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Toyota details design of fuel cell system in Mirai; work on electrode catalysts

While other major automakers have either introduced (Hyundai, Honda) or are in serious development of new hydrogen fuel cell vehicles for the market, Toyota continues to take the point in not just promoting, but also supporting the broader technical (and infrastructure) development required for a large-scale realization of hydrogen-based electromobility.

At the 2015 CES, Toyota announced royalty-free use of approximately 5,680 fuel-cell-related patents held globally, including critical technologies developed for the Mirai fuel cell vehicle. (Earlier post.) At the SAE 2015 World Congress, Toyota presented a set of four technical papers detailing some of the technology innovations used in Mirai fuel cell stack. (Earlier post.) And again at this year’s 2016 SAE World Congress, Toyota presented three more papers: one detailing the development of Mirai’s Toyota Fuel Cell System (TCFS) and two dealing with the critical issues of the fuel cell catalysts.

The Toyota papers were part of the larger World Congress technical session on practical hydrogen fuel cell technology: PFL 720, Advances in Fuel Cell Vehicle Applications, chaired by Jesse Schneider of BMW.

Toyota Fuel Cell System (TFCS)

The 2015 papers provided technical details on the high performance fuel-cell (FC) stack; specific insights into FC separator, and stack manifold; Mirai’s newly developed boost converter; and the new high-pressure hydrogen storage system with innovative carbon fiber windings.

The 2016 paper by Hasegawa et al. describes the details of the larger Toyota Fuel Cell System (TFCS) from the standpoints of improved efficiency and reliability, as well as the simplification of the fuel cell (FC) system.

FC Boost Converter. Hybrid systems are a core technology that Toyota is adopting in every form of its next-generation vehicles; the Mirai uses the same motor, power control unit (PCU), and hybrid battery as existing Toyota hybrids. To enable this technology sharing in the Mirai, Toyota had to make a design advance from Toyota’s earlier Toyota FCHV-adv fuel cell vehicle, launched on a limited lease basis in 2008 (earlier post) with the introduction of an FC Boost Converter.

In the 2008 vehicle, the fuel cell stack and inverter were directly connected, using the same voltage; this required dedicated designs for both the traction motor and inverter to match the unique characteristics of the FC (i.e., low voltage and high current).

By contrast, Mirai’s FC boost converter adjusts the voltage difference between the motor and inverte, enabling the adoption of a traction motor and inverter already in mass production. It also eliminated the restrictions caused by the gap between the FC and traction motor voltages, allowing the number of layered cells to be optimized and reducing the size of the FC stack.

FC System Configuration. Compared to the FC system in the FCHV-adv, the FC system in the Mirai is simplified and more reliable. The most important modifications included achieving a humidifier-less system by eliminating the external humidifier; changing the type of the air compressor; consolidating the functions of the valves (the stack inlet shut valve and the flow diverter shut valve, as well as the stack outlet shut valve and the pressure adjustment valve); eliminating the hydrogen diluter, and reducing the number of hydrogen tanks from four to two.

Configuration of TFCS in the Mirai. Hasegawa et al. Click to enlarge.
In PEM fuel cells, the humidity of the electrolyte membrane must be controlled to ensure sufficient proton conductivity; this function is usually performed by an external humidifier.

To eliminate the external humidifier, Toyota migrated water generated at the cathode to the anode where it uniformly distributed the water onto the surface of the anode membrane electrode assembly (MEA). This was done by developing innovative technology for the stack structure as well as modifying the anode operating conditions.

For the stack, Toyota:

  1. Reduced the thickness of the electrolyte membrane, promoting the diffusion of the water in the air system.

  2. Humidified the system using moisture at the anode—this humidifies the cathode inlet by flowing H2 and air in counter directions.

  3. Reduced evaporation by increasing coolant flow at the cathode inlet.

At the anode, Toyota increased the amount of H2 circulation based on driving conditions. Reducing the anode inlet pressure after ensuring the required amount of circulation promotes the evaporation of moisture and enhances the movement of generated water onto the anode surfaces.

Fuel economy. Mirai has a lower hydrogen storage capacity than the Toyota FCHV-adv. To achieve a similar cruising range as a gasoline vehicle, the fuel economy of the Mirai was improved by 20% compared to the Toyota FCHV-adv.

Two of the main measures enabling this are the reduction in H2 permeability due to crossover, and the use of a Roots-type blower for the air compressor instead of the conventional scroll type.

H2 crossover refers to the permeation of hydrogen from the anode through the electrolyte membrane to the cathode, creating an H2 partial pressure gradient between the anode and cathode. The amount of crossover is proportional to the permeability coefficient of the electrolyte membrane.

