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The unique combination of high electronic conductivity, surface area, and tunability in MXenes makes them ideal electrocatalysts for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR); however, existing literature predominantly focuses on their electrocatalytic performance as opposed to their long-term electrocatalytic stability, such as how nitride MXene HER or ORR activity varies over time. A key experiment to understanding the stability is evaluating the overpotential, Tafel slope, and exchange current density of the MXene after prolonged immersion in solvents typically used for MXene storage. Herein, we investigate the electrocatalytic behavior of a nanolayered Ti4N3Tx nitride MXene toward HER and ORR after a period of 230 days in various solvents, including water, ethanol, N–N dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile, and hexane. Specifically, we evaluate the overpotential, Tafel slope, and limiting current density after the 230 day period as metrics of the material stability and degradation. Remarkably, the HER activity of the nanolayered Ti4N3Tx MXene remains unaltered after 230 days of immersion, with consistent overpotential and Tafel slope values, regardless of the solvent. We observe a minimal amount of TiO2 formation, as suggested by Raman spectroscopy and scanning electron microscopy imaging. However, for ORR, the overpotential, Tafel slope, and limiting current density change after the immersion time. Additionally, in all the solvents, the number of electrons transferred per molecule of product shifts from 4 to 2, indicating a mechanistic change. These findings suggest that the effect of immersion solvent on MXene electrocatalysis is correlated with the type of reaction and is likely a strong function of the terminal group. In summary, these findings can be exploited to establish MXene-solvent electrocatalytic relationships to broaden their utilization in electrocatalysis and facilitate their integration in solvent-based applications. [ABSTRACT FROM AUTHOR]

Nitrogen reduction reaction (NRR) is used to convert nitrogen (N2) to ammonia (NH3) at ambient conditions. In general, acidic electrolytes are used to provide protons (H+), however, this leads to low selectivity due to H2 formation via the hydrogen evolution reaction (HER). Recently, it is shown that Ti2N MXene is active and selective for NRR in acidic electrolyte, and operates through a Mars‐van Krevelen (MvK) mechanism. However, the effect of pH on its catalytic performance is not well understood. Here, experiments are performed in varying pH values and relations are developed between pH and performance. The findings of this study indicate that the lowest NRR onset occurs within pH 2 systems exhibiting a nonlinear trend. However, as pH decreases, overpotential decreases, and kinetics increase due to H+ participation. FE values are found to increase as pH increased. These findings will be extended to other materials to design optimal electrolytic systems for NRR. [ABSTRACT FROM AUTHOR]

Two‐dimensional (2D) carbide and nitride MXenes possess properties that are desirable for a broad range of electrocatalytic applications including the hydrogen evolution reaction (HER). These properties include high surface area, hydrophilicity, heterogeneity of redox‐active transition metals, and tunable surface functionalities allowing for low HER overpotentials. In this paper, we report on the cathodic etching and −O/−OH functionalization of hybrid Ti3CN upon the application of an external potential for improved HER performance and show that the active sites for HER on this MXene catalyst are located primarily on the −O− and −OH functional groups. The overpotential for the hybrid Ti3CN improves by 350 mV upon in‐situ −O/−OH functionalization and etching, reaching −0.46 V vs. RHE at a current density of 10 mA cm−2, much lower than those reported for the benchmark Ti3C2 carbide MXene. These results provide a path forward to tuning the electrocatalytic activity of MXenes and related electrocatalysts for water splitting. [ABSTRACT FROM AUTHOR]

Current climate issues can be partially remedied through the inclusion of renewable energy sources. However, these energy sources suffer from the need for highly efficient energy storage systems. To this end, studies have been conducted on developing energy storage materials that can provide high energy and power densities. Two‐dimensional (2D) carbide and nitride MXenes have the potential to provide both if their mechanism of charge storage is understood. Here, we use in situ/operando Raman spectroelectrochemistry to investigate in real time the charge storage mechanism of the benchmark Ti3C2 MXene in acidic and neutral media. We found that during charge/discharge cycling in acidic media, protons are pulled towards the surface, leading to a reversible reaction with the surface termination groups leading to a switch from −O− to −O(OH) to −O− chemistry, indicative of a pseudocapacitive mechanism. In neutral media, no pseudocapacitive behavior is observed and it is found that the material exhibits an electrostatic double layer charge storage mechanism through attracting sodium ions towards the surface for transient adsorption processes. Taking into consideration the mechanism we propose, the Ti3C2 MXene can exhibit a pseudocapacitance of 358 F/g within a voltage window of 1.35 V. Ultimately, these fundamental insights can be used to design electrode materials with both high energy and power densities. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This article addresses the question of how West Edmonton Mall (WEM) may be a new and distinctive component of retail spatial structure by analysing the retail and service attributes of the mall in the context of (1) Edmonton's retail structure at the regional shopping centre level, and (2) the internal structure of the mall. It is suggested that WEM has added a new level to Edmonton's retail hierarchy, rivalling the downtown as the city's primate centre. The analysis of tenant changes leads to the conclusion that these changes tend to reflect a movement from unconventional to conventional tenant mix, and that WEM is not much different, other than in sheer size, from most large regional shopping centres. The only unique structural characteristic appears to be the existence of enclaves not usually found in other malls. L'auteur etudie en quoi le 'West Edmonton Mall' pourrait etre un element nouveau et distinct de la structure spatiale du commerce de detail en analysant les caracteristiques des magasins de detail et des comptoirs de services du WEM dans le contexte (1) de la structure de commerce de detail d'Edmonton au niveau du centre commercial regional, d'une part; et (2) de la structure interne du mall, d'autre part. Il suggere que le WEM represente un nouvel echelon dans la hierarchie du commerce de detail d'Edmonton et qu'il rivalise avec le centre-ville. L'analyse du roulement des commercants-locataires l'amene a conclure que la tendance des changements suggere un passage du locataire non-conventionnel au locataire conventionnel et que, en dehors de ses dimensions, le WEM ne serait pas tres different de la plupart des grands centres commerciaux regionaux. La seule caracteristique structurale specifique serait l'existence d'enclaves qui ne se trouvent habituellement pas dans d'autres centres.

West Edmonton Mall (WEM) is an internationally known shopping mall/leisure complex comprising approximately 4 million sq. ft. of leasable retail and service space. It contains more than 600 shops, plus a hotel and leisure and recreational facilities that are unmatched in any previous shopping centre. The superimposition of a single, massive development on a pre-existing urban structure has created both opportunities and problems for Edmonton, while the emergence of such an urban phenomenon, unprecedented in scale and character, should raise new questions and stimulate new thinking among academics. This introductory article provides a context for the articles that follow by outlining the developmental history of WEM and assessing its significance as a megamall prototype, and by identifying themes to be addressed in detail in the subsequent articles. Le'West Edmonton Mall' (WEM) est un ensemble commercial et de loisirs mondialement connu; son espace locatif en commerces et services depasse 4 millions de pieds carres. Il contient plus de six cents magasins, un hotel et des installations de loisirs que l'on ne rencontre pas souvent dans un centre commercial. La superposition d'un projet de cette envergure sur la structure urbaine existante presente des avantages et des inconvenients pour Edmonton. L'emergence de ce nouveau phenomene urbain selon cette echelle et ces caracteristiques devrait soulever de nouvelles questions et stimuler une nouvelle reflexion parmi les universitaires. Cet article d'introduction fournit le contexte pour les etudes qui suivent en retracant l'histoire du developpement du WEM, en evaluant sa signification comme prototype de megacentre commercial, et en relevant les themes traites dans les articles subsequents.