On January 2, 2020, the Ministry of Industry and Information Technology issued specifications for the comprehensive utilization of used batteries, proposing to establish a comprehensive traceability management platform and conduct full-process information collection on a series of processes including battery production, use, scrapping, recycling, and utilization. At the same time, the new national standard for waste battery recycling requires battery manufacturers to "sell one and collect one", and the battery industry is ushering in a huge change.

　　Battery industry experts believe that the reshuffle of the power battery industry will accelerate, and a complete improvement in the industry will have to wait until the explosive consumption of new energy vehicles occurs in the later period. In addition, some leading battery companies have been making frequent moves recently and are exploring new application areas such as electric ships.

　　Optimal control of hydrogen concentration in the fuel cell anode is one of the key performance parameters that affects the efficiency and durability of fuel cell electric vehicles. Installing a hydrogen concentration sensor is a common practice for hydrogen concentration management in the anode of fuel cell systems. However, the current hydrogen concentration sensor chip has not yet overcome the impact of moisture and the technology is not mature enough. The use of hydrogen concentration sensors is extremely limited. Therefore, it is necessary to develop a new hydrogen concentration estimator to maintain the hydrogen concentration in the fuel cell hydrogen system pipeline (including the stack anode chamber) at a certain level. This article shares Hyundai Motor Company's NEXO fuel cell anode hydrogen concentration estimator and estimator-based purge controller.

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　　Vehicle fuel cell systems usually include fuel cell stacks, air supply systems, hydrogen supply systems, and thermal management systems. To improve fuel cell efficiency and durability, material breakthroughs and technology updates are essential. Among them, the most effective but dangerous technology is the optimization of anode hydrogen concentration. Due to the limitations of hydrogen concentration sensors for fuel cell vehicles in terms of accuracy, life, cost, reliability and other factors, in order to maintain the hydrogen concentration at a certain level during fuel cell operation, the widely used method is the equivalent Q value (EQV) method. That is, the output current that changes with time in the fuel cell is multiplied and integrated by the weighting factor, as shown in the following formula.

　　Hyundai NEXO fuel cell system architecture

　　When the equivalent Q value is higher than the target level, the anode performs a purge operation to increase the hydrogen concentration. However, this method is an indirect method to maintain the hydrogen concentration within a certain range, and is greatly affected by environmental conditions and driving conditions. To deal with these uncertainties, Hyundai Motor Company has done a lot of work to revise the weighting factors in the equivalent Q-value method. Therefore, to obtain the true anode hydrogen concentration that is closest to what is measured by the hydrogen sensor, Hyundai developed the Hydrogen Concentration Estimator (HCE) based on thermodynamics, fluid dynamics, mathematical modeling, and sensors within the fuel cell system. In addition, based on the hydrogen concentration estimator, Hyundai Motor Company designed and developed anode voltage regulation controller and purge controller to achieve optimal control operation of hydrogen concentration, and conducted a large number of experiments on fuel cell vehicles.

　　Hydrogen concentration control design concept

　　The usage rate of hydrogen from on-board hydrogen storage bottles is mainly divided into three types, one is the available usage rate, and the other two are unavailable usage rates. First, the available usage rate is the hydrogen usage (more than 90%) used by the fuel cell to generate electricity to drive the vehicle forward and operate the system-related actuators, defined as the amount of hydrogen required to generate electricity/total hydrogen amount.

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　　The first type of hydrogen unavailability is related to the diffusion of hydrogen across the proton membrane to the cathode (hydrogen infiltration), where the cathode hydrogen flows with air to the air outlet manifold. Theoretically, the amount of hydrogen permeation is proportional to the difference in hydrogen partial pressure between the cathode and anode. The greater the pressure difference, the greater the amount of hydrogen permeation. The nitrogen that diffuses from the cathode to the anode across the proton membrane will be recirculated through the hydrogen supply system pipeline (such as a hydrogen circulation pump), and will evolve over time, causing the hydrogen concentration in the hydrogen system pipeline to decrease. Therefore, the accumulated nitrogen in the hydrogen pipeline should be removed. Discharge to the air outlet manifold to increase the anode hydrogen concentration, an operation called purging. But during purge, hydrogen will also be discharged together with nitrogen and water vapor, which is the second source of unavailable hydrogen usage. In order to improve efficiency, the above two unavailable hydrogen usage rates need to be minimized, and it is not advisable to maintain the hydrogen concentration (hydrogen partial pressure) in the hydrogen supply system pipeline at an ultra-high level. However, the large power generation command triggered by the driver will result in the inability to supply enough hydrogen (the partial pressure of hydrogen is low), thereby permanently damaging the fuel cell electrodes. Therefore, it is particularly necessary to maintain a higher hydrogen concentration range to prevent damage, but at the same time, hydrogen penetration will also increase, resulting in reduced efficiency. There are compromises and trade-offs between efficiency and durability. To achieve both high efficiency and high durability of fuel cells, the hydrogen concentration should be optimally controlled, which requires embedding a real-time hydrogen concentration estimator in the hydrogen supply system.

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