Proceedings of a Workshop
Advances, Challenges, and Long-Term Opportunities in Electrochemistry: Addressing Societal Needs
Proceedings of a Workshop—in Brief
Advances in electrochemistry are enabling new developments in energy storage, energy conversion, catalysis, synthesis, separations, and instrumentation. The workshop Advances, Challenges, and Long-Term Opportunities in Electrochemistry: Addressing Societal Needs, held in Washington, DC, on November 18–19, 2019, provided a venue for scientists in various sectors to discuss electrochemistry applications and the future of the field. Specifically, the workshop reviewed emerging applications of electrochemistry; discussed instrumentation, educational, human-resource, and other needs to enable advances in electrochemistry; and highlighted new technologies and processes that could be developed in light of breakthroughs in fundamental and applied research in electrochemistry. Ultimately, the workshop explored how electrochemistry could transform technologies related to various applications. This Proceedings of a Workshop—in Brief summarizes the presentations and discussions that took place during the workshop. The workshop videos and presentations are available online.1
ELECTROCHEMISTRY: PAST, PRESENT, AND FUTURE
Larry Faulkner, president emeritus of The University of Texas at Austin, opened the workshop by reflecting on the long history of electrochemistry and highlighting some of the early experiments and advances. He emphasized that electrochemistry is intimately connected to fundamental scientific processes in that it connects electrical energy with the material world and focuses on a central chemical process, electron transfer. The operational range of electrochemical systems is enormous: electrochemical systems already support large segments of advanced economies yet present opportunities for investigation of new science on an atomistic scale. He noted that over his career, the field of electrochemistry has “renewed” itself several times and is poised to do so again.
Faulkner discussed some of the challenges that the world faces today and the solutions that electrochemistry offers. Vehicle electrification and development of energy-storage capacity for the electric power grid could advance an energy economy that is sustainable and help to combat climate change. Electrochemistry can also be instrumental in developing greener methods for chemical manufacturing and methods for capturing carbon from waste streams. And it has enabled mobile electronic devices and tools and offers exciting possibilities for new chemical products.
Faulkner stated that future directions of electrochemistry in three categories—energy storage, energy conversion, and electrosynthesis—are the focus of this workshop.2 Although research and development have continued over the years to improve the economics, performance, and safety of batteries, he emphasized that further advances are needed to electrify all types of vehicles and to store the energy needed for proper power-grid management. He added that advances in batteries and fuel cells will require research on composition and design, kinetics of critical processes, and life-limiting processes in cells and their packaging. Faulkner noted that advances in energy conversion and electrosynthesis depend on a better understanding of complex reactions at the electrodes. The most promising direction might be in the ability to investigate events at the electrodes on an atomic scale—research that could lead to new understanding of catalytic processes, he said. He concluded by emphasizing that “a serious program of research in basic science of electrochemistry is essential for scientific leadership and, in the end, for the well-being and security of the nation.”
2 energy storage was a workshop topic, discussion was focused on small-scale energy storage rather than grid-scale storage. Fuel-cell technologies were not specifically addressed.
SESSION I: APPLICATIONS OF ELECTROCHEMISTRY
Stanley Whittingham, Distinguished Professor of Chemistry and Materials Science and Engineering at the State University of New York at Binghamton and 2020 Nobel Laureate in chemistry, discussed the development of multivalent systems for energy systems. Today, lithium-ion batteries dominate portable and grid applications. Various metal oxides have been used for the cathode, but the most commonly used today is Li[NiMnCoAl]O2 (NMC). Whittingham said that the goal is to develop a multivalent (multielectron) cathode, which would reduce the amount of transition metal needed (and the cost) and increase energy density by 50–70% without changing the size or weight of the system. He posed the questions, however, of whether researchers can devise a cathode whose structure can tolerate a two-electron change and a system that can tolerate the voltage changes.
Whittingham described his research on vanadium oxides that has demonstrated that two lithium ions can be reversibly intercalated into a crystalline lattice without damaging the structure (Whittingham et al. 2018). His research has also shown that sodium ions can be intercalated, but the results were not as promising as those with lithium ions. Although magnesium has been suggested as a possible multivalent-ion option, Whittingham has seen no evidence yet that magnesium can support more than one-electron transfers, and it readily grows dendrites, moves slowly in the system, and incurs a high voltage penalty. His research suggests that calcium might be a more attractive option than magnesium. He noted that theoretical predictions have advanced to the point where they can be relied on in predicting lattice structures that allow fast diffusion of various cations in them. He closed by emphasizing the need for fundamental studies of transport, thermodynamics, and structure prediction.
Esther Takeuchi, Distinguished Professor and William and Jane Knapp Chair in Energy and the Environment in the Departments of Materials Science and Chemical Engineering and Chemistry at the State University of New York at Stony Brook with a joint appointment at Brookhaven National Laboratory, continued the discussion of energy storage by demonstrating how in situ and operando methods can advance understanding of highly complex electrochemical systems. She began by defining in situ methods as ones that measure a property or material in an intact system and operando methods as ones that probe a system while it is in operation. She emphasized that operando methods are valuable because they can give insights into kinetics.
