Editorial pubs.acs.org/cm
Materials + Energy + International Collaboration = Fundamental Insights Toupin, Brousse and Beĺ anger: New Members of the Chemistry of Materials’ 1k Club entitled “Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor”, was published in 2004 and represents an early ripple that grew into the present tidal wave of research in the area of materials for applications in energy generation and storage.1,2 The goal of this work was a fundamental understanding of the charge storage mechanisms of MnO2, which then became the foundation for the subsequent development of MnO2 and other oxide materials for the storage of charge. Daniel Bélanger (DB) of the Université du Québec à Montréal hosted Thierry Brousse (TB) as a sabbatical visitor from the Université de Nantes, and Mathieu Toupin (MT) was a graduate student in Bélanger’s lab. CM: At what stage of your academic career were you when you submitted this article to Chemistry of Materials? DB: The paper was submitted more than 15 years after the beginning of my academic career, and about 10 years after I started to work on electrochemical capacitors (during a sabbatical leave at Los Alamos). Prior to this paper, our laboratory was mainly investigating conducting polymers, and in the early 2000s, we were inspired by a paper of John Goodenough, in which the pseudocapacitive properties of manganese dioxide were described.3 An interesting feature of this material was that it could be easily synthesized (using what could be referred to as a chemical toolkit). TB: At that time, I was assistant professor at the Université de Nantes, just after having spent a sabbatical year in the lab of my colleague, Daniel Bélanger at the Université du Québec à Montréal. I had previously been working on negative electrodes for Li-ion batteries (containing Sn, Si, alloys, among others) when Daniel offered me the possibility to start a new research topic on electrochemical capacitors, so-called supercapacitors. This was a unique opportunity for me to get involved in this topic of the team of a specialist. Coming back to Nantes by the end of 2002, I continued to interact with Daniel and spent several stays in Montréal every year in order to develop our scientific collaboration. In addition I must say it was also a great human adventure in addition to the scientific project. My two co-authors are real friends not only from a professional side but very much so from a personal one as well. I used to travel several times a year to Montréal, and we have 33 papers in common and have co-supervised 4 Ph.D. students, as well as many Master students. CM: Who was the other author on the paper, Mathieu Toupin, and at what stage was he? TB: As mentioned earlier, Daniel was the head of the lab in Montréal. Mathieu Toupin was the Master student with whom I started working on supercapacitors. Mathieu was exactly the right person to work on this topiche was skilled, clever and
As part of our ongoing series of interviews with authors of papers in Chemistry of Materials that have been cited 1000 times or more, we (CM) spoke with Mathieu Toupin, Thierry Brousse, and Daniel Bélanger, authors of the latest publication to accumulate more than 1000 citations (Figure 1). The paper,
Figure 1. Latest members of the Chemistry of Materials’ 1k Club, Daniel Bélanger, Thierry Brousse, and Matthieu Toupin. © 2016 American Chemical Society
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DOI: 10.1021/acs.chemmater.6b01645 Chem. Mater. 2016, 28, 2883−2885
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note that our first paper on MnO2, also published in Chemistry of Materials, has also received a relatively high number of citations (673).4 TB: Since 2004, only a few true pseudocapacitive materials have been discovered. It even seems researchers have forgotten what this word means since a lot of battery-type materials are presented as such in many papers, probably out of frustration at not being able to discover more possible pseudocapacitive electrodes. It seems people are not asking themselves the right questions. At the time we performed our experiments for the article, we knew exactly what we were looking for: if MnO2 was a pseudocapacitive material, we might see a change in the mean oxidation state of Mn upon cycling the electrode. We started with the electrochemical signature of MnO2, which “looked” capacitive, and then tried to figure out what mechanism was leading to such behavior. These days, researchers seem to be focused on electrode engineering and performance, but only a few studies have tried to start with the concept of using multivalent cations, followed by determination of the conditions which could give rise to pseudocapacitive behavior. Recently we demonstrated this concept with FeWO4, and we tried to emphasize the role of iron in this crystallographic structure.5 CM: If you had to put your finger on it, what made your paper special? TB: In the field of supercapacitors, this work was one of the first steps toward the coupling of electrochemical techniques and analytical techniques in order to elucidate the charge storage mechanism. There was growing interest in doing so for lithium ion battery electrodes, but due to big differences in time constants, it had been difficult to translate to supercapacitors. This success was the main innovation of our paper. MT: The aspect that made this work special to me was the honest and clear explanation of the characterization work that connected the dots between concepts known at the time. Each of the concepts were already out there, including the valence shift from +3 to +4, the high capacitance of thin film electrodes, and the involvement of protons or cations. Addressing all of these parameters at once allowed for the settling of some debates on how supercapacitors function, from a fundamental standpoint. CM: What are you most happy about when you reread this article? DB: We identified the limitations of the material, and the findings in our paper could provide ideas for approaches and research directions that could be used to improve the electrochemical performance, and eventual development of commercial devices using this material as active electrode material, for example, in a hybrid electrochemical capacitor or an aqueous battery. TB: This paper is the summary of what a group of people can do when they trust each other. I have a clear memory of intense brainstorming before setting up the experiments. It was very difficult to organize the project correctly, so that we would cycle the electrodes the right way, polarize them, and then transfer the samples to the XPS spectrometer. The three of us were quite busy and concentrated on our tasks. We really wanted to show the evidence of pseudocapacitive charge storage in MnO2, and we succeeded. This was a happy and exciting period of my career as a researcher. This was also the time to share good meals and drinks with my co-workers. MT: Up until recently, this work serves as a reference for fellow scientists with respect to the interpretation of their own
