ArticlesMaterials towards carbon-free, emission-free and oil-free mobility: hydrogen fuel-cell vehicles—now and in the future
Abstract
In the past, material innovation has changed society through new material-induced technologies, adding a new value to society. In the present world, engineers and scientists are expected to invent new materials to solve the global problem of climate change. For the transport sector, the challenge for material engineers is to change the oil-based world into a sustainable world. After witnessing the recent high oil price and its adverse impact on the global economy, it is time to accelerate our efforts towards this change.
Industries are tackling global energy issues such as oil and CO2, as well as local environmental problems, such as NOx and particulate matter. Hydrogen is the most promising candidate to provide carbon-free, emission-free and oil-free mobility. As such, engineers are working very hard to bring this technology into the real society. This paper describes recent progress of vehicle technologies, as well as hydrogen-storage technologies to extend the cruise range and ensure the easiness of refuelling and requesting material scientists to collaborate with industry to fight against global warming.
1. Introduction
Mobility is one of the most basic desires of a human being. Even in the prehistoric age, there are many evidences of people travelling all over the world. Once the society was exposed to an affordable and easy means of mobility, the ‘automobile’, the idea of personal and quick mobility was enthusiastically accepted all over the world. The growth of automobile population is still very high. Typically, once a person attains a certain economic threshold, the desire to acquire a personal means of mobility becomes very strong. It is estimated that the vehicle population will be doubled within the next one or two decades (figure 1). However, since the late twentieth century, we have faced negative aspects of this very attractive mobility
— local pollution, | |||||
— accidents, | |||||
— global warming (CO2), and | |||||
— energy shortage. | |||||
Figure 1. Material innovation and society.
Local pollution and accidents are now well tackled owing to technological developments, such as catalysts and airbags, although improvement is still necessary in some areas. But global warming (CO2) and energy shortage are now becoming more serious and confronted globally.
The energy demand of mobility favours only one side: ‘oil’. Although energy diversification is inevitable, escaping from the oil-based world, for the automobile, is not easy owing to a high energy density of petrol or diesel oil. Nonetheless, we are forced to re-realize the uncertainty of oil price and its influence on the world economy, as observed during 2007–2008.
The automobile industry has been working hard to introduce ‘non’-oil-based automobiles, but has not yet been widely successful because of the strong advantage of oil-based fuel and the cost differences that exist between those so-called alternative fuel technologies and conventional oil-burning technologies. However, recent progress of electric propulsion systems, led by the hybrid electric car highlights two kinds of pure electric cars as a near-future alternative automobile—the battery electric vehicle (BEV) and the fuel-cell vehicle (FCV).
2. What is the future of mobility?
What should future mobility be like? This needs to be discussed because future transport requires increased/new infrastructures and further material research, resulting in a huge cost to the society in terms of money and resources.
‘Efficiency’ (energy/man-km) is a commonly used term in discussions pertaining to future transport, but in addition, the ‘quality of mobility’ (QOM) should be discussed more for the future quality of life. The reason behind ‘why people spend relatively large amounts of money to purchase an automobile’ needs to be considered and is thought to be as follows. QOM is not easily quantifiable, but to make the discussion simple, QOM is defined as ‘the time to reach a destination from home’. Figure 2 shows an image where a comparison is drawn between public transport and a private passenger car based on typical daily activities (commute, shopping, meeting someone). In many cities, the commuting time is shorter by car, except during certain peak hours when use of the car is less advantageous owing to traffic congestion and difficulty in parking. On the other hand, the efficiency of the public transport is heavily dependent on traffic density (i.e. number of people on board). As a result, the efficiency of the public transport drops in the countryside owing to under-capacity operation, and so does the QOM, owing to the low frequency of the service.
Figure 2. Quality of mobility and car.
There is a need to discuss ways to improve mobility, but the superiority of private cars cannot change in many cities and countries. This aspect needs to be considered when cities redesign future traffic. However, as previously described, the negative impact of this mobility must be overcome, otherwise we would be forced to sacrifice the QOM. At least, the energy efficiency must be improved by improving the fuel economy (alternative fuel or new power trains such as fuel cells) and by the improvement of traffic management in cities to avoid traffic jams and congestion, in turn preventing the deterioration of efficiency and QOM. An ideal scenario of future mobility would be one where, on the one hand, public transport and private cars help each other in the city by sharing the road with optimum efficiency and QOM, while on the other hand, the usage of private cars becomes the main mobility tool in the countryside.
