Percolative metal-organic framework/carbon composites for hydrogen storage Shuqian Xie a , Jiann-Yang Hwang b , Xiang Sun b , Shangzhao Shi b , Zheng Zhang b , Zhiwei Peng b , Yuchun Zhai a, * a School of Materials and Metallurgy, Northeastern University, No.11, Lane 3, Wenhua Road, Heping District, Shenyang, Liaoning 110819, China b Department of Materials Science and Engineering, Michigan Technological University,1400 Townsend Drive, Houghton, MI 49931, USA highlights  We study hydrogen adsorption over MOFs by introducing an external electric field.  We synthesize activated carbon additive and MOF composites.  The hydrogen uptake capability is increased with help from the PMN-PT.  Electrostatic interaction between hydrogen and adsorbent can be built by external electric field. article info Article history: Received 10 August 2013 Received in revised form 30 November 2013 Accepted 15 December 2013 Available online 25 December 2013 Keywords: Hydrogen adsorption Electric field MOF Polarization abstract Percolative Metal-organic framework/Carbon (MOFAC) composites are synthesized by IRMOF8 (iso- reticular metal-organic frameworks) directly depositing on activated carbon via heterogeneous nucle- ation. Carbon content is calculated by TGA (Thermogravimetric analysis) tests. XRD (X-ray diffraction) and FESEM (Field emission-scanning electron microscope) are carried out to characterize the structures of the samples. BET surface areas and the pore size distribution are measured. The dielectric constant is measured with impedance analyzer and a specially designed sample holder. The dielectric constants of the MOFAC composites rise with increasing the carbon content, and the composites possess the insulator econductor transition as the carbon content increases from 17.77 wt% to 22.2 wt%. The composites are further tested for hydrogen storage capability under assist of the PMN-PT (single crystal lead magnesium niobateelead titanate) generated electric field. With help from the PMN-PT, the hydrogen uptake capability is increased about 31.5% over the MOFAC3 (MOF-Carbon composite with 22.2 wt% of carbon) without PMN-PT, which is elucidated by the charge distribution mechanisms. The improved storage is due to a stronger electrostatic interaction between IRMOF8 and hydrogen molecule caused by field polarization. Meanwhile, rapid adsorption/desorption kinetics and total reversibility on the samples are observed in the present or absence of external electric field. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The largest obstacle for utilizing hydrogen energy for trans- portation applications is the reversible storage of hydrogen gas in vehicles. With respect to the gas physisorption, various adsorbents such as activated carbon [1e3], carbon nanostructures [4e6], and zeolites [7] have been extensively studied over the past few de- cades. However, at ambient temperature, none of these adsorbents can be used for transportation applications. Recently, a new class of porous materials, namely metal-organic frameworks (MOFs) has been developed which have an extremely large surface area and potential applications as adsorbents [8]. The weak adsorption en- ergy limits MOF’s real hydrogen storage applications although promising storage capacity has been achieved at 77 K [9e11]. Hydrogen spillover has remarkably increased the storage capacity of IRMOF8 to 4 wt% at room temperature [12,13], however, the strong interaction energy results in problems such as slow adsorption/desorption kinetics, and questionable reversibility. Our group has reported a novel way to increase hydrogen adsorption enthalpy in carbon [14,15] and oxide materials [16] by using an external electric field. Promising enhancement has been achieved and our computational work [17] also supports these observations. Moreover, a publication [18] from another group also demonstrated the storage enhancement effect of an electric field in * Corresponding author. Tel./fax: þ86 24 8368 7731. E-mail address: yuchunzhai93@gmail.com (Y. Zhai). Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.12.072 Journal of Power Sources 253 (2014) 132e137