Takeuchi stated that charge transfer (transport) must be considered on multiple length scales: atom or molecule, crystallite, particle or aggregate, and electrode. All affect how a system functions and ultimately determine how much energy can be realized from a battery. She used her work with magnetite to illustrate how beneficial insights can be gained by using multiple probes to investigate a system on various scales (Abraham et al. 2016; Bruck et al. 2016). Using several spectroscopic techniques and modeling approaches, her team revealed specific ion-transport pathways in the material, found that the electrochemistry depends on the crystallite size, showed that finely divided materials aggregate in the electrode, and concluded that the goal is to minimize agglomeration because it limits transport within the electrode. Takeuchi noted that operando x-ray absorption spectroscopy can provide particularly useful insight into oxidation state, structure, and coordination environment and be used to map the influence of the local environment on electrochemical activity. Using that technique, her team was able to observe reactions during the lithiation of magnetite (Huie et al. 2018, 2019).
Energy-dispersive x-ray diffraction (EDXRD) has proved to be a valuable technique for investigating electrochemical systems on the scale of the electrode (Bruck et al. 2019), Takeuchi said. Research using EDXRD has shown that electrodes in some cases need to be redesigned to enable ion access and electrochemical reaction throughout the entire electrode. EDXRD has also been used to investigate electrode component design, has shown that binders used to create the electrodes are important to consider, and led Takeuchi and her team to consider the development of conductive binders to minimize the effect on transport of electrons throughout the electrode. Takeuchi concluded by reiterating the importance of investigating electrochemical systems on multiple length scales with multiple methods; doing so provides insight not only into design of the materials but into design of the components that govern transport and kinetics of the system.
Veronica Augustyn, assistant professor and University Faculty Scholar at North Carolina State University, concluded the session on energy storage by discussing the fundamentals and applications of electrochemical capacitors, which store energy by the electrosorption or chemisorption of ions at high-surface-area electrodes. Electrochemical capacitors are not limited by faradaic charge transfer or solid-state mass transfer and offer high power densities, excellent kinetics, intermediate energy densities, and long lifetimes. Augustyn noted that the high energy efficiency at high power translates into much less heat generation, an important property. The market for electrochemical capacitors is much smaller than that for lithium-ion batteries but is likely to grow.
Carbon has typically been used as the material of choice for electrochemical capacitors because it has relatively high conductivity, offers large surface areas, has low density, provides good corrosion resistance, and is inexpensive, Augustyn said. However, more recent efforts have focused on creating hybrid systems that combine an activated carbon electrode with, for example, a graphite or lithium-titanate anode. Unfortunately, those hybrids still have the power limitations exhibited by battery electrodes. Thus, the electrochemical-capacitor field has focused on increasing energy without decreasing power by, for example, modifying the electrolyte or identifying pseudocapacitive materials that can allow charge transfer via chemisorption or intercalation reactions. Augustyn emphasized the importance of intercalation pseudocapacitance that has
been achieved with nanostructured materials because this mechanism provides a pathway for achieving high power and high energy density. She noted that understanding capacitive mechanisms under confinement is crucial and concluded by highlighting the new operando methods that can be used to study the electrochemical interface—research that could lead to fundamental discoveries that would make the high-power–high-energy electrochemical system a reality.
Marc Koper, professor of surface chemistry and catalysis at Leiden University, began the discussion of energy conversion by highlighting some requirements of proton-coupled electron transfers at electrodes. He noted that when multiple electrons need to be transferred the process proceeds in a stepwise fashion with the formation of intermediates—catalyst-bound intermediates if catalysis is involved. He continued that the process is sensitive to pH because protons are typically transferred with electrons, and his research has shown that the optimal pH for a specific reaction is equal to the pKa of the specific acid–base equilibrium.
Applying the electron-transfer concept, Koper next discussed his research on the electrocatalytic reduction of carbon dioxide. Carbon monoxide and formic acid can be formed from a two-electron transfer to carbon dioxide, and he was interested in what determined which product was formed. His work with metalloporphyrins demonstrated that the metal center determined which product was favored with these catalysts (Göttle and Koper 2018). Iron and cobalt centers led to carbon monoxide via a metal-activated pathway, and indium, tin, and rhodium led to formic acid or formate via a hydride-activated pathway. Next, he turned his attention to the reduction of carbon monoxide on copper, a reaction that had been shown to yield methane and ethylene. His team investigated two copper electrodes—Cu(111) and Cu(100)—and found that carbon monoxide was reduced to methane on the Cu(111) electrode and that carbon monoxide was reduced to ethylene on the Cu(100) electrode (Schouten et al. 2011). They demonstrated that ethylene was formed via a dimer of carbon monoxide and that this intermediate was favored only in the binding sites available on the Cu(100) electrode (Pérez-Gallent et al. 2017). He noted that pH plays a role in the reactions described and that recent work with the reduction of carbon dioxide using gold electrodes has emphasized the importance of pH, pH gradients, and mass transport in determining what products are generated.
