RESUMEN
The electrochemical CO2 reduction reaction (CO2RR) to produce methanol (CH3OH) is an attractive yet challenging approach due to a lack of selective electrocatalysts. An immobilized cobalt phthalocyanine (CoPc) molecular catalyst has emerged as a promising electrocatalyst for CH3OH synthesis, demonstrating decent activity and selectivity through a CO2-CO-CH3OH cascade reaction. However, CoPc's performance is limited by its weak binding strength toward the CO intermediate. Recent advancements in molecular modification aimed at enhancing CO intermediate binding have shown great promise in improving CO2-to-CH3OH performance. In this Perspective, we discuss the competitive binding mechanism between CO2 and CO that hinders CH3OH formation and summarize effective molecular modification strategies that can enhance both the binding of the CO intermediate and the conversion of the CO2-to-CH3OH activity. Finally, we offer future perspectives on optimization strategies to inspire further research efforts to fully unlock the potential for methanol synthesis via the CO2RR using molecular catalysts.
RESUMEN
Studying the interactions between CO2 sorbents and electrocatalysts for the electrochemical CO2 reduction reaction (e-CO2RR) can offer viable strategies to advance the development of the Reactive Capture of CO2 (RCC). In this report we studied the effect of amines on the performance of the [Co(PDI-Py)] catalyst for the e-CO2RR. The presence of amines shifts the onset potential for the e-CO2RR more positive and increases the catalytic activity while maintaining the high Faradaic efficiency (≥90%) for CO production.
RESUMEN
Cobalt phthalocyanine (CoPc) is a promising molecular catalyst for aqueous electroreduction of CO2, but its catalytic activity is limited by aggregation at high loadings. Codeposition of CoPc onto electrode surfaces with the coordinating polymer poly(4-vinylpyridine) (P4VP) mitigates aggregation in addition to providing other catalytic enhancements. Transmission and diffuse reflectance UV-vis measurements demonstrate that a combination of axial coordination and π-stacking effects from pyridyl moieties in P4VP serve to disperse cobalt phthalocyanine in deposition solutions and help prevent reaggregation in deposited films. Polymers lacking axial coordination, such as Nafion, are significantly less effective at cobalt phthalocyanine dispersion in both the deposition solution and in the deposited films. SEM images corroborate these findings through particle counts and morphological analysis. Electrochemical measurements show that CoPc codeposited with P4VPonto carbon electrode surfaces reduces CO2 with higher activity and selectivity compared to the catalyst codeposited with Nafion.
RESUMEN
Nitrate (NO3-) is a common nitrogen-containing contaminant in agricultural, industrial, and low-level nuclear wastewater that causes significant environmental damage. In this work, we report a bioinspired Cr-based molecular catalyst incorporated into a redox polymer that selectively and efficiently reduces aqueous NO3- to ammonium (NH4+), a desirable value-added fertilizer component and industrial precursor, at rates of â¼0.36 mmol NH4+ mgcat-1 h-1 with >90% Faradaic efficiency for NH4+. The NO3- reduction reaction occurs through a cascade catalysis mechanism involving the stepwise reduction of NO3- to NH4+ via observed NO2- and NH2OH intermediates. To our knowledge, this is one of the first examples of a molecular catalyst, homogeneous or heterogenized, that is reported to reduce aqueous NO3- to NH4+ with rates and Faradaic efficiencies comparable to those of state-of-the-art solid-state electrocatalysts. This work highlights a promising and previously unexplored area of electrocatalyst research using polymer-catalyst composites containing complexes with oxophilic transition metal active sites for electrochemical nitrate remediation with nutrient recovery.
RESUMEN
Electrochemical reduction of CO2 using Cu catalysts enables the synthesis of C2+ products including C2H4 and C2H5OH. In this study, Cu catalysts were fabricated using plasma-enhanced atomic layer deposition (PEALD), achieving conformal deposition of catalysts throughout 3-D gas diffusion electrode (GDE) substrates while maintaining tunable control of Cu nanoparticle size and areal loading. The electrochemical CO2 reduction at the Cu surface yielded a total Faradaic efficiency (FE) > 75% for C2+ products. Parasitic hydrogen evolution was minimized to a FE of â¼10%, and a selectivity of 42.2% FE for C2H4 was demonstrated. Compared to a line-of-sight physical vapor deposition method, PEALD Cu catalysts show significant suppression of C1 products compared to C2+, which is associated with improved control of catalyst morphology and conformality within the porous GDE substrate. Finally, PEALD Cu catalysts demonstrated a stable performance for 15 h with minimal reduction in the C2H4 production rate.
