Organic Additive-derived Films on Cu Electrodes Promote Electrochemical CO2 Reduction to C2+ Products Under Strongly Acidic Conditions.

Electrochemical CO2 reduction (CO2 R) at low pH is desired for high CO2 utilization; the competing hydrogen evolution reaction (HER) remains a challenge. High alkali cation concentration at a high operating current density has recently been used to promote electrochemical CO2 R at low pH. Herein we report an alternative approach to selective CO2 R (>70 % Faradaic efficiency for C2+ products, FEC2+ ) at low pH (pH 2; H3 PO4 /KH2 PO4 ) and low potassium concentration ([K+ ]=0.1 M) using organic film-modified polycrystalline copper (Modified-Cu). Such an electrode effectively mitigates HER due to attenuated proton transport. Modified-Cu still achieves high FEC2+ (45 % with Cu foil /55 % with Cu GDE) under 1.0 M H3 PO4 (pH≈1) at low [K+ ] (0.1 M), even at low operating current, conditions where HER can otherwise dominate.

Strategies for breaking molecular scaling relationships for the electrochemical CO2 reduction reaction.

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.

Enhancing a Molecular Electrocatalyst's Activity for CO2 Reduction by Simultaneously Modulating Three Substituent Effects.

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 ∼0.2 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.

Determining the coordination environment and electronic structure of polymer-encapsulated cobalt phthalocyanine under electrocatalytic CO2 reduction conditions using in situ X-Ray absorption spectroscopy.

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.

Electrocatalytic CO2 reduction by a cobalt bis(pyridylmonoimine) complex: effect of acid concentration on catalyst activity and stability.

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.

Iridium(III)-Based Metal-Organic Frameworks as Multiresponsive Luminescent Sensors for Fe3+, Cr2O72-, and ATP2- in Aqueous Media.

Three iridium(III)-based metal-organic frameworks (MOFs), namely [Cd3{Ir(ppy-COO)3}2(DMF)2(H2O)4]·6H2O·2DMF (1), [Cd3{Ir(ppy-COO)3}2(DMA)2(H2O)2]·0.5H2O·2DMA (2), and [Cd3{Ir(ppy-COO)3}2(DEF)2(H2O)2]·8H2O·2DEF (3) (ppy-COOH = methyl-3-(pyridin-2-yl)benzoic acid, DMF = N,N-dimethylformamide, DMA = N,N-dimethylacetamide, DEF = N,N-diethylformamide), have been synthesized and characterized. Single-crystal structural determinations reveal that compounds 1-3 are isostructural, showing a three-dimensional framework structure with (3,6) connected rtl topologyin whose trimers of {Cd3(COO)6} are cross-linked by Ir(ppy-COO)33-. The structures are completely different from those of other Ir(III)-based MOFs. Compound 1 was selected for a detailed study on sensing properties. The excellent luminescence as well as good water stability of 1 makes it a highly selective and sensitive multiresponsive luminescent sensor for Fe3+ and Cr2O72-. The detection limits are 67.8 and 145.1 ppb, respectively. Compound 1 can also be used as an optical sensor for selective sensing of adenosine triphosphate (ATP2-) over adenosine diphosphate (ADP2-) and adenosine monophosphate (AMP2-) in aqueous solution. This is the first example of iridium(III)-based MOFs for the optical detection of Fe3+, Cr2O72-, and ATP2-. More interestingly, the luminescent composite film doped with 1% (w/w) of compound 1, 1@PMMA (PMMA = poly(methyl methacrylate)), can be successfully prepared, which endows efficient sensitivity for Fe3+ and Cr2O72- detection and thus provides great potential for future applications.

Defective Metal-Organic Frameworks Incorporating Iridium-Based Metalloligands: Sorption and Dye Degradation Properties.

Artificial control and engineering of metal-organic framework (MOF) crystals with defects can endow them with suitable properties for applications in gas storage, separation, and catalysis. A series of defective iridium-containing MOFs, [Zn4 (µ4 -O)(Ir-A)2(1-x) (Ir-B)2x ] (ZnIr-MOF-dx ), were synthesized by doping heterostructured linker Ir-BH3 into the parent [Zn4 (µ4 -O)(Ir-A)2 ] (ZnIr-MOF), in which Ir-AH3 represents [Ir(ppy-COOH)3 ] (ppyCOOH=3-(pyridin-2-yl)benzoic acid) and Ir-BH3 is [Ir(ppy-COOH)2 (2-pyPO3 H)] (2-pyPO3 H2 =2-pyridylphosphonic acid). Samples with different degrees of defects were characterized by SEM, IR and NMR spectroscopy, powder XRD measurements, and thermal and elemental analyses. ZnIr-MOF-d0.3 was selected as a representative for gas (N2 , CO2 ) or vapor (H2 O, alcohol) sorption studies. The results demonstrate that defective ZnIr-MOF-d0.3 possesses multiple pore size distributions, ranging from micro- to mesopores, unlike the parent material, which shows a uniform micropore distribution. The hydrophilicity of the interior surface is also increased after defect engineering. As a result, ZnIr-MOF-d0.3 shows an enhanced adsorption capability toward n-butanol, relative to that of the parent compound. Optical studies reveal that both ZnIr-MOF and ZnIr-MOF-d0.3 have low band gaps (2.35 and 2.40 eV), corresponding to semiconductors. ZnIr-MOF-d0.3 exhibits dramatically increased photocatalytic efficiency for dye degradation.

Introduction of functionality, selection of topology, and enhancement of gas adsorption in multivariate metal-organic framework-177.

Metal-organic framework-177 (MOF-177) is one of the most porous materials whose structure is composed of octahedral Zn4O(-COO)6 and triangular 1,3,5-benzenetribenzoate (BTB) units to make a three-dimensional extended network based on the qom topology. This topology violates a long-standing thesis where highly symmetric building units are expected to yield highly symmetric networks. In the case of octahedron and triangle combinations, MOFs based on pyrite (pyr) and rutile (rtl) nets were expected instead of qom. In this study, we have made 24 MOF-177 structures with different functional groups on the triangular BTB linker, having one or more functionalities. We find that the position of the functional groups on the BTB unit allows the selection for a specific net (qom, pyr, and rtl), and that mixing of functionalities (-H, -NH2, and -C4H4) is an important strategy for the incorporation of a specific functionality (-NO2) into MOF-177 where otherwise incorporation of such functionality would be difficult. Such mixing of functionalities to make multivariate MOF-177 structures leads to enhancement of hydrogen uptake by 25%.

Exfoliated layered copper phosphonate showing enhanced adsorption capability towards Pb ions.

Copper 5-(2-bromothienyl)phosphonate (1) with a layered structure is obtained via solvothermal reaction. The layers can be successfully exfoliated using a "top-down" approach, resulting in 2D nanosheets. The exfoliated sample shows an enhanced adsorption capability to the Pb(II) ions in aqueous solution compared with that of the bulk material.