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.
|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.
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