Abstract
One of the major drawbacks in Lithium-air batteries is the sluggish kinetics of the oxygen reduction reaction (ORR). In this context, better performances can be achieved by adopting a suitable electrocatalyst, such as MnO2. Herein, we tried to design nano-MnO2 tuning the final ORR electroactivity by tailoring the doping agent (Co or Fe) and its content (2% or 5% molar ratios). Staircase-linear sweep voltammetries (S-LSV) were performed to investigate the nanopowders electrocatalytic behavior in organic solvent (propylene carbonate, PC and 0.15 M LiNO3 as electrolyte). Two percent Co-doped MnO2 revealed to be the best-performing sample in terms of ORR onset shift (of ~130 mV with respect to bare glassy carbon electrode), due to its great lattice defectivity and presence of the highly electroactive γ polymorph (by X-ray diffraction analyses, XRPD and infrared spectroscopy, FTIR). 5% Co together with 2% Fe could also be promising, since they exhibited fewer diffusive limitations, mainly due to their peculiar pore distribution (by Brunauer–Emmett-Teller, BET) that disfavored the cathode clogging. Particularly, a too-high Fe content led to iron segregation (by energy dispersive X-ray spectroscopy, EDX, X-ray photoelectron spectroscopy, XPS and FTIR) provoking a decrease of the electroactive sites, with negative consequences for the ORR.
Highlights
In the last few years, rechargeable metal-air batteries (MABs, as lithium-air or zinc-air ones) have gained renovate attention due to their feasibility as both electrochemical energy storage and conversion devices [1,2]. Due to their potentially very high theoretical energy density, they can be considered as one of the most promising technologies in the energetic field. Their engineering and commercialization have been significantly hindered by their scarce cycle life because of the sluggish kinetics of either the cathodic oxygen reduction reaction (ORR) or the anodic oxygen evolution reaction (OER) [4,5]
We evaluated the possible gain in terms of ORR onset shift towards less cathodic values, the rate of the electronic transfer, though the slope of the linear sweep voltammetries and the assessment of the electron transfer pathway by Tafel slopes
X-ray powder diffraction (XRPD) and IR analyses clearly showed the peculiar polymorphic composition by changing the doping agent, i.e. from a combination of α and γ phases for both pure and Co-doped MnO2 to ε and ramsdellite (RAM, as a minority one) polymorphs for Fe-MnO2, due to Mn4+ gradual substitution by Fe3+ ions. For the latter, an increase of dopant amount led to the formation of a segregated iron oxide shell, as seen by FEG-SEM/EDX and X-ray photoelectron spectroscopy (XPS) techniques, that may hinder the potential electrocatalytic behavior
Summary
In the last few years, rechargeable metal-air batteries (MABs, as lithium-air or zinc-air ones) have gained renovate attention due to their feasibility as both electrochemical energy storage and conversion devices [1,2] Due to their potentially very high theoretical energy density (around 3600 Wh kg−1, which is almost eight-fold times the value reported for Li-ion cells [1,3]), they can be considered as one of the most promising technologies in the energetic field. Their engineering and commercialization have been significantly hindered by their scarce cycle life because of the sluggish kinetics of either the cathodic oxygen reduction reaction (ORR) or the anodic oxygen evolution reaction (OER) [4,5]. The fabrication of highly efficient ORR and OER catalysts is of paramount importance to improve the cyclic stability and longevity of these devices [2], since these materials must both facilitate the decomposition of LiO2/Li2O2 and control the side reactions
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