The design of off-line activation and performance recovery methods has become a compulsory course that affects the performance, cost and life of fuel cells. At present, what are the mainstream practical performance recovery methods and how Toyota performs fuel cell performance recovery. This paper shares the mainstream fuel cell performance recovery methods at home and abroad.

According to the national standardGB/T 38914-2020 "Vehicle Proton Exchange Membrane Fuel Cell Stack Service Life Test Evaluation Method" definition, service life representationCumulative service time from the start of use to the time when the volt-ampere characteristic decays to the specified minimum. In addition, the standard proposes that the evaluation criteria for the service life of the vehicle fuel cell stack is from the beginning of the volt-ampere curve.(BOL) to the final volt-ampere curve (EOL), inFuel cell at reference currentaverage voltage per sectionAttenuation10%. Previously, the U.S. Department of Energy(DOE) Technical Indicators for 2020 Fuel Cell Development also provide for vehicle fuel cell life, I .e., based on steady-state polarization curves after durability testing.Rated powerAttenuation does not exceed10%.

FigureScreenshot of the national standard "Vehicle Proton Exchange Membrane Fuel Cell Stack Service Life Test Evaluation Method"

According to reports, Toyota in the fuel cell development plan for the second generation.Mirai is set to face a life span of no more than 15 years.8.8%@ 2.2A/cm2 and7.7%@ 0.2A/cm2 voltage decay target. Toyota's official website shows that the fuel cell stack guarantee time of the second-generation Mirai fuel cell vehicle is the earlier of the new car registration 5 years and the endurance 100000 kilometers.In addition to material-related, life and use conditions, recovery methods are also related..The performance loss of fuel cells has both recoverable and irreversible losses.. As the name implies, reversible loss means that the performance loss can be eliminated by means of recovery to return to the state before the loss, mainly.catalyst surface oxide,CO、SO3^-,SO2adsorption of other impuritiescause, usually throughlow potential reductionRemoval of catalyst surface impurities. Irreversible loss mainly referscaused by degradation of materials such as catalysts and supportsloss of voltage. Due to the permanent change or damage of the material, the material transmission channel including water, gas and electricity is blocked, and the performance recovery cannot be achieved.

FigureReversible and irreversible losses in fuel cells

Usually, performance recovery or activation methods are used after the stack is assembled, tested and used.For example, the first activation is carried out after the reactor production line, and the activation is carried out after long-term use and storage.Since these methods are essentially unified and mutually applicable, they are collectively referred to herein as performance recovery.. Japan's New Energy Industry Technology Comprehensive Development Plan(NEDO) In the target plan for mass production of fuel cell stacks in 2035, it is proposed that the target offline activation time for fuel cell stacks in 2025 is20Mins, 2030 target offline activation time is10Mins, 2035 to achieve the target offline activation time is0MINS. It can be seen that the method design of performance recovery shows a pivotal role in the mass production and use of the reactor, which is closely related to life, performance and cost.

This paper shares the current mainstream fuel cell performance recovery methods at home and abroad.01soaking method

In the process of fuel cell testing and operation, it is inevitable to produce various types of attenuation, including reversible attenuation and irreversible attenuation. in facingIn the 2020 fuel cell project plan, the U.S. Department of Energy (DOE) proposed a performance recovery method for the soaking method, which can be used inWithout any external electronic loadunder the circumstances,Immersion through nitrogen, air and hydrogenThe catalyst surface oxide is reduced to recover the reversible loss. During the recovery process, the intake humidity of both the cathode and the anode is maintained.100%, and the back pressure was maintained at an absolute pressure of 150 kPa. If the recovery operation is performed before the laboratory test, the battery temperature during the recovery process can be set to the temperature of other test conditions in the next step.

02Load Control

Load control refers to the load control of the fuel cell through external professional electronic equipment, mainly including.Control currentandControl voltageTwo kinds. At present, the method used more in fuel cell stack enterprises is.Load Cyclemethod, I .e. by controlling the current(or voltage method) The voltage is cycled between a high potential and a low potential many times. When the cathode reaches the oxidation potential, the catalyst forms an oxide film. When the cathode reaches the reduction potential, the oxide film is reduced and removed, and the potential cycle is realized.catalyst oxidation/Reduction and impurity cleaningto achieve the effect of performance recovery.

Figure The principle of high and low potential cycle recovery of fuel cell(Toyota)

A typical case of this approach is Toyota, whose second generationThe Mirai fuel cell stack adopts the load cycle method to achieve performance recovery during the production line and long-term use, and the recovery conditions are as follows. According to the data, by using the load cycle method, the recovery time of the second band Mirai fuel cell stack is lower than that of the first generation.70%.

Figure Second GenerationMirai fuel cell stack recovery method

The performance recovery method of load cycling also appears in the protocols proposed by other authorities, such as the European Joint Research Center(JRC) and DOE. Among them, in 2015 JRC proposed to make fuel cells in0.45Vand0.75VLoad cycle condition of voltage cycle, as shown in the figure below;2009DOE is proposing to make fuel cells inOCVOpen circuit and0.55VThe condition of the voltage cycle.

Figure JRC Recovery Method

03EmptyLow voltage operation can effectively reduce the oxide on the catalyst surface to achieve performance recovery.However, under the condition of high metering ratio of traditional methods, low voltage is generally accompanied by higher current density, resulting in greater electrical power and thermal power, which puts forward high requirements for load and heat dissipation.. Therefore, by reducing the air flow rate and lowering the voltage to the reduction potential, the generated power and the heat generation power can be simultaneously reduced. In addition, hydrogen protons will recombine at the cathode to produce hydrogen under under-empty recovery, further creating a reducing atmosphere, which can achieve a multiplier effect.

Figure Implementation Method of Underempty Low Voltage(take 0.4V as an example)

Care must be taken to ensure adequate hydrogen supply during underemptying recovery,Eliminate hydrogen deficiencyphenomenon. The common practice of underemptying recovery is to reduce the air flow to a specified lower level while the current remains constant, and then increase the air flow again to restore the voltage to the normal level. This process can be repeated multiple times. According to public reports, using the heat generation effect of under-empty operation, Toyota has been in two generations.Mirai passenger cars have been successfully applied to the low temperature start process. At present, a large number of patents in Hyundai and Toyota have appeared in the use of under-empty to achieve performance recovery entries, the method is expected to be further substantive applications in the future.

Figure Principle of cathode hydrogen production under under empty condition

04electrochemical hydrogen pumpThe electrochemical hydrogen pump method uses an external power supply to decompose the hydrogen into the anode into protons, which are transferred to the cathode and then reduced to hydrogen.The cathode is usually filled with inert gas or maintains the inlet closed and the outlet open. Similar to the undervoid recovery method, the nature of the electrochemical hydrogen pump recovery method isReducing atmosphere created by cathode hydrogen pump. After the electrochemical hydrogen pump is restored, the fuel cell is usually purged with an inert gas such as nitrogen to avoid the generation of a hydrogen void interface. The recovery effect of the electrochemical hydrogen pump is better, which can achieve a uniform recovery effect between the cells and the battery power generation plane, and the hydrogen consumption is less. The disadvantage of this method is the need to use an additional external power supply, so the general.More use in the laboratory.

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