Electrochemical CO2 Reduction Reaction: Comprehensive Strategic Approaches to Catalyst Design for Selective Liquid Products Formation.

The escalating concern regarding the release of CO2 into the atmosphere poses a significant threat to the contemporary efforts in mitigating climate change. Amidst a multitude of strategies for curtailing CO2 emissions, the electrochemical CO2 reduction presents a promising avenue for transforming CO2 molecules into a diverse array of valuable gaseous and liquid products, such as CO, CH3OH, CH4, HCO2H, C2H4, C2H5OH, CH3CO2H, 1-C3H7OH and others. The mechanistic investigations of gaseous products (e. g. CO, CH4, C2H4, C2H6 and others) broadly covered in the literature. There is a noticeable gap in the literature when it comes to a comprehensive summary exclusively dedicated to coherent roadmap for the designing principles for a selective catalyst all possible liquid products (such as CH3OH, C2H5OH, 1-C3H7OH, 2-C3H7OH, 1-C4H9OH, as well as other C3-C4 products like methylglyoxal and 2,3-furandiol, in addition to HCO2H, AcOH, oxalic acid and others), selectively converted by CO2 reduction. This entails a meticulous analysis to justify these approaches and a thorough exploration of the correlation between materials and their electrocatalytic properties. Furthermore, these insightful discussions illuminate the future prospects for practical applications, a facet not exhaustively examined in prior reviews.

Redox Replacement of Silver on MOF-Derived Cu/C Nanoparticles on Gas Diffusion Electrodes for Electrocatalytic CO2 Reduction.

Bimetallic tandem catalysts have emerged as a promising strategy to locally increase the CO flux during electrochemical CO2 reduction, so as to maximize the rate of conversion to C-C-coupled products. Considering this, a novel Cu/C-Ag nanostructured catalyst has been prepared by a redox replacement process, in which the ratio of the two metals can be tuned by the replacement time. An optimum Cu/Ag composition with similarly sized particles showed the highest CO2 conversion to C2+ products compared to non-Ag-modified gas-diffusion electrodes. Gas chromatography and in-situ Raman measurements in a CO2 gas diffusion cell suggest the formation of top-bound linear adsorbed *CO followed by consumption of CO in the successive cascade steps, as evidenced by the increasingνC-H bands. These findings suggest that two mechanisms operate simultaneously towards the production of HCO2 H and C-C-coupled products on the Cu/Ag bimetallic surface.

B-Cu-Zn Gas Diffusion Electrodes for CO2 Electroreduction to C2+ †Products at High Current Densities.

Electroreduction of CO2 to multi-carbon products has attracted considerable attention as it provides an avenue to high-density renewable energy storage. However, the selectivity and stability under high current densities are rarely reported. Herein, B-doped Cu (B-Cu) and B-Cu-Zn gas diffusion electrodes (GDE) were developed for highly selective and stable CO2 conversion to C2+  products at industrially relevant current densities. The B-Cu GDE exhibited a high Faradaic efficiency of 79 % for C2+  products formation at a current density of -200 mA cm-2 and a potential of -0.45 V vs. RHE. The long-term stability for C2+ formation was substantially improved by incorporating an optimal amount of Zn. Operando Raman spectra confirm the retained Cu+ species under CO2 reduction conditions and the lower overpotential for *OCO formation upon incorporation of Zn, which lead to the excellent conversion of CO2 to C2+ products on B-Cu-Zn GDEs.

A Metal-Organic Framework derived Cux Oy Cz Catalyst for Electrochemical CO2 Reduction and Impact of Local pH Change.

Developing highly efficient and selective electrocatalysts for the CO2 reduction reaction to produce value-added chemicals has been intensively pursued. We report a series of Cux Oy Cz nanostructured electrocatalysts derived from a Cu-based MOF as porous self-sacrificial template. Blending catalysts with polytetrafluoroethylene (PTFE) on gas diffusion electrodes (GDEs) suppressed the competitive hydrogen evolution reaction. 25 to 50 wt % teflonized GDEs exhibited a Faradaic efficiency of ≈54 % for C2+ products at -80 mA cm-2 . The local OH- ions activity of PTFE-modified GDEs was assessed by means of closely positioning a Pt-nanoelectrode. A substantial increase in the OH- /H2 O activity ratio due to the locally generated OH- ions at increasing current densities was determined irrespective of the PTFE amount.

MOF Derived Co3O4@Co/NCNT Nanocomposite for Electrochemical Hydrogen Evolution, Flexible Zinc-Air Batteries, and Overall Water Splitting.

