A Conversation with Al Bard
F
ather of Modern Electrochemistry is a term often applied to Allen (Al) J. Bard, Professor of Chemistry, University of Texas at Austin. For nearly five decades he has remained a great visionary in identifying key areas in energy research (Figure 1). From fundamental electrochemistry to
solar energy conversion. ECL involves the production of light through the electrochemical generation of radical ion intermediates, and Daniels said something like it is easy to produce light from electricity, but much more difficult to produce electricity (or chemicals) from light. I think I answered that I agreed, but I did not see how one could run ECL backward. However, I started to think about this and concluded that homogeneous photochemistry was unlikely to be an efficient route for the conversion of solar energy, because recombination would be too fast, and I think earlier and even current work supports this. It was only when I read the Honda−Fujishima paper (DOI: 10.1038/238037a0) that I started to think about the use of semiconductors for this purpose. I was scheduled to go on a sabbatical in Paris in 1973, so I could not get started on it right away. I did partly, however, use my time there to learn about the field, especially the excellent papers on electrochemistry at semiconductor electrodes by Heinz Gerischer. EL: What were some of the major challenges that you encountered in carrying out energy research in the 1970s and 1980s? How did you overcome these challenges? Bard: I was fortunate to have an excellent graduate student, Ken Hardee, who was interested in this problem, and we started to work on it. A first challenge was that almost all work had been done with TiO2 single crystal (rutile). Single crystals were good for fundamental studies because they tended to be relatively pure and more amenable to theoretical treatment; however, they were expensive and not available for many materials that we might want to try. So early on we decided to try polycrystalline materials that we could synthesize and then use to fabricate thin films. In fact, our first paper published in 1975 (DOI: 10.1149/1.2134312) was a description of TiO2 that was deposited by chemical vapor deposition (CVD). This worked surprisingly well, and we could replicate the Honda− Fujishima experiments with this. A well-recognized problem with rutile was its large band gap, 3 eV, which meant it absorbed only about 4% of the solar spectrum. Thus, we began looking for oxides with smaller band gaps; Ken was the first one to make and try out the PEC on hematite (1976, DOI: 10.1149/1.2132984). Another challenge was relating the properties of semiconductors, especially the band energies and intermediate levels of surface state energies, to the electrochemical properties. The thermodynamic electrochemical window available in water is too small to map most semiconductors of interest in solar energy conversion; so, one must use nonaqueous solvents, e.g. acetonitrile, for such studies in 1975 (DOI: 10.1021/ ja00859a007). An excellent postdoc of mine, Steve Frank, undertook the study of TiO2 and found a level about 0.7 eV below the conduction band edge. An additional advantage of
Figure 1. With Prof. Allen J. Bard during a recent visit to University of Texas at Austin. (Photo courtesy of P. Kamat).
electrochemiluminescence, from photoinduced processes at electrodes to dye sensitization, from semiconductor particle photocatalysis to single-crystal semiconductor photoelectrochemistry, he has made seminal contributions since the 1970s. He has nurtured photoelectrochemistry for nearly five decades and trained several generations of students and postdoctoral researchers who have become successful energy researchers. His lifelong research contributions have been recognized through the Priestley Medal, the American Chemical Society’s highest honor. In 2011 he was also awarded the National Medal of Science by President Barak Obama. As the first oil crisis appeared in the 1970s, Prof. Al Bard took the initiative to engage in renewable energy research and publish a series of papers related to semiconductor photoelectrochemistry and photocatalysis. These early papers provided fundamental principles of photoelectrochemical/ photocatalytic conversion of light energy into electrical energy or chemical energy. Be it a design of a new solar cell or an energy storage device of water-splitting and CO2 reduction, we find his published work highly informative when carrying out modern-day energy research (Figure 2). The following conversation provides some insights into the visionary world of Prof. Allen Bard. EL (ACS Energy Letters): How did you become interested in photoelectrochemistry and energy conversion during the early years of your career? Bard: I first became interested in this as a result of a question after a seminar on electrogenerated chemiluminescence (ECL) I was giving at the University of Wisconsin. It took place in the middle 1960s, and the questioner was the distinguished physical chemist Farrington Daniels, who had long been interested in © XXXX American Chemical Society
Received: June 29, 2017 Accepted: July 7, 2017
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DOI: 10.1021/acsenergylett.7b00566 ACS Energy Lett. 2017, 2, 1746−1748
Energy Focus
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ACS Energy Letters