Koper provided some final thoughts on needed research. Scientists still do not completely understand the role of the electrolyte or what happens near the electrode surface. He said that spectroscopic studies of the interface could provide valuable information and added that theory and computer simulation could also play important roles. He concluded that fundamental research also is needed to understand long-term stability or instability of electrodes.
Ismaila Dabo, associate professor in the Department of Materials Science and Engineering at the Pennsylvania State University, next provided some examples of the use of computer simulations to understand electrochemical systems. First, however, Dabo provided a perspective on the challenge of supplying enough energy for the growing population, that is, achieving global energy sustainability. Using three factors—extraction intensity, energy intensity, and gross domestic product per capita—he calculated that the pace of the transition to renewable energy sources must be greater than 2% per year to achieve sustainability. Although there are reasons to be optimistic with the advent, for example, of hybrid and electric vehicles, there are reasons to worry given that batteries and capacitors that can power heavy-duty vehicles, which are large consumers of energy, are not yet commercially available.
Dabo posed the question, How can we produce chemical fuels in a sustainable manner? He stated that one answer is by developing systems that use artificial photosynthesis, and computer simulations can help to understand and predict the performance of such systems. As an example, Dabo described his work with fuel cells. A problem with fuel cells is the dissolution of the catalytic nanoparticles from the electrodes, which leads to poor durability. One can use computer simulations to model the electrochemical interface; such simulations can help scientists understand which surface sites are the most susceptible to electrodissolution. Using a method called quantum-continuum embedding, his team was able to predict the influence of voltage on clustering trends at bimetallic electrodes. He hopes to use the model to investigate the stability of complex bimetallic and shape-controlled nanoparticles.
He closed by describing his research that uses first-principles computational approaches to accelerate screening of materials for photocatalysts. Using his computational screening approach and taking into account potential availability, his team nominated 30 candidates for testing. Thus far, 12 candidates have been synthesized and tested for photochemical activity. Although some candidates performed as predicted, others did not; this prompted Dabo and his team to refine their predictions and screening criteria. He concluded, however, that his work has demonstrated the potential of using computer simulations to guide research in this field.
Thomas Jaramillo, associate professor of chemical engineering at Stanford University and director of the SUNCAT Center for Interface Science and Catalysis, closed the session on energy conversion by discussing the design of new catalysts and processes for sustainable production of fuels and chemicals. He noted that the rapidly decreasing price of electricity from renewable resources has created new interest in using electrochemical processes for chemical production. The question, however, is, How does one create a new paradigm in an industry that has spent decades building a complex manufacturing infrastructure that is based almost exclusively on fossil resources. Jaramillo said that one potential opportunity is using direct sunlight or electricity to convert abundantly available molecules—such as nitrogen, carbon dioxide, and water—into valuable
products. He noted, however, that any such process must be cost-effective to be considered for scale-up and to motivate research and development in this area.
Jaramillo described three case examples involving the production of hydrogen, carbon-based compounds, and ammonia to illustrate progress. First, he stated that a recognized challenge in the production of hydrogen from electrolysis of water is the use of expensive precious-metal catalysts. His research team has therefore investigated and developed non–precious-metal catalysts, such as nanostructured molybdenum sulfides and cobalt phosphides, that show promising activity, some with commercial applications (Ng et al. 2015; King et al. 2019). Second, Jaramillo discussed his research on carbon dioxide electrocatalysis. Here, the challenge is to design a selective catalyst so that one does not produce a vast mixture of virtually inseparable carbon compounds. Using insights from their work with copper catalysts, his team created a series of new catalysts with improved activity and selectivity in converting carbon dioxide or carbon monoxide to carbon–carbon coupled oxygenates (that is, molecules that have at least two carbons and one oxygen that can be used as fuels or chemical building blocks for other products). His team also developed a gold–copper catalyst that substantially enhanced activity and selectivity for the production of ethanol and propanol compared with pure gold or pure copper catalysts (Morales-Guio et al. 2018). They found that electrode surface area can affect selectivity and created high-surface-area copper catalysts that show nearly 100% selectivity for producing a mixture of ethanol, acetate, and acetaldehyde. Finally, his team created a copper–silver electrode that steered selectivity almost exclusively toward acetaldehyde production (Wang et al. 2020). Jaramillo emphasized that scientists will need to advance mechanistic understanding of carbon-dioxide reduction to develop new catalysts and cost-effective processes.
Jaramillo closed by discussing his research on an electrochemical process for ammonia synthesis that uses a stepwise lithium-cycling process to circumvent hydrogen evolution (McEnaney et al. 2017). This research is of substantial interest because of the desire to replace the Haber-Bosch process, the primary industrial process for making ammonia to be used in manufacturing plant fertilizer and other commodity chemicals today. He noted that one challenge in this research is to detect small amounts of ammonia given its prevalence in the environment; much work has therefore been devoted to developing rigorous protocols for ammonia measurements that can be coupled to new processes (Andersen et al. 2019).
