Suppressed H2 Evolution Research Articles

Hydrogen-Bond-Network Breakdown Boosts Selective CO2 Photoreduction by Suppressing H2 Evolution.

Conventional strategies for highly efficient and selective CO2 photoreduction focus on the design of catalysts and cocatalysts. In this study, we discover that hydrogen bond network breakdown in reaction system can suppress H2 evolution, thereby improving CO2 photoreduction performance. Photosensitive poly(ionic liquid)s are designed as photocatalysts owing to their strong hydrogen bonding with solvents. The hydrogen bond strength is tuned by solvent composition, thereby effectively regulating H2 evolution (from 0 to 12.6 mmol g-1 h-1). No H2 is detected after hydrogen bond network breakdown with trichloromethane or tetrachloromethane as additives. CO production rate and selectivity increase to 35.4 mmol g-1 h-1 and 98.9 % with trichloromethane, compared with 0.6 mmol g-1 h-1 and 26.2 %, respectively, without trichloromethane. Raman spectroscopy and theoretical calculations confirm that trichloromethane broke the systemic hydrogen bond network and subsequently suppressed H2 evolution. This hydrogen bond network breakdown strategy may be extended to other catalytic reactions involving H2 evolution.

Hydrogen‐Bond‐Network Breakdown Boosts Selective CO2 Photoreduction by Suppressing H2 Evolution

AbstractConventional strategies for highly efficient and selective CO2 photoreduction focus on the design of catalysts and cocatalysts. In this study, we discover that hydrogen bond network breakdown in reaction system can suppress H2 evolution, thereby improving CO2 photoreduction performance. Photosensitive poly(ionic liquid)s are designed as photocatalysts owing to their strong hydrogen bonding with solvents. The hydrogen bond strength is tuned by solvent composition, thereby effectively regulating H2 evolution (from 0 to 12.6 mmol g−1 h−1). No H2 is detected after hydrogen bond network breakdown with trichloromethane or tetrachloromethane as additives. CO production rate and selectivity increase to 35.4 mmol g−1 h−1 and 98.9 % with trichloromethane, compared with 0.6 mmol g−1 h−1 and 26.2 %, respectively, without trichloromethane. Raman spectroscopy and theoretical calculations confirm that trichloromethane broke the systemic hydrogen bond network and subsequently suppressed H2 evolution. This hydrogen bond network breakdown strategy may be extended to other catalytic reactions involving H2 evolution.

Tunable product selectivity on demand: a mechanism-guided Lewis acid co-catalyst for CO2 electroreduction to ethylene glycol

This work illustrates the feasibility of using Lewis acid/base co-catalysts to change the established chemical reaction mechanism of an electrocatalyst to form a new, chemically predictable, more valuable product in high yield.

Sustainable Iron Production by Low Temperature Electrolysis of Fe2O3 Colloidal Electrode

Fe is one of the most important metals that has been used in nearly every construction of man. Currently, Fe is manufactured by a carbothermic reduction by which Fe2O3 ores and carbon are reduced at >2000°C in a blast furnace. This conventional method is environmentally hazardous as it emits almost 2 tons of CO2 per every ton of crude iron produced, accounting for 5% of global CO2 emission. With the goal of zero CO2 emission and sustainable development, more advanced technologies of Fe manufacturing are urgently required. Herein, we introduce a novel and sustainable Fe production using a low temperature electrolysis (100°C) of Fe2O3 colloidal electrode. The Fe2O3 colloid with an electronic-ionic conductive network was successfully fabricated which contains Fe2O3 electrochemically active species, carbon and NaOH solution. Carbon acts as an electronic conductor while NaOH solution serves as an ionic conductor. This electrode design helps increase reactive areas for the Fe2O3 reduction process resulting in fast reaction rate. In addition, some organic and inorganic additives were added to enhance an electrolysis efficiency via promoting the reaction and suppressing H2 evolution parasitic reaction. This method can produce high purity Fe powder with a high current efficiency of 95%. Our zero CO2 emission method is much greener, and less energy consumptive than the conventional carbothermic process, which potentially revolutionizes the iron and steel industry that has a history of environmental consciousness for many decades.

