10.2. Zero Emission Vehicles
The concept of zero-emission vehicles is typically attributed to the transportation options that do not result in any harmful emissions during vehicle operation. Harmful emissions are defined as those known to have a negative impact on the environment or human health. They can include carbon dioxide, carbon monoxide, nitrogen and sulfur oxides, ozone, various hydrocarbons, volatile organic compounds (VOC), heavy metals in volatile forms (e.g., lead, mercury, etc.), and particulate matter.
Typical examples of zero-emission vehicles are electric (battery-powered) cars, electric trains, hydrogen-fueled vehicles, and human / animal powered transportation (e.g., bicycles, velomobiles, carriages, etc.). The battery technology for electric vehicles is based on charge/discharge cycles, meaning that the battery is charged beforehand using an electricity source and is discharged during vehicle operation. Because electricity production may involve some emissions, there is also a concept of well-to-wheel emissions, which includes not only operating emissions, but also those associated with the fuel source and other stages of the vehicle operating cycle. So, the "zero-emission" term is conditional in that sense.
The hydrogen-fueled vehicles are typically based on fuel cell technology, which imply electrochemical conversion of the fuel energy into electricity (as opposed to combustion). As a result, the only emissions of fuel cell operation are water and heat, which are not classified as harmful and therefore allow placing the fuel cell transport vehicles in the zero-emission category. The same as electric vehicles, fuel cell vehicles shift the emissions to the stage of fuel production. Thus, manufacturing of hydrogen gas via reforming of natural gas results in CO2 emissions, which must be taken into account in the life cycle assessment.
However, there is a possibility of designing a sustainable zero-emission lifecycle for electric and hydrogen vehicles, if electricity for recharging the batteries is supplied from renewable sources such as wind, solar, hydro-power converters, and the hydrogen to power fuel cells is produced via electrolysis or other emission-free technologies.
The energy conversion technologies that support the electric vehicles rely heavily on special chemistry and materials necessary to facilitate the efficient charge transfer processes. Understanding the components and principle of those technologies is important to foresee potential barriers on the way to their wide implementation and commercialization. The following learning materials will provide you with the basic knowledge on how the battery and fuel cell systems work.
Li-ion battery technology for cars
A schematic representation of a generic Li-ion battery is given in Figure 10.1. Roughly, Li-ion cell consists of three layers: electrode 1 (cathode) plate (usually lithium cobalt oxide), electrode 2 (anode) plate (usually carbon), and a separator. The electrodes inside the battery are submerged in an electrolyte, which provides for Li+ ion transfer between the anode and cathode. The electrolyte is typically a lithium salt in an organic solvent.
During the charging process, a DC current is used to withdraw Li+ ions from the cathode and to partially oxidize the cathode compound:
LiCoO2 → Li1-xCoO2 + xLi+ + xe-
The released Li+ ions migrate through electrolyte towards the anode, where they become absorbed in the porous carbon structure:
xLi+ + xe- + C6 → xLiC6
At the same time, electrons travel through the external circuit (electrolyte is not electron conductive).
During the battery discharge, the reverse process takes place. Li+ ions spontaneously return to the cathode, where electrochemical reduction occurs.
Please watch this short video for an animated illustration of the Li-ion battery principle:
If we compare the energy densities of the typical rechargeable Li-ion battery (~ 0.875 MJ/kg weight) and regular gasoline fuel (~46 MJ/kg), we can see that the gasoline beat battery electricity in potential to deliver power at least by a factor of 50. Thinking that typical engine is normally used at ~50% capacity, to match the capabilities of the internal combustion engine, the Li-ion battery has to be made at least 20 times more efficient, or the size of the on-board battery should be increased 20 times, which is a prohibitive option.
Limitations of the Li-ion batteries are rooted in the material properties.
For example, the LiCoO2 ⇔ Li1-xCoO2 conversion is only reversible with x<0.5, which limits the depth of the charge-discharge cycle. But, with a wider variety of materials available, research is underway to develop new generations of Li-ion batteries.
For example, take a look at Sigma Aldrich website, which lists multiple alternatives for cathode, anode, electrolyte, and solvents.
|Relatively high energy density and potential of finding even better formulations||Circuit protection needed to avoid damaging high voltage / current|
|No need for priming - new battery is ready to operate||Aging - battery gradually loses its capacity even if not in use|
|Low self-discharge (compared to other types of batteries)||Toxic chemicals are subject to regulations|
|Low maintenance||High cost of materials and manufacturing process|
|Capability to generate high current / power||Technology is not fully mature; varying components and chemicals|
Supplemental Reading on Li-ion Battery Technology:
- Goodenough, J.B. and Park, K.S., The Li-Ion Rechargeable Battery: A Perspective, J. Am. Chem. Soc., 2013, 135 (4), pp 1167–1176.
