# Coaxial nickel cobalt selenide/nitrogen-doped carbon nanotube array as a three-dimensional self-supported electrode for electrochemical energy storage

## TL;DR

A new three-dimensional electrode structure is developed for energy storage, offering high capacity and long cycle life.

## Contribution

A one-step method to create a coaxial nickel cobalt selenide/nitrogen-doped carbon nanotube array as a self-supported electrode.

## Key findings

- The coaxial array electrode shows over 7 times higher areal capacity than conventional structures.
- The device achieves a volumetric capacity of 22.5 C cm−3 and retains 86% after 10,000 cycles.
- It delivers a volumetric energy density of 4.9 mW h cm−3 and a power density of 208.1 mW cm−3.

## Abstract

Herein, we propose a one-step urea pyrolysis method for preparing a nitrogen-doped carbon nanotube array grown on carbon fiber paper, which is demonstrated as a three-dimensional scaffold for constructing a nickel cobalt selenide-based coaxial array structure. Thanks to the large surface area, interconnected porous structure, high mass loading, as well as fast electron/ion transport pathway of the coaxial array structure, the nickel cobalt selenide/nitrogen-doped carbon nanotube electrode exhibits over 7 times higher areal capacity than that directly grown on carbon fiber paper, and better rate capability. The cell assembled by a nickel cobalt selenide/nitrogen-doped carbon nanotube positive electrode and an iron oxyhydroxide/nitrogen-doped carbon nanotube negative electrode delivers a volumetric capacity of up to 22.5 C cm−3 (6.2 mA h cm−3) at 4 mA cm−2 and retains around 86% of the initial capacity even after 10 000 cycles at 10 mA cm−2. A volumetric energy density of up to 4.9 mW h cm−3 and a maximum power density of 208.1 mW cm−3 are achieved, and is comparable to, if not better than, those of similar energy storage devices reported previously.

An energy storage device with a voltage window of 1.6 V is developed using three-dimensional coaxial array electrodes, and exports a volumetric energy density of 4.9 mW h cm−3 at 4 mA cm−2.