Visualization of CO2 electrolysis using optical coherence tomography.

Electrolysers offer an appealing technology for conversion of CO2 into high-value chemicals. However, there are few tools available to track the reactions that occur within electrolysers. Here we report an electrolysis optical coherence tomography platform to visualize the chemical reactions occurring in a CO2 electrolyser. This platform was designed to capture three-dimensional images and videos at high spatial and temporal resolutions. We recorded 12 h of footage of an electrolyser containing a porous electrode separated by a membrane, converting a continuous feed of liquid KHCO3 to reduce CO2 into CO at applied current densities of 50-800 mA cm-2. This platform visualized reactants, intermediates and products, and captured the strikingly dynamic movement of the cathode and membrane components during electrolysis. It also linked CO production to regions of the electrolyser in which CO2 was in direct contact with both membrane and catalyst layers. These results highlight how this platform can be used to track reactions in continuous flow electrochemical reactors.

Catalyst Aggregation Matters for Immobilized Molecular CO2RR Electrocatalysts.

Here, we detail how the catalytic behavior of immobilized molecular electrocatalysts for the CO2 reduction reaction (CO2RR) can be impacted by catalyst aggregation. Operando Raman spectroscopy was used to study the CO2RR mediated by a layer of cobalt phthalocyanine (CoPc) immobilized on the cathode of an electrochemical flow reactor. We demonstrate that during electrolysis, the oxidation state of CoPc in the catalyst layer is dependent upon the degree of catalyst aggregation. Our data indicate that immobilized molecular catalysts must be dispersed on conductive supports to mitigate the formation of aggregates and produce meaningful performance data. We leveraged insights from this mechanistic study to engineer an improved CO-forming immobilized molecular catalyst─cobalt octaethoxyphthalocyanine (EtO8-CoPc)─that exhibited high selectivity (FECO ≥ 95%), high partial current density (JCO ≥ 300 mA/cm2), and high durability (ΔFECO < 0.1%/h at 150 mA/cm2) in a flow cell. This work demonstrates how to accurately identify CO2RR active species of molecular catalysts using operando Raman spectroscopy and how to use this information to implement improved molecular electrocatalysts into flow cells. This work also shows that the active site of CoPc during CO2RR catalysis in a flow cell is the metal center.

High-Efficiency Perovskite Solar Cells with Sputtered Metal Contacts.

Sputter deposition produces dense, uniform, adhesive, and scalable metal contacts for perovskite solar cells (PSCs). However, sputter deposition damages the other layers of the PSC. We here report that the damage caused by sputtering metal contacts can be reversed by aerial oxidation. We support this claim by making PSCs sputtered with Au contacts that exhibit higher efficiencies (18.7%) and stabilities than those made with thermally evaporated Au contacts (18.4%). We performed a series of experiments that show that the post-sputtering oxidation step reconstructs the molecular order of the hole transport layer (HTL) and reverses Au atom diffusion into the HTL. This potential restoration was previously neglected in PSC fabrication recipes because metal contact deposition is generally performed after the HTL oxidation. This result is important for scaling PSCs because sputtering is a superior method for manufacturing optimal-quality coatings or large-area devices.

Conversion of Reactive Carbon Solutions into CO at Low Voltage and High Carbon Efficiency.

Electrolyzers are now capable of reducing carbon dioxide (CO2) into products at high reaction rates but are often characterized by low energy efficiencies and low CO2 utilization efficiencies. We report here an electrolyzer that reduces 3.0 M KHCO3(aq) into CO(g) at a high rate (partial current density for CO of 220 mA cm-2) and a CO2 utilization efficiency of 40%, at a voltage of merely 2.3 V. These results were made possible by using: (i) a reactive carbon solution enriched in KHCO3 as the feedstock instead of gaseous CO2; (ii) a cation exchange membrane instead of an anion exchange membrane, which is common to the field; and (iii) the hydrogen oxidation reaction (HOR) at the anode instead of the oxygen evolution reaction (OER). The voltage reported here is the lowest reported for any CO2 to CO electrolyzer that operates at high current densities (i.e., a partial current density for CO greater than 200 mA cm-2) with a CO2 utilization efficiency of greater than 20%. This study highlights how the choice of feedstock, membrane, and anode chemistries affects the rate and efficiency at which CO2 is converted into products.

