Safe and powerful energy storage devices are becoming increasingly important. Charging times of seconds to minutes, with power densities exceeding those of batteries, can in principle be provided by electrochemical capacitors--in particular, pseudocapacitors. Recent research has focused mainly on improving the gravimetric performance of the electrodes of such systems, but for portable electronics and vehicles volume is at a premium. The best volumetric capacitances of carbon-based electrodes are around 300 farads per cubic centimetre; hydrated ruthenium oxide can reach capacitances of 1,000 to 1,500 farads per cubic centimetre with great cyclability, but only in thin films. Recently, electrodes made of two-dimensional titanium carbide (Ti3C2, a member of the 'MXene' family), produced by etching aluminium from titanium aluminium carbide (Ti3AlC2, a 'MAX' phase) in concentrated hydrofluoric acid, have been shown to have volumetric capacitances of over 300 farads per cubic centimetre. Here we report a method of producing this material using a solution of lithium fluoride and hydrochloric acid. The resulting hydrophilic material swells in volume when hydrated, and can be shaped like clay and dried into a highly conductive solid or rolled into films tens of micrometres thick. Additive-free films of this titanium carbide 'clay' have volumetric capacitances of up to 900 farads per cubic centimetre, with excellent cyclability and rate performances. This capacitance is almost twice that of our previous report, and our synthetic method also offers a much faster route to film production as well as the avoidance of handling hazardous concentrated hydrofluoric acid.

Conductive two-dimensional titanium carbide ‘clay’ with high volumetric capacitance

The particularly difficult subject of predicting the swelling behavior of clay minerals is addressed by a combination of molecular dynamics and Monte Carlo sampling techniques. The introduced algorithm essentially mimics the experimental determination of the water adsorption isotherm and quantitatively predicts clay swelling for a montmorillonite-type clay including such details as the occurrence of hydrated states and hysteresis. Furthermore, important insights into the underlying mechanism of clay swelling from the one-layer to the two-layer hydrate are derived. It turns out that, for this case, clay swelling proceeds via the migration of counterions that are initially bound to the mineral surface to the central interlayer plane where they become fully hydrated. The extent of clay swelling strongly depends on the charge locus. This information appears to be transferable to other clay types.

Why clays swell

Geological disposal is the preferred option for the final storage of high-level nuclear waste and spent nuclear fuel in most countries. The selected host rock may be different in individual national programs for radioactive-waste management and the engineered barrier systems that protect and isolate the waste may also differ, but almost all programs are considering an engineered barrier. Clay is used as a buffer that surrounds and protects the individual waste packages and/or as tunnel seal that seals off the disposal galleries from the shafts leading to the surface.

Bentonite and bentonite/sand mixtures are selected primarily because of their low hydraulic permeability in a saturated state. This ensures that diffusion will be the dominant transport mechanism in the barrier. Another key advantage is the swelling pressure, which ensures a self-sealing ability and closes gaps in the installed barrier and the excavation-damaged zone around the emplacement tunnels. Bentonite is a natural geological material that has been stable over timescales of millions of years and this is important as the barriers need to retain their properties for up to 106 y.

In order to be able to license a final repository for high-level radioactive waste, a solid understanding of how the barriers evolve with time is needed. This understanding is based on scientific knowledge about the processes and boundary conditions acting on the barriers in the repository. These are often divided into thermal, hydraulic, mechanical, and (bio)chemical processes. Examples of areas that need to be evaluated are the evolution of temperature in the repository during the early stage due to the decay heat in the waste, re-saturation of the bentonite blocks installed, build-up of swelling pressure on the containers and the surrounding rock, and degradation of the montmorillonite component in the bentonite. Another important area of development is the engineering aspects: how can the barriers be manufactured, subjected to quality control, and installed?

Geological disposal programs for radioactive waste have generated a large body of information on the safety-relevant properties of clays used as engineered barriers. The major relevant findings of the past 35 y are reviewed here.

The Use of Clay as an Engineered Barrier in Radioactive-Waste Management a Review

Clay and non-clay minerals in the pharmaceutical industry: Part I. Excipients and medical applications

Clay and man: clay raw materials in the service of man

The Swelling Behaviour of Clays

Bentonite as a containment barrier for the disposal of highly radioactive wastes

Nineteen drill core samples of lower Dogger opalinum shale from wells drilled in connection with a tunnel project near Brugg, northern Switzerland, were investigated. The shale is a well known swelling rock that has caused problems in underground construction work. Swelling pressures determined under constant volume conditions to obtain maximum values were 0.7 to 2.2 N/ram 2. The samples contained 37-59% clay-size material and about 35% quartz, 7-18% carbonate minerals, and about 5% feldspar, pyrite, and organic matter. In addition to kaolinite, illite, and chlorite, the clay-size fraction also contained mixed-layer illite/smectite with about 30% swelling layers.The specific surface area of the clay fraction was 135 m2/g. The specific surface charge of the clay (6.7 x 104 esu/cm2), the ion concentration in the pore fluid of the specimen after the swelling test (10 -2 mmole/cm3), the valence of the ions in the double layer of the clay particles (+ 1), and the half distance between the clay plates in the specimen (8- 15/~) allowed the calculation of the swelling pressure for each sample according to the Gouy double layer theory. The mean value of the calculated swelling pressures was found to be of the same order of magnitude as the measured values, indicating that the technique can be used where cylindrical or rectangular spec- imens are not available for direct measurement.

Max Müller-Vonmoos

Papers

The Swelling Behaviour of Clays

Bentonite as a containment barrier for the disposal of highly radioactive wastes

Swelling pressure calculated from mineralogical properties of a jurassic opalinum shale, switzerland