RESUMEN
The electrochemical CO2 reduction reaction (CO2RR) in gas-fed flow electrolyzers using gas diffusion electrodes (GDEs) generates industrially relevant activities and provides a promising approach for carbon recycling. Developing effective catalyst systems on GDEs is critical for achieving high activities. Catalyst-polymer composites (CPCs) formed between immobilized molecular catalysts and coordinating polymers exhibit positive synergies for the enhancement of CO2RR activity. However, previous studies of CPCs have been primarily confined to liquid reaction platforms, and there are few examples of translating CPCs to GDE architectures. This suggests a knowledge gap exists in translating between the two platforms. Herein, we identify and bridge that gap by demonstrating a case study for the (poly-4-vinylpyridine)-encapsulated cobalt phthalocyanine (CoPc-P4VP) CPC. We identify a major knolwedge gap in the overlooked factor of CPC's hydrophobicity, which plays a significant role in gas-fed CO2RR but is often neglected in fundamental studies conducted on the liquid reaction platform. We bridge this gap by correlating catalyst hydrophobicity in liquid CO2RR with activity in gas-fed CO2RR by means of water contact angle measurements. Our case study underscores the importance of incorporating an engineering perspective into CPC studies and the necessity to consider hydrophobicity in CPC design and evaluation. This approach will hopefully accelerate the applied studies of this group of promising catalytic materials in gas-fed CO2 electrolysis.
RESUMEN
The electrocatalytic CO2 reduction reaction (CO2RR) is a promising strategy for converting CO2 to fuels and value-added chemicals using renewable energy sources. Molecular electrocatalysts show promise for the selective conversion of CO2 to single products with catalytic activity that can be tuned through synthetic structure modifications. However, for the CO2RR by traditional molecular catalysts, beneficial decreases in overpotentials are usually correlated with detrimental decreases in catalytic activity. This correlation is sometimes referred to as a "molecular scaling relationship". Overcoming this inverse correlation between activity and effective overpotential remains a challenge when designing new, efficient molecular catalyst systems. In this perspective, we discuss some of the concepts that give rise to the molecular scaling relationships in the CO2RR by molecular catalysts. We then provide an overview of some reported strategies from the last decade for breaking these scaling relationships. We end by discussing strategies and progress in our own research designing efficient molecular catalysts with redox-active ligands that show high activity at low effective overpotentials for the CO2RR.
RESUMEN
The electrochemical CO2 reduction reaction (CO2RR) is an attractive method for capturing intermittent renewable energy sources in chemical bonds, and converting waste CO2 into value-added products with a goal of carbon neutrality. Our group has focused on developing polymer-encapsulated molecular catalysts, specifically cobalt phthalocyanine (CoPc), as active and selective electrocatalysts for the CO2RR. When CoPc is adsorbed onto a carbon electrode and encapsulated in poly(4-vinylpyridine) (P4VP), its activity and reaction selectivity over the competitive hydrogen evolution reaction (HER) are enhanced by three synergistic effects: a primary axial coordination effect, a secondary reaction intermediate stabilization effect, and an outer-coordination proton transport effect. We have studied multiple aspects of this system using electrochemical, spectroscopic, and computational tools. Specifically, we have used X-ray absorption spectroscopy measurements to confirm that the pyridyl residues from the polymer are axially coordinated to the CoPc metal center, and we have shown that increasing the σ-donor ability of nitrogen-containing axial ligands results in increased activity for the CO2RR. Using proton inventory studies, we showed that proton delivery in the CoPc-P4VP system is controlled via a proton relay through the polymer matrix. Additionally, we studied the effect of catalyst, polymer, and graphite powder loading on CO2RR activity and determined best practices for incorporating carbon supports into catalyst-polymer composite films.In this Account, we describe these studies in detail, organizing our discussion by three types of microenvironmental interactions that affect the catalyst performance: ligand effects of the primary and secondary sphere, substrate transport of protons and CO2, and charge transport from the electrode surface to the catalyst sites. Our work demonstrates that careful electroanalytical study and interpretation can be valuable in developing a robust and comprehensive understanding of catalyst performance. In addition to our work with polymer encapsulated CoPc, we provide examples of similar surface-adsorbed molecular and solid-state systems that benefit from interactions between active catalytic sites and a polymer system. We also compare the activity results from our systems to other results in the CoPc literature, and other examples of molecular CO2RR catalysts on modified electrode surfaces. Finally, we speculate how the insights gained from studying CoPc could guide the field in designing other polymer-electrocatalyst systems. As CO2RR technologies become commercially viable and expand into the space of flow cells and gas-diffusion electrodes, we propose that overall device efficiency may benefit from understanding and promoting synergistic polymer-encapsulation effects in the microenvironment of these catalyst systems.
