Covalently Grafting Graphene onto Si Photocathode to Expedite Aqueous Photoelectrochemical CO2 Reduction

Silicon semiconductor functionalized with molecular catalysts emerges as a promising cathode for photoelectrochemical (PEC) CO₂ reduction reaction (CO₂RR). However, the limited kinetics and stabilities remains a major hurdle for the development of such composites. We herein report an assembling strategy of silicon photocathodes via chemically grafting a conductive graphene layer onto the surface of n⁺-p Si followed by catalyst immobilization. The covalently-linked graphene layer effectively enhances the photogenerated carriers transfer between the cathode and the reduction catalyst, and improves the operating stability of the electrode. Strikingly, we demonstrate that altering the stacking configuration of the immobilized cobalt tetraphenylporphyrin (CoTPP) catalyst through calcination can further enhance the electron transfer rate and the PEC performance. At the end, the graphene-coated Si cathode immobilized with CoTPP catalyst managed to sustain a stable 1-Sun photocurrent of −1.65 mA cm⁻² over 16 h for CO production in water at a near neutral potential of −0.1 V vs. reversible hydrogen electrode. This represents a remarkable improvement of PEC CO₂RR performance in contrast to the reported photocathodes functionalized with molecular catalysts.     

Exclusive Formation of Formic Acid from CO2 Electroreduction by a Tunable Pd‐Sn Alloy

Conversion of carbon dioxide (CO₂) into fuels and chemicals by electroreduction has attracted significant interest, although it suffers from a large overpotential and low selectivity. A Pd‐Sn alloy electrocatalyst was developed for the exclusive conversion of CO₂ into formic acid in an aqueous solution. This catalyst showed a nearly perfect faradaic efficiency toward formic acid formation at the very low overpotential of −0.26 V, where both CO formation and hydrogen evolution were completely suppressed. Density functional theory (DFT) calculations suggested that the formation of the key reaction intermediate HCOO* as well as the product formic acid was the most favorable over the Pd‐Sn alloy catalyst surface with an atomic composition of PdSnO₂, consistent with experiments.     

A Hybrid Co Quaterpyridine Complex/Carbon Nanotube Catalytic Material for CO2 Reduction in Water

Associating a metal‐based catalyst to a carbon‐based nanomaterial is a promising approach for the production of solar fuels from CO₂. Upon appending a Co^II quaterpyridine complex [Co(qpy)]²⁺ at the surface of multi‐walled carbon nanotubes, CO₂ conversion into CO was realized in water at pH 7.3 with 100 % catalytic selectivity and 100 % Faradaic efficiency, at low catalyst loading and reduced overpotential. A current density of 0.94 mA cm^?2 was reached at ?0.35 V vs. RHE (240 mV overpotential), and 9.3 mA cm^?2 could be sustained for hours at only 340 mV overpotential with excellent catalyst stability (89 095 catalytic cycles in 4.5 h), while 19.9 mA cm^?2 was met at 440 mV overpotential. Such a hybrid material combines the high selectivity of a homogeneous molecular catalyst to the robustness of a heterogeneous material. Catalytic performances compare well with those of noble‐metal‐based nano‐electrocatalysts and atomically dispersed metal atoms in carbon matrices.     

Surface Reconstruction of Ultrathin Palladium Nanosheets during Electrocatalytic CO2 Reduction

A surface reconstructing phenomenon is discovered on a defect-rich ultrathin Pd nanosheet catalyst for aqueous CO₂ electroreduction. The pristine nanosheets with dominant (111) facet sites are transformed into crumpled sheet-like structures prevalent in electrocatalytically active (100) sites. The reconstruction increases the density of active sites and reduces the CO binding strength on Pd surfaces, remarkably promoting the CO₂ reduction to CO. A high CO Faradaic efficiency of 93 % is achieved with a site-specific activity of 6.6 mA cm⁻² at a moderate overpotential of 590 mV on the reconstructed 50 nm Pd nanosheets. Experimental and theoretical studies suggest the CO intermediate as a key factor driving the structural transformation during CO₂ reduction. This study highlights the dynamic nature of defective metal nanosheets under reaction conditions and suggests new opportunities in surface engineering of 2D metal nanostructures to tune their electrocatalytic performance.     

