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.
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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:
How do Lithium-ion batteries work? (9:29)
Transcript: How do Lithium-ion batteries work? (9:29)
Exploring Lithium-Ion Batteries: How they work, Recharge, and Degrade by: Branch Education
It's crazy every second you use your smartphone, there's a chemical reaction,
like a baking soda volcano happening inside of it.
It looks like a solid device without many moving parts, but its true!
Inside the battery there's chemical a reaction that is continuously running and without it,
your phone would just be dead, which is something we’re all familiar with.
Let’s investigate this lithium-ion battery. How does it power your smartphone,
what happens when you recharge it, and probably what we’re all wonder
Why does your battery die earlier and earlier in the day?
To answer these questions, let's open up this battery and look inside.
How does your battery power your smartphone
So first, how does your battery power your smartphone? Let’s start from what we know.
All batteries have a positive terminal and a negative terminal
and supply power or electricity to our portable devices.
So, Electricity is essentially a flow of electrons and in our smartphone.
Electrons which are negatively charged flow from the negative terminal
and run things like the speakers or the display and then end up at the positive terminal.
So then, where does this flow of electrons come from?
Well, this is a lithium ion battery, so the electrons come from the element lithium.
At the negative terminal, which is technically called the anode,
lithium is stored between layers of carbon graphite, similar to the graphite in your pencil.
Graphite has a nifty crystal structure of layered planes
that allows for the lithium to be wedged in between each of the layers.
The technical term for this is intercalation.
Graphite functions as kind-of-like a stable storage space for lithium atoms.
Ok- Moving on, one inherent property of the element lithium is that it doesn’t like it’s outer-most electron,
and it wants to give it up.
When there is an available path from the negative terminal to the positive terminal,
this electron separates from the lithium and starts heading to the other side.
At the same time, the lithium leaves the graphite,
and becomes positively or +1 charged and is now called a lithium ion.
FYI- an ion is just fancy word for an atom
who has lost or gained an electron, and thus is charged.
When a lot of lithium atoms leave the graphite at the same time,
a flow of electrons is built up.
So, let's now jump to the positive terminal, which is technically called the cathode.
Here we have Cobalt that has lost some electrons to oxygen,
thus making the Cobalt positive, or +4 charged.
As a result, it wants to gain back an electron.
So, when we connect the negative and positive terminals to our smartphone,
the electrons flow from the lithium which wants to give up an electron,
through the circuits and components in the smartphone
and to the cobalt which wants to gain an electron.
Now here we run into a small issue.
With the flow of electrons from the negative to the positive terminal,
the cobalt side grows more and more negatively charged,
and the other side positively charged.
Yes, the electrons do want to flow in this direction,
but at the same time electrons don’t like to flow to an area
that is growing more and more negatively charged.
This is because opposite charges attract, and similar charges repel.
So, to fix this, we give the now positively charged lithium ions
that recently left the graphite, a path to move to the other side.
This path is called an electrolyte, and its function allows for lithium-ions
to migrate over from one side to the other, while not allowing the electrons to move through it.
When lithium gets to the cobalt side,
it again wedges itself, or intercalates with the cobalt and oxygen to become Lithium Cobalt Oxide.
The lithium isn't regaining its electron-
that electron went to the cobalt, it's just balancing out the charge build up.
Recap
Let's quickly recap. Here is a full battery.
Throughout the day lithium atoms leave the graphite layers
and separate from their electrons to become lithium ions.
The electrons flow from the negative terminal through the circuits and components in the smartphone
and into the positive terminal to join the cobalt atoms.
At the same time, the lithium-ions travel
through the electrolyte in order to neutralize the charge build up and keep the reaction going.
Here's the chemical formula for the reaction.
Thus, at the end of the day almost all of the lithium has left the graphite layers,
and joined the cobalt to become lithium cobalt oxide, and your battery is now running on empty.
Recharging
Now that the battery is empty, let's recharge it.
We plug in our smartphone and when we do this the USB charger
applies a higher force on a flow of electrons in the opposite direction.
Electrons are pulled out of the cobalt,
thus returning cobalt to its +4 state and kicking out the lithium ions.
On the other side, electrons are forced onto the graphite,
which pulls the lithium through the electrolyte, and back into the layers of graphite.
As you see it’s the exact opposite of the earlier reaction, which is why this battery is rechargeable.
The lithium and its electrons move in one direction when you use the phone,
and the opposite when you charge it back up.
Ok, so now let's rewind and add a few more details of note.
First, these two sides can’t touch, If the anode and cathode were to touch,
and if there were any lithium left in the graphite,
the chemical reaction would accelerate uncontrollably and cause a fire or often a small explosion.
Thus, a non-conductive semipermeable separator
that allows the lithium-ions to pass through is placed in the middle.
