Engineering Cu surfaces for the electrocatalytic conversion of CO2: Controlling selectivity toward oxygenates and hydrocarbons.

In this study we control the surface structure of Cu thin-film catalysts to probe the relationship between active sites and catalytic activity for the electroreduction of CO2 to fuels and chemicals. Here, we report physical vapor deposition of Cu thin films on large-format (∼6 cm2) single-crystal substrates, and confirm epitaxial growth in the <100>, <111>, and <751> orientations using X-ray pole figures. To understand the relationship between the bulk and surface structures, in situ electrochemical scanning tunneling microscopy was conducted on Cu(100), (111), and (751) thin films. The studies revealed that Cu(100) and (111) have surface adlattices that are identical to the bulk structure, and that Cu(751) has a heterogeneous kinked surface with (110) terraces that is closely related to the bulk structure. Electrochemical CO2 reduction testing showed that whereas both Cu(100) and (751) thin films are more active and selective for C-C coupling than Cu(111), Cu(751) is the most selective for >2e- oxygenate formation at low overpotentials. Our results demonstrate that epitaxy can be used to grow single-crystal analogous materials as large-format electrodes that provide insights on controlling electrocatalytic activity and selectivity for this reaction.

High-Throughput in Situ Pressure Analysis of Lithium-Ion Batteries.

Many degradation processes in lithium-ion batteries are accompanied by gas evolution and therefore lead to an increase in internal cell pressure. This causes serious safety concerns for state-of-the-art lithium-ion batteries, calling for a thorough investigation of the origin and the magnitude of such processes. Herein we introduce a multichannel in situ pressure measurement system that allows for the high-throughput quantification of gas evolution under realistic battery conditions. The capability of the system was demonstrated through its application on Li4Ti5O12 half cells. The pressure changes could be divided into an irreversible and a reversible part, where the latter is caused by the deposition and dissolution of elemental lithium during cycling. Comparison of the measured and the theoretical reversible pressure changes showed a close match, indicating the high accuracy of the system. Additionally, the irreversible part observed in the pressure changes was attributed to gas evolution, as confirmed by complementary measurements using differential electrochemical mass spectrometry. To show the practicality of the system, the temperature dependence of gas evolution in Li1+xNi0.6Co0.2Mn0.2O2 full cells was investigated. Enhanced gas evolution was observed at elevated temperature, which is partly attributed to the thermal decomposition of the conducting salt LiPF6.

Electrochemical CO2 reduction on Au surfaces: mechanistic aspects regarding the formation of major and minor products.

In the future, industrial CO2 electroreduction using renewable energy sources could be a sustainable means to convert CO2 and water into commodity chemicals at room temperature and atmospheric pressure. This study focuses on the electrocatalytic reduction of CO2 on polycrystalline Au surfaces, which have high activity and selectivity for CO evolution. We explore the catalytic behavior of polycrystalline Au surfaces by coupling potentiostatic CO2 electrolysis experiments in an aqueous bicarbonate solution with high sensitivity product detection methods. We observed the production of methanol, in addition to detecting the known products of CO2 electroreduction on Au: CO, H2 and formate. We suggest a mechanism that explains Au's evolution of methanol. Specifically, the Au surface does not favor C-O scission, and thus is more selective towards methanol than methane. These insights could aid in the design of electrocatalysts that are selective for CO2 electroreduction to oxygenates over hydrocarbons.

Electrocatalytic conversion of carbon dioxide to methane and methanol on transition metal surfaces.

Fuels and industrial chemicals that are conventionally derived from fossil resources could potentially be produced in a renewable, sustainable manner by an electrochemical process that operates at room temperature and atmospheric pressure, using only water, CO2, and electricity as inputs. To enable this technology, improved catalysts must be developed. Herein, we report trends in the electrocatalytic conversion of CO2 on a broad group of seven transition metal surfaces: Au, Ag, Zn, Cu, Ni, Pt, and Fe. Contrary to conventional knowledge in the field, all metals studied are capable of producing methane or methanol. We quantify reaction rates for these two products and describe catalyst activity and selectivity in the framework of CO binding energies for the different metals. While selectivity toward methane or methanol is low for most of these metals, the fact that they are all capable of producing these products, even at a low rate, is important new knowledge. This study reveals a richer surface chemistry for transition metals than previously known and provides new insights to guide the development of improved CO2 conversion catalysts.

Insights into the electrocatalytic reduction of CO2 on metallic silver surfaces.

The electrochemical reduction of CO2 could allow for a sustainable process by which renewable energy from wind and solar are used directly in the production of fuels and chemicals. In this work we investigated the potential dependent activity and selectivity of the electrochemical reduction of CO2 on metallic silver surfaces under ambient conditions. Our results deepen our understanding of the surface chemistry and provide insight into the factors important to designing better catalysts for the reaction. The high sensitivity of our experimental methods for identifying and quantifying products of reaction allowed for the observation of six reduction products including CO and hydrogen as major products and formate, methane, methanol, and ethanol as minor products. By quantifying the potential-dependent behavior of all products, we provide insights into kinetics and mechanisms at play, in particular involving the production of hydrocarbons and alcohols on catalysts with weak CO binding energy as well as the formation of a C-C bond required to produce ethanol.

Gas Evolution in Lithium-Ion Batteries: Solid versus Liquid Electrolyte.

Gas evolution in conventional lithium-ion batteries using Ni-rich layered oxide cathode materials presents a serious issue that is responsible for performance decay and safety concerns, among others. Recent findings revealed that gas evolution also occurred in bulk-type solid-state batteries. To further clarify the effect that the electrolyte has on gassing, we report in this work-to the best of our knowledge-the first study comparing gas evolution in lithium-ion batteries with NCM622 cathode material and different electrolyte types, specifically solid (ß-Li3PS4 and Li6PS5Cl) versus liquid (LP57). Using isotopic labeling, acid titration, and in situ gas analysis, we show the presence of O2 and CO2 evolution in both systems, albeit with different cumulative amounts, and possible SO2 evolution for the lithium thiophosphate-based cells. Our results demonstrate the importance of considering gas evolution in solid-state batteries, especially the formation and release of highly corrosive SO2, due to side reactions with the electrolyte.

Origin of Carbon Dioxide Evolved during Cycling of Nickel-Rich Layered NCM Cathodes.

Gas formation caused by parasitic side reactions is one of the fundamental concerns in state-of-the-art lithium-ion batteries because gas bubbles might block local parts of the electrode surface, hindering lithium transport and leading to inhomogeneous current distributions. Here, we elucidate on the origin of CO2, which is the dominant gaseous species associated with the layered lithium nickel cobalt manganese oxide (NCM) cathode, by implementing isotope labeling and electrolyte substitution in differential electrochemical mass spectrometry-differential electrochemical infrared spectroscopy measurements. Li2CO3 on the NCM surface was successfully labeled with 13C via a process that involves its removal followed by intentional growth. In situ gas analytics on such NCM samples with 13C-labeled Li2CO3 clearly indicate that Li2CO3 decomposition contributes to CO2 evolution, especially during the first charge. At the same time, the greater contribution of electrolyte decomposition was indicated by the large amount of 12CO2 observed. Employment of butyronitrile as the electrolyte solvent in further measurements helped determine that the majority of electrolyte decomposition occurs via a reaction that involves the lattice oxygen of NCM.