Catholyte‐Free Electrocatalytic CO2 Reduction to Formate

Electrochemical reduction of carbon dioxide (CO₂) into value‐added chemicals is a promising strategy to reduce CO₂ emission and mitigate climate change. One of the most serious problems in electrocatalytic CO₂ reduction (CO₂R) is the low solubility of CO₂ in an aqueous electrolyte, which significantly limits the cathodic reaction rate. This paper proposes a facile method of catholyte‐free electrocatalytic CO₂ reduction to avoid the solubility limitation using commercial tin nanoparticles as a cathode catalyst. Interestingly, as the reaction temperature rises from 303 K to 363 K, the partial current density (PCD) of formate improves more than two times with 52.9 mA cm^?2, despite the decrease in CO₂ solubility. Furthermore, a significantly high formate concentration of 41.5 g L^?1 is obtained as a one‐path product at 343 K with high PCD (51.7 mA cm^?2) and high Faradaic efficiency (93.3 %) via continuous operation in a full flow cell at a low cell voltage of 2.2 V.     

Synthesis,characterization, luminescent,and catalytic performance of a dinuclear triazenido–silver complex

Our group has developed a series of molecular electrocatalysts for hydrogen generation based on triazenido–metal complexes (such as cobalt, copper, etc.). In this paper, we present the electrocatalytic performance of a new dinuclear silver complex, [Ag₂(L)₂], formed by reaction of the triazenido ligand, 1-[(2-carboxyethyl)benzene]-3-[benzimidazole]triazene (HL) with AgNO₃. The electrocatalytic systems based on this silver complex can afford 91.23 and 473 moles of hydrogen per mole of catalyst per hour (mol H₂/mol catalyst/h) from acetic acid at overpotential (OP) of 991.6 mV and an aqueous buffer at an OP of 837.6 mV, respectively. Electrochemical investigations show both the silver center and the triazenido ligand, HL, play important roles in determining the catalytic activities of the electrocatalytic system. Additionally, the triazenido ligand (HL) can serve as a fluorescent sensor for Ag⁺.     

Directly Knitted Hierarchical Porous Organometallic Polymer-Based Self-Supported Single-Site Catalyst for CO2 Hydrogenation in Water

In recent times, heterogenization of homogeneous molecular catalysts onto various porous solid support structures has attracted significant research focus as a method for combining the advantages of both homogeneous as well as heterogeneous catalysis. The design of highly efficient, structurally robust and reusable heterogenized single-site catalysts for the CO₂ hydrogenation reaction is a critical challenge that needs to be accomplished to implement a sustainable and practical CO₂-looped renewable energy cycle. This study demonstrated a heterogenized catalyst [Ir-HCP-(B/TPM)] containing a molecular Ir-abnormal N-heterocyclic carbene (Ir-aNHC) catalyst self-supported by hierarchical porous hyper-crosslinked polymer (HCP), in catalytic hydrogenation of CO₂ to inorganic formate (HCO₂⁻) salt that is a prospective candidate for direct formate fuel cells (DFFC). By employing this unique and first approach of utilizing a directly knitted HCP-based organometallic single-site catalyst for CO₂-to-HCO₂⁻ in aqueous medium, extremely high activity with a single-run turnover number (TON) up to 50816 was achieved which is the highest so far considering all the heterogeneous catalysts for this reaction in water. Additionally, the catalyst featured excellent reusability furnishing a cumulative TON of 285400 in 10 cycles with just 1.6 % loss in activity per cycle. Overall, the new catalyst displayed attributes that are important for developing tangible catalysts for practical applications.     

固载Ru基催化剂上二氧化碳加氢合成甲酸Ⅱ反应条件对催化剂性能的影响

The synthesis of formic acid from carbon dioxide and hydrogen using a silica immobilized ruthenium catalyst as precursor has been studied in different reaction conditions. The results revealed that the TOF （turn over frequency） of HCOOH achieved 1481.5 h^-1 on immobilized ruthenium catalyst near the critical pressure point of CO2 with H2 pressure of 4.0 MPa, reaction temperature of 80℃ and PPh3/Ru molar ratio of 6：1. The reaction activity of immobilized catalyst was higher than that of homogeneous catalyst, and the immobilized catalyst also offered the practical advantages such as easy separation and reuse.     