Asunto(s)
Dióxido de Carbono , Polímeros , Dióxido de Carbono/química , Catálisis , Electrodos , Hidrógeno/químicaRESUMEN
The electrocatalytic activity for CO2 reduction is greatly enhanced for Co complexes with pyridyldiimine-based ligands through the stepwise integration of three synergistic substituent effects: extended conjugation, electron-withdrawing ability, and intramolecular electrostatic effects. The stepwise incorporation of these effects into the catalyst structures results in a series of complexes that show an atypical inverse scaling relationship for CO2 reduction-the maximum activity of the resulting catalysts increases as the onset potentials are driven positive due to the ligand electronic substituent effects. Incorporating all three effects simultaneously into the catalyst structure results in a Co complex [Co(PDI-PyCH3+I-)] with dramatically enhanced activity for CO2 reduction, operating with over an order of magnitude higher activity (TOFcat = 4.1 × 104 s-1) and â¼0.2 V more positive catalytic onset (Eonset = -1.52 V vs Fc+/0) compared to the parent complex, an intrinsic activity parameter TOF0 = 6.3 × 10-3 s-1, and >95% Faradaic efficiency for CO production in acetonitrile with 11 M water. This makes [Co(PDI-PyCH3+I-)] among the most active molecular catalysts reported for the CO2 reduction reaction. Our work highlights a promising catalyst design strategy for molecular CO2RR catalysts in which catalytic ability is enhanced by tuning three synergistic substituent effects simultaneously in a single catalyst structure.
RESUMEN
Vanadium-doped cobalt oxide materials have emerged as a promising class of catalysts for the oxygen evolution reaction. Previous studies suggest vanadium doping in crystalline Co spinel materials tunes the electronic structure and stabilizes surface intermediates. We report a CoV2O4 material that shows good activity for the oxygen evolution reaction. However, postmortem characterization of the catalyst material shows dissolution of vanadium resulting in an amorphous CoOx material, suggesting that this vanadium-free material, and not CoV2O4, is the active catalyst. This study highlights the importance of postmortem characterization prior to mechanistic and computational analysis for this class of materials.
RESUMEN
Encapsulating cobalt phthalocyanine (CoPc) within the coordinating polymer poly-4-vinylpyridine (P4VP) results in a catalyst-polymer composite (CoPc-P4VP) that selectively reduces CO2 to CO at fast rates at low overpotential. In previous studies, we postulated that the enhanced selectively for CO over H2 production within CoPc-P4VP compared to the parent CoPc complex is due to a combination of primary, secondary, and outer-coordination sphere effects imbued by the encapsulating polymer. In this work, we perform in situ electrochemical X-ray absorption spectroscopy measurements to study the oxidation state and coordination environment of Co as a function of applied potential for CoPc, CoPc-P4VP, and CoPc with an axially-coordinated py, CoPc(py). Using in situ X-ray absorption near edge structure (XANES) we provide experimental support for our previous hypothesis that Co changes from a 4-coordinate square-planar geometry in CoPc to a mostly 5-coordinate species in CoPc(py) and CoPc-P4VP. The coordination environment of CoPc-P4VP is potential-independent but pH-dependent, suggesting that the axial coordination of pyridyl groups in P4VP to CoPc is modulated by the protonation of the polymer. Finally, we show that at low potential the oxidation state of Co in the 4-coordinate CoPc is different from that in the 5-coordinate CoPc(py), suggesting that the primary coordination sphere modulates the site of reduction (metal-centered vs. ligand centered) under catalytically-relevant conditions.
RESUMEN
The selective and efficient electrochemical reduction of CO2 to single products is crucial for solar fuels development. Encapsulating molecular catalysts such as cobalt phthalocyanine within coordination polymers such as poly-4-vinylpyridine leads to dramatically increased activity and selectivity for CO2 reduction. In this study, we use a combination of kinetic isotope effect and proton inventory studies to explain the observed increase in activity and selectivity upon polymer encapsulation. We provide evidence that axial-coordination from the pyridyl moieties in poly-4-vinylpyridine to the cobalt phthalocyanine complex changes the rate-determining step in the CO2 reduction mechanism accounting for the increased activity in the catalyst-polymer composite. Moreover, we show that proton delivery to cobalt centers within the polymer is controlled by a proton relay mechanism that inhibits competitive hydrogen evolution. These mechanistic findings provide design strategies for selective CO2 reduction electrocatalysts and serve as a model for understanding the catalytic mechanism of related heterogeneous systems.