Because Toyota reduced the thickness of the electrolyte membrane by

two-thirds to realize the external humidifier-less system, crossover increased substantially. To counter this, and to improve fue economy, Toyota changed the anode control by optimizing H2 partial pressure control and H2 circulation control.

The H2 partial pressure required to prevent degradation of the anode catalyst due to insufficient H2 was reduced by using the H2 circulation pump to ensure an even H2 concentration at the anode. Along with enhanced membrane physical properties, this measure improved fuel economy by roughly 6.5% while achieving both the humidifier-less system and a reduction in H2 crossover.

To encourage the widespread adoption of FCVs, it is necessary to reduce the vehicle price, build an extensive hydrogen infrastructure, and enhance vehicle appeal. The technologies described in this paper enhanced the freedom of vehicle packaging by reducing the size of the system, and ensured that the Mirai has an equivalent cruising range to gasoline vehicles.

—Hasegawa et al.

Work on the Electrode Catalyst

The other two Toyota papers presented during the World Congress session took a less vehicle-specific view, and addressed the large general problem of enhancing the performance of the electrode catalyst, as well as providing insight on the degradation process.

Mizutani et al. reported on two examples of efforts to improve the performance of a platinum-cobalt (PtCo) catalyst in fuel cell vehicles. PtCo alloy nanoparticles have demonstrated much better activity over traditional Pt nanoparticles—but mainly towards the reduction of activation overpotential. An issue with their use is operation at high current when mass transfer process becomes the limitation.

  • By lowering acid treatment voltage, the team enhanced the effectiveness of the removal of unalloyed cobalt, leading to less Co dissolution during cell operation and about 40% higher catalyst mass activity.

  • The use of nonporous carbon support material promoted mass transfer and resulted in substantial drop of overpotential at high current and low humidity. This may suggest an effective strategy towards the development of fuel cell systems that operate without additional humidification, the authors said.

In his paper, Kato reported on an in situ transmission electron microscopy (TEM) method that enables real time, high-resolution observation of carbon-supported platinum nanoparticles in liquid electrolyte under working conditions.

On the cathode of a fuel cell, protons and oxygen molecules receive electrons, resulting in a chemical reaction that forms water molecules. To promote this chemical reaction, platinum particles of several nanometers are used as a catalyst. Because platinum is a rare element and expensive, it is necessary to reduce its use.

One of the factors behind the large usage of platinum is its degradation. It is known that platinum nanoparticles degrade while the fuel cell is generating electricity, thus reducing its output. Therefore, it is necessary to elucidate this degradation mechanism and implement a countermeasure. It would not be an overstatement to say that the very quality of automobiles depends on nanoscale analysis technologies. However, the mechanism behind the degradation phenomenon of platinum nanoparticles has not been fully understood. This is largely due to the technical difficulty in directly observing the nanometer-sized platinum particles in liquid electrolyte under applied electrochemical potentials.


By improving the design of the Micro Electro Mechanical Systems (MEMS) sample holder, the migration and aggregation of neighboring platinum nanoparticles could be visualized consistently and correlated to applied electrode potentials during aging process (i.e., cyclic voltammetry cycles).

Hypotheses of platinum nanoparticle degradation mechanism. Based on in-situ TEM observations, Kato observed platinum nanoparticle degradation via dissolution into the electrolyte and aggregation. Using the improved MEMS chip, he could stably perform cyclic voltammetry measurements. This enables the analysis of the relationship between the potential control and platinum nanoparticle degradation in detail based on real-time observation of degradation. Kato (2016) Click to enlarge.


  • Hasegawa, T., Imanishi, H., Nada, M., and Ikogi, Y. (2016) “Development of the Fuel Cell System in the Mirai FCV,” SAE Technical Paper 2016-01-1185 doi: 10.4271/2016-01-1185

  • Mizutani, N. and Ishibashi, K. (2016) “Enhancing PtCo Electrode Catalyst Performance for Fuel Cell Vehicle Application,” SAE Technical Paper 2016-01-1187 doi: 10.4271/2016-01-1187

  • Kato, H. (2016) “In-Situ Liquid TEM Study on the Degradation Mechanism of Fuel Cell Catalysts,” SAE Int. J. Alt. Power. 5(1) doi: 10.4271/2016-01-1192


Account Deleted

@ Roger it is as if you don’t know that Tesla has already build a dense global network of supercharger stations that will be nearly everywhere by the end of 2016 and certainly everywhere by 2020 when the first autonomous vehicles will be ready for driverless transport of anything.

