Structure models and nano energy system design for proton exchange membrane fuel cells in electric energy vehicles

Structure models and nano energy system design for proton exchange membrane fuel cells in electric energy vehicles

Renewable and Sustainable Energy Reviews 67 (2017) 160–172 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journa...

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Renewable and Sustainable Energy Reviews 67 (2017) 160–172

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Structure models and nano energy system design for proton exchange membrane fuel cells in electric energy vehicles Yong Li a,c,n, Jie Yang b, Jian Song c a Key Laboratory of Dynamics and Control of Flight Vehicle, Ministry of Education, School of Aerospace Engineering, Beijing Institute of Technology, Beijing 100081, PR China b E & M School, Beihang University, Beijing 100191, PR China c State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 10 June 2015 Received in revised form 16 May 2016 Accepted 9 September 2016

Electric vehicles require fuel cells with a highly specific energy for the purpose of environmental protection and energy saving. However, proton exchange membrane vehicle fuel cells (PEMFC) face problems in terms of energy conversion efficiency, power density, costs and lifespan. This paper reviews key technical issues regarding the application of vehicle PEMFC especially the integration of nano-electrocatalytic energy system with high-performance electrolyte membranes. It also discusses the relation between vehicle PEMFC membrane structures and electrode performance revealing the nanostructured system model and the membrane electrode interface characterization. Manipulation of vehicle PEMFC electrode structure and quantitative characterization of the nanoscale catalyst interface are summarized aiming at improving Pt utilization efficiency, ionic conductivity and nano membrane electrode performance. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Fuel cell electric vehicles Proton exchange membrane fuel cells Structure models Energy conversion Energy system design Vehicle fuel cells function design

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Nanofilms and electrode interface models of vehicle PEMFC . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Nano energy system design of durable catalyst with lower content of Pt . . . . . . . . . . . . . . . . 4. Design, construction and tunnel control of proton exchange membrane . . . . . . . . . . . . . . . . . 5. Controllable construction and interface evolution of membrane electrode and catalyst layer 6. Model, interface properties and nano energy system design of proton exchange membrane. 7. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The use of traditional energy resources faces a number of problems like fluctuating oil prices, turmoil in oil-producing regions and depleting oil reserves. If the world fails to find practical alternative energy resources, the future development of society will be greatly limited. There are many kinds of alternative energy

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sources such as hydro, wind, tidal, geothermal, biomass, solar and hydrogen (fuel cell) [1,2], and these renewable energies are able to recycle. Since electric vehicles aim to protect the environment, hydrogen with zero emissions has become an important emerging energy. Applications of hydrogen include direct combustion and fuel cells. In terms of pollution control, efficiency and applicability, the fuel cell will become an important energy option for the future

n Corresponding author at: Key Laboratory of Dynamics and Control of Flight Vehicle, Ministry of Education, School of Aerospace Engineering, Beijing Institute of Technology, Beijing 100081, PR China. E-mail address: [email protected] (Y. Li).

http://dx.doi.org/10.1016/j.rser.2016.09.030 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

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[2,3]. The fuel cell of electric vehicles combines fuel with cells to produce a reliable energy system, which has a continuous supply of fuel generating power continuously, with no environmental pollution. Additionally, electric vehicles have greater energy efficiency and operating stability than traditional internal combustion engines. Proton exchange membrane fuel cells (PEMFC), as a focus of new energy vehicles, provide the world’s industrial development with an excellent opportunity for energy transformation [3,4]. Compared with phosphoric acid, molten carbonate and solid oxide fuel cell, PEMFC features include a long lifespan, light weight, small size, high specific power, low operating temperature and a simple design [4,5]. Its working temperature is 70–100 °C [6], and the working efficiency is up to 0.45–0.55 [7,8], which is particularly suitable to serve as an electric vehicle power[9]. Therefore, the market potential is attracting the world’s attention, and there is a wide application prospect. Researchers around the world are competing for PEMFC power systems and PEMFC stack, and in the next ten years, the world is expected to have one million fuel cell vehicles in operation [10]. PEMFC is a new energy system without combustion that directly turns chemical energy from fuel into electrical energy through an electrochemical reaction. PEMFC has positive and negative poles separated by electrolytes [11]. Inside a vehicle PEMFC, hydrogen in the fuel and oxygen in the oxidant react respectively at positive and negative poles to produce water while generating the current as shown in Fig. 1 [12]. When vehicle PEMFC works,

Fig. 1. PEMFC structure and diagram [12].

