Electrocatalysis and electrochemistry, the inseparable components of low-carbon economy

Transition to low-carbon economy implies development of hydrogen energetics. This, in its turn, requires enhancement of the systems of water electrolysis for producing “green” hydrogen and improvement of fuel elements for using hydrogen as fuel. The chemical processes in these systems engage electrocatalysts. Dr. Aleksandr Oshchepkov, researcher of Boreskov Institute of Catalysis, spoke about the studies and discoveries concerning the development of electrocatalytic systems of energy conversion.

What is the difference between electrocatalysis and heterogeneous catalysis?

“An additional factor appears in the electrochemical systems, which is called the electrode potential. By varying it one can change the energetics of the process. The basic difference between electrocatalysis and catalysis is that in the former at least one stage occurs with electron transfer”, noted Oshchepkov.

Only the material with either electronic or hole conductivity can be an electrode, and the electrolyte must provide ionic conductivity.

“As a result a complex system appears in which the presence of electrolyte significantly complicates conducting any in situ and operando measurements. Besides, the complex organization of molecules in the double electric layer near the electrode surface significantly complicates the understanding of the mechanism of any electrocatalytic processes”, said the scientist.

Stable and active electrocatalysts for the systems with proton-exchange membranes

The main types of electrocatalytic systems of energy conversion with proton-exchange membranes are the plants for water electrolysis and fuel elements. Electrolysis is based on decomposition of water on the electrodes with formation of hydrogen and oxygen. Water that flows to anode is oxidized releasing protons of hydrogen that go further through the membrane, and electrons that go along the outside chain. The protons of hydrogen on cathode meet the electrons and form molecular hydrogen. In case of fuel element the processes are inverted – oxygen and hydrogen are fed to the electrodes, and water and electricity are produced in the output.

All these processes require electrocatalysts. Typically, they are the particles of metals fixed at the surface of the carbon carrier and supported over the surface of the membrane. The hydrogen reactions in acidic medium occur with the use of electrocatalysts based on platinum, and they are rather trouble-free. But in the oxygen reactions everything is more complicated, says Aleksandr Oshchepkov. First, a larger load of noble metals is needed, and second, in the course of this work the systems degrade faster and require replacement.

“The commercial catalyst for the reaction of oxygen isolation in acidic medium is iridium oxide that demonstrates relatively high stability. The catalysts based on ruthenium feature much higher activity, but they are less stable. We decided to determine why these systems differ so much in their properties, and conducted a series of studies with the use of in situ and operando spectroelectrochemical methods”, said the scientist.

A research group headed by Prof. Elena Savinova (Strasbourg University, France) used photoelectron spectroscopy with the use of synchrotron radiation, which allowed them to study the chemical composition of the metal particles on the surface of electrocatalyst in real time. By changing photon energy it is possible to conduct nondestructive analysis of the materials with depth varying.

The researchers revealed that the reaction of oxygen isolation occurs by anion mechanism on the surface of iridium oxide, whereas in case of ruthenium oxide cation mechanism is realized.

“The electron state of iridium does not change within the course of the process and corresponds to iridium oxide up to rather high anode potentials, which confirms its stability. On the contrary, examination of ruthenium oxide showed significant changes in the chemical composition of the surface of electrocatalyst with a change in potential, and the stable form of RuO2 produces various compounds, in particular, soluble ones. One can suppose that for making more stable and active electrocatalysts it is necessary to provide the realization of cation mechanism of the reaction of oxygen isolation, but at the same time to make sure that during the process no soluble compounds formed”, explained Oshchepkov.

Electrocatalytic systems based on anion-exchange membranes are the future of energetics

According to Aleksandr Oshchepkov, so far there are no commercial examples of using the systems of energy conversion with the anion-exchange membranes, but the experts admit that it is the thing of the future, and the transition from the systems with proton-exchange membranes should begin in the nearest years.

“The advantage of the anion-exchange membranes is that replacement of the acidic medium with basic one allows using non-noble metals as electrocatalysts, and they are more readily available and produced in sufficient volumes. A recent analysis showed that the use of electrolysers based on anion-exchange membranes would help to produce hydrogen, the cost of which is lower as compared with traditional basic electrolysers and devices based on proton-exchange membranes”, he noted.

These systems have a reverse problem – the oxygen reactions occur faster, and the hydrogen ones slow down. The activity drops by two orders of magnitude with transition from the acidic to basic medium even for platinum catalyst, considered the best for hydrogen reactions. Nickel is the most active among the non-noble metals, but the dispersion of parameters of its activity measured in different groups exceeds two orders of magnitude. Until recently there was no explanation why it was so.

The joint research of scientists from Boreskov Institute of Catalysis and Strasbourg University showed that if the nickel electrodes are prepared with various concentrations of nickel (hydr)oxide on the surface, then the activity in the reactions of isolation and oxidation of hydrogen changes dramatically. The highest activity is reached in the case of surface completion by the (hydr)oxides of about 35%.

“Depending on what composition of the electrode surface we have, either metal nickel, or nickel with content of oxygen compounds on the surface, the change of the shape of current-potential curve helps us to judge of the changes of the surface properties of electrodes. They correlate greatly with the data of in situ methods, in particular, with spectroscopy of Raman scattering that showed that only nickel alpha-hydroxide forms on the metal nickel with low potentials, and in the presence of stable oxygen-containing compounds the characteristic oscillations of nickel beta-hydroxide appear. Nobody controlled the degree of surface completion with (hydr)oxides before, which caused incomprehension of the course of such a wide dispersion of parameters of activity”, told Oshchepkov.

Natrium borane solves the problem of hydrogen storage

Another joint research of the scientists of Boreskov Institute of Catalysis with their French colleagues concerns the problem of hydrogen storage. Typically hydrogen is used in liquefied state, which implies restrictions on the ways of its transportation and storage increasing its cost. One of the possible solutions is the use of solid natrium borane. The standard approach is based on borane hydrolysis with isolation of hydrogen that can be used further in fuel elements. But the scheme can be further improved so that the liquid solution of natrium borane in base can be used right away as fuel.

“First, liquid fuel is easy to store, transport and use. Second, the thermodynamic potential of borane oxidation exceeds the potential of hydrogen oxidation. Therefore, due to direct use we can get an advantage in power of fuel element on borane up to 25%. However, this requires development of efficient electrocatalysts based on non-noble metals for the reaction of borane oxidation”, underlined the researcher.

According to Oshchepkov, if borane is oxidized without going into very high anode potentials, the nickel electrodes demonstrate high activity, and the lower is the concentration of oxides on its surface, the higher is the activity.

“Nickel electrocatalysts are much more stable in the reaction of borane oxidation than the systems based on noble metals. The processes of oxidation occur with much lower potentials, which means that they require less energy to activate the reaction occurrence. The highest potential of the direct borane fuel element is much higher with the use of nickel electrocatalyst as compared with the anodes based on noble metals. Under similar conditions of operation we can get an advantage in power of fuel element with nickel anode up to 20%”, said Aleksandr Oshchepkov.