(Invited) Challenge of Industrializing Novel Molten Salt Electrochemical Processes
Authors: Ito Yasuhiko , Nishikiori Tokujiro , Tsujimura Hiroyuki 
Published: February 2020

Taking into account the advantageous characteristics of molten salt systems, traditional industrial electrochemical processes have been developed, and currently the role of molten salts in the fields of energy, environment and materials is becoming more and more important. Against the above background, we are now taking up the challenge of industrializing various novel molten salt electrochemical processes (MSEPs) that we have invented and developed. Four of these processes are described here.

Electrolytic synthesis of ammonia from water and nitrogen

The production of NH3 is crucial since it is an essential ingredient in the manufacture of fertilizer and many other important chemicals. At present, almost all NH3 is synthesized by the Haber-Bosch process on a large scale at high temperatures and under high pressures. For the distributed use of NH3, a new process must be created that can be operated at a lower synthesis temperature and pressure. Furthermore, it is now necessary to reduce CO2
emissions, which requires us to create a new ammonia synthesis process that can be carried out without causing any CO2 emission. Given the above circumstances, we propose the direct electrolytic synthesis of NH3 from H2O and N2 under atmospheric pressure using molten salt electrolytes, and are currently developing this process for industrialization. The principle of this process is as follows: The nitride ion (N3-) is produced in an electrolyte by cathodic reduction of N2 gas: 1/2N2 + 3e- →. N3-. When water vapor is supplied to the electrolyte, N3- and H2O react to produce NH3 and O2-: N3- + 3/2H2O → NH3 + 3/2O2-. The produced oxide ion (O2-) is anodically oxidized at the anode to evolve O2 gas: O2-
→1/2O2 + 2e-. Thus, overall reaction is expressed as 1/2N2 + 3/2H2O →NH3 + 3/4O2.The process will also be able to play a very important role in a future hydrogen energy system.

E
lectrochemical formation of carbon film

The electrochemical formation of carbon film can be achieved either by anodic oxidation of a C2
2- ion or by cathodic reduction of a CO3
2- ion dissolved in a molten salt. Coating with dense and adherent carbon film can be achieved by anodic oxidation of C2
2- ion. The obtained carbon-coated metal actually has the features advantageous for a fuel cell separator, a current collector of a Li-ion secondary battery/capacitor, and a substitute for fluoric resin film used in a semi-conductor/liquid crystal production line. Also, as we can even coat the surface of a screw thread with a thin and dense adherent carbon film as well as the inside of a porous body, this process can be used for producing corrosion-resistant fastening components (bolts and nuts) and metallic porous body . On the other hand, the carbon film can be obtained by a cathodic reduction of a CO3
2- ion. The carbon film obtained by a cathodic reduction of CO3
2- ion usually has a micro-porous structure, which is an advantageous feature for application to an electric double-layer capacitor or a super-capacitor.

P
lasma induced discharge electrolysis to form nanoparticles

Even when one electrode is outside the electrolytic bath, plasma-induced stationary discharge makes electrolysis possible under certain conditions. With this discharge electrolysis method, we have thus far obtained various types of metal and alloy nanoparticles. It was noticed that the primary nanoparticles sized at 5-20 nm are first formed just under the discharge and aggregated to form secondary nanoparticles with diameters at around 100 nm. This growth process proceeds in the area just under the discharge during the initial stage of the electrolysis. Based on this observation, in order to obtain finer and more uniform nanoparticles, we constructed a rotating disk anode-type electrolytic cell and confirmed its availability, choosing Ni nanoparticle formation as an example. Utilizing this process, recycling and effective reuse of various crucial metals can be attained simultaneously.

Recycling of crucial metals using ‘bifunctional ’ electrode

In a conventional rare earth recycling method, the rare earth elements extracted and recovered separately through the process are finally changed to oxide again and converted to corresponding metals by a molten salt electrolysis. We propose novel process by which we can skip all the intermediate, complicated, and expensive steps and obtain the desired rare earth metal directly from the scrap by using only molten salt electrolysis. The principle is based on an alloy formation potential difference between Nd-Ni and Dy-Ni alloys, for instance. In this process, a ‘bifunctional’ electrode which functions as a counter anode for cathodic alloy formation and as a counter cathode for anodic dissolution, respectively, can play an important role.