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hydrogen is supplied to the anode and air to the cathode. At the negative electrode, hydrogen is decomposed into H þ and an electron. H þ enters the electrolyte, and the electrons move towards the positive pole through the external circuit. At the cathode, oxygen in the air and hydrogen ions in the electrolyte absorb electrons arriving at the positive electrode to form water. When the supply of fuel and oxidizer is continuous, PEMFC can continuously generate electricity [13,14]. Hydrogen is the fuel for vehicle PEMFC, and oxygen obtained from the air is the oxidant for PEMFC. Hydrogen with a high electrochemical reactivity can be derived from oil, natural gas and methane [15,16]. PEMFC stack uses polymer as its electrolyte membrane allowing H þ to pass through to reach the cathode where it is reduced [17]. The core structure of vehicle PEMFC consists of a membrane, electrode, catalyst layer, gas diffusion layer and bipolar plate. The membrane is a polymer film for proton conduction composed of two surfaces, which are the catalytic layers of positive and negative electrodes. This means that between the negative and the positive electrode, there is a polymer proton exchange membrane. There is also a gas diffusion layer between the electrode, catalyst layer and the bipolar plate [18]. The proton exchange membrane is able to transfer hydrogen ion and isolate the gas. Appropriate moisture is helpful for hydrogen ion conduction, but too much water will stay in the negative electrode and affect the oxygen delivery as shown in Fig. 2 [19]. However, inadequate water management will make the film too dry increasing the impedance for hydrogen ions and deteriorating the proton conductivity. The review shows that thermal management, water management and PEMFC stack models significantly improve the performance of PEMFCs, thus supporting longer mileage and service life, while providing new ideas for the PEMFC stack design of renewable energy vehicles. Compared with other power PEMFC, PEMFC stack are advantageous in addressing the requirements raised by PEMFC electric vehicles, such as, long mileage, high-current charging, and safety. Therefore, PEMFCs are reliable and feasible for the deployment in electric energy vehicles. To address the demand for vehicles using PEMFC energy with high-performance electrodes, this paper discusses nano energy system and the structural design for PEMFC electrodes while giving special attention to three characteristics: thermal management, water management and PEMFC stack. The structural management model, PEMFC stack mechanism and the construction process of stack electrodes of PEMFC technology are introduced. By analyzing heat and water transmission and characterizing the stack structure, energy management system, surface and interface property at the different scales and levels, this paper reveals the intrinsic link between how an energy management system is structured and performs. It combines the management system model with the application of high-performance stack electrodes

Fig. 2. Proton exchange membrane structures with nanoscale dynamic interface [19].

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to highlight the compatibility between nanoscale and macroscale. By addressing progress in the applicability of renewable energy and the sustainability of energy technology during recent years, this paper provides scientific and experimental support for the practicality of PEMFC energy. The rest of the paper is organized as follows. Section 2 introduces the nano film and electrode models used in PEMFC; Section 3 outlines the main methods for designing durable catalyst with less Pt or non-Pt; Section 4 briefs on the construction and tunnel control of membrane; Section 5 discusses the interface evolution process which is valuable for construction control; Section 6 is focusing on the interface properties and nano energy system design, including density functional theory and comparison of Pt-based PEMFC, non-Pt PEMFC and PEMFC stack. Section 7 makes the conclusion.

