Organic Electrocatalysts Research Articles

Crystalline 1D Linear Conjugated Polymers with a Quinoidal Unit As Metal‐Free Electrocatalysts for Oxygen Reduction

AbstractCrystalline and soluble 1D linear conjugated polymers (LCPs) have garnered considerable interest as cost‐effective and efficient metal‐free electrocatalysts for the oxygen reduction reaction (ORR) due to their high exposure of catalytic sites and ease of processing. However, difficulties remain in achieving excellent ORR performance for 1D LCPs by conventional molecular design. Herein, it is demonstrated the utilization of quinoidal units as a promising strategy to develop 1D LCPs for ORR. The incorporation of quinoidal unit not only results in polymers with strong inter‐ and intra‐molecular interactions and low‐lying LUMO (lowest unoccupied molecular orbital) energy levels, but also provides polymers with good crystallinity and commendable charge carrier mobilities. The electronic properties of these polymers are fine‐tuned through the introduction of additional heteroatoms and/or substituents in the monomers and comonomers to understand and optimize their ORR catalytic activity. Evaluation of their catalytic performances reveals a remarkable half‐wave potential and limiting current density of up to 0.74 V (vs reversible hydrogen electrod) and 7.50 mA cm−2, respectively. Notably, these performances are achieved without the addition of carbon nanomaterials as support. This study offers a new insight into the design of highly efficient metal‐free organic electrocatalysts.

Rational Design of Organic Electrocatalysts for Hydrogen and Oxygen Electrocatalytic Applications.

Efficient electrocatalysts are pivotal for advancing green energy conversion technologies. Organic electrocatalysts, as cost-effective alternatives to noble-metal benchmarks, have garnered attention. However, the understanding of the relationships between their properties and electrocatalytic activities remains ambiguous. Plenty of research articles regarding low-cost organic electrocatalysts started to gain momentum in 2010 and have been flourishing recently though, a review article for both entry-level and experienced researchers in this field is still lacking. This review underscores the urgent need to elucidate the structure-activity relationship and design suitable electrode structures, leveraging the unique features of organic electrocatalysts like controllability and compatibility for real-world applications. Organic electrocatalysts are classified into four groups: small molecules, oligomers, polymers, and frameworks, with specific structural and physicochemical properties serving as activity indicators. To unlock the full potential of organic electrocatalysts, five strategies are discussed: integrated structures, surface property modulation, membrane technologies, electrolyte affinity regulation, and addition of anticorrosion species, all aimed at enhancing charge efficiency, mass transfer, and long-term stability during electrocatalytic reactions. The review offers a comprehensive overview of the current state of organic electrocatalysts and their practical applications, bridging the understanding gap and paving the way for future developments of more efficient green energy conversion technologies.

Exploiting the NADP+/NADPH-like Hydride-Transfer Redox Cycle with Bis-Imidazolium-Embedded Heterohelicene for Electrocatalytic Hydrogen Evolution Reaction.

Generation of clean energy in a viable manner demands efficient and sustainable catalysts. One prospective method of clean energy generation is the electrochemical hydrogen evolution reaction (HER). Over the years, various transition metal-based complexes/polymeric organic materials were utilized in HER. However, the use of a redox-active small organic molecule as a catalyst for HER has not been explored well. The requirements of a strongly acidic solution, very high overpotential, and stability under acidic conditions pose several challenges for applying organic electrocatalysts for HER. Considering these challenges, herein, we demonstrated an NADP+-like organic system (NADP+ = nicotinamide adenine dinucleotide phosphate), a bis-imidazolium-fused heterohelicene, which acts as a catalyst for HER with mild acid (acetic acid) as a proton source at moderate overpotential. The unique structural backbone of this dicationic heterohelicene allowed to exploit the NADP+/NADPH-type (NADPH = reduced nicotinamide adenine dinucleotide phosphate) hydride transfer-based redox cycle efficiently under the applied conditions, where the NADPH-like hydride intermediate transfers the hydride to the proton of the mild acid to generate H2. The Faradaic efficiency and turnover number for the present HER were achieved up to 85 ± 5% and 50 ± 3, respectively. In addition, the maximum turnover frequency, TOFmax, value of 410 s-1 was observed, which is around 400 times that obtained for the existing reported NADP+-like organic compounds used as catalysts for HER. Thorough mechanistic studies were conducted experimentally and computationally to establish a plausible catalytic cycle. This advancement could help in designing efficient organic electrocatalysts for HER from a mild proton source.

