Recent experimental evidence suggests that the rate and mechanisms of many electrocatalytic reactions depend on electrolyte pH and the identity of the alkali metal cation present in an alkaline electrolyte. In particular, the rate of the hydrogen oxidation reaction, important in hydrogen fuel cells, is 2-3 orders of magnitude slower in an alkaline electrolyte than in an acid electrolyte, even on the most active platinum catalyst. While it is well known that many anions effect the rates of electrocatalytic reactions, through their specific adsorption and blocking of active sites on the electrode surface, the mechanism by which alkali metal cations exert their effects is unknown. Both experiment and density functional theory modeling of the electrode-electrolyte interface are used in this dissertation to better understand how pH and alkali metal cations effect electrocatalytic reactions. Density functional theory calculations show that alkali metal cation specific adsorption is favorable at low potentials to many electrode surfaces, including platinum, in an alkaline electrolyte. Once on the surface, these alkali metal cations show only a small interaction with adsorbed hydrogen, but a significant weakening of adsorbed hydroxide. These results explain an experimentally observed anomalous shift in the low potential features of cyclic voltammograms measured on Pt(110), Pt(100), and stepped Pt surfaces with increasing pH, which correlate with the pH dependence of the rate of the hydrogen oxidation reaction. The rate of the hydrogen oxidation reaction is experimentally measured in alkaline electrolytes, and is found to depend on the alkali metal cation present, following the trend Li > Na > K > Cs. The density functional theory calculated trend in the effect of these cations on hydroxide adsorption matches the trend in rate, supporting that adsorbed hydroxide may be an intermediate in the hydrogen oxidation reaction. To design highly active hydrogen oxidation/evolution catalysts, both hydrogen adsorption strength and hydroxide adsorption strength must be considered.
It is well known that the hydrogen reaction rates on Pt and other electrocatalysts are fast at low pH but 1-2 orders of magnitude slower at high pH. Although the electrochemical hydrogen evolution and oxidation reactions (HER and HOR) are arguably the best-understood reactions in electrocatalysis, the anomalous effect of pH on reaction kinetics has defied simple explanation for decades. This longstanding puzzle not only limits applied catalyst design, but also exposes gaps in the fundamental understanding of electrocatalysis by showing that singular adsorption descriptors (e.g., the hydrogen binding energy) cannot describe kinetic effects across electrolytes. The goal of this work is to examine the possible sources of the strong pH dependence in hydrogen electrocatalysis. This dissertation delves in logical progression into the interfacial processes that govern this anomalous pH dependence, beginning with fundamental experimental and theoretical investigations of the chemical and physical processes occurring at the interface, and ending with the incorporation of "double-layer dopants" able to further enhance the kinetics of highly active electrocatalysts in alkaline media. In this work, we have applied electroanalytical techniques to the reversible hydrogen reaction on single crystal Pt surfaces to gain insight into the role of coadsorbed species, the potential of zero charge (Epzc), transition state barrier heights, and "double-layer dopants" in the alkaline hydrogen reaction mechanism. Combining experimental results with microkinetic modeling, we determined that adsorbed hydroxide being an active participant in the alkaline hydrogen reaction mechanism is thermodynamically unfeasible, and therefore the anomalous effect of pH on kinetics must come from changes in kinetic parameters rather than from changes in thermodynamic binding energies. Focused on interfacial water mobility as the basis for such changes in kinetic parameters, we performed kinetic isotope effect (KIE) voltammetry measurements to quantify the importance of solvent mobility on the overall hydrogen reaction kinetics. Large KIE values of up to 3.4 for HOR in KOD on Pt(111) compared to no measurable effects in DClO4 confirmed that interfacial water mobility drives the pH dependence of the reversible hydrogen reaction. With this insight, we further assessed the impact of interfacial electric field strength, as described by the electrode's Epzc, on electrochemical reaction kinetics in the context of hydrogen electrocatalysis. Previous findings from our group demonstrated the use of molecular interfacial additives such as adsorbed caffeine for enhancing the alkaline HER/HOR kinetics, possibly by disrupting the double layer structure and affecting the orientation and dynamics of water. Hence, using caffeinated Pt as a model surface, we correlated reaction kinetics to the electrode's Epzc and to the solvent mobility. The activity of composite Pt surfaces was found to correlate with the proximity of the