The chemistry of rechargeable aqueous Zn/MnO2 batteries in a neutral or mildly acidic electrolyte has drawn great attention recently due to the fact that all of the components of a Zn/MnO2 battery are safe, abundant, and sustainable. MnO2-based cathodes are promising for zin ion battery (ZIB) applications due to their high theoretical capacity and voltage. However, they suffer from severe capacity fading after prolonged charge-discharge cycles, and the reaction mechanism of the MnO2 cathode remains a topic of discussion. This thesis concerns two important subject matters: synthesis of manganese dioxide catalyst with the micro-wave-assisted hydrothermal method and electrodeposition methods, followed by electrochemical characterization of the synthesized active manganese-based material by various electrochemical and structural analysis. The first section of this thesis examined the basic electrodeposition mechanism of MnO2 films on different electrodes using the cyclic voltammetry technique, beginning by reproducing previously published experiments and then varying the chemical composition of aqueous electrodeposition solutions to provide meaning for the potential role of cations in the intercalation mechanisms. In the following section, we conducted a systematic exploration of a subset of the electrodeposition parameter space in order to lay the groundwork for our next work on the effect of Zn2+ in neutral electrolytes. Due to the scarcity of electroanalytic work in neutral electrolytes, work in an acidic environment at a Pt electrode was chosen as a benchmark case. Finally, we conducted cycling tests in the operating potential range to determine the effect of Zn2+ on the electrochemical behavior of MnOx-based ZIB cathodes. We specifically carried out battery-like tests in electrolytes without and with added Zn2+, for MnOx films electrodeposited potentiostaticaly from the electrolytes. To confirm the importance of MnSO4 and ZnSO4 co-presence in the electrolyte, we performed split-cell experiments in 0.1 M Na2SO4 and 1 M ZnSO4 neutral solutions. Finally, birnessite-type MnO2 films appear to be a viable cathode material for ZIB applications, as they exhibit reversible Zn2+ intercalation and good stability in a moderate ZnSO4 aqueous electrolyte containing a MnSO4 additive that inhibits irreversible cathode deactivation. In the second part of this work, we examine the electrochemical performance of drop-cast α-MnO2 electrodes, prepared by the hydrothermal method. We explored the structural and morphological evolution of α-MnO2 electrodes in detail using XRD, Raman spectroscopy, and SEM-EDS mapping. This material is commonly used for ZAB cathodes, thus we investigated cyclic voltammetry tests in KOH solution, which is typically used for electrocatalytic ORR tests, but here in the absence of O2, as is the case in ZIBs. Then, we also conducted cycle voltammetry measurements in the operating potential range in various near neutral solutions, as in the previous section, to examine electrochemical behavior of α-MnO2 for ZIB applications. In the end, cycling behavior of hydrothermally produced cryptomelane-type α-MnO2 is better in Zn solutions as compared to Zn-alone solutions, but rapid capacity loss is still observed in both cases.
Le batterie Zn/MnO2 ad elettrolita acquoso, neutro o alcalino, stanno vivendo negli ultimi decenni un rinnovato interesse a causa dei loro componenti sicuri, facilmente reperibili e sostenibili. In particolare, i catodi a base di biossido di manganese (MnO2) risultano essere promettenti nell’ambito della tecnologia agli ioni di zinco grazie alla loro elevata capacità teorica e alla loro presunta capacità di intercalare lo ione zinco in maniera analoga ai catodi delle batterie agli ioni di litio. Tuttavia, essi mostrano ancora oggi un notevole degrado nelle prestazioni dopo aver subito prolungati cicli di carica e scarica e non possono essere sfruttati in una produzione su larga scala. Questa tesi tratta di due importanti aspetti legati ai catodi a base MnO2: in primo luogo si parlerà della sintesi del biossido di manganese, ottenuto tramite un processo idrotermico a microonde e un processo elettrochimico; in seguito si tratterà della caratterizzazione strutturale ed elettrochimica del catalizzatore sintetizzato. Nella prima sezione della tesi si tratterà della sintesi di strati sottili di MnO2 tramite elettrodeposizione, sfruttando la voltammetria ciclica. Nella fase iniziale del lavoro sono stati riprodotti diversi esperimenti di letteratura per validare il protocollo di elettrodeposizione; successivamente sono stati cambiati sia i parametri di elettrodeposizione sia le composizioni delle soluzioni elettrolitiche per studiare il ruolo di diversi cationi nel dibattuto processo di intercalazione. Data la mancanza di letteratura specifica sull’argomento, sono stati presi come riferimento degli esperimenti svolti in ambiente acido con un elettrodo di platino. Dopo aver caratterizzato elettroanaliticamente i processi di elettrodeposizione e di intercalazione, i film di biossido di manganese elettrodepositato potenziostaticamente hanno subito vari cicli di carica e scarica, in presenza o meno di Zn2+, per verificare le loro prestazioni in condizioni applicative. Le misure nella cella a tre elettrodi sono state infine confrontate con i test svolti in una split-cell, confermando la reversibilità dell’intercalazione dello ione zinco; inoltre, è stata verificata anche la buona stabilità dell’elettrodo a base di MnO2 in ambiente neutro. Nella seconda parte della tesi sono state studiate le prestazioni di un elettrodo a base di criptomelano (α-MnO2) sintetizzato tramite un processo idrotermico. L’evoluzione morfologica e strutturale del catalizzatore è stata studiata usando la diffrazione ai raggi X (XRD), la spettroscopia Raman e la microscopia elettronica a scansione (SEM-EDS). Dato che il criptomelano è un materiale sfruttato anche nei catodi per le batterie zinco-aria, la caratterizzazione elettrochimica è stata svolta sia in ambiente alcalino che neutro, ma in assenza di ossigeno dato che l’obiettivo di questa tesi è studiare il catalizzatore nell’ambito dei catodi ad intercalazione per tecnologie Zn-ione.