Metal-Free Carbon-Based Covalent Organic Frameworks with Heteroatom-Free Units Boost Efficient Oxygen Reduction.

Accurate identification of carbon-based metal-free electrocatalyst (CMFE) activity and enhancing their catalytic efficiency for O2 conversion is an urgent and challenging task. This study reports a promising strategy to simultaneously develop a series of covalent organic frameworks (COFs) with well-defined heterocyclic-free biphenyl or fluorenyl units. Unlike heteroatom doping, the developed method not only supplies methyl-induced molecular configuration to promote activity, but also provides a direct opportunity to identify heteroatom-free carbon active centers. The introduction of methyl groups (MGs) with reversible valence bonds into a pristine biphenyl-based COF results in an excellent performance with a half-wave potential of 0.74V versus the reversible hydrogen electrode (RHE), which is among the highest values for CMFE-COFs as oxygen reduction reaction (ORR) electrocatalysts. Combined with in situ Raman spectra and theoretical calculations, the MG-bound skeleton (DAF-COF) is found to produce ortho activation, confirming the ortho carbon (site-5) adjacent to MGs as active centers. This may be attributed to the opening and binding of MGs, which effectively regulate the molecular configuration and charge redistribution, as well as improve charge transfer and reduce the energy barrier. This study provides insight into the design of highly efficient metal-free organic electrocatalysts via the regulation of valence bonds.

Digitally controlled organic electrocatalyst for water electrolysis

Rapid growth of information processing and storing technology enforces the rapid relevancy reduction of predecessor technology. Electronic waste accumulation and energy scarcity in the future can be tackled through waste re-utilization. This study intends to develop a digitally controllable organic electrocatalyst based on CD-R. The developed catalyst hydrogen evolution reaction rate was evaluated on electrolysis test in saltwater solvent. The mechanistic view of the kinetics was built from the functional group, phase, and morphological characterization followed by surface charge analysis through digital imaging technique. The result shows that digital information alters electrocatalytic performance of CD shavings and undamaged CD. The half-loaded data CD shavings produced 449101 ppm H 2 which surpass fully-loaded CD shavings with 421879 ppm H 2 . All CD shavings final H 2 were higher than the undamaged CD final concentration. The size of the data loaded on CD determines the surface polarity pattern which enabling digital control. The ununiformed CD shaving kinetics is strongly depends on its size and shape which deviates the role of loaded data size. The strong interchanging surface polarity of fully-loaded CD is effective to interact with negative and positive water ion cluster. In conclusion, this study has enabled electrocatalyst kinetic control through digital information. • Digitally controllable electrocatalyst is possible to be designed. • Digital information alters the electrocatalytic performance. • CD electrocatalyst surface polarity is tunable. • Loaded data size determine the surface polarity series.

Covalent organic frameworks based on electroactive naphthalenediimide as active electrocatalysts toward oxygen reduction reaction

Developing organic electrocatalysts toward the oxygen reduction reaction (ORR) that avoid heteroatom doping processes and high-temperature carbonization is of great significance for the maturing of fuel cell applications. Herein, a series of two-dimensional imide-based covalent organic framework (COFs) electrocatalysts toward the ORR is reported. The hydrodynamic electrochemical study reveals that 3.5 electrons are exchanged during the ORR indicating that the process catalyzed by these COFs has a clear preference for the 4-electron reduction pathway. The COFs contain conjugated electroactive napthalenediimide (NDI) moieties that provides the active sites for the electrocatalysis and promotes the formation of COFs with face-to-face π-π stacked structures to provide intrinsic porosity and large surface areas. These COFs can be essentially considered as an organized pattern of active sites embedded in the pore walls of the COF. The choice of suitable comonomers with variable distortions from planarity offers the possibility of obtaining these electroactive COFs with similar redox ability but different degrees of porosity and interlaminar spacing. This work evidences a new insight into developing novel families of electrocatalysts from COFs. Structure and stacking fashion of the COF-systems are investigated on the basis of DFT calculations, as well as the photoabsorption spectra of the representative molecular entities and a proof-of-concept rationalization of the intermediate steps of the ORR mechanism.

