CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface
18 May 2018-Science (American Association for the Advancement of Science)-Vol. 360, Iss: 6390, pp 783-787
TL;DR: A copper electrocatalyst at an abrupt reaction interface in an alkaline electrolyte reduces CO2 to ethylene with 70% faradaic efficiency at a potential of −0.55 volts versus a reversible hydrogen electrode (RHE).
Abstract: Carbon dioxide (CO 2 ) electroreduction could provide a useful source of ethylene, but low conversion efficiency, low production rates, and low catalyst stability limit current systems. Here we report that a copper electrocatalyst at an abrupt reaction interface in an alkaline electrolyte reduces CO 2 to ethylene with 70% faradaic efficiency at a potential of −0.55 volts versus a reversible hydrogen electrode (RHE). Hydroxide ions on or near the copper surface lower the CO 2 reduction and carbon monoxide (CO)–CO coupling activation energy barriers; as a result, onset of ethylene evolution at −0.165 volts versus an RHE in 10 molar potassium hydroxide occurs almost simultaneously with CO production. Operational stability was enhanced via the introduction of a polymer-based gas diffusion layer that sandwiches the reaction interface between separate hydrophobic and conductive supports, providing constant ethylene selectivity for an initial 150 operating hours.
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TL;DR: It is envisaged that the rational design of molecular–heterogeneous hybrid catalysts for simultaneously achieving enhanced efficiency and stability will become an important area of research.
Abstract: Anthropogenic CO2 emissions engender a severe threat to the global ecosystem and consequently are a growing concern associated with traditional means of energy production. Direct electrocatalytic reduction of CO2 into energyrich fuels and value-added chemical feedstocks provides a promising route to realize a carbon-neutral energy cycle, and at the same time, store the electricity generated from intermittent renewableenergy sources. Thermodynamically, the CO2 reduction reaction (CO2RR) suffers from a high energy barrier of CO2•− formation (-1.90 V vs. SHE, pH = 7) and low selectivity due to the competing hydrogen evolution reaction (HER). Kinetically, multi-proton-coupled electron transfer steps upon CO2RR render sluggish reaction processes [1]. To tackle the above issues, significant progress has been achieved on the optimization of catalysts, products, and systems. The ultimate viability of this technology is contingent upon the exploitation of lowcost catalytic systems capable of providing high energy efficiency and conversion rate. Catalysts for CO2RR can be generally categorized into heterogeneous or molecular materials (Fig. 1, bottom) [2]. Nanostructured heterogeneous catalysts especially metal nanoparticles (e.g. Cu, Ag, Au, Sn) are among the most popularly utilized heterogeneous materials owing to the ease of synthesis and enhanced performance compared with bulk materials. Tuning the surface structure and composition of nanocatalysts is extremely crucial because most mass transfer processes occur on its surface. Although a vast number of researches have focused on modifying the nanomaterials from different angles, the inherent inactive properties of the atoms inside the material still limit its overall performance. Such a different behavior between surface and inside atoms also makes the mechanistic investigations difficult. Thus, the synthesis of heterogeneous catalysts with high atomic utilization and clear catalytic mechanism remains a great challenge. Molecular catalysts are typically comprised of organometallic complexes, their activity and selectivity are primarily determined by organic ligands coordinated with metal centers [3].This family of catalysts possesses the advantages of highly exposed active sites and better-understood catalytic reaction mechanism. However, molecular catalysts tend to be plagued by inferior stability as they can easily aggregate tominimize the surface energy. It is envisaged that the rational design of molecular–heterogeneous hybrid catalysts for simultaneously achieving enhanced efficiency and stability will become an important area of research. Recently, single-atom catalysts (SACs) with isolated metal atoms anchored by covalent coordination are emerging as a new frontier in the catalysis community. Similar to molecular catalysts, SACs possess a well-defined and specific atomic structure which can offer high selectivity towards certain intermediates adsorption/desorption during CO2RR. Besides, their atomically dispersed nature can support a metal utilization up to 100%, resulting in high activity [4]. Metal-N-C (e.g. Fe, Ni, Co) based SACs have shown the state-of-the-art efficiency for CO2-to-CO production. Other SACs consist of noble metals (e.g. Au, Pd, Ru) are also promising and deserve more attention in the future research. Hence, the development of SACs paves a new way to design, as well as to understand, heterogeneous catalysis from the molecular angle and build a bridge between heterogeneous and homogeneous catalysis. CO2RR can generate at least 16 different gas and liquid products depending on a variety of reaction pathways (Fig. 1, middle) [5]. C1 products including carbon monoxide and formate, via two electron transfer reactions, have been produced with high efficiency on a wide range of catalysts such as metalN-C, Sn, In, Ag, Pd, etc. Other products such as methane, methanol, ethylene, ethanol and propanol, via multiple electron transfer reactions, have been generated with much lower Faradic efficiencies typically on Cu-based electrodes. This mainly stems from the competing formation of C–C, C–H and C– O bonds and the additional reaction barrier associated with a key step regarding C–C bond formation. Whilst the multicarbon products have a wide application market, in part due to their high energy density, their production efficiency via CO2RR need to be improved to become economically viable. To achieve this goal, the rational design and synthesis of catalysts, especially for Cu-based materials, with desired electronic and morphological properties are pivotal [6]. Different from the well-established formation mechanism for C1 products, the reaction pathways for C2+ products are more complex and highly dependent on the catalyst surface and intermediates.