3. Comparison of two pure electric vehicles
BEVs and FCVs share almost the same components, such as electric motors and power controllers or inverters, to control the speed of the vehicle. However, big differences remain in the main energy source. While BEVs uses the energy stored in the battery, FCVs use the electricity generated by the fuel cell, with the primary energy stored in the hydrogen tank.
Although similar in components, characteristics of the BEV and the FCV are quite different owing to the difference in energy density of the battery and the hydrogen tank (figure 3). The BEV’s cruising range is restricted because of the small quantity of energy stored in the battery.
Figure 3. Energy density.
On the other hand, the FCV has a longer cruising range because of its high energy density and the easiness of refuelling. Battery-technology development has focused on increasing the energy density, but there still remains a very big gap between the battery pack and the hydrogen tank.
The automobile industry is of the view that there is a clear difference of use between the two technologies and hence both can coexist (figure 4). While the BEV is suitable for short and small mobilities, the FCV is for larger or long-range vehicles (US Department of Energy 2008).
Figure 4. Positioning of the BEV and FCV. EV, electric vehicle; FCHV, fuel-cell hybrid vehicle; PHV, plug-in hybrid vehicle.
4. Fuel-cell vehicles; carbon-free, emission-free and oil-free vehicles and its progress
(a) Recent progress of fuel-cell vehicles
In order to accomplish FCV commercialization, five major issues (figure 5) should be solved, as stated previously on many occasions. At present, the industry claims to have solved most of the technical issues and the main focus is on reducing the cost for commercialization.
Figure 5. Recent progress and remaining issues.
The Fuel Cell Commercialization Conference of Japan (an initiative bringing together the automobile industry and the energy industry) has announced that they are now targeting the start of FCV commercialization by 2015.
(b) Potential of hydrogen
Hydrogen FCVs do not emit CO2 or hazardous emissions during operation. However, hydrogen is the secondary energy carrier, and is at present mostly produced from carbon-based primary energy sources. So, the CO2 emission is solely owing to the hydrogen production process. Currently, industries are mass-producing hydrogen several ways. The oil industry is one of the biggest users of hydrogen where, on the one hand, hydrogen is produced by steam-reforming natural gas or oil residuals. On the other hand, hydrogen is also produced by electrolysis while producing chemical products, wherein hydrogen is a by-product. As such, it is difficult to calculate the efficiency in the latter case. A common view of hydrogen vision is shown in figure 6. It is economical to start producing hydrogen with the existing facilities that oil refineries currently have, and gradually increase production through either carbon capture and sequestration or renewable-energy production. This could be accelerated either based on the needs of society to reduce carbon or by new ways of hydrogen production from renewable sources.
Figure 6. Hydrogen vision. HV, hybrid vehicle; PHV, plug-in hybrid vehicle; EV, electric vehicle.
Figures 7 and 8 show the latest comparison of well-to-wheel efficiency from natural gas to vehicle propulsion energy.
Figure 7. Efficiency comparison of power train. FCHV, fuel-cell hybrid vehicle; EV, electric vehicle; HV, hybrid vehicle; ICE, internal combustion engine.
Figure 8. Comparison of CO2 reduction potential. FCHV, fuel-cell hybrid vehicle; HV, hybrid vehicle; EV electric vehicle.
Owing to the latest improvement in vehicle efficiency, FCVs consume the least amount of primary energy. This shows the advantage of FCVs, even if they use hydrogen from fossil fuels.
5. Materials to be prepared for mass introduction
(a) Materials innovations for fuel-cell stacks
The use of expensive platinum as a catalyst is commonly stated as one of the weakest points of fuel cells. However, recent progress in the development of platinum alloy and structured catalysts shows the potential to reduce the amount of platinum usage towards a level similar to the early years of the current three-way catalyst. But the understanding of the mechanism of catalyst deterioration and migration into the membrane surface has to be developed further before commercialization and mass usage.
In addition, catalysts other than platinum are the most required innovation we expect from scientists. Currently, carbon alloys or metal oxide catalysts are developed through laboratory research, but are not as active as platinum.
(b) Materials innovations for hydrogen storage
This has long been thought of as one of the biggest hurdles for hydrogen FCVs. It is still a difficult challenge, but the cruising range of certain FCVs has now exceeded conventional petrol vehicles with the development of 70 MPa high-pressure carbon fibre composite tanks, in addition to the improvements in fuel-cell efficiency. However, this is not a sustainable solution since the current high-pressure tank uses expensive carbon fibres in large quantities, thus making it expensive and not easy to produce in large numbers. We still need a cheap, safe and easily producible hydrogen-storage material and, yet again, we are expecting an innovative and intelligent breakthrough from the scientific community.