Synthesis and characterization of manganese dioxide cathodes for Zn-ion batteries
OKSAS, ABDURRAHMAN SAFA
2020/2021
Abstract
The chemistry of rechargeable aqueous Zn/MnO2 batteries in a neutral or mildly acidic electrolyte has drawn great attention recently due to the fact that all of the components of a Zn/MnO2 battery are safe, abundant, and sustainable. MnO2-based cathodes are promising for zin ion battery (ZIB) applications due to their high theoretical capacity and voltage. However, they suffer from severe capacity fading after prolonged charge-discharge cycles, and the reaction mechanism of the MnO2 cathode remains a topic of discussion. This thesis concerns two important subject matters: synthesis of manganese dioxide catalyst with the micro-wave-assisted hydrothermal method and electrodeposition methods, followed by electrochemical characterization of the synthesized active manganese-based material by various electrochemical and structural analysis. The first section of this thesis examined the basic electrodeposition mechanism of MnO2 films on different electrodes using the cyclic voltammetry technique, beginning by reproducing previously published experiments and then varying the chemical composition of aqueous electrodeposition solutions to provide meaning for the potential role of cations in the intercalation mechanisms. In the following section, we conducted a systematic exploration of a subset of the electrodeposition parameter space in order to lay the groundwork for our next work on the effect of Zn2+ in neutral electrolytes. Due to the scarcity of electroanalytic work in neutral electrolytes, work in an acidic environment at a Pt electrode was chosen as a benchmark case. Finally, we conducted cycling tests in the operating potential range to determine the effect of Zn2+ on the electrochemical behavior of MnOx-based ZIB cathodes. We specifically carried out battery-like tests in electrolytes without and with added Zn2+, for MnOx films electrodeposited potentiostaticaly from the electrolytes. To confirm the importance of MnSO4 and ZnSO4 co-presence in the electrolyte, we performed split-cell experiments in 0.1 M Na2SO4 and 1 M ZnSO4 neutral solutions. Finally, birnessite-type MnO2 films appear to be a viable cathode material for ZIB applications, as they exhibit reversible Zn2+ intercalation and good stability in a moderate ZnSO4 aqueous electrolyte containing a MnSO4 additive that inhibits irreversible cathode deactivation. In the second part of this work, we examine the electrochemical performance of drop-cast α-MnO2 electrodes, prepared by the hydrothermal method. We explored the structural and morphological evolution of α-MnO2 electrodes in detail using XRD, Raman spectroscopy, and SEM-EDS mapping. This material is commonly used for ZAB cathodes, thus we investigated cyclic voltammetry tests in KOH solution, which is typically used for electrocatalytic ORR tests, but here in the absence of O2, as is the case in ZIBs. Then, we also conducted cycle voltammetry measurements in the operating potential range in various near neutral solutions, as in the previous section, to examine electrochemical behavior of α-MnO2 for ZIB applications. In the end, cycling behavior of hydrothermally produced cryptomelane-type α-MnO2 is better in Zn solutions as compared to Zn-alone solutions, but rapid capacity loss is still observed in both cases.| File | Dimensione | Formato | |
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Descrizione: Synthesis and characterization of manganese dioxide cathodes for Zn-ion batteries
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https://hdl.handle.net/10589/177819