Progress and Prospect of Organic Electrocatalysts in Lithium-Sulfur Batteries.

Lithium−sulfur (Li−S) batteries featured by ultra-high energy density and cost-efficiency are considered the most promising candidate for the next-generation energy storage system. However, their pragmatic applications confront several non-negligible drawbacks that mainly originate from the reaction and transformation of sulfur intermediates. Grasping and catalyzing these sulfur species motivated the research topics in this field. In this regard, carbon dopants with metal/metal-free atoms together with transition–metal complex, as traditional lithium polysulfide (LiPS) propellers, exhibited significant electrochemical performance promotions. Nevertheless, only the surface atoms of these host-accelerators can possibly be used as active sites. In sharp contrast, organic materials with a tunable structure and composition can be dispersed as individual molecules on the surface of substrates that may be more efficient electrocatalysts. The well-defined molecular structures also contribute to elucidate the involved surface-binding mechanisms. Inspired by these perceptions, organic electrocatalysts have achieved a great progress in recent decades. This review focuses on the organic electrocatalysts used in each part of Li−S batteries and discusses the structure–activity relationship between the introduced organic molecules and LiPSs. Ultimately, the future developments and prospects of organic electrocatalysts in Li−S batteries are also discussed.

Substituents and the induced partial charge effects on cobalt porphyrins catalytic oxygen reduction reactions in acidic medium

Charge states at the catalytic interface can intensely alter the charge transfer mechanism and thus the oxygen reduction performance. Two symmetric cobalt porphyrins with electron deficient 2,1,3-benzothiadiazole (BTD) and electron-donating propeller-like triphenylamine (TPA) derivatives have been designed firstly, to rationally generate intramolecular partial charges, and secondly, to utilize the more exposed molecular orbitals on TPA for enhancing the charge transfer kinetics. The catalytic performance of the two electrocatalysts was examined for oxygen reduction reactions (ORR) in acidic electrolyte. It was found that BCP1/C with two BTD groups showed greater reduction potential but less limiting current density as compared to BCP2/C bearing BTD-TPA units. The reduced potential of BCP2/C was proposed to the introduction of the electron-donating ability of TPA, which may decrease the adsorption affinity of oxygen to the cobalt center. Both dipole-induced partial charge effect and the more exposed cation orbitals of the 3D structural TPA were proposed to contribute to the increased response current of BCP2/C. In addition, BCP2/C attained more than 80% of H2O2 generation in acidic solution, which may also relate to the structural effect. These findings may provide new insight into the structural design of organic electrocatalysts and deep understanding on the interfacial charge transfer mechanism for ORR.

Robust protein based organic electrocatalyst for hydrogen production through instant noodle wastewater electrolysis

Natural resources carbonization doping using heavy atom is the common method to synthesize organic electrocatalyst for waste to hydrogen energy conversion. This study provides one-step solution to synthesize organic electrocatalyst using enzymatic protein of Lumbricus Rubellus. L,Rubellus extract and its combination with graphite flakes in powder form are tested as electrocatalyst for instant noodle wastewater electrolysis and pre-hydrolyzed instant noodle wastewater by direct mixing. Pre-hydrolysis has doubled the hydrogen production rate for each tested catalyst. Lumbricus Rubellus extract without graphite flake mix performs better in pre-hydrolyzed wastewater. Graphite flakes form polar substrates in pre-hydrolyzed wastewater. Positive substrates inhibit the hydrogen evolution reaction of pre-hydrolyzed wastewater by electron deposition. Negative substrate inactivates L,Rubellus protein due to competitive inhibition.

Electrochemically Triggered Dynamics within a Hybrid Metal-Organic Electrocatalyst.

