Densification Pressure Optimization of MOF-808-Based Membranes for Lithium Metal Batteries

The use of metal–organic frameworks (MOFs) in hybrid electrolytes for lithium (Li) metal batteries has grown in prominence in recent years, primarily due to the chemical tunability of the MOF’s pore structures, which can directly influence Li–ion transport properties. The most attractive form factor for a MOF electrolyte is a thin, flexible membrane, which requires the application of pressure to increase the contact between the MOF particles. Herein, a systematic study of the influence of pressure on the properties of MOF-808-based membranes is presented. It is shown that when a dry, roll-pressed membrane is subjected to pressure ≥120 MPa, a total loss of crystallinity and a significant loss of porosity is observed. Alternatively, a slurry-cast membrane, compressed under controlled pressures, can maintain crystallinity and porosity while decreasing the interparticle void space. Interesting, the conductivity of the membranes infiltrated with liquid electrolyte is not greatly affected by the pressure applied, though ultimately it is shown that for cycling with Li metal, compressed membranes with compact particles are preferred. This study highlights the critical importance of controlling the pressure applied to MOF-based membranes during fabrication and during cell assembly and lays out the foundation for further investigation of how to optimize membrane fabrication for hybrid electrolytes that use MOFs as the dominate component

Probing Edge Site Reactivity of Oxidic Cobalt Water Oxidation Catalysts

Differential electrochemical mass spectrometry (DEMS) analysis of the oxygen isotopologues produced by ¹⁸O-labeled Co-OEC in H₂¹⁶O reveals that water splitting catalysis proceeds by a mechanism that involves direct coupling between oxygens bound to dicobalt edge sites of Co-OEC. The edge site chemistry of Co-OEC has been probed by using a dinuclear cobalt complex. ¹⁷O NMR spectroscopy shows that ligand exchange of OH/OH₂ at Co­(III) edge sites is slow, which is also confirmed by DEMS experiments of Co-OEC. In borate (B_i) and phosphate (P_i) buffers, anions must be displaced to allow water to access the edge sites for an O–O bond coupling to occur. Anion exchange in P_i is slow, taking days to equilibrate at room temperature. Conversely, anion exchange in B_i is rapid (<i>k</i>_assoc = 13.1 ± 0.4 M^–1 s^–1 at 25 °C), enabled by facile changes in boron coordination. These results are consistent with the OER activity of Co-OEC in B_i and P_i. The P_i binding kinetics are too slow to establish a pre-equilibrium sufficiently fast to influence the oxygen evolution reaction (OER), consistent with the zero-order dependence of P_i on the OER current density; in contrast, B_i exchange is sufficiently facile such that B_i has an inhibitory effect on OER. These complementary studies on Co-OEC and the dicobalt edge site mimic allow for a direct connection, at a molecular level, to be made between the mechanisms of heterogeneous and homogeneous OER

Probing Edge Site Reactivity of Oxidic Cobalt Water Oxidation Catalysts

Differential electrochemical mass spectrometry (DEMS) analysis of the oxygen isotopologues produced by ¹⁸O-labeled Co-OEC in H₂¹⁶O reveals that water splitting catalysis proceeds by a mechanism that involves direct coupling between oxygens bound to dicobalt edge sites of Co-OEC. The edge site chemistry of Co-OEC has been probed by using a dinuclear cobalt complex. ¹⁷O NMR spectroscopy shows that ligand exchange of OH/OH₂ at Co­(III) edge sites is slow, which is also confirmed by DEMS experiments of Co-OEC. In borate (B_i) and phosphate (P_i) buffers, anions must be displaced to allow water to access the edge sites for an O–O bond coupling to occur. Anion exchange in P_i is slow, taking days to equilibrate at room temperature. Conversely, anion exchange in B_i is rapid (<i>k</i>_assoc = 13.1 ± 0.4 M^–1 s^–1 at 25 °C), enabled by facile changes in boron coordination. These results are consistent with the OER activity of Co-OEC in B_i and P_i. The P_i binding kinetics are too slow to establish a pre-equilibrium sufficiently fast to influence the oxygen evolution reaction (OER), consistent with the zero-order dependence of P_i on the OER current density; in contrast, B_i exchange is sufficiently facile such that B_i has an inhibitory effect on OER. These complementary studies on Co-OEC and the dicobalt edge site mimic allow for a direct connection, at a molecular level, to be made between the mechanisms of heterogeneous and homogeneous OER

Electron-Transfer Studies of a Peroxide Dianion

A peroxide dianion (O₂^2–) can be isolated within the cavity of hexacarboxamide cryptand, [(O₂)⊂mBDCA-5t-H₆]^2–, stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O₂)⊂mBDCA-5t-H₆]^2– with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler–Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O₂^2– occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O₂^2– within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal–air batteries and the use of [(O₂)⊂mBDCA-5t-H₆]^2– as a chemical reagent