Electrocatalysts Derived from Copper Complexes Transform CO into C2+ Products Effectively in a Flow Cell.

Electrochemical reactors that electrolytically convert CO2 into higher-value chemicals and fuels often pass a concentrated hydroxide electrolyte across the cathode. This strongly alkaline medium converts the majority of CO2 into unreactive HCO3 - and CO3 2- byproducts rather than into CO2 reduction reaction (CO2RR) products. The electrolysis of CO (instead of CO2 ) does not suffer from this undesirable reaction chemistry because CO does not react with OH- . Moreover, CO can be more readily reduced into products containing two or more carbon atoms (i. e., C2+ products) compared to CO2 . We demonstrate here that an electrocatalyst layer derived from copper phthalocyanine (CuPc) mediates this conversion effectively in a flow cell. This catalyst achieved a 25 % higher selectivity for acetate formation at 200 mA/cm2 than a known state-of-art oxide-derived Cu catalyst tested in the same flow cell. A gas diffusion electrode coated with CuPc electrolyzed CO into C2+ products at high rates of product formation (i. e., current densities ≥200 mA/cm2 ), and at high faradaic efficiencies for C2+ production (FEC2+ ; >70 % at 200 mA/cm2 ). While operando Raman spectroscopy did not reveal evidence of structural changes to the copper molecular complex, X-ray photoelectron spectroscopy suggests that the catalyst undergoes conversion to a metallic copper species during catalysis. Notwithstanding, the ligand environment about the metal still impacts catalysis, which we demonstrated through the study of a homologous CuPc bearing ethoxy substituents. These findings reveal new strategies for using metal complexes for the formation of carbon-neutral chemicals and fuels at industrially relevant conditions.

An industrial perspective on catalysts for low-temperature CO2 electrolysis.

Electrochemical conversion of CO2 to useful products at temperatures below 100 °C is nearing the commercial scale. Pilot units for CO2 conversion to CO are already being tested. Units to convert CO2 to formic acid are projected to reach pilot scale in the next year. Further, several investigators are starting to observe industrially relevant rates of the electrochemical conversion of CO2 to ethanol and ethylene, with the hydrogen needed coming from water. In each case, Faradaic efficiencies of 80% or more and current densities above 200 mA cm-2 can be reproducibly achieved. Here we describe the key advances in nanocatalysts that lead to the impressive performance, indicate where additional work is needed and provide benchmarks that others can use to compare their results.

CO2 electrochemical catalytic reduction with a highly active cobalt phthalocyanine.

Molecular catalysts that combine high product selectivity and high current density for CO2 electrochemical reduction to CO or other chemical feedstocks are urgently needed. While earth-abundant metal-based molecular electrocatalysts with high selectivity for CO2 to CO conversion are known, they are characterized by current densities that are significantly lower than those obtained with solid-state metal materials. Here, we report that a cobalt phthalocyanine bearing a trimethyl ammonium group appended to the phthalocyanine macrocycle is capable of reducing CO2 to CO in water with high activity over a broad pH range from 4 to 14. In a flow cell configuration operating in basic conditions, CO production occurs with excellent selectivity (ca. 95%), and good stability with a maximum partial current density of 165 mA cm-2 (at -0.92 V vs. RHE), matching the most active noble metal-based nanocatalysts. These results represent state-of-the-art performance for electrolytic carbon dioxide reduction by a molecular catalyst.

Molecular electrocatalysts can mediate fast, selective CO2 reduction in a flow cell.

Practical electrochemical carbon dioxide (CO2) conversion requires a catalyst capable of mediating the efficient formation of a single product with high selectivity at high current densities. Solid-state electrocatalysts achieve the CO2 reduction reaction (CO2RR) at current densities ≥ 150 milliamperes per square centimeter (mA/cm2), but maintaining high selectivities at high current densities and efficiencies remains a challenge. Molecular CO2RR catalysts can be designed to achieve high selectivities and low overpotentials but only at current densities irrelevant to commercial operation. We show here that cobalt phthalocyanine, a widely available molecular catalyst, can mediate CO2 to CO formation in a zero-gap membrane flow reactor with selectivities > 95% at 150 mA/cm2 The revelation that molecular catalysts can work efficiently under these operating conditions illuminates a distinct approach for optimizing CO2RR catalysts and electrolyzers.