It is demonstrated that the electroactivity of the oxygen reduction reaction (ORR) of Pt depends on the structure
of a support. Highly conductive two-dimensional titanium carbide (Ti3C2) was selected as the support for Pt
because of the expected strong metal-support interaction (SMSI) between Pt and Ti. To control the edge-to-basal
ratio, the number of Ti3C2 layers was modulated by exfoliation. Pt nanoparticles (4 nm) were loaded on three
different Ti3C2 supports including multi-, few-, and mono-layered Ti3C2 (22L-, 4L-, and 1L-Ti3C2, respectively).
The edge-to-basal ratio of layered Ti3C2 increased as the number of layers increased. The edge-dominant support
(22L-Ti3C2) donated more electrons to Pt than the basal-dominant supports (4L-Ti3C2 and 1L-Ti3C2). As a result,
electron-rich Pt with less d-band vacancies (e.g., Pt/22L-Ti3C2) showed higher ORR activity. In addition, the
electron transfer from the support to Pt inducing the strong interaction between Pt and Ti improved the durability
of the ORR electroactivity of Pt.
Decades of electrochemical CO2 reduction research have led to established rules about the product selectivity, i.e., bare tin yields formic acid as the main product. Here, we present Sn nanoparticles supported on carbon nanotubes (CNTs) in a hollow fiber (Sn-CHF), which produce CO with 10 times higher selectivity than formate. Density functional theory calculations reveal that a strong interfacial field induced by the carbon support enhances the rate-limiting CO2 adsorption and thus CO production on Sn nanoparticles, whereas the field-insensitive formate and hydrogen production routes were completely suppressed and occurred mainly from carbon sites. Modification of the interfacial electric field via exchange of the electrolyte-containing cation from Li+ to Cs+ induces an unprecedented 2 orders of magnitude change in the CO current while keeping the other products almost unchanged. This work demonstrates how electrochemical selectivity rules can be modulated by controlling the interfacial field, thus opening up new windows for electrocatalyst design.
Developing electrocatalysts that are stable and efficient for CO2 reduction is important for constructing a carbon-neutral energy cycle. New approaches are required to drive input electricity toward the desired CO2 reduction reaction (CO2RR) rather than the competitive hydrogen evolution reaction (HER). In this study, we have used quantum mechanics to demonstrate that the space confinement formed in the gaps of adjacent gold or silver nanoparticles can be used to improve the Faradaic efficiency of CO2RR to CO. This behavior is due to the space confinement stabilizing *COOH, which is the key intermediate in the CO2RR. However, space confinement has almost no effect on *H, which is the key intermediate in the HER. Possible experimental approaches for the preparation of this type of gold or silver electrocatalyst have been proposed.
Copper (Cu) offers a means for producing value‐added fuels through the electrochemical reduction of carbon dioxide (CO2), i.e., the CO2 reduction reaction (CO2RR), but designing Cu catalysts with significant Faradaic efficiency to C2+ products remains as a great challenge. This work demonstrates that the high activity and selectivity of Cu to C2+ products can be achieved by atomic‐scale spacings between two facets of Cu particles. These spacings are created by lithiating CuOx particles, removing lithium oxides formed, and electrochemically reducing CuOx to metallic Cu. Also, the range of spacing (ds) is confirmed via the 3D tomographs using the Cs‐corrected scanning transmission electron microscopy (3D tomo‐STEM), and the operando X‐ray absorption spectra show that oxidized Cu reduces to the metallic state during the CO2RR. Moreover, control of ds to 5–6 Å allows a current density exceeding that of unmodified CuOx nanoparticles by about 12 folds and a Faradaic efficiency of ≈80% to C2+. Density functional theory calculations support that ds of 5–6 Å maximizes the binding energies of CO2 reduction intermediates and promotes C–C coupling reactions. Consequently, this study suggests that control of ds can be used to realize the high activity and C2+ product selectivity for the CO2RR.
Designing and preparing highly active and stable nanostructured Pt-based catalysts with ultralow Pt loading are still challenging for electrochemical applications such as water electrolysis and fuel cells. Here we report for the first time an in situ electrochemical process to synthesize Pt-MoO3−x nanoflakes (NFs) overgrown on commercial bulk MoS2 by employing a facile and simple electrochemical method without using any expensive precious metal salts. The overgrowth of Pt-MoO3−x NFs on the bulk MoS2 surface is conducted by applying electrical energy to the bulk MoS2 and using Pt counter electrode dissolution in an acidic solution. In spite of their 10 times lower Pt loadings compared to commercial Pt black (Alfa Aesar), the synthesized Pt-MoO3−x NFs demonstrate excellent catalytic performance with a Pt mass activity of 2.83 A/mgPt at the overpotential of 100 mV for electrochemical hydrogen evolution reaction (HER), an approximately 4 times higher value than the value of 0.76 A/mgPt at the overpotential of 100 mV for commercial Pt black. We hypothesize that the outstanding HER characteristics of Pt-MoO3−x NFs are related to the existence and increase of Pt-MoO3 interfacial sites and oxygen vacancy sites such as Mo5+ in the Pt-MoO3−x NF structures. In addition, our density functional theory (DFT) calculations demonstrate that Pt and O sites at Pt and MoO3 interfaces and O sites at defective MoO3−x in the Pt-MoO3−x NFs contribute to accelerate the HER.