Phil Baran, Darlene Shiley Professor in the Department of Chemistry of the Scripps Research Institute, began the discussion by emphasizing the need to make synthetic organic electrochemistry mainstream. He noted that several years ago his laboratory was trying to synthesize a dimeric indole alkaloid. Out of desperation more than a sense of sustainability, they used a constant-potential electrolysis to produce the dimerized product. That success made them question why electrochemical approaches were not used more commonly in organic synthesis. Baran noted that an advantage of such approaches is that they are innately scalable, which is borne out by the production of various commercial chemicals, and he emphasized the untapped potential in the use of electrochemistry to create a vast array of products.
There have been several barriers to more mainstream adoption of electrosynthesis, Baran said. Education has been a challenge; organic chemists are typically not familiar with the reactions that can be accomplished by using electrochemistry. Lack of standardized equipment has also posed a problem; however, Baran’s team collaborated with a company to create an apparatus to address the problem. He emphasized that the biggest hurdle is a perception that the opportunities offered by synthetic organic electrochemistry are simply not interesting enough; that is, there is no compelling reason to change from a traditional approach. To counter that perception, his laboratory demonstrated new transformations to high-value products (Horn et al. 2016); more sustainable, higher-yielding transformations than current approaches (O’Brien et al. 2014); and new selectivity (Kawamata et al. 2017) with the use of electrochemical approaches.
To close, Baran described two recent success stories regarding electrosynthesis. First, his laboratory developed an electrochemical amination by using a nickel catalyst (Kawamata et al. 2019). He noted that unraveling the mechanism allowed them to improve the process and increase yields and the array of potential products. Second, his laboratory developed a replacement for the Birch reduction that was inspired by lithium-ion battery chemistry (Peters et al. 2019). Their process involves a lithium-ion electroreduction that is simple, sustainable, and safe and that can be used to produce a vast array of products, including carbocycles, heterocycles, and natural-product derivatives. The reaction has a broad scope and can be used for deoxygenations, epoxide and furan openings, reductive olefinations, tandem aziridine openings, and benzyl deprotections. Most important, this electroreduction eliminates the expense and safety concerns associated with the use of lithium metal and liquid ammonia required for a Birch reduction. As a final note, Baran emphasized the importance of collaboration and acknowledged that his work would not have been possible without his collaborators.
Song Lin, Howard Milstein Faculty Fellow and assistant professor of chemistry at Cornell University, continued the discussion on electrosynthesis by describing his research on electrochemical alkene azidation. He was interested in exploring electrochemical approaches because they offer a new avenue for reaction discovery given precise potential control, direct generation of radical intermediates, integration of multiple reduction–oxidation events in the same reaction system, external temporal control of a reaction as it proceeds, and opportunities for chemoselectivity and stereoselectivity. He noted that his team used fundamental knowledge of organic chemistry to guide the discovery of new catalysts for electrosynthesis but also used electrochemistry to discover new reactions and to gain a deeper and better understanding of the organic reaction mechanisms.
Lin was interested in diazides because they can be easily reduced to diamines, many of which are chemically or biologically important. In the first phase of the research, his team conducted control experiments to shed light on key radical events, searched for an electrocatalyst that could promote diazidation, and identified a manganese catalyst that promotes highly efficient and selective alkene diazidation (Fu et al. 2017). Lin noted the broad scope of the reaction and the vast array of diazides that can be created with high yield by using their approach. His team was also able to develop a tandem diazidation-reduction protocol for diamine synthesis. In the second phase of his research, his team conducted mechanistic studies of electrochemical azidooxygenation by using spectroscopic techniques, kinetic analysis, and computational analysis (Siu et al. 2018). The insights gained were then used to develop a second-generation approach to alkene diazidation with even broader applicability (Siu et al. 2019). In closing, Lin stated that electrochemistry has great potential for aiding discovery of new reactions and creation of new molecules that are important to society; scientists have only scratched the surface of all the possibilities.
Kevin Moeller, professor of chemistry at Washington University in St. Louis, next described four examples from his work that demonstrate unique opportunities that electrochemistry offers to synthetic chemists. He agreed with Baran and Lin that a compelling story has been needed to thrust electrosynthesis into the mainstream—that is, a reaction that synthetic chemists want or need that cannot be conducted in any other way—and that is what his research is designed to accomplish. For the first example, Moeller stated that the goal was to develop synthetic routes to lactam-based peptidomimetics that allow one to shape conformational probes. They needed a method for functionalizing an amino acid and found that anodic electrochemistry offered a perfect solution: it allowed them to oxidize a variety of molecules with high selectivity without having to vary reaction conditions and use multiple chemical oxidants (Tong et al. 1998). In the second example, Moeller’s team investigated the radical cation intermediates that are central to oxidative cyclization reactions and used electrochemistry to conduct structure–activity studies that allowed them to identify problematic oxidative steps and possible solutions for avoiding unsuccessful transformations (Feng et al. 2017). Moeller stated that the third example highlights a totally new synthetic challenge; the goal was to be able to monitor binding interactions between small molecules and biological targets in real time. To do so, they created a microelectrode array in which each surface site can be uniquely functionalized to probe the activity of a molecule with a receptor (Graaf and Moeller 2015). Moeller noted that one benefit of that approach is that they can recover molecules from any electrode in an array so that they can be characterized—a situation that provides an unprecedented level of quality control. In the final example, Moeller showed a proof-of-principle experiment for a new avenue to selectivity that involves depositing a molecular recognition element onto an electrode and thus attracting a specific molecule to the surface to undergo the desired reaction. The idea is to create a surface-based molecular recognition event that would alter the course of the reaction, that is, favor one product over another. He closed by emphasizing that electrochemistry has been and remains an essential tool for his research.
