Investigation of Si-Coated Multiwalled Carbon Nanotubes as Potential Electrodes for Multivalent Metal-Ion Electrochemical Energy Storage Systems

Abstract

In the present paper, we report the chemical vapor deposition of multiwall carbon nanotubes (MWCNTs) coating on stainless steel substrates and the coverage of the individual CNTs by a silicon (Si) thin layer. The study's goal is to create and evaluate electrochemically Si/MWCNT composites with varying Si loadings to act both as a mechanical support and electrical conducting path to the current collector. MWCNTs incorporated efficiently into the Si layer on their surface and aided Si's poor stability while maintaining its high-capacity values. The Si/ MWCNT composite electrodes operated in a wide potential window of 1.7 V with improved Q/V ratios (charge to voltage) and retention over cycling compared to plain MWCNTs and Si electrodes, despite the similar surface area. In addition, the composite electrodes displayed a larger integration area in cyclic voltammograms, distinct charge/discharge plateaus, and wider discharge potential, indicating a higher energy storage capacity and high relative voltammetric charge retention. In particular, the Si/ MWCNT electrodes demonstrated relative voltammetric charge retentions of 99.3% and 68% at 100 mV s −1 and stable Q/V ratios of 231 and 209 mF cm −2 at a current density of 2 mA cm −2 with retention maintained at 100% after 30 cycles in ZnSO 4 and Al 2 (SO 4 ) 3 aqueous electrolytes, respectively. Although the Q/V ratios may have distinct origins, we assumed that both electrolytes incorporate a mix of surface and bulk contributions. A combination of both ionic diffusion and pseudocapacitive behavior is displayed. Combined charge storage mechanisms are present at different electrode states, indicating the electrolyte ions' adsorption (surfacecontrolled) and insertion (diffusion-controlled) in the active sites of the composites. The results suggested that the pseudocapacitive nature combined with the reversible diffusion of ions during redox reactions worked synergistically to increase contributions to the overall Q/V ratios. The storage of ions is facilitated by the small ionic radius of Al 3+ (0.53 Å) and Zn 2+ (0.75 Å) ions. Diffusion coefficient values of Al 3+ (4.58 × 10 −11 cm 2 s −1 ) are higher than those of Zn 2+ (3.16 × 10 −11 cm 2 s −1 ) during oxidation but lower at reduction processes (6.02 × 10 −11 and 7.1 × 10 −11 cm 2 s −1 ). As a result, the Al 3+ ions are less effective while extracted with reduced reversibility than Zn 2+ ions. Agglomerations of smaller ions stored in electrodes might confine the extraction process and possibly the overall stability over intense cycling. The faradaic performance on the one side coupled with the semi-infinite diffusion and pseudocapacitive behavior could offer a promising utilization as working electrodes in Zn 2+ and Al 3+ ion hybrid energy storage systems.