Herein, we introduce a permeable carbon nanotube hollow-fiber electrode incorporated with SnO2 nanoparticles (SnO2–CHE) and propose a new type of gas-phase operational mode. Highly efficient electrochemical syngas production from CO2 is made possible by switching the operating mode from liquid phase to gas phase. The operation of SnO2–CHE in the conventional liquid-phase mode yielded a H2/CO ratio higher than 4.59, and the maximum jCO was only 2.16 mA/cm2 at −0.88 V (vs RHE) due to the low solubility and limited mass transfer of CO2 in liquid electrolytes. On the other hand, SnO2–CHE operated under the newly designed gas-phase mode achieved a H2/CO ratio ranging from 1.22 to 4.11 with a maximum jCO of 7.42 mA/cm2 at −0.76 V (vs RHE), which is proper for direct post-conversion processes. Therefore, this work could offer a new avenue for electrochemical syngas production using a nonprecious metal-based hollow-fiber type electrode, which allows for a large electrode surface area and high CO2 availability in gas-phase operation.
Electrochemical CO2 reduction (CO2RR) has received much attention for its ability to generate value-added chemicals from a molecule that would otherwise be a waste end-product. Numerous studies have emerged in the past decades, but the renewable and sustainable carbon-neutral CO2 reduction process is yet to be industrialized. Here, we review the progress and bottlenecks of the electrochemical CO2 reduction technologies over the past 15 years (2004–2018) to examine whether CO2RR process is to be applicable in a large-scale. Although the techno-economic analysis and pilot plants based on liquid-phase electrolysis have shown some positive results, current densities of the liquid-phase electrochemical CO2 reduction are well below what techno-economic analyzes have projected due to its intrinsic limitations of solubility. On the other hand, the gas-phase electrolysis of CO2 has shown superior performance parameters compared to the liquid phase electrolysis, especially in the current densities, showing commercial viability although its techno-economic analysis is yet to be performed. Herein, we offer some perspectives and guidelines where future research in CO2 electrolysis should aim. Based on the performance parameters obtained from the lab-scale gas-phase reactions, we believe that the current negative outlooks towards the industrial feasibility of the CO2 electrolysis system could turn to positive views.
Alkali metal ion intercalation into TiO2 photocatalysts is a well-known method for obtaining enhanced photocatalytic properties. Herein, an effective electrochemical lithiation method is proposed for the synthesis of lithium-intercalated TiO2 nanoparticles (NPs) suitable for high performance photocatalytic production of hydrogen. The electrochemical lithiation of TiO2induces partial reduction of Ti ions in the crystal lattices, which introduces additional energy states within the band edge of TiO2, resulting in the improved optical properties. Consequently, Li-intercalated TiO2 NPs exhibit almost 60-times improved hydrogen generation activity under both UV and visible light exposure compared to those of commercial TiO2 NPs (P25).
Conversion of carbon dioxide (CO2) to useful chemicals is considered a potential solution to resolve the climate threat. In this study, a facile metal-organic framework (MOF)-mediated strategy to obtain an efficient electrocatalyst for the synthesis of methane (CH4) is suggested. Cu-based MOF-74 was chosen as the precursor, which was electrochemically reduced to obtain Cu nanoparticles (NPs). The porous structure of the MOF serves as a template for the synthesis of isolated Cu NP clusters with high catalytic activities and high efficiencies for the CH4 production in the electrochemical CO2 reduction reaction. The MOF-derived Cu NPs demonstrate high Faradaic efficiency (>50%) for the CH4 production with suppressed C2 production, and 2.3-fold higher methanation activity at −1.3 V (vs. RHE) compared to the commercial Cu NPs.
Herein, the development of a cost‐effective system is reported for the mass production of electrochemically exfoliated graphene (EEG) using multiple graphite–stainless‐steel electrodes (multicells) in a series configuration and its application to heat transfer. Exfoliation using series‐configured multicells leads to the production of high‐quality graphene (a few layers of graphene sheets with a low oxygen content and a high C/O ratio of 16.2) at a rate of 30 g per half hour (one‐batch). Furthermore, EEG paper is fabricated by the vacuum filtration of the EEG dispersion, and further thermal annealing and mechanical‐compression processes are used to investigate the effects of heat and pressure on the thermal conductivities of the EEG paper. EEG paper with wide (100–1000 W m−1 K−1) and narrow (100–200 W m−1 K−1) ranges of thermal conductivity is obtained when thermally annealed and mechanically compressed, respectively, highlighting the high quality of the massively produced and solution processable graphene. This approach provides a cost‐effective process for the mass production of graphene, as well offering a feasible route to highly thermally conductive graphene paper for heat‐management applications, such as heat‐dissipating media in light‐emitting‐diode displays, and electronic and photonic devices.