SO42− mediated CO2 activation on metal electrode for efficient CO2 electroreduction

The most difficulty of efficient CO2 electroreduction lies in the activation step that turns CO2 into the CO2− radicals or other intermediates that can be further converted. To overcome this bottleneck, many efforts have been devoted to develop electrocatalysts to lower the overpotential of CO2 activation, but they inevitably involve complicated or/and unscalable synthesis methods. Here we reported that the SO42− ion modified metal foils, including Zn, Ag, Au and Cu, demonstrated ∼6.5, 1.4, 14 and 2.5 times of CO faradaic efficiency in K2SO4 electrolyte compared to KHCO3 electrolyte at −1.05 V vs. RHE. According to both experimental and DFT studies, the electron-rich metal-SO4 adlayer can activate CO2 by bending the linear molecule of CO2 through donating electrons to the antibonding orbital of CO2, thus facilitating the formation of key intermediate *CO2−. Moreover, the adsorption of H+ on metal sites is restrained by the metal-SO4 adlayer, leading to a suppressed H2 evolution activity. From a broader perspective, many catalysts can benefit from this approach to achieve more efficient CO2 electrochemical reduction.

A Tin Oxide-Coated Copper Foam Hybridized with a Gas Diffusion Electrode for Efficient CO2 Reduction to Formate with a Current Density Exceeding 1 A cm-2.

The electrochemical CO2 reduction reaction (CO2 RR) is a promising strategy for closing the carbon cycle. Increasing the current density ( J) for CO2 RR products is a critical requirement for the social implementation of this technology. Herein, nanoscale tin-oxide-modified copper-oxide foam is hybridized with a carbon-based gas-diffusion electrode (GDE). Using the resultant electrode, the Jformate is increased to -1152mA cm-2 at -1.2V versus RHE in 1m KOH, which is the highest value for CO2 -to-formate electrolysis. The formate faradaic efficiency (FEformate ) reaches ≈99% at -0.6V versus RHE. The achievement of ultra-high-rate formate production is attributable to the following factors: i) homogeneously-modified Sn atoms suppressing H2 evolution and ii) the hydrophobic carbon nanoparticles on GDEs penetrating the macroporous structure of the foam causing the increase in the thickness of triple-phase interface. Additionally, the FEformate remains at ≈70% under a high J of -1.0 A cm-2 for more than 20h.

Ultra-efficient N2 electroreduction achieved over a rhodium single-atom catalyst (Rh1/MnO2) in water-in-salt electrolyte

Electrocatalytic nitrogen reduction reaction (NRR) is an appealing strategy for green ammonia synthesis. Despite tremendous efforts, current NRR performances of most catalysis systems remain far below the targets for practical applications. Herein, a highly active and selective NRR catalysis system is reported by using an Rh single-atom catalyst in water-in-salt electrolytes (WISE). The developed single-atomic Rh on MnO2 (Rh1/MnO2) catalyst in 9 m K2SO4 presents a superior NH3 yield of 271.8 μg h−1 mg−1 and Faradaic efficiency of 73.3%, far exceeding that in dilute electrolyte and representing one of the best NRR performances on record. Multiple operando XAS, in situ FTIR/Raman spectroscopic characterizations together with the theoretical calculations unravel that WISE enables Rh1/MnO2 to exhibit suppressed H2 evolution, increased N2 enrichment on catalyst surface, and enhanced N2 activation/hydrogenation on active Rh sites.

Solar-Driven Syngas Production Using Al-Doped ZnTe Nanorod Photocathodes.

