Publications

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Rechargeable Zn/MnO2 alkaline batteries are a promising technology for grid storage applications since they are safe, low cost, and considered environmentally friendly. Here, a commercial ceramic sodium ion conductor which is impervious to zincate [Zn(OH)42−], a contributor to MnO2 cathode failure, is evaluated as the battery separator. As received, the ionic conductivity of this separator was measured with electrochemical impedance spectroscopy to be 3.5 mS cm−1, while its thickness is 1.0 mm, resulting in large total membrane resistance of 25.3 Ω. Reducing the thickness of the ceramic to 0.5 mm provided for a decreased resistance of 9.8 Ω. Crossover experiments conducted using inductively coupled plasma - mass spectrometry measurements failed to measure any Zn(OH)42− transport indicating a diffusion coefficient that is at least two orders of magnitude less than that for the commercial cellophane and Celgard separators. For 5% DOD at a C/5 rate, the cycle lifetime was increased by over 22% using the 0.5 mm thick ceramic separator compared to traditional Celgard and cellophane separators. Scanning electron microscopy/energy dispersive X-ray spectroscopy and X-ray diffraction characterization of cycled electrodes showed limited amounts of zinc species on the cathode utilizing the ceramic separator, consistent with its prevention of Zn(OH)42− transport.

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Nickel-doped α-MnO2 nanowires (Ni-α-MnO2) were prepared with 3.4% or 4.9% Ni using a hydrothermal method. A comparison of the electrocatalytic data for the oxygen reduction reaction (ORR) in alkaline electrolyte versus that obtained with α-MnO2 or Cu-α-MnO2 is provided. In general, Ni-α-MnO2 (e.g., Ni-4.9%) had higher n values (n = 3.6), faster kinetics (k = 0.015 cm s-1), and lower charge transfer resistance (RCT = 2264 Ω at half-wave) values than MnO2 (n = 3.0, k = 0.006 cm s-1, RCT = 6104 Ω at half-wave) or Cu-α-MnO2 (Cu-2.9%, n = 3.5, k = 0.015 cm s-1, RCT = 3412 Ω at half-wave), and the overall activity for Ni-α-MnO2 trended with increasing Ni content, i.e., Ni-4.9% > Ni-3.4%. As observed for Cu-α-MnO2, the increase in ORR activity correlates with the amount of Mn3+ at the surface of the Ni-α-MnO2 nanowire. Examining the activity for both Ni-α-MnO2 and Cu-α-MnO2 materials indicates that the Mn3+ at the surface of the electrocatalysts dictates the activity trends within the overall series. Single nanowire resistance measurements conducted on 47 nanowire devices (15 of α-MnO2, 16 of Cu-α-MnO2-2.9%, and 16 of Ni-α-MnO2-4.9%) demonstrated that Cu-doping leads to a slightly lower resistance value than Ni-doping, although both were considerably improved relative to the undoped α-MnO2. The data also suggest that the ORR charge transfer resistance value, as determined by electrochemical impedance spectroscopy, is a better indicator of the cation-doping effect on ORR catalysis than the electrical resistance of the nanowire. (Figure Presented).

Rechargeable alkaline Zn/MnO2 batteries are being developed for use as cost-effective grid-scale energy storage devices. Previous studies have shown that limiting the depth of discharge (DOD) of the MnO2 cathode extends cell lifetime while still providing a cost-effective battery system. Herein, a comprehensive study of triethanolamine (TEA) as an additive in Zn/MnO2 limited DOD batteries is provided by examining the effect of TEA in full cells as well as independently on the cathode, anode, separator, and electrolyte. Improvement in cycle-ability of the cathode (on average, 80% of cycled capacity remains after 191 cycles without TEA, 568 cycles with TEA) and a decrease in ionic zinc mobility across Celgard 3501 (7.91 × 10-5 cm2/min without TEA, 3.56 × 10-5 cm2/min with TEA) and Cellophane 350P00 (3.26 × 10-5 cm2/min without TEA, 4.74 × 10-6 cm2/min with TEA) separators upon the addition of TEA are demonstrated. However, TEA increased both the reduction potential of Zn (-0.68 V vs. Hg/HgO without TEA, -0.76 V with TEA) and the solubility of Zn2+ (0.813 M without TEA, 1.023 M with TEA). Overall, the addition of TEA extended the lifetime of limited DOD cells on average by 297%.

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