While other major automakers have either introduced (Hyundai, Honda) or are in serious development of new hydrogen fuel cell vehicles for the market, Toyota continues to take the point in not just promoting, but also supporting the broader technical (and infrastructure) development required for a large-scale realization of hydrogen-based electromobility.
At the 2015 CES, Toyota announced royalty-free use of approximately 5,680 fuel-cell-related patents held globally, including critical technologies developed for the Mirai fuel cell vehicle. (Earlier post.) At the SAE 2015 World Congress, Toyota presented a set of four technical papers detailing some of the technology innovations used in Mirai fuel cell stack. (Earlier post.) And again at this year’s 2016 SAE World Congress, Toyota presented three more papers: one detailing the development of Mirai’s Toyota Fuel Cell System (TCFS) and two dealing with the critical issues of the fuel cell catalysts.
The Toyota papers were part of the larger World Congress technical session on practical hydrogen fuel cell technology: PFL 720, Advances in Fuel Cell Vehicle Applications, chaired by Jesse Schneider of BMW.
Toyota Fuel Cell System (TFCS)
The 2015 papers provided technical details on the high performance fuel-cell (FC) stack; specific insights into FC separator, and stack manifold; Mirai’s newly developed boost converter; and the new high-pressure hydrogen storage system with innovative carbon fiber windings.
The 2016 paper by Hasegawa et al. describes the details of the larger Toyota Fuel Cell System (TFCS) from the standpoints of improved efficiency and reliability, as well as the simplification of the fuel cell (FC) system.
FC Boost Converter. Hybrid systems are a core technology that Toyota is adopting in every form of its next-generation vehicles; the Mirai uses the same motor, power control unit (PCU), and hybrid battery as existing Toyota hybrids. To enable this technology sharing in the Mirai, Toyota had to make a design advance from Toyota’s earlier Toyota FCHV-adv fuel cell vehicle, launched on a limited lease basis in 2008 (earlier post) with the introduction of an FC Boost Converter.
In the 2008 vehicle, the fuel cell stack and inverter were directly connected, using the same voltage; this required dedicated designs for both the traction motor and inverter to match the unique characteristics of the FC (i.e., low voltage and high current).
By contrast, Mirai’s FC boost converter adjusts the voltage difference between the motor and inverte, enabling the adoption of a traction motor and inverter already in mass production. It also eliminated the restrictions caused by the gap between the FC and traction motor voltages, allowing the number of layered cells to be optimized and reducing the size of the FC stack.
FC System Configuration. Compared to the FC system in the FCHV-adv, the FC system in the Mirai is simplified and more reliable. The most important modifications included achieving a humidifier-less system by eliminating the external humidifier; changing the type of the air compressor; consolidating the functions of the valves (the stack inlet shut valve and the flow diverter shut valve, as well as the stack outlet shut valve and the pressure adjustment valve); eliminating the hydrogen diluter, and reducing the number of hydrogen tanks from four to two.
|Configuration of TFCS in the Mirai. Hasegawa et al. Click to enlarge.
To eliminate the external humidifier, Toyota migrated water generated at the cathode to the anode where it uniformly distributed the water onto the surface of the anode membrane electrode assembly (MEA). This was done by developing innovative technology for the stack structure as well as modifying the anode operating conditions.
For the stack, Toyota:
Reduced the thickness of the electrolyte membrane, promoting the diffusion of the water in the air system.
Humidified the system using moisture at the anode—this humidifies the cathode inlet by flowing H2 and air in counter directions.
Reduced evaporation by increasing coolant flow at the cathode inlet.
At the anode, Toyota increased the amount of H2 circulation based on driving conditions. Reducing the anode inlet pressure after ensuring the required amount of circulation promotes the evaporation of moisture and enhances the movement of generated water onto the anode surfaces.
Fuel economy. Mirai has a lower hydrogen storage capacity than the Toyota FCHV-adv. To achieve a similar cruising range as a gasoline vehicle, the fuel economy of the Mirai was improved by 20% compared to the Toyota FCHV-adv.
Two of the main measures enabling this are the reduction in H2 permeability due to crossover, and the use of a Roots-type blower for the air compressor instead of the conventional scroll type.
