Metal-free Electrocatalysis Research Articles

Coordinatively fluxional diazo-based organo-electrocatalyst for conversion of CO2 to C2 and C3 products

The conversion of carbon dioxide (CO2) into valuable chemicals, specifically C2 and C3, through metal-free electrocatalysis remains a formidable challenge. Breaking away from traditional transition metal complexes, the focus is on designing and selecting efficient organic catalysts. In this pursuit, a diazo-based organic bulky ligand emerges as a promising candidate, offering a solution that is both sustainable and renewable. The key feature of this ligand is its low-lying π* (LUMO), enabling it to readily accept an electron in an electrochemical environment when a potential is applied. The synthesized Diazo-based ligands have been meticulously characterized using various techniques, including 1H NMR, 13C NMR, UV-Vis, and IR spectroscopy. This diazo-based ligand serves as an electrocatalyst, undergoing reduction to a triplet diradical that acts as a nucleophile. In an aqueous medium, it forms an adduct with CO2, leading to the generation of a formyl radical. This radical further couples to produce acetic acid and acetone with efficiencies of 19.6% and 24.2%, respectively, at pH 5.5. To provide a deeper understanding, we present a proposed mechanism pathway supported by in-situ UV-Vis spectroscopy and a comprehensive Density Functional Theory (DFT) study. These findings mark a significant step forward in the field of metal-free electrocatalysis, offering a sustainable approach to the conversion of CO2 into valuable chemicals, contributing to the development of renewable and environmentally friendly systems.

Side-Chain Modification in Conjugated Polymer Frameworks for the Electrocatalytic Oxygen Evolution Reaction.

Conjugated polymer frameworks (CPFs) have recently sparked tremendous research interest due to their broad potentials in various frontline application areas such as photocatalysis, sensing, gas storage, energy storage, etc. These framework materials, without sidechains or functional groups on their backbone, are generally insoluble in common organic solvents and less solution processable for further device applications. There are few reports on metal-free electrocatalysis, especially oxygen evolution reaction (OER) using CPF. Herein, we have developed two triazine-based donor-acceptor conjugated polymer frameworks by coupling a 3-substituted thiophene (donor) unit with a triazine ring (acceptor) through a phenyl ring spacer. Two different sidechains, alkyl and oligoethylene glycol, were rationally introduced into the 3-position of thiophene in the polymer framework to investigate the effect of side-chain functionality on the electrocatalytic property. Both the CPFs demonstrated superior electrocatalytic OER activity and long-term durability. The electrocatalytic performance of CPF2, which achieved a current density of 10 mA/cm2 at an overpotential (η) of 328 mV, is much superior to CPF1, which reached the same current density at an overpotential of 488 mV. The porous and interconnected nanostructure of the conjugated organic building blocks, which allowed for fast charge and mass transport processes, could be attributed to the higher electrocatalytic activity of both CPFs. However, the superior activity of CPF2 compared to CPF1 may be due to the presence of a more polar oxygen-containing ethylene glycol side chain, which enhances the surface hydrophilicity, promotes better ion/charge and mass transfer, and increases the accessibility of the active sites toward adsorption through lower π-π stacking compared to hexyl side chain present in CPF1. The DFT study also supports the plausible better performance toward OER for CPF2. This study confirms the promising potentiality of metal-free CPF electrocatalysts for OER and further sidechain modification to improve their electrocatalytic property.

Scalable production of reduced graphene oxide via biowaste valorisation: an efficient oxygen reduction reaction towards metal-free electrocatalysis

The development of environment-friendly, scalable, and low-cost electrocatalysts for an efficient ORR is important for green energy harvesting. This article deals with the scalable production of N-doped rGO for demonstrating better ORR activity.

Nitrogen-Doped Microporous Carbons Synthesized from Indole-Based Copolymer Spheres for Supercapacitors and Metal-Free Electrocatalysis

Indole-based copolymer spheres serve as a precursor for the facile and easily scalable synthesis of a series of nitrogen-rich microporous carbons. The influence of chemical activation and carboniza...

