Core Technology of Fuel Cell Vehicles

The new generation of environmentally friendly vehicles, known as fuel cell vehicles, use abundant hydrogen as fuel instead of traditional fossil fuels, and emit only water and no CO2. Fuel cell vehicles are driven by electric motors, which can combine quietness with good driving performance, and have a shorter fuel filling time, and ensure a range similar to that of internal combustion engine vehicles. Various automobile manufacturers are now actively carrying out R&D and promotion of fuel cell vehicles. The structure, design and control of Toyota's fuel cell system (TFCS) and fuel cell stack are introduced. It focuses on a core technology of the TFCS, namely "water management control technology", and the design process based on the fuel cell stack and the visualization and measurement technology of the internal state of the fuel cell stack.

0?Preface

In recent years, as the greenhouse effect on the earth is increasing and oil resources are being depleted, the issue of energy security (in particular, the stable supply of energy, etc.) has been highlighted, and the new energy vehicles that do not produce CO2 during operation have attracted a great deal of attention. Toyota has recently established a "zero CO2 emission target" and is currently conducting research to increase the proportion of new energy vehicles sold by 2050 (Fig. 1).

FCVs are characterized by the following features: (1) hydrogen as fuel, which can be produced from a wide range of energy sources, including fossil fuels; (2) only water is emitted during driving; (3) because the main drive unit is an electric motor, quietness and good drivability are taken into account; and (4) they have a shorter fuel-filling time, and at the same time ensure a range comparable to that of a vehicle with an internal combustion engine. The range of the vehicle is similar to that of an internal combustion engine vehicle. At present, all sectors of society are eagerly looking forward to the practical realization of this type of environmentally friendly vehicle. Considering the many advantages of FCVs, researchers believe that FCVs can also meet the needs of medium- and long-distance transportation (Figure 2). Toyota was the first company in the world to market and sell a mass-produced FCV, the MIRAI, in 2014. In addition, in 2018, Toyota launched the new fuel cell city bus "SORA" (Fig. 3), which utilizes the same fuel cell system, and is gradually conducting a validation review of light-duty trucks (Fig. 4).

1?Toyota Fuel Cell System

Toyota has positioned hybrid technology as the core technology for new energy vehicles, replacing the engine of the hybrid system with a fuel cell system and the fuel tank with Toyota Fuel Cell System (TFCS) (Figure 5).

The fuel cell system consists of a fuel cell stack that generates electricity, a hydrogen system that supplies hydrogen fuel, an air system that supplies oxygen, and a cooling system (Fig. 6). The power generated by the fuel cell stack is supplied to high-voltage systems such as the main drive motor and high-voltage battery through a fuel cell boost converter (Fig. 7). In terms of electrolyte conductivity, which has a significant impact on the power generation of the fuel cell stack, its sensitivity varies significantly with the relative humidity of the surrounding environment. Moreover, the water generated during the reaction process affects the fuel supply process within the fuel cell stack, and thus the management of the generated water can be critical. This paper discusses the design and system control based on water management in fuel cell stacks.

2?Fuel Cell Stacks

Fuel cell stacks are designed to obtain the required electrical energy by designing the electrode area of a single cell and the number of single cells. In general, a single cell consists of components such as a membrane electrode assembly (MEA), which serves as a reaction site for hydrogen and oxygen, a micropermeable layer (MPL), a gas diffusion layer (GDL), a gas channel for supplying hydrogen and air from the outside, and a spacer plate (Fig. 8).

Toyota has achieved a high density of the fuel cell system by improving the fuel cell flow channels and MEA? In addition, the connection members of the cell have been simplified by the effective application of the internal spring mechanism of the single cell. At the same time, the thinning of the cell itself has reduced the volume size. Moreover, with the adjustment of the spacer material, the total weight of the cell was effectively reduced, resulting in a high power density (3.1?kW/L? vs. 2.0?kW/kg, Fig. 9). The results showed that the amount of platinum catalyst used for the fuel cell electrodes was also reduced (Fig. 10). Not only that, the surface treatment process of the spacer was also adapted from electroplated gold treatment to the cheaper polymerized amorphous carbon (PAC) plating in order to avoid reducing the contact resistance and to ensure corrosion resistance, thus significantly reducing the cost.

2.1?High current densification

Battery performance is determined by the loss of theoretical starting voltage (overvoltage). Overvoltage can be divided into three general categories: "activation overvoltage" originating from the catalytic reaction, "resistance overvoltage" originating from the movement of electrons and protons, and "concentration overvoltage" originating from the reaction process. "(Figure 11). In the case of polymer electrolyte fuel cell (PEFC), since the water generated during the power generation process is in liquid phase, the concentration overvoltage is further deteriorated due to the obstruction of gas diffusion in the single cell. On the other hand, in the high-temperature region where vapors are easily formed, the resistive overvoltage, which is the resistance to the movement of protons, increases accordingly as the relative humidity near the electrolyte decreases. Based on the above analysis, the design and control of components for the water generated in the power generation process is essential to realize the high current density of the fuel cell, which is the core concept of the fuel cell water management technology.