2. Nanofilms and electrode interface models of vehicle PEMFC PEMFC is a complex system involving polymers, membrane materials, thermodynamics, electrochemistry, interface and nanoenergy. Due to the continuous operation of the vehicle PEMFC, if heat generated cannot be timely released, its internal temperature will gradually rise lowering the strength, efficiency and output voltage of vehicle PEMFC [20,21]. Therefore, attention should be paid to the polymer film thermodynamics and interfacial effect, which is the core of PEMFC technology. Between the positive and negative electrodes is the polymer proton exchange membrane, through which H þ move to the negative electrode to react and create H2O [22–24]. Under heavy load of electric vehicles, internal current density increases, electrochemical reaction strengthens and more water is generated. At this point, if water is not drained, the negative pole will be flooded, and the normal electrochemical reaction is destroyed resulting in PEMFC failure as shown in Fig. 3 [9]. Therefore, humidity regulation inside PEMFC and drainage control at the anode is the key to the development of high-power and high-performance the vehicle PEMFC system. Currently, PEMFC poles use Pt, and the catalyst layer is based on the nano PtC core-shell structure as shown in Fig. 4 [9]. The catalyst layer is where the electrochemical reaction takes place. In order to accelerate the reaction, the catalyst layer needs to have a larger area. Reducing the Pt particle size is generally a method to increase the reaction area. But if the resultant water is not removed quickly from the anode, the reaction area will be flooded and reduced [25,26]. Although the Pt catalyst layer, with superior performance, can be used in electric vehicles, its reserves are scarce and production is difficult, which forms barriers for commercialization of PEMFC electric vehicles [27,28]. Dramatically reducing the Pt

Fig. 3. Nano-membrane water generation mechanism of PEMFC [9].

Fig. 4. Membrane interface and Pt-C nano-catalyst core-shell structure of PEMFC [9].

content in nanocomposite catalyst while maintaining high catalyst activity, selectivity and long life has become the core of catalyst research and film design for vehicle PEMFC. The vehicle PEMFC electrodes are porous nanomaterials made of carbon nanotubes or graphene, which supports the proton exchange membrane and catalyst layer while strengthening its mechanical and thermodynamic stability [29,30]. However, the structure of a highly active Pt catalyst is too complicated, which makes it too expensive. For a practical vehicle PEMFC power, design and characterization of a catalyst with less Pt is critical [31]. Research [32–34] has been conducted on catalysts, electrode interface thermodynamics and kinetic energy. Chan et al. [35] studied the nano-catalyst, electrolyte membrane and anode control as shown in Fig. 5. They analyzed the compatibility and interaction between vehicle PEMFC materials, discussed the relationship between the interface structure and catalytic properties and regarded the core-shell structure as an ideal solution [32–35]. Therefore, on the basis of significant demand for vehicle PEMFC [36,37], PEMFC polymer nanofilms, catalyst and interface effects [36–38], we will review the design and characterization of lowcost nano catalyst, the formation mechanism of selective nano channel and the transport efficiency of ions and electrons.

3. Nano energy system design of durable catalyst with lower content of Pt Important issues for vehicle PEMFC film and interface include: how to choose the right core metal for the catalytic layer [39], the reaction of Pt atoms on the surface [40], the change of the electronic structure [41–43], maintain catalytic activity and easily obtain the structure [44,45]. Feng et al. [46] designed a core-shell structure, as shown in Fig. 6, and characterized the impact of sublayers of metal on the surface Pt atoms and the chemical properties of polymer film. They obtained the relation between catalyst composition and properties. Geim et al. [47] investigated the impact of non-Pt core on the Pt shell regarding surface properties and the behavior of intermediates (H, CO, O) as shown in Fig. 7. They characterized the catalyst activity and stability, reduced the Pt usage to 0.2 of current level and operated for more than 2000 h smoothly. Yamauchi et al. [48] used Pt-Co electrode model and differential electrochemical mass spectrometry to discover the difference between the ideal surface (model catalyst) and the real surface (nano catalyst). They examined the methanol electro-oxidation reaction and oxygen

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Fig. 5. PEMFC regulatory model [35] (a) anode film and protons (b) nanowire regulation model (c) anode structure construction (d) ions and electrons transmission.

Fig. 6. Core-shell structure design and characterization [42] (a) characterization (b) model.