So there are none important rural transit routes left that Tesla has not already covered or will very soon cover. Each supercharger station is typically connected to a 1.4Mwatt power line enabling up to 12 simultaneous Tesla’s to charge 120kW each or say 3 autonomous heavy duty trucks to charge at 480kw each.

BYD has already sold thousands of large busses with 350kwh batteries and the 500kwh battery we need for a heavy duty truck is really no big deal either in volume or weight.

We will probably use the super durable lithium titanate chemistry so expect 5000 kg for the battery pack. That is a lot but you can save a 1000 kg by not using a driver cabin and another 700 kg or so because the electric moter is less heavy than the comparable diesel and complex transmission and exhaust system. Then you also save a 200 gallons or 800kg diesel tank. So this truck that is meant to transport up to 12000 kg will weight some 2500 kg above a diesel truck. So what? Use aluminum instead of steel and we save 2500 kg. That will be more expensive but we have saved the drives salary and the high cost of diesel and the cost of making a cabin with life support. Aluminum also last longer than steel that rust. The latter is important because this driverless truck will be doing about 16*50 = 800 miles per day 350 days per year or 280,000 miles per year! Not possible with a human operated truck but it is with a driverless BEV truck.


Multiple, up-to-date, Norwegian type electrolysers, compressors and storage tanks can efficiently produce and store as much H2 as required, at the same physical location.

The technology exist to truck compressed H2 to smaller adjacent H2 distribution substations; in smaller cities/villages or within adjacent large cities.

Making, compressing, storing and distributing clean H2, on a 24/7 basis is not a real problem.

Producing enough REs (Hydro, Solar, Wind, Nuke and many others) to produce clean H2 is not a problem either.

The cost of mass produced clean H2 can be reduced by 50% to 75% with acceptable subsidies, for the first 10+ years or the first 1000+ large H2 stations and 10,000 substations or so.

Funding could come from progressive but drastic reduction of current Oil Wars. USA could progressively pull out of M-E and North Africa and restrain entrance of those (well known) trouble makers.

It is doable but USA may need better governance? It may come by the end of the current year.

Of course, no real numbers from real manufacturers offered, why bother? The argument would fall apart. No attempt to show the real cost, it would be more clearly uneconomic.


@ e-c-i.c:

All new NPPs are drastically uneconomical but many countries are still building many to close their CPPs and lower GHG and pollution and/or to meet their 24/7 e-energy needs.

Many posters are supporting uneconomical nuclear energy for similar reasons and they are not 100% wrong.

NPPs were not so uneconomical 50 to 60 years ago when the price of a family house was less than $20K instead of $400+K today.

Many other products went the other way and became very affordable. Flat HDTVs, lithium batteries (all sizes), FCs, digicams, LED lights, printers, computers, are a few examples. BEVs and FCEVs will soon be added to the above list.

I agree Harvey, FCVs have become more affordable. The question is when will they compete with BEVs and PHEVs? Not just the car, but the whole package needed for passenger transport - car, fuel infrastructure, fuel.

What's in the product pipeline that gives us sufficient confidence to spend hundreds of millions to billions on the enterprise?

As I said, lets name some nameplate capacities and pencil it out.


The arrival of FCEVs and clean H2 stations may come with the arrival of new up-dated NPPs.

The main driver (in both cases) may be main necessities rather than cost?

1) to drive around for up to 500 miles per refill, in all weather with clean fuel, at the same/equivalent fuel cost as for ICEVs or about $4/gal to $6/gal equivalent.

2. to do away with ICEVs produced GHGs and pollution, oil imports and most polluting bio-fuel production.

3. to replace lower cost but highly polluting CPPs and NGPPs with higher cost clean NPPs for base line 24/7 energy, to reduce GHGs and pollution.

FCEVs and clean H2 stations may come first, but both are required.

Harvey, what leads you to believe that the target fuel for H2 cost parity is gasoline, when the ZEV competition is electricity?

Where is the evidence that these H2 stations will be cost-competitive?

It's fine to speculate that the taxpayer could foot the bill in lieu of military spending, (it is at least be a better moral position) but back in reality, in a capitalist society (not to mention a militaristic one) where things have to actually economically compete, let's see a back of the envelope calculation that makes the case for fitness.

Even if the electricity for the electrolysis was free, it still wouldn't pencil out.


@ e-c-i-c:

Clean all weather extended range vehicles may be more a question of necessity than a question of H2 cost.

We had the first Green Xmas for the past (XX) years and the average temperature has been rising at n increasing rate for the last 10+ years. Ski places have been going bankrupt.