Mass transfer, kinetics, and mechanism of electrochemical CO2 reduction have been explored on a model mesostructure of highly-ordered copper inverse opal (Cu-IO), which was fabricated by Cu electrodeposition in a hexagonally-closed packed polystyrene template. As the number of Cu-IO layers increases, the formation of C2 products such as C2H4 and C2H5OH was significantly enhanced at reduced overpotentials (∼200 mV) compared to a planar Cu electrode. At the thickest layer, we observe for the first time the formation of acetylene (C2H2), which can be generated through a kinetically slow reaction pathway and be a key descriptor in the unveiling of the CC coupling reaction mechanism. Based on our experimental observation, a plausible reaction pathway in Cu mesostructures is rationalized.
This chapter introduces copper (Cu) catalysts for the electrochemical reduction of carbon dioxide (CO2) in aqueous media. Cu is the only metallic electrode capable of electrochemically converting CO2 into hydrocarbons and alcohols with significant faradaic efficiencies. However, there are still challenges pertaining to reaction selectivity, efficiency and catalyst stability that need to be overcome before Cu can be applied to industrial-scale CO2 reduction. Previous experimental and theoretical works have suggested that tuning the binding energy of the key reaction intermediates by nanostructuring the Cu surface can play an important role in achieving this end. Therefore, this chapter focuses on the role of nanostructured Cu catalysts such as nanoparticles, oxide-derived Cu and Cu composites for the efficient and selective CO2 reduction to target products.
Recently, many experimental and theoretical efforts are being intensified to develop high-performance catalysts for electrochemical CO2 conversion. Beyond the catalyst material screening, it is also critical to optimize the surrounding reaction medium. From vast experiments, inclusion of room-temperature ionic liquid (RTIL) in the electrolyte is found to be beneficial for CO2 conversion; however, there is no unified picture of the role of RTIL, prohibiting further optimization of the reaction medium. Using a state-of-the-art multiscale simulation, we here unveil the atomic origin of the catalytic promotion effect of RTIL during CO2 conversion. Unlike the conventional belief, which assumes a specific intermolecular coordination by the RTIL component, we find that the promotion effect is collectively manifested by tuning the reaction microenvironment. This mechanism suggests the critical importance of the bulk properties (e.g., resistance, gas solubility and diffusivity, viscosity, etc.) over the detailed chemical variations of the RTIL components in designing the optimal electrolyte components, which is further supported by our experiments. This fundamental understanding of complex electrochemical interfaces will help in the development of more advanced electrochemical CO2 conversion catalytic systems in the future.
Metal-based layered structures are promising structures for use as photocatalysts, but ones that are capable of enabling a complete conversion of carbon dioxide (CO2) into a value added carbon monoxide (CO) are still limited. In this paper, a quadruple metal-based layered structure, composed of aluminium (Al), gallium (Ga), magnesium (Mg), and nickel (Ni), is reported which allows the photocatalytic conversion of CO2 into CO with a high selectivity close to 100% in the presence of water. The shifted oxidation states on the Ni and Mg ions than bivalent states lead to an increment in electronegativity for their neighboring oxygen (O) while the Ga and Al ions maintain their trivalent states, thereby enabling the O to adsorb a high amount of CO2. Furthermore, the quadruple metal-based layered structure without any use of scavengers is proven to give an approximately two-fold increase in photocatalytic activity compared to those with bi or triple metal-based structures.
Electrolyte cation size is known to influence the electrochemical reduction of CO2 over metals; however, a satisfactory explanation for this phenomenon has not been developed. We report here that these effects can be attributed to a previously unrecognized consequence of cation hydrolysis occurring in the vicinity of the cathode. With increasing cation size, the pKa for cation hydrolysis decreases and is sufficiently low for hydrated K+, Rb+, and Cs+ to serve as buffering agents. Buffering lowers the pH near the cathode, leading to an increase in the local concentration of dissolved CO2. The consequences of these changes are an increase in cathode activity, a decrease in Faradaic efficiencies for H2 and CH4, and an increase in Faradaic efficiencies for CO, C2H4, and C2H5OH, in full agreement with experimental observations for CO2 reduction over Ag and Cu.
