Heterogenized Pyridine-Substituted Cobalt(II) Phthalocyanine Yields Reduction of CO2 by Tuning the Electron Affinity of the Co Center.

Conversion of CO2 to reduced products is a promising route to alleviate irreversible climate change. Here we report the synthesis of a Co-based phthalocyanine with pyridine moieties (CoPc-Pyr), which is supported on a carbon electrode and shows Faradaic efficiency ∼90% for CO at 490 mV of overpotential (-0.6 V vs reversible hydrogen electrode (RHE)). In addition, its catalytic activity at -0.7 V versus RHE surpasses other Co-based molecular and metal-organic framework catalysts for CO2 reduction at this bias. Density functional theory calculations show that pyridine moieties enhance CO2 adsorption and electron affinity of the Co center by an inductive effect, thus lowering the overpotential necessary for CO2 conversion. Our study shows that CoPc-Pyr reduces CO2 at lower overpotential and with higher activity than noble metal electrodes, such as silver.

Investigating the Role of Copper Oxide in Electrochemical CO2 Reduction in Real Time.

Copper oxides have been of considerable interest as electrocatalysts for CO2 reduction (CO2R) in aqueous electrolytes. However, their role as an active catalyst in reducing the required overpotential and improving the selectivity of reaction compared with that of polycrystalline copper remains controversial. Here, we introduce the use of selected-ion flow tube mass spectrometry, in concert with chronopotentiometry, in situ Raman spectroscopy, and computational modeling, to investigate CO2R on Cu2O nanoneedles, Cu2O nanocrystals, and Cu2O nanoparticles. We show experimentally that the selective formation of gaseous C2 products (i.e., ethylene) in CO2R is preceded by the reduction of the copper oxide (Cu2OR) surface to metallic copper. On the basis of density functional theory modeling, CO2R products are not formed as long as Cu2O is present at the surface because Cu2OR is kinetically and energetically more favorable than CO2R.

Effects of temperature and gas-liquid mass transfer on the operation of small electrochemical cells for the quantitative evaluation of CO2 reduction electrocatalysts.

In the last few years, there has been increased interest in electrochemical CO2 reduction (CO2R). Many experimental studies employ a membrane separated, electrochemical cell with a mini H-cell geometry to characterize CO2R catalysts in aqueous solution. This type of electrochemical cell is a mini-chemical reactor and it is important to monitor the reaction conditions within the reactor to ensure that they are constant throughout the study. We show that operating cells with high catalyst surface area to electrolyte volume ratios (S/V) at high current densities can have subtle consequences due to the complexity of the physical phenomena taking place on electrode surfaces during CO2R, particularly as they relate to the cell temperature and bulk electrolyte CO2 concentration. Both effects were evaluated quantitatively in high S/V cells using Cu electrodes and a bicarbonate buffer electrolyte. Electrolyte temperature is a function of the current/total voltage passed through the cell and the cell geometry. Even at a very high current density, 20 mA cm-2, the temperature increase was less than 4 °C and a decrease of <10% in the dissolved CO2 concentration is predicted. In contrast, limits on the CO2 gas-liquid mass transfer into the cells produce much larger effects. By using the pH in the cell to measure the CO2 concentration, significant undersaturation of CO2 is observed in the bulk electrolyte, even at more modest current densities of 10 mA cm-2. Undersaturation of CO2 produces large changes in the faradaic efficiency observed on Cu electrodes, with H2 production becoming increasingly favored. We show that the size of the CO2 bubbles being introduced into the cell is critical for maintaining the equilibrium CO2 concentration in the electrolyte, and we have designed a high S/V cell that is able to maintain the near-equilibrium CO2 concentration at current densities up to 15 mA cm-2.

General Thermal Texturization Process of MoS2 for Efficient Electrocatalytic Hydrogen Evolution Reaction.

Molybdenum disulfide (MoS2) has been widely examined as a catalyst containing no precious metals for the hydrogen evolution reaction (HER); however, these examinations have utilized synthesized MoS2 because the pristine MoS2 mineral is known to be a poor catalyst. The fundamental challenge with pristine MoS2 is the inert HER activity of the predominant (0001) basal surface plane. In order to achieve high HER performance with pristine MoS2, it is essential to activate the basal plane. Here, we report a general thermal process in which the basal plane is texturized to increase the density of HER-active edge sites. This texturization is achieved through a simple thermal annealing procedure in a hydrogen environment, removing sulfur from the MoS2 surface to form edge sites. As a result, the process generates high HER catalytic performance in pristine MoS2 across various morphologies such as the bulk mineral, films composed of micron-scale flakes, and even films of a commercially available spray of nanoflake MoS2. The lowest overpotential (η) observed for these samples was η = 170 mV to obtain 10 mA/cm(2) of HER current density.

Tailoring Copper Nanocrystals towards C2 Products in Electrochemical CO2 Reduction.

Favoring the CO2 reduction reaction (CO2RR) over the hydrogen evolution reaction and controlling the selectivity towards multicarbon products are currently major scientific challenges in sustainable energy research. It is known that the morphology of the catalyst can modulate catalytic activity and selectivity, yet this remains a relatively underexplored area in electrochemical CO2 reduction. Here, we exploit the material tunability afforded by colloidal chemistry to establish unambiguous structure/property relations between Cu nanocrystals and their behavior as electrocatalysts for CO2 reduction. Our study reveals a non-monotonic size-dependence of the selectivity in cube-shaped copper nanocrystals. Among 24 nm, 44 nm and 63 nm cubes tested, the cubes with 44 nm edge length exhibited the highest selectivity towards CO2RR (80 %) and faradaic efficiency for ethylene (41 %). Statistical analysis of the surface atom density suggests the key role played by edge sites in CO2RR.

A direct thin-film path towards low-cost large-area III-V photovoltaics.

III-V photovoltaics (PVs) have demonstrated the highest power conversion efficiencies for both single- and multi-junction cells. However, expensive epitaxial growth substrates, low precursor utilization rates, long growth times, and large equipment investments restrict applications to concentrated and space photovoltaics (PVs). Here, we demonstrate the first vapor-liquid-solid (VLS) growth of high-quality III-V thin-films on metal foils as a promising platform for large-area terrestrial PVs overcoming the above obstacles. We demonstrate 1-3 µm thick InP thin-films on Mo foils with ultra-large grain size up to 100 µm, which is ~100 times larger than those obtained by conventional growth processes. The films exhibit electron mobilities as high as 500 cm²/V-s and minority carrier lifetimes as long as 2.5 ns. Furthermore, under 1-sun equivalent illumination, photoluminescence efficiency measurements indicate that an open circuit voltage of up to 930 mV can be achieved, only 40 mV lower than measured on a single crystal reference wafer.