SESSION II: EDUCATIONAL, RESOURCE, AND OTHER NEEDS TO ADVANCE ELECTROCHEMICAL SCIENCE AND APPLICATIONS—A PANEL DISCUSSION
Panelists: Jeffrey Dick, University of North Carolina; Thomas Jaramillo, Stanford University; Shelley Minteer, University of Utah; Andrew Rappe, University of Pennsylvania; Esther Takeuchi, State University of New York at Stony Brook; and Bill Tumas, National Renewable Energy Laboratory
Baran moderated the panel discussion and began by prompting the panelists to provide an “elevator pitch” to demonstrate the importance of electrochemistry. Tumas stated that the world is undergoing a transformation in how energy is generated, stored, and used, and electrochemistry will be key to the innovations needed to make this transformation. He noted, for example, that it is not simply electrifying vehicles but electrifying our industries and creating a circular economy. Takeuchi emphasized that electrochemistry is the interface between humans and technology in that it offers the ability to store electricity effectively and use it on demand. Minteer stated that electrochemistry will play an important role in health science and services by helping to create wearable diagnostic devices that will provide needed data on health and fitness. Dick noted that electrochemistry now offers the opportunity to study reactions on an atomic or molecular scale and added that although much of the discussion has focused on making chemical bonds, electrochemistry also offers the potential for breaking bonds, which is important for environmental remediation.
Baran asked the panelists about educating the future generation of electrochemists, a topic that resonated with the workshop attendees, who spent the majority of the session discussing various aspects of this topic. Rappe noted the need for an integrated science approach that provides the next generation with all the relevant understanding and capabilities. Jaramillo agreed that electrochemistry needs to be better integrated into the existing curriculum but added that instructors need to be teaching the electrochemistry of the next 50 years rather than what has happened in the last 50 years. Takeuchi emphasized the importance of research experience to excite the next generation and the need to expose students to the multidisciplinary aspect of electrochemistry. Rappe agreed that a robust course that leads students into the laboratory at the earliest stage possible is ideal; students will be more motivated to return to the classroom because they recognize what knowledge they need to enhance their ability to perform research. A workshop participant echoed the need to educate students early about electrochemistry and consider research not only in academe but in industry, where students can begin to understand the applications of research results. Tumas agreed that incorporating the practical aspects—the
science of manufacturing—into the curriculum is important. Rappe stated that all science undergraduates would benefit greatly from having one or two engineering courses. A workshop participant, noting that the discussion had been focused on undergraduates, would encourage integrating high-school students into team research and focusing not only on the top achievers but on the B students who might be overlooked. A great research experience can be a powerful motivator to improve achievement. Another workshop participant countered that the best time to reach “the future generation” is in middle school and encouraged programs that pair graduate students with middle-school students as mentors.
Minteer voiced concerns about the lack of degree programs in electrochemistry, particularly in comparison with other countries. Several panelists acknowledged that electrochemistry is often an afterthought in some courses and then never taught again. Several workshop participants echoed the concern about the lack of rigorous teaching of electrochemistry in this country and emphasized the need for a competitive electrochemistry curriculum that puts the United States on par with countries that have been granting degrees in electrochemistry since 1900. Several participants discussed the need to develop an online course dedicated to electrochemistry and hoped that a professional organization, such as The Electrochemical Society, might sponsor such an activity. Others raised concerns about offering opportunities to disadvantaged students and increasing diversity in the field. Augustyn, who collaborates with universities in Africa, noted that a global passion for electrochemistry is growing and that development of an open-source course supported by a professional society would help to address issues of access and bring greater diversity to the field.
Baran asked the audience about instrumentation needed to advance the field. A participant responded that x-ray spectroscopic techniques are needed that can be used under aqueous conditions and provide both spatial and chemical resolution so that interfaces can be better probed. Another participant added that spatial resolution and chemical resolution are important but that temporal resolution is also important. Scientists need to understand how the instruments that use high-energy photons perturb the electrochemical systems under operando conditions, another participant said, because much time is spent in trying to understand those perturbations. As a final note on this topic, one participant emphasized the need for complex, centralized light sources and the need for institutes that have critical microscopy instruments equipped with electrochemical cells.