The electrocatalytic conversion of furanic compounds, i.e. mainly furfural and 5-hydroxymethylfurfural, has recently emerged as a potentially scalable technology for both oxidation and hydrogenation processes because of its highly valuable products. However, its practical application in industry is currently limited by low catalytic activity and product selectivity. Thus, a better understanding of the catalytic reactions as well as a strategy for the catalyst design can bring solutions for a complete and selective conversion into desired products. In this perspective, we review the status and challenges of electrocatalytic oxidation and hydrogenation of furanic compounds, including thermodynamics, voltammetric studies, and bulk electrolysis with important reaction parameters (i.e., catalyst, electrolyte, temperature, etc.) and reaction mechanisms. In addition, we introduce methods of energy-efficient electrocatalytic furanic synthesis by combining yields of anodic and cathodic reactions in a paired reactor or a reactor powered by a renewable energy source (i.e., solar energy). Current challenges and future opportunities are also discussed, aiming at industrial applications.
Herein we describe a combined experimental and computational study of electrochemical glycerol oxidation in acidic media on Pt(111) and Pt(100) electrodes. Our results show that glycerol oxidation is a very structure-sensitive reaction in terms of activity and, more surprisingly, in terms of selectivity. Using a combination of online HPLC and online electrochemical mass spectrometry, we show that on the Pt(111) electrode, glyceraldehyde, glyceric acid, and dihydroxyacetone are products of glycerol oxidation, while on the Pt(100) electrode, only glyceraldehyde was detected as the main product of the reaction. Density functional theory calculations show that this difference in selectivity is explained by different binding modes of dehydrogenated glycerol to the two surfaces. On Pt(111), the dehydrogenated glycerol intermediate binds to the surface through two single Pt–C bonds, yielding an enediol-like intermediate, which serves as a precursor to both glyceraldehyde and dihydroxyacetone. On Pt(100), the dehydrogenated glycerol intermediate binds to the surface through one double Pt═C bond, yielding glyceraldehyde as the only product. Stripping and in situ FTIR measurements show that CO is not the only strongly bound adsorbed intermediate of the oxidation of glycerol, glyceraldehyde, and dihydroxyacetone. Although the nature of this adsorbate is still unclear, this intermediate is highly resistant to oxidation and can only be removed from the Pt surface after multiple scans.
There are a number of recent reports on the use of oxidation/reduction cycling of Cu surfaces to improve their selectivity for ethylene formation in the aqueous CO2 reduction reaction. Here, the oxidation/reduction process is examined in detail. It is found that the faradaic efficiencies for both ethylene and ethanol are enhanced after oxidation/reduction cycling in the presence of halide anions. Specifically, cycling of the electrode in the presence of chloride, bromide, or fluoride anions allows for an ethylene faradaic efficiency of approximately 15.2 %, a factor of 1.5 higher than that for polycrystalline copper (at −1.0 V vs. RHE). The faradaic efficiency for ethanol is also enhanced from 2.65 to approximately 7.6 %. The effects of electrochemical oxidation/reduction with the chloride anion were investigated by using in situ Raman spectroscopy, and the changes in the surface morphology of copper were monitored by using SEM. Consistent with prior reports, it is observed that during the oxidation part of the cycle, anodic corrosion forms a Cu2O layer, which consists of cubical crystals of about 150 nm. During the reduction sweep, it is converted to metallic copper, which forms irregular Cu nanoparticles of around 20 nm in diameter. The enhancement in ethylene formation is presumably attributed to the formation of grain boundaries, which may serve as active sites.
In the last few years, there has been increased interest in electrochemical CO2 reduction (CO2R). Many experimental studies employ a membrane separated, electrochemical cell with a mini H-cell geometry to characterize CO2R catalysts in aqueous solution. This type of electrochemical cell is a mini-chemical reactor and it is important to monitor the reaction conditions within the reactor to ensure that they are constant throughout the study. We show that operating cells with high catalyst surface area to electrolyte volume ratios (S/V) at high current densities can have subtle consequences due to the complexity of the physical phenomena taking place on electrode surfaces during CO2R, particularly as they relate to the cell temperature and bulk electrolyte CO2 concentration. Both effects were evaluated quantitatively in high S/V cells using Cu electrodes and a bicarbonate buffer electrolyte. Electrolyte temperature is a function of the current/total voltage passed through the cell and the cell geometry. Even at a very high current density, 20 mA cm−2, the temperature increase was less than 4 °C and a decrease of <10% in the dissolved CO2 concentration is predicted. In contrast, limits on the CO2 gas–liquid mass transfer into the cells produce much larger effects. By using the pH in the cell to measure the CO2 concentration, significant undersaturation of CO2 is observed in the bulk electrolyte, even at more modest current densities of 10 mA cm−2. Undersaturation of CO2 produces large changes in the faradaic efficiency observed on Cu electrodes, with H2 production becoming increasingly favored. We show that the size of the CO2 bubbles being introduced into the cell is critical for maintaining the equilibrium CO2 concentration in the electrolyte, and we have designed a high S/V cell that is able to maintain the near-equilibrium CO2 concentration at current densities up to 15 mA cm−2.