Syngas, traditionally produced from fossil fuels and natural gases at high temperatures and pressures, is an essential precursor for chemicals utilized in industry. Solar-driven syngas production can provide an ideal pathway for reducing energy consumption through simultaneous photoelectrochemical CO2 and water reduction at ambient temperatures and pressures. This study performs photoelectrochemical syngas production using highly developed Al-doped ZnTe nanorod photocathodes (Al:ZnTe), prepared via an all-solution process. The facile photo-generated electrons are transferred by substitutional Al doping on Zn sites in one-dimensional arrays to increase the photocurrent density to −1.1 mA/cm2 at −0.11 VRHE, which is 3.5 times higher than that for the pristine ZnTe. The Al:ZnTe produces a minor CO (FE ≈ 12%) product by CO2 reduction and a major product of H2 (FE ≈ 60%) by water reduction at −0.11 VRHE. Furthermore, the product distribution is perfectly switched by simple modification of Au deposition on photocathodes. The Au coupled Al:ZnTe exhibits dominant CO production (FE ≈ 60%), suppressing H2 evolution (FE ≈ 15%). The strategies developed in this study, nanostructuring, doping, and surface modification of photoelectrodes, can be applied to drive significant developments in solar-driven fuel production.

High-Efficiency N2 Electroreduction Enabled by Se-Vacancy-Rich WSe2-x in Water-in-Salt Electrolytes.

Electrocatalytic nitrogen reduction reaction (NRR) is a promising approach for renewable NH3 production, while developing the NRR electrocatalysis systems with both high activity and selectivity remains a significant challenge. Herein, we combine catalyst and electrolyte engineering to achieve a high-efficiency NRR enabled by a Se-vacancy-rich WSe2-x catalyst in water-in-salt electrolyte (WISE). Extensive characterizations, theoretical calculations, and in situ X-ray photoelectron/Raman spectroscopy reveal that WISE ensures suppressed H2 evolution, improved N2 affinity on the catalyst surface, as well as an enhanced π-back-donation ability of active sites, thereby promoting both activity and selectivity for the NRR. As a result, an excellent faradaic efficiency of 62.5% and NH3 yield of 181.3 μg h-1 mg-1 is achieved with WSe2-x in 12 m LiClO4, which is among the highest NRR performances reported to date.

Using Light and Electrons to Bend Carbon Dioxide: Developing and Understanding Catalysts for CO2 Conversion to Fuels and Feedstocks.

Our global society generates an unwieldy amount of CO2 per unit time. Therefore, the capture of this greenhouse gas must involve a diverse set of strategies. One solution to this problem is the conversion of CO2 into a more useful chemical species. Again, a multiplicity of syntheses and products will be necessary. No matter how elegant the chemistry is, commercial markets often have little use for a small set of compounds made in tremendous yield. Following this reasoning, the Bocarsly Research Group seeks to develop new electrochemical and photochemical processes that may be of utility in the conversion of CO2 to organic compounds. We focus on investigating proton-coupled charge transfer mechanisms that produce both C1 and carbon-carbon bonded products (C2+).In early work, we considered the reduction of CO2 to formate at electrocatalytic indium and tin electrodes. These studies demonstrated the key role of surface oxides in catalyzing the reduction of CO2. This work generated efficient systems for the formation of formate and paved the way to studies using non-copper, intermetallic electrocatalysts for the generation of C2+ species. Most notable is the efficient formation of oxalate at an oxidized Cr3Ga electrode. Oxalate has recently been suggested as a potential nonfossil, alternate organic feedstock.Separately, we have focused on the electrocatalytic effects of pyridine on the reduction of CO2 in aqueous electrolyte. These studies demonstrated that electrodes that normally yield a low hydrogen overpotential (Pd and Pt) show suppressed H2 evolution and strongly enhanced activity for CO2 reduction in the presence of pyridinium. Methanol was observed to form in high Faradaic yield at low overpotential using this system. The 6-electron, 6-proton reduction of CO2 in the presence of pyridinium was intriguing, and significant effort was placed on understanding the mechanism of this reaction both on metal electrodes and on semiconducting photocathodes. P-GaP electrodes were found to provide exceptional behavior for the formation of methanol using only light as the energy source.The pyridinium studies highlighted the role of protons in the overall reduction of CO2, stimulating our interest in the chemistry of MnBr(bpy)(CO)3 and related compounds. This complex was reported to electrochemically reduce CO2 to CO. We saw these reports as an opportunity to study the detailed nature of the proton-coupled electron transfer (PCET) mechanism associated with CO2 reduction. Our investigation of this system revealed the role of hydrogen-bonding in CO2 reduction and pointed the way for the construction of a photochemical process for CO generation using a [(bpy)(CO)3Mn(CN)Mn(bpy)(CO)3]+ photocatalyst.Based on our studies to date, it appears likely that heterogeneous systems can be assembled to convert CO2 into products that are "beyond C2 products." This may open up new practical chemistry in the area of fossil-based replacements for both synthesis and fuels. Systems with pragmatic efficiencies are close to reality. Electrochemical reactors using heterogeneous electrocatalysts show the stability and product selectivity needed to generate industrial opportunities. Continued growth of mechanistic understanding is expected to facilitate the chemical design of cogent systems for the taming of CO2.

