Halide Perovskites Breathe Too: The Iodide-Iodine Equilibrium and Self-Doping in Cs2SnI6.

The response of an oxide crystal to the atmosphere can be personified as breathing-a dynamic equilibrium between O2 gas and O2- anions in the solid. We characterize the analogous defect reaction in an iodide double-perovskite semiconductor, Cs2SnI6. Here, I2 gas is released from the crystal at room temperature, forming iodine vacancies. The iodine vacancy defect is a shallow electron donor and is therefore ionized at room temperature; thus, the loss of I2 is accompanied by spontaneous n-type self-doping. Conversely, at high I2 pressures, I2 gas is resorbed by the perovskite, consuming excess electrons as I2 is converted to 2I-. Halide mobility and irreversible halide loss or exchange reactions have been studied extensively in halide perovskites. However, the reversible exchange equilibrium between iodide and iodine [2I-(s) ↔ I2(g) + 2e-] described here has often been overlooked in prior studies, though it is likely general to halide perovskites and operative near room temperature, even in the dark. An analysis of the 2I-(s)/I2(g) equilibrium thermodynamics and related transport kinetics in single crystals of Cs2SnI6 therefore provides insight toward achieving stable composition and electronic properties in the large family of iodide perovskite semiconductors.

Rational Design of Polymers for Selective CO2 Reduction Catalysis.

A series of copolymers comprising a terpyridine ligand and various functional groups were synthesized toward integrating a Co-based molecular CO2 reduction catalyst. Using porous metal oxide electrodes designed to host macromolecules, the Co-coordinated polymers were readily immobilized via phosphonate anchoring groups. Within the polymeric matrix, the outer coordination sphere of the Co terpyridine catalyst was engineered using hydrophobic functional moieties to improve CO2 reduction selectivity in the presence of water. Electrochemical and photoelectrochemical CO2 reduction were demonstrated with the polymer-immobilized hybrid cathodes, with a CO:H2 product ratio of up to 6:1 compared to 2:1 for a corresponding "monomeric" Co terpyridine catalyst. This versatile platform of polymer design demonstrates promise in controlling the outer-sphere environment of synthetic molecular catalysts, analogous to CO2 reductases.

Mechanical Characterization of Human Brain Tissue and Soft Dynamic Gels Exhibiting Electromechanical Neuro-Mimicry.

Synthetic hydrogels are an important class of materials in tissue engineering, drug delivery, and other biomedical fields. Their mechanical and electrical properties can be tuned to match those of biological tissues. In this work, hydrogels that exhibit both mechanical and electrical biomimicry are reported. The presented dual networks consist of supramolecular networks formed from 2:1 homoternary complexes of imidazolium-based guest molecules in cucubit[8]uril and covalent networks of oligoethylene glycol-(di)methacrylate. The viscoelastic properties of human brain tissues are also investigated. The mechanical properties of the dual network gels are benchmarked against the human tissue, and it is found that they both are neuro-mimetic and exhibit cytocompatibility in a neural stem cell model.

Cucurbit[8]uril-Derived Graphene Hydrogels.

The scalable production of uniformly distributed graphene (GR)-based composite materials remains a sizable challenge. While GR-polymer nanocomposites can be manufactured at a large scale, processing limitations result in poor control over the homogeneity of hydrophobic GR sheets in the matrices. Such processes often result in difficulties controlling stability and avoiding aggregation, therefore eliminating benefits that might have otherwise arisen from the nanoscopic dimensions of GR. Here, we report an exfoliated and stabilized GR dispersion in water. Cucurbit[8]uril (CB[8])-mediated host-guest chemistry was used to obtain supramolecular hydrogels consisting of uniformly distributed GR and guest-functionalized macromolecules. The obtained GR hydrogels show superior bioelectrical properties over identical systems produced without CB[8]. Utilizing such supramolecular interactions with biologically derived macromolecules is a promising approach to stabilize graphene in water and avoid oxidative chemistry.

Nanoscale Carbon Modified α-MnO2 Nanowires: Highly Active and Stable Oxygen Reduction Electrocatalysts with Low Carbon Content.

