I. Abstract of the Paper’s Core Research

This study employs a CVD‑grown substrate consisting of 40 μm‑high, vertically aligned carbon nanotube arrays. A monodisperse cobalt ferrite DMF‑based nanoink was formulated, and aerosol jet printing was used to achieve uniform three‑dimensional encapsulation of the nanoparticles. By comparing drop‑coating with varying numbers of print passes (5, 10, and 15) and examining the electrochemical performance of symmetric versus asymmetric devices, the study comprehensively elucidates the relationships among deposition process, microstructural morphology, and energy‑storage performance. The key findings are as follows:

Mechanism of Deep and Uniform Impregnation and Forming in Aerosols

Sheath‑gas high pressure drives micron‑sized droplets to penetrate the interstitial spaces of carbon nanotubes, resulting in uniform deposition of CoFe₂O₄ nanoparticles across the entire length, from the bottom to the top of the tubes. In contrast, drop‑coating leads to material accumulation only at the array’s apex, leaving the middle and base regions devoid of active material and wasting a significant portion of the pore channels.

Precise control of the number of printed layers for regulating active‑ingredient loading.

5 layers: thin, uniform coating yields the highest specific capacitance; 10 layers: optimal balance between loading amount and pore structure, resulting in the best areal capacitance; 15 layers: nanoparticle agglomeration blocks the pores, hindering ion transport and leading to a performance decline; drop‑coating achieves a high total loading but with extremely poor distribution.

Asymmetric devices significantly broaden the voltage window.

An asymmetric structure is constructed by depositing CoFe₂O₄ on one electrode and pure VACNTs on the other; in an aqueous electrolyte, the voltage window reaches 1.76 V, whereas it is only 1.0 V for a symmetric device, and the square‑voltage effect significantly enhances the energy density.

A leapfrog improvement in electrochemical performance

10 printings of asymmetric microcapacitors: the areal capacitance is 0.59 mF·cm⁻², a 13-fold improvement over devices made solely from carbon nanotubes; the areal energy density is 0.65 mWh·cm⁻², and the power density is 5.48 mW·cm⁻², representing increases of 43-fold and 15-fold, respectively.

Digital graphic partitioning and deposition capability

CAD allows custom definition of single‑sided or double‑sided deposition regions for interdigitated electrodes, enabling active material to be printed exclusively on the target electrode and automatically fabricating asymmetric devices—without the need for masks or secondary patterning steps.

II. Complete Preparation Procedure for CoFe₂O₄–VACNT Microcapacitors

1) Oxidative insulation of the silicon substrate, followed by evaporation of Cr/Au current collectors, a Ti buffer layer, and an Fe catalytic layer;

2) A PECVD tube furnace was used to grow a 40-μm-thick array of vertically aligned carbon nanotubes at 850°C, with interdigitated electrodes.

3) Monodisperse CoFe₂O₄ nanoparticles were synthesized via the micelle method, and a stable aerosol ink was formulated using DMF.

4) Optomec aerosol equipment, ultrasonic atomization, carrier gas at 35 sccm, sheath gas at 70 sccm, substrate temperature at 50°C;

5) Set printing cycles of 5, 10, and 15, respectively; single-sided printing is for asymmetric components, while double-sided printing is for symmetric ones.

6) The ink’s polymeric shell is removed by annealing at 200°C, and the electrochemical workstation performs comprehensive CV and GCD tests.

III. Five Core Process Advantages of Aerosol Deposition (Compared with Drop‑Coating and Spin‑Coating)

Uniform, all‑encompassing coating of the three‑dimensional carbon framework

High-speed focusing enables droplets to penetrate the micron-scale gaps between carbon nanotubes, ensuring uniform distribution of the active material from bottom to top and fully leveraging the entire specific surface area of the carbon nanotubes.

Digital partitioning–controlled deposition enables the one-step fabrication of asymmetric devices.

Pseudocapacitive materials can be directly printed on a single-sided interdigitated electrode, eliminating the need for stepwise lithography and masking processes, and enabling flexible, tunable device architectures.

Load capacity is linearly and precisely controllable.

By precisely controlling the loading of active substances through the number of printing cycles, it can be tailored to meet two distinct application requirements: high areal density and high specific mass.

Maskless direct-write microfabrication, compatible with on-chip microelectrodes.

The minimum printable line width can reach 10 μm, making it compatible with millimeter-scale interdigitated micro‑energy‑storage chips while avoiding contamination of the electrode isolation regions.

Gentle low-temperature forming, with no damage to the carbon nanotube structure.

