Room-temperature sodium–sulfur batteries have attracted wide interest due to their high energy density and high natural abundance. Polysulfide dissolution and irreversible Na2S conversion are challenges to achieving high battery performance. Herein, we utilize a metal–organic framework-derived Co-containing nitrogen-doped porous carbon (CoNC) as a catalytic sulfur cathode host. A concentrated sodium electrolyte based on sodium bis­(fluorosulfonyl)­imide, dimethoxyethane, and bis­(2,2,2-trifluoroethyl) ether is used to mitigate polysulfide dissolution. We tune the amount of Co present in the CoNC carbon host by acid washing. Significant improvement in reversible sulfur conversion and capacity retention is observed with a higher Co content in CoNC, with 600 mAh g–1 and 77% capacity retention for CoNC and 261 mAh g–1 and 56% capacity retention for acid-washed CoNC at cycle 50 at 80 mAh g–1. Post-mortem X-ray photoelectron spectroscopy, transmission electron microscopy, and selected area electron diffraction suggest that CoS is formed during cycling in place of Co nanoparticles and CoN4 sites. Raman spectroscopy suggests that CoS exhibits a catalytic effect on the oxidation of Na2S. Our findings provide insights into understanding the role Co-based catalysts play in sulfur batteries.

The electrochemical nitrate reduction reaction (NO3 –RR) offers two-fold advantagesrestoring balance to the global nitrogen cycle and a less energy intensive pathway to the production of ammonia. We report the results of voltammetric and spectroscopic measurements examining NO3 –RR on Cu and Cu-alloyed electrodes (CuAg, CuSn, and CuPt) in an alkaline medium. Electrochemical results demonstrate that the overpotential for the NO3 –RR is ∼120 mV less on the CuAg catalyst as compared to the Cu-only catalyst. In situ surface enhanced Raman spectroscopy (SERS) obtained from these two Cu samples shows that the presence of dilute Ag maintains the Cu surface in a more reduced state (Cu(I)) during the course of NO3 –RR, while the neat Cu surface is heavily oxidized during NO3 –RR in an alkaline medium. Consistent with this behavior, the CuSn alloy also stabilizes Cu(I) on the electrode surface and results in increased NO3 –RR rates. Alternatively, the CuPt alloy does not yield a stabilized Cu(I) component and consequently results in NO3 –RR rates lower than those for neat Cu. These results indicate that alloying Cu with different metals can tune the nitrate reduction activity by making the Cu atoms more resistant to oxidation to Cu(II) and stabilizing the Cu atoms in lower oxidation states.

We investigate the effect of pretreatment of Li metal electrodes with chloroethylene carbonate (CEC). In comparison with either untreated or fluoroethylene carbonate (FEC)-treated Li surfaces, the CEC-treated electrodes exhibit smaller overpotentials and greater stability in symmetric cell cycling using a highly concentrated acetonitrile-LiTFSI-based electrolyte. The origin of the more facile cycling behavior is associated with a thicker SEI originating from a condensation reaction of N- and O-containing reduction products. Full cells constructed using the CEC treatment exhibit cycling behaviors equivalent to those seen using the FEC treatment. Replacing the LiTFSI electrolyte with LiFSI diminishes the salubrious effect of surface treatment by either FEC or CEC due to consequences arising from the more complete decomposition of the FSI– anion upon Li metal exposure.

Magnetometry is used in conjunction with voltammetry, microscopy, and photoemission spectroscopy to examine the effect of different preparations on the activity of pyrolyzed Co-based catalysts for the oxygen reduction reaction. Preparations involving only a metal–organic framework (MOF)-based precursor contain primarily paramagnetic species with the addition of a small amount of nonparamagnetic Co consistent with the presence of a small amount (5%) of Co nanoparticles. Addition of the surfactant cetyltrimethyl­ammonium bromide (CTAB) to the MOF precursor results in the presence of ca. 50% nanoparticles in the sample, likely due to the agglomeration of Co during catalyst precursor synthesis. Changes in the amount of Co nanoparticles do not correlate with major changes in ORR parameters such as E 1/2 or peroxide formation amount in either acidic or basic electrolyte.