Research on oxygen reduction gives traction to solar power Important insights can be drawn from the study of natural catalysts, which can then be applied in the development of artificial systems. We spoke to Dr Dennis Hetterscheid about the work of the Cu4Energy project in studying molecular copper catalysts for water oxidation and oxygen reduction, reactions which are central to the performance of fuel cells The majority of artificial catalysts have heterogenous metal surfaces, which react via relatively simple mechanisms, yet typically energy is lost during the process. The underlying mechanisms need to be modified if these energy losses are to be reduced, as Dr Dennis Hetterscheid explains. “More degrees of freedom are required, more complexity, in order to reduce barriers.That cannot be achieved with a simple, flat metal surface,” he says. Nature builds catalysts in an entirely different way to artificial systems, using for example an enzyme called laccase. “The active site of laccase contains three copper atoms, it’s called a trinuclear copper centre, and the environment of this copper centre is completely controlled. So it’s perfectly oriented, there are gas channels, water channels and polar channels, to and away from the active site,” explains Dr Hetterscheid. “That’s perfectly aligned. Researchers have previously shown that the laccase enzyme is an excellent electrocatalyst for the oxygen reduction reaction.” This is a central part of the motivation behind Dr Hetterscheid’s work in the Cu4Energy project. Based at the University of Leiden in the Netherlands, Dr Hetterscheid and his colleagues in the project are drawing inspiration from nature in the study of molecular catalysts. “We aim to understand how a laccase does this, then look at how we can do this in the lab with simple molecules. If we can understand that, then at some point we could potentially implement that knowledge in the development of electrolysers and fuel cells,” he outlines. Attention is currently focused primarily on fundamental research around two main reactions, namely water oxidation (WO) and oxygen reduction (OR), both of which are key reactions in terms of the performance of electrolysers and fuel cells. “A lot of energy loss in electrolysers and fuel cells is related to the oxygen reduction and water oxidation reactions in those systems,” says Dr Hetterscheid.
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A lot of energy in research is currently devoted to improving the efficiency and overall performance of these kinds of artificial systems, reinforcing the wider relevance of the project’s work. Dr Hetterscheid believes much can be learned in this respect by studying the superior performance of natural catalysts. “We aim to understand how natural enzymes do it – and then to see whether we can make molecular catalysts that react in
Catalytic cycle The relative inefficiency of artificial catalysts is typically attributable to one particular step in the process, which researchers in the project aim to address by treating and modifying molecules, looking to gain new insights into the mechanisms behind the catalytic reaction. The molecules themselves consist of copper atoms and a surrounding ligand, which both determines the electron density of the metal
The active site of laccase contains three copper atoms, it’s called a trinuclear copper centre, and the environment of this copper centre is completely controlled. So it’s perfectly oriented, there are gas channels, water channels and polar channels, to and away from the active site very similar ways,” he continues. Researchers are investigating the fundamental processes involved in catalysis, aiming to build a deeper picture of the factors that influence the speed and efficiency of a reaction. “We’re looking at things like electron transfer, proton transfer, proton-coupled electron transfer, and at the overall catalytic cycle,” says Dr Hetterscheid.
and also imposes geometric constraints. “By changing the structure of the ligand, we can tune what happens in the metal,” outlines Dr Hetterscheid. Researchers aim to investigate laccase molecules, and to develop what Dr Hetterscheid calls functional models, which react in the same way. “We look at molecular compounds, and these are really developed so
A typical potential energy landscape of the water oxidation reaction mediated by a simple catalyst. For such a system it is difficult to reduce reaction barriers without creating new ones. The arrows symbolize that a thermodynamic sink is created at the *OH intermediate when one tries to reduce the potential energy of the *OOH intermediate.”