The printing and post-processing temperatures are low, resulting in well-preserved vertical carbon nanotube structures that do not collapse or tilt.

IV. Five Core Technological Innovations of This Study

Innovation 1: Aerosol Deep-Impregnation Three-Dimensional Carbon Nanotube Process

It is demonstrated that aerosol droplets can penetrate thick VACNT arrays, thereby addressing the inherent limitation of drop‑coating, which results in deposition only at the surface layer.

Innovation 2: Number of Printing Channels – Quantitative Matching System for Energy Storage Performance

Establish a database of microstructural and capacitive parameters corresponding to 5, 10, and 15 printing cycles, and identify two optimal process windows.

Innovation: A New Paradigm of Asymmetric Micro-Supercapacitors with Direct Aerosol Printing

Single‑device, single‑step processing enables differentiation of positive and negative electrode materials, significantly streamlining the fabrication process for heterogeneous energy‑storage devices.

Innovative Dual-Metal Ferrite–Composite Three-Dimensional Carbon-Based High-Efficiency Energy Storage System

By integrating carbon-based double-layer capacitance with the pseudocapacitance of cobalt ferrite, both the capacitance and the voltage window are synergistically enhanced.

Innovation 5: Microscopic Immersion—Theory of Electrochemical Performance Correlation

From the perspective of ion-accessible specific surface area, this study elucidates how the deposition morphology influences charge–discharge performance, thereby providing a theoretical basis for optimizing three-dimensional electrode fabrication processes.

V. Current Industry Challenges and Corresponding Solutions

Challenge 1: During drop-casting and spin-coating, the active material deposits only at the tips of the carbon nanotubes, leaving the internal channels unoccupied.

✅ Aerosol sheath‑gas–driven droplets penetrate deep into the array, with carbon nanotubes uniformly loaded with nanoparticles along their entire length, fully leveraging their three-dimensional specific surface area.

Challenge 2: Conventional fabrication methods struggle to pattern asymmetric energy-storage electrodes, necessitating additional lithographic masking.

✅CAD‑specific single‑sided electrode printing area enables one‑step fabrication of asymmetric structures, reducing manufacturing steps and costs.

Challenge 3: Uncontrolled loading of active materials leads to excessive agglomeration, which clogs pores and results in capacity degradation.

✅ By linearly tuning the loading via the number of printing passes, an optimal balance between load and porosity is achieved in just 10 print cycles.

Challenge 4: On‑chip microelectrodes demand high precision, and coating can easily lead to material overflow, causing electrode short circuits.

✅ High-precision aerosol-focused beam, depositing material exclusively in the target area with no excess material in the interdigitated gaps.

Challenge 5: Symmetric aqueous capacitors suffer from a narrow voltage window and relatively low energy density.

✅ Aerosol-based unidirectional deposition constructs an asymmetric system, with the operating voltage nearly doubled and the energy density significantly increased.

VI. Future Diversified Industrial Application Scenarios

1 On-chip Micro-Energy Storage for IoT Chips

MEMS sensors and passive RF chips integrate miniature supercapacitors, enabling a miniaturized, long‑life power supply unit.

2 Flexible Wearable Physiological Sensing Patch

A flexible‑substrate, gel‑electrolyte‑based 3D printing technique for energy‑storage electrodes, enabling lightweight monitoring without external batteries.

3 Miniature Implantable Medical Devices

Low‑volume, high‑power microcapacitors, suitable for short‑term power supply in minimally invasive monitoring and micro‑drug‑delivery chips.

4 Flexible Printed Electronics Rapid Prototyping Platform

Rapidly iterate on various load conditions and symmetric/asymmetric energy storage devices, thereby shortening the R&D cycle for electrode materials.

5 Onboard Micro Power Supply for Micro-Robots

A compact drive mechanism is paired with an ultra-thin energy-storage array to meet the demands of instantaneous high‑power discharge.

Image and Text Guide

Table 1. EDX Elemental Distribution and CoFe₂O₄ Mass Loading (Drop‑Coating vs. AJP with Different Numbers of Passes)

Figure 1. Complete fabrication process of the CoFe₂O₄–VACNT 3D MSC

Figure 2. Cross-sectional SEM + EDX elemental mapping: AJP 5/10/15 passes vs drop-casting

Figure 3. High-magnification SEM cross-section of the VACNT midsection

Figure 4. Electrochemical characterization of electrodes and asymmetric devices (optimization of AJP pass count)

Figure 5. Comparison of Symmetric vs. Asymmetric Devices (10 Passes)