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Development of next-generation proton-conducting fuel cells (PCFC) and novel electrolyte materials

 
 
In our lab, we are working on developing fuel cells that incorporate inorganic protonic conductors as the electrolyte material, ultimately aiming to design and propose novel energy systems that integrate these technologies. 
Currently, the fuel cells with the highest power generation efficiencies are solid oxide fuel cells (SOFC) that use oxide ions as the charge carriers. In our lab, we aim to further improve efficiency using proton-conducting ceramic fuel cells (PCFC), which replace the oxide-ion-conducting electrolytes with proton-conducting electrolytes that have a perovskite structure (ABO3). These perovskite materials exhibit maximum protonic conductivity at 400-600˚C, lower than the 800-1000˚C temperature regime that conventional SOFCs are operated at. Such a reduction in operation temperature would enable smaller-scale energy systems that are better suited for decentralized generation. In addition, PCFCs generate water vapor at the air electrode rather than at the hydrogen electrode, resulting in higher fuel utilization and a subsequent improvement in cell voltage. We aim to exploit these advantageous characteristics of PCFCs by developing new electrolyte materials and catalytically active electrode materials, and by deepening our understanding of the electrode reaction mechanisms. 
We have focused on developing novel proton-conducting solid electrolyte materials and on transforming these materials into thin-film electrolytes. Specifically, we are investigating the transport properties of charge carriers in these materials and at the interfaces, and using the insight gained from these studies to better control the transport properties and thereby improve the performance of actual devices. In recent years, we used the non-perovskite lanthanum-based proton-conducting oxide La28 – xW4+xO54+3x/2 (LWO), in hopes of addressing the issue of leakage current that occurred when using conventional perovskite materials. We successfully demonstrated that LWO can suppress hole conductivity and enable higher power generation efficiency than other protonic conductors. 
 
 

 
Fig. Investigation of novel electrolyte materials and PCFC cell design

 

Relevant Equipment

Pulsed Laser Deposition (PLD)

 
We use the PLD to deposit thin layers of solid oxide thin films. A pulsed laser is directed towards a sintered target of the desired composition, to produce a plume that is deposited onto the substrate placed above. Thin-film electrolytes that are several hundred nm to several μm thick can be formed.
 

 
    
Fig. SPM-KFM equipment (external view)
 

Thin-film surface characterization equipment (controlled atmosphere SPM-KFM)

 We use SPM-KFM to measure the potential distribution and transport properties, for example of the interfaces of layered thin-films deposited via PLD. Controlled-atmosphere scanning probe microscopy (SPM) and atomic force microscopy (AFM) are used to observe surface morphology, and Kelvin probe force microscopy (KFM) is used to measure surface potentials. 
 

Fig. SPM-KFM equipment (external view)

 

Fuel cell power generation equipment

 The power generation performance of fabricated fuel cells is tested using this double-chamber generation equipment.

Fig. Power generation equipment (external view)

 

Electrosynthesis via proton-conducting solid electrolyte fuel cells

 Ammonia is considered a promising next-generation hydrogen energy carrier due to its high volumetric hydrogen content and its ease of liquefaction at relatively ambient conditions. Currently ammonia is synthesized industrially via the Haber-Bosch process, which is energy-intensive and reliant on fossil fuels. This process requires nitrogen and hydrogen to be reacted at high temperatures and pressures; to achieve these harsh synthesis conditions, the process is conducted in large centralized production facilities. A more environmentally benign ammonia synthesis method could be realised by moving towards smaller-scale, decentralized production powered by renewable energy. 
              One such promising green ammonia synthesis method is an electrosynthesis process that uses protonic conductors to electrochemically convert water and nitrogen into ammonia. 
 
Anodic reaction: H2O → 1/2O2 + 2H+ + 2e-
Cathodic reaction; N2 + 6H+ + 6e- → 2NH3
Net reaction:  3H2O + N2 → 2NH3 + 3/2O2
 

Fig. Schematic diagram of ammonia electrosynthesis using protonic conductors
 

 In our lab, we are developing ammonia electrosynthesis cells that employ Y-doped BaCeO3, a proton-conducting ceramic material that operates at around 500˚C. We are focusing on developing cathode catalytic materials with controlled microstructural properties, aiming to improve the ammonia synthesis rate. In addition, we are also working towards a better understanding of the ammonia electrosynthesis reaction mechanism, of which much remains unknown. 
 

 
Fig. Schematic of a proposed ammonia electrosynthesis reaction mechanism (TEM image of reduced BCY-Ru)

 

Relevant Equipment

Ammonia electrosynthesis equipment

 
 Two quartz tubes are sealed onto either side of the cell. Wet hydrogen is flowed to the anode side, while nitrogen is flowed to the cathode side.
 