A wide array of systems, ranging from enzymes to synthetic catalysts, exert adaptive motifs to maximize their functionality. In a related manner, select metal-organic frameworks (MOFs) and similar systems exhibit structural modulations under stimuli such as the infiltration of guest species. Probing their responsive behavior in situ is a challenging but important step toward understanding their function and subsequently building functional systems. In this report, we investigate the dynamic behavior of an electrocatalytic Mn-porphyrin-containing MOF system (Mn-MOF). We discover, using a combination of electrochemistry and in situ probes of UV-vis absorption, resonance Raman, and infrared spectroscopy, a restructuration of this system via a reversible cleavage of the porphyrin carboxylate ligands under an applied voltage. We further show, by combining experimental data and DFT calculations, as a proof of concept, the capacity to utilize the Mn-MOF for electrochemical CO2 fixation and to spectroscopically capture the reaction intermediates in its catalytic cycle. The findings of this work and the methodology developed open opportunities in the application of MOFs as dynamic, enzyme-inspired electrocatalytic systems.

Metal-Free Hydrogen-Bonded Polymers Mimic Noble Metal Electrocatalysts.

The most active and efficient catalysts for the electrochemical hydrogen evolution reaction (HER) rely on platinum, a fact that increases the cost of producing hydrogen and thereby limits the widespread adoption of this fuel. Here, a metal-free organic electrocatalyst that mimics the platinum surface by implementing a high work function and incorporating hydrogen-affine hydrogen bonds is introduced. These motifs, inspired from enzymology, are deployed here as selective reaction centres. It is shown that the keto-amine hydrogen-bond motif enhances the rate-determining step in proton reduction to molecular hydrogen. The keto-amine-functionalized polymers reported herein evolve hydrogen at an overpotential of 190 mV. They share certain key properties with platinum: a similar work function and excellent electrochemical stability and chemical robustness. These properties allow the demonstration of one week of continuous HER operation without notable degradation nor delamination from the carrier electrode. Scaled continuous-flow electrolysis is reported and 1 L net molecular hydrogen is produced within less than 9 h using 2.3 mg of polymer electrocatalyst.

Hydrogen production from instant noodle wastewater by organic electrocatalyst coated on PVC surface

The potential of electron-donating capability in methoxy groups of antioxidant containing protein (ACAP) as organic catalyst is restricted by its low isoelectric point. The goal of this study is to construct endure ACAP based metal-free organic catalyst for hydrogen production from electrolysis of noodle wastewater. The ACAP was coated thermomechanically on PVC sheet and its performance was tested during electrolysis of noodle wastewater. The morphological analysis, phase analysis, and elemental analysis of coated materials have shown a simultaneous pattern with electrolysis performances. The use of graphite flake to cover turmeric ACAP obstructs the electron to attack directly the positive charge of ACAP so that the electrocatalytic endurance increases while maintaining the hydrogen production rate. The combination of phenolic and enzymatic ACAPs is found to have the slowest reaction rate and lowest hydrogen production. The phenolic compound inhibits the enzymatic reaction.

Novel Fully Organic Water Oxidation Electrocatalysts: A Quest for Simplicity.

Despite the growing need for readily available and inexpensive catalysts for the half-reactions involved in water splitting, water oxidation and reduction electrocatalysts are still traditionally based on noble metals. One long-standing challenge has been the development of an oxygen evolution reaction catalyzed by easily available, structurally simple, and purely organic compounds. Herein, we first generalize the performance of the known N-ethyl-flavinium ion to a number of derivatives. Furthermore, we demonstrate an unprecedented application of different pyridinium and related salts as very simple, inexpensive water oxidation organocatalysts consisting of earth-abundant elements (C, H, O, and N) exclusively. The results establish the prospects of heterocyclic aromatics for further design of new organic electrocatalysts for this challenging oxidation reaction.

