Editorial pubs.acs.org/cm
Chemistry of Materials’ 1k Club: Klaus-Dieter Kreuer. Establishing the Connection Between Materials and Proton Conductivity
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cases, an inherent part of forming an uninterrupted proton trajectory. This makes proton conduction a complex phenomenon of extended ensembles with the involvement of a large number of nontrivially coupled collective coordinates. In the Chemistry of Materials article, I tried to break down the latter into three effective coordinates (q, Q, and S), which provided a reasonable setting for discussing proton conduction phenomena. Admittedly, discourses in this field are still scarce and progress is slow. At least, we have seen phenomenological descriptions of the proton conduction mechanism in oxides (see Figure 1), in imidazole (prototypical for heterocyclic systems), in solid salts of oxo-acids, and, very recently, in phosphoric acid, the compound with the highest intrinsic proton conductivity. These insights were obtained through quantum chemical molecular dynamics simulations, which sometimes even revealed some controlling interaction. Nonetheless, even almost 20 years after the Chemistry of Materials article, a quantitative theory of proton conduction phenomena is still missing. At the same time we have seen increasing interest in proton conducting materials for electrochemical applications (especially fuel cells), and this is the apparent reason for the high citation record of the Chemistry of Materials paper. Connecting the fundamentals of proton conduction phenomena with the phenomenology of proton conducting materials and their applications was another aspect of this article, indeed. But more than this very small initial step there was the steady effort of a growing community working to combine high proton conductivity with many other properties required by the envisioned (potential) application which led to the development of new useful proton conducting materials. As a consequence, nonaqueous proton conductivity in heterocycle and phosphonic functionalized polymers was discovered. Stable proton conducting ceramics based on BaZrO3 were developed, and solid acids were implemented as electrolyte material of medium temperature fuel cells. More recently, the development of proton conducting hydrocarbon membranes has reached a level where they may substitute for perfluoro-sulfonic-acid (PFSA) membranes such as the well-established Nafion used in many electrochemical applications. CM: If you had to put your finger on it, what made your review special? What are you most happy about when you reread this paper? KDK: Well, as pointed out above, I do not really see that the high citation record immediately indicates the paper is special. But, when rereading the yellowed pages, I understand that students, in particular, starting to work in the field like to read the article. It connects considerations concerning the molecular scale (conduction mechanisms) with the phenomenology of materials which are classified in a meaningful way. It may thus
s part of our ongoing series highlighting those papers published in Chemistry of Materials that have been cited 1000 times or more, we were very happy to interview Dr. Klaus-Dieter Kreuer of the Max-Planck-Institut für Festkörperforschung (Figure 1). His 31-page review from 1996, entitled “Proton Conductivity: Materials and Applications” is a monumentous landmark publication that has, as judged by its citation record, underpinned applications in fuel cells, batteries, and other energy storage media, as well as research on sensors, ionic liquids, and fundamentals such as proton transport within confined media such as carbon nanotubes.1 The review has been cited 1420 times (Google Scholar) and 1112 times (Thomson-Reuters/Web of Science). We (CM) asked Dr. Kreuer (KDK) a few questions regarding this high-impact paper, to try to glean some insights into what made this review special. CM: At what stage of your academic career were you when you submitted this review to Chemistry of Materials? KDK: It was only about three years after I had returned to academia (Max Planck Institute for Solid State Research) from industry (Endress + Hauser, a Swiss-German company) where I had enjoyed a lot of freedom doing R&D on electrochemical sensing, albeit with a very high dependence on external factors such as the general economic situation. You may remember that Germany had just been unified, with all the insecurities arising from that. In this situation, I greatly appreciated the long-term research perspective offered by the Max Planck Institute for Solid State Research. Looking back, I think of Hermann Hesse (who grew up in the Stuttgart area) writing in his poem Steps “A magic dwells in each beginning”, describing the way I felt when I wrote this article.2 It was very exciting seeing how the contours of a more general and deeper understanding of proton conduction phenomena slowly emerging from the diversity of the sometimes confusing or even contradicting experimental and simulation findings. This is the feeling I simply wanted to share with the community when I wrote this article. CM: Given the high citation record of this article (and many of your others), a significant amount of research has been impacted by your findings over the years. Where did you think the field was headed when you wrote the paper? In your opinion, how has the field of proton conduction evolved since? KDK: Frankly speaking, at that time I had hoped that we would soon see models describing proton conduction phenomena in a quantitative fashion. The great beauty of Marcus theory (MT) that achieved this for electron transfer reactions in solutions surely inspired Mark Tuckerman (NYU) et al. to use the term “pre-solvation” in their pioneering simulation work on proton conduction in water3 and Noam Agmon (Hebrew University, Jerusalem) putting forward the socalled “Moses mechanism”.4 However, as already adumbrated in the Chemistry of Materials article, structural or solvent rearrangements do not only accompany proton transfer reactions (as described by Tuckerman). They are, in most © 2014 American Chemical Society
Published: December 9, 2014 6651
dx.doi.org/10.1021/cm504118j | Chem. Mater. 2014, 26, 6651−6652
Chemistry of Materials
Editorial
Figure 1. Left: 1k Club member, Dr. Klaus-Dieter Kreuer. Right: Still shots of simulations of proton conduction within solids. The movies from which these images were taken can be found in the Supporting Information of this editorial.
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help students to form a framework for their own work. What really makes me happy is that students talk to me at conferences or send e-mails asking specific questions about the many aspects touched upon in this article. I have the feeling that this piece of work defined the box within which we are still discussing proton conduction phenomena. CM: What is your advice to young scientists trying to discover the next breakthrough in materials science? KDK: In the context of this interview, I am tempted to say: Do not think of impact when you perform and publish scientific work! Science is much more than that! Materials science is empirical science, and a lot depends on the reliability of the data on which your conclusions are based. Reproduce the results you have obtained until you have sufficient statistical evidence. Do not ignore results simply because they do not match the modelthey could be the source of new insights. Just accept that the most you can achieve at a certain stage is consistency of available data and developed concepts, being well aware of Karl Popper’s insight that hypotheses can only be falsified.5 “Intellectual fun” is a great driving force in science. Trying to keep the right balance between enthusiasm and skepticism may be a good means to become a happy and successful scientist. I also learned that this is only possible in a suitable environment. I would especially like to encourage supervisors and tutors of my age to pursue what Jürgen Habermas calls “herrschaf tsf reier” (“f ree of domination”) discourse. Communication free of any kind of asymmetrical power relation is an essential element to the progress science.6
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AUTHOR INFORMATION
Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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REFERENCES
(1) Kreuer, K. D. Chem. Mater. 1996, 8, 610−641. (2) Hermann Hesse won the Nobel Prize for Literature in 1946. This poem, “Steps” is translated from the original German version entitled “Stufen”. (3) Tuckerman, M. E.; Marx, D.; Parrinello, M. Nature 2002, 417, 925. (4) Agmon, N. Chem. Phys. Lett. 1995, 244, 456. (5) Popper, K. The Logic of Scientific Discovery; Taylor & Francis: New York, 2005. First published in German as Logik der Forschung; Verlag von Julius Springer: Vienna, Austria. A lead online reference into Popper’s concept of falsifiability can be found here: http://plato. stanford.edu/entries/popper/. (6) A lead reference into Theorie des kommunikative Handelns can be found here: http://de.wikipedia.org/wiki/Theorie_des_ kommunikativen_Handelns.
Jillian M. Buriak, Editor-in-Chief Carlos Toro, Managing Editor
ASSOCIATED CONTENT
S Supporting Information *
Movies as described in Figure 1. This material is available free of charge via the Internet at http://pubs.acs.org. 6652
dx.doi.org/10.1021/cm504118j | Chem. Mater. 2014, 26, 6651−6652
