Tsinghua team develops zinc-air fuel cell stack with high power density
29 December 2013
Researchers at Tsinghua University have developed a high-power-density zinc-air fuel cell (ZAFC) stack using an inexpensive manganese dioxide (MnO2) catalyst with potassium hydroxide (KOH) electrolyte. As reported in a paper in the Journal of Power Sources, they achieved peak power in a ZAFC stack as high as 435 mW cm-2.
They also reported that the time required for voltages to reach steady state and for current step-up or step-down are in milliseconds, indicating that the ZAFC could be applied to vehicles with rapid dynamic response demands.
Zinc-air technology, although offering high energy density—about twice the gravimetric density (Wh/kg) and three times the volumetric density (Wh/L) of Li-ion technology—has been generally limited to low-power, non-rechargeable applications.
Many efforts are focused on batteries and fuel cells, which can be used for EVs propulsion and large-scale energy storage. However, the existing batteries and hydrogen fuel cells cannot meet all the market needs; for example: low price, high safety, longer lifetime, and zero pollution. For electrochemical power sources, it is worth noting that zinc possesses a unique set of characteristics as anode material, including low equilibrium potential, electrochemical reversibility, stability in aqueous electrolytes, good conductivity, low equivalent weight, high specific energy, and high volumetric density. Moreover, zinc has other merits, such as, abundant resources, low cost, low toxicity, easy storage and safe handling.
Up to now various types of zinc air batteries and fuel cells have been developed, such as primary battery, electrically rechargeable battery and fuel cell (mechanically rechargeable battery). However, the primary battery is not applicable to EVs and energy storage, and the electrically rechargeable battery has a relatively short lifetime.
…Overall, the [mechanically rechargeable] ZAFC has great potential for EVs propulsion … Currently, the performance of ZAFC still cannot meet the commercialization requirement. The power densities of ZAFC with manganese dioxide (MnO2) catalyst reported in previous researches are quite low, mostly in the range of 50-100 mW cm-2. This falls far behind PEMFC. MnO2 is one of the typical oxygen reduction catalysts, and has a reasonably high catalytic activity for oxygen reduction in alkaline electrolyte. In this study, a ZFAC stack with inexpensive MnO2 catalyst was developed to study the factors affecting the stack performance and maximize power density.
—Pei et al.
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| Basic principle of ZAFC. Click to enlarge. |
The ZAFC comprises an anode plate, cathode plate and bi-polar plates fabricated from graphite; each fuel cell has an active surface area of 215 cm2.
The air cathode has three layers: an active layer produced form a mixture of active carbon powder with manganese oxide powder and PTFE binder; a woven nickel mesh current collector layer; and a hydrophobic layer of Teflon film.
The team tested two configurations of the anode chamber; at the bottom of the zinc pellet beds is a gap or mesh layer allowing KOH electrolyte and small particles to fall out of the pellets bed.
Zinc pellets (average size 1 mm) are fed to the anode chamber uniformly and intermittently by a mechanical device above each stack. The KOH electrolyte is contained in a separate storage tank, and driven by a magnetic pump for circulation. Zinc pellets automatically enter the anode chamber trough from the upper slit with the flowing electrolyte. Discharge products (potassium zincate) are carried out by the electrolyte.
Ambient air is fed through the inlet and distributed between the unit cells by an electric fan. Outflow from the unit cells is combined in the outlet header and then sent through the stack outlet.
For testing, they assembled a 5-cell stack and then several 2-cell stacks. Conclusions drawn from the testing include:
Peak power density of the ZAFC can reach 435 mW cm-2 at 0.86V, 510 mA cm-2.
Optimizing the filled state of zinc pellets and decreasing contact resistance can improve the fuel cell performance significantly. Developing a surface conductive air cathode is particularly important.
Location of the cell, flow state of the electrolyte and air are the main factors affecting the performance and uniformity of the ZAFC stack.
Time needed for voltage to reach steady state in response to both step-up and step-down are in milliseconds.
Resources
Pucheng Pei, Ze Ma, Keliang Wang, Xizhong Wang, Mancun Song, Huachi Xu (2014) “High performance zinc air fuel cell stack,” Journal of Power Sources, Volume 249, Pages 13-20 doi: 10.1016/j.jpowsour.2013.10.073
Judging from the chemical reaction, this is not a fuel cell, it is a glorified Zinc battery.
The consumable energy source is Zinc!
Zn ore + energy +l osses = Zn
Zn + other reactants + losses = K2Zn(OH)4 + byproducts + energy
This is NOT useful from a total energy consumption perspective. There are too many unavoidable losses. Making Zinc to oxidize in a battery is a waste of otherwise good energy that should be used more directly
Posted by: Jus7tme | 29 December 2013 at 11:54 AM
It all gets down to if the potassium zincate can be recycled and how much energy is required as such. The one time refining of zinc from virgin ore in that case would not be a factor in th energy balance.
Posted by: Mannstein | 30 December 2013 at 05:16 AM
Same difference. MIning Zince blende (Zn,Fe)S is probably worse than electrolysis or some other reduction of K2Zn(OH)4, but in either case the energy losses will be worse than that of a good rechargeable battery.
That's the point. This type of Zinc battery offers no value ta a car.
Posted by: Jus7tme | 30 December 2013 at 10:54 AM
Not very user friendly?
Posted by: HarveyD | 30 December 2013 at 01:50 PM
1 mm pellets of metallic zinc could be poured like fine gravel, or blown with air.
I recall calculating the efficiency of a zinc cycle system but I can't find the numbers now. Even at 40% or so, the cheapness of electricity makes the raw energy input quite a bit cheaper than petroleum.
Posted by: Engineer-Poet | 30 December 2013 at 05:26 PM
The obfuscators are hard at work, I see. The point remains that using Zinc as an energy storage medium, and specifically for automobile battery, is worse (more energy loss, more CO2 emitted per usable kWh) than using electricity directly in a Lithium-ion battery, which does not require the Lithium to be removed and chemically reduced after each use.
And as usual, it is necessary to point out that a state-of-the-art disel-based hybrid is more efficient than either of the two aforementioned methods.
Posted by: Jus7tme | 31 December 2013 at 11:02 AM
No obfuscation. Efficiency isn't everything driving range as well as cost of the battery probably play a much larger role.
Posted by: Mannstein | 31 December 2013 at 02:59 PM
So long as the energy input is clean and cheap, the losses from converting to metallic zinc and back won't be a problem. Of course, this means that it won't work if you're forced to use something that gets a feed-in tariff; the self-contradictions of the proposed "renewable economy" are many and glaring.
The amount of zinc required for large amounts of storage (days of US grid demand) is an issue, but things like trailerable ZAFC range extenders ought to be manageable just fine. The issue is price and time arbitrage; if you can afford a Tesla-class battery and also the time to charge it every couple hours on the road, Li-ion will do. If your budget manages only a Leaf, the as-needed ZAFC trailer will likely be the better deal for you and give you better time to your destination as well.
Posted by: Engineer-Poet | 31 December 2013 at 05:12 PM
Zinc Air Fuel Cells are now past the development stage and can be used for various applications. One great application is the electrification of the developing world. One model is here explained: http://sinovoltaics.com/solar-energy-storage/zinc-air-fuel-cell-challenges-solar-battery-systems/
Posted by: Peter de Groot | 16 December 2015 at 12:12 AM