A Water-Soluble Sodium Pectate Complex with Copper as an Electrochemical Catalyst for Carbon Dioxide Reduction

A selective noble-metal-free molecular catalyst has emerged as a fruitful approach in the quest for designing efficient and stable catalytic materials for CO₂ reduction. In this work, we report that a sodium pectate complex of copper (PG-NaCu) proved to be highly active in the electrocatalytic conversion of CO₂ to CH₄ in water. Stability and selectivity of conversion of CO₂ to CH₄ as a product at a glassy carbon electrode were discovered. The copper complex PG-NaCu was synthesized and characterized by physicochemical methods. The electrochemical CO₂ reduction reaction (CO₂RR) proceeds at −1.5 V vs. Ag/AgCl at ~10 mA/cm² current densities in the presence of the catalyst. The current density decreases by less than 20% within 12 h of electrolysis (the main decrease occurs in the first 3 h of electrolysis in the presence of CO₂). This copper pectate complex (PG-NaCu) combines the advantages of heterogeneous and homogeneous catalysts, the stability of heterogeneous solid materials and the performance (high activity and selectivity) of molecular catalysts.     

Synthesis,characterization and electrocatalytic performance of a dinuclear triazenidosilver(I) complex for hydrogen production

Our group has developed a series of molecular electrocatalysts for hydrogen generation based on triazenido–metal complexes (cobalt, copper, etc.). In this paper, we first present the electrocatalytic performance of a new dinuclear silver complex, [Ag₂(L)₂], formed by the reaction of the triazenido ligand 1‐[(2‐carboxymethyl)benzene]‐3‐[(2‐methoxy)benzene]triazene (HL) with AgNO₃. At room temperature, the silver complex shows photoluminescence at 653 nm. The electrocatalytic systems based on this silver complex can afford 106.57 and 1536.36 moles of hydrogen per mole of catalyst per hour from acetic acid at an overpotential (OP) of 991.6 mV and from a neutral aqueous buffer (pH = 7.0) at an OP of 837.6 mV, respectively. Electrochemical investigations show that both silver ion and triazenido ligand play a role in determining the catalytic activities of the electrocatalytic system.     

Rapid Selective Electrocatalytic Reduction of Carbon Dioxide to Formate by an Iridium Pincer Catalyst Immobilized on Carbon Nanotube Electrodes

An iridium pincer dihydride catalyst was immobilized on carbon nanotube‐coated gas diffusion electrodes (GDEs) by using a non‐covalent binding strategy. The as‐prepared GDEs are efficient, selective, durable, gas permeable electrodes for electrocatalytic reduction of CO₂ to formate. High turnover numbers (ca. 54 000) and turnover frequencies (ca. 15 s^?1) were enabled by the novel electrode architecture in aqueous solutions saturated in CO₂ with added HCO₃^?.     

Electrocatalytic CO2 Reduction with a Half‐Sandwich Cobalt Catalyst: Selectivity towards CO

We present herein a Cp*Co(III)‐half‐sandwich catalyst system for electrocatalytic CO₂ reduction in aqueous acetonitrile solution. In addition to an electron‐donating Cp* ligand (Cp*=pentamethylcyclopentadienyl), the catalyst featured a proton‐responsive pyridyl‐benzimidazole‐based N,N‐bidentate ligand. Owing to the presence of a relatively electron‐rich Co center, the reduced Co(I)‐state was made prone to activate the electrophilic carbon center of CO₂. At the same time, the proton‐responsive benzimidazole scaffold was susceptible to facilitate proton‐transfer during the subsequent reduction of CO₂. The above factors rendered the present catalyst active toward producing CO as the major product over the other potential 2e/2H⁺ reduced product HCOOH, in contrast to the only known similar half‐sandwich CpCo(III)‐based CO₂‐reduction catalysts which produced HCOOH selectively. The system exhibited a Faradaic efficiency (FE) of about 70% while the overpotential for CO production was found to be 0.78 V, as determined by controlled‐potential electrolysis.     

Supramolecular Enhancement of Electrochemical Nitrate Reduction Catalyzed by Cobalt Porphyrin Organic Cages for Ammonia Electrosynthesis in Water**

The electrochemical nitrate (NO₃⁻) reduction reaction (NO₃RR) to ammonia (NH₃) represents a sustainable approach for denitrification to balance global nitrogen cycles and an alternative to traditional thermal Haber-Bosch processes. Here, we present a supramolecular strategy for promoting NH₃ production in water from NO₃RR by integrating two-dimensional (2D) molecular cobalt porphyrin ( CoTPP ) units into a three-dimensional (3D) porous organic cage architecture. The porphyrin box CoPB-C8 enhances electrochemical active site exposure, facilitates substrate–catalyst interactions, and improves catalyst stability, leading to turnover numbers and frequencies for NH₃ production exceeding 200,000 and 56 s⁻¹, respectively. These values represent a 15-fold increase in NO₃RR activity and 200-mV improvement in overpotential for the 3D CoPB-C8 box structure compared to its 2D CoTPP counterpart. Synthetic tuning of peripheral alkyl substituents highlights the importance of supramolecular porosity and cavity size on electrochemical NO₃RR activity. These findings establish the incorporation of 2D molecular units into 3D confined space microenvironments as an effective supramolecular design strategy for enhancing electrocatalysis.     

