Jens Ulstrup

Bio: Jens Ulstrup is an academic researcher from Technical University of Denmark. The author has contributed to research in topics: Electron transfer & Scanning tunneling microscope. The author has an hindex of 33, co-authored 114 publications receiving 6072 citations. Previous affiliations of Jens Ulstrup include University College Dublin.

The effect of intramolecular quantum modes on free energy relationships for electron transfer reactions

Abstract: A general quantum mechanical description of exothermic electron transfer reactions is formulated by treating such reactions as the nonradiative decay of a ’’supermolecule’’ consisting of the electron donor, the electron acceptor, and the polar solvent. In particular, the role of the high‐frequency intramolecular degrees of feedom on the free energy relationship for series of closely related reactions was investigated for various model systems involving displacement of potential energy surfaces, frequency shift, and anharmonicity effects. The free energy plots are generally found to pass through a maximum and to be asymmetric with a slower decrease in the transition probability with increasing energy of reaction. For high‐frequency intramolecular modes this provides a rationalization of the experimental observation of ’’activationless’’ regions. Isotope effects are discussed as also are the oscillatory free energy relationships, predicted for low temperatures and high frequencies, and which are analogous t...

Extracellular polymeric substances are transient media for microbial extracellular electron transfer

Abstract: Microorganisms exploit extracellular electron transfer (EET) in growth and information exchange with external environments or with other cells. Every microbial cell is surrounded by extracellular polymeric substances (EPS). Understanding the roles of three-dimensional (3D) EPS in EET is essential in microbiology and microbial exploitation for mineral bio-respiration, pollutant conversion, and bioenergy production. We have addressed these challenges by comparing pure and EPS-depleted samples of three representative electrochemically active strains viz Gram-negative Shewanella oneidensis MR-1, Gram-positive Bacillus sp. WS-XY1, and yeast Pichia stipites using technology from electrochemistry, spectroscopy, atomic force microscopy, and microbiology. Voltammetry discloses redox signals from cytochromes and flavins in intact MR-1 cells, whereas stronger signals from cytochromes and additional signals from both flavins and cytochromes are found after EPS depletion. Flow cytometry and fluorescence microscopy substantiated by N-acetylglucosamine and electron transport system activity data showed less than 1.5% cell damage after EPS extraction. The electrochemical differences between normal and EPS-depleted cells therefore originate from electrochemical species in cell walls and EPS. The 35 ± 15-nm MR-1 EPS layer is also electrochemically active itself, with cytochrome electron transfer rate constants of 0.026 and 0.056 s-1 for intact MR-1 and EPS-depleted cells, respectively. This surprisingly small rate difference suggests that molecular redox species at the core of EPS assist EET. The combination of all the data with electron transfer analysis suggests that electron "hopping" is the most likely molecular mechanism for electrochemical electron transfer through EPS.

Mechanisms of Proton Conductance in Polymer Electrolyte Membranes

Abstract: We provide a phenomenological description of proton conductance in polymer electrolyte membranes, based on contemporary views of proton transfer processes in condensed media and a model for heterogeneous polymer electrolyte membrane structure. The description combines the proton transfer events in a single pore with the total pore-network performance and, thereby, relates structural and kinetic characteristics of the membrane. The theory addresses specific experimentally studied issues such as the effect of the density of proton localization sites (equivalent weight) of the membrane material and the water content of the pores. The effect of the average distance between the sulfonate groups, which changes during membrane swelling, is analyzed in particular, and the factors which determine the temperature dependence of the macroscopic membrane conductance are disclosed. Numerical estimates of the specific membrane conductivity obtained from the theory agree very well with typical experimental data, thereby confirming the appropriateness of the theoretical concepts. Moreover, the versatility of the models offers a useful and transparent frame for combining the analysis of both experimental data and the results of molecular dynamics simulations.