Enhanced CO2 electroreduction to formate over tin coordination polymers via amino-functionalization

Electroreduction of CO2 into value-added products relies on the availability of highly active/selective and robust catalysts. Herein, we report a strategy to controllably enhance the catalytic activity of tin coordination polymers (Sn-CPs) for CO2 electroreduction via framework functionalization. By adjusting ligands, amino groups (-NH2) were successfully introduced into the framework of Sn-CPs, enabling the enhanced CO2 adsorption capacity and effectively suppressed H2 evolution (H2 Faraday efficiency (FE) < 10%) with lower overpotentials. With the highest surface Sn2+ content and most –NH2 functional groups in the framework among three Sn-CPs, Sn-DaPTA (tin 2,5-diaminoterephthalate) delivered the highest formate FE of 85% with a current density of 11 mA/cm2 at −0.8 V (vs. RHE) and maintained stable performance during 18 reuse cycles. This work opens a new avenue for promoting the CO2 electroreduction performance of CPs via amino-functionalization.

Mechanism of galvanic reduction of selenate oxyanions and surface immobilization by nano zero-valent iron aggregates under anaerobic conditions: towards high electron efficiency

Elucidation of the galvanic mechanism of selenate reduction by nZVI leads to &gt;99.95% removal and record electron efficiency by suppressing H2 evolution via in situ Se doping.

Oxide-derived Tin Nanospheres for Efficient CO2 Electroreduction to Formate

Rational design and engineering of electrocatalysts for electrochemical CO2 reduction (CO2RR) is crucial to effectively, selectively converting CO2 into carbon-neutral chemicals and fuels. In this work, we have developed hierarchical tin oxide nanosphere electrocatalysts to boost the production of formate. Three-dimensional (3D) SnO2 spheres constructed from small crystalline nanoparticles exhibited 71-81% Faradaic efficiencies (FE) and energy efficiency (40-50%) toward formate production at partial current densities of 22-72.4 mA/cmgeo 2 from -0.9 V to -1.3 V vs. the reversible hydrogen electrode (RHE). Such hierarchical sphere-like catalysts outperformed non-hierarchical SnO2 nanoparticles and commercially benchmarking catalysts, achieving a peak 81% FE at superior formate current density of ~50 mA/cmgeo 2 at -1.2 V vs. RHE. The spherical oxide-derived Sn catalysts were highly stable up to 36 hours, preserving long-term partial current density of ~ 45 mA/cmgeo 2 and 68% FE for formate. Such an outstanding performance can be credited to the 3D hierarchical structure, which offers enlarged surface area, larger number of electrochemically active sites, and strongly suppressed H2 evolution. Our results may provide more guidelines for the introduction of hierarchical porosity into CO2RR electrocatalysts for large-scale applications.

