Fuel Cell (FC) Technology in Toyota's New Mirai

  There were 4,035 attendees at the 2021 Society of Automotive Engineers of Japan (JSAE) Annual Congress (Spring) that was held online from May 26 to 28, 2021. This is more than the 3,408 attendees at the 2020 JSAE Congress (Autumn), which was held online due to the COVID-19 pandemic.

  Toyota Motor Corporation (Toyota) gave a number of presentations on the fuel cell vehicle (FCV) system installed on the all-new Mirai, and introduced the development contents of various elemental technologies. This report will introduce some of them.

• The all-new Mirai, launched by Toyota in December 2020, has a smaller stack which fits under the hood, three high-pressure hydrogen tanks including in the center tunnel to increase the amount of hydrogen loaded, and improved fuel efficiency to increase the cruising range by 30%, from 650km to 850km.
• The components of the FC (fuel cell) unit, even the auxiliary components such as the hydrogen mixing pipe, have been optimized using simulations to reduce the operating noise of the main shutoff valve, pump, compressor, and other components, and improve the quietness of the system, thereby reducing the size and cost as well as improving the merchantability.
• The stack case, which protects the cell layers in the FC stacks, has been downsized by revising the structure and construction method. The cells have improved power generation by improving the physical properties of the electrode materials and reduced cost by reducing the amount of precious metal used, while the cell structure has been revised to improve manufacturability.
• The high-pressure valves, high-pressure pressure reducing valves, high-pressure temperature sensors, and other components which make up the high-pressure hydrogen storage system are also downsized reducing the mass and cost.
• The high-pressure hydrogen tank uses high-strength, high-elasticity carbon fiber to reduce the number of CFRP layers by 7%, increasing the volume and reducing the weight of the tank, while increasing productivity by speeding up the winding process and automating tank quality measurements. The mass efficiency of the high-pressure hydrogen tank has been improved by 5% to achieve 6.0wt%.

All-new Mirai launched in December 2020	FC system of the all-new Mirai

(Source: Toyota)

References

  The reference numbers, titles, and names of speakers of the presentations referred to in this report are as follows. (All speakers belong to Toyota Motor Corporation.)

20215124 “Development of the High-Performance Fuel Cell Electrode for New FCV, Establishment of High Performance and Durability”, Kenji Tsubosaka
20215125 “Fuel Cell Channel Design using 3D Numerical Model”, Daisuke Hayashi
20215126 “Design of Fuel Cell that Enable High-Speed Production for New FCV”, Taiki Kuwahara
20215127 “Structure and Equipping of FC Stack for New FCV”, Yuma Takabatake
20215128 “Development of Physical Model of Fuel Cell System and Application for Control Design”, Nobukazu Mizuno
20215130 “Development of Hydrogen Mixing Pipe for New FCV by using Two-Phase Flow Simulation”, Hideyuki Arai
20215131 “Development of Noise Reduction for the New Type FCV”, by Ryota Akaboshi
20215133 “Development of High-Pressure Hydrogen Tank for New FCV”, Sogo Goto
20215134 “Development of the Hydrogen Pressure Vessel's Fatigue Failure Model Considering CFRP Failure Modes”, Tsuyoshi Nishihara
20215135 “Development of the High-Pressure Hydrogen Storage System Parts for New FCV”, by Koji Kida

Related reports:
Development of Fuel Cell Vehicles (FCVs) and Expansion of Applications (Apr. 2021)
Smart Energy Week 2021: Electrification-related Technologies (FC edition) (Mar. 2021)

  In the all-new Mirai launched by Toyota in December 2020, the stack was downsized so that it can be installed under the hood, and by adding one high-pressure hydrogen tank in the available center tunnel, the hydrogen loading capacity was increased by 1 kg. In addition, the vehicle’s performance was improved by 1.5%, the unit performance was improved by 6.2%, and the control was improved by 2.3%, for a total of 10% improvement in fuel efficiency, realizing a 30% improvement in range from 650km to 850km.

  Moreover, while the development of the FC system for the first-generation Mirai launched in 2014 made progress in ensuring sub-freezing starting performance and durability reliability, the FC system for the all-new Mirai has improved quietness to enhance its merchantability and user convenience.

