Evaluation of Surface, Bulk, Electrochemical and Desalination Properties of Ti3C2Tx Membranes Used for Selective Ion Removal from Brine.
Authors: Bootwala Yousuf Zoher , Zaman Wahiduz , Hatzell Kelsey B. , Hatzell Marta C. 
Published: February 2020

Transition metal carbides, carbonitrides, and nitrides or MXenes represent a large family of 2D materials derived from a class of ternary ceramic materials known as MAX described by the general formula Mn+1 AXn where n=1,2,or 3 and M represents an early transition metal, A is a group A element, X is carbon and/or nitrogen. MAX phase materials are layered hexagonal materials with P63/mmc symmetry and are composed of metal carbide and/or nitride layers(Mn+1Xn) twinned with respect to each other and separated by an A-layer1.There are approximately 70 known MAX phases that have been experimentally produced and have numerous potential applications. More recently, they have attracted interest for both gas and aqueous phase separations due to their high stability in aqueous environment, flexibility, high surface area and conductivity2.This includes organic contaminant removal from water, energy storage and gas separation2.The layered structure of MXene allows the inter-layered spacing to be tuned to allow rejection of selective ions based on their size and charge. When compared to graphene oxide membranes, MXene membranes have higher water flux due to 2-3 layers of water molecules in the spacing between the layers3.
Here we will explore the surface and bulk properties of pristine Ti3C2Tx membranes, and Ti3C2Tx membranes with ion based intercalants (K+, Li+).In situ surface science (ambient pressure XPS and FTIR) will be presented in various relevant conditions(e.g.temperature,relative humidity). In addition, the electrochemical properties of the membranes (conductivity, resistance, capacitance) will be presented. Finally, experimental insight into salt separations will be presented. The experiments were carried out using a H-Cell apparatus and the variation in the selectivity and electrochemical capacity of Ti3C2Tx under an applied electrochemical field was evaluated in different salt solutions. The interlayer spacing was also fixed by passing a specific ion of a certain size through the membrane and then the membrane was tested with different salt solutions (NaCl, KCl, CaCl2 and mixtures of NaCl+CaCl2) to test for selectivity. The electrochemical capacity and the columbic efficiency were higher with ions with smaller hydrated radii (e.g. Potassium has the smallest hydrated radii and thus had the highest capacity and efficiency ) and this can be attributed to both the pore size of the membrane and the difference in size of the ions. In conclusion, the technical feasibility of the selectivity and the capacity of MXene membranes is demonstrated.

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