Organic terpyridine molecule as an efficient cocatalyst for metal–free CO2 photoreduction mediated by mesoporous graphitic carbon nitride

Metal-free catalytic conversion of CO2 with high selectivity and efficiency is promising for achieving carbon neutrality. However, nonmetallic catalytic sites for highly selective and efficient CO2 photoconversion are still challenging. In this work, organic terpyridine molecules are developed as highly active cocatalysts for visible-light-driven CO2 reduction with a CO evolution rate of 1.92 × 104 μmol g−1h−1 and 99.8% selectivity in aqueous solution, which is among the highest CO evolution rates for metal-free CO2 photoreduction. Mechanistic studies revealed that efficient electron transfer from organic light-absorbing photosensitizer to terpyridine molecules was mediated by mesoporous g-C3N4 (mpg-C3N4), with no need of much efforts to optimize inherent fast charge recombination of pristine mpg-C3N4. The synergistic effect of mpg-C3N4 and terpyridine molecule suppressed H2 evolution process from CB of mpg-C3N4 and promoted CO2 reduction at terpyridine sites. These results pave a feasible way to achieve robust metal-free CO2 photoreduction by employing organic molecular hybrid photocatalysis.

Site- and Spatial-Selective Integration of Non-noble Metal Ions into Quantum Dots for Robust Hydrogen Photogeneration

Summary Artificial photosystems consisting of semiconductor quantum dots (QDs) and non-noble metal ions (e.g., Fe2+, Co2+, Ni2+) show intrinsic advantages in efficient and cost-effective hydrogen (H2) evolution. However, well-controlled integration of these metal ions into ultra-small QDs is a major challenge. Here, we present a two-step strategy to realize site- and spatial-selective integration of earth-abundant Fe2+ ions in CdSe QDs; i.e., CdSe QDs are modified with a thin and anisotropic ZnS shell, and partial Zn2+ ions are selectively substituted by Fe2+ via cation exchange. The anisotropic ZnS layer not only passivates CdSe core but also works as an ideal medium to intimately anchor Fe2+. The multifunctional CdSe/Zn1−xFexS QD exhibits extraordinary activity and stability toward H2 photogeneration. Specifically, more than 880 mL (∼39,300 μmol) H2 gas is produced from 6 mL of aqueous solution during a consecutive 172 h (>1 week) experiment, thus achieving a turnover number of >600,000 per QD.

Enhanced photocatalytic CO2 reduction with suppressing H2 evolution via Pt cocatalyst and surface SiO2 coating

The photocatalytic CO2 reduction to chemical feedstocks represents a promising route for the utilization of stranded renewable resources, but faces poor activity and selectivity resulting from the fast recombination of photogenerated charge carriers and competing hydrogen (H2) evolution reaction. Herein, we report the synthesis of core-shell structured Pt/SiO2@SiO2 composites via electrostatic self-assembly and surface SiO2 coating for photocatalytic CO2 reduction with Ru(bpy)32+ (abbreviated as Ru, bpy = 2′2-bipyridine) as photosensitizer and Pt as cocatalyst under visible light irradiation (λ > 420 nm). Photoluminescence results illustrate that Pt cocatalyst enhances the transfer of photogenerated electrons from the excited Ru, thereby improving photocatalytic performance. Further surface coating SiO2 prevents Pt from directly contacting with H2O and ameliorates CO2 adsorption ability of catalyst, thus suppressing H2 evolution and increasing CO selectivity, which also endows the catalyst with enhanced stability. This work would provide an instructive guideline in regulating the selectivity and stability of using transition metal complexes as photosensitizers for photocatalytic CO2 reduction through the cocatalyst loading and surface coating strategy.

Photocatalytic Reductive Defluorination of Fluorinated Compounds in Aqueous Alcohol Suspensions of a Metal‐loaded Titanium(IV) Oxide

AbstractSelective elimination of the fluorine element in organic fluorinated compounds under mild conditions is desired in order to resolve the environment and energy resource issues. Selective reductive defluorination of fluorine‐containing organic compounds in 2‐propanol‐water suspensions of metal‐loaded titanium(IV) oxide (TiO2) photocatalysts is described in this paper. The effects of different types of metal co‐catalysts, adsorption behavior of fluorobenzene (FB), stoichiometry, redox balance, scope, chemoselectivity and reaction temperature were examined. Photocatalytic defluorination of FB occurred over platinum‐loaded TiO2 at room temperature, and benzene and fluoride ion were produced with a high stoichiometry without degradation of the benzene structure, while hydrogen (H2) production occurred as a competitive electron‐consuming reaction. A slight elevation in the reaction temperature greatly accelerated defluorination and suppressed H2 evolution, resulting in FB defluorination free from H2 production at 333 K. Kinetic parameters of FB defluorination and H2 evolution were determined, and the results obtained are explained on the basis of these parameters.