40 citations
40 citations
TL;DR: It is reported that a nanodendrite configuration for Cu catalyst can improve the electrocatalytic performance of Cu catalysts, especially multicarbon product formation, while suppressing HER and methane production.
Abstract: The electrochemical conversion of carbon dioxide (CO2) to fuels and chemicals is an opportunity for sustainable energy research that can realize both renewable energy storage and negative carbon cycle feedback. However, the selective generation of multicarbon products is challenging because of the competitive hydrogen evolution reaction (HER) and protonation of the reacting adsorbate. Copper-based materials have been the most commonly studied catalysts for CO2 electroreduction due to their ability to produce a substantial amount of C2 products. Here, we report that a nanodendrite configuration can improve the electrocatalytic performance of Cu catalysts, especially multicarbon product formation, while suppressing HER and methane production. The abundant conductive networks derived from the fractal copper dendritic structures with a high electrochemically active surface area (ECSA) facilitate electron transport and mass transfer, leading to superior kinetics for the formation of multicarbon products from CO2 electroreduction. As a result, approximately 70-120% higher ethylene and 60-220% higher C3 (n-PrOH and propanal) yields with lower onset potentials were produced over Cu nanodendrites compared to the initial Cu particles. This work opens an avenue for promoting CO2 electrochemical reduction to multicarbon products by catalyst configuration modulation.
39 citations
TL;DR: In this paper, the authors achieved ionomer-lean Pt surfaces by masking the Pt nanoparticles of the Pt/C catalyst with an alkanethiol, which achieved a low population of ionomers on the Pt surface while preserving the fast proton transport in the catalyst layer.
Abstract: Ionomer films even only a few nanometers thick in the catalyst layer of a polymer electrolyte membrane fuel cell are detrimental to the power performance at low Pt loadings due to their large oxygen transport resistance. Therefore, removing the ionomer films on the Pt surface for oxygen transport while preserving them on the carbon surface for proton transport can be an ideal ionomer distribution. Herein, we achieve ionomer-lean Pt surfaces by masking the Pt nanoparticles of the Pt/C catalyst with an alkanethiol. Due to the weakening of the ionomer/Pt interaction by the molecular mask, a low population of ionomers on the Pt surface is achieved while preserving the fast proton transport in the catalyst layer. The alkanethiol is then electrochemically removed from the catalyst layer, recovering the catalytic activity of the Pt. Electrochemical analyses showed a reduced oxygen transport resistance through the ionomer film and a consequent enhancement of the power performance. This molecular masking strategy marks the beginning of the nano-scale control of ionomer distribution in the development of advanced fuel cells.
39 citations
TL;DR: In this paper, the sulfonate groups of the polytetrafluoroethylene (PTFE) backbone of Nafion® were quantified using an X-ray fluorescence (XRF) spectroscopic protocol.
Abstract: Gas diffusion electrodes (GDEs) mediate the transport of reagents, products, and electrons in electrochemical reactors designed to reduce CO2 into fuels or chemicals. While the ratio of ionomer to electrocatalyst in the precursor catalyst ink is typically assumed not to change after being deposited on the GDE, we show herein that this assumption is likely not valid. Moreover, we discovered that the faradaic efficiency for formate, which is considered to be inconsequential relative to CO when using Ag electrocatalysts, can be modulated by 20% by a mere 5 wt% change in GDE Nafion® content. We were able to resolve these small differences in GDE composition by developing an X-ray fluorescence (XRF) spectroscopic protocol that quantifies the sulfonate groups appended to the polytetrafluoroethylene (PTFE) backbone of Nafion®. Using this protocol, we were able to determine how to precisely control the relative amount of ionomer to electrocatalyst for each GDE. We also found that maintaining a uniform ionomer–catalyst composition across the entire GDE can likely be done more effectively with automated spray coating than with manual deposition methods. We recommend following these procedures in order to generate reproducible CO2RR performance parameters in flow cells.
39 citations
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