DAFCs. The method and concepts developed are generally applicable for studying oxidation of other liquid fuels, such as, methanol and ethanol as their oxidation reactions also involve multiple intermediates and pathways.

Acknowledgements

This research was funded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0000805 and by the Division of Chemical Sciences, Geosciences and Biosciences Division, US Department of Energy under contract DE-SC0012704.