Facile synthesis of hierarchical flower-like Ag/Cu2O and Au/Cu2O nanostructures and enhanced catalytic performance in electrochemical reduction of CO2

Novel, hierarchical, flower-like Ag/Cu2O and Au/Cu2O nanostructures were successfully fabricated and applied as efficient electrocatalysts for the electrochemical reduction of CO2. Cu2O nanospheres with a uniform size of ∼180 nm were initially synthesized. Thereafter, Cu2O was used as a sacrificial template to prepare a series of Ag/Cu2O composites through galvanic replacement. By varying the Ag/Cu atomic ratio, Ag0.125/Cu2O, having a hierarchical, flower-like nanostructure with intersecting Ag nanoflakes encompassing an inner Cu2O sphere, was prepared. The as-prepared Agx/Cu2O samples presented higher Faradaic efficiencies (FE) for CO and relatively suppressed H2 evolution than the parent Cu2O nanospheres due to the combination of Ag with Cu2O in the former. Notably, the highest CO evolution rate was achieved with Ag0.125/Cu2O due to the larger electroactive surface area furnished by the hierarchical structure. The same hierarchical flower-like structure was also obtained for the Au0.6/Cu2O composite, where the FECO (10%) was even higher than that of Ag0.125/Cu2O. Importantly, the results reveal that Ag0.125/Cu2O and Au0.6/Cu2O both exhibit remarkably improved stability relative to Cu2O. This study presents a facile method of developing hierarchical metal-oxide composites as efficient and stable electrocatalysts for the electrochemical reduction of CO2.

Boosting Electrochemical Reduction of CO2 at a Low Overpotential by Amorphous Ag‐Bi‐S‐O Decorated Bi0 Nanocrystals

AbstractBimetal‐S‐O composites have been rarely researched in electrochemical reduction of CO2. Now, an amorphous Ag‐Bi‐S‐O decorated Bi0 catalyst derived from Ag0.95BiS0.75O3.1 nanorods by electrochemical pre‐treatment was used for catalyzing eCO2RR, which exhibited a formate FE of 94.3 % with a formate partial current density of 12.52 mA cm−2 at an overpotential of only 450 mV. This superior performance was attributed to the attached amorphous Ag‐Bi‐S‐O substance. S could be retained in the amorphous region after electrochemical pre‐treatment only in samples derived from metal‐S‐O composites, and it would greatly enhance the formate selectivity by accelerating the dissociation of H2O. The existence of Ag would increase the current density, resulting in a higher local pH, which made the role of S in activating H2O more significantly and suppressed H2 evolution more effectively, thus endowing the catalyst with a higher formate FE at low overpotentials.

Boosting Electrochemical Reduction of CO2 at a Low Overpotential by Amorphous Ag‐Bi‐S‐O Decorated Bi0 Nanocrystals

Bimetal-S-O composites have been rarely researched in electrochemical reduction of CO2 . Now, an amorphous Ag-Bi-S-O decorated Bi0 catalyst derived from Ag0.95 BiS0.75 O3.1 nanorods by electrochemical pre-treatment was used for catalyzing eCO2 RR, which exhibited a formate FE of 94.3 % with a formate partial current density of 12.52 mA cm-2 at an overpotential of only 450 mV. This superior performance was attributed to the attached amorphous Ag-Bi-S-O substance. S could be retained in the amorphous region after electrochemical pre-treatment only in samples derived from metal-S-O composites, and it would greatly enhance the formate selectivity by accelerating the dissociation of H2 O. The existence of Ag would increase the current density, resulting in a higher local pH, which made the role of S in activating H2 O more significantly and suppressed H2 evolution more effectively, thus endowing the catalyst with a higher formate FE at low overpotentials.