(Invited) Reduced Graphene Oxides with Engineered Defects for Efficient Electrochemical Reduction of Dinitrogen to Ammonia in Wide pH Range

Electrochemical nitrogen fixation under mild conditions is highly demanded yet remains a grand challenge. Herein we report metal-free electrocatalysis of aqueous N2 reduction to produce NH3 over reduced graphene oxide (DrGO) with tuned defects. The defect sites in DrGO, consisting of unsaturated carbon [single vacancy (SV), double vacancy (DV), and –COOH], were examined, and showed an improved NH3 selectivity due to the strong binding of N2 instead of H. In addition to improved selectivity, the calculated free energies for N2 reduction reaction at DrGO-COOH and DrGO-DV sites suggest that the thermodynamic ovepotentials of these metal-free catalysts are comparable to the most efficient transition metal-based catalysts reported thus far. Our nonmetallic and dopant-free catalysts can convert N2 to NH3 at a faradaic efficiency of up to 22.0% at -0.116 V (versus the reversible hydrogen electrode vs. RHE) in 0.1 M HCl and 10.8% at -0.166 V (vs. RHE) in 0.1 M KOH, surpassing most earlier reported catalysts. An NH3 formation rate exceeding 7.3 μg·h-1 mg-1 was achieved at low overpotentials in both acidic and alkaline environments, comparable to the values shown by metal electrocatalysts under similar conditions.

Reduced graphene oxides with engineered defects enable efficient electrochemical reduction of dinitrogen to ammonia in wide pH range

Electrochemical nitrogen fixation under mild conditions is highly demanded yet remains a grand challenge. Herein we report metal-free electrocatalysis of aqueous N2 reduction to produce NH3 over reduced graphene oxide (DrGO) with tuned defects. The defect sites in DrGO, consisting of unsaturated carbon [single vacancy (SV), double vacancy (DV), and –COOH], were examined, and showed an improved NH3 selectivity due to the strong binding of N2 instead of H. In addition to improved selectivity, the calculated free energies for N2 reduction reaction at DrGO-COOH and DrGO-DV sites suggest that the thermodynamic overpotentials of these metal-free catalysts are comparable to the most efficient transition metal-based catalysts reported thus far. Our nonmetallic and dopant-free catalysts can convert N2 to NH3 at a faradaic efficiency of up to 22.0% at −0.116 V (versus the reversible hydrogen electrode vs. RHE) in 0.1 M HCl and 10.8% at −0.166 V (vs. RHE) in 0.1 M KOH, surpassing most earlier reported catalysts. An NH3 formation rate exceeding 7.3 μg h−1 mg−1 was achieved at low overpotentials in both acidic and alkaline environments, comparable to the values shown by metal electrocatalysts under similar conditions.

Enhanced reduction of nitrate by noble metal-free electrocatalysis on P doped three-dimensional Co3O4 cathode: Mechanism exploration from both experimental and DFT studies

Developing green, sustainable and economical materials for emerging electrochemical remediation of nitrate (NO3−) contaminant have attracted immense research interest. In this study, a cost-effective and stable three-dimensional P-doped Co3O4/nickel foam (NF) was prepared for electrocatalytic reduction of NO3− in water. The doping of P could replace the lattice oxygen in Co3O4 and lead to the enhancements in Co3+ percentage and empty d orbitals. As a result, the relative energy for Volmer reaction and the adsorption energy of atomic H* decreased to be −0.73 eV and −3.60 eV, respectively. Furthermore, the higher electrochemical active area and lower interface impedance of Co3O4 were attained after P doping. The P2.1-Co3O4/NF cathode outperformed Co3O4/NF in the NO3− reduction rate (8.45 times faster) and contribution of atomic H* (2.37 folds increase in the ratio). And almost 100% N2 selectivity can be obtained in the presence of 2500 mg L−1 chloride ions. Generally, this study provided a new paradigm for designing more practical non-metallic catalyst for NO3− reduction.