Toward the goal of clean and sustainable energy source, the development of a trifunctional electrocatalyst is a boon for energy storage and conversion devices such as regenerative fuel cells and metal-air batteries. MOF-derived semiconducting-metallic core-shell electrocatalyst Co3O4@Co/NCNT (NCNT = nitrogen-doped carbon nanotube), which was shown to catalyze oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), is also found to be an active electrocatalyst for hydrogen evolution reaction (HER) with a low overpotential of 171 mV. Here, the HER activity of Co3O4@Co/NCNT is presented and is shown as highly efficient and robust trifunctional electrocatalyst. The detailed theoretical calculation has found N-center of Co-N4 moiety to be the H+ binding active site and thus proves Co3O4@Co/NCNT to be active for HER. Further, the ORR and OER bifunctionality of Co3O4@Co/NCNT helped in fabricating secondary Zn-air battery with high power density of 135 mW/cm2. Also, an all-solid-state flexible and wearable battery with Co3O4@Co/NCNT as cathode and electrodeposited Zn on carbon fiber cloth as anode was shown to withstand its performance even under stressed conditions. Finally, the material being trifunctional in nature was used both as an anode and cathode material for the electrolysis of water, which was powered by the Zn-air batteries with Co3O4@Co/NCNT as the cathode material. It is believed that the development of a trifunctional catalyst would help in wide commercialization of regenerative fuel cells.

Guest-Responsive Reversible Electron Transfer in a Crystalline Porous Framework Supported by a Dynamic Building Node.

We demonstrate a redox-active, crystalline donor-acceptor (D-A) assembly in which the electron transfer (ET) process can be reversibly switched. This ET process, induced by a guest-responsive structural transformation at room temperature, is realized in a porous, metal-organic framework (MOF), having anthracene (D)-naphthalenediimide (A) as struts. A control MOF structure obtained by a solvent-assisted linker exchange (SALE) method, replacing an acceptor strut with a neutral one, supported the switchable electronic states in the D-A MOF. Combined investigations with X-ray diffraction, spectroscopy, and theoretical analyses revealed the dynamic metal paddle-wheel node as a critical unit for controlling structural flexibility and the corresponding unprecedented ET process.

Soft Porous Crystal Based upon Organic Cages That Exhibit Guest-Induced Breathing and Selective Gas Separation.

Soft porous crystals (SPCs) that exhibit stimuli-responsive dynamic sorption behavior are attracting interest for gas storage/separation applications. However, the design and synthesis of SPCs is challenging. Herein, we report a new type of SPC based on a [2 + 3] imide-based organic cage (NKPOC-1) and find that it exhibits guest-induced breathing behavior. Various gases were found to induce activated NKPOC-1 crystals to reversibly switch from a "closed" nonporous phase (α) to two porous "open" phases (ß and γ). The net effect is gate-opening behavior induced by CO2 and C3 hydrocarbons. Interestingly, NKPOC-1-α selectively adsorbs propyne over propylene and propane under ambient conditions. Thus, NKPOC-1-α has the potential to separate binary and ternary C3 hydrocarbon mixtures, and the performance was subsequently verified by fixed bed column breakthrough experiments. In addition, molecular dynamics calculations and in situ X-ray diffraction experiments indicate that the gate-opening effect is accompanied by reversible structural transformations. The adsorption energies from molecular dynamics simulations aid are consistent with the experimentally observed selective adsorption phenomena. The understanding gained from this study of NKPOC-1 supports the further development of SPCs for applications in gas separation/storage because SPCs do not inherently suffer from the recyclability problems often encountered with rigid materials.

Co3 O4 @Co/NCNT Nanostructure Derived from a Dicyanamide-Based Metal-Organic Framework as an Efficient Bi-functional Electrocatalyst for Oxygen Reduction and Evolution Reactions.

There has been growing interest in the synthesis of efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER), for their potential use in a variety of renewable energy technologies, such as regenerative fuel cells and metal-air batteries. Here, a bi-functional electrocatalyst, derived from a novel dicyanamide based nitrogen rich MOF {[Co(bpe)2 (N(CN)2 )]⋅(N(CN)2 )⋅(5 H2 O)}n [Co-MOF-1, bpe=1,2-bis(4-pyridyl)ethane, N(CN)2- =dicyanamide] under different pyrolysis conditions is reported. Pyrolysis of the Co-MOF-1 under Ar atmosphere (at 800 °C) yielded a Co nanoparticle-embedded N-doped carbon nanotube matrix (Co/NCNT-Ar) while pyrolysis under a reductive H2 /Ar atmosphere (at 800 °C) and further mild calcination yielded Co3 O4 @Co core-shell nanoparticle-encapsulated N-doped carbon nanotubes (Co3 O4 @Co/NCNT). Both catalysts show bi-functional activity towards ORR and OER, however, the core-shell Co3 O4 @Co/NCNT nanostructure exhibited superior electrocatalytic activity for both the ORR with a potential of 0.88 V at a current density of -1 mA cm-2 and the OER with a potential of 1.61 V at 10 mA cm-2 , which is competitive with the most active bi-functional catalysts reported previously.