Copper is a unique electrocatalyst for CO2 reduction, since it is one of the few catalysts able to produce methane, ethylene and ethane from CO2 with decent faradaic efficiencies. Here we report on the design and synthesis of a new non-copper-containing catalyst able to reduce CO2 to C1 to C5 hydrocarbons. This catalyst was designed by combining a metal that binds CO strongly, Pd, with a metal that binds CO weakly, Au, in an attempt to tune the binding energy of CO. We show that a mixture of C1–C5 hydrocarbons and soluble products are produced from an onset potential of −0.8 VRHE. We propose that the higher hydrocarbons are formed via a polymerization of –CH2 groups adsorbed on the catalyst surface.
Carbon materials are frequently used as supports for electrocatalysts because they are conductive and have high surface area. However, recent studies have shown that these materials can contain significant levels of metallic impurities that can dramatically alter their electrochemical properties. Here, the electrocatalytic activity of pure graphite (PG), graphene oxide (GO), and carbon nanotubes (CNT) dispersed on glassy carbon (GC) are investigated for the electrochemical CO2 reduction reaction (CO2RR) in aqueous solution. It was observed that GO and CNT dispersed on GC all exhibit significant electrochemical activity that can be ascribed to impurities of Ni, Fe, Mn, and Cu. The level of Cu in GO can be particularly high and is the cause for the appearance of methane in the products produced over this material when it is used for the CO2RR. Washing these supports in ultrapure nitric acid is effective in removing the metal impurities and results in a reduction in the electrochemical activity of these forms of carbon. In particular, for GO, nearly all of the catalytically relevant metals can be removed. Electrochemical deposition of Cu on GO and PG supported on GC, and on GC itself, increased both the electrochemical activity of these materials and the production of methane via the CO2RR. Particularly high rates of methane formation per unit of Cu mass were obtained for Cu electrodeposited on GO and PG supported on GC. We suggest that this high activity may be due to the preferential deposition of Cu onto defects present in the graphene sheets comprising these materials.
Density functional theory (DFT) calculations are performed to investigate the energetics of the CO2 electrochemical reduction on metal (M) porphyrin-like motifs incorporated into graphene layers. The objective is to develop strategies that enhance CO2 reduction while suppressing the competitive hydrogen evolution reaction (HER). We find that there exists a scaling relation between the binding energy of the catalyst to hydrogen and that to COOH, a key intermediate in the reduction of CO2 to CO; however, the M–H bond is stronger than the M–COOH bond, driving the reaction toward the HER rather than the reduction of CO2 to CO. This scaling relation holds even with axial ligation to the metal cation coordinated to the porphyrin ring. When 4f lanthanide or 5f actinide elements are used as the reactive center, the scaling relation still holds but the M–COOH bond is stronger than the M–H bond, and the reaction favors the reduction of CO2 to CO. By contrast, there is no scaling relation between the binding energy of the catalyst to H and that to OCHO, the key intermediate in CO2 reduction to formic acid. Interestingly, we find that coordination of a ligand to an unoccupied axial site can make the M–OCHO bond stronger than the M–H bond, resulting in preferential formic acid formation. This means that the axial ligand effectively enhances CO2 reduction to formic acid and suppresses the HER. Our DFT calculations have also identified several promising electrocatalysts for CO2 reduction to HCOOH with almost zero overpotentials.
The electrochemical conversion of carbon dioxide and water into useful products is a major challenge in facilitating a closed carbon cycle. Here we report a cobalt protoporphyrin immobilized on a pyrolytic graphite electrode that reduces carbon dioxide in an aqueous acidic solution at relatively low overpotential (0.5 V), with an efficiency and selectivity comparable to the best porphyrin-based electrocatalyst in the literature. While carbon monoxide is the main reduction product, we also observe methane as by-product. The results of our detailed pH-dependent studies are explained consistently by a mechanism in which carbon dioxide is activated by the cobalt protoporphyrin through the stabilization of a radical intermediate, which acts as Brønsted base. The basic character of this intermediate explains how the carbon dioxide reduction circumvents a concerted proton–electron transfer mechanism, in contrast to hydrogen evolution. Our results and their mechanistic interpretations suggest strategies for designing improved catalysts.