In closing, Baran asked the audience about the challenges facing electrochemistry. One workshop participant noted that more attention needs to be paid to data processing and analysis, which can be a bottleneck in research. Tumas noted the challenges of doing electrochemistry in brackish water. He also emphasized the need to use products from both electrodes. Takeuchi noted the often burdensome nature of collaborating with companies and hoped that a better infrastructure could be developed to promote information transfer. A workshop participant noted that the field appears to be too tied to metrics for their own sake at the expense of determining whether a process might be good enough for a particular application. Jaramillo agreed that we need to remember the big picture. Kat Stephan, an editor of Joule, said that she would appreciate feedback on what people thought were the appropriate metrics to report in the literature. Bruce Garrett, director of the Chemical Sciences, Geosciences, and Biosciences Division in the Office of Basic Energy Sciences of the US Department of Energy, concluded the discussion by noting that he was less concerned about whether a particular metric had been met than about what was learned about fundamental mechanisms; that understanding is what will ultimately lead to better systems.
SESSION III: ELECTROCHEMICAL APPLICATIONS ON THE HORIZON
Shelley Minteer, Dale and Susan Poulter Endowed Chair of Biological Chemistry in the Department of Chemistry at the University of Utah, began the session by discussing next-generation applications of bioelectrochemistry. She defined bioelectrochemistry as the use of electrochemistry to study a biological system or the use of a biological system to do electrochemistry. In studying biological systems, she noted, much of the focus has been on material design and on making electrodes smaller, more durable, and more biocompatible.
Minteer stated that she is interested in bioelectrocatalysis—the use of enzymes or microorganisms as catalysts—and described research in this field. Bioelectrocatalysis has been used to create biosensors, such as blood glucose monitors for people who have diabetes, and to develop microbial fuel cells. The difference between a traditional fuel cell and a biofuel cell is that the metal catalyst has been replaced with a biological catalyst. Today, people are experimenting with using microbial fuel cells to treat wastewater; these systems use the oxidation of organic materials in the waste to produce electricity and thus create energy-efficient wastewater treatment. To improve performance of these fuel cells, scientists are examining how to engineer an enzyme that normally has a single substrate so that it can be used with multiple substrates, that is, to bioengineer an enzyme so that it can accomplish multiple steps in a process. Another application of biofuel cells, Minteer noted, is energy harvesting for wearable electronics that can sense molecules of interest, such as metabolites or toxins in biological fluids. As a final example, Minteer described her research that uses biocatalysts to generate ammonia as an intermediate in the synthesis of such interesting products as chiral amines (Mutti et al. 2011).
In closing, Minteer listed several challenges. First, scientists need to increase understanding of the electron-transfer pathways between electrodes and microorganisms or enzymes; research today involves a trial-and-error approach rather than one that relies on knowledge-based design. Second, materials that promote electron transfer are needed. Third, scientists need to improve stability of devices in various environments, and this emphasizes the need for device engineering that brings together material science, reactor design, and synthetic biology. She concluded by saying that scientists need to start integrating all the systems, for example, integrating bioreactors with biosensors.
David Muller, Samuel B. Eckert Professor of Engineering in the School of Applied and Engineering Physics at Cornell University, described the application of advanced electron microscopy and spectroscopy to characterize electrochemical systems with high spatial resolution. Today, he said, scientists have the ability to see individual atoms in chemical species and determine composition and bonding at atomic resolution. He noted, however, that resolution is dose-limited by radiation damage in many electrochemically active systems. Muller stated that cryogenic electron microscopy— a technique borrowed from the biological sciences to study three-dimensional structures—has been valuable in examining solid–liquid interfaces. As an example, he described a study that used cryogenic scanning transmission electron microscopy to examine the solid-electrolyte interphase in a lithium-metal battery and found two structurally and chemically distinct dendrite structures, one of which could help to explain a loss of battery capacity (Zachman et al. 2018). Muller next highlighted a new detector—the electron microscope pixel-array detector—developed in his group that substantially increases resolution at a much lower dose and makes it possible to characterize large numbers of catalysts rapidly (Jiang et al. 2018). Another advance, Muller said, is the development of electrochemical cells that can be used with spectroscopic imaging techniques to allow study of a system on a nanoscale in situ as it operates and can be used for direct imaging and studying the electrochemical double layer associated with electrode surfaces (Holtz et al. 2014). He concluded by noting that the ability of electron microscopy to resolve and isolate various types of local atomic structures at potential catalytic sites and to quantify their distributions is especially valuable given the inhomogeneous nature of many electrochemical systems.