Epzc, measured by CO displacement, to the equilibrium potential of HER/HOR, but larger KIE values measured on caffeinated Pt at high pH indicate that this is not a causal relationship and Epzc is not a mechanistic descriptor of the alkaline hydrogen reaction kinetics. The use of "double-layer dopants" such as caffeine represents a new approach for designing not just HER/HOR catalysts with increased activity but also experiments that may bring light to interfacial phenomena dictating reaction kinetics. Hence, to uncover the mechanistic origin of the observed enhancement in alkaline HER/HOR kinetics, we have performed single-crystal voltammetry measurements of Pt(111) electrodes covered with surface additives in buffered electrolytes of varying pH. The results show a decrease/increase transition in kinetics around the additive's pKa, suggesting that double-layer dopants reduce transition state barriers for HER/HOR by regulating the interfacial pH. We have also made steps towards determining the relationship between Epzc and interfacial water structure through in-situ XAS measurements on caffeinated Pt and Au electrodes. Finally, we propose paths forward for improving the mechanistic understanding of how specific interactions between the surface and species in solution affect macroscopic rates, which include combining single-crystal voltammetry, electroanalytical chemistry, in-operando spectroscopy, atomic-scale DFT calculations, and other molecular "double layer dopants".
This book is based on a graduate course and suitable as a primer for any newcomer to the field, this book is a detailed introduction to the experimental and computational methods that are used to study how solid surfaces act as catalysts. Features include: First comprehensive description of modern theory of heterogeneous catalysis Basis for understanding and designing experiments in the field Allows reader to understand catalyst design principles Introduction to important elements of energy transformation technology Test driven at Stanford University over several semesters
Photoelectrocatalysis: Fundamentals and Applications presents an in-depth review of the topic for students and researchersworking on photoelectrocatalysis-related subjects from pure chemistry to materials and environmental chemistry inorder to propose applications and new perspectives. The main advantage of a photoelectrocatalytic process is the mildexperimental conditions under which the reactions are carried out, which are often possible at atmospheric pressure androom temperature using cheap and nontoxic solvents (e.g., water), oxidants (e.g., O2 from the air), catalytic materials (e.g.,TiO2 on Ti layer), and the potential exploitation of solar light. This book presents the fundamentals and the applications of photoelectrocatalysis, such as hydrogen production fromwater splitting, the remediation of harmful compounds, and CO2 reduction. Photoelectrocatalytic reactors and lightsources, in addition to kinetic aspects, are presented along with an exploration of the relationship between photocatalysisand electrocatalysis. In addition, photocorrosion issues and the application of selective photoelectrocatalytic organictransformations, which is now a growing field of research, are also reported. Finally, the advantages/disadvantages andfuture perspectives of photoelectrocatalysis are highlighted through the possibility of working at a pilot/industrial scale inenvironmentally friendly conditions. Presents the fundamentals of photoelectrocatalysis Outlines photoelectrocatalytic green chemistry Reviews photoelectrocatalytic remediation of harmful compounds, hydrogen production, and CO2 reduction Includes photocorrosion, photoelectrocatalytic reactors, and modeling along with kinetic aspects
This book honors Professor. John O’M. Bockris, presenting authoritative reviews on some of the subjects to which he made significant contributions – i.e., electrocatalysis, fuel cells, electrochemical theory, electrochemistry of single crystals, in situ techniques, rechargeable batteries, passivity, and solar-fuels – and revealing the roles of electrochemical science and technology in achieving a sustainable society. Electrochemistry has long been an object of study and is now growing in importance, not only because of its fundamental scientific interest but also because of the central role it is expected to play in a future sustainable society. Professor John O’M. Bockris contributed greatly to various aspects of fundamental and applied electrochemistry – such as the structure of the double layer, kinetics and mechanism of the electrochemistry of hydrogen and oxygen, electrocatalysis, adsorption and electrochemical oxidation of small organic molecules, fuel cells, electrocrystallization, theoretical electrochemistry, new methods, photoelectrochemistry, bioelectrochemistry, corrosion and passivity, hydrogen in metals, ionic solutions and ionic liquids, and molten silicates and glasses, as well as socio-economic issues such as the hydrogen economy – for over half a century from 1945 until his retirement in 1997.
