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Abstract Porous MXene-polymer composites have gained attention due to their low density, large surface area, and high electrical conductivity, which can be used in applications such as electromagnetic interference shielding, sensing, energy storage, and catalysis. High internal phase emulsions (HIPEs) can be used to template the synthesis of porous polymer structures, and when solid particles are used as the interfacial agent, composites with pores lined with the particles can be realized. Here, we report a simple and scalable method to prepare conductive porous MXene/polyacrylamide structures via polymerization of the continuous phase in oil/water HIPEs. The HIPEs are stabilized by salt flocculated Ti 3 C 2 T x nanosheets, without the use of a co-surfactant. After polymerization, the polyHIPE structure consists of porous polymer struts and pores lined with Ti 3 C 2 T x nanosheets, as confirmed by scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy. The pore size can be tuned by varying the Ti 3 C 2 T x concentration, and the interconnected Ti 3 C 2 T x network allows for electrical percolation at low Ti 3 C 2 T x loading; further, the electrical conductivity is stable for months indicating that in these composites, the nanosheets are stable to oxidation at ambient conditions. The polyHIPEs also exhibit rapid radio frequency heating at low power (10 °C s −1 at 1 W). This work demonstrates a simple approach to accessing electrically conductive porous MXene/polymer composites with tunable pore morphology and good oxidation stability of the nanosheets. 

The importance and widespread need for accurate pH monitoring necessitates the fabrication of new pH sensors with high sensitivity that can be used in a variety of environments. However, typical pH sensors have certain limitations ( e.g. , glass electrodes are fragile and require consistent upkeep, colorimetric pH strips are single use and inaccurate). Herein, we examine the pH-response of multilayers consisting of Ti 3 C 2 T x nanosheets and polycations fabricated using layer-by-layer (LbL) assembly. The MXene sheets themselves are pH-responsive due to their hydroxyl surface groups, and this effect may be amplified with the choice of an appropriate polycation. Specifically, the performance of multilayers assembled with the strong electrolyte poly (diallyldimethylammonium) (PDADMA) or pH-sensitive branched polyethylenimine (BPEI) is compared. As expected, the use of a pH-sensitive constituent leads to a 464% increase in pH sensitivity (116 kΩ pH −1 unit vs. 25 kΩ pH −1 unit) as compared to PDADMA. This is due to the conformational changes that BPEI undergoes with (de)protonation as pH changes. Further comparisons with reduced graphene oxide (rGO), which is far less pH responsive, confirm the unique pH responsivity of MXene nanosheets themselves. The ability to enhance response to particular stimuli by changing the constituent polycation demonstrates promise for future use of MXenes in resistive sensors for a variety of stimuli.