Dissolves water through a chemical change

Materials research for the production of hydrogen

Hydrogen is tomorrow's oil. And easy to obtain, because after all, only the water molecule has to be split electrolytically - if possible with green electricity. But what seems so simple is actually quite complicated.

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The National Hydrogen Strategy is intended to make green hydrogen marketable and to prepare the Federal Republic of Germany's entry into a hydrogen economy. "Because technologies for the production, transport and use of green hydrogen hold considerable potential for added value for the German economy - and the possibility of designing environmentally friendly areas that have the greatest impact on the climate today: industry, transport and heat supply," it says in one Call for an ideas competition by the Federal Ministry of Education and Research.

2018 | OriginalPaper | Book chapter

Electrolysis of water

Hydrogen is considered a long-term chemical energy carrier, especially since the electrolysis of water allows the use of wind energy, solar power, water and tidal power. The chapter summarizes the state of the art for electrolytic hydrogen generation.

Ideas are indeed in demand, because the electrolysis of water known from school lessons (H.2O), whereby hydrogen (H) is produced at one electrode and oxygen (O) at the other, has its pitfalls - applied on a large scale - and has therefore played a rather subordinate role so far. "The global H2-Production uses natural gas (40%), coal (18%) and water electrolysis (4%). The catalytic splitting of water on semiconductors or with algae hydrogenases does not play a technical role ", says Peter Kurzweil in" Chemistry "(page 316).

In fact, water electrolysis is the ideal way to generate pure hydrogen, especially if it uses green electricity from renewable energy sources such as photovoltaics and wind turbines. The by the discharge of H+Hydrogen produced by ions is initially present as atomic hydrogen. Two neutral hydrogen atoms combine to form one H.2-Molecule. But this chemical reaction can be kinetically inhibited: "The inhibition can be lifted by a catalytic effect of the electrode material or the surface structure of the electrode. The same applies to the formation of oxygen O."2 at the anode. Different electrode materials provide different values ​​for the reaction overvoltage "(" Experimental introduction to electrochemistry ", page 362).

The worse the reaction, the worse the profitability. The authors D. Mitra and S. R. Narayanan from the Department of Chemistry, Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles illuminate details in the journal "Topics in Catalysis". For example, water electrolysis can be carried out in both acidic and alkaline media. "A wide range of materials, including metals, transition metal oxides and hydroxides, are stable in alkaline media; expensive coatings and catalysts made from noble metals such as platinum are required to achieve comparable stability in acidic water electrolysis."

In the electrolysis of aqueous solutions, water is always decomposed; the added acid, base or salt solution serves solely to increase the electrical conductivity, because pure water is an insulator. "Peter Kurzweil, Otto K. Dietlmeier," Elektrochemische Speicher ", page 381.

As a result, the system of alkaline water electrolysis is attractive from the point of view of capital costs, but the production of electrolysis hydrogen negatively affects energy costs, say Mitra and Narayanan. A large part of the comparatively poor energy inefficiency - 60 to 80 percent, based on the calorific value, see chapter "Conventional processes for hydrogen production" in "CO2 and CO - Sustainable Carbon Sources for the Circular Economy ", page 29 - can be traced back to the process of oxygen development in water electrolysis. The search is therefore on cost-effective, efficient and long-lasting electrocatalysts for this electrochemical reaction, which takes place via the generation and conversion of intermediate products into oxygen.

"These surface reactions on the electrode are usually slow and are facilitated by catalytic metal oxide surfaces and the application of large anodic overpotentials," report Mitra and Narayana. Industrial water electrolysis has so far relied on nickel as the base material for catalysts due to the inherent chemical stability of nickel under anodic conditions in alkaline electrolytes. Because even if electrocatalysts based on ruthenium oxide and iridium oxide show low overpotentials for the oxygen evolution reaction (OER), these electrocatalysts are unstable in alkaline media and also prohibitively expensive. However, according to the California researchers, there is evidence that a variety of alternative catalyst materials based on transition metal perovskite and spinel oxides as well as layered double hydroxides containing iron, nickel and cobalt are also showing promise. Carbon-based catalysts containing nitrogen and phosphorus would also have shown remarkable electrocatalytic activity. However, carbon-based catalysts are prone to degradation in long-term operation, since carbon can be oxidized to graphitic acid and other carboxylic acids.

Electrocatalysts - applied in a thin layer or finely distributed on an electrode carrier - lower the activation energy or activation overvoltage and thus accelerate the desired electrode reaction. Side reactions should be inhibited. The oxygen deposition causes an oxide film on the surface. Metals with several oxidation states are the most effective catalysts, especially iridium dioxide. "Peter Kurzweil, Otto K. Dietlmeier," Elektrochemische Speicher ", page 416.