Biopolymer Electrolytes: Fundamentals and Applications in Energy Storage provides the core fundamentals and applications for polyelectrolytes and their properties with a focus on biopolymer electrolytes. Increasing global energy and environmental challenges demand clean and sustainable energy sources to support the modern society. One of the feasible technologies is to use green energy and green materials in devices. Biopolymer electrolytes are one such green material and, hence, have enormous application potential in devices such as electrochemical cells and fuel cells. Features a stable of case studies throughout the book that underscore key concepts and applications Provides the core fundamentals and applications for polyelectrolytes and their properties Weaves the subject of biopolymer electrolytes across a broad range of disciplines, including chemistry, chemical engineering, materials science, environmental science, and pharmaceutical science
Electrocatalysis in Balancing the Natural Carbon Cycle Explore the potential of electrocatalysis to balance an off-kilter natural carbon cycle In Electrocatalysis in Balancing the Natural Carbon Cycle, accomplished researcher and author, Yaobing Wang, delivers a focused examination of why and how to solve the unbalance of the natural carbon cycle with electrocatalysis. The book introduces the natural carbon cycle and analyzes current bottlenecks being caused by human activities. It then examines fundamental topics, including CO2 reduction, water splitting, and small molecule (alcohols and acid) oxidation to prove the feasibility and advantages of using electrocatalysis to tune the unbalanced carbon cycle. You’ll realize modern aspects of electrocatalysis through the operando diagnostic and predictable mechanistic investigations. Further, you will be able to evaluate and manage the efficiency of the electrocatalytic reactions. The distinguished author presents a holistic view of solving an unbalanced natural carbon cycle with electrocatalysis. Readers will also benefit from the inclusion of: A thorough introduction to the natural carbon cycle and the anthropogenic carbon cycle, including inorganic carbon to organic carbon and vice versa An exploration of electrochemical catalysis processes, including water splitting and the electrochemistry CO2 reduction reaction (ECO2RR) A practical discussion of water and fuel basic redox parameters, including electrocatalytic materials and their performance evaluation in different electrocatalytic cells A perspective of the operando approaches and computational fundamentals and advances of different electrocatalytic redox reactions Perfect for electrochemists, catalytic chemists, environmental and physical chemists, and inorganic chemists, Electrocatalysis in Balancing the Natural Carbon Cycle will also earn a place in the libraries of solid state and theoretical chemists seeking a one-stop reference for all aspects of electrocatalysis in carbon cycle-related reactions.
Electrochemical surface science (EC-SS) is the natural advancement of traditional surface science (where gas–vacuum/solid interfaces are studied) to liquid (solution)/electrified solid interfaces. Such a merging between two different disciplines—i.e., surface science (SS) and electrochemistry—officially advanced ca. three decades ago. The main characteristic of EC-SS versus electrochemistry is the reductionist approach undertaken, inherited from SS and aiming to understand the microscopic processes occurring at electrodes on the atomic level. A few of the exemplary keystone tools of EC-SS include EC-scanning probe microscopies, operando and in situ spectroscopies and electron microscopies, and differential EC mass spectrometry (DEMS). EC-SS indirectly (and often unconsciously) receives a great boost from the requirement for rational design of energy conversion and storage devices for the next generation of energetic landscapes. As a matter of fact, the number of material science groups deeply involved in such a challenging field has tremendously expanded and, within such a panorama, EC and SS investigations are intimately combined in a huge number of papers. The aim of this Special Issue is to offer an open access forum where researchers in the field of electrochemistry, surface science, and materials science could outline the great advances that can be reached by exploiting EC-SS approaches. Papers addressing both the basic science and more applied issues in the field of EC-SS and energy conversion and storage materials have been published in this Special Issue.