2.2?Reduction of concentration overvoltage

In the low temperature and normal operation temperature region, the water generated from power generation will be retained in the cell flow channel, GDL, MPL and MEA on the air pole side, which generates concentration overvoltage. Normally, liquid water tends to accumulate in the GDL and MEA, which are not in contact with the gas flow path. However, the cell runner structure of the fuel cell stack fitted in Toyota's MIRAI model utilizes a 3D fine-mesh lattice structure. While optimizing oxygen supply and discharging liquid water, the hydrophilic nature of the partition surface directs liquid water to the runner surface, thereby reducing the concentration overvoltage (Figures 12 and 13). In addition, within the GDL, optimization was achieved by adjusting the ratio of carbon fiber to binder. As for the MPL, the gas diffusivity was improved by about two times by reducing the permeability pressure by realizing coarse granularity of the carbon black particles, which in turn reduced the concentration overvoltage.

2.3?Reduction of resistive overvoltage

In order to ensure the proton conductivity of the electrolyte in PEFC, it is necessary to keep the environment around the electrolyte in a moist state. In a conventional fuel cell system, water generated in the reaction can be discharged through a humidifier, returned to the fuel cell stack and humidified. The TFCS fitted to the MIRAI model can be structurally simplified to improve reliability. Toyota eliminated this type of humidifier in order to reduce costs, and designed each component based on the self-humidification concept, thereby realizing high-temperature performance similar to that of conventional models (Fig. 14). The self-humidification mechanism works by humidifying the air at the inlet of dry air through a hydrogen gas pole. This design approach not only takes into account the individual components, but also integrates with systems such as cooling water flow and hydrogen circulation pump flow.

When a fuel cell operates at high temperatures, the air pole inlet humidity can be relatively low. In the vicinity of the catalyst inside the MEA?, the proton conductivity will gradually deteriorate, which in turn will increase the resistive overvoltage. Externally, the effective surface area of the catalyst decreases, deteriorating the fuel cell performance. The effective surface area of the catalyst is kept constant by increasing the functional groups of the encapsulated catalyst electrolyte. While improving proton conductivity, the surface area of the catalyst can be effectively increased even in low humidity environments by optimizing the electrolyte/carrier carbon ratio and solidifying the catalyst carrier carbon. This measure also enables optimization of the shape of the single-cell flow channel, which effectively suppresses drying tendencies at the air-pole inlet. In addition to the design process for the above components, the optimization of the system's own operating conditions enables stable operation of the single-cell power generation process even under high-temperature environments, thus minimizing the possibility of overvoltage (15, Fig. 16).

On the other hand, free radical concentration occurs as a result of power generation in fuel cells under low humidity conditions, leading to gradual deterioration of the electrolyte chemistry. At the same time, thin-filming causes a decrease in mechanical properties, which in turn leads to problems such as film cracking. The countermeasures taken by the researchers include the addition of free radical quenching materials to the electrode to reduce ferrous ion contamination, and the use of 3D fine-mesh flow channels to homogenize the pressure on the electrode surface to ensure its durability (Fig. 17).

3?Water management control of the fuel cell stack

To keep the power generation performance of the fuel cell stack in the optimal state, the researchers adjusted the operating conditions of the fuel cell based on the alternating current impedance (AC impedance) method and by measuring the resistance of the MEA components with an on-board device.

3.1?Water content measurement based on AC impedance method

Fig. 18 illustrates the equivalent circuit of a conventional fuel cell. In the figure, Rohm is the resistance of the electrolyte membrane, Rvoid is the resistance of the GDL, and Rion is the resistance of the electrolyte. These resistances vary with water content. When in a moderately wet state, the resistance of each part is kept low. During the cooling process, the diffusion resistance increases due to the large amount of liquid water inside the GDL, so the Rvoid value increases accordingly. On the contrary, in the state of low water content such as during high temperature operation, Rohm and Rion increase and resistive overvoltage is generated.

The DC command current of the fuel cell boost converter (FIG. 7) is measured by overlapping two sinusoidal current values of high frequency and low frequency, and Rohm is calculated from the impedance value (HFR) measured by overlapping high frequency sinusoidal current. On the other hand, Rvoid is calculated from LFR, Rohm and Rion.

3.2?Self-humidification control of the fuel cell stack

When the TFCS is operated at high temperature, the operating conditions of the hydrogen pole are changed for water management. In order to efficiently distribute water to the surface of the hydrogen pole, the amount of hydrogen circulation can be increased by controlling the hydrogen pump according to the relevant operating conditions. After securing the necessary amount of hydrogen circulation, the water on the surface of the hydrogen pole is continuously flowed by reducing the inlet pressure of the hydrogen pole. As a result of the above countermeasures, the environment near the catalyst is moist, and even without external humidification, the ambient temperature during system operation can be effectively increased (Fig. 19).

