Engineering the microstructures of manganese dioxide coupled with oxygen vacancies for boosting aqueous ammonium-ion storage in hybrid capacitors

The aqueous ammonium ion (NH₄⁺) is a promising charge carrier in virtue of its safety, environmental friendliness, abundant resources and small hydrated ionic size. The exploration of NH₄⁺ host electrodes with good reversibility and large storage capacity to construct high-performance ammonium-ion hybrid capacitors (AIHCs), however, is still in its infancy. Herein, a facile etching technique is put forward to produce oxygen-deficient MnO₂ (O_d-MnO₂) as the electrode material for NH₄⁺ storage. According to the experimental and theoretical calculation results, the etching process not only creates more porosity, offering abundant active sites, but also generates abundant oxygen vacancies, which modify the structure of pristine MnO₂, enhance charge storage capacity and boost ion diffusion kinetics. Consequently, O_d-MnO₂ can deliver a specific capacity of 155 mAh·g⁻¹ at 0.5 A·g⁻¹ and a good long-term cycling stability with 86.8% capacity maintained after 10,000 cycles at 5.0 A·g⁻¹. Additionally, the NH₄⁺ storage mechanism was evidenced by several ex-situ characterization analyses. To examine the actual implementation of O_d-MnO₂ as a positive electrode for NH₄⁺ full device, AIHCs are assembled with activated carbon functionalized with Fe₃O₄ nanoparticles (Fe₃O₄@AC) as a negative electrode. A high specific capacitance of 184 F·g⁻¹ at 0.5 A·g⁻¹, satisfactory energy density of 102 Wh·kg⁻¹ at 500 W·kg⁻¹, a low self-discharge rate and good cycling durability after 10,000 cycles are attained. The electrochemical performance of these AIHCs is comparable to or surpass those of traditional supercapacitors with metal ions as charge carriers, highlighting the advantages of structural modification in enhancing the NH₄⁺ storage performance. Graphical abstract: (Figure presented.)