- Etacheri, V., Marom, R., Elazari, R., Salitra, G., and Aurbach, D., Challenges in the development of advanced Li-ion batteries: a review, Energy & Environmental Science, 2011 (9), 3243-3262.
Fuel Cell Technology for Cars
Fuel cell is similar to battery in the electrochemical principle of energy conversion, but different in operational design. Instead of storing the reagents and products of chemical reactions inside, like batteries do, fuel cells operate on continuous inflows/outflows of reagents and products. In that sense, they are not limited by discharge time and can generate electricity non-stop as long as fuel is supplied. Hydrogen is the best-proven fuel for fuel cells, although its storage and supply imposes some constraints on this technology.
A schematic representation of a hydrogen/oxygen fuel cell is given in Figure 10.2. The main components of the fuel cell include: membrane electrode assembly, which consists of a proton-exchange membrane and electrodes (anode and cathode) attached to the membrane on each side, gas diffusion layers, bipolar plates, and supporting structure. The fuel cell electrodes contain dispersed catalyst particles (usually platinum), which are necessary to promote electrochemical reaction.
A hydrogen-powered fuel cell combines hydrogen with oxygen in the electrochemical reaction to produce water and electricity. In case of direct contact of these gases, the reaction H2 + ½ O2 = H2O is very active and generate significant amount energy (under certain conditions – explosion). In a fuel cell, hydrogen is separated from oxygen by a proton conductive membrane, so, in order to react, it is forced to transform into ionic form by losing electrons:
H2 -> 2H+ + 2e- - this reaction occurs on the cell anode.
Further, the formed hydrogen ions (protons) are transferred through the proton-exchange membrane, while electrons are transferred through the external circuit, where they can be harvested as electric current. Once reaching the cathode, protons (H+) react with oxygen molecules consuming electrons from circuit and producing water:
2H+ + ½O2 + 2e- -> H2O - this reaction occurs on the cell cathode.
As long as the supply of reagents, hydrogen and oxygen gases is maintained, the process continuously generates electric energy and water.
Please watch the animated illustration of this process in the following video:
The productivity of this simple process, i.e., how much electricity a single fuel cell can produce, is limited by a few factors. First is the proton conductivity of the membrane. The membrane consists of a special polymer (for example, sulfonated tetrafluoroethylene, Nafion®) which performs as an ionic conductor only under specially controlled temperature and humidity regime. This and other polymers produced for such applications are quite expensive. Second, the platinum (Pt) catalyst is necessary to provide sufficiently fast kinetics of the electrochemical reactions. Platinum is a noble metal, which has high cost and limited availability.
When it works, the fuel cell process is very efficient (80-90% efficiency) and can generate electricity pollution free and with no mechanical degradation to the cell components.
|No recharging required, so the power can be generated away from electricity sources||Costly components, especially platinum catalysts|
|Hydrogen-fueled fuel cells do not pollute: the only exhaust is water||High sensitivity to temperature (slow start-up when cold, degrade when hot)|
|Compact cell size and possibility of stacking to fit applications of various scale||High sensitivity to impurities in fuel; catalyst is easily poisoned|
|High efficiency even at low power levels||Hydrogen supply infrastructure is not developed|
|No noise||On-board hydrogen storage is a challenge|
|Low toxicity (compared to batteries)|
For quite a while, battery- and fuel-cell-operated cars were parallel track for future implementation of electric automotive engines, and the advancement of one or the other depended on breakthroughs in materials and device efficiency.
To overview the current status and trends in these technologies, please refer to the following reading.
Book: National Research Council. Transitions to Alternative Vehicles and Fuels. Washington, DC: The National Academies Press, 2013. Sections 2.5 and 2.6. (See E-Reserves in Canvas.)
Please read Section 2.5 to learn about the status and promise of the battery-powered vehicles.
Please read Section 2.6, "Hydrogen Fuel Cell Electric Vehicles to learn about the status and promise of the hydrogen engines for cars.
Based on the above reading, try to shape your opinion on the following question: Which type of electric vehicles in your opinion may have a better future – fuel cell or battery? Find specific arguments, pros and cons, to support it. In this lesson activity, you will be asked to perform an investigation to compare these technologies based on some common metrics. For more details, see Summary and Activity section.