similar electrochemical reaction) (DOI: 10.1021/ja00465a065), and in 1978 a new pathway with acetic acid to form methane (DOI: 10.1021/ja00475a049). It also occurred to us that we could make these particle reactions more efficient by giving the surface of the particle differential reactivity. In 1978 we could do this by depositing platinum on the TiÒ 2 and could carry this out photochemically with the electrons that resulted from acetic acid oxidation (DOI: 10.1021/ja00481a059). This platinized powder was later used for many other reactions, and powders of different semiconductors, like CdS and WO3, also worked, and other metals, like Cu, could also be deposited. The scope of application of various powders in heterogeneous photocatalysis was wide indeed. For example, Kraeutler and Harold Reiche in 1979 showed that radical polymerization could be carried out. Fu-Ren (Frank) Fan demonstrated photosensitization of TiO2 by phthalocyanine in the oxidation of hydroquinone (DOI: 10.1021/ja00514a056). A review of the principles and much of this work appeared in 1980.1 A student, Wendell Dunn, in 1981 showed that the stirred powder suspensions, “slurry electrodes”, with an inert electrode contact, could serve as PEC electrodes (DOI: 10.1149/1.2127378). Perhaps the most interesting of our heterogeneous photocatalysis experiments was one that provided APEC equivalent of the famous Urey−Miller spark experiment, where Dunn and Aikawa showed that platinized TiO2 produces amino acids under irradiation of a mixture of water, methane, and ammonia (DOI: 10.1021/ja00413a020). Several of the concepts of homogeneous photocatalysis were patented, and these were licensed to allow larger scale studies, for example in removing toxic chemicals from water; these were interesting to me because they demonstrated the possibilities and problems of larger-scale PEC operations. For example, the obvious problem of the intermittency of terrestrial solar irradiation meant that capital equipment was used only about one-third of the time. Thus, in the water purification experiments, where it was desirable to continuously treat a waste stream, it was better to employ artificial lamps and operate around the clock. The cost of pumping was also clearly a non-negligible factor. In the end, the demonstration project was a success in terms of decomposing toxic waste but did not compete economically with alternative processes, e.g. simple adsorption on activated carbon. EL: The current focus of photocatalysis is in the area of water splitting and CO2 reduction. We still have a long way to go to design economically sustainable, practical devices. Would you mind commenting on the potential of photocatalysis as part of the energy storage portfolio? Bard: This is a difficult question, because it depends importantly on economic factors and available alternative energy technologies. Let me go back to the early 1990s and discuss this in the context of work being done then. At this time, PEC concepts had matured considerably, and the melding of electrochemistry, photochemistry, and semiconductor physics resulted in a better understanding of the processes and possibilities; for example, at the University of Texas a consortium of faculty, students, and postdocs formed to address the possibility of practical water splitting, funded by the Gas Research Institute (GRI). Among the members of this group were Marye Anne Fox, Tom Mallouk, and Prashant Kamat. Many useful concepts emerged, e.g. that of “integrated chemical systems”, in which a more complex system involving semiconductor, catalyst, etc. was needed to obtain desired results. One result of this collaboration was the publication of an account in a special issue of Accounts of Chemical Research
Figure 2. Pivotal contributions of Prof. Bard in the area of semiconductor photoelectrochemistry and photocatalysis provide the basis for modern-day energy research.
the aprotic solvent was the availability of a wide range of redox couples that could be employed. Such solvents were used fairly frequently in later studies, for example to improve the stability of the semiconductor under the radiation. Understanding the factors that control the stability of the semiconductor/liquid interface was another challenge. I was spending a sabbatical at Caltech as a Fairchild scholar at the same time that Mark Wrighton, who had independently been making important advances in this field at MIT, was also visiting. One morning, Mark came to my office in 1977 to discuss this problem, and we came up with a fairly simple thermodynamic justification of the stability (DOI: 10.1149/1.2133140). Heinz Gerischer, a leader in research in this field for many years, came up with a similar proposal at about the same time. EL: You were the pioneer in establishing the radical formation and chemical transformations in UV-irradiated TiO2 particle systems. The chemical reactions you studied at semiconductor surfaces became the basis of photocatalysis. What led you to identif y the strength of the photocatalysis f ield so early? Bard: We began to think about the possibility of scaling up the photoelectrochemical cells to do chemistry. It really seemed that the cost of materials, e.g. electrodes and separators, as well as the known high capital costs of electrochemical cells, would make practical utilization pretty difficult, especially at the low efficiencies that were being obtained with the current semiconductors. It occurred to us that it might be possible for powders of semiconductors to carry out similar photochemical reactions and simplify scale-up, for example in solar ponds. We understood the possible difficulties with using small particles, such as the lack of suitable interfacial fields for efficient separation of carriers, but we thought it was worth a try. We used P-25 TiO2, which is relatively inexpensive and which consists of anatase and rutile particles in the range of nm in μm aggregates. Steve Frank in 1977 looked at the behavior of aqueous cyanide solutions under artificial and solar irradiation and demonstrated quite efficient removal of cyanide (DOI: 10.1021/ja00443a081). These experiments were quite easy, and so when Bernhardt Kraeutler came as a postdoc we decided to try to extend these experiments in 1977. For example, one could use the same strategy to decompose acetate and form ethane and what we called the photo-Kolbe reaction (after a 1747
