Enhancing Long-Term Durability of Electrochemical Reactors Producing Formate from CO2 and Water Designed for Integration with Solar Cells.

Artificial photosynthetic cells producing organic matter from CO2 and water have been extensively studied for carbon neutrality, and the research trend is currently transitioning from proof of concept using small-sized cells to large-scale demonstrations for practical applications. We previously demonstrated a 1 m2 size cell in which an electrochemical (EC) reactor featuring a ruthenium (Ru)-complex polymer (RuCP) cathode catalyst was integrated with photovoltaic cells. In this study, we tackled the remaining issue to improve the long-term durability of cathode electrodes used in the EC reactors, demonstrating high Faradaic efficiencies exceeding 80% and around 60% electricity-to-chemical energy-conversion efficiencies of a 75 cm2 sized EC reactor after continuous operation for 3000 h under practical conditions. Introduction of a pyrrole derivative containing an amino group in the RuCP coupled with UV-ozone treatment to create carboxyl groups on the carbon supports effectively reduced the detachment of the RuCP catalyst by forming a strong amide linkage. A newly developed chemically resistant graphite adhesive prevented the carbon supports from peeling off of the conductive substrates. In addition, highly durable anodes composed of IrOx-TaOy/Pt-metal oxide/Ti were adopted. Even though the EC reactor was installed at an inclined angle of 30°, which is approximately the optimal angle for receiving more solar energy, the crossover reactions were sufficiently suppressed because the porous separator film impeded the transfer of oxygen gas bubbles from the anode to the cathode. The intermittent operation improved the energy-conversion efficiency because the accumulated bubbles were removed at night.

Positive Feedback Mechanism to Increase the Charging Voltage of Li-O2 Batteries.

The large overpotential of nonaqueous Li-O2 batteries when charging causes low round-trip efficiency and decomposition of the electrode materials and electrolyte. The origins of this overpotential have been enthusiastically explored to date; however, a full understanding has not yet been reached because of the complexity of multistep reaction mechanisms. Here, we applied structural and electrochemical analysis techniques to investigate the reaction step that results in the increase of the overpotential when charging. Rietveld refinement of ex situ powder X-ray diffraction showed that a Li-deficient phase of Li2O2, Li2-xO2, formed when discharging and was present over the course of charging. The galvanostatic intermittent titration technique revealed that the rate-determining process in the first step of charging was a solid-solution type of delithiation. The chemical diffusion coefficient of Li+ ions in Li2-xO2, DLi, decreases as the cell voltage increases, which in turn leads to a decrease in the oxidation rate of Li2-xO2. Under galvanostatic conditions, the deceleration of oxidation induces further increase of the cell voltage; therefore, an intrinsic mechanism of positive feedback to increase the cell voltage occurs in the first step. The results demonstrate that the continuity of the first step can be extended by the suppression of changes in any of the elements of the positive feedback loop, i.e., the oxidation rate, cell voltage, or DLi.

Electrochemical CO2 reduction over nanoparticles derived from an oxidized Cu-Ni intermetallic alloy.

Oxide-derived Cu-Ni (3-32 at%-Ni) alloy nanoparticles with a size of 10 nm enhance selectivity for ethylene and ethanol formation over oxide-derived Cu nanoparticles by electrochemical CO2 reduction. X-ray absorption spectroscopy measurements suggest that Ni (generally recognized as an element to avoid) is in a mixed phase of oxidized and metallic states.

Synergistic Effect of Binary Electrolyte on Enhancement of the Energy Density in Li-O2 Batteries.

Enhancement of the discharge capacity of lithium-oxygen batteries (LOBs) while maintaining a high cell voltage is an important challenge to overcome to achieve an ideal energy density. Both the cell voltage and discharge capacity of an LOB could be controlled by employing a binary solvent electrolyte composed of dimethyl sulfoxide (DMSO) and acetonitrile (MeCN), whereby an energy density 3.2 times higher than that of the 100 vol % DMSO electrolyte was obtained with an electrolyte containing 50 vol % of DMSO. The difference in the solvent species that preferentially solvates Li+ and that which controls the adsorption-desorption equilibrium of the discharge reaction intermediate, LiO2, on the cathode/electrolyte interface provides these unique properties of the binary solvent electrolyte. Combined spectroscopic and electrochemical analysis have revealed that the solvated complex of Li+ and the environment of the cathode/electrolyte interface were the determinants of the cell voltage and discharge capacity, respectively.

Publisher Correction: Negative differential resistance as a critical indicator for the discharge capacity of lithium-oxygen batteries.

The original version of this Article contained an error in the title, which was previously incorrectly given as 'Negative differential resistance as a critical indicator for the discharge capacity of lithium-oxygene batteries'. The correct version states 'lithium-oxygen' in place of 'lithium-oxygene'. This has been corrected in both the PDF and HTML versions of the Article.

Negative differential resistance as a critical indicator for the discharge capacity of lithium-oxygene batteries.

In non-aqueous lithium-oxygen batteries, the one-electron reduction of oxygen and subsequent lithium oxide formation both occur during discharge. This lithium oxide can be converted to insulating lithium peroxide via two different pathways: a second reduction at the cathode surface or disproportionation in solution. The latter process is known to be advantageous with regard to increasing the discharge capacity and is promoted by a high donor number electrolyte because of the stability of lithium oxide in media of this type. Herein, we report that the cathodic oxygen reduction reaction during discharge typically exhibits negative differential resistance. Importantly, the magnitude of negative differential resistance, which varies with the system component, and the position of the cathode potential relative to the negative differential resistance determined the reaction pathway and the discharge capacity. This result implies that the stability of lithium oxide on the cathode also contributes to the determination of the reaction pathway.

A highly efficient Li2O2 oxidation system in Li-O2 batteries.

A novel indirect charging system that uses a redox mediator was demonstrated for Li-O2 batteries. 4-Methoxy-2,2,6,6-tetramethylpiperidinyl-1-oxyl (MeO-TEMPO) was applied as a mediator to enable the oxidation of Li2O2, even though Li2O2 is electrochemically isolated. This system promotes the oxidation of Li2O2 without parasitic reactions attributed to electrochemical charging and reduces the charging time.