The discovery of electrocatalysts that can efficiently reduce CO2 to fuels with high selectivity is a subject of contemporary interest. Currently, the available analytical methods for characterizing the products of CO2 reduction require tens of hours to obtain the dependence of product distribution on applied potential. As a consequence, there is a need to develop novel analytical approaches that can reduce this analysis time down to about an hour. We report here the design, construction, and operation of a novel differential electrochemical mass spectrometer (DEMS) cell geometry that enables the partial current densities of volatile electrochemical reaction products to be quantified in real time. The capabilities of the novel DEMS cell design are demonstrated by carrying out the electrochemical reduction of CO2 over polycrystalline copper. The reaction products are quantified in real time as a function of the applied potential during linear sweep voltammetry, enabling the product spectrum produced by a given electrocatalyst to be determined as a function of applied potential on a time scale of roughly 1 h.
Electrocatalytic hydrogenation of 5‐hydroxymethylfurfural (HMF) is studied on solid metal electrodes in acidic solution (0.5 M H2SO4) by correlating voltammetry with on‐line HPLC product analysis. Three soluble products from HMF hydrogenation are distinguished: 2,5‐dihydroxymethylfuran (DHMF), 2,5‐dihydroxymethyltetrahydrofuran (DHMTHF), and 2,5‐dimethyl‐2,3‐dihydrofuran (DMDHF). Based on the dominant reaction products, the metal catalysts are divided into three groups: (1) metals mainly forming DHMF (Fe, Ni, Cu, and Pb), (2) metals forming DHMF and DMDHF depending on the applied potentials (Co, Ag, Au, Cd, Sb, and Bi), and (3) metals forming mainly DMDHF (Pd, Pt, Al, Zn, In, and Sb). Nickel and antimony are the most active catalysts for DHMF (0.95 mM cm−2 at ca. −0.35 VRHE and −20 mA cm−2) and DMDHF (0.7 mM cm−2 at −0.6 VRHE and −5 mA cm−2), respectively. The pH of the solution plays an important role in the hydrogenation of HMF: acidic condition lowers the activation energy for HMF hydro‐genation and hydrogenates the furan ring further to tetrahydrofuran.
The electrochemical reduction of CO2 is a reaction of much current interest as a possible reaction for energy storage. In this paper, we show that on electrodeposited palladium on platinum, a good formic acid oxidation catalyst, the onset potential for CO2 reduction to formic acid is dramatically reduced in comparison to bulk palladium. Two different reaction pathways are observed; a pathway at low overpotential in which formic acid is produced from either direct bicarbonate reduction or from the reduction of CO2 generated from bicarbonate near the surface, and a pathway at more negative potentials where formic acid is produced from direct CO2 reduction. Furthermore, we show that reversible formic acid oxidation and CO2 reduction is possible on this catalyst, although unfortunately the processes are hindered by poisoning of the catalyst, most likely by CO.
A new electrocatalytic method for the selective electrochemical oxidation of sorbitol to fructose and sorbose is demonstrated by using a platinum electrode promoted by p‐block metal atoms. By the studying a range of C4, C5 and C6 polyols, it is found that the promoter interferes with the stereochemistry of the polyol and thereby modifies its reactivity.
Antimony irreversibly adsorbed on a carbon supported platinum electrode oxidizes glycerol selectively to dihydroxyacetone with a lower onset potential (ca. 150 mV) and a higher peak current density (ca. 170 %) compared to clean Pt/C. Pb, In, and Sn also promote the catalytic activity of glycerol oxidation, however the reaction pathway towards the primary alcohol oxidation remains unchanged. Higher surface coverage by adatoms on Pt/C generally increases the activity of glycerol oxidation.
We present a comparative study of the activity and selectivity of Rh/C nanoparticles and Sn‐modified Rh/C nanoparticles towards electrocatalytic nitrate reduction in sulfuric acid. Electrochemical techniques, combined with more direct analytical techniques such as mass spectrometry and ion chromatography, were applied to analyse the products obtained during the reaction. Online electrochemical mass spectrometry was employed to detect volatile products, such as nitric oxide (NO), nitrous oxide (N2O) and dinitrogen (N2). The combination of online sample collection to the electrochemical cell and offline ion chromatography allows the quantitative analysis of non‐volatile products, such as ammonium () and hydroxylamine (NH3OH+). Non‐volatile products can be detected during nitrate reduction at Rh/C electrodes. The catalytic activity of Rh/C electrodes can be enhanced by Sn modification. N2O is the dominant volatile product at SnRh/C electrodes. is the main ionic product at the Rh/C electrodes, whereas modification by Sn also leads to the formation of NH3OH+.
It has been suggested that peroxide could act as an enhancing agent for gold‐catalyzed oxidation reactions in the aqueous phase. Here, we probe the potential role of peroxide in the catalytic oxidation of glycerol on gold in the aqueous phase by electrochemical techniques and find that neither the presence of oxygen nor peroxide leads to a significant enhancement of the overall oxidation activity or product selectivity at low temperature. In alkaline media, there appears to be a small suppression of the formation of glyceraldehyde in the presence of hydrogen peroxide but there is no major influence on the overall oxidation pathway. In neutral and acidic media, glycerol oxidation on gold has low activity.
