Carbon xerogels electrochemical oxidation and correlation with their physico-chemical properties

Abstract

The electrochemical oxidation of carbon represents a relevant limitation for the application of carbon-based materials at the electrodes of several electrochemical devices, like fuel cells, metal-air batteries, water electrolyzers or supercapacitors. Understanding the key influencing parameters is thus of paramount importance to prevent and avoid the degradation of carbon materials. In the present investigation, carbon xerogels (CXGs) as a model of synthetic amorphous carbon, were studied on the basis of different surface chemistry and structure. Electro-oxidation experiments consisted of potential holding (1.2 V vs. RHE) on electrodes prepared with CXGs. The results indicate that the presence of oxygen groups (-C-O) and a higher ordering degree hinder the carbon oxidation reaction. A sub-structural parameter considering both the amount of oxygen-free carbon atoms (C-C + C*-C-O) from X-ray photoelectron spectroscopy, and the disordering degree from Raman spectra (in terms of I_D/I_G) is here proposed as a new factor to evaluate the tendency of an amorphous carbon material, like xerogels, to be electrochemically oxidized.

Introduction

Advanced carbon-based materials are playing a key role nowadays in the development of low-cost and environmentally friendly electrochemical devices devoted to the conversion and storage of energy, such as fuel cells, water electrolyzers, advanced batteries, solar cells and supercapacitors [[1], [2], [3]]. Over the past decades, the rise of novel carbon materials (graphene, nanotubes, nanofibers, etc.) characterized by remarkable physico-chemical and surface properties, has been responsible for the success of more efficient electrodes, adding high value to the electrochemical conversion of energy [4]. In particular, the very low density of carbon gels and their easily tunable porous texture have attracted a great interest within the scientific community [5,6]. Since Pekala et al. first reported the synthesis of carbon gels [7], plenty of works have been aimed to optimize the porous structure of different types of gels to be fitted in specific applications [6,[8], [9], [10], [11], [12]]. Moreover, great efforts have been done to reduce the production cost of high quality carbon gels, as recently revised by Rojas-Cervantes [13].

Regardless the more or less graphitic nature of carbon materials, they are prone to electro-oxidation at highly positive potentials. The equilibrium potential of the carbon oxidation reaction (COR) to carbon dioxide is 0.207 V vs. NHE at 25 °C [14]. In the presence of electron acceptor species, like water (present in aqueous electrolytes and/or electrode environments) or oxygen (as in the case of the positive electrode of fuel cells, metal-air batteries, electrolyzers, etc.), the occurrence of relatively high positive potentials (>1 V vs. RHE) unavoidably results in a certain degree of COR, and thus, the degradation and eventual modification of the electrode initial properties. At the positive electrode of an electrochemical cell, potentials more positive than the reversible potential for COR, may occur under normal operation (charge/discharge processes, current transients with time, etc.). Besides, some abnormal working conditions and/or situations, such as sudden start-stop cycles or the starvation of reactant species, can be exceedingly damaging for carbon materials at the electrodes [15]. A typical example is the carbon corrosion at the cathode of polymer electrolyte fuel cells, which is a major concern towards the development of stable systems based on this technology [[16], [17], [18]].

Understanding the correlation between carbon features and oxidation kinetics is thus of high relevance to ameliorate electrode degradation [19]. The COR is a complex process involving several steps and the role of carbon matrix characteristics is not yet well understood. Up to date, the main known carbon features conditioning its electro-oxidation are correlated with high surface area, large graphitic interlayer spacing, high amount of oxygen species and low amount of graphitic domains [[20], [21], [22]]. The electrochemical oxidation of carbon has been investigated from different points of view [19,[23], [24], [25], [26], [27]], but the reaction mechanism is not yet clear. As a consequence, only a few strategies have been proposed to ameliorate the degradation of carbon. Most promising approaches consist of reducing the surface area [28], controlling the morphology or structure [29,30] or modifying the chemical composition at the surface [31,32]. Still, the individual contribution of each parameter is not well defined. Empirical investigations can lead to a general understanding based on a specific carbon morphology. For further details on kinetic models and interpretation, it is recommended to consult the work of Gallagher and Fuller [33].

Carbon xerogels (CXGs) are of great interest, from a quality-to-cost point of view, among the different carbon gels typologies (mainly comprising also aerogels and cryogels) [12]. CXGs, though possessing a certain ordering degree in the short-range, are not graphitic materials [[34], [35], [36]]. This fact, together with their relatively highly developed porosity, makes them more prone to oxidation processes than graphite-like carbons, such as carbon nanotubes or nanofibers [37]. The main scope of the present work is to examine the electrochemical oxidation behavior of CXGs differing in porosity and surface chemistry. A cross-analysis between physico-chemical properties and electrochemical behavior is discussed, aimed to design strategies to minimize degradation phenomena on porous amorphous carbonaceous materials.