Forming a B[sbnd]B bond in boron carbon nitride composite: A way for metal free electrocatalyst for oxygen reduction reaction in alkaline medium

Unmanageable of oxygen electrochemistry on cathode side of fuel cells creates a major hindrance to designing a cell device. Effectively, Platinum alone cannot afford the oxygen reduction reaction (ORR) due to several worries such as unaffordable cost, stability, scarcity and intermediate tolerance etc. that would hurdle the development of fuel cell technology. In order to demonstrate similar efforts, we discuss a synthesis of boron carbon nitride (BCN) nanosheets and its composite by thermal and solvothermal method respectively. The BCN nanosheets also show the formation of a homonuclear BB bond, which are important for enhancing oxygen affinity. Since the chemistry of BB bond involves higher bond energy, more reactivity or unusual gas adsorption behavior, thus making such sites promising towards stronger electrophilic interactions such as oxygen adsorption followed by its reduction. As synthesized BCN exhibits an interesting onset potential of ∼0.94 V in electroreduction of oxygen. More importantly, upon composite formation with carbon black, BCN shows further enhancement in the ORR kinetics with an onset potential of 1.01 V and a current density of 5 mA/cm2, which is very much comparable with Pt/C. An interesting phenomenon of activating O2 reduction using BCN has been systematically studied to unravel noble metal-free electrocatalysis.

N,P co-doped hollow carbon nanofiber membranes with superior mass transfer property for trifunctional metal-free electrocatalysis

Carbon-based metal-free electrocatalysts have inspired extensive efforts to explore their applications in many nontrivial electrochemical reactions, such as oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER), by virtue of the integrated advantages including low cost, sustainability, longevity, and multifunctionality. Herein, N,P co-doped hollow carbon nanofiber (N,P–HCNF) membranes were facilely prepared via coaxial electrospinning technology, which are bestowed with a hierarchical porous architecture, turbostratic structures, and abundant catalytically active sites such as doping, defects, and edges. Benefiting from structural features of the one-dimensional (1D) carbon hollow nanoarchitecture, which affords plentiful active sites, continuous conducting pathways, and benign mass transfer channels, the resultant catalyst reveals an excellent trifunctional electrocatalytic activity for ORR, OER, and HER. Impressively, it exhibits one of the best metal-free bifunctional electrocatalytic activities in oxygen electrocatalysis as characterized by a low potential deviation (ΔE) of 0.73 V between the half-wave potential (E1/2) for ORR and the potential reaching 10 mA cm−2 (Ej=10) for OER. Significantly, further investigations demonstrate that the effect of mass transfer makes a great difference to electrocatalytic activity, mainly through enlarged specific surface area to affect intrinsic catalytic activity and the ionic resistance in pores. This work sheds light on the design, fabrication, and regulation of highly active metal-free electrocatalysts with abundant active sites and tuned pore structures for electrocatalysis and other applications.

(Invited) Precious Metal-Free Electrocatalysis: Accomplishments and Challenges

In this short presentation, we will concentrate on entirely precious metal-free electrocatalysts as potential alternatives to the incumbent platinum group metal (PGM) materials for electrochemical reactions occurring in acidic, relevant to the energy conversion and storage: (i) oxygen reduction reaction (ORR), (ii) hydrogen evolution reaction (HER) and, time permitted, (iii) hydrogen peroxide generation (“two-electron ORR”). In the first part of this presentation, we will offer a summary of the development of non-precious metal ORR catalysts, emphasizing in particular the approaches that aim at improving the ORR active-site density and understanding of the origins of electrocatalytic activity of non-precious metal catalysts in oxygen reduction as a prerequisite for further development of this class of catalysts. We will focus on metal-nitrogen-carbon (M-N-C) catalysts obtained via the high-temperature treatment of organic precursors of carbon and nitrogen and inorganic precursors of a transition metal, which have been under development at Los Alamos for over a decade and have consistently exhibited the highest ORR activity in both electrochemical-cell and fuel cell testing to date. The second part of this presentation will be devoted to less frequently studied electrocatalysts based on small organic molecules as materials for hydrogen evolution reaction and hydrogen peroxide generation in a two-electron ORR. Unlike the oxygen reduction reaction catalysts in the first part of the talk, these HER/two-electron ORR catalysts are entirely metal-free and have been obtained by a low-temperature synthesis route. In addition to exhibiting a promising activity in electrochemical testing these catalysts show good performance durability over several days of continuous operation. The density functional theory (DFT) calculations have been performed to gain an insight into the reaction mechanisms of HER and two-electron ORR on various conformations of one selected organic molecule. The calculations have revealed thermodynamically favorable reaction pathways and structure-activity relationships, which can guide future synthesis of this new class of electrocatalysts. Acknowledgements Financial support of this research by the U.S. Department of Energy via Fuel Cell Technologies Office (FCTO) and by Los Alamos National Laboratory via Laboratory Directed Research and Development (LDRD) program is gratefully acknowledged.