Fig. 7. Design, modification and characterization of durable catalyst nanostructure [47].

reduction reaction and made catalyst with Pt core and Co shell as shown in Fig. 8. They also established the relationship between structure and property and evaluated the activity and stability of the catalytic structure. In order to develop a fuel cell energy system that includes highly dispersed, stable and active catalysts, both the interaction between the non-Pt catalyst and a unique structure for energy storage are necessary. Geim et al. [47] optimized fuel cell performance through designing a successful non-Pt catalyst nanostructure and surface property. This design changed the composite structure of non-Pt catalyst through doping and modification, resulting in rapid charging under a large current. Additionally, their design reduces the use of Pt as a catalyst for the fuel cell electrode, which is another topic that is worthy of further study. Under the principles of non-Pt catalyst doping, surface modification, and porous nanostructure, Yamauchi et al. [48] designed a layered non-Pt catalyst model with high specific surface to examine the impact of specific capacitance, specific power, and cycle performance. They integrated a porous non-Pt catalyst electrode with a current collector to create an energy storage system with composite structures. This is an innovative design for the concept of energy storage and for the combination of non-Pt catalyst and PEMFC. The heterogeneous multilayer structure is perfect for the physical and chemical properties of non-Pt catalyst. The work of these scientists prompts the need for further study of vehicle fuel cell technology with high energy density.

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Fig. 8. Design and characterization of Co-Pt bimetallic catalyst nanostructure [47].

4. Design, construction and tunnel control of proton exchange membrane Currently, the proton exchange membrane is designed to transfer ions and isolate the reactants. However, it is difficult to completely isolate the reactant molecules (particularly methanol) resulting in a “hybrid” potential effect [48] that significantly lowers energy conversion efficiency. The key to solving this problem is to regulate the nanochannels of the membrane [49], which results in a membrane that promotes ion transmission and blocks methanol [50]. Feng et al. [51] used nano-porous Ru shell to match with Pt core as shown in Fig. 9. Their results showed that the highly selective channels influence the cell life and costs directly. Therefore, core-shell nanostructure construction and interface functions are vital technologies [52]. By optimizing the design of the Pt-Ru coreshell structure, they significantly reduced the usage of Pt [53] and obtained a selective membrane for ion transport [54]. Kwon et al. [55] doped inorganic nano-ion conductor in the electrolyte membrane to form nano channel in the microstructures, as shown in Fig. 10. By ways of interfacial polymerization, electrostatic adsorption and spatial construction, membrane electrode interface can be regulated. Compared with

Nafion, the cost of new film is reduced by 32%, performance improved by 30% and the ionic conductivity increased by 150%. The lifespan is more than 3600 h. This idea is a unique contribution to the field in the sense that it analyzes non-Pt catalyst electrodes from the perspectives of PEMFC stack, which combines a novel structure with the theory of energy mechanics. This work is therefore very useful and has the potential to advance the use of electric vehicle around the globe.

5. Controllable construction and interface evolution of membrane electrode and catalyst layer Abruna et al. [56] proposed the idea to design Pt-Co nanostructured membrane electrode and catalyst layer. It established the characterization model for the relationship between structure and properties. They used electrodeposition, self-assembly and template technology, as shown in Fig. 11. They matched the structure with the ion transport behavior to obtain the hydrophobic gradient distribution. Kulikovsky et al. [57] calculated the reaction heat for the oxygen reduction and methanol oxidation, examined the impact of Pt-based core-shell nanostructures on

Fig. 9. Porous Pt-Ru core-shell structure design and characterization of selective nano-Ru tunnel [51].

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Fig. 10. Octahedral and tetrahedral ion channel of proton exchange membrane and electrode [55].

Fig. 11. Structure, interface evolution and characterization of membrane electrode and catalyst layer [56].

performance and built theoretical prediction models before and after aging, as shown in Fig. 12. They used quantum chemistry and molecular dynamics simulations in analyzing the optimized components. By constructing nanoporous membrane diffusion layer, the relationship between porosity and electronic conductivity were obtained. Selman [58] established the constitutive relation in microscopic dynamics for oxygen reduction and methanol oxidation, revealed the influence of nanoporous membrane structure on proton transfer and fluid retention and built core-shell structure. By interfacial polymerization and electrostatic self-assembly, they clarified the mechanism of ion transfer channels and obtained novel membrane, as shown in Fig. 13. The permeability of the novel membrane for methanol is lower than Nafion membrane, and the catalyst activity is 5 times higher than conventional Pt-C catalyst. Feng et al. [59] studied the adsorption properties and thermodynamic properties of graphene-based Pt, proposed a high-

performance proton transport membrane model and established a method of fabricating graphene-based polymer. Their new membrane has a methanol permeation coefficient 3–6 times lower than Nafion membrane. They assessed different working conditions for electrocatalyst with low content of Pt, as shown in Fig. 14. This technology is helpful to manufacture electrocatalyst with high activity and stability. It addresses the control problem of the interface membrane.

