1. Studies of Reversible Hydrogen Binding in Nano-sized Materials
Pēteris Lesničenoks1 ,Jānis Zemītis1, Jānis Kleperis2,Georgs Čikvaidze2, Reinis Ignatāns2, Marius Urbonavičius3, Simona Tučkute3 , Darius Milčius3
1 - Faculty of Materials Science and Applied Chemistry, Riga Technical University, Riga, Latvia;
2 - Institute of Solid State Physics of University of Latvia, Kengaraga Street 8, Riga LV1063, Latvia;
3 – Lithuanian Energy Institute - Center for Hydrogen Energy Technologies, Breslaujos str. 3, LT Kaunas , Lithuania
Introduction:
Increasing demand and consumption of energy is fuelling the research for more efficient and less polluting way to manufacture, store and use energy. Hydrogen is considered as ecologically friendly energy carrier with high energy density. To fully integrate it in energy circulation, it is important to find efficient enough and safe hydrogen storage system. So far high pressure storage tanks, liquefied hydrogen storage tanks, metal hydrides, some metal-organic frameworks and mesoporous materials are being investigated [1–4]. The aim established by DOE (USA) is to develop material that reaches at least 10 wt% of stored hydrogen until 2015 [5].
The interest in porous materials and materials with high surface area as hydrogen storage materials in not a completely new field, but the problem of storage medium with low weight is still an active topic of research The big challenge for carbon nanoporous materials as hydrogen storage media is to find a structure with tunable porosity and very high specific surface area, where hydrogen adsorbs strongly enough on the surface as to form a thermodynamically stable arrangement but not too strongly so that reversible fast loading/unloading kinetics are possible[6]. Graphene was discovered in 2004 by Geim and Novoselov [7] - the one atom thick layer of carbon atoms tightly packed into a graphite two-dimensional structure, having many extraordinary properties, also high surface specific area, a decisive factor for hydrogen storage applications - to 2630 m2/g.  
Experimental: Sample preparation: To obtain few layer graphene (FLG), the electrochemical exfoliation were used, taking graphite industrial waste road as working electrode. Different pulse sequence, amplitude, filling is used to find optimal parameters of exfoliation process. Important step is purification – single sheet material is lightest and can be easy separated with centrifuge or sedimentation. For higher precision samples were exposed in Ar/H2 gas flow at 300°C for 3 hours – guarantying higher degree of reduction.
Sample characterization: The initial samples were characterized by X-ray diffraction, SEM (Hitachi S-400N), elemental composition of samples was determined by EDS analysis. FTIR spectra were obtained with Hyperion 80 experimental device. FTIR spectra were obtained in transmitance mode using chamber consisting of stainless steel and KBr and KSR – 5 (Thallium Bromide-Iodide) glasses. Suspended material flakes in ethanol or acetone were evaporated on KSR-5 glass at RT and then vacuumed and heated up to 120 – 140 degrees using resistance heating near outer glass surface. Then samples were exposed to hydrogen atmosphere at 0,2 bar and allowed to cool down to RT. Zeolite – Clinoptilolite sample was charecterized by xrd as natura clinoptilolite with some ferrerite impurities. Ion exchange performed with MgCl2 and deionized water solution. Due to different hydrogen loading pressure (5bar) and exposure to air different results are expected.
XRD of exfoliated FLG powder
FLG sheet stacks obtained with electrochemical exfoliation.
Mg FLG mapping data for atomic distribution on particle
Reduced FLG powder
Mapping data for reduced particles, their EDS spectra and Raman spetra of reduced FLG
Gas analyser data for zeolite after hydrogenation in Severt type device PCT – PRO 200
FTIR spectra of once , twice hydrogenated exfoliated FLG film and FLG film before hydrogenation (top to bottom left- side preference)
Hydrogen
EDS spectra of Mg FLG
Horiba gas anayser data fot Mg FLG after hydrogenation at 300°C
Results:
SEM pictures show opened few layer graphite/graphene structures, with higher surface area than if it would be stacked together – determined by BET method - Mg FLG 0.43 m2/g and Reduced FLG m2/g. Calculated thickness from SEM pictures are around nm as mutilayer material up to 200nm thickness, containing voids between graphene sheets of couple nm up to several µm. From the SEM pictures the quality of graphene / graphite type structures it could be determined that chemical exfoliation produces open FLG structures. Reducing process breaks up the stacks of material, leaving open structures deep in the particles, but also exposing single sheets of FLG.
X ray diffraction shows graphitic structure with additional impurity peaks from production.
EDAX XRF analysis shows elements with larger atomic number as Na, The spectra showed mostly Fe, Si and some Ti impurities in graphite substrate material. EDS Analysis for FLG show sulfur impurities and some clorine, as well as Al, Si, Fe, and Ti.
Obtained multistacks of graphene sheets were characterized with Raman scattering was used to identify the presence of graphene. Preliminary experiments were done to test hydrogen absorption capability in graphene sheet stacks, using thermal absorption/desorption methods. Two different samples were collected from both – exfoliation and plasma methods – the light fraction of material that floated on the top of solution, and heavy fraction which sank at the bottom of the solution.
In the Horiba EMGA – 830AC gas analyzer a study of hydrogenated Mg FLG samples was done. During burning up of the samples at different rates but reaching 6 kW and around 3000 degrees, the absolute hydrogen content in samples was determined: from 0,39 wt% – 0,46 wt% hydrogen in the sample.
For the zeolite (Clinoptiloloite) samples – hydrogen uptake from 0.6 wt% to 1.4 wt% was observed. Exposure to air for multiple days did not affect hydrogen content significantly, possibly due to hydrogen exchange with water although oxygen significant increase was not observed.
Discussion: With graphene in FLG structures hydrogen can interact using physisorption and chemisorption forces; another possibility is to exploit intercalation of hydrogen between FLG sheet stacks (distance between sheets is important) [6]. As it is highly possible with IR spectroscopy, if hydrogen bonding has happened through physical bonding – dipole, induction etc. Then it is possible to find no change in IR spectrum, if the energetic sum of bond has not changed. As reported previously, not doped graphne sheets show hydrogen storage possibilities around 0,55 wt% [13] but there is promise that doping could allow 49% increase of storage capabilities for FLG / graphene. Clinoptilolite shows a promising tendency for higher hydrogen storage capacity, but further studies is needed.
Conclusions: It has been shown that exfoliated few layer graphene (FLG) can be used for hydrogen storage if continuous research of metal – intercalated FLG structures show promising results. Hydrogen loading at 5 bar, opposed to hydrogen flow or 2 bar loading - show adsorption values from 0,6 to 1,4 wt%.
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Energy level diagram for the graphene–hydrogen system. The energy is in eV per H atom [12],
Contacts:
Dr. Phys. Janis Kleperis:
B. Sc. Ing Peteris Lesnicenoks:
Acknowledgment : Authors acknowledge Latvian Council of Science Cooperation Project No. 666/2014 for financial support.
Results partialy obtained in colaboration with LEI at Kaunas with support from the COST Action MP1103 " Nanostructured materials for solid‐state hydrogen storage ".