In vitro alternatives: Evaluation and recommendations for the reconstructed 3D skin micronucleus assay (RSMN assay): 72h protocol

Three compounds with nitrocarbazole frameworks were synthesized and their electrochemical reversibility as organic electrocatalysts was studied by cyclic voltammetry. The electrochemical reversibility and oxidation-reduction potential of the compounds were greatly affected by their substituents. The oxidation-reduction potential of the compound with an electron-donating group was negative, while that of the compound with an electron-withdrawing group on the carbazole framework was positive. The electrocatalytic oxidation activities of the nitrocarbazole compounds were investigated through cyclic voltammetry and controlled potential electrolysis at room temperature. The electrocatalysts showed excellent selectivity for p-methoxybenzyl alcohol, converting it to the corresponding aldehyde through electro-oxidation with just 2.5 mol% of the electrocatalysts presented. The electrocatalysts maintained their excellent electroredox activity following recycling.

Synthesis of nitrocarbazole compounds and their electrocatalytic oxidation of alcohol

Three compounds with nitrocarbazole frameworks were synthesized and their electrochemical reversibility as organic electrocatalysts was studied by cyclic voltammetry. The electrochemical reversibility and oxidation-reduction potential of the compounds were greatly affected by their substituents. The oxidation-reduction potential of the compound with an electron-donating group was negative, while that of the compound with an electron-withdrawing group on the carbazole framework was positive. The electrocatalytic oxidation activities of the nitrocarbazole compounds were investigated through cyclic voltammetry and controlled potential electrolysis at room temperature. The electrocatalysts showed excellent selectivity for p-methoxybenzyl alcohol, converting it to the corresponding aldehyde through electro-oxidation with just 2.5 mol% of the electrocatalysts presented. The electrocatalysts maintained their excellent electroredox activity following recycling.

An Organic Catalyst for Li-O2 Batteries: Dilithium Quinone-1,4-Dicarboxylate.

Solid organic electrocatalysts have hardly been tested in Li-O2 batteries. Here, a new solid organic electrocatalyst, dilithium quinone-1,4-dicarboxylate (Li2 C8 H2 O6 ) is presented, which is expected to overcome the shortcomings of inorganic catalysts. The function-oriented synthesis is low cost and low polluting. The electrocatalytic performance is evaluated by following the degradation of Li2 O2 during the charge process in a Li-O2 cell through in situ XRD and operando synchrotron radiation powder XRD (SR-PXD) measurements. The results indicate that the electrocatalytic activity of Li2 C8 H2 O6 is similar to that of commercial Pt. The Li2 O2 decomposition in a cell with Li2 C8 H2 O6 catalyst follows a pseudo-zero-order reaction, virtually without any side reactions. These results provide an insight into the development of new organic catalysts for the oxygen evolution reaction (OER) in Li-O2 batteries.

The Reduction of Oxygen on Iron(II) Oxide/Poly(3,4-ethylenedioxythiophene) Composite Thin Film Electrodes

Conducting polymers (CPs) are currently being investigated for use in many applications owing to their abilities to catalyze a wide range of electrochemical reactions and act as an effective electrode support for various inorganic and organic electrocatalyst materials. Here, we have found that the deposition of poly(3,4-ethylenedioxythiophene) (PEDOT) through the use of an established base-inhibited chemical vapor-phase polymerization (VPP) procedure using an iron(III) tosylate oxidant results in the co-deposition of electrocatalytic iron(II) oxide species within the film. The presence of these species accounts for the 2-electron reduction of hydrogen peroxide that occurs on these electrodes during the series 4-electron oxygen reduction reaction. Furthermore, this realization leads to the possibility of fabricating thin film inorganic/CP composites of various compositions through careful choice of oxidant in a facile, one-step process. A combination of in situ Raman (487.77nm laser) and in situ UV–Vis spectroscopy was used to probe the oxidation state of PEDOT in the thin film composite electrodes while reducing oxygen in alkaline conditions. These measurements show that the 2-electron electroreduction of hydrogen peroxide (or HO2−) occurs only on the iron(II) oxide species in a reaction that is facilitated by an effective electron transfer from the delocalized electron orbitals of the PEDOT matrix. This approach could potentially be used in situ to monitor the electrocatalyst/electrode interface quality of conducting polymer-supported electrocatalysts.