  The output density of the FC stack in the first-generation Mirai released in 2014 was 3.5kW/L, but the stack on the all-new Mirai released in 2020 is more compact and lighter with a volume reduction from 33L to 24L and a mass reduction from 41kg to 24kg. The maximum output has increased by 15% from 114kW to 128kW, and the output density has increased to 5.4kW/L.

FC system overview FC boost converter A compact, high-efficiency, high-capacity converter that boosts the voltage of the FC stack to 650V. Toyota's first use of silicon carbide (SiC), a next-generation semiconductor material, realizes high efficiency and high output. FC Stack Compact size and world's top-level power density by integration of FC stack with improved FC boost converter, achieving under-hood installation. Drive battery High efficiency and compactness realized by using a Li-ion battery that stores the energy recovered during deceleration and assists the output of the FC stack during acceleration. Motor Uses electricity produced by the FC stack and from the drive battery to propel the vehicle. High pressure hydrogen tanks (3) Tanks that stores hydrogen as fuel achieve world-class tank storage performance.

Power density of FC stack 2008 model → 1st generation FC stack 3.5kW/L, 33L/41kg → 2nd generation FC stack 5.4kW/L, 24L/24kg (Vertical axis: Volumetric power density, Horizontal axis: Mass power density)

(Source: Toyota)

Components of the FC unit (Source: Toyota, 2021 JSAE Annual Congress (Spring) - presentation materials)

  As shown in the figure, the components of the FC unit form an integrated unit of the FC stack and boost converter mounted on the upper surface of the aluminum frame, and an electric air compressor, electric water pump, electric pump for hydrogen circulation, and an electric compressor for the air conditioner are mounted on the lower surface of the aluminum frame.

  The integrated FC unit is mounted on a suspension member via mounting brackets, and these vehicle mountings provide comfort and maneuverability.

  Since FCVs have no engine noise, there is an issue that the operating noise of FC auxiliary equipment can be heard at low vehicle speeds when the background noise level is low.

  FCV noise includes airborne noise and structure-borne noise during various situations of start/stop, acceleration, and deceleration. To produce electricity by chemically reacting hydrogen and oxygen in the fuel cell stack, air compressors to supply large amounts of air, hydrogen injectors to supply hydrogen, hydrogen pumps to circulate the hydrogen, main shut-off valves to seal the high-pressure hydrogen gas in the tanks, the booster converter, and other components are necessary and these are the various sources of vibration and noise.

  The hydrogen pump and FCDC (voltage converter), which were conventionally mounted under the floor, are mounted under the hood in the front of the vehicle together with the stack, and the operating noise can be reduced by increasing the distance between the driver's ear and auxiliary equipment.

  In addition, the air compressor integrated with the FC stack is a turbo pump equipped with a speed-increasing part, which generates vibrations due to rotation so in addition to vibration isolation for the air compressor, measures are taken to ensure rigidity while considering the spring constant of the FC mount.

FC system configuration (Source: Toyota) Air (green line, from left): Air cleaner - Air compressor - Bypass valve - Muffler, or Air cleaner - Air compressor - Inlet valve - FC stack - Pressure control valve - Muffler Coolant (blue line): Radiator - FC stack Hydrogen (red line, from right): Filling port - Hydrogen tank X 3 units - Main shutoff valve - High pressure regulator - Injector - FC stack - Exhaust valve - Hydrogen pump

  In addition to the FC stack that generates electricity, the FC system consists of an air system that supplies oxygen, a hydrogen system that supplies water vapor and hydrogen as fuel, and a cooling system that cools the FC stack, as well as various pumps, valves, injectors, regulators, and other auxiliary equipment.

  In the case of auxiliary equipment for FC systems, even parts such as the hydrogen mixing pipe are developed and optimized using simulation.

Exterior view of FC stack (Photo by MarkLines at the Toyota booth, Smart Energy Week 2021)

  The FC stack is the central part of the fuel cell, and holds the cells in which electrodes and separators are integrated, and generates electricity by supplying hydrogen, air, and cooling water. In addition, auxiliary components such as pumps, compressors, and valves are installed around the stack to make up the FC system.