Newly developed electrochemical synthesis of Co-based layered double hydroxides: toward noble metal-free electro-catalysis

A novel and reproducible protocol allows the electro-synthesis of LDHs for electro-catalytic applications.

Doping of Carbon Materials for Metal-Free Electrocatalysis.

Carbon atoms in the graphitic carbon skeleton can be replaced by heteroatoms with different electronegative from that of the carbon atom (i.e., heteroatom doping) to modulate the charge distribution over the carbon network. The charge modulation can be achieved via direct charge transfer with an electron acceptor/donor (i.e., charge transfer doping) or through introduction of defects (i.e., defective doping). Various doping strategies, including heteroatom doping, charge-transfer doping, and defective doping, have now been devised for modulating the charge distribution of numerous graphite carbon materials to impart new properties to carbon materials. Consequently, carbon nanomaterials with defined doping have recently become prominent members in the carbon family, promising for a variety of applications, including catalysis, energy conversion and storage, environmental remediation, and important chemical production and industrial processes. The purpose of this review is to present an overview on the doping of carbon materials for metal-free electrocatalysis, especially the development of doping strategies and doping-induced structure and property changes for potential catalytic applications. Current challenges and future perspectives in the doped carbon-based metal-free catalyst field are also discussed.

Experimental and Computational Study of Dopamine as an Electrochemical Probe of the Surface Nanostructure of Graphitized N-Doped Carbon

N-doped carbon nanomaterials have received increased attention from electrochemists because of their applications in the metal-free electrocatalysis of important redox processes. In this work, a se...

Correction to: Carbon-Based Metal-Free Electrocatalysis for Energy Conversion, Energy Storage, and Environmental Protection

The original version of this article unfortunately contained mistakes. The title was incorrect. The corrected title is given above.

Carbon-Based Metal-Free Electrocatalysis for Energy Conversion, Energy Storage, and Environmental Protection

Carbon-based metal-free catalysts possess desirable properties such as high earth abundance, low cost, high electrical conductivity, structural tunability, good selectivity, strong stability in acidic/alkaline conditions, and environmental friendliness. Because of these properties, these catalysts have recently received increasing attention in energy and environmental applications. Subsequently, various carbon-based electrocatalysts have been developed to replace noble metal catalysts for low-cost renewable generation and storage of clean energy and environmental protection through metal-free electrocatalysis. This article provides an up-to-date review of this rapidly developing field by critically assessing recent advances in the mechanistic understanding, structure design, and material/device fabrication of metal-free carbon-based electrocatalysts for clean energy conversion/storage and environmental protection, along with discussions on current challenges and perspectives.Graphical

Simultaneous reduction and nitrogen functionalization of graphene oxide using lemon for metal-free oxygen reduction reaction