Dynamic Entangled Porous Framework for Hydrocarbon (C2-C3) Storage, CO2 Capture, and Separation.

Storage and separation of small (C1-C3) hydrocarbons are of great significance as these are alternative energy resources and also can be used as raw materials for many industrially important materials. Selective capture of greenhouse gas, CO2 from CH4 is important to improve the quality of natural gas. Among the available porous materials, MOFs with permanent porosity are the most suitable to serve these purposes. Herein, a two-fold entangled dynamic framework {[Zn2 (bdc)2 (bpNDI)]⋅4DMF}n with pore surface carved with polar functional groups and aromatic π clouds is exploited for selective capture of CO2 , C2, and C3 hydrocarbons at ambient condition. The framework shows stepwise CO2 and C2 H2 uptake at 195 K but type I profiles are observed at 298 K. The IAST selectivity of CO2 over CH4 is the highest (598 at 298 K) among the MOFs without open metal sites reported till date. It also shows high selectivity for C2 H2 , C2 H4 , C2 H6 , and C3 H8 over CH4 at 298 K. DFT calculations reveal that aromatic π surface and the polar imide (RNC=O) functional groups are the primary adsorption sites for adsorption. Furthermore, breakthrough column experiments showed CO2 /CH4 C2 H6 /CH4 and CO2 /N2 separation capability at ambient condition.

Redox-Active Metal-Organic Frameworks: Highly Stable Charge-Separated States through Strut/Guest-to-Strut Electron Transfer.

Molecular organization of donor and acceptor chromophores in self-assembled materials is of paramount interest in the field of photovoltaics or mimicry of natural light-harvesting systems. With this in mind, a redox-active porous interpenetrated metal-organic framework (MOF), {[Cd(bpdc)(bpNDI)]⋅4.5 H2 O⋅DMF}n (1) has been constructed from a mixed chromophoric system. The µ-oxo-bridged secondary building unit, {Cd2 (µ-OCO)2 }, guides the parallel alignment of bpNDI (N,N'-di(4-pyridyl)-1,4,5,8-naphthalenediimide) acceptor linkers, which are tethered with bpdc (bpdcH2 =4,4'-biphenyldicarboxylic acid) linkers of another entangled net in the framework, resulting in photochromic behaviour through inter-net electron transfer. Encapsulation of electron-donating aromatic molecules in the electron-deficient channels of 1 leads to a perfect donor-acceptor co-facial organization, resulting in long-lived charge-separated states of bpNDI. Furthermore, 1 and guest encapsulated species are characterised through electrochemical studies for understanding of their redox properties.

Stoichiometry-controlled two flexible interpenetrated frameworks: higher CO2 uptake in a nanoscale counterpart supported by accelerated adsorption kinetics.

Here, we report the synthesis, structural characterizations, and gas storage properties of two new 2-fold interpenetrated 3D frameworks, {[Zn2(bpdc)2(azpy)]·2H2O·2DMF}n (1) and {[Zn3(bpdc)3(azpy)]·4H2O·2DEF}n (2) [bpdc = 4,4'-biphenyldicarboxylate; azpy = 4,4'-azobipyridine], obtained from the same set of organic linkers. Furthermore, 1 has been successfully miniaturized to nanoscale (MOF1N) of spherical morphology to study size dependent adsorption properties through a coordination modulation method. The two different SBUs, dinuclear paddle-wheel {Zn2(COO)4} for 1 and trinuclear {Zn3(µ2-OCO)2(COO)4 }for 2, direct the different network topologies of the frameworks that render different adsorption characteristics into the systems. Both of the frameworks show guest induced structural transformations as supported by PXRD studies. Adsorption studies of 1 and 2 show CO2 selectivity over several other gases (such as N2, H2, O2, and Ar) under identical experimental conditions. Interestingly, MOF1N exhibits significantly higher CO2 storage capacity compared to bulk crystals of 1 and that can be attributed to the smaller diffusion barrier at the nanoscale that is supported by studies of adsorption kinetics in both states. Kinetic measurement based on water vapor adsorption clearly distinguishes between the rate of diffusion of bulk (1) and nanospheres (MOF1N). The respective kinetic rate constant (k, s(-1)) for MOF1N (k = 1.29 × 10(-2) s(-1)) is found to be considerably higher than 1 (k = 7.1 × 10(-3) s(-1)) as obtained from the linear driving force (LDF) model. This is the first account where a new interpenetrated MOF has been scaled down to nanoscale through a coordination modulation method, and their difference in gas uptake properties has been correlated through a higher rate of mass diffusion as obtained from kinetics of adsorption.