6. Model, interface properties and nano energy system design of proton exchange membrane The catalyst systems in practical application are often too complex and difficult to observe the detailed structure of the catalytic interface with existing characterization techniques. It hinders the study of mechanism and structure-activity

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Fig. 12. Nanoporous model of Pt core-shell structure before and after aging [57].

Fig. 13. Technical ideas for membranes with selective nanochannels for ion and proton transport [58].

Fig. 14. Graphene-based Pt adsorption and selective nanochannel [59].

relationship [60–62]. Catalytic performance of metal catalysts are often closely related to the composition, size and morphology of metals as well as the nature of carrier and additives [63,64]. Performance optimization of the catalyst depends on the thorough

understanding of how these parameters affect the catalytic properties and the effective control of these parameters [65–67]. The present studies use an ideal single crystal surface as a model for characterization with different techniques. It promotes the development of vehicle PEMFC technology [68–70]. However, there is a gap between interface science and the real catalyst in terms of pressure and materials [71–73], which has been pushing new ideas in polymer interface [74–76]. With this context, more studies began to try the nanocatalysts with high specific surface area. It aims at understanding the complex catalytic mechanism [77,78]. For precious metal oxide nanoparticle catalyst, there is interface synergy between metal and oxide, but it is difficult to characterize the fine structure of the interface [79]. PEMFC has a high-energy conversion efficiency and environmental friendliness, which is ideal for electric vehicles [80,81]. However, the commercialization of fuel cell vehicle must resolve the issue of high cost of Pt-C catalyst. Researchers are using in situ spectroscopy [82], X-ray diffraction [83], fluorescence analysis [84], electron microscopy [85] and spectroscopy techniques [86], in combination with theoretical simulation of graphene membrane electrode to try to reveal the mechanism of changes in the interface and the interface evolution [87,88]. Under working conditions, some researchers investigated the catalytic activity for grapheme-Pt [89,90], while others used onsite alternative current impedance technique [91,92] for study of

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the internal resistance of the electrode [92,93] and Warburg impedance [94,95] and explored the factors for electrode stability [96,97]. By simulating the real applications, assessment of vehicle PEMFC performance was conducted for more than 5,000 h [98,99]. For metal catalysts with metal-oxide interface synergy [100–102], it is found that different metal oxide has different performance [103,104]. Yang et al. [105] designed the hollow frame structure, in which the molecules can reach the catalyst surface from three directions, improving Pt utilization, as shown in Fig. 15. They investigated the influence of membrane pore and aggregation on proton transfer and built a controllable structure. Using molecular dynamics simulation, they obtained the behavior of ordered molecules, electron and ion transport. Zheng et al. [106] prepared nano-catalysts and used high-resolution transmission electron microscopy, synchrotron radiation X-ray absorption spectroscopy and high-sensitivity low energy ion scattering spectroscopy to study the Pt-FeNi(OH)x interface synergistic mechanism for the oxidation of CO. They further developed the synthetic method for Pt-based catalyst, extending catalyst reactivity interface from one dimension to three dimensions, as shown in Fig. 16. The ratio of active Pt atoms against the total number of Pt atoms is up to 50%. The new catalyst achieves the oxidation of CO at room temperature, but also catalyzes the selective oxidation of CO in hydrogenrich condition as well as the elimination of H2 in oxygen-rich condition. The lifespan is up to two months. Zheng et al. [106] built the Fe3 þ -OH-Pt interface and parsed the fine structure, which significantly improves the catalytic activity. They found OH is the active substance for CO oxidation. They observed the introduction of Ni2 þ could help solve the dehydration of interface. Zheng et al. [106] found that Ni2 þ and Fe3 þ together form a stable structure. It greatly improved the life of the catalyst, as shown in Fig. 17. The new nano-catalysts significantly improve the utilization of Pt and reducing the cost. At room temperature, CO is converted 100%. They fabricated a core-shell [email protected] model, which is smaller in Pt particle size and similar to industrial Pt catalyst performance [107], can be directly tested under real conditions [108]. Sub-monolayer characteristics of FeOH are suitable for spectroscopic characterization [108,109]. The work of Zheng et al. showed how nanoscience promotes catalytic interface