6. Hydrogen-storage technologies now and in the future
(a) State of the art
Figure 9 shows state-of-the-art hydrogen-storage technologies on the scale of gravimetric density and volumetric density, together with the target. There is a big gap between the target and the current status. In addition, many technologies are still at the laboratory level, because of which, only a few technologies, such as high-pressure tanks and liquid-hydrogen tank systems, have been tested in real vehicle conditions. Requirements for vehicle conditions are provided in the list in figure 10.
Figure 9. Current hydrogen storage. MOF, metal organic framework; ICE, internal combustion engine; BCC, body-centred cubic.
Figure 10. List of requirements.
‘Why is hydrogen storage difficult?’ This question is related to the energy required to conceal hydrogen in a small area. There are several ways to conceal hydrogen in small areas, but all of them fall either under physical or chemical ways. Current tank systems, both high pressure and liquid temperature, are based on physical ways of storing hydrogen. Physical ways have a big advantage as the energy required to extract hydrogen for use is very low. However, the tank itself is very expensive and complicated since the system has to maintain extreme pressure and temperature conditions, i.e. either 70 MPa or 20 K.
Other ways of storing hydrogen is by chemical bonding, wherein the hydrogen is stored in the lattices of host atoms. There are many challenges in this direction, but for chemical storage, the energy required to store hydrogen in a small area is small, but the energy needed to extract hydrogen for use (ΔH) is relatively large because it involves breaking the chemical bond in order to release hydrogen. Efforts to find a material with a low ΔH are ongoing (figure 11).
Figure 11. Schematic picture of hydrogen storage.
Liquid hydrogen is another good candidate as it has characteristically good gravimetric and volumetric density. In fact, in the industry, the gas delivery system commonly uses liquid hydrogen. However, energy required for liquefying hydrogen is much larger than in theory and is normally not recoverable. Furthermore, flash gas during the refuelling and boil off gas while the system is idling and not consuming gas does not make this technology very attractive.
(b) New development trends
Current development trends are shown in figure 12. The emphasis is on lowering the pressure for metal hydride tank systems and lowering the temperature for chemical hydride tank systems.
Figure 12. Directions of developments.
A new approach is extending the temperature range above the liquid hydrogen temperature and the room temperature.
Hydrogen absorbing alloy (metal hydride) and its improved hybrid tank system are also very attractive, but material cost and high-performance material development stall implementation (Darkrim et al. 1999; Lamari et al. 2000; Bénard & Chahine 2001; Gardiner et al. 2004; Kojima et al. 2005, 2006; Mori et al. 2005a,b,c; Bhatia & Myers 2006; Kabbour et al. 2006; Poirier et al. 2006; Furukawa et al. 2007; Liu et al. 2007; Vasiliev et al. 2007; Richard et al. 2009a,b).
7. Important message for material scientists
Tank weight and volume is a function of pressure, temperature, material performance and bulk density. Most of the hydrogen-storage material discussion is on the gravimetric and volumetric index. However, influences of the material characteristics other than these performances are ignored easily.
Figure 13 shows the influence of the physical property of the material into the tank system. This shows the effect of the different parameters on the hydrogen adsorption tank system size and weight.
Figure 13. Influence of physical property. LT, low temperature (or cryo temperature range around 77 K); RT, room temperature.
Pressure and bulk density, rather than the adsorption performance, are more influential factors for tank size and weight. An example is shown for the case of the adsorption system. There are other characteristics, and these are different from one system to another. It is important for both scientists and engineers to work together from the early stage of the material development to meet the required material target (Mori & Hirose 2009; Richard et al. 2009c).
8. Conclusion
In order to tackle the current problems, it is very urgent to bring new technologies such as the fuel cell into the real world as quickly and as much as possible. Engineers and scientists need to communicate with each other more often and deeply than ever to accelerate developing innovative materials in order to bring those technologies into real life.
— Future mobility must meet to improve energy efficiency, QOM and reduce emissions and global warming gas, thus giving a good QOM. | |||||
— FCVs have very high energy efficiency and retain good usability, such as range and short charging time, so that they reduce global warming gas emissions and oil usage. | |||||
— FCVs are now in the process of commercialization. Cost reduction is the key for this technology so that the material innovations (fuel stack and hydrogen storage) are essential for the mass introduction of this new technology. | |||||
— For the hydrogen-storage-material development, physical properties, such as bulk density, are very important for the size and weight of the tank system. | |||||
— Scientists and engineers must work closely to accelerate material development and bring new materials into commercialization, thus being effective for climate change. | |||||
Footnotes
One contribution of 13 to a Discussion Meeting Issue ‘Energy materials to combat climate change’.