my teacher in this new field. Additionally we were matched quite well and quickly found our way to work together. I think Daniel made us work together intentionally. He came with the ideas and the three of us managed to lead it to some reality. Chemistry of Materials: Where is Mathieu now? DB: Mathieu Toupin completed a Ph.D. in materials science and now holds a position as a Research Officer at the National Research Council of Canada in the field of materials for energy technologies. CM: Given the high citation record of this article, 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 this article? DB: Frankly speaking, we had no idea at that time where the field was heading, and we were more interested in finding how the MnO2 electrode was working when it was cycled. We wanted to know the nature of the redox/chemical processes occurring when the manganese dioxide electrode was used within its stability range of potentials. This was important because of the “capacitive”-like (also called pseudocapacitive) shape of the cyclic voltammogram. A change of the redox state of manganese in the oxide was expected, and XPS was used to probe these redox states. Our initial efforts with thick film electrodes were not successful and no change of the oxidation state of manganese was detected (the results of these experiments were described and discussed in the paper). But from the literature, we hypothesized that some phenomenon (possibly charge redistribution, self-discharge) was preventing the detection of the change of oxidation state of the manganese. At this point, we decided to use thin films, and the results obtained with thin films constituted the main findings of the paper. TB: There was a trend to discover new pseudocapacitive materials, a concept not fully understood even now by everybody in the field. People were trying to get out of expensive ruthenium based compounds. John B. Goodenough (yet again!) published the first paper in 1999 about the pseudocapacitive behavior of MnO2 in a neutral aqueous electrolyte.3 This work was a breakthrough, but the scientific evidence for pseudocapacitance was lacking. In order to obtain this evidence, spectroscopic techniques were needed to check the evolution of the mean manganese oxidation state, which was quite a challenge. MT: At the time, the insertion of protons in ruthenium oxides even in bulk was performing at a high level as a pseudocapacitive material, and thus the race for a cheaper transition oxide material was on. The widespread belief back then was that the charge mechanism of MnO2 was similar to that of the ruthenium oxides, and that the proton could diffuse into the bulk of the material. Hence all our efforts to improve the latter phenomenon fell short with respect to increasing the performance. This outcome encouraged us to concentrate our efforts on figuring out what was going on at the mechanistic level, to explain these results and help find clues to move forward. CM: In your opinion, how has this particular research field evolved ever since? DB: The field (restricted here to MnO2 for electrochemical capacitors) has only slowly evolved in the past ten years. A lot of related materials and new synthetic methods have been developed but there has been no major breakthrough. The electrochemical performance of MnO2 is more or less in the range of that obtained in the early 2000s. It is also important to 2884
DOI: 10.1021/acs.chemmater.6b01645 Chem. Mater. 2016, 28, 2883−2885
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XPS data when MnO2 is studied in more complex forms and morphologies within nanostructured materials, for instance. CM: What’s your advice to young scientists trying to discover the next breakthrough in material science? DB: By analogy to a pool, try to avoid jumping in where there are too many people. In addition, read the literature in other/closely related fields because you might derive from there, ideas or approaches that will be helpful to better understand your materials/systems under investigation. Finally, do not try to predict if your paper will be highly cited. As scientists, we all believe that our work is very important. Let other people determine if that is the case, and if your work ends up being useful to others, then your paper may eventually become highly cited. TB: Just come back to the basics and try to imagine new concepts using these basics without following any trend. Sharing your work with other scientists is also the key point to check if your ideas are going toward the right directions. MT: I will have to go with what my thesis advisor, who I will not name (!), used to torture me when discussing disappointing results: “I understand that it did not work as we hoped, but now you have to figure out why...”. Sometimes, or really most of the time, I have to admit that I was somewhat annoyed when I was sent back to the lab to find answers as to why we were in a dead end. Why could not I shoot for the next best thing instead? But the diligence and effort did, and do, pay off.
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Carlos Toro, Managing Editor Jillian M. Buriak, Editor-in-Chief 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) Toupin, M.; Brousse, T.; Bélanger, D. Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor. Chem. Mater. 2004, 16, 3184−3190. (2) Reference 1 has been cited 1046 times as per Web of Science, and 1267 times as per Google Scholar, as of April 22, 2016. (3) Lee, H. Y.; Goodenough, J. B. Supercapacitor Behavior with KCl Electrolyte. J. Solid State Chem. 1999, 144, 220−223. (4) Toupin, M.; Brousse, T.; Bélanger, D. Influence of Microstucture on the Charge Storage Properties of Chemically Synthesized Manganese Dioxide. Chem. Mater. 2002, 14, 3946−3952. (5) Goubard-Bretesché, N.; Crosnier, O.; Payen, C.; Favier, F.; Brousse, T. Nanocrystalline FeWO4 as a pseudocapacitive electrode material for high volumetric energy density supercapacitors operated in an aqueous electrolyte. Electrochem. Commun. 2015, 57, 61−64.
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DOI: 10.1021/acs.chemmater.6b01645 Chem. Mater. 2016, 28, 2883−2885
