Switchable Charge Storage Mechanism via in Situ Activation of MXene Enables High Capacitance and Stability in Aqueous Electrolytes.

The need for reliable renewable energy storage devices has become increasingly important. However, the performance of current electrochemical energy storage devices is limited by either low energy or power densities and short lifespans. Herein, we report the synthesis and characterization of multilayer Ti4N3Tx MXene in various aqueous electrolytes. We demonstrate that Ti4N3Tx can be electrochemically activated through continuous cation intercalation over a 10 day period using cyclic voltammetry. A wide operating window of 2 V is maintained throughout activation. After activation, capacitance at 2 mV s-1 increases by 300%, 140%, and 500% in 1 M H2SO4, 1 M MgSO4, and 1 M KOH, respectively, while maintaining ∼600 F g-1 at 2 mV s-1 after 50000 cycles in 1 M H2SO4. This activation process is possibly attributed to the unique morphology of the multilayered material, allowing cation intercalation to increase access to redox-active sites between layers. This work adds to the growing repository of electrochemically stable MXenes reported for aqueous energy storage applications. These findings offer a reliable option for reliable energy storage devices with potential applications in large-scale grid storage and electric vehicles.

Superior cyclability of high surface area vanadium nitride in salt electrolytes.

High surface area vanadium nitrides (VNs) have been extensively studied as materials for aqueous supercapacitors due to the high initial capacitance in alkaline media at low scan rates. However, low capacitance retention and safety limit their implementation. The use of neutral aqueous salt solutions has the potential to mitigate both of these concerns, but is limited in analysis. Hence, we report on the synthesis and characterization of high surface area VN as a supercapacitor material in a wide variety of aqueous chlorides and sulfates using Mg2+, Ca2+, Na+, K+, and Li+ ions. We observe the following trend in the salt electrolytes: Mg2+ > Li+ > K+ > Na+ > Ca2+. Mg2+ systems provide the best performance at higher scan rates with areal capacitances of 294 µF cm-2 in 1 M MgSO4 over a 1.35 V operating window at 2000 mV s-1. Furthermore, VN in 1 M MgSO4 maintained a 36% capacitance retention from 2 to 2000 mV s-1 compared to 7% in 1 M KOH. Capacitance in 1 M MgSO4 and 1 M MgCl2 increased to 121% and 110% of their original values after 500 cycles and maintained capacitances of 589 and 508 µF cm-2 at 50 mV s-1 after 1000 cycles, respectively. In contrast, in 1 M KOH the capacitance decreases to 37% of its original value, reaching only 29 F g-1 at 50 mV s-1 after 1000 cycles. The superior performance of the Mg system is attributed to a reversible surface 2 e- transfer pseudocapacitive mechanism between Mg2+ and VNxOy. These findings can be used to further the field of aqueous supercapacitors to build safer and more stable energy storage systems that can charge quicker compared to KOH systems.

Hydrogen evolution reaction mechanism on Ti3C2 MXene revealed by in situ/operando Raman spectroelectrochemistry.

MXenes have shown great promise as electrocatalysts for the hydrogen evolution reaction (HER), but their mechanism is still poorly understood. Currently, the benchmark Ti3C2 MXene suffers from a large overpotential. In order to reduce this overpotential, modifications must be made to the structure to increase the reaction rate of the H+/e- coupled transfer steps. These modifications heavily depend on understanding the HER mechanism. To remedy this, in situ/operando Raman spectroelectrochemistry combined with density functional theory (DFT) calculations are utilized to probe the HER mechanism of the Ti3C2 MXene catalyst in aqueous media. In acidic electrolytes, the -O- termination groups are protonated to form Ti-OH bonds, followed by protonation of the adjacent Ti site, leading to H2 formation. DFT calculations show that the large overpotential is due to the lack of an optimum balance between O and Ti sites. In neutral electrolytes, H2O reduction occurs on the surface and leads to surface protonation, followed by H2 formation. This results in an overcharging of the structure that leads to the observed large HER overpotential. This study provides new insights into the HER mechanisms of MXene catalysts and a pathway forward to design efficient and cost-effective catalysts for HER and related electrochemical energy conversion systems.