3.3 Water management control during high-temperature operation of the fuel cell

Water management is performed by controlling parameters such as the flow rate of the hydrogen pump and the water temperature of the fuel cell in the MIRAI model based on the impedance value obtained by the measurement method. FIG. 20 shows the evaluation results when the vehicle is driven at high speed on a steeper ramp while water management control is performed. FIG. 21, on the other hand, illustrates the evaluation results when the vehicle is driven at high speeds on a steeper ramp under conditions where water management control is not performed. Under the condition of water management control, the Rohm value is stable and the increase in cooling water temperature is suppressed, and the output power of the fuel cell stack can be obtained. On the other hand, under the condition without water management control, the impedance value varied greatly due to the influence of cooling water temperature, and the same output power could not be ensured. In this case, the cell characteristics of the fuel cell stack faced the same problem, i.e., the impedance value was high in the full current region and the specified voltage could not be output. This phenomenon can be considered as one of the reasons for the increase in resistance over voltage of the electrolyte membrane and other components (Fig. 22). In addition, the heating condition of the fuel cell stack gradually increases due to the voltage reduction, which in turn leads to an increase in the cooling water temperature. This result indicates that the water content of the electrolyte and electrolyte film has decreased, resulting in the fuel cell power generation characteristics facing further deterioration.

From the above analysis, it can be seen that the water management control can keep the electrolyte membrane and other components in a stable state and be wetted, and at the same time, improve the power generation characteristics of the fuel cell stack, and can effectively inhibit the increase of the cooling water temperature.

3.4 Water management control during startup at 0°C

The main problem faced by the fuel cell system during startup at 0°C is the freezing of the residual water inside the fuel cell system and the water generated during the power generation process, which prevents the timely supply of hydrogen and oxygen to the MEA for operation. The worst case scenario is that the fuel cell will not be able to generate electricity properly.

Fig. 23 shows a flowchart of the system control in a 0?°C environment. The concept of water management in a fuel cell system at 0?°C is to ensure that the gas supply system operates properly during startup. When the water is about to freeze, a "fast warm-up" control system is used to warm up the fuel cell system to above 0?°C. The system control flow chart shows the system control flow in a 0?°C environment.

3.5?Water content reduction control

The water content in the power generation area of the fuel cell stack can be calculated by measuring the impedance value, and the water content in the GDL can be managed by utilizing the Rvoid. The water content reduction control controls parameters such as cooling water temperature, air flow rate, hydrogen circulation volume, etc. during operation and when the system is stopped, and adjusts the impedance value appropriately so that the fuel cell can achieve a smooth startup without facing problems due to gas diffusion even when the startup is performed in an environment of less than 0?℃ (Fig. 24).

3.6?Rapid Warm-up Control

When the temperature of the fuel cell stack is below 0?°C, the power generation characteristics are lower than in normal operation. At the same time, the fuel cell stack is unable to achieve continuous power generation due to the gradual freezing of the generated water (Fig. 25). Therefore, when the temperature at cold start is below 0?°C, it is necessary to keep the temperature of the fuel cell stack above 0?°C in order to be able to continue power generation.

When a fuel cell stack generates electricity, heat generation occurs simultaneously with various types of energy losses. When the fuel cell stack is in normal operation, the heat generation should be minimized and the fuel cell stack should be operated efficiently. In order to realize the rapid warming of the fuel cell stack, the amount of air required for the reaction process should be reduced, and then the concentration overvoltage should be gradually increased (Fig. 26).

Fig. 27 illustrates rapid warm-up control in a -15?°C temperature environment. According to the actual vehicle evaluation results at a fuel cell temperature of -15?°C, the fuel cell stack can generate power from 8?s after system calibration. Since a certain output power has to be maintained on the one hand, and the voltage has to be slowly reduced on the other hand to increase the heat generation of the fuel cell stack, the output power of the fuel cell is finally controlled to be 5~90?kW. In addition, it has been confirmed that the fuel cell stack can be warmed up to more than 0?℃ in about 32?s.

The fuel cell stack can be warmed up to more than 0?℃ in about 32?s.

4 Conclusion

This paper focuses on water management, which is a core technology of fuel cell systems. Using visualization and measurement technology, quantitative processing is realized, and the technology is effectively applied to the design and system control process of fuel cell stacks. Water management is a key technology for fuel cell stacks, and we will explain the operation mechanism of fuel cell stacks based on the relevant principles in order to promote the miniaturization, low cost, and performance improvement of fuel cell stack systems.

Note: This article was published in the Journal of Automotive and New Power, Vol. 3, No. 3, 2020

Author: [Japanese]? Imaishi Kaiyuki, etc.

Finishing: Peng Huimin

Edited by Wusait

This article comes from the author of Automotive House Car, and does not represent the viewpoint position of Automotive House.