How are the reaction mechanisms verified
The fundamental role of chemistry in electrocatalytic water splitting
The transition to a sustainable energy economy requires electrocatalytic methods to convert and store electrical energy in and to chemical energy and raw materials. A research team from the TU Berlin, the ETH Zurich, the National Research Council of Italy under the direction of the FHI has now discovered fundamentally new aspects of the reaction mechanism of one of the most important electrocatalytic processes, the oxygen evolution reaction. The results are in Nature released.
The electrocatalytic evolution of oxygen, i.e. the reactive conversion of water molecules into oxygen, protons and electrons, is a key electrochemical process in the transition to an energy economy based on renewable electricity. Because with an increasing share of volatile renewable energy sources (such as wind and solar energy), energy storage solutions are required to absorb intermittent fluctuations in power and ensure a reliable energy supply. One of the most flexible solutions to this problem is the conversion and storage of electrical energy into chemical fuels using protons and electrons, since chemical fuels store a lot of energy and can be used where and when it is needed. However, one major hurdle in this approach has been the identification of electrocatalysts for converting water into molecular oxygen - the oxygen evolution reaction - that provides the protons and electrons to make these fuels. In efforts to develop improved electrocatalysts, experts have long assumed that the electrocatalytic oxygen evolution reaction can be understood using a standard theory of non-catalytic electron transfer reactions that has been firmly established in textbooks and developed over many decades. A research team, including members of the Inorganic Chemistry and Interfacial Science departments at the Fritz Haber Institute, decided to check this assumption. Surprisingly, the scientists found that the electrochemical catalysis of the oxygen evolution reaction is actually much more similar to traditional thermochemical catalysis than previously assumed. This insight enables tools and concepts from thermochemical catalysis to be applied to their electrochemical counterparts for the first time.
"It is important to understand how electrocatalysts work at the most fundamental level, because this is the only way to improve them in the future. It became increasingly clear to us that the traditional picture of what drives electrocatalytic reactions is incomplete," explains Peter Strasser, one of the co-authors from the Technical University of Berlin. He adds: "Researchers typically assume that the oxygen evolution reaction is controlled by the direct action of the electrical potential on the reaction coordinate. This is a very different idea than thermochemical catalysis, in which chemical bond formation and - resolution is controlled by the surface chemistry ".
In an in Nature published study, the team reports how one of the most successful classes of oxygen evolution catalysts, iridium oxides, works. They carried out synchrotron-based operando X-ray spectroscopy at BESSY II in Berlin and Petra III in Hamburg to investigate how iridium oxides behave during the electrocatalytic evolution of oxygen. With these experiments, they were able to observe the electrical potential and the surface chemistry at the same time. With the knowledge gained from these experiments, they built models of the catalyst surfaces on an atomic scale, which were used in quantum mechanical simulations of the reaction in the high-performance computing center in Stuttgart. "In agreement with the measurements, the simulations showed that the reaction rate depends exponentially on the surface coverage of the oxidative charge," says Travis Jones from the Fritz Haber Institute. "The simulations also recorded the change in table tilt, a key feature of iridium oxide, and traced them back to a change in how the oxidative charge reacts to the electrical potential; and not, as previously assumed, to a change in the molecular reaction mechanism," explains Simone Piccinin, one of the co-authors of the National Research Council of Trieste, Italy. These investigations led the researchers to suspect that the reaction is controlled by the surface chemistry and not by the potential directly acting on the reaction coordination. By developing a laboratory-based method to quantify charge accumulation, the team was able to study a range of materials and found that they all behaved the same way. Detre Teschner from the Fritz Haber Institute explains this: "It turned out that the role of the potential was to oxidize the surface and that the charge accumulated through this oxidation controlled the rate of the reaction, similar to thermal catalysis."
After seeing that the charge seemed to mediate the electrocatalytic rate, the researchers set out to find a way to control the catalyst charge regardless of the potential to test their results. "We needed a chemical way to change how much charge the catalysts can store. We quickly realized that we could do that by replacing some of the oxygen on the surface with chlorine, since chlorine cannot be oxidized. to store additional charge, "says Javier Pérez-Ramírez from ETH Zurich. The Zurich team used its expertise in halogen chemistry to produce a range of catalysts with varying amounts of chlorine. As expected, the amount of charge the catalysts could store varied with the amount of chlorine on them. Electrocatalytic tests of these new materials verified their behavior in the oxygen evolution reaction and agreed with the team's predictions. "Seeing how changing a catalyst's ability to store charge predictably changed its catalytic activity gave us confidence in the results. We believe that this will apply to a huge class of electrocatalysts, and we plan to to use this new knowledge to design and test new materials, "says Travis Jones from the Fritz Haber Institute.
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