And this electrolyte isn't an effective barrier because it's a liquid
The second thing to note is that the graphite and cobalt peroxide
aren’t good at collecting or distributing the electrons.
Thus, a conductive copper layer is added next to the graphite,
and a conductive aluminum layer next to the cobalt peroxide.
These two layers or sheets are called collectors.
Ok, onto third, these animations are showing 100% of the lithium moving from
the anode to the cathode, and back.
But in reality, there will always be some percentage
of lithium that remains in the anode, cathode, and electrolyte
despite the battery being fully charged or discharged respectively.
Continuing to fourth, in order to maximize the capacity of the battery,
and allow the battery to fit into your smartphone,
all these layers are folded and wrapped into a rectangular prism package.
Ugh, I know this is a lot, but fifth and final,
in order to regulate the flow of electricity,
additional circuitry is added to the top of the battery.
This circuitry prevents overcharging and damage to the battery.
So, then the final topic.
Why does your battery's max capacity reduce over time?
There are several reasons,
one of which is that sometimes lithium and the incoming electron
react with electrotye and organic solvent to form
compounds that are called a solid electrolyte interphase or SEI
SEI's irreversibly consume lithium and the electrolyte,
thus reducing the overall quantity of lithium and
thereby reducing the max capacity of your battery.
Another reason is that when you fully discharge your battery until it’s dead,
it can result in too much lithium on the cobalt side,
which causes the irreversible generation of Lithium oxide and Cobalt (II) Oxide.
These compounds are stuck in that state which thereby reduces the amount
lithium and cobalt for future use.
So, one tip is to not let your battery run until its empty.
It’s better to recharge your battery at
30 or 40% then to let it run until its dead.
That about wraps it up.
When it comes to batteries,
there are hundreds of different chemistries and compounds that allow them to work,
but they all work on similar principles.
You just need three materials,
one that wants electrons,
one that wants to give up electrons,
and then a path for the build up of charge to neutralize.
Thanks for watching! Here are 3 questions I’m going to leave you with.
Discuss them in the comments. Also, ask questions in the comments!
If you do it I will pin the top questions for further discussion.
Don’t forget to subscribe and tell your
friends and family about something you learned today.
This episode is about lithium ion smartphone batteries,
and it branches to electric vehicle batteries
discussed by Learn Engineering, we recommend you take a look!
It also connects to galvanic and voltaic cells,
Chemical bonds & electronegativity
and lemon batteries.
Post your comments with further questions, answers, and thoughts.
And Remember conceptual simplicity and structural complexity.
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.
| Advantages | Limitations |
|---|---|
| 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 a 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:
Honda's video guide to Hydrogen fuel cell technology in cars (3:31)
Transcript: Honda's video guide to Hydrogen fuel cell technology in cars (3:31)
Fuel cell cars, which run on electricity produced from compressed hydrogen, emit zero harmful emissions and could be the future of motoring. They can be just as fast, practical, and can travel as far as a conventional petrol or diesel engine car. But the technology is very different. And importantly, the only thing that comes out of the exhaust is water vapor.
Rather than having a petrol or diesel tank like a conventional car, the fuel cell car has a tank that stores compressed hydrogen as a gas, hydrogen is used as an energy carrier so that the fuel cell car can produce its own electricity on board rather than storing it in batteries.
This compressed hydrogen is expanded and then fed into the fuel cell stack. The fuel cell stack is like a tiny electric power station.
Inside it, the hydrogen combines with oxygen from the air to generate electricity and water as a byproduct.
Water vapor is the fuel cell car's only emission. The electricity created inside the fuel cell stack is used to power the electric motor, which is in turn used to drive the
[Music]
car. The fuel cell stack is made up of hundreds of individual cells stacked together like a loaf of bread. In fact, each cell is like a sandwich with a membrane electrode assembly or MEA between two separators or bipolar plates.
The MEA is made up of a proton exchange membrane or PEM which sits between hydrogen and oxygen electrode layers and gas diffusion layers.
In each cell, hydrogen gas passes over the hydrogen electrode. Each hydrogen atom is converted into a hydrogen ion in a catalytic reaction releasing an electron in the process.
The hydrogen ions then pass through the electrolytic membrane where they bond with oxygen ions straight from the atmosphere. The previously emitted electrons from the hydrogen molecules arrive at the oxygen electrode via an external circuit.
The released electrons create a flow of direct electrical current in the external circuit and water is generated at the oxygen electrode as a byproduct.
This water is drained from the system and exits the car as water vapor via the exhaust. Because the electricity is generated from hydrogen and oxygen, no carbon dioxide or other pollutants are emitted from the car. It's the ultimate in clean performance.
Honda's FCX Clarity is the world's first production fuel cell car and is already on sale in the US and Japan.
[Music]
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.
| Advantages | Limitations |
|---|---|
| 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.
Reading Assignment:
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.