Effect of surface oxides on hydrogen storage of activated carbon

Abstract

The influence of surface oxides of activated carbon (AC) was experimentally studied to ascertain the adsorption characteristics of hydrogen. Super activated carbon was prepared by chemical activation in nitrogen at 1073 K, using KOH as the activating agent and litchi wood as the precursor. The activated carbon was oxidized by nitric acid or hydrogen peroxide at various temperatures. Porous texture of all ACs was characterized by nitrogen adsorption at 77 K using an automatic adsorption system. The amounts of acidic surface oxides of oxidized ACs were determined by the Boehm's titration method and X-ray photoelectron spectroscopy (XPS). The hydrogen adsorption was accurately measured by a volumetric adsorption apparatus at 77 K up to 0.1 MPa and 303 K up to 6 MPa, respectively. Experimental results revealed that specific surface area and micropore volume of oxidized ACs slightly changed after oxidation at lower temperatures (below 323 K), however, drastically decreased after oxidation at 373 K. The total surface acidity greatly increases due to the wet oxidation. The total acidity of the AC oxidized by 3N HNO₃ at 373 K is about 29 times as large as that of the original AC. The hydrogen capacity of oxidized ACs is significantly suppressed when the acidic group amounts are larger than 0.8 mmol/g.

Introduction

Hydrogen is considered as the future fuel for on-board applications because it burns cleanly without producing pollutants and can be produced from renewable energy sources. However, the main problem with the utilization of hydrogen as a transportation fuel is the storage of hydrogen which presently cannot meet the storage targets (gravimetric capacity and volumetric capacity) set by the US Department of Energy (DOE) for on-board storage systems [1]. The conventional methods (liquefaction and compression) have been used to store hydrogen. Nevertheless, these methods pose problems such as large boil-off loss, the need for heavy containers, safety and high capital cost. Various materials have been explored as the candidates for hydrogen storage including metal hydrides [2], [3], chemical hydrides [4], [5], and adsorbent carbons [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16].

Various carbonaceous materials, including nanotubes [6], [7] graphite nanofibers [6], metal organic frameworks [8], [9], and traditional activated carbons (ACs), have been widely investigated experimentally and theoretically as potential adsorbents in hydrogen storage because of their large surface area, microporous structure, low mass density, and fast adsorption/desorption kinetics. Furthermore, AC has the advantage of high surface area, availability, and low cost, compared with other carbonaceous materials such as carbon nanotubes and carbon nanofibers [11]. The influence of the various physical properties of the carbon adsorbent such as the surface area, micropore volume, pore size distribution on hydrogen storage capacity has been investigated extensively [12], [13], [14], [15], [16]. Pores with two [15] or three [16] times the diameter of the hydrogen molecule have been demonstrated to be the optimum size for hydrogen storage, because high gas density was obtained in the narrow pores from theoretical calculations. Furthermore, hydrogen capacity exhibits a linear correlation with specific surface area and micropore volume of carbonaceous materials [12], [13], [14].

Surface complexes onto carbon samples were also considered as key factor in hydrogen adsorption. Agarwal et al. [17] reported that hydrogen capacity increased with increasing amounts of the surface acidity of the ACs. On the contrary, some papers [18], [19], [20], [21] indicated that oxygen functional groups repressed hydrogen uptake on ACs. Georgakis et al. [18] evaluated theoretically the adsorption capacity of hydrogen on microporous carbonaceous materials and reported that hydrogen adsorption is always higher in the pure materials than in the oxygenated structures. They used a slit-shaped pore model to simulate the solid structure of microporous active carbon. However, there are millions of model pores inside structure of activated carbon. A sophisticated experiment should be conducted to verify the effect of oxygen functional groups. Takagi et al. [19] investigated hydrogen adsorption properties of activated carbon fibers (ACFs) oxidized with (NH₄)S₂O₈, and found that the amount of hydrogen adsorbed on the ACF samples were remarkably decreased by oxidation. Zhao et al. [20] used HNO₃ to introduce various oxygen functional groups on the carbon surface. They found that the presence of functional groups has a detrimental effect on the maximum amount of hydrogen adsorption. Bleda-Martínez et al. [21] studied electrochemical storage of hydrogen in carbon materials and reported that the higher the amount of surface oxygen groups the lower is the hydrogen uptake. Clearly, the discrepancy amongst various theoretical predictions and experimental results for hydrogen adsorption on oxidized ACs remains to be elucidated.

In this study, litchi wood-based AC samples with high porosity were prepared using the KOH activation method. The prepared AC samples were oxidized by HNO₃ and hydrogen peroxide. The amounts of the surface oxides of oxidized ACs were determined by the Boehm's titration method and X-ray photoelectron spectroscopy (XPS). Hydrogen storage measurements were performed by a volumetric apparatus. Effects of porous texture and functional groups of ACs on hydrogen adsorption were investigated.