Charge Transfer Processes in Condensed Media

Abstract: 1 Introduction.- 1.1 Nature of Elementary Chemical Processes.- 1.2 Development of Theories for Elementary Chemical Processes.- 1.3 Chemical Reactions as a Class of Radiationless Processes.- 2 Multiphonon Representation of Continuous Media.- 2.1 Nature of Solvent Configuration Fluctuations.- 2.2 Interaction with Ionic Charges.- 2.3 Relation to Macroscopic Parameters.- 3 Quantum Mechanical Formulation of Rate Theory.- 3.1 Elements of Scattering Theory.- 3.2 Channel States and Nature of the Perturbation.- 3.3 Evaluation of Transition Matrix Elements.- 3.3.1 Harmonic Oscillator Representation.- 3.4 The Role of a Continuous Vibration Spectrum.- 3.5 Relation to Experimental Data.- 3.5.1 The Electronic Factor.- 3.5.2 Intramolecular and Medium-induced Electronic Relaxation.- 3.6 Lineshape of Optical Transitions.- 4 The Effect of Intramolecular Modes.- 4.1 Special Features of Electron Transfer Processes.- 4.2 Quantum Modes in Electron Transfer Reactions.- 4.2.1 Displaced Potential Surfaces..- 4.2.2 Effects of Frequency Changes.- 4.2.3 Effects of Anharmonicity.- 4.3 Relation to Experimental Data.- 5 Semiclassical Approximations.- 5.1 One-Dimensional Nuclear Motion.- 5.1.1 Classical Nuclear Motion.- 5.1.2 Nuclear Quantum Effects.- 5.2 Many-Dimensional Nuclear Motion.- 5.3 Relation to Experimental Data.- 5.3.1 Outer Sphere Electron Transfer Processes.- 5.3.2 Nucleophilic Substitution Reactions.- 6 Atom Group Transfer Processes.- 6.1 General Features of Nuclear Motion.- 6.2 Semiclassical Approaches to Atom Group Transfer.- 6.3 Quantum Mechanical Formulation of Atom Group Transfer.- 6.3.1 Nuclear Tunnelling between Bound States.- 6.3.2 Adiabatic and Nonadiabatic AT.- 6.3.3 Relation to the Gamov Tunnelling Factor.- 6.4 Relation to Experimental Data.- 7 Higher Order Processes.- 7.1 Higher Order Processes in Chemical ET Reactions.- 7.2 Theoretical Formulation of Higher Order Rate Probability.- 7.2.1 Semiclassical Methods..- 7.2.2 The Effect of High-Frequency Modes..- 7.2.3 Adiabatic Second Order Processes.- 7.2.4 Quantum Mechanical Formulation.- 7.3 Relation to Experimental Data.- 8 Electrochemical Processes.- 8.1 Fundamental Properties of Electrochemical Reactions.- 8.1.1 The nonuniform dielectric medium.- 8.1.2 The continuous electronic spectrum.- 8.1.3 Adiabaticity effects in many-potential surface systems.- 8.2 Quantum Mechanical Formulation of Electrode Kinetics.- 8.2.1 Metal electrodes.- 8.2.2 Semiconductor electrode.- 8.3 Relation to Experimental Data.- 8.3.1 The current-voltage relationship.- 8.3.2 The nature of the substrate electrode.- 8.3.3 The electrochemical hydrogen evolution reaction (her).- 8.4 Electrode Processes at Film Covered Electrodes.- 8.4.1 Tunnelling mechanisms.- 8.4.2 Mobility mechanisms.- 9 Application of the Rate Theory to Biological Systems.- 9.1 General.- 9.2 Specific Biological Electron Transfer Systems.- 9.2.1 Primary Photosynthetic Events.- 9.2.2 Bioinorganic ET Reactions.- 9.3 Electronic Conduction in Biological Systems.- 9.4 Conformational Dynamics.- A1.- A1.1 Derivation of the Sum Rules(eq.(2.49)).- A1.2 Derivation of Eq.(2.56).- References.

Electron Transfer in Chemistry and Biology: An Introduction to the Theory

Abstract: Some Theoretical Prerequisites. The Tunnel Effect in Physical, Chemical and Biological Processes. Elements of Dielectric Theory. Charge Transfer in Solids. The Simplest Chemical Process: Electron Transfer. Some Selected Experimental Data for Simple Electron Transfer Reactions. Towards More Precise Electron Transfer Theory. Optical Charge Transfer in Allowed and Forbidden Transitions. Elements of Proton and Other Light-Atom Transfer Theory. The Electrochemical Process. Elements of Long-Range and Multi-Level Electron Transfer. Stochastic Views in Chemical Rate Theory. Elements of Charge Transfer in Biological Systems. Perspectives and Outlook. Appendices. Index.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Combining theory and experiment in electrocatalysis: Insights into materials design

Abstract: BACKGROUND With a rising global population, increasing energy demands, and impending climate change, major concerns have been raised over the security of our energy future. Developing sustainable, fossil-free pathways to produce fuels and chemicals of global importance could play a major role in reducing carbon dioxide emissions while providing the feedstocks needed to make the products we use on a daily basis. One prospective goal is to develop electrochemical conversion processes that can convert molecules in the atmosphere (e.g., water, carbon dioxide, and nitrogen) into higher-value products (e.g., hydrogen, hydrocarbons, oxygenates, and ammonia) by coupling to renewable energy. Electrocatalysts play a key role in these energy conversion technologies because they increase the rate, efficiency, and selectivity of the chemical transformations involved. Today’s electrocatalysts, however, are inadequate. The grand challenge is to develop advanced electrocatalysts with the enhanced performance needed to enable widespread penetration of clean energy technologies. ADVANCES Over the past decade, substantial progress has been made in understanding several key electrochemical transformations, particularly those that involve water, hydrogen, and oxygen. The combination of theoretical and experimental studies working in concert has proven to be a successful strategy in this respect, yielding a framework to understand catalytic trends that can ultimately provide rational guidance toward the development of improved catalysts. Catalyst design strategies that aim to increase the number of active sites and/or increase the intrinsic activity of each active site have been successfully developed. The field of hydrogen evolution, for example, has seen important breakthroughs over the years in the development of highly active non–precious metal catalysts in acid. Notable advancements have also been made in the design of oxygen reduction and evolution catalysts, although there remains substantial room for improvement. The combination of theory and experiment elucidates the remaining challenges in developing further improved catalysts, often involving scaling relations among reactive intermediates. This understanding serves as an initial platform to design strategies to circumvent technical obstacles, opening up opportunities and approaches to develop higher-performance electrocatalysts for a wide range of reactions. OUTLOOK A systematic framework of combining theory and experiment in electrocatalysis helps to uncover broader governing principles that can be used to understand a wide variety of electrochemical transformations. These principles can be applied to other emerging and promising clean energy reactions, including hydrogen peroxide production, carbon dioxide reduction, and nitrogen reduction, among others. Although current paradigms for catalyst development have been helpful to date, a number of challenges need to be successfully addressed in order to achieve major breakthroughs. One important frontier, for example, is the development of both experimental and computational methods that can rapidly elucidate reaction mechanisms on broad classes of materials and in a wide range of operating conditions (e.g., pH, solvent, electrolyte). Such efforts would build on current frameworks for understanding catalysis to provide the deeper insights needed to fine-tune catalyst properties in an optimal manner. The long-term goal is to continue improving the activity and selectivity of these catalysts in order to realize the prospects of using renewable energy to provide the fuels and chemicals that we need for a sustainable energy future.