The production of green hydrogen also suffers from the quality of green electricity. "One reason for this is that the cyclical electricity from the sun and wind quickly pushes the materials involved to their limits due to the dynamic load, so that inexpensive catalyst materials in particular quickly lose their activity," explains Michael Bron from the Institute for Chemistry at the University of Halle- Wittenberg a basic problem. His working group has now discovered a method with which both the stability and the activity of inexpensive nickel hydroxide electrodes can be significantly increased. To increase the stability of the material, the hydroxide is usually heated up to 300 degrees Celsius in order to partially convert it into nickel oxide. And at even higher temperatures, the hydroxide is also completely destroyed, the Wittenbergers had taken from the specialist literature. "We wanted to see that with our own eyes and gradually heated the material in the laboratory to 1000 degrees Celsius," says Bron.

Active oxide defects on catalyst particles

As expected, the researchers were able to observe a change in the individual particles with an electron microscope as the temperature increased, as reported in ACS Catalysis. The particles transformed into nickel oxide, grew together to form larger structures and, at very high temperatures, formed patterns that are reminiscent of zebra crossings. However, to their surprise, the electrochemical test showed a consistently high activity of the particles, which should actually no longer have been usable for electrolysis. The general rule in electrolysis is that large surfaces and thus smaller structures are more active. "We therefore attribute the high activity of our much larger particles to an effect that surprisingly only occurs at high temperatures - the formation of active oxide defects on the particles," says Bron. And since the heated particles still generated 50 percent more electricity than the untreated particles even after repeated measurements after 6000 cycles, Bron and his team are sure that they have come across a promising approach. In a next step, they want to use X-ray diffraction to understand more precisely why these defects increase activity so much. They are also looking for ways to manufacture the new material in such a way that smaller structures can be retained even after the heat treatment.

Expensive precious metals can be replaced

Chemists at the Technical University of Berlin have now published in the journal "Nature Communication" the molecular mode of action of special nickel oxide catalysts, which are even superior to conventional noble metal catalysts. Proton Exchange Membrane (PEM) electrolysers use the very expensive and rare precious metal iridium as a catalyst at the anode, where oxygen is formed in the coupled process of hydrogen production. "If one assumes the capacity targets for hydrogen production up to 2030 formulated in the hydrogen strategy of the federal government, we would have to use the entire annual iridium production in the world - to cover a few percent of the German energy demand," says Peter Strasser, head of the electrochemistry department Electrochemical energy conversion at the TU Berlin. Together with his team, Peter Strasser is researching precious metal-free catalysts made from nickel and iron oxides for electrolysers that work in alkaline conditions. The catalysts now developed in Berlin accelerate the electrolysis of water in such a way that they are superior to iridium, as the publication shows.

Atomic investigations improve understanding of electrocatalysis

A group of scientists from the Max Planck Institute for Iron Research (MPIE), the Helmholtz Institute Erlangen-Nuremberg for Renewable Energies (HI-ERN), the Friedrich-Alexander University Erlangen-Nuremberg (FAU) and the Ruhr University Bochum (RUB ) had already found out two years ago through the use of high-resolution microscopy methods that the first atomic layers on the surface of electrocatalysts show chemical changes that determine the efficiency of the catalyst. By optimizing the surface it is possible to accelerate the water electrolysis, as the group reported in "Nature Catalysis". She used atom probe tomography to reveal the three-dimensional structure of the first few layers of atoms made from electrochemically grown iridium oxide, an efficient electrocatalyst for the oxygen evolution reaction. In doing so, they revealed the formation of trapped, non-stoichiometric Ir-O species during oxygen evolution. These species gradually transform into IrO2, which leads to improved stability, but at the same time also to a decrease in activity.

Water splitting observed in the nano range

The water electrolysis can therefore be accelerated with catalyst surfaces optimized for this purpose. However, so far it has only been insufficiently observable how the chemical process of the decomposition of water into hydrogen and oxygen takes place at the molecular level. A team led by Katrin Domke, independent Boehringer Ingelheim "Plus 3" group leader at the Max Planck Institute for Polymer Research (MPI-P) in Mainz, has now developed a new method for the first time the process of water splitting on a gold surface could be examined with a spatial resolution of less than 10 nm. "We were able to prove experimentally that smooth surfaces split water less energy-efficiently than surfaces with roughness in the nanometer range," says Katrin Domke. "With our pictures we follow the catalytic activity of the reactive centers during the first steps of water splitting."

Chemical nanoimaging

For their method, the scientists combined Raman scattering with scanning tunneling microscopy: by scanning a nanometer-thin gold tip illuminated with laser light over the surface to be examined, the Raman signal is amplified by a kind of antenna effect directly at the tip by many powers of ten. On the one hand, this enables only a few molecules to be measured. On the other hand, the strong focus of the light by the tip leads to a spatial resolution of less than ten nanometers. The specialty of the apparatus is that it can be operated under real conditions. "We were able to show that when water is split, two different gold oxides are formed at such a rough point - i.e. a reactive center (Au2O3 and Au2O), which could represent the important intermediate products in the separation of the oxygen atom from the hydrogen atoms, "says Domke. The scientists have published the results of their investigations in the journal" Nature Communications ".