Editors' Choice—Electrocatalyzed Oxygen Reduction at Manganese Oxide Nanoarchitectures: From Electroanalytical Characterization to Device-Relevant Performance in Composite Electrodes

Abstract

We assess the effect of the pore-solid architecture of cryptomelane-type manganese oxide (MnOx) xerogels and aerogels on electrocatalysis of the oxygen-reduction reaction (ORR) using three different electrochemical test platforms. Rotating-disk electrode measurements at ink-cast films of carbon + MnOx show that both MnOx nanoarchitectures exhibit comparable intrinsic ORR activity for four-electron reduction with a low onset overpotential (∼310 mV). The MnOx xerogel and aerogel powders were also incorporated into practical powder-composite electrodes that include carbon and polymer binder. When evaluated in an air-breathing three-electrode electroanalytical cell, the aerogel-based composite electrode exhibits an overpotential lowered by ∼50 mV compared to the xerogel-based analog. The superior performance of the aerogel-based composite is also demonstrated in zinc-air button cells, with up to 100 mV improvement in discharge voltage at moderate-to-challenging current densities (5-125 mA cm -2 ). We ascribe the enhanced activity of the MnOx aerogel-based composite to a more uniform dispersion of the aerogel powder within the carbon/binder matrix, as verified by focused-ion beam−scanning electron microscopy and elemental mapping. The results reported herein highlight the importance of assessing the translation of electrocatalytic activity from fundamental measurements to technologically relevent electrode structures.

Translating Materials-Level Performance into Device-Relevant Metrics for Zinc-Based Batteries

“…[12][13][14][15][16] Because so many challenges must be solved to realize high-performance Zn batteries with Li-ion competitive cycle life, the characteristics of individual cell components-Zn anode, alkaline electrolyte composition, separators, cathodes (particularly for rechargeable Zn-air)-are often first evaluated with classical electroanalytical measurements in simplified cells or three-electrode configurations. Fundamental studies may provide valuable information, yet caution must be exercised when attempting to extrapolate results from such experiments to projected cell and pack performance, 17 particularly in the case of advances toward rechargeability.…”

Sustainable Electrocatalytic Architectures Enable Rechargeable Zinc–Air Batteries with Low Voltage Hysteresis

“…We previously showed that aerogel pore−solid architectures enhance electrocatalytic activity because they express amplified surface-to-bulk ratios and nanometric secondary particle sizes on the order of 10 nm. 29,30,57 An added benefit of aerogel materials is that they can be readily ground into fine powders, which disperse homogenously into the powder-composite electrode structures already used commercially for air cathodes. 57 The combination of a high active surface area and uniform distribution of the electrocatalyst within the conductive carbon/polymer matrix translates to improved electrochemical activity compared with conventionally prepared catalysts or those that form hard agglomerates but otherwise comprise nanoscale domains.…”

“…The maximum power density with the Gortex membrane in place is 53 mW cm −2 at 0.95 V, whereas without the membrane present the cell delivers a maximum power density of 180 mW cm −2 at 0.73 V, a performance comparable to or higher than previously reported Zn−air cells using Mnbased catalysts for ORR. 57,66,67 These results indicate that for practical cells to operate at a higher power density than is typical with commercial Zn−air batteries (already considered "low"), one must improve not only cathode/catalyst performance but also O 2 flux and moisture management using advanced hydrophobic membranes. Potentiostatic impedance spectroscopy of a Zn−air cell containing a CC-Ni2Fe+Mn cathode was measured with a DC bias at specified voltages within a practical Zn−air battery discharge operation (1.3, 1.2, and 1.1 V).…”

“…We prepare powder-composite air cathodes comprising pre-formed Ni 2 FeO x aerogels and cryptomelane-type α-MnO 2 aerogels. We previously showed that aerogel pore–solid architectures enhance electrocatalytic activity because they express amplified surface-to-bulk ratios and nanometric secondary particle sizes on the order of 10 nm. ,, An added benefit of aerogel materials is that they can be readily ground into fine powders, which disperse homogenously into the powder-composite electrode structures already used commercially for air cathodes . The combination of a high active surface area and uniform distribution of the electrocatalyst within the conductive carbon/polymer matrix translates to improved electrochemical activity compared with conventionally prepared catalysts or those that form hard agglomerates but otherwise comprise nanoscale domains.…”

“…Here, we demonstrate simple-to-fabricate Co-free air cathodes functionalized with sustainable, aerogel-based electrocatalysts that show a low ΔV of 700 mV and a high discharge voltage in comparison with previously reported Zn−air cathodes. 3,14,17,19,20,36−56 The electrocatalysts, down-selected from our prior three-electrode investigations of ORR and OER performance, are α-MnO 2 for efficient ORR 57 and Ni 2 FeOx primarily for OER, 29 although nickel-ferrite aerogels also express a bifunctional character. 30 We evaluate these electrocatalyst-functionalized air cathodes versus 3D-Zn-sponge anodes in prototype Zn−air cells.…”