Influence of acid-base catalysis on the textural and thermal properties of carbon aerogel monoliths

Highlights

• Carbon aerogel monoliths (CAMs) with high strength and low thermal conductivity were prepared through acid-basic catalysis.

• The phenolic oligomers formed by acid catalysis were fastened tightly to become robust skeleton through later base catalysis.

• The failure strength of pure CAMs can reach 1.76 MPa with the thermal conductivity at 2000 °C of 0.809 W/(m•K).

Abstract

The preparation of carbon aerogel monoliths (CAMs) with low density, low thermal conductivity and high strength concurrently is a challenge, which restricts their application in the field of ultra-high thermal insulation. Acid and basic two-step catalysis was successfully used for preparing the low density CAMs with both high strength and low thermal conductivity. The linear phenolic oligomers originated from acid catalysis were fastened tightly to become robust skeleton architecture through later crosslinking polymerization resulted from base catalysis. The strong nanoporous structure can afford the capillary force during ambient drying and shrink little to obtain low density CAMs with high specific surface area, and endows the CAMs with high compressive strength. With no need to prepare composites, the failure strength and elastic modulus of pure CAMs can reach 1.76 MPa and 29.73 MPa, respectively, with the specific surface area of 689 m²/g and pore volume of 2.122 cm³/g. Moreover, the thermal insulation property of this CAM is very outstanding with the thermal conductivity at 2000 °C under 0.1 MPa argon atmosphere 0.809 W/(m•K), which is lower than commercially available carbon aerogel filled foam and carbon foam.

Introduction

Since silica aerogels were invented in 1931 by Kistler, aerogels have developed into a big family including inorganic aerogels [1], organic aerogel [[2], [3], [4]], carbon aerogel [5,6], metal aerogels [7,8], composite aerogel [9,10] and so on. Among diverse aerogels, carbon aerogels possess many superior characters such as great physical and chemical stability, high electric conductivity and high infrared extinction coefficient, so they have potential application prospects in the fields of electrodes [[11], [12], [13], [14], [15]], catalyst supports [16,17], adsorption [[18], [19], [20], [21]] and thermal insulation materials [[22], [23], [24]]. Especially, carbon aerogel monoliths (CAMs) are extensively considered as the next generation ultra-high temperature thermal insulation materials (up to 2000 °C) due to their versatile excellent properties such as low thermal conductivity, low density and good thermal stability in inert atmospheres [23,25]. The low thermal conductivity of CAMs are contributed to the carbonaceous nature for shielding heat exchange from infrared radiation wave and the unique three-dimensional nanoporous network structure, which can suppress gaseous heat conduction and reduce solid heat transfer [22,26].

CAMs are conventionally prepared through catalytic gelling, supercritical drying and carbonizing processes [27]. Drying process is a key to preparing aerogel. Only if the skeleton strength can afford the capillary force, the nanoporous architecture can be perfectly retained and the obtained CAMs possess low density, high specific surface area and low thermal conductivity. Compared with supercritical drying [5], ambient pressure drying is low cost and safe [28,29], but the capillary force resulted from the volatilization of solvents during ambient drying is strong [30]. And so, to improve the skeleton strength is necessary for preparing low density CAMs [26]. Besides, intact monolithic appearance derived from great mechanical strength is also an important feature of CAMs required for heat insulation application [23,31]. High mechanical performances generally conflict with low density and high specific surface area of CAMs [32]. Hence, improving mechanical properties with simultaneously maintaining high porosity and low thermal conductivity during ambient drying is a crucial and fundamental challenge towards achieving practical application [32,33]. According to the differences of catalysts, catalytic gelling can be divided into acid catalysis and basic catalysis. The common acid catalysts include nitric acid, hydrochloric acid and acetic acid [34]. The basic catalysts include ammonia solution, sodium hydroxide, sodium carbonate [20], hexamethylenetetramine [35] and so on. However, the properties of CAMs prepared through acid catalysis alone or basic catalysis alone are unsatisfactory. For example, the CAMs synthesized via acid catalysis have high density and high thermal conductivity despite high strength. While, the mechanical strength of CAMs prepared via basic catalysis is generally poor. Therefore, it is a great challenge to obtain low density CAMs with both low thermal conductivity and high strength. It's like that there is a “trade-off” phenomenon between thermal conductivity and mechanical strength.

To improve the skeleton strength of organic gel, many efforts have been directed toward the challenge [36]. Reuß et al. used hydrochloric acid as acid catalyst to synthesize aerogels while the resulting carbon aerogels had high density and small specific surface area [37]. To improve specific surface area, Gang-Ping Wu et al. synthesized carbon aerogels through inorganic template method under ambient drying, but removing templates was time-consuming and easily destroyed the aerogels [38]. Mei-Fang Yan et al. used different base catalysts as sodium carbonate to control the porous structure, but the mechanical strength of synthesized carbon aerogels was poor [39]. Recently, Xianfeng Jia et al. employed linear phenolic resin and hexamethylenetetramine as carbon precursor and basic catalyst, respectively, to prepare low density as low as 0.07 g/cm³, and robust CAMs with mechanical strength of 0.9–5.0 MPa and low thermal conductivity of 0.032–0.069 W/(m•K). Their excellent properties could be attributed to a strong nanometer network [32]. As mentioned in these literatures, the amount and type of catalysts have a significant effect on the character of carbon aerogels. Acid and alkaline two-step catalysis method is generally used to prepare inorganic silica aerogels through adjusting hydrolysis and condensation reaction [1,[40], [41], [42]]. According to some literatures, phenol and aldehyde monomers tended to quickly form linear phenolic oligomers under the condition of acid catalysis while they were inclined to produce web structure prepolymers within basic catalysis [34,39,[43], [44], [45], [46]]. Hence, acid catalysis and basic catalysis may be combined to adjust the nanopores and skeleton architecture.

In this work, acid and alkaline two-step catalysis method for the first time was used for the preparation of robust CAMs with low density and low conductivity. Nitric acid played the role of acid catalyst and hexamethylenetetramine acted as basic catalyst and crosslinked agent. Through catalyzing first by acid and then by base, linear phenolic chains created by acid catalysis were interconnected into become robust three-dimension nanoporous architecture stabilized by nodes resulted from base catalyst. The typical structure endowed the resulting CAMs not only high strength but also high porosity and good heat insulation. The preparation processes including acid/basic two-step catalysis, solvothermal treatment, ambient drying and carbonization were simple, high-safety and low-cost, which meant that the CAMs could be fabricated for large-scale industrial production. The obtained CAMs with high porosity and high strength have wide application prospects in thermal insulation, supports for electrochemical catalysts and adsorption materials.