The Material
One of the naturally occurring oxides of manganese, manganese (iii) oxide is a solid material with a wide array of uses. Valued for its strength and durability, manganese sesquioxide is responsible for more than 89% of mined manganite ore by mass. It also exists as the hydrate, Mn2O3.H2O which is much more reactive. Synthetic production is via oxidation of the primary oxide MnO2, with subsequent dehydration. It occurs in two crystal states, ɑ and ɣ, which are complimentary in terms of affording physical properties.
This article looks into the use of manganese (iii) oxide in the sensing and energy storage areas. A previous article has discussed the use of Mn2O3 in water treatment and industrial remediation.
Energy Storage: Anodes
Manganese oxide is a popular material choice for electrical energy storage - conventionally thought of as batteries. More correctly, capacitive energy storage has used transition metal compounds from the very outset. Researchers have demonstrated that high purity manganese oxide fibres can possess capacitance values in excess of 350 F g-1 at a 0.5 A g-1 current density. Multiple methods of production are available, including with the absence of surfactants(1). In these fibres, the key to the superior performance as a capacitor relies heavily on the purity - and that purity is heavily influenced by the manufacturing process. During annealing, great care has to be taken to ensure no rapid temperature changes, so as to avoid uneven formation of the manganese oxide in its crystalline form(2).
In general, however, manganese sesquioxide is employed alongside other materials in the energy storage space. For example, in advanced lithium based electrical storage systems, manganese oxide may be combined with graphene to produce composite anodes(3). Such anodes not only outperform non-composite forms, but they are physically stronger, allowing for much longer service lives. Composite anodes are not limited to carbon based identities - research has shown that silica can be mixed with manganese sesquioxide to produce a well performing anode - which led to an overall better storage capacity(4). Much contemporary research looks into the combination of metal oxides with other materials in order to find highly performing new chemistries. The capacitance value for manganese sesquioxide puts it firmly in the vicinity of being a significant part of our energy future.
Not to be outdone with superior performance as a capacitor-like material, research has shown that when manganese sesquioxide is incorporated into a metal organic framework (MOF), the entire system is able to exhibit supercapacitor-like behavior(5). The advantage here is that not only can manganese (iii) oxide be used to modulate the performance of the electrode within the storage device, but it can confer advantageous properties towards moving charge into/out of the device. This has potential applications in so-called ‘fast charging’ applications.
Energy Storage: Cathodes
As opposed to anodes, there is much more in the way of research on manganese sesquioxide as a cathodic material - also in energy storage. Multivalent zinc ion cells (batteries) that have used manganese sesquioxide as a cathode have been demonstrated to provide a cell with a high capacity and long lifetime. Working via a mechanism which permits the temporary storage of a zinc ion on the surface of the sesquioxide cathode, some systems have been shown to keep performing well in excess of 2,000 complete charge/discharge cycles(6).
Composite manganese sesquioxide and zinc oxide electrodes are readily formed when the components are annealed at 600 °C, and when employed as cathodes in aqueous rechargeable battery systems, reversible capacity values of 111.9 mAh g-1 have been reported(7). Such high performance has been attributed to a synergistic effect between the manganese and zinc - acceptable and even useful in providing more options for battery chemistries. This novel type of battery system leans on the known chemistry of zinc cells, adding manganese sesquioxide to produce a new battery chemistry which has significant potential for large scale battery sizes, with high power densities. Other research looks at manganese sesquioxide in its nanoscale form - at which it behaves as a superconductor(8). The deposition of manganese sesquioxide onto carbon nanospheres has also been shown as a highly effective material as a cathode in a zinc ion aqueous battery system(9), with authors particularly praising the novel material’s cycling capability. Higher capacity for cycling means a cell will last longer, enhancing its ‘green’ credentials. Interestingly, pH in the electrolyte in such aqueous zinc ion systems is increased during cycling - but is reduced slightly in proximity to the manganese sesquioxide electrolyte(10).
An advantage of manganese sesquioxide is its price. Compared to superstar performers such as iridium, which is rarer than gold and thus even more expensive, sesquioxide offers excellent performance at a highly competitive price.