 
Fig. Ammonia electrosynthesis equipment (external view)
 

FTIR with long optical path length gas cell 

 We conduct isotopic analyses of the reaction products formed when introducing deuterium into the electrosynthesis reaction. Ammonia isotopes are detected using this Fourier transform infrared (FTIR) spectrophotometer coupled to a gas cell with an optical path length of 8 m.
 

 
Fig. Long optical path length gas cell (external view)
 

FTIR reaction tracking equipment

In ammonia synthesis reactions, dissociation of the strong nitrogen triple bond is considered a potential limiting step. By setting our electrosynthesis cells into a closed gas-circulating system, we can observe how the infrared peaks corresponding to nitrogen adsorption onto the catalyst change when potential is applied. From these results, we aim to elucidate the relationship between the applied potential and the strength of the nitrogen bond.
 

 
Fig. FTIR reaction tracking equipment (external view)
 

Chemical-looping combustion: development of oxygen carrier materials and hydrogen production process

 
Chemical-looping combustion (CL) relies on using separate reactors to conduct the reduction and oxidation reactions of metal oxides, to obtain products like hydrogen, carbon dioxide, and nitrogen from separate compartments, all while simultaneously recovering heat. A CL system consists of a reduction reactor and an oxidation reactor, within which the following chemical reactions occur:
 
(Reduction reactor) CmHn + (2m+1/2n) MO + heat → 2m M + H2O + CO2
(Oxidation reactor)M + 1/2 O2 → MO + heat
 
In CL, lattice oxygen atoms within the metal oxide are used as the oxygen source. Low grade heat is used for the reduction reaction, while high grade heat is produced via the oxidation reaction. This results in an energy conversion system that can undergo both fuel combustion and heat recovery by coupling exothermic and endothermic reactions. Moreover, by introducing water vapor into the reduction reactor, hydrogen can also be produced. These versatile applications have led to CL gathering interest in recent years as a route towards environmentally friendly energy systems.
Our lab is working on developing metal oxide particles that can exhibit high activity and high stability during continuous redox cycles, given that these parameters are critical in determining the efficiencies and lifetimes of CL systems. Alongside fundamental research on particle synthesis and characterization, we are also conducting CL reaction experiments using a fluidized bed. The synthesized carrier particles are fluidized within the reactor and evaluated under various conditions based on the outlet gas composition and kinetic analysis, to determine the reaction mechanisms and develop high efficiency systems.
 

   
Fig. Energy conversion/storage technologies using redox reactions of metal oxides (L: 2 reactors, R: 3 reactors)

 

Fig. Energy conversion/storage technologies using redox reactions of metal oxides (application)

 
 We are also working on hydrogen evolution via the thermal decomposition of methane, and on alternative carrier particles that would enable carbon dioxide utilization. We are investigating a system that performs both hydrogen evolution via methane decomposition, and carbon monoxide synthesis by reacting the carbon remaining on the particle surfaces after methane decomposition with carbon dioxide, in two separate reactors.
 

Relevant equipment

Chemical looping evaluation equipment

 In addition to developing carrier particles and evaluating their characteristics, we conduct chemical looping reaction experiments using a fluidized bed.
 

 
Fig. Chemical looping evaluation equipment (experiment in progress)
 

Fluidized bed containing carrier particles

 
Natural minerals like ilmenite (FeTiO₃) are commonly used as the metal oxide particles for CL. While natural minerals can be considered beneficial for their low cost, they demonstrate low reactivity due to their small surface areas. To artificially develop high performance particles, research efforts have been directed towards composite particles consisting of metal oxides particles and support particles. Below is a SEM image of an Al2O3 supported Fe2O3 artificial particle.
 

 
Fig. Fe-Al2O3 artificial particle
 
Fig. Fluidized bed reactor containing carrier particles
 

Cost engineering for the development and evaluation of novel energy systems

 Thinking ahead to the future implementation of energy conversion systems that will contribute to the realization of a low-/zero- carbon society, we are working on energy system designs, including fuel cell designs and chemical looping process designs, and analyzing the economic viability of such systems.
              Technologies for improved fuel cell power generation performance, cell design, and manufacturing processes are being continually and rapidly developed. In our lab, we use a technical evaluation method, termed “cost engineering”, to develop data structure and evaluation methods that can adapt to these technological improvements in real-time. Using this evaluation method, we investigate the techno-economic potentials of future fuel cell technologies and hydrogen production processes via water electrolysis.
              By crafting scenarios that link these cutting-edge technologies to society, and unveiling the technological challenges for implementation, we aim to propose pathways for the development of materials and energy devices.
 

 
Fig. Methods to evaluate the implementation and improvement of new technologies and innovations