Lithium-Induced Oxygen Vacancies in MnO₂@MXene for High-Performance Zinc–Air Batteries

Abstract

The traditional methods for creating oxygen vacancies in materials present several challenges and limitations, such as high preparation temperatures, limited oxygen vacancy generation, and morphological destruction, which hinder the application of transition metal oxides in the field of zinc–air batteries (ZABs). In order to address these limitations, we have introduced a pioneering lithium reduction strategy for generating oxygen vacancies in δ-MnO2@MXene composite materials. This strategy stands out for its simplicity of implementation, applicability at room temperature, and preservation of the material’s structural integrity. This research demonstrates that aqueous Ov-MnO2@MXene-5, with introduced oxygen vacancies, exhibits an outstanding oxygen reduction reaction (ORR) activity with an ORR half-wave potential reaching 0.787 V. DFT calculations have demonstrated that the enhanced activity could be attributed to adjustments in the electronic structure and alterations in adsorption bond lengths. These adjustments result from the introduction of oxygen vacancies, which in turn promote electron transport and catalytic activity. In the context of zinc–air batteries, cells with Ov-MnO2@MXene-5 as the air cathode exhibit outstanding performance, featuring a significantly improved maximum power density (198.3 mW cm–2) and long-term cycling stability. Through the innovative strategy of introducing oxygen vacancies, this study has successfully enhanced the electrochemical catalytic performance of MnO2, overcoming the limitations associated with traditional methods for creating oxygen vacancies. Consequently, this research opens up new avenues and directions for nonprecious metal catalyst application in ZABs.

Ammonium ion intercalation and oxygen-rich vacancies in birnessite-type MnO2 for supercapacitors and oxygen evolution applications

“…S3(a) and (b) (ESI†), in the O 1s spectra of C-A-MnO 2 and δ-MnO 2 , the peaks close to 533.3 eV were attributed to H–O–H (hydrated oxygen), and the peaks between 531.0 and 532.0 eV corresponded to Mn–O–H (surface adsorbed oxygen), and there was no significant shift in the binding energies of the four samples. 14,36,56,57 The Mn–O–Mn (lattice oxygen) in δ-MnO 2 was located at 529.4 eV, but the peak of A-MnO 2 shifted a little to the higher binding energy at 529.7 eV, which further proved that the introduction of NH 4 + ions changed the δ-MnO 2 coordination structure. According to a previous report, the degree of deficiency of oxygen can be evaluated from the contents of adsorbed and hydrated oxygen on the surface.…”

“…The presence of oxygen vacancies improved the conductivity, promoted redox reactions and contributed to enhancing electrochemical properties. 14 From the peak area comparison, the contents of Mn 3+ and Mn 4+ in δ-MnO 2 were about 65.5% and 34.5%, while the contents of Mn 3+ and Mn 4+ in A-MnO 2 were about 77.6% and 22.4% after NH 4 Cl treatment. According to the reaction process (7), the presence of Mn 3+ in MnO 2 required the generated certain oxygen vacancies to keep its overall charge balance.…”

“…5a, the result indicated that C-A-MnO 2 and δ-MnO 2 exhibited strong and symmetric resonance centered at g = 2.003, and the signal intensities after NH 4 + ions treatment were both larger than the signal intensity of δ-MnO 2 , which proved more oxygen vacancies existed in C-A-MnO 2 . 14 It had the most contents of oxygen vacancies at the NH 4 + ions concentration of 1 M (Fig. 5b).…”

“…12,13 Besides, TMOs can also be used as potential catalysts for the oxygen evolution reaction (OER) due to their tunable catalytic activity, abundance of active sites and good stability. 14 In particular, MnO 2 is considered as a promising positive material due to some desirable characteristics, for example, rich resources, high theoretical capacity (1370 F g À1 ), low price, non-toxicity and environmental friendliness. [15][16][17][18][19] There are six general crystal forms in MnO 2 : a-MnO 2 , b-MnO 2 , d-MnO 2 , l-MnO 2 and R-MnO 2 .…”

“…12,13 Besides, TMOs can also be used as potential catalysts for the oxygen evolution reaction (OER) due to their tunable catalytic activity, abundance of active sites and good stability. 14…”