Carbon-coated α-MnO2 nanowires (C-MnO2 NWs) were prepared from α-MnO2 NWs by a two-step sucrose coating and pyrolysis method. This method resulted in the formation of a thin, porous, low mass-percentage amorphous carbon coating (<5 nm, ≤1.2 wt % C) on the nanowire with an increase in single-nanowire electronic conductivity of roughly 5 orders of magnitude (α-MnO2, 3.2 × 10-6 S cm-1; C-MnO2, 0.52 S cm-1) and an increase in surface Mn3+ (average oxidation state: α-MnO2, 3.88; C-MnO2, 3.66) while suppressing a phase change to Mn3O4 at high temperature. The enhanced physical and electronic properties of the C-MnO2 NWs-enriched surface Mn3+ and high conductivity-are manifested in the electrocatalytic activity toward the oxygen reduction reaction (ORR), where a 13-fold increase in specific activity (α-MnO2, 0.13 A m-2; C-MnO2, 1.70 A m-2) and 6-fold decrease in charge transfer resistance (α-MnO2, 6.2 kΩ; C-MnO2, 0.9 kΩ) were observed relative to the precursor α-MnO2 NWs. The C-MnO2 NWs, composed of ∼99 wt % MnO2 and ∼1 wt % carbon coating, also demonstrated an ORR onset potential within 20 mV of commercial 20% Pt/C and a chronoamperometric current/stability equal to or greater than 20% Pt/C at high overpotential (0.4 V vs RHE) and high temperature (60 °C) with no additional conductive carbon.

Insights into the spontaneous formation of hybrid PdO x /PEDOT films: electroless deposition and oxygen reduction activity.

Hybrid palladium oxide/poly(3,4-ethylenedioxythiophene) (PdO x /PEDOT) films were prepared through a spontaneous reaction between aqueous PdCl4 2- ions and a nanostructured film of electropolymerized PEDOT. Spectroscopic and electrochemical characterization indicate the presence of mixed-valence Pd species as-deposited (19 ± 7 at% Pd0, 64 ± 3 at% Pd2+, and 18 ± 4 at% Pd4+ by X-ray photoelectron spectroscopy) and the formation of stable, electrochemically reversible Pd0/α-PdO x active species in alkaline electrolyte and furthermore in the presence of oxygen. The elucidation of the Pd speciation as-deposited and in solution provides insight into the mechanism of electroless deposition in neutral aqueous conditions and the electrocatalytically active species during oxygen reduction in alkaline electrolyte. The PdO x /PEDOT film catalyses 4e- oxygen reduction (n = 3.97) in alkaline electrolyte at low overpotential (0.98 V vs. RHE, onset potential), with mass- and surface area-based specific activities competitive with, or superior to, commercial 20% Pt/C and state-of-the-art Pd- and PEDOT-based nanostructured catalysts. The high activity of the nanostructured hybrid PdO x /PEDOT film is attributed to effective dispersion of accessible, stable Pd active sites in the PEDOT matrix.

Electrodeposited MnO(x)/PEDOT Composite Thin Films for the Oxygen Reduction Reaction.

Manganese oxide (MnOx) was anodically coelectrodeposited with poly(3,4-ethylenedioxythiophene) (PEDOT) from an aqueous solution of Mn(OAc)2, 3,4-ethylenedioxythiophene, LiClO4 and sodium dodecyl sulfate to yield a MnOx/PEDOT composite thin film. The MnOx/PEDOT film showed significant improvement over the MnOx only and PEDOT only films for the oxygen reduction reaction, with a >0.2 V decrease in onset and half-wave overpotential and >1.5 times increase in current density. Furthermore, the MnOx/PEDOT films were competitive with commercial benchmark 20% Pt/C, outperforming it in the half-wave ORR region and exhibiting better electrocatalytic selectivity for the oxygen reduction reaction upon methanol exposure. The high activity of the MnOx/PEDOT composite is attributed to synergistic charge transfer capabilities, attained by coelectrodepositing MnOx with a conductive polymer while simultaneously achieving intimate substrate contact.

Electrodeposited Ni(x)Co(3-x)O4 nanostructured films as bifunctional oxygen electrocatalysts.

Nanostructured Ni(x)Co(3-x)O4 films serve as effective electrocatalysts for both the oxygen reduction and oxygen evolution reactions in alkaline electrolyte.