An adsorbate biased dynamic 3D porous framework for inverse CO2 sieving over C2H2.

Separating carbon dioxide (CO2) from acetylene (C2H2) is one of the most critical and complex industrial separations due to similarities in physicochemical properties and molecular dimensions. Herein, we report a novel Ni-based three-dimensional framework {[Ni4(µ3-OH)2(µ2-OH2)2(1,4-ndc)3](3H2O)}n (1,4-ndc = 1,4-naphthalenedicarboxylate) with a one-dimensional pore channel (3.05 × 3.57 Å2), that perfectly matches with the molecular size of CO2 and C2H2. The dehydrated framework shows structural transformation, decorated with an unsaturated Ni(ii) centre and pendant oxygen atoms. The dynamic nature of the framework is evident by displaying a multistep gate opening type CO2 adsorption at 195, 273, and 298 K, but not for C2H2. The real time breakthrough gas separation experiments reveal a rarely attempted inverse CO2 selectivity over C2H2, attributed to open metal sites with a perfect pore aperture. This is supported by crystallographic analysis, in situ spectroscopic inspection, and selectivity approximations. In situ DRIFTS measurements and DFT-based theoretical calculations confirm CO2 binding sites are coordinatively unsaturated Ni(ii) and carboxylate oxygen atoms, and highlight the influence of multiple adsorption sites.

Gaining the Freedom of Scalable Gas Diffusion Electrodes for the CO2 Reduction Reaction.

Gas diffusion electrodes (GDEs) in CO2 reduction reaction (CO2RR) alleviate the mass transfer limitation of gaseous reagents, which is beneficial for reducing CO2 into valuable chemicals. GDEs offer higher current densities compared to electrodes immersed in the electrolyte. Disclosing the roles of different structural parameters in tuning the performance of the GDEs is essential to exert the potential of catalysts and to meet potential large-scale industrial applications of the CO2RR. A novel layer structure for the airbrush-type spray fabrication of GDEs was designed and optimised, comprising a carbon-based gas-diffusion layer, a PEEK fabric, a Ni mesh, a carbon-integrated catalyst layer, and a PTFE top layer. It was shown that adjusting the carbon material in the gas diffusion and the catalyst layer impacts the selectivity of the CO2RR due to the modulation of the pore network. This work disclosed a practical and scalable but also an easily transferable pathway for preparing GDEs and offered an idea of how to tune the significant parameters of GDEs for optimising their CO2RR performance.

Trace CO2 capture by an ultramicroporous physisorbent with low water affinity.

CO2 accumulation in confined spaces represents an increasing environmental and health problem. Trace CO2 capture remains an unmet challenge because human health risks can occur at 1000 parts per million (ppm), a level that challenges current generations of chemisorbents (high energy footprint and slow kinetics) and physisorbents (poor selectivity for CO2, especially versus water vapor, and/or poor hydrolytic stability). Here, dynamic breakthrough gas experiments conducted upon the ultramicroporous material SIFSIX-18-Ni-ß reveal trace (1000 to 10,000 ppm) CO2 removal from humid air. We attribute the performance of SIFSIX-18-Ni-ß to two factors that are usually mutually exclusive: a new type of strong CO2 binding site and hydrophobicity similar to ZIF-8. SIFSIX-18-Ni-ß also offers fast sorption kinetics to enable selective capture of CO2 over both N2 (S CN) and H2O (S CW), making it prototypal for a previously unknown class of physisorbents that exhibit effective trace CO2 capture under both dry and humid conditions.

Counter anion dependent gradual spin transition in a 1D cobalt(ii) coordination polymer.

One Co(ii) SCO coordination polymer [Co(enbzpy)(µ1,5-dca)]n(PF6)n (·PF6) has been isolated and characterised structurally and magnetostructurally. The properties of ·PF6 are compared with the reported perchlorate analogue [Co(enbzpy)(µ1,5-dca)]n(ClO4)n (·ClO4). The gradual spin transition in ·PF6 in contrast to an abrupt spin transition with a hysteresis loop in ·ClO4 has been analysed in terms of structural factors.