This year (2016) the average temperature has gone up:

1) + 5.3C for January
2) + 5.5C for February
3) + 9.2C f0r March
4) + 4.7C for December.

Canada CO2 will never go down 30% between 2005 and 2030. It may go up.


Forgot to add that, with warmer temperatures, clean Hydro electricity consumption went down, over 12 months ago:

1. by 18% in December 2015.
2. by 13% in January 2016
3. by 17% in February 2016
4. by 12% in March 2016

If this keeps up, our forecasted surplus may last until 2032 instead of 2027, unless the switch to EVs an FCEVs is accelerated and/or planed Hydro developments and Wind Farms are delayed by 10 to 15 years.

Our Hydro grid could support 100+ large H2 stations and or 200+ quick charge large EV stations with current surpluses.

I don't dispute your weather reporting Harvey. Funnies email signature I ever read (in a gallows sort of way) was from a Canadian who ended every message with "I'm all for global warming!"

But none of that changes the economics of hydrogen.

Whether you pay at the pump, or pay in your tax bill, the cost will be ruinous. If there were no options, you might have a case. But the option is waiting 30 minutes for a charge a few times a year when you drive more than 200-300 miles in a day.

Most people will opt for a cheaper daily drive, and a small rest break while on vacation.

If you can afford the alternative, fabulous.


For now and until 2020 or so, we will continue with our excellent (55 mpg and 41 mpg) Toyota Prius and Camry HEVs.

After 2020, the vehicles of choice will depend on the availability of clean H2 stations and/or ultra quick charging facilities and/or the initial cost of all weather extended range (350+ miles) BEVs.

We survived with $5+/gallon gasoline and we would survive with $5 to $8/Kg clean H2 from low cost surplus Hydro electricity.

Using unused surplus Hydro electricity to produce clean H2 would increase our sole Hydro producer profits and dividend to our provincial government and to us!

I sincerely hope you see $5-$8 kg H2 from hydro. But given $13.50 - $16.50 H2 from methan in California, I don't see it happening in 5 years. Or 10, or 15. There just isnt't going to be sufficient competitive pressure, even if the tech makes it possible.


How about $7.33/Kg with a new process (on GCC) with average high price USA electricity.

If that process works as claimed, the H2 cost with 1/10 clean electricity cost, could be between $4 to $6/Kg (by 2020 or so).

Does that figure take into account site capitalization, finance charges, operating expenses and profit margin?

David Freeman

Both eci and HarveyD are (generally) wrong when it comes to long-distance heavy transport.

Batteries just don't have the energy density for long-distance travel, and the fast-charging infrastructure would be immense. The only real gamechanger would be aluminum-air (either recyclable or rechargeable) batteries.

While there's a better chance of hydrogen PEM fuel cells becoming an option (there's plenty of room for those tanks) the hassle of hydrogen just doesn't seem worth it.

Why not just use existing fuels with next-generation SOFC, later migrating to bio- and syn-fuels? Even if warmup takes 'forever', you can easily carry a battery pack capable of powering the truck for the first bit of a long-distance journey. Basically the architecture would look like an EREV diesel-electric locomotive (with batteries).

If you really want to go hydrogen, why not just burn it in an ICE? There's no need for a fuel cell, and you could even move to an optimized engine like Mazda's hydrogen Wankel. In a serial hybrid mode there's no need for the massive torque of a diesel engine. Heck, you could probably even justify using a 'light-weight' fuel-efficient engine with traditional fuels in a serial hybrid mode - forget fuel cells, hydrogen, or massive battery packs...

DF> If you really want to go hydrogen, why not just burn it in an ICE?

Because an ICE is not efficient enough and you wouldn't get the required range. Lots of literature out on that.

DF> Both eci and HarveyD are (generally) wrong when it comes to long-distance heavy transport.

Harvey and I are on opposite sides of the argument. I haven't suggested that batteries are suitable for long haul cargo transport applications. BEV trucks are in use for short haul drayage, and they work fine.


We are still a very long way from practical long haul heavy battery e-trucks and appropriate quick charging facilities. Neither is impossible, but something close to 10-10-10 batteries and 400+ KW chargers would be required. The required technology enhancement may not arrive before 2030/2035.

On the other hand, electrified heavy long haul trucks equipped with appropriate size FCs + SS H2 tanks + super caps for acceleration and braking energy recovery could be mass produced today, or as soon as enough H2 stations are installed, starting with California, Washington, Oregon, BC, Germany, Denmark, Japan, UK and many other US States and EU countries before 2020.

The same applies to long range FC buses.

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