Currently, energy shortages and ecological protection are gaining widespread attention in various countries, and the massive use of fossil fuels is further worsening environmental problems. Among various energy sources and technologies to replace fossil fuels, hydrogen energy and fuel cells are considered promising solutions to achieve zero-pollution emissions [1]. The automobile industry is an important industry in many countries, and automobiles are also a necessity in people’s daily lives. At present, traditional fuel vehicles still occupy a large share of the market, which will produce many air pollutants and greenhouse gases in the driving process. Replacing fossil fuels as power sources for vehicles with clean energy sources such as hydrogen and electricity can greatly reduce pollutant and greenhouse gas emissions [2].

The fuel cells discussed  in this paper are proton exchange membrane fuel cells (PEMFCs), which use hydrogen energy as the energy source to generate electricity. The PEMFC directly converts the chemical energy contained in hydrogen into electricity, heat, and water [3]. The fuel cell suffers from a slow dynamic response [4] and is difficult to adapt to complex driving conditions [5]. The chemical reaction of the hydrogen in the fuel cell which supplies electrical energy is often smaller than the rate of change of the load. At the same time, rapid acceleration and deceleration and frequent start-stop operations during driving will affect the durability of the fuel cell. Based on these characteristics, fuel cells are often used in hybrid energy storage systems with other energy sources, such as batteries and ultracapacitors, for power applications ^[6][7][6,7]. Additionally, fuel cells are widely used in hybrid power systems with other energy sources. It is useful to reduce the consumption of hydrogen, reduce the size of fuel cells and increase the economy of hybrid power systems ^[8][9][8,9]. Fuel cell-based hybrid systems are widely used not only in fuel cell hybrid vehicles but also in other transportation equipment, such as unmanned aerial vehicles (UAVs) and trams [10]. This shows that hydrogen energy is playing an increasingly important role in the transportation industry.

Fuel cell hybrid vehicles usually use fuel cells as the main power source and are equipped with batteries or ultracapacitors as auxiliary energy sources. The working conditions of automobiles driving on the road are very complex. They often face various emergencies, and the required power demand will also have large fluctuations and sudden changes. However, if only fuel cells are used as the energy source, the output of large fluctuations in power can reduce the life of the fuel cell [11]. Therefore, the role of the auxiliary energy source is necessary. Batteries and ultracapacitors can play good roles in auxiliary energy sources. Batteries can recover excess energy and provide power to the system simultaneously with fuel cells when the load demand power is high. The ultracapacitor has the characteristics of a fast dynamic response, fast energy recovery, and high specific power, which can play the role of a timely response in the face of rapid changes in load demand [12]. At present, there are three main system structures of fuel cell hybrid vehicles. The first type is a hybrid system composed of fuel cells and batteries. The second is a hybrid system composed of fuel cells and ultracapacitors. The final type is a hybrid system composed of fuel cells, batteries, and ultracapacitors. The power system structure has been studied and analyzed for different types of fuel cell hybrid vehicles [13].

EMSs play a significant role in the performance and efficiency of fuel cell hybrid vehicles [14]. Its main objective is distributing power between different energy sources while achieving two goals: first, reducing hydrogen consumption or minimizing equivalent energy consumption [15]; and second, extending fuel cell life, which also means increasing the economy of the hybrid system [16]. A large number of energy management strategies are focused on these two optimization goals. The first type of EMSs is the rule-based method. This usually requires obtaining the power map of the fuel cell to obtain the highest efficiency operating point. It can also adjust the power distribution of the fuel cell and the energy storage system (battery or ultracapacitor), according to the power system state. However, it has many disadvantages, such as the parameters that are affected by the test operating conditions, lacking adaptability to different operating conditions, and the control results are not optimal. Optimization-based energy management strategies are some of the most studied types and can be divided into two categories: online optimization strategies; and offline optimization strategies. Among them, the real-time optimal energy management strategy based on model predictive control (MPC) has been widely discussed in the past two years. The third category are the learning-based energy management strategies. Its basic idea is to use large data sets of real-time and historical information to train the parameters of the strategy to obtain optimal control ^[17][17]. An energy management strategy based on intelligent vehicle interconnection technology has also been recently proposed by researchers and is discussed in this paper. The classification of common energy management strategies for fuel cell hybrids is shown in Figure 1. Regardless of the type of energy management strategy, the core of the optimization is represented by two aspects: optimizing energy consumption; and extending the life of fuel cells and other components [18].