A surface structural preference for (1 0 0) terraces of fcc metals is displayed by many bond-breaking or bond-making reactions in electrocatalysis. Here, this phenomenon is explored in the electrochemical oxidation of dimethyl ether (DME) on platinum. The elementary C–O bond-breaking step is identified and clarified by combining information obtained from single-crystal experiments and density functional theory (DFT) calculations. Experiments on Pt(1 0 0), Pt(5 1 0), and Pt(10 1 0) surfaces show that the surface structure sensitivity is due to the bond-breaking step, which is unfavorable on step sites. DFT calculations suggest that the precursor for the bond-breaking step is a CHOC adsorbate that preferentially adsorbs on a square ensemble of four neighboring atoms on Pt(1 0 0) terraces, named as “the active site”. Step sites fail to strongly adsorb CHOC and are, therefore, ineffective in breaking C–O bonds, resulting in a decrease in activity on surfaces with increasing step density. Our combined experimental and computational results allow the formulation of a new mechanism for the electro-oxidation of DME as well as a simple general formula for the activity of different surfaces toward electrocatalytic reactions that prefer (1 0 0) terrace active sites.
Electrocatalytic hydrogenation of 5‐hydroxymethylfurfural (HMF) to 2,5‐dihydroxymethylfuran (DHMF) or other species, such as 2,5‐dimethylfuran, on solid metal electrodes in neutral media is addressed, both in the absence and in the presence of glucose. The reaction is studied by combining voltammetry with on‐line product analysis by using HPLC, which provides both qualitative and quantitative information about the reaction products as a function of electrode potential. Three groups of catalysts show different selectivity towards: (1) DHMF (Fe, Ni, Ag, Zn, Cd, and In), (2) DHMF and other products (Pd, Al, Bi, and Pb), depending on the applied potential, and (3) other products (Co, Au, Cu, Sn, and Sb) through HMF hydrogenolysis. The rate of electrocatalytic HMF hydrogenation is not strongly catalyst‐dependent because all catalysts show similar onset potentials (−0.5±0.2 V) in the presence of HMF. However, the intrinsic properties of the catalysts determine the reaction pathway towards DHMF or other products. Ag showed the highest activity towards DHMF formation (up to 13.1 mM cm−2 with high selectivity> 85 %). HMF hydrogenation is faster than glucose hydrogenation on all metals. For transition metals, the presence of glucose enhances the formation of DHMF and suppresses the hydrogenolysis of HMF. On poor metals such as Zn, Cd, and In, glucose enhances DHMF formation; however, its contribution in the presence of Bi, Pb, Sn, and Sb is limited. Remarkably, in the presence of HMF, glucose hydrogenation itself is largely suppressed or even absent. The first electron‐transfer step during HMF reduction is not metal‐dependent, suggesting a non‐catalytic reaction with proton transfer directly from water in the electrolyte.
To overcome the shortcomings of electroanalytical methods in analyzing the ionic reaction products that are either electrochemically inert or lack distinct electrochemical/spectroscopic fingerprints, we suggest combining voltammetry with ion chromatography by applying online sample collection to the electrochemical cell and offline ion chromatographic analysis. This combination allows a quantitative analysis including the potential dependence of the product distribution in a straightforward way. As a proof-of-concept example, we discuss the formation of ionic reaction products from nitrate reduction on Pt and Sn-modified Pt electrode in acid. On the Pt electrode, ammonia was the only identifiable product. After Sn modification of the Pt electrode, a change in selectivity was observed to hydroxylamine as the dominant product. Moreover, the rate determining step of nitrate reduction (reduction to nitrite) was enhanced by Sn modification of the Pt electrode, and a significant concentration of nitrite was evidenced on a Pt electrode with a high coverage of Sn species. The suggested combination of voltammetry and online ion chromatography hence proves very useful in the quantitative elucidation of electrocatalytic reactions with different ionic products.
The electrochemical reduction of CO2 on copper is an intensively studied reaction. However, there has not been much attention for CO2 reduction on copper in alkaline electrolytes, because this creates a carbonate buffer in which CO2 is converted in HCO3 − and the pH of the electrolyte decreases. Here, we show that electrolytes with phosphate buffers, which start off in the alkaline region and, after saturation with CO2, end up in the neutral region, behave differently compared to CO2 reduction in phosphate buffers which starts off in the neutral region. In initially alkaline buffers, a reduction peak is observed, which is not seen in neutral buffer solutions. In contrast with earlier literature reports, we show that this peak is not due to the formation of a CO adlayer on the electrode surface but due to the production of formate via direct bicarbonate reduction. The intensity of the reduction peak is influenced by electrode morphology and the identity of the cations and anions in solution. It is found that a copper nanoparticle-covered electrode gives a rise in intensity in comparison with mechanically polished and electropolished electrodes. The peak is observed in the SO4 2−-, ClO4 −-, and Cl−- containing electrolytes, but the formate-forming peak is not seen with Br− and I−.