H2 crossover refers to the permeation of hydrogen from the anode through the electrolyte membrane to the cathode, creating an H2 partial pressure gradient between the anode and cathode. The amount of crossover is proportional to the permeability coefficient of the electrolyte membrane.Because Toyota reduced the thickness of the electrolyte membrane by
two-thirds to realize the external humidifier-less system, crossover increased substantially. To counter this, and to improve fue economy, Toyota changed the anode control by optimizing H2 partial pressure control and H2 circulation control.
The H2 partial pressure required to prevent degradation of the anode catalyst due to insufficient H2 was reduced by using the H2 circulation pump to ensure an even H2 concentration at the anode. Along with enhanced membrane physical properties, this measure improved fuel economy by roughly 6.5% while achieving both the humidifier-less system and a reduction in H2 crossover.
To encourage the widespread adoption of FCVs, it is necessary to reduce the vehicle price, build an extensive hydrogen infrastructure, and enhance vehicle appeal. The technologies described in this paper enhanced the freedom of vehicle packaging by reducing the size of the system, and ensured that the Mirai has an equivalent cruising range to gasoline vehicles.—Hasegawa et al.
Work on the Electrode Catalyst
The other two Toyota papers presented during the World Congress session took a less vehicle-specific view, and addressed the large general problem of enhancing the performance of the electrode catalyst, as well as providing insight on the degradation process.
Mizutani et al. reported on two examples of efforts to improve the performance of a platinum-cobalt (PtCo) catalyst in fuel cell vehicles. PtCo alloy nanoparticles have demonstrated much better activity over traditional Pt nanoparticles—but mainly towards the reduction of activation overpotential. An issue with their use is operation at high current when mass transfer process becomes the limitation.
By lowering acid treatment voltage, the team enhanced the effectiveness of the removal of unalloyed cobalt, leading to less Co dissolution during cell operation and about 40% higher catalyst mass activity.
The use of nonporous carbon support material promoted mass transfer and resulted in substantial drop of overpotential at high current and low humidity. This may suggest an effective strategy towards the development of fuel cell systems that operate without additional humidification, the authors said.
In his paper, Kato reported on an in situ transmission electron microscopy (TEM) method that enables real time, high-resolution observation of carbon-supported platinum nanoparticles in liquid electrolyte under working conditions.
On the cathode of a fuel cell, protons and oxygen molecules receive electrons, resulting in a chemical reaction that forms water molecules. To promote this chemical reaction, platinum particles of several nanometers are used as a catalyst. Because platinum is a rare element and expensive, it is necessary to reduce its use.
One of the factors behind the large usage of platinum is its degradation. It is known that platinum nanoparticles degrade while the fuel cell is generating electricity, thus reducing its output. Therefore, it is necessary to elucidate this degradation mechanism and implement a countermeasure. It would not be an overstatement to say that the very quality of automobiles depends on nanoscale analysis technologies. However, the mechanism behind the degradation phenomenon of platinum nanoparticles has not been fully understood. This is largely due to the technical difficulty in directly observing the nanometer-sized platinum particles in liquid electrolyte under applied electrochemical potentials.—Kato
By improving the design of the Micro Electro Mechanical Systems (MEMS) sample holder, the migration and aggregation of neighboring platinum nanoparticles could be visualized consistently and correlated to applied electrode potentials during aging process (i.e., cyclic voltammetry cycles).
Hasegawa, T., Imanishi, H., Nada, M., and Ikogi, Y. (2016) “Development of the Fuel Cell System in the Mirai FCV,” SAE Technical Paper 2016-01-1185 doi: 10.4271/2016-01-1185
Mizutani, N. and Ishibashi, K. (2016) “Enhancing PtCo Electrode Catalyst Performance for Fuel Cell Vehicle Application,” SAE Technical Paper 2016-01-1187 doi: 10.4271/2016-01-1187
Kato, H. (2016) “In-Situ Liquid TEM Study on the Degradation Mechanism of Fuel Cell Catalysts,” SAE Int. J. Alt. Power. 5(1) doi: 10.4271/2016-01-1192