Miguel Modestino, assistant professor in the Department of Chemical and Biomolecular Engineering at New York University, next discussed the opportunities to use electrochemistry to create sustainable chemical manufacturing; he focused on basic commodity chemicals. He noted that separations are an extremely energy-intensive process in manufacturing and listed a few processes—olefin production, carbon dioxide capture, and water purification—in which electrochemical approaches have been applied successfully. However, he described his research to improve an electrochemical method for synthesizing adiponitrile—an intermediate in the production of nylon 6,6, a high-volume commodity chemical—and create a method that has high selectivity, high throughput, and high energy-conversion efficiency (Blanco et al. 2019). His team first had to overcome some key challenges, such as the poor stability of electrolytes, the low solubility of reactants in the electrolytes, and the low selectivity given multiple reaction pathways. They found that they could control the reaction dynamics and enhance mass transport by using pulsed electrosynthesis. That is, they could modulate the concentration of various species at the electrode interface where the chemistry happens, drive specific pathways in the reaction mechanism, and thus increase selectivity and production rates. His team then optimized the reaction conditions by using data-driven models. Modestino noted that machine-learning tools can allow scientists to understand fundamental mechanisms and accelerate research. He concluded that this case study has broad implications for the chemical industry and can serve as a model for organic synthesis that uses electrochemical methods. He noted, however, that there is a need to build a bridge between the electrochemistry community and the reaction-engineering community, not only with respect to knowledge but with respect to tools.
Matt Sigman, Peter J. Christine S. Stang Presidential Endowed Chair of Chemistry and Distinguished Professor at the University of Utah, continued the discussion begun by Modestino on the opportunities for data science to optimize electrochemical processes. He described the process for using data science in organic chemistry—first collecting empirical data and data mining, next developing or estimating molecular descriptors and property sets, then conducting supervised learning by using regression tools, and finally converting the “math” to mechanisms and translating to new systems. He noted that his group typically describes the molecular features by using density functional theory or quantum mechanically derived parameters, but they also use hybrid parameters that combine vibrational frequencies. He illustrated those approaches with two case studies.
The first case study involved identification of a small-molecule catalyst for the enzymatic electrochemical oxidation of glycerol and optimization of the catalyst (Hickey et al. 2014). They were able to identify a satisfactory catalyst (TEMPO-NH2)3 and then sought to understand how the structural features correlated with catalytic activity so that they could predict catalytic activity and eliminate the need for catalyst screening. Studying the structure–function relationships enabled them to design TEMPO derivatives for various diverse applications. The second case study involved designing better electrolytes for flow batteries (Sevov et al. 2015). First, they identified a promising anolyte candidate and then synthesized several analogues to serve as a training set for developing a statistical model of stability. Sigman noted that they used several properties—such as infrared frequencies, steric parameters, and electrochemical potentials—as model inputs. After a few refinements, the model was able to predict an anolyte that had good stability and high redox potential.
Sigman concluded by listing several applications of data science to chemistry, including understanding structure–function relationships, designing synthetic routes, optimizing reaction protocols and autonomous reactions, and predicting molecular features and novel reactions. He noted, however, that there are issues with data quality and quantity; if one wants to use a sophisticated learning algorithm, one needs “big data.” Other issues include designing experiments to ensure that data are statistically relevant, using appropriate methods for feature selection, and training electrochemists in data science. Perhaps, he said, the most important problem is data bias: what does not “work” does not get published. Sigman closed by emphasizing that electroanalytical tools are especially useful in that they provide a rich dataset and information on kinetics and thermodynamics.
3 is (2,2,6,6-tetramethyl-1-piperidinyl)oxidanyl.
Jeffrey Dick, assistant professor in the Department of Chemistry at the University of North Carolina at Chapel Hill, built on Sigman’s closing comment regarding the importance of electroanalytical tools and discussed the measurement of single entities with electroanalytical methods. Dick defined an entity broadly to mean any one thing—something as big as the Earth or as small as a single atom. He stated that it is important to measure single entities because misconceptions arise when ensemble averages are measured. Fortunately, scientists now have an array of techniques to study the chemical and physical properties of single entities, and these new techniques take measurement science toward a digital era in which the limit of detection is one entity (Goines and Dick 2020). He emphasized, however, that scientists need to correlate collisions with microscopy to move the field forward.
One question that Dick has tried to address in his research is how small a nanoparticle can be observed. Controlling mass transport and using electrocatalytic amplification, he has been able to demonstrate electrodeposition of small platinum clusters and nanoparticles on ultramicroelectrodes (Glasscott and Dick 2018). He noted that detecting single entities typically requires nanoelectrodes to enhance sensitivity and described several methods for making them, including using lasers to “pull” wires to small diameters, using scanning electrochemical cell microscopy to activate only a small portion of an electrode, and confining contents to nanodroplets. In closing, Dick stated that gaining a fundamental understanding of inner-sphere reactions is important for all applications—energy storage, energy conversion, and electrosynthesis—and that the electrochemical techniques described provide an avenue for learning new truths at this level. He added that resolution in electrochemistry is limited only by the smallest electrode that one can make.
Karthish Manthiram, Warren K. Lewis Career Development Professor in Chemical Engineering at the Massachusetts Institute of Technology, closed the session by discussing the opportunities for using electrocatalysis and electrosynthesis to advance a sustainable chemical industry. He stated that the chemical industry is responsible for 7% of global greenhouse-gas emissions, and the need to decrease its carbon footprint has been an important motivating factor for his research. Accordingly, he has focused on developing synthetic routes for chemicals by using nitrogen, carbon dioxide, and water as reactants; he discussed three examples.