RESUMEN
A previously reported cobalt complex featuring a tetraimidazolyl-substituted pyridine chelate is an active water oxidation electrocatalyst with moderate overpotential at pH 7. While this complex decomposes rapidly to a less-active species under electrocatalytic conditions, detailed electrochemical studies support the agency of an initial molecular catalyst. Cyclic voltammetry measurements confirm that the imidazolyl donors result in a more electron-rich Co center when compared with previous pyridine-based systems. The primary changes in electrocatalytic behavior of the present case are enhanced activity at lower pH and a marked dependence of catalytic activity on pH.
RESUMEN
A Co complex with a redox-active bis(pyridylmonoimine) ligand has been prepared and shows catalytic activity for electrochemical CO2 reduction in acetonitrile. Addition of a proton source such as water or trifluoroethanol dramatically improves the activity and stability of the molecular catalyst. The Co complex reduces CO2 to CO selectively at -1.95 V vs. Fc+/0 in the presence of high concentrations of water. The activity of the Co complex for CO2 reduction compares favorably to other molecular Co-based catalysts in acetonitrile solutions.
RESUMEN
Rotating disk electrodes (RDEs) are widely used in electrochemical characterization to analyze the mechanisms of various electrocatalytic reactions. RDE experiments often make use of or require collection and quantification of gaseous products. The combination of rotating parts and gaseous analytes makes the design of RDE cells that allow for headspace analysis challenging due to gas leaks at the interface of the cell body and the rotator. In this manuscript we describe a new, hermetically sealed electrochemical cell that allows for electrode rotation while simultaneously providing a gastight environment. Electrode rotation in this new cell design is controlled by magnetically coupling the working electrode to a rotating magnetic driver. Calibration of the RDE using a tachometer shows that the rotation speed of the electrode is the same as that of the magnetic driver. To validate the performance of this cell for hydrodynamic measurements, limiting currents from the reduction of a potassium ferrocyanide (K4[Fe(CN)6]·3H2O) were measured and shown to compare favorably with calculated values from the Levich equation and with data obtained using more typical, nongastight RDE cells. Faradaic efficiencies of â¼95% were measured in the gas phase for oxygen evolution in alkaline media at an Inconel 625 alloy electrocatalyst during rotation at 1600 rpm. These data verify that a gastight environment is maintained even during rotation.
RESUMEN
Well-defined mixed-metal [CoMn3 O4 ] and [NiMn3 O4 ] cubane complexes were synthesized and used as precursors for heterogeneous oxygen evolution reaction (OER) electrocatalysts. The discrete clusters were dropcasted onto glassy carbon (GC) and indium tin oxide (ITO) electrodes, and the OER activities of the resulting films were evaluated. The catalytic surfaces were analyzed by various techniques to gain insight into the structure-function relationships of the electrocatalysts' heterometallic composition. Depending on preparation conditions, the Co-Mn oxide was found to change metal composition during catalysis, while the Ni-Mn oxides maintained the NiMn3 ratio. XAS studies provided structural insights indicating that the electrocatalysts are different from the molecular precursors, but that the original NiMn3 O4 cubane-like geometry was maintained in the absence of thermal treatment (2-Ni). In contrast, the thermally generated 3-Ni develops an oxide-like extended structure. Both 2-Ni and 3-Ni undergo structural changes upon electrolysis, but they do not convert into the same material. The observed structural motifs in these heterogeneous electrocatalysts are reminiscent of the biological oxygen-evolving complex in Photosystemâ II, including the MMn3 O4 cubane moiety. The reported studies demonstrate the use of discrete heterometallic oxide clusters as precursors for heterogeneous water oxidation catalysts of novel composition and the distinct behavior of two sets of mixed metal oxides.
Asunto(s)
Compuestos de Manganeso/química , Metales/química , Óxidos/química , Oxígeno/química , Catálisis , Electrodos , Iones/química , Oxidación-ReducciónRESUMEN
Diverse transition metal hydroxide nanostructures were synthesized by laser-induced hydrolysis in a liquid precursor solution for alkaline oxygen evolution reaction (OER). Several active OER catalysts with fine control of composition, structure, and valence state were obtained including (Lix)[Ni0.66Mn0.34(OH)2](NO3)(CO3) · mH2O, Lix[Ni0.67Co0.33(OH)2](NO3)0.25(ORO)0.35 · mH2O, etc. An operate overpotential less than 0.34 V at current density of 10 mA cm(-2) was achieved. Such a controllable laser-chemical route for assessing complex nanostructures in liquids opens many opportunities to design novel functional materials for advanced applications.
