A joint effort of scientists fr om the Laboratory for Structures and Functional Properties of Molecular Systems (NSU Physics Department), Boreskov Institute of Catalysis SB RAS and University of Kyoto, provided an in-depth molecular-scale picture of proton conductivity mechanism in an acid-free proton-exchange membrane based on a porous metal-organic framework and urea. The results were published in one of the most influential chemistry journal worldwide, “The Journal of the American Chemical Society”.
The relevance of this research relates to the scientists' attempts to change the development vector of energy based on oil and coal. Creating fuel cells with two primary components, hydrogen and oxygen, increases their durability (solving the energy conservation problem due to minimal degradation of the material inside the cells) making them more environmentally friendly.
The operating principle is to connect pre-prepared components of gases (that are stored in cylinders) inside the cell. The primary component is the proton-exchange membrane that passes only hydrogen ions that are formed at the anode of the cell. In this process, hydrogen ions pass through the membrane and interact with oxygen to form water. This creates an electromotive force that can be used to operate various devices. The membrane must be moisture resistant and impermeable by other substances.
Although this new generation of fuel electrochemical cells will solve the chemical problem of preserving and transmitting energy at a distance, the optimal structure for the material of the fuel cell‘s key element had not been identified. In addition to a high conductivity rate and charge density, the material for the membrane must meet a number of requirements, such as stability and operational safety. Novosibirsk scientists solved this problem working with colleagues from the University of Kyoto (Japan). They created an effective membrane without the use of toxic acid components. As a rule, various microporous carriers are used to create highly efficient proton-conducting membranes and acid groups are introduced, either in the form of guest acid molecules (such as HCl or H2SO4) or as functional groups. In such conditions even with water used as a carrier, there is a danger of acid escape from the pores and destruction of both the membrane and the entire device.
Scientists at Kyoto University created a prototype for a suitable membrane using metal-organic frameworks with urea embedded in the pores. The Novosibirsk scientists used solid state nuclear magnetic resonance to demonstrate that in the presence of water molecules, urea microchannels (MOF-74) inside the framework and is capable of forming a network of hydrogen bonds suitable for efficient charge transfer.
Daniil Kolokolov, Senior Research Fellow at the NSU Laboratory explained,
This is an important achievement. Through experiment, we were able to understand the proton conductivity mechanism and demonstrate that with open metal centers in the framework, urea molecules coordinate the center and in this state can form fairly strong hydrogen bonds with guest water molecules. This does not happen in an ordinary urea-water solution. We were able to prove that the resulting hydrogen bond structure, formed in the channel of the framework, is responsible for the charge transfer mechanism. It is important to note that, unlike the usual method for using acids, urea is a much more environmentally friendly reagent and it will not evaporate easily and corrode the fuel cell when overheated.
Scientists were developing proton-exchange membranes in the 1960s for the space program and military applications. However, the resulting prototypes were too expensive to produce them on a massive scale. Interest in membranes returned in the 1990s and in recent decades new membranes made of cheaper and more effective materials started to appear.
Today, the creation of efficient fuel cells will not only solve challenges related to powering spacecraft, but provide more affordable and environmentally friendly electrification for infrastructure (ie. location systems) in remote areas including the Far North wh ere it is necessary to be protected from external conditions (wind, sun, temperature).