Ti2N nitride MXene evokes the Mars-van Krevelen mechanism to achieve high selectivity for nitrogen reduction reaction.

We address the low selectivity problem faced by the electrochemical nitrogen (N2) reduction reaction (NRR) to ammonia (NH3) by exploiting the Mars-van Krevelen (MvK) mechanism on two-dimensional (2D) Ti2N nitride MXene. NRR technology is a viable alternative to reducing the energy and greenhouse gas emission footprint from NH3 production. Most NRR catalysts operate by using an associative or dissociative mechanism, during which the NRR competes with the hydrogen evolution reaction (HER), resulting in low selectivity. The MvK mechanism reduces this competition by eliminating the adsorption and dissociation processes at the sites for NH3 synthesis. We show that the new class of 2D materials, nitride MXenes, evoke the MvK mechanism to achieve the highest Faradaic efficiency (FE) towards NH3 reported for any pristine transition metal-based catalyst-19.85% with a yield of 11.33 µg/cm2/hr at an applied potential of - 250 mV versus RHE. These results can be expanded to a broad class of systems evoking the MvK mechanism and constitute the foundation of NRR technology based on MXenes.

Holdups in Nitride MXene's Development and Limitations in Advancing the Field of MXene.

As nanomaterials are becoming a key component in various electronics, 2D nanomaterials are emerging and attracting tremendous attention in the scientific community due to their unique physical, chemical, and structural properties. In recent years, a new family of 2D carbides and nitrides, known as MXenes, has become the center of attention for many electrochemical energy storage and conversion systems. While nitride MXenes have some publications centered around them, the overwhelming majority revolve around carbide and their direct application to systems without understanding the underlying mechanism behind their performance. The lack of publications in both of these fields, nitrides and mechanistic understanding, causes a major stopgap in MXene research and needs to be remedied in order to truly utilize their potential for future electronics and energy conversion systems. In this work, the limited works on nitride MXenes and the applications of in situ/operando characterization techniques in understanding the underlying mechanisms of energy storage and conversion in MXenes are reviewed, major progress and remaining challenges in both fields are identified, recommendations on how to circumvent the challenges and limitations are provided, and finally, new research directions that must be performed to advance the field of 2D carbide and nitride MXenes are proposed.

Effect of CO2 and H2O on the behavior of shale gas confined inside calcite [104] slit-like nanopore: a molecular dynamics simulation study.

The effect of CO2 and H2O on the behavior of shale gas confined in calcite [104] slit-like nanopore is investigated using molecular dynamics simulation technique. The study is relevant as the advancement of enhance gas recovery (EGR) technologies requires in-depth atomistic understanding of the hydrocarbons, water, carbon dioxide, and other fracturing fluid constituents inside the nanopores of shale gas reservoirs. We are considering carbon dioxide (CO2) because it is an attractive displacing agent for enhanced gas recovery and has the potential to be an "exotic" fracturing fluid. Water (H2O) is considered as it is the major component of water-based fracturing fluids. The structural and dynamical properties of the confined species are computed. The results indicate that the presence of CO2 and H2O in the nanopore drastically affects the behavior of shale gas. The shale gas molecules that were tightly packed near the pore wall displaced towards the center by CO2 and H2O molecules. A new layer of carbon dioxide and water is formed near the pore wall. Further investigation reveals that CO2 molecules align themselves flat near the surface, whereas H2O molecules have directional orientation with oxygen atoms of water molecules pointing towards to wall. The predicted lateral (in-plane) self-diffusion coefficient values of methane, ethane, carbon dioxide and water indicate complex dynamics inside the pore. The investigation shows the fastest dynamics for methane gas followed by ethane. Both CO2 and H2O are almost immobile. The increase in temperature in the range from 300 to 450 K does not appear to have any significant impact on the behavior of the molecules inside the pore. The adsorption energies show that both CO2 and H2O have stronger interactions with calcite [104] surface than shale gas molecules. Graphical abstract TOC- Shale gas extraction process is shown on the left side. In the right side is a calcite [104] nanopore (H = 2.2 nm) filled with shale gas and fracking chemicals.