Inspire by the vision of finding a simple and green method for simultaneous reduction and nitrogen (N)-functionalization of graphene oxide (GO), a N-rich reduced graphene oxide (rGO) has been synthesized through a facile and ecofriendly hydrothermal strategy while most of the existing methods are involving with multiple steps and highly toxic reducing agents that are harmful to human health and environment. In this paper, the simultaneous reduction and N-functionalization of GO using as available lemon juice (denoted as Lem-rGO) for metal-free electrocatalysis towards oxygen reduction reaction (ORR) is described. The proposed method is based on the reduction of GO using of the reducing and the N-precursor capability of ascorbic acid and citric acid as well as the nitrogenous compounds, respectively, that containing in lemon juice. The resultant Lem-rGO has higher reduction degree, higher specific surface area and better crystalline nature with N-incorporation than that of well investigated ascorbic acid and citric acid treated rGO. As a result, it shows better ORR electrocatalytic activity in respect to the improved onset potential, electron transfer rate and kinetics than those typical rGO catalysts. Moreover, it shows a significant tolerance to the anodic fuels and durability than the Pt/C during ORR.

Carbon-based catalysts for metal-free electrocatalysis

Carbon-based electrocatalysts have been developed to replace noble metal catalysts for low-cost renewable generation and storage of clean energy through metal-free electrocatalysis.

3D graphene preparation via covalent amide functionalization for efficient metal-free electrocatalysis in oxygen reduction

3D and porous reduced graphene oxide (rGO) catalysts have been prepared with sp3-hybridized 1,4-diaminobutane (sp3-DABu, rGO-sp3-rGO) and sp2-hybridized 1,4-diaminobenzene (sp2-DABe, rGO-sp2-rGO) through a covalent amidation and have employed as a metal-free electrocatalyst for oxygen reduction reaction (ORR) in alkaline media. Both compounds have used as a junction between functionalized rGO layers to improve electrical conductivity and impart electrocatalytic activity to the ORR resulting from the interlayer charge transfer. The successful amidation and the subsequent reduction in the process of catalyst preparation have confirmed by X-ray photoelectron spectroscopy. A hierarchical porous structure is also confirmed by surface morphological analysis. Specific surface area and thermal stability have increased after successful the amidation by sp3-DABu. The investigated ORR mechanism reveals that both functionalized rGO is better ORR active than nonfunctionalized rGO due to pyridinic-like N content and rGO-sp3-rGO is better ORR active than rGO-sp2-rGO due to higher pyridinic-like N content and π-electron interaction-free interlayer charge transfer. Thus, the rGO-sp3-rGO has proven as an efficient metal-free electrocatalyst with better electrocatalytic activity, stability, and tolerance to the crossover effect than the commercially available Pt/C for ORR.

Substrate-Induced Synthesis of Nitrogen-Doped Holey Graphene Nanocapsules for Advanced Metal-Free Bifunctional Electrocatalysts

The development of efficient metal-free electrocatalysts for oxygen electrocatalysis is of great significance for various energy conversion devices. Herein, novel nitrogen-doped holey graphene nanocapsules (NHGNs) are reported prepared by self-assembly of graphene oxide nanosheets on the surface of amino-functionalized silica template and NH3 activation with simultaneously enhanced nitrogen doping and etching of nanopores in graphene, followed by template etching. The silica template is demonstrated to show a substrate-enhanced effect on nitrogen doping and etching of nanopores in graphene based on density functional theory calculations. Benefiting from the large surface area, unique pore distribution, and high surface functionality of nitrogen doping, the resulting NHGNs exhibit superior bifunctional electrocatalytic activity and durability for both oxygen reduction reaction and oxygen evolution reaction, which is similar to that of the commercial Pt/C and RuO2 electrocatalysts, respectively. This work presents an advance in developing new nitrogen-doped graphene species for highly efficient metal-free electrocatalysis.

Carbon-Based Metal-Free Catalysts for Electrocatalysis beyond the ORR.

Besides their use in fuel cells for energy conversion through the oxygen reduction reaction (ORR), carbon-based metal-free catalysts have also been demonstrated to be promising alternatives to noble-metal/metal oxide catalysts for the oxygen evolution reaction (OER) in metal-air batteries for energy storage and for the splitting of water to produce hydrogen fuels through the hydrogen evolution reaction (HER). This Review focuses on recent progress in the development of carbon-based metal-free catalysts for the OER and HER, along with challenges and perspectives in the emerging field of metal-free electrocatalysis.