DOI: 10.1021/acsenergylett.7b00566 ACS Energy Lett. 2017, 2, 1746−1748
Energy Focus
ACS Energy Letters devoted to a number of “Holy Grails” in chemistry, like solar energy to produce hydrogen. This account discussed factors in producing a successful PEC system.2 I think many of the ideas in this paper, even after more than 20 years, are still relevant. Humans have relied on solar energy in the form of biomass and later stored fossil fuels for most of their existence. Unless we discover new forms of energy that can be tapped, solar energy in a variety of configurations is still probably the energy of the future. EL: Many young researchers aspire to engage in energy research. Could you please provide some tips to these young scientists on how to be successf ul? Bard: I can try, but only with a few caveats. For one, it depends how we define “successful”. I would define it as having performed good science, i.e. answered an important and interesting question or having assembled and tested a useful device, preferably one that is better than the existing state-ofthe-art. Second, with an understanding that this advice comes from someone who grew up in a very different science culture than that which currently exists. By this I mean that a scientist tries to tell as complete a story about the research as possible, calling attention to any still unsolved problems in the interpretation, and is honest in questions of statistical validity and reproducibility.3 I’m afraid the current style is more like that of a used-car salesman. Finally, I do not think my advice is specific for energy research. In general, I do not think one should choose research based on societal impact, especially in a field as large and complex as “energy”. Really having a societal impact here is quite improbable. I would also avoid the “fad of the week”, e.g., graphene, nanoparticles, etc. One should select the problem the same way one selects a mate, i.e. really love the research and rather be doing that than almost anything else! Moreover, one must recognize that a good problem may be plagued with lots of difficulties and experiments that “do not work”. However, one gets a beautiful result or discovery once in a while, and that makes it all worth it.
Biography
Allen J. Bard was born in New York City on December 18, 1933 and grew up and attended public schools there, including the Bronx High School of Science (1948−1951). He attended The City College of the College of the City of New York (CCNY) (B.S., 1955) and Harvard University (M.A., 1956, Ph.D., 1958). Dr. Bard joined the faculty at The University of Texas at Austin (UT) in 1958 and has spent his entire career there. He has been the Hackerman-Welch Regents Chair in Chemistry at UT since 1985. He spent a sabbatical in the CNRS lab of Jean-Michel Savéant in Paris in 1973 and a semester in 1977 at the California Institute of Technology, where he was a Sherman Mills Fairchild Scholar. He was also a Baker lecturer at Cornell University in the spring of 1987 and the Robert Burns Woodward visiting professor at Harvard University in 1988. He has worked as mentor and collaborator with 99 Ph.D. students, 18 M.S. students, over 200 postdoctoral associates, and numerous visiting scientists. He has published 988 peer-reviewed research papers, 75 book chapters, and other publications, and he has received over 23 patents. He has authored three books: Chemical Equilibrium (1966), Electrochemical MethodsFundamentals and Applications (1980, 2nd Ed., 2001, with L. R. Faulkner), and Integrated Chemical Systems: A Chemical Approach to Nanotechnology (1994). He served as Editor-in-Chief of the Journal of the American Chemical Society 1982−2001. His many awards include the ACS Priestley Medal (2002), the Welch Foundation Award in Chemistry (2004), the Wolf Foundation Prize (2008), the 2011 National Medal of Science (2011), and The Enrico Fermi Award (2013). His research interests involve the application of electrochemical methods to the study of chemical problems and include investigations in scanning electrochemical microscopy, electrogenerated chemiluminescence, and photoelectrochemistry.
Prashant V. Kamat, Editor-in-Chief, ACS Energy Letters
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University of Notre Dame, Notre Dame, Indiana 46556, United States
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00566.
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REFERENCES
(1) Bard, A. J. Photoelectrochemistry. Science 1980, 207, 139−144. (2) Bard, A. J.; Fox, M. A. Artificial Photosynthesis - Solar Splitting of Water to Hydrogen and Oxygen. Acc. Chem. Res. 1995, 28, 141−145. (3) National Academy of Sciences, National Academy of Engineering, and Institute of Medicine. On Being a Scientist: A Guide to Responsible Conduct in Research, 3rd ed.; The National Academies Press: Washington, DC, 2009; https://doi.org/10.17226/12192.
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AUTHOR INFORMATION
ORCID
Prashant V. Kamat: 0000-0002-2465-6819 Notes
Views expressed in this Energy Focus are those of the author and not necessarily the views of the ACS. The author declares no competing financial interest. 1748
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