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research. Composite nanoparticles with controllable structure and composition can be used to study the catalyst interface effects [108–110]. It is applicable in studying other catalytic systems and interface effects [110–112]. It will contribute to the development of cheaper and practical PEMFC electric vehicles. Kim et al. [23] used density functional theory (DFT) to study the impact of structure, interface and morphology of non-Pt catalyst on the stack's performance. Their work aims to develop and apply different scale evolution technology to the construction of non-Pt electrodes. Under different electrode models, they studied the relationship between structures and the transmission of heat and water. To examine the relationship between pores and electrical conductivity and resistance, they used different non-Pt catalyst to fabricate multiple nano-scale porous diffusion layers in an electrode. At the same time, they designed a three-dimensional adjustable network to alter electrode structure, which consisted of orientation, arrangement, and observation of the performance in

Fig. 16. Schematic of development from one-dimensional to three-dimensional catalyst [106].

Fig. 15. Pt-Ni hollow frame structure and nanoscale characterization [105].

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Fig. 17. Structure and characterization of Pt-FeNi(OH)x nanoparticles catalyst [106]: (a) core-shell structure and interleaving structure, (b) HAADF-STEM images, ((c)- (d)) HAADF-STEM images before and after aging, (e) STEM-EDS images of Pt, Fe, Ni, Fe-Ni and Pt-Fe-Ni nanoparticles.

an electrode. Their research of non-Pt catalyst’s impact on heat and water are good for the development of orderly structure and function control for electrodes. Experiments that were conducted throughout this process also revealed the evolution of structure and interface through characterization technologies on different scale. Kim et al. [21] used DFT to test the performance of nanoribbons under non-equilibrium state with high temperature, large current and found the scale effect has changed the dislocation mechanism fundamentally. The non-Pt catalyst constitutive theory cannot characterize the strain gradient effect for pores. Macroscale model and stress/strain distribution are obtainable through DFT analysis. DFT technology can track the evolution of crystal structure and defects of non-Pt catalyst under fast charging. Meanwhile, there is another idea to solve problem of the film nano-structure and catalytic activity. We can find the ideal material to replace Pt-C catalytic structure. For example, graphene is an ideal substitute material option. Scholars [107–109] have tried to explore how to enhance the catalytic activity of oxygen reduction reaction (ORR) of nitrogen doped graphene material to replace the existing Pt-C catalyst. Wei et al. [110] made significant progress in the development of low-cost vehicle PEMFC catalyst and used

nitrogen doped graphene (NG) materials. Through the association of “NG molecular structure - NG conductivity - ORR catalytic activity,” they acquired three NG materials with high stability and good electrical conductivity. Thus, the material has excellent ORR catalytic activity, as shown in Fig. 18. They used layered material (LM), interposed the graphene and pyridine monomer by polymerization and pyrolysis and got NG material with nitrogen at more than 90%. ORR activity of the NG material was 53 times that of traditional methods [111] and is possible to replace Pt-C catalyst in a vehicle PEMFC engine [112]. LM layer is flat and closed space, overcoming not only the problem of poor conductivity of traditional open system and low activity [111–113], but also the low synthesis efficiency and high cost [112–114]. The study implies that catalyst cost of vehicle PEMFC may be solved [115–120]. Academia shall grasp the advantages of existing technology [120–125] and development opportunities to meet the practical challenges [126–130]. On one hand, continue to accelerate the pace of vehicle fuel cell laboratory research [131–135] and advanced nano-interface technology [136–140]. On the other hand, special attention should be paid to reducing vehicle fuel cell cost with practical technologies [141–149].

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Fig. 18. Layered structure of graphene nano-scale model and catalyst synthesis [110].

