Modified anodic aluminum oxide template for vertically aligned and laterally connected carbon tube arrays

My first PhD research project aimed at the preparation of vertically aligned nitrogen-doped carbon tubes via chemical vapor deposition (CVD) onto a commercial aluminum oxide (AAO) template. I can recount several challenges associated with the hard template process: After high temperature carbonization (>800°C), the chemical resistance of the alumina hard template towards mild etching agents including NaOH and HCl increased; in addition to the carbon tube formation in shape of the surface of the pores, a carbon layer covered the inter-pore space on top and bottom of the AAO template and had to be removed; the removal of the AAO template without a polymer matrix anchoring the tubes caused agglomeration, with the tubes sticking on each other on the outer walls. This agglomeration restricted the available surface area to the tubes’s inner surfaces…

At a later stage, during my thesis write up, I encountered the recent articles of Guowen Meng and coworkers from Hefei Institute of Physical Sciences and I was particularly intrigued by his teams  innovative approach to create a three-dimensional (3D) carbon tube (CT) grid (3D-CTG): a modified anodic aluminum oxide template allowed to keep the inter carbon tube distance post etching by introducing lateral connection channels between neighboring vertically aligned tubes. (1)

Image 1: Schematic illustration of synthetic procedures of the 3D-CT grid: CVD growth of 3D-CT/3D-AAO composite and subsequent etching to remove AAO template produces 3D-CT grid with laterally interconnected vertical CTs.

3D-AAO: Adding lateral pore connections to AAO

The key to this breakthrough lies in modification of the AAO template with lateral pores connecting adjacent vertical channels. These templates can be prepared from aluminum films containing trace metal impurities (<1%), with copper being a suitable choice, as described in the works of Thompsons (2), Inoue (3), and Vereecken (4) on anodizing impurity-containing aluminum films. Anodization of pure aluminum leads to the formation of vertical pores separated by aluminum oxide, which can serve as template for vertical carbon tubes. When copper impurities are present in the aluminum, the metal phases are incorporated locally into the aluminum oxide pore walls. It is hypothesized that during anodization the metal impurities accumulate beneath the aluminum layer if their free energy of oxide formation is higher than that of aluminum. When the concentration of the metal impurity surpasses a critical threshold, they are incorporated int the oxide layer. Residual metals in the pore walls is either etched away by the electrolyte or subsequent wet-chemical etching, leaving behind horizontal pores. (Image 1)

Low temperature CVD Process

The chemical vapor deposition process is conducted in a gas mixture of acetylene in argon at 650°C, forming a uniform carbon layer on the AAO template. This relatively low CVD temperature offers a crucial advantage: the AAO template remain etchable, which becomes significantly more challenging after CVD processes at temperatures exceeding 800°C, as I observed during my PhD work. The article of Guowen Meng and coworkers reported that amorphous carbon on the surface must be removed using argon palm etching to expose the AAO template and clear the inter-tube distances. Following this, the AAO template was selectively etched using 6 M NaOH at 60°C. Alternatively, the sample could be soaked in nick has to be removed via etching argon plasma to give access to the AAO template and remove the carbon on the inter tube distances. They reported to have etched the AAO template in 6 M NaOH at 60°C to selectively remove the AAO template, to obtain the 3D-CTG.

Applications in miniaturized electronic equipment

The interconnected carbon tube films have been demonstrated as electrode components for application in catalysis and electronics, showcasing competitive results with conventional benchmarks in both fields. A key electronic component in portable electronics is the filter capacitor, which is used in filter circuits for alternating current (AC) / direct current (DC) conversion. The drawback commercial aluminum electrolytic capacitors (AECs) for AC line filtering, is their large space requirements due to their low volumetric capacitances, which prevents device miniaturization. A promising alternative are electric double-layer capacitors (EDLCs), which have higher areal and volumetric capacitance and are widely used in energy storage. However, the frequency response for ac-line filtering requires their phase angles to be lower than -81°, a condition rarely met by the EDLCs. The EDLCs with interconnected carbon tube network prepared with help of AAO template combine the advantages of both worlds of capacitors: 1. Phase angles below -80° and 2. High areal and volumetric capacitances. The reported three dimensional interconnected carbon tube array is a materials breakthrough for the miniaturization of filtering capacitors with high capacitance.

Image 2: a) Phase angle versus frequency of 3D-CT-10 with a thickness of 10 μm in comparison with commercial AEC (Panasonic, Japan, 6.3 V/330 μF). Reproduced with data from (1). b) LSV curves of nitrogen-oped N-CTs, ultra-low platinum loaded 3D-CTs-1.04 compared with Pt/C (20 wt%) benchmark. Reproduced with data from (5).

Applications in catalysis

Another field for the application of the tubular carbon structured array (3D-CTG) is catalysis, in which it was utilized as a freestanding support for a platinum (Pt) single-atom catalyst (SAC). (5) Although Pt SACs with with ultra low loadings have demonstrated excellent performance in the hydrogen evolution reaction (HER), they are typically in powder form and require immobilization using polymeric binders to form electrodes. The freestanding carbon tube array overcomes this limitation, offering vertically aligned channels (diameter: 200 nm) and lateral channels (diameter 50-120 nm) that facilitate fast electrolyte diffusion and rapid detachment of hydrogen bubbles. The freestanding nitrogen-doped carbon tubular supports were loaded with platinum. (Pt@N-CTs)

The carbon tube array supports with a 1.04 wt% platinum loading (Pt@N-CTs-1.04) outperform Pt/C (20 wt%) reference catalyst at high current densities for the HER, achieving 1.0 A/cm² at an overpotential of 157.9 mV , which is substantially lower than the 221.5 mV overpotential determined for the commercial Pt/C (20 wt%) cathode. (Image 2) This impressive performance is attributed to the superior conductivity provided by the 3D-interconnected nitrogen-doped carbon tube network, favorable mass transport conditions within the channels, and highly exposed actives enabled by the polymeric binder-free cathode composition. Conclusively, the ampere-level current density achieved by the Pt@N-CTs-1.04 cathode highlights commercial prospects of the 3D-CTG support in the hydrogen production.