First, Manthiram discussed an electrochemical process for a low-carbon ammonia synthesis. He noted that ammonia is an extremely important commodity chemical given its use in many synthetic processes. Unfortunately, generation of ammonia is responsible for a substantial portion of greenhouse gases emitted from the chemical industry; ammonia is typically produced by the reaction of nitrogen and hydrogen at high temperatures and pressures. However, Manthiram stated, a much milder electrochemical process that uses nitrogen and water as reactants is theoretically possible. His group was able to develop a lithium-mediated continuous electrochemical process for ammonia synthesis (Lazouski et al. 2019). Second, Manthiram described an electrochemical process that uses carbon dioxide at room temperature to carboxylate benzylic carbon–nitrogen bonds (Yang et al. 2019). The advantage of the process is that it does not require metal reductants, sacrificial anodes, or column chromatography for product purification. Third, he described an electrochemical epoxidation process that involves an oxygen-atom transfer reaction that uses water and a manganese oxide electrocatalyst (Jin et al. 2019). One of the benefits of his process is that it also provides an inexpensive source of hydrogen. In closing, he noted that the processes described constitute a starting point and that much more needs to be done to develop new routes to enable sustainable chemical synthesis.
Carol Bessel, acting division director for chemistry at the National Science Foundation, provided a few closing remarks. She touched on the diversity of topics discussed and stated that the workshop has set the stage for what we can do now and what we would like to do in the future. The federal funding agencies, she noted, are extremely interested in the tools and workforce capabilities needed to bring the technologies highlighted here to fruition. Bessel appreciated the conversation on educating and motivating future generations and agreed that online tools and curricula would be great resources. She emphasized that big challenges lie ahead and that conquering them will require a multidisciplinary approach; team science will move us forward.
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DISCLAIMER: This Proceedings of a Workshop—in Brief was prepared by Ellen Mantus as a factual summary of what occurred at the workshop. The statements recorded here are those of the individual workshop participants and do not necessarily represent the views of all participants, the planning committee, the Chemical Sciences Roundtable, or the National Academies of Sciences, Engineering, and Medicine. To ensure that this proceedings meets institutional standards for quality and objectivity, it was reviewed in draft form by Carol Bessel, National Science Foundation; Shannon S. Stahl, University of Wisconsin–Madison; and Esther S. Takeuchi, State University of New York at Stony Brook. The review comments and draft manuscript remain confidential to protect the integrity of the process.
Planning committee members were Héctor D. Abruña, Cornell University; Phil Baran, Scripps Research Institute; Carol A. Bessel, National Science Foundation; Yet-Ming Chiang, Massachusetts Institute of Technology; Anne Co, The Ohio State University; Raul Miranda, US Department of Energy; and Cynthia Zoski, The University of Texas at Austin.
ABOUT THE CHEMICAL SCIENCES ROUNDTABLE
The Chemical Sciences Roundtable provides a neutral forum to advance the understanding of issues in the chemical sciences and technologies that affect government, industry, academic, national laboratory, and nonprofit sectors and the interactions among them and to furnish a vehicle for education, exchange of information, and discussion of issues and trends that affect the chemical sciences. The Roundtable accomplishes its objectives by holding annual meetings of its members and by organizing webinars and workshops on relevant important topics.
Chemical Sciences Roundtable members are Linda Broadbelt (Co-Chair), Northwestern University; Mark E. Jones (Co-Chair), The Dow Chemical Company; Tina Bahadori, US Environmental Protection Agency; Brian Baynes, MODO Global Technologies; Michael R. Berman, Air Force Office of Scientific Research; Carol Bessel, National Science Foundation; Martin Burke, University of Illinois at Urbana-Champaign; Michelle Chang, University of California, Berkeley; Miles Fabian, National Institutes of Health; Michael J. Fuller, Chevron Energy Technology Company; Laura Gagliardi, University of Minnesota; Bruce Garrett, US Department of Energy; Franz Geiger, Northwestern University; Carlos Gonzalez, National Institute of Standards and Technology; Malika Jeffries-El, Boston University; Jack Kaye, National Aeronautics and Space Administration; Mary Kirchhoff, American Chemical Society; Robert E. Maleczka, Jr., Michigan State University; David Myers, GCP Applied Technologies; Timothy Patten, National Science Foundation; Nicola Pohl, Indiana University Bloomington; Ashutosh Rao, US Food and Drug Administration; Leah Rubin Shen, Office of Senator Chris Coons; and Jake Yeston, American Association for the Advancement of Science.
This activity was supported by the National Science Foundation under Grant CHE-1546732 and the US Department of Energy under Grant DE-FG02-07ER15872. Any opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of any organization or agency that provided support for the project.
Suggested citation: National Academies of Sciences, Engineering, and Medicine. 2020. Advances, Challenges, and Long-Term Opportunities in Electrochemistry: Addressing Societal Needs: Proceedings of a Workshop—in Brief. Washington, DC: The National Academies Press. https://doi.org/10.17226/25760.
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