A Redox-Mediated Zinc–Air Fuel Cell

Abstract

Zinc–air batteries (ZABs) have recently attracted revived interest. However, critical issues pertaining to the labile zinc anode and sluggish air cathode have yet to be adequately addressed. Here, we demonstrate a redox-mediated zinc–air fuel cell (RM-ZAFC) to tackle the above problems. Upon operation, the complex cobalt triisopropanolamine serves as an electrolyte-borne electron carrier and homogeneous catalyst to boost the 4e– oxygen reduction reaction in a separate gas diffusion tank, which makes the system free of a sophisticated air electrode. With mediation by the ultrafast reaction with a phenazine derivative, zinc could be liberated from the electrode to a separate “fuel” tank at high utilization (>90%), making it feasible to be “refueled” after it is depleted. Above all, RM-ZAFC has the combined advantages of both ZABs and alkaline fuel cells and can operate with high energy density, good flexibility, scalability and safety at low cost and thus is promising for various energy storage applications.

Towards a mechanically rechargeable solid fuel flow battery based on earth-abundant materials

“…Zinc composite blends (e.g., Zn, carbon black, and polyvinylidene fluoride), along with greater concentrations of KOH, may limit the potentially harmful homogeneous formation of ZnO and the precipitation of ZnO onto undesired surfaces (e.g., cell tubing). 7,8 Hydrogen evolution also occurs in the presence of Zn in alkaline solutions; though the presence of 2-HNQ (or generally any mediator) does not necessarily affect the rate of hydrogen evolution, in this specific system, hydrogen evolution appears to be slower than 2-HNQ reduction, as evinced by Figures 3a and 3c. These analyses indicate that 2-HNQ is reduced in the electrolyte due primarily to its interactions with Zn, which, in turn, is corroded.…”

“…For example, by mechanically exchanging zinc in a modular fashion, the metal may necessitate less stringent specifications than are required for conventional immobile metal electrodes, increasing materials flexibility; in a similar manner, the overall energy capacity may scale volumetrically as, in principle, the entire energetic metal surface can be accessed (i.e., less stringent electrode thickness constraints). Within the SFFB, charging is designed to be exclusively conducted by mechanical recharging via facile periodic SEF exchange (i.e., no electrochemical charging occurs); [6][7][8] this may simplify the recharging process-rather than replacing entire cells, only fuels need be exchanged (i.e., less inactive material replacement).…”

“…In recent years, there have been several reports that describe electrochemical processes that use solubilized RMs to transfer charge between physically separated reactants and an electrode for the purpose of energy storage and conversion. [7][8][9][10][11][12][13][14] Wang and coworkers have pioneered rechargeable mediated flow batteries with solid active materials in external tanks via redoxtargeting-the process of selecting or engineering RMs by modulating their redox potential to favorably exchange electrons with a solid (insoluble) material. 9,10 Deng and coworkers have also demonstrated a fuel cell targeting biomass byproducts of pulp and paper processing using iron (Fe)-based mediators, while Stahl and coworkers have extensively studied mediated fuel cell architectures with both organic and inorganic redox chemistries.…”

Toward a Mechanically Rechargeable Solid Fuel Flow Battery Based on Earth-Abundant Materials

“…25 days of galvanostatic discharge (Figure 4c). We anticipate that the Zn utilization may be greater if the cell had continued running; additional strategies (e.g., increased KOH concentrations and carbon− zinc slurries) may further extend the utilization of the SEF, 7,8 though material compatibility concerns (e.g., wetted components) may arise at more extreme pH values. Like all metal−air batteries, the theoretical energy density is high, but the form factor and operating conditions dictate what is practically achievable.…”

“…In recent years, there have been several reports that describe electrochemical processes that use solubilized RMs to transfer charge between physically separated reactants and an electrode for the purpose of energy storage and conversion. − Wang et al . have pioneered rechargeable mediated flow batteries with solid active materials in external tanks via redox targetingthe process of selecting or engineering RMs by modulating their redox potential to favorably exchange electrons with a solid (insoluble) material. , Deng et al have also demonstrated a fuel cell targeting biomass byproducts of pulp and paper processing using iron (Fe)-based mediators, while Stahl et al have extensively studied mediated fuel cell architectures with both organic and inorganic redox chemistries. , Recent demonstrations by Perez-Antolin et al and Zhang et al have also shown that similar architectures (e.g., refillable primary batteries with semi-solid slurries, zinc–air flow batteries leveraging redox-mediated ORR) are promising. , Here, we leverage these previous research efforts to further explore the implementation of metallic SEFs into modified metal–air systems; we particularly focus on RM–SEF interactions in the storage tank (Figure ). We first use in situ microelectrode voltammetry, post-mortem X-ray diffractometry, and post-mortem optical microscopy to assess the efficacy of RM–SEF pairingsidentifying 2-hydroxynaphthoquinone (2-HNQ) and zinc (Zn) as a favorable combination.…”