Integrated Bismuth Oxide Ultrathin Nanosheets/Carbon Foam Electrode for Highly Selective and Energy-Efficient Electrocatalytic Conversion of CO2 to HCOOH

Electroreduction of CO₂ into formic acid (HCOOH) is of particular interest as a hydrogen carrier and chemical feedstock. However, its conversion is limited by a high overpotential and low stability due to undesirable catalysts and electrode design. Herein, an integrated 3D bismuth oxide ultrathin nanosheets/carbon foam electrode is designed by a sponge effect and N-atom anchor for energy-efficient and selective electrocatalytic conversion of CO₂ to HCOOH for the first time. Benefitting from the unique 3D array foam architecture for highly efficient mass transfer, and optimized exposed active sites, as confirmed by density functional theory calculations, the integrated electrode achieves high electrocatalytic performance, including superior partial current density and faradaic efficiency (up to 94.1 %) at a moderate overpotential as well as a high energy conversion efficiency of 60.3 % and long-term durability.     

Tailoring Electrochemical CO2 Reduction on Copper by Reactive Ionic Liquid and Native Hydrogen Bond Donors

Electrochemical CO₂ reduction (CO₂RR) on copper (Cu) shows promise for higher-value products beyond CO. However, challenges such as the limited CO₂ solubility, high overpotentials, and the competing hydrogen evolution reaction (HER) in aqueous electrolytes hinder the practical realization. We propose a functionalized ionic liquid (IL) which generates ion-CO₂ adducts and a hydrogen bond donor (HBD) upon CO₂ absorption to modulate CO₂RR on Cu in a non-aqueous electrolyte. As revealed by transient voltammetry, electrochemical impedance spectroscopy (EIS), and in situ surface-enhanced Raman spectroscopy (SERS) complemented with image charge augmented quantum-mechanical/molecular mechanics (IC-QM/MM) computations, a unique microenvironment is constructed. In this microenvironment, the catalytic activity is primarily governed by the IL and HBD concentrations; former controlling the double layer thickness and the latter modulating the local proton availability. This translates to ample CO₂ availability, reduced overpotential, and suppressed HER where C₄ products are obtained. This study deepens the understanding of electrolyte effects in CO₂RR and the role of IL ions towards electrocatalytic microenvironment design.     

Studies on preparation of metalloporphyrin‐functionalized PVI and PVI/SiO2 via axial coordination reaction

Cobalt tetraphenylporphyrin (CoTPP) was bond on the side chains of poly(N‐vinylimidazole) (PVI) and immobilized on PVI/SiO₂ via axial coordination reactions, respectively, leading to the soluble metalloporphyrin‐functionalized PVI (denoted as CoTPP–PVI) and the particles CoTPP–PVI/SiO₂. The above two target products were characterized using spectroscopy methods such as infrared spectrum, electronic absorption spectrum, and fluorescence emission spectrum. And the effects of various factors on the bonding reaction and immobilization reaction of CoTPP were studied in detail. The experimental results show that CoTPP–PVI and CoTPP–PVI/SiO₂ can be prepared favorably through axial coordination reaction of CoTPP with the side imidazole groups of PVI and PVI/SiO₂. In addition, the soluble CoTPP–PVI has all the spectral characteristics of CoTPP. As compared to CoTPP, the adsorption spectra of CoTPP–PVI exhibit red shift obviously. In the immobilization process of CoTPP on PVI/SiO₂, the higher the concentrations of CoTPP and the reaction temperature, the greater the immobilization amount of CoTPP is. Copyright © 2009 John Wiley & Sons, Ltd.     

