|Electrochemical storage of hydrogen in activated carbon made from brown coal|
|By Amandeep S. Oberoi, PhD student, School of Engineering (Aerospace, Mechanical and Manufacturing Engineering), RMIT University, Melbourne|
|Continuous power production and energy supply has been a major challenge in the field of renewable energy generation for a long time. One solution to the problem is that hydrogen obtained from renewables can be utilised in a solar electricity supply system as a long-term energy store to enable the system to supply energy continuously on a year round basis. Yet a challenge remains in finding a safe, economical, lightweight and compact form of hydrogen storage.|
|Conventional hydrogen storage systems include storage of hydrogen as a gas at high pressure in a pressure vessel, as a cryogenic liquid at very low temperatures, and in solid-state form in metal and chemical hydrides. Current hydrogen storage systems have various restrictions in terms of volumetric and gravimetric energy densities, net-energy losses, and cost-effectiveness. Electrochemical storage of hydrogen in solid media, in particular carbon-based materials, is a promising alternative storage method, and has been actively researched over the past decade.|
Recent research at RMIT University has led to the novel concept of a ‘proton flow battery’. In the proton flow battery, a solid hydrogen storage electrode is integrated into a single Proton Exchange Membrane (PEM) cell that can operate reversibly as an electrolyser to split water or as a fuel cell to generate electricity. In electrolyser mode, hydrogen ions, that is, protons, emerging from the membrane enter the solid storage directly, and then react with electrons and the atoms of the storage material to form a hydride without producing hydrogen gas. Hence in principle, a proton flow battery would be an ideal device to store surplus electrical energy from a solar energy system for resupply at times of low or zero solar input
|Electrochemical storage of hydrogen in activated carbon (aC) electrodes as part of a reversible fuel cell in a proton flow battery, offers a potentially attractive option for storing surplus electrical energy from inherently variable solar and wind energy resources. Running in charge mode, intermittent electricity is fed to a Proton Exchange Membrane (PEM) electrolyser that splits water into hydrogen and oxygen. Running in supply mode, stored hydrogen is fed to a PEM fuel cell, along with oxygen from the air, and electricity is generated once again. This system promises to have a roundtrip energy efficiency comparable to lithium ion batteries, while having higher gravimetric and volumetric energy densities. |
|Figure 1: Proton flow battery.|
|Previous work on the proton flow battery at RMIT has investigated the performance of two novel electrode materials. The first was a novel composite metal hydride-nafion polymer material, which established the feasibility of the proton flow battery concept. However, the use of metal hydride had certain limitations, namely the high cost and mass of the metal alloy powder. |
The second electrode was a composite of nafion and activated carbon. Activated carbons have attracted considerable research interest as a solid-state hydrogen storage medium because of their high internal surface area, high pore volume, and light weight. However, the initial electrode design was limited by poor hydrogen transport into the ultramicropores of the activated carbon particles.
My PhD research has been conducted under the supervision of Professor John Andrews, with funding support from BCIA. My research involved an investigation of the use of porous activated carbon electrodes and sulphuric acid as a liquid proton conductor.
Activated carbon (aC) powders made from Victorian brown coal were supplied by Monash University, Melbourne, Australia (Professor Alan Chaffee’s group), and those from phenolic resin were obtained from CIC Energigune, Spain. These powders were activated after potassium hydroxide (KOH) addition and the percentage of KOH was varied to obtain different samples with a range of micropore volumes and surface areas (activation done by Lachlan Ciddor, PhD student of Prof. Alan Chaffee, Monash University, and Dr M. Karthik, research associate, CIC Energigune, Spain).
Their proton conductivity was measured by electrochemical impedance spectroscopy, double layer capacitance with cyclic voltammetry using a split flat coin cell (shown in Figure 2), and hydrogen storage capacity by galvanostatic charging and discharging in a three-electrode electrolytic cell (shown in Figure 3) with 1 M sulphuric acid as electrolyte.
|Figure 2: Split flat coin cell used to measure double layer capacitance [Oberoi A. 2015].|
|Figure 3: Three-electrode electrolytic cell used to measure hydrogen storage capacity.|
|Importantly, all the sample aC electrodes have shown promising results in terms of mass% of hydrogen electrochemically and electro-statically stored within them. Specifically, hydrogen storage capacity of aC samples made from Victorian brown coal was found to be in the range of 0.58%–1.36% mass, which is comparable with the capacity of commercial metal hydride-based hydrogen storage cylinders.|
My PhD research has contributed experimental data on reversible electrochemical storage of hydrogen in solid carbon-based material in acid electrolytes, including activated carbons made from brown coal. It has also enhanced scientific understanding of the factors, especially pore volume and surface area, influencing hydrogen storage inactivated carbons. Further work is still needed however, to prove the technical feasibility of a proton flow battery incorporating a carbon-based electrode.
|Amandeep Oberoi was successful in obtaining a scholarship from BCIA in 2014 to support his research. Amandeep has submitted his PhD thesis, which is currently being examined. BCIA congratulates Amandeep on this accomplishment, and wishes him every success in his professional career.|