  The FC stack is a stacked structure of 330 cells, and the gaskets between the cells are required to seal hydrogen gas, air, and cooling water, and to prevent seal failure due to cell stacking misalignment, even during shock input.

  The cell layers are held together by the fastening load of the stack case and end plates, its reaction force, and frictional constraints due to the friction coefficient between the cells.

  The stack case is supported by a load that compresses the cells in the stacking direction in order to apply surface pressure to the seals and electrodes of each stacked cell, and it also needs to be equipped with mounting points for attaching auxiliary components to the periphery. It needs to be a strong member with a complex shape, so an aluminum die casting process is used.

  In the new FC system, the upper and lower parts of the stack case are die-cast separately and by joining with FSW (friction stir welding), equal distance between the stacked cells and the stack case is ensured and an increase in part size due to the die-casting drafting slope is suppressed, thereby realizing downsizing.

Structure of FC stack	Stack case construction method

(Source: Toyota, JSAE 2021 Congress (Spring) presentation materials)

  A cell is a combination of a separator (a metal plate with a complex pattern on its surface to form a gas flow path and a sealing surface) and a membrane electrode assembly (MEA) that integrates a solid polymer membrane, electrode, catalyst, and gas diffusion layer. A large number of these cells are stacked to form a stack.

  When hydrogen, air, and cooling water are supplied from the manifold to the 330 cells stacked in the FC stack, a sealing function is required to prevent the media from mixing. The hydrogen, air, and cooling water are separated by seal bonding inside the cells and gasket sealing between the cells.

Cutaway view of the cell (Photo by MarkLines at the Toyota booth, Smart Energy Week 2021)	Cell structure of the new FC stack (Source: Toyota, JSAE 2021 Congress (Spring) presentation materials)

Seal bonding structure of FC cell (Source: Toyota, JSAE 2021 Congress (Spring) presentation materials)

  As shown in the figure on the right, the seal bonding structure of the FC cell is different between the 2014 and 2020 models, with the EPDM rubber replaced by a three-layer plastic sheet.

  The three-layer sheet is made by laminating a hot melt adhesive layer on both sides of a polyethylene naphthalate base material, and exhibits adhesiveness upon heating/cooling.

  EPDM rubber required a processing time of more than 10 minutes per sheet for filling and vulcanization, but because the the three-layer sheets use UV cured adhesion and thermal plasticity adhesion, cell production can be accomplished in seconds per sheet.

  In the joining process of the electrode and the three-layer sheet, UV-curing adhesive is first applied to the edge of the electrode by screen printing. The three-layer sheet is laminated on overlapping the adhesive, and then the adhesive is cured by irradiating UV light to bond the electrode and the three-layer sheet together to form a single unit. Since the processing time required for each process is only a few seconds, continuous production on the order of seconds is possible.

  The junction of the electrode and the three-layer sheet is sandwiched between two separators, and the thermoplastic resin of the hot-melt adhesive layer of the three-layer sheet instantaneously develops adhesiveness through a continuous hot press/cold press operation. The cell is completed when the separator and the three-layer sheet are thermoplastically bonded.

Joining process of electrode and 3-layer sheet	Cell production process flow

(Source: Toyota, JSAE 2021 Congress (Spring) presentation materials)

  In the cell, it is necessary to simulate mass transport and electrochemical phenomena to ensure oxygen transport to and drainage from the electrodes. In the case of development such that air is convected into the electrodes by installing a resistance section with a narrowed cross-sectional area flow path in the groove channel, the cell shape is optimized through power generation simulation by considering factors including oxygen concentration, voltage, and pressure drop.

Platinum dispersed inside a porous carbon carrier (Source: Toyota) Top: 1st generation solid carbon carrier Bottom: 2nd generation porous carbon carrier (Green: Ionomer, Red: Pt on surface, Blue: Pt inside)

  The electrodes of the new FC system are designed to improve electric power generation by improving the material properties of the electrodes.

  For the catalyst layer, Toyota innovated the materials of the catalyst and ionomer (meaning ion + polymer), and adopted porous carbon as the catalyst carbon carrier instead of a solid carbon support. Catalyst activity improved 50% by dispersing platinum inside the carrier.

