Enhanced performance of molecular electrocatalysts for CO2 reduction in a flow cell following K+ addition

Self-replenishing Ni-rich stainless-steel electrode toward oxygen evolution reaction at ampere-level

In the past few decades, tremendous attention has been devoted to enhancing the activity of oxygen evolution reaction (OER) catalysts for hydrogen production, while the cost and long-term stability of catalysts, which can play an even more important role in industrialization, have been much less emphasized. Herein, we engineered an OER electrode from abundant stainless steel (SS) via facile approaches, and the obtained electrode consists of a Ni-rich oxide surface layer with a Fe-rich metal substrate. An outstanding activity was observed with an overpotential of 316 mV at 100 mA cm-2 in 1 M KOH electrolyte. Additionally, an electrode self-replenishing concept is proposed in which a Ni-rich catalyst layer can be regenerated from a metallic substrate due to the difference in diffusion and dissolution rates of metal oxides/hydroxides, and this regeneration is validated by various characterizations. A recorded degradation rate of 0.012 was observed at 1000 mA cm-2 for 1000 h. The facile engineering of OER electrodes from SS combined with the self-replenishing catalyst can potentially address the cost, activity, and long-term stability barriers.

Dynamic protonation of ligand sites in molecular catalysts enhances electrochemical CO2 reduction

Molecular catalysts with functional group decorations are promising for electrocatalytic CO2 reduction to produce valuable chemicals and fuels. Using nickel phthalocyanine derivatives with cyano, methoxy, and dimethylamino groups, this study unveils why decorating molecular catalysts with either electron-donating or electron-withdrawing groups can enhance their activity. Notably, the dimethylamino group-decorated catalyst demonstrated stable and nearly 100% CO2-to-CO reduction selectivity over a wide potential range and high CO partial current densities up to 300 milliamperes per square centimeter. Theoretical and in situ spectroscopic analyses revealed the critical role of dynamic protonation of ligand sites in activating the metal center, which can be facilitated by the decoration of electron-withdrawing groups. Conversely, electron-donating groups, although requiring higher energy for protonation, enhance the synergy between metal centers and protonated sites, favoring the formation of key *COOH intermediates and improving CO selectivity at higher bias. This study underscores the importance of dynamic protonation of ligand sites in optimizing functionalized molecular catalysts for enhanced CO2RR activity.

Tuning the microenvironment of immobilized molecular catalysts for selective electrochemical CO2 reduction

The electrochemical CO2 reduction reaction (CO2RR), as a novel technology, holds great promise for carbon neutrality. Immobilized molecular catalysts are considered efficient CO2RR catalysts due to their high selectivity and fast electron transfer rates. However, at high current densities, changes in the microenvironment of molecular catalysts result in a decrease in the local CO2 concentration, leading to suboptimal catalytic performance. This work describes an effective strategy to control the local CO2 concentration by manipulating the hydrophobicity. The obtained catalyst exhibits high CO selectivity with a Faradaic efficiency (FE) of 96% in a membrane electrode assembly. Moreover, a consistent FE exceeding 85% could be achieved with a total current of 0.8 A. Diffusion impedance testing and interface characterization confirm that the enhanced hydrophobicity of the catalyst layer leads to an increase in the thickness of the Nernst diffusion layer and an expansion of the three-phase interface, thereby accelerating CO2 adsorption to enhance the performance.

CO2-to-CO Conversion with Over 10 % Efficiency Using Earth Abundant System in a Single-Compartment Reactor with Oxygen Tolerant Mn Complex Catalyst

The direct conversion of CO2 in the presence of O2 to value-added chemicals is a potentially important cost-effective solar-driven CO2 reduction technology. The present work demonstrates the solar-powered conversion of CO2 to CO with greater than 10 % efficiency using a Mn complex cathode and an Fe-Ni anode in a single-compartment reactor without an ion exchange membrane in conjunction with a Si solar cell. Reactors separated by ion exchange membranes are typically used to prevent any effects of oxygen generated by the counter electrode. However, the present Mn complex catalyst maintained its activity even in the presence of 15 % O2. Operando surface-enhanced Raman spectroscopy established that the present Mn catalyst preferentially reacted with CO2 without adsorbing O2. This unique characteristic enabled solar-driven CO2 reduction using a single-compartment reactor. In contrast, catalytic metals such as Ag tend to lose activity in such systems as a consequence of reaction with oxygen produced at the anode.

Alkali metal cations enhance CO2 reduction by a Co molecular complex in a bipolar membrane electrolyzer

The electrochemical reduction of CO2 is a promising pathway for converting CO2 into valuable fuels and chemicals. The local environment at the cathode of CO2 electrolyzers plays a key role in determining activity and selectivity, but currently some mechanisms are still under debate. In particular, alkali metal cations have been shown to enhance the selectivity of metal catalysts, but their role remains less explored for molecular catalysts especially in high-current electrolyzers. Here, we investigated the enhancement effects of cations (Na+, K+, Cs+) on Co phthalocyanine (CoPc) in a state-of-the-art reverse-biased bipolar membrane electrolyzer. When added to the anolyte, these cations increased the Faradaic efficiency for CO, except in the case of Na+ in which the effect was transient, but the effects are convoluted with the transport process through the membrane. Alternatively, these cations can also be added directly to the cathode as chloride salts, allowing the use of a pure H2O anolyte feed, leading to sustained improved CO selectivity (61% at 100 mA cm-2 after 24 h). Our results show that cation addition is a simple yet effective strategy for improving the product selectivity of molecular electrocatalysts, opening up new avenues for tuning their local environment for CO2 reduction.This article is part of the discussion meeting issue 'Green carbon for the chemical industry of the future'.

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.