Water Oxidation Catalysis Beginning with CuCo2S4: Investigation of the True Electrochemically Driven Catalyst

The exploitation of efficient and stable water oxidation catalysts is a pressing challenge to solve the energy crisis. Herein, flower‐like CuCo₂S₄ microspheres were successfully synthesized and used as an effective water oxidation catalyst. CuCo₂S₄/NF (NF=nickel foam) affords electrocatalytic water oxidation activity, with a current density of 20 mA cm^?2 at a low overpotential of 260 mV. The overpotential value is lower than that of benchmark RuO₂/NF (overpotential of 340 mV at a current density of 20 mA cm^?2). The water oxidation activity increases linearly before nonlinearly improving with increasing pH; this indicates that the substrate changes from water to hydroxyl. The CuCo₂S₄/NF catalyst is demonstrated to be a real water oxidation catalyst based on diverse experiments.     

Axial Modification of Cobalt Complexes on Heterogeneous Surface with Enhanced Electron Transfer for Carbon Dioxide Reduction

Efficient electron communication between molecular catalyst and support is critical for heterogeneous molecular electrocatalysis and yet it is often overlooked during the catalyst design. Taking CO₂ electro-reduction on tetraphenylporphyrin cobalt (PCo) immobilized onto graphene as an example, we demonstrate that adding a relay molecule improves the interfacial electron communication. While the directly immobilized PCo on graphene exhibits relatively poor electron communications, it is found that diphenyl sulfide serves as an axial ligand for PCo and it improves the redox activity of PCo on the graphene surface to facilitate the generation of [PCo]^.- active sites for CO₂ reduction. Thus, the turnover frequencies of the immobilized Co complexes are increased. Systematic structural analysis indicates that the benzene rings of diphenyl sulfide exhibit strong face-to-face stacking with graphene, which is proposed as an efficient medium to facilitate the interfacial electron communication.     

Solvent-mediated outer-sphere CO2 electro-reduction mechanism over the Ag111 surface

The electrocatalytic CO₂ reduction reaction (CO₂RR) is one of the key technologies of the clean energy economy. Molecular-level understanding of the CO₂RR process is instrumental for the better design of electrodes operable at low overpotentials with high current density. The catalytic mechanism underlying the turnover and selectivity of the CO₂RR is modulated by the nature of the electrocatalyst, as well as the electrolyte liquid, and its ionic components that form the electrical double layer (EDL). Herein we demonstrate the critical non-innocent role of the EDL for the activation and conversion of CO₂ at a high cathodic bias for electrocatalytic conversion over a silver surface as a representative low-cost model cathode. By using a multiscale modeling approach we demonstrate that under such conditions a dense EDL is formed, which hinders the diffusion of CO₂ towards the Ag111 electrocatalyst surface. By combining DFT calculations and ab initio molecular dynamics simulations we identify favorable pathways for CO₂ reduction directly over the EDL without the need for adsorption to the catalyst surface. The dense EDL promotes homogeneous phase reduction of CO₂via electron transfer from the surface to the electrolyte. Such an outer-sphere mechanism favors the formation of formate as the CO₂RR product. The formate can undergo dehydration to CO via a transition state stabilized by solvated alkali cations in the EDL.

In addition to the commonly accepted inner-sphere mechanism for e⁻ transfer, we show that an outer-sphere electron transfer from the cathode to CO₂ is operable at high overpotentials.     

Controlled Synthesis of Ni-Doped MoS2 Hybrid Electrode for Synergistically Enhanced Water-Splitting Process

The development of high-efficiency, low-cost, and earth-abundant electrocatalysts for overall water splitting remains a challenge. In this work, Ni-modified MoS₂ hybrid catalysts are grown on carbon cloth (Ni-Mo-S@CC) through a one-step hydrothermal treatment. The optimized Ni-Mo-S@CC catalyst shows excellent hydrogen evolution reaction (HER) activity with a low overpotential of 168 mV at a current density of 10 mA cm⁻² in 1.0 m KOH, which is lower than those of Ni-Mo-S@CC (1:1), Ni-Mo-S@CC (3:1), and pure MoS₂. Significantly, the Ni-Mo-S@CC hybrid catalyst also displays outstanding oxygen evolution reaction (OER) activity with a low overpotential of 320 mV at a current density of 10 mA cm⁻², and remarkable long-term stability for 30 h at a constant current density of 10 mA cm⁻². Experimental results and theoretical analysis based on density functional theory demonstrate that the excellent electrocatalytic performance can be attributed mainly to the remarkable conductivity, abundant active sites, and synergistic effect of the Ni-doped MoS₂. This work sheds light on a unique strategy for the design of high-performance and stable electrocatalysts for water-splitting electrolyzers.