  As for the ionomer, a high oxygen-permeable ionomer has been adopted to improve oxygen permeability by a factor of three, and a molecular structure with a higher density of acid functional groups has improved proton conductivity by a factor of 1.2.

  Other innovations in electrodes, including electrolyte membranes and diffusion layers, have improved the output per electrode unit area by 15%, contributing to a volumetric output density of 5.4kW/L of the FC stack.

  The company is also working to reduce the cost of electrodes; by increasing the current density and the power generation area utilization rate through innovations of the cell flow path structure and electrodes, cell size was reduced by 20%. By increasing the current, the number of stacked cells was reduced. By using a thinner electrolyte film and a more active catalyst, the amount of platinum used per unit output was reduced by 58%. Cost has been reduced to a quarter.

Improvement of power output per unit area of FC stack electrode 15% increase in output per electrode unit area (Vertical axis: Cell voltage, Horizontal axis: Current density)	Reduction in the use of Pt amount 2008 model → 1st generation: 72% reduction → 2nd generation: 58% reduction (Vertical axis: Amount of Pt per output)

(Source: Toyota)

  In a conventional FCV, the hydrogen tanks are mounted under the floor, but in the new FCV, the stack is installed under the front hood, so the tank layout utilizes the center tunnel, and the vehicle is equipped with three tanks of the same diameter. The mass efficiency of the tank itself has also been improved, increasing the amount of available hydrogen by 21%, thus extending the range of the FCV.

  The mass efficiency of the high-pressure hydrogen tank on board the all-new Mirai has been improved by 5% compared to the conventional model to achieve 6.0 wt%.

High-pressure hydrogen tanks for FCVs	Trend of mass efficiency of high-pressure hydrogen tanks

(Source: Toyota, JSAE 2021 Congress (Spring) presentation materials)

  The new FCV utilizes the center tunnel to mount a 1.5m long tank, increasing the tank volume by 15% from 122L to 142L in total without sacrificing interior space.

  When mounting the tank in the center tunnel, a neck-mounted system is used because the conventional band fixtures are not strong enough to restrain the tank against the G forces of light collision levels in the longitudinal direction.

  The neck mount method secures the holding force by fixing the base of the tank on the valve side with a bracket, restrains it up to the point of self-propelled collision, and beyond that, it is designed to be detached before the tank is damaged.

  In addition, the neck-mounted system eliminates the need for a spring mechanism installed next to the tank to accommodate tank expansion and contraction, which also contributes to ensuring capacity in the center tunnel.

High-pressure hydrogen tank fixture method	Neck-mounted fixture structure

(Source: Toyota, JSAE 2021 Congress (Spring) presentation materials)

Configuration of the high-pressure hydrogen storage system (Source: Toyota, JSAE 2021 Congress (Spring) presentation materials)

  The high-pressure hydrogen storage system consists of high-pressure hydrogen tanks and a pressure reduction mechanism. The high-pressure hydrogen supplied from the three tanks is reduced in two stages by high-pressure pressure reducing valves and injectors, and then supplied to the FC stack.

  Regarding the high-pressure valve, for the new FC system the manufacturing process was revised to improve manufacturability and reduce costs.

  The aluminum body of the valve, which is gray in the lower left figure, is made of continuous casting material and employs a near-net forging process to improve the material yield by 20%.

  The stainless steel parts in the figure are colored yellow and are made using an MIM (Metal Injection Molding) process, which enables complex shapes and improves the material yield by 35%.

  The orange in-unions and out-unions in the figure are made of hydrogen-resistant AUS316L-H2, which is not easy to process, so low-frequency vibration cutting (LFV) is used to shorten the processing time and reduce the overall cost by 75%, according to the company.

Structure and materials of the high-pressure valves	High-pressure valve manufacturing processes

(Source: Toyota, JSAE 2021 Congress (Spring) presentation materials)

  The seal in the piston of the high-pressure pressure reducing valve must withstand tens of millions of sliding movement cycles and maintain the seal, so precision cutting and surface treatment are required. However, the new FC system, by cutting the body side with a high-rigidity lathe, has a simplified two-layer structure instead of a three-layer structure reducing the cost by 66% and the mass by 15%.