EU Research
Cu4Energy
Biomimetic Copper Complexes for Energy Conversion Reactions
Project Objectives
that we can investigate different intermediates and look at what the mechanism is like,” he says. These factors will have a major influence on the speed and efficiency of a catalytic reaction, which are of course important considerations in terms of performance. Dr Hetterscheid says the speed and efficiency of a catalyst are often inversely related. “Catalysts that operate very fast typically require an additional driving force to do so. That driving force – that overpotential – results in energy losses, so a catalyst that shows high catalytic rates may not necessarily be a catalyst that is energy efficient,” he explains. Measuring the efficiency of a catalytic reaction is not entirely straightforward however, as every catalyst works differently “We want to achieve the highest possible turnover frequency, at the lowest possible overpotential.” This depends to a significant degree on a deeper understanding of the structure of natural catalysts, which forms an important part of the project’s overall agenda. Alongside achieving enhanced catalytic rates, Dr Hetterscheid and his colleagues also aim to understand the underlying factors behind those higher rates. “It’s not only the high active rates that’s a deliverable from the project, but also the knowledge of how and why we get those high active rates,”
he stresses. A major research objective for Dr Hetterscheid is to find a reversible catalyst for oxygen reduction and water oxidation, and also to understand the finer details, laying the foundations for future applications. “We want to understand how we can get such a catalyst, and what makes a natural system like laccase such a good redox catalyst,” he continues.
Catalytic activity The Cu4Energy project itself received funding for five years, and Dr Hetterscheid says that there is still a lot to achieve over the remaining two years of the term. Nevertheless, Dr Hetterscheid is also keen to explore other avenues of research. “I’m not just interested in the catalytic side of enzymes, but also the complete way of how enzymes tune catalytic activity. One of the things I find very interesting is how water molecules are perfectly arranged in hydrogen-bonding networks,” he says. “Those water molecules that are effectively in a confined environment will have a totally different reactivity, a totally different chemistry, to the water that other electro-chemists are currently using in fuel cell electrolysers. So I’m very interested in understanding and harnessing those types of features.”
The aim of the proposal is to significantly increase our fundamental understanding of the design principles for molecular oxygen reduction (OR) and water oxidation (WO) catalysts and to deliver new and very active molecular copper catalysts for OR and WO at the end of the project. Experiments will be carried out wherein the structure of the catalyst is linked to the observed catalytic activity and the potential energy surface of the catalytic cycle. The proposal is in particular focused on the rate-determining step of the catalytic reaction, as improvements here will directly lead to enhanced catalytic rates. A functional model system of the copper enzyme Laccase will be designed to study the rate limiting proton-and-electron-coupled O–O bond scission reaction, which is the rate limiting step in OR by Laccase.
Project Funding
Funded by the ERC-StG-2014 - ERC Starting Grant Cu4Energy.
Contact Details
Assistant Professor, Dr Dennis Hetterscheid, PhD. Mathematics and Natural Sciences Leiden Institute of Chemistry LIC/Catalysis & Surface Chemistry Science Campus Einsteinweg 55 2333 CC Leiden Room number EE4.19 T: +31 71 527 4545 E: d.g.h.hetterscheid@chem.leidenuniv.nl W: http://lic.leidenuniv.nl/spotlight/dennishetterscheid
Dr Dennis Hetterscheid, PhD.
Dennis Hetterscheid has obtained his PhD at the Radboud University of Nijmegen under the supervision of Prof. Bas de Bruin. He then moved to the Massachusetts Institute of Technology where he worked in the lab of Prof. Richard R. Schrock, and to the University of Amsterdam where he worked with Prof. Joost N. H. Reek. Since 2013 Dennis is an assistant professor in physical chemistry of sustainable energy at Leiden University. The main research theme in his group is to understand and mimic bioinorganic multi-electron processes that are relevant to a future energy infrastructure. The thermodynamic pathway of the oxygen reduction reaction is the inverse of the water oxidation reaction. In case of an ideal catalyst, where the potential energy surface is flat, one would expect to find activity for both reactions at a very low overpotential.
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