Enhancing capacitive deionization performance and cyclic stability of nitrogen-doped activated carbon by the electro-oxidation of anode materials

Abstract

Electrode materials with high desalination capacity and long-term cyclic stability are the focus of capacitive deionization (CDI) community. Understanding the causes of performance decay in traditional carbons is crucial to design a high-performance material. Based on this, here, nitrogen-doped activated carbon (NAC) was prepared by pyrolyzing the blend of activated carbon powder (ACP) and melamine for the positive electrode of asymmetric CDI. By comparing the indicators changes such as conductivity, salt adsorption capacity, pH, and charge efficiency of the symmetrical ACP–ACP device to the asymmetric ACP–NAC device under different CDI cycles, as well as the changes of the electrochemical properties of anode and cathode materials after long-term operation, the reasons for the decline of the stability of the CDI performance were revealed. It was found that the carboxyl functional groups generated by the electro-oxidation of anode carbon materials make the anode zero-charge potential (Epzc) shift positively, which results in the uneven distribution of potential windows of CDI units and affects the adsorption capacity. Furthermore, by understanding the electron density on C atoms surrounding the N atoms, we attribute the increased cyclic stability to the enhanced negativity of the charge of carbon atoms adjacent to quaternary-N and pyridinic-oxide-N.

1. Introduction

Water scarcity around the world calls for a low-cost, energy-saving, and environmentally friendly desalination technology for sewage and brine treatment. Different from traditional membrane separation and thermal evaporation technologies, in capacitive deionization (CDI), charged ions are reversibly stored in electrical double layers (EDLs), which are formed on the interface between porous carbon materials and the electrolyte when a limited voltage is applied to electrodes [1]. Based on this mechanism, CDI has significant advantages such as low energy consumption as well as no secondary pollution.

The main cost of CDI is focused on the replacement and maintenance of electrode materials and membrane components. Improving longevity of electrode materials means that it is necessary to clarify the causes of the deterioration of the cycle stability of CDI and provide powerful strategies based on this. Carbon electrode oxidation is the most key impactor leading to the decline of CDI desalination performance [2,3].

Because of low cost and abundance, conventional carbon materials still play a key role in CDI community, such as graphene [4], carbon aerogel (CA) [5], carbon nanotube [6], and activated carbon powders (ACPs) [7,8]. Cohen et al. [9] believed that the anode is gradually oxidized in the long-term test and that oxygen-containing functional groups formed on the surface of carbon electrode. At the same time, the pore wall will collapse, leading to the decrease of average pore diameter and relative specific surface area, as a result of unstable performance. In addition, Shapira et al. [10] claimed that the oxygen reduction reaction at the negative electrode may be the trigger for the parasitic surface oxidation of the positive carbon electrode. Srimuk et al. [11] further traced it to the negative effect of hydrogen peroxide from the reduction of oxygen on the negative electrode. To alleviate the deterioration of carbon, more and more efforts have been down, such as reducing the application potential window [12], periodically alternating the voltage [13] and modifying the surface of electrode [14]. Introducing heteroatoms, such as nitrogen, sulfur, or boron, into carbon matrix structures is proved to be effective in changing the microstructure and physicochemical properties of carbonaceous materials [15–17]. Among them, the nitrogenization of carbons is considered as a most feasible and adjustable approach to enhance the performance of CDI desalination and supercapacitors [18,19]. Through altering the pyrolysis temperatures in post-treatment methods that direct calcine carbons with N-sources, the species of N-containing functional groups and their content will be the corresponding change [20,21]. The other way is the in-situ synthesis approach, of which the pyrolysis material is both acted as the carbon and nitrogen source [22,23]. The latter method can provide various morphologies and higher atom percentage of nitrogen content of nitrogen-doped activated carbon (NAC) by using specific precursors.

It was found that N-doping could improve the hydrophilicity and ...