Versatile chemical transformations of surface functional groups in 2D transition-metal carbides (MXenes) open up a new design space for this broad class of functional materials. We introduce a general strategy to install and remove surface groups by performing substitution and elimination reactions in molten inorganic salts. Successful synthesis of MXenes with O, NH, S, Cl, Se, Br, and Te surface terminations, as well as bare MXenes (no surface termination) was demonstrated. These MXenes show distinctive structural and electronic properties. For example, the surface groups control interatomic distances in the MXene lattice, and Tin+1Cn (n = 1, 2) MXenes terminated with Te2− ligands show a giant, (>18%) in-plane lattice expansion compared to the bulk TiC lattice. Nb2C MXenes exhibited surface-group-dependent superconductivity.

Reduction of U(VI) to U(IV) as the result of direct or indirect microbial activity is currently being explored for in situ remediation of subsurface U plumes, under the assumption that U(IV) solubility is controlled by the low-solubility mineral uraninite (U(IV)-dioxide). However, recent characterizations of U in sediments from biostimulated field sites, as well as laboratory U(VI) bioreduction studies, report on the formation of U(IV) species that lack the U═O(2)═U coordination of uraninite, suggesting that phases other than uraninite may be controlling U(IV) solubility in environments with complexing surfaces and ligands. To determine the controls on the formation of such nonuraninite U(IV) species, the current work studied the reduction of carbonate-complexed U(VI) by (1) five Gram-positive Desulfitobacterium strains, (2) the Gram-negative bacteria Anaeromyxobacter dehalogenans 2CP-C and Shewanella putrefaciens CN32, and (3) chemically reduced 9,10-anthrahydroquinone-2,6-disulfonate (AH(2)QDS, a soluble reductant). Further, the effects of 0.3 mM dissolved phosphate on U(IV) species formation were explored. Extended X-ray absorption fine structure (EXAFS) spectroscopy analysis demonstrated that the addition of phosphate causes the formation of a nonuraninite, phosphate-complexed U(IV) species, independent of the biological or abiotic mode of U(VI) reduction. In phosphate-free medium, U(VI) reduction by Desulfitobacterium spp. and by AH(2)QDS resulted in nonuraninite, carbonate-complexed U(IV) species, whereas reduction by Anaeromyxobacter or Shewanella yielded nanoparticulate uraninite. These findings suggest that the Gram-positive Desulfitobacterium strains and the Gram-negative Anaeromyxobacter and Shewanella species use distinct mechanisms to reduce U(VI).

Perovskite oxides are an important class of oxygen evolution reaction (OER) catalysts in alkaline media, despite the elusive nature of their active sites. Here, we demonstrate that the origin of the OER activity in a La 1−x Sr x CoO 3 model perovskite arises from a thin surface layer of Co hydr(oxy)oxide (CoO x H y ) that interacts with trace-level Fe species present in the electrolyte, creating dynamically stable active sites. Generation of the hydr(oxy)oxide layer is a consequence of a surface evolution process driven by the A-site dissolution and O-vacancy creation. In turn, this imparts a 10-fold improvement in stability against Co dissolution and a 3-fold increase in the activity−stability factor for CoO x H y / LSCO when compared to nanoscale Co-hydr(oxy)oxides clusters. Our results suggest new design rules for active and stable perovskite oxide-based OER materials.

Covalent organic frameworks (COFs) with redox-active units are a class of ideal materials for electrochemical-energy-storage devices. A novel two-dimensional (2D) PDC–MA–COF with redox-active triazine units was prepared via aldehyde–amine condensation reaction by using 1,4-piperazinedicarboxaldehyde (PDC) and melamine (MA) as structural units, which possessed high specific surface area (S BET = 748.2 m2 g–1), narrow pore width (1.9 nm), large pore volume (1.21 cm3 g–1), and high nitrogen content (47.87%), for pseudocapacitance application. The interlayer C–H···N hydrogen bonding can “lock” the relative distance between two adjacent layers to avoid an interlayer slip, which is more conducive to maintaining the ordered pore structure of the COF and improving a fast charge transfer between the electrode interface and triazine units. The PDC–MA–COF exhibited an excellent electrochemical performance with the highest specific capacitance of 335 F g–1 along with 19.71% accessibility of the redox-active triazine units in a three-electrode system and 94 F g–1 in a two-electrode system at 1.0 A g–1 current density. Asymmetric supercapacitor of PDC–MA–COF//AC assembled using PDC–MA–COF and activated carbon (AC) as positive and negative electrode materials, respectively, exhibited a high energy density of 29.2 W h kg–1 with a power density of 750 W kg–1. At the same time, it also showed an excellent cyclic stability and could retain 88% of the initial capacitance after 20 000 charge–discharge cycles, which was better than those of the most of the analogous materials reported previously. This study provided a new strategy for designing redox-active COFs for pseudocapacitive storage.

Fluorescent COFs with large π-conjugated building units and inherent rigid structure have important potential for chemosensing detection of target molecules or ions based on turn-on and turn-off modes.