  The high-pressure temperature sensor is a sensor placed inside the tank. By revising the structural members and changing from stainless steel to plastic materials, the number of parts and mass were reduced, providing a cost reduction of 25% and a mass reduction of 90%.

High pressure reducing valve (3-layer → 2-layer structure)	Structure of the high pressure temperature sensor

(Source: Toyota, JSAE 2021 Congress (Spring) presentation materials)

High-pressure hydrogen tank installed in the all-new Mirai (Photo by MarkLines at the Toyota booth, Smart Energy Week 2021)

  The all-new Mirai is equipped with three 70 MPa, Type 4 (a pressure vessel with plastic liner overwrapped by carbon-fiber composite material) high-pressure hydrogen tanks, but with a common diameter and different lengths.

  The tank consists of a plastic liner on the innermost layer to contain hydrogen gas, a CFRP (carbon fiber reinforced polymer) layer next to withstand high pressure, and the outermost layer is covered with impact-resistant GFRP (glass fiber reinforced polymer).

  The carbon fiber is 4% stronger and more elastic than before, and the CFRP layers have been reduced by 7% to increase the volume and reduce the weight of the tank.

Specifications of the high-pressure hydrogen tanks	Structure of high-pressure hydrogen tank

(Source: Toyota, JSAE 2021 Congress (Spring) presentation materials)

  The CFRP layer is formed in a high-speed filament winding process using tow prepreg (TPP), in which the tow (bundle of carbon fibers) is impregnated with epoxy resin as a binder in an uncured state.

  There are two types of filament winding (FW) processes used: hoop winding to strengthen the cylindrical part and helical winding to strengthen the dome part.

  CFRP accounts for the majority of the mass and cost of high-pressure hydrogen tanks, and its impact on weight and cost reductions is significant.

TPP layering patterns	Carbon fiber winding process

(Source: Toyota, JSAE 2021 Congress (Spring) presentation materials)

  The cross-sectional shape of the high-pressure hydrogen tank was improved in the 2014 model by (1) concentrating the hoop winding to improve the efficiency of strengthening the cylindrical part, (2) changing the high-angle helical layer, which has low pressure resistance efficiency with the discontinuous liner to a more efficient hoop layer, and (3) reducing the surface pressure that CFRP receives from the inlet boss by increasing the diameter of the inlet boss. All of which enabled reduction of CFRP by 40%.

  Since CFRP having a structure of carbon fiber impregnated with polymers has a complex fracture mode, the company is working on a fatigue fracture prediction method after conducting fatigue tests on actual tanks.

  In observations of cross-sections after the actual tank fatigue tests, (1) fiber fracture in the transverse direction and (2) delamination modes were confirmed.

Tank cross-section	Cross-section after tank fatigue test

(Source: Toyota, JSAE 2021 Congress (Spring) presentation materials)

Production process of high-pressure hydrogen tanks (Source: Toyota, JSAE 2021 Congress (Spring) presentation materials)

  As shown in the figure, the production process for high-pressure hydrogen tanks includes a liner process, a FW (filament winding) process, a valve assembly process, and an inspection process.

  In the FW (filament winding) process, a bundle of carbon fibers is impregnated with resin as a binder in an uncured state, and the tow prepreg (TPP) is used to form the CFRP layer by high speed filament winding. 3500m of carbon fiber is wound while applying internal pressure to the low-rigidity plastic liner and controlling the tension on the carbon fibers.

  Productivity has been tripled by speeding up equipment operation to reduce processing time by 50%, and by automating quality measurements to reduce measurement time by 90%.

  In terms of ensuring quality in the FW process, the inspections of all the layers that had been manual operations were automated.

  Since it is difficult to discriminate the part stacked on top of the laminate, light is irradiated during winding and scattering is used to distinguish the top from the lower layers, reducing the measurement time by 90%.

Overview of FW (filament winding) equipment	Quality measurement of hoop winding

(Source: Toyota, JSAE 2021 Congress (Spring) presentation materials)

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Keywords
Toyota, MIRAI, FCV, Fuel Cell, FC Stack, Hydrogen Tank, High Pressure Valve, Electric

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