RESUMEN
Objective comparisons of electrocatalyst activity and stability using standard methods under identical conditions are necessary to evaluate the viability of existing electrocatalysts for integration into solar-fuel devices as well as to help inform the development of new catalytic systems. Herein, we use a standard protocol as a primary screen for evaluating the activity, short-term (2 h) stability, and electrochemically active surface area (ECSA) of 18 electrocatalysts for the hydrogen evolution reaction (HER) and 26 electrocatalysts for the oxygen evolution reaction (OER) under conditions relevant to an integrated solar water-splitting device in aqueous acidic or alkaline solution. Our primary figure of merit is the overpotential necessary to achieve a magnitude current density of 10 mA cm(-2) per geometric area, the approximate current density expected for a 10% efficient solar-to-fuels conversion device under 1 sun illumination. The specific activity per ECSA of each material is also reported. Among HER catalysts, several could operate at 10 mA cm(-2) with overpotentials <0.1 V in acidic and/or alkaline solutions. Among OER catalysts in acidic solution, no non-noble metal based materials showed promising activity and stability, whereas in alkaline solution many OER catalysts performed with similar activity achieving 10 mA cm(-2) current densities at overpotentials of ~0.33-0.5 V. Most OER catalysts showed comparable or better specific activity per ECSA when compared to Ir and Ru catalysts in alkaline solutions, while most HER catalysts showed much lower specific activity than Pt in both acidic and alkaline solutions. For select catalysts, additional secondary screening measurements were conducted including Faradaic efficiency and extended stability measurements.
RESUMEN
Transient hydride ligands bridging two or more iron centers purportedly accumulate on the iron-molybdenum cofactor (FeMoco) of nitrogenase, and their role in the reduction of N2 to NH3 is unknown. One role of these ligands may be to facilitate N2 coordination at an iron site of FeMoco. Herein, we consider this hypothesis and describe the preparation of a series of diiron complexes supported by two bridging hydride ligands. These compounds bind either one or two molecules of N2 depending on the redox state of the Fe2(µ-H)2 unit. An unusual example of a mixed-valent Fe(II)(µ-H)2Fe(I) is described that displays a 10(6)-fold enhancement of N2 binding affinity over its oxidized congener, quantified by spectroscopic and electrochemical techniques. Furthermore, these compounds show promise as functional models of nitrogenase as substantial amounts of NH3 are produced upon exposure to proton and electron equivalents. The Fe(µ-H)Fe(N2) sub-structure featured herein was previously unknown. This subunit may be relevant to consider in nitrogenases during turnover.
Asunto(s)
Hidrógeno/química , Compuestos de Hierro/química , Nitrógeno/química , Amoníaco/síntesis química , Amoníaco/química , Amoníaco/metabolismo , Sitios de Unión , Técnicas Electroquímicas , Hidrógeno/metabolismo , Compuestos de Hierro/síntesis química , Compuestos de Hierro/metabolismo , Modelos Moleculares , Estructura Molecular , Molibdeno/química , Molibdeno/metabolismo , Nitrógeno/metabolismo , Nitrogenasa/química , Nitrogenasa/metabolismo , Oxidación-ReducciónRESUMEN
Base metal, molecular catalysts for the fundamental process of conversion of protons and electrons to dihydrogen, remain a substantial synthetic goal related to a sustainable energy future. Here we report a diiron complex with bridging thiolates in the butterfly shape of the 2Fe2S core of the [FeFe]-hydrogenase active site but with nitrosyl rather than carbonyl or cyanide ligands. This binuclear [(NO)Fe(N2S2)Fe(NO)2](+) complex maintains structural integrity in two redox levels; it consists of a (N2S2)Fe(NO) complex (N2S2=N,N'-bis(2-mercaptoethyl)-1,4-diazacycloheptane) that serves as redox active metallodithiolato bidentate ligand to a redox active dinitrosyl iron unit, Fe(NO)2. Experimental and theoretical methods demonstrate the accommodation of redox levels in both components of the complex, each involving electronically versatile nitrosyl ligands. An interplay of orbital mixing between the Fe(NO) and Fe(NO)2 sites and within the iron nitrosyl bonds in each moiety is revealed, accounting for the interactions that facilitate electron uptake, storage and proton reduction.