When manufacturing electrodes, particle size matters. Not only does it play a part in the physical production of the electrode, but also when it comes to internal resistance. Manganese sesquioxide is easily milled, leading to excellent physical density - and therefore a greater density for capacitance(11). Density, relating to size, can also have an impact on the final dimensions of a battery - a crucial consideration. This has been corroborated by researchers looking into rapid charge/discharge zinc cells, where a combined sesquioxide and graphite cathode was used and they concluded that particle size had a profound impact on the capacitive properties at the electrode’s surface(12). Additionally, they claimed that the combined sesquioxide and graphite system was “ultra stable” when cycled.
Finally, Mn2O3 is able to operate above 450 °C in a cathode environment(13). This is particularly important for large scale charge-discharge settings, where high temperatures are often reached. MnO2 is not as highly performing in this regard.
A much more in-depth review of manganese sesquioxide in zinc ion cells can be found(14).
Sensors
Sensing technologies are perhaps a logical continuation of using manganese sesquioxide as an electrode. Whereas an electrode allows the passage of charged moieties to pass, a sensor will - when affected by an external stimulus - modulate the charge being passed across it. The basic idea of a manganese oxide bearing sensor is that given an electrical circuit with some Mn2O3 in it and exposed, a current response will be noticed when stimulated. This is a crucial and important part of many modern industrial processes.
The major application of manganese oxide in the sensing space is for humidity. Manganese sesquioxide has been demonstrated as a rugged and reliable method of detecting moisture in a given setting(15). In this, the response is an increase in direct current resistance when local humidity levels are higher. Furthermore, trials showed that the system was effective in an alternating current system, meaning more potential applications
The basic construction of manganese sesquioxide sensors (especially for humidity) is a paste of the oxide supported on alumina. Operating temperatures around 25 °C are common, with response ranges up to 90% relative humidity. Response times are between two and five minutes, on average(16). Manganese sesquioxide sensors have found good use despite concerns of conductivity and microstructure-related predicted limitations.
Further developments for humidity sensing with manganese sesquioxide have been realised when the oxide has been doped onto indium oxide films(17). highly effective and regeneratable humidity sensors can be produced, which are very sensitive. The advantage to combining the sesquioxide and indium oxide is that not only does it produce an even more sensitive sensor for humidity, but they are much easily regenerated - reducing lifetime costs. Further evidence of manganese sesquioxide as a good sensor material is provided when it is combined with zinc oxide. One of the most important uses for manganese sesquioxide is for the detection of formaldehyde. This is an application where manganese sesquioxide and zinc oxide are used together(18). Much like the indium oxide mentioned previously, the combined zinc-manganese system recovers much faster and has an excellent response time. Gas detection is particularly important ahead of and during the shipping of perishable goods.
Combining the idea of sensing and an electrode, manganese sesquioxide can be formed into nanosheets and deposited onto nickel foil. From there, this material is highly effective at the sensing of N-hydroxysuccinimide(19). This application is, perhaps, indirect as N-hydroxysuccinimide itself is used to detect enzymes (and thus protein fragments) in biological research settings, by immobilising the enzyme itself.
In a more industrial setting, manganese sesquioxide sensors have been shown as effective on the detection of liquefied petroleum gas (LPG). The oxide was employed in the form of nanoparticles, formed at 450 °C, with excellent response rates to 1,000 ppm LPG. The authors note, however, that sensing with Mn2O3 alone was not as highly performing as when moderate amounts of ZnO was also present(20).
Summary
- Manganese sesquioxide is an oxide of manganese with a broad range of uses, from ceramics to water purification
- As a cathode material, Mn2O3 has wide applications including at high temperatures, with superior performance especially when combined with other materials
- Building on its cathode uses, manganese sesquioxide has applications in energy storage, where its unique resistive and conductive properties mean that it is a candidate for exciting new battery chemistries as a cathode, paving the way to a lower carbon future
- Sensing of moisture and pollutant chemicals is important industrially - manganese sesquioxide has been used in sensors to detect water, formaldehyde and liquefied petroleum gas
References
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