Liquid alkaline (AFC) and anion exchange membrane fuel cells (AEMFC) are among the most promising solutions for converting chemical energy into electrical one. However, today there are several problems that prevent their commercialization, one of which is the use of expensive Pt metal as electrocatalyst for the electrode reactions, in particular at the cathode. However, it was shown that it is possible to use transition metal oxides instead of Pt as catalysts in the oxygen reduction reaction (ORR) at the cathode of an AFC or AEMFC. It was previously established that oxides containing Mn3+ in their composition possess high catalytic activity in the reaction of oxygen reduction in an alkaline medium . However, it has not yet been established which of the specific characteristics of the oxide (composition, crystal structure, etc.) determine its catalytic activity in ORR.
Spinels are an interesting subject of research. These compounds have the formula AB2O4, where B is cation of 3d metal such as manganese. These compounds are interesting in that by changing cation A it is possible to change the oxidation state of manganese and to change the distance between the catalytic centers (which is presumably a descriptor of catalytic activity). In this case, the crystal structure of these compounds remains the same. Even though this class of compounds has been actively studied, there are still questions related to the relationship between the spinel composition and the electrocatalytic activity in ORR.
Spinels of the composition Mn3O4, MgMn2O4, CdMn2O4, LiMn2O4 were synthesized and characterized by BET, SEM and powder X-ray diffraction. The specific surface area of the obtained compounds determined with N2 adsorption (within BET approximation) ranged from 16.5 for Mn3O4 to 39.4 m2/g for CdMn2O4. The BET surface area of MgMn2O4 and LiMn2O4 compounds is equal to 22 and 21 m2/g correspondingly.
The obtained samples were investigated electrochemically (by the methods of cyclic voltammetry, CV, and by the method of a rotating disk electrode, RDE). Considering that the shape of CVs, and in particular the position of red-ox peaks (corresponding to Mn3+/Mn4+ transition) are very sensitive to the structure of the interface, it is interesting to note that CVs of all spinel oxides (with the exception of LiMn2O4) are rather similar and resemble the CV of MnOOH. This may indicate that degradation of the samples occurs when they are placed in an electrolyte and when a potential is applied. During decomposition, a product like MnOOH can be formed on the electrode surface explaining the shape of CVs in the supporting electrolyte. For proving this observation, we also set up an experiment to study the stability of these compounds, when, after studying the ORR, the electrode was placed back into an oxygen-free alkaline solution. It turned out that the shape of the CV before and after ORR measurements for CdMn2O4 and Mn3O4 was significantly different, which may indicate the degradation of the oxides. In addition, for all spinel oxides the charges calculated from СVs were compared with theoretical ones, the latter estimated from the BET surface area and Mn cation density in the oxide crystals (not shown). It turned out that for LiMn2O4 and MgMn2O4, the experimental and the theoretical charge differ significantly, which can also be related to the surface degradation (making the second and subsequent atomic layers accessible). From the polarization curves for the reduction of oxygen and hydrogen peroxide, it can be seen that Mn3O4, MgMn2O4, and CdMn2O4 have similar activities, which may also indicate degradation and that a low-activity degradation product is formed on the surface. The electrical activity of spinels is significantly lower than that of Mn2O3 and is comparable to that of MnOOH. To sum up, we can conclude that, for the spinel class compounds, degradation of samples likely occurs in 1 M NaOH.
The reported study was funded by RFBR, project number 18-29-12064 mk.
 A. Bonnefont, A. E. Baranchikov, and E. E. Levin, “Rationalizing the Influence of the Mn ( IV )/ Mn ( III ) Red-Ox Transition on the Electrocatalytic Activity of Manganese Oxides in the Oxygen Reduction Reaction,” no. December 2015, 2016.
|Affiliation of speaker||Lomonosov Moscow State University|
|Position of speaker||student|
|Publication||Impact Factor journals|