Table 1 Characteristics of different PEMFC technologies. PEMFC technologies

Life (cycle)

Energy density (Wh/L)

Self-discharge rate (%/month)

References

non-Pt catalyst PEMFC Pt catalyst PEMFC PEMFC stack

2100  2300

300  450

7  12

[78,80,97,102,119]

2600 2900 500  650

59

[72,82,85,105,121]

1100  1400

11  14

[56,64,71,108,127]

to limit the diffusion of free radicals inside an electrode so as to improve its durability. Non-Pt catalyst is a cost-effective option for PEMFCs, and related researches in this field have developed quickly. The integration between a nano energy system and PEMFCs will benefit the commercialization of non-Pt catalyst electrodes.

7. Conclusions and outlook 350  550

An advantage of PEMFC is that it has no memory effect. Therefore its capacity will not be affected if the cell is recharged without first discharging completely. However, the PEMFC might be damaged for over recharging [78,80,97,102,119]. In contrast with PEMFC using non-Pt catalyst, the PEMFC using Pt catalyst have a lower self-discharge rate, longer life and higher volumetric specific energy. At given output power, the weight and size of nonPt catalyst PEMFC are smaller than conventional Pt catalyst PEMFC [72,82,85,105,121]. The self-discharge rate of non-Pt catalyst PEMFC is higher than 8%. After being idle for one to two weeks, the PEMFC can work normally. The performance and safety of the battery mainly depends on the design of electrodes. Table 1 is a comparison of Pt catalyst PEMFC, non-Pt catalyst PEMFC, and PEMFC stack in terms of different parameters, which show that Ptbased PEMFCs are advantageous regarding energy density, life span, and self-discharge rate. However, PEMFC also faces the problems of cell balancing and consistency, because a PEMFC stack is composed of many individual cells. The theoretical life span of a PEMFC stack is basically decided by the shortest life span of a single cell. Only when individual PEMFCs perform consistently and evenly can the stack’s life approach that of a single cell. Typically, a single PEMFC can be used for more than 2600 times, but a PEMFC stack can only be used for around 1100–1400 times [56,64,71,108,127]. With their rapid development in the past decade, powered PEMFCs have gained preponderance in the market of renewable energy vehicles. They have also become increasingly competitive for renewable energy applications. Although the research and studies of PEMFC has achieved significant progress, there are still obstacles for large-scale application in most vehicles. The major obstacle is the durability or long term stability. If such problem can be solved according to various application requirements, the emphasis for future research will be changed. For example, the structure of cells can be designed to take full advantage of the nano-scale effects to protect non-Pt catalyst from degradation. Atomic doping method can be utilized to prevent polarization, which is helpful to improve the stability of PEMFC electrodes. In addition, crosslink reaction can be deployed

As a focus of new energy vehicles, PEMFC provides the world’s industrial development with an excellent opportunity for energy transformation. Compared with phosphoric acid, molten carbonate and solid oxide fuel cell, PEMFC features with a long lifespan, light weight, small size, high specific power, low operating temperature and a simple design. Constantly revealing the relation between vehicle PEMFC nanomaterials structure and power is helpful to achieve high performance nano-catalyst and controllable electrolyte membrane with new designs. Furthermore, carry out functional studies for vehicle PEMFC nanomaterials (such as high durability) as well as the development of practical nanoelectro-catalyst. For example, fabricate catalyst with Pt surface and non-Pt core reduces the usage of Pt. For future study, academia is trying to introduce self-assembly, surface reduction, segregation and adsorption-induced annealing method into vehicle PEMFC in order to obtain microstructure with maximum density of catalyst and high stability. In reviewing the development path of vehicle PEMFC, there are many experiences and lessons worth learning. To accelerate the pace of vehicle PEMFC research, we should focus on the catalytic activity, improve power generation efficiency, enhance environmental performance, prolong life and reduce costs. The design and synthesis of different nanocomposite catalysts, film and the electrode is vital to explore patterns of polymer state and catalyst gradient distribution. Use X-ray diffraction, electron microscopy and spectroscopy techniques to characterize membrane, electrode interface and structure morphology. Continuously combine theoretical research to develop interface diagnostic methods and techniques. Explore the impact of nanostructures on the transfer and transmission of electrons and ions will lay the foundation for PEMFC electric vehicle with high performance and long life.

Acknowledgment This work was supported by the National Natural Science Foundation of China (10972037) and the Science Fund of State Key Laboratory of Automotive Safety and Energy (KF11071).

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