This Full Paper addresses the electrocatalytic hydrogenation of glucose to sorbitol or 2‐deoxysorbitol on solid metal electrodes in neutral media. Combining voltammetry and online product analysis with high‐performance liquid chromatography (HPLC), provides both qualitative and quantitative information regarding the reaction products as a function of potential. Three groups of catalysts clearly show affinities toward: (1) hydrogen formation [on early transition metals (Ti, V, Cr, Mn, Zr, Nb, Mo, Hf, Ta, We, and Re) and platinum group metals (Ru, Rh, Ir, and Pt)], (2) sorbitol formation [on late transition metals (Fe, Co, Ni, Cu, Pd, Au, and Ag) and Al (sp metal)], and (3) sorbitol and 2‐deoxysorbitol formation [on post‐transition metals (In, Sn, Sb, Pb, and Bi), as well as Zn and Cd (d metals)]. Ni shows the lowest overpotential for the onset of sorbitol formation (−0.25 V) whereas Pb generates sorbitol with the highest yield (<0.7 mM cm−2). Different from a smooth Pt electrode, a large‐surface‐area Pt/C electrode hydrogenates glucose to sorbitol from −0.21 V with relatively low current. This emphasizes the importance of the active sites and the surface area of the catalyst. The mechanism to form 2‐deoxysorbitol from glucose and/or fructose is discussed according to the observed reaction products. The yield and selectivity of hydrogenated products are highly sensitive to the chemical nature and state of the catalyst surface.
This paper addresses the hydrolysis of cellobiose to glucose and its further decomposition with electrochemically generated acid (H+) on a platinum electrode, and with electrochemically generated hydroxyl radicals (OH.) on boron‐doped diamond (BDD). The results are compared with the hydrolysis promoted by conventional acid (H2SO4) and OH. (from Fenton’s reaction) and supported by product analysis by using online HPLC (for soluble products) and online electrochemical mass spectrometry (for CO2). Cellobiose hydrolysis follows a first‐order reaction, which obeys Arrhenius’ law over the temperature range from 25–80 °C with different activation energies for the acid‐ and radical‐promoted reaction, that is, approximately 118±8 and 55±1 kJ mol−1, respectively. The high local acidity with electrochemically generated H+ on the Pt electrode increases the rate of glucose formation, however, the active electrode (PtOx) interacts with glucose and decomposes it further to smaller organic acids. In addition, O2 formed during the oxygen evolution reaction (OER) lowers the selectivity of glucose by forming side‐products. OH. generated on a BDD electrode first hydrolyzes the cellobiose to glucose, but rapidly attacks the aldehyde on glucose, which is further decomposed to smaller aldoses and finally formaldehyde, which is subsequently oxidized electrochemically to formic acid.
A carbon supported platinum electrode in a bismuth saturated solution at a carefully chosen potential is capable of oxidizing glycerol to dihydroxyacetone with 100% selectivity. In the absence of bismuth, the primary alcohol oxidation is dominant. Using a combination of online HPLC and in situ FTIR, it is shown that Bi blocks the pathway for primary oxidation but also provides a specific Pt–Bi surface site poised for secondary alcohol oxidation.
In heterogeneous catalysis and electrocatalysis, adsorbed carbon monoxide typically acts as a poison or poisoning intermediate in the oxidation of alcohols. However, gold as an (electro)catalyst often exhibits unexpected properties. Here we show that carbon monoxide irreversibly adsorbed on a Au(111) surface in aqueous alkaline media can act as a promoter for the electrocatalytic oxidation of certain alcohols, in particular methanol. In comparison with bare Au(111), the onset potential for methanol oxidation is significantly lower in the presence of adsorbed CO, and formation of the main methanol oxidation products—formaldehyde and formic acid—is enhanced. By studying the effect of adsorbed CO on the oxidation of other alcohols on gold, we conclude that the presence of adsorbed CO promotes beta-hydrogen elimination, that is, C–H bond breaking. Apart from its importance to gold catalysis, this is an unanticipated example of promotion effects by co-adsorbed small molecules in electrocatalysis.
This paper addresses the oxidation mechanism of glycerol on Au and Pt electrodes under different pH conditions. Intermediates and/or reaction products were detected by using an online high‐performance liquid chromatography technique (for soluble products) and online electrochemical mass spectrometry (for CO2). In alkaline media, the main product of glycerol oxidation on the Pt electrode is glyceric acid produced via glyceraldehyde. Glyceric acid is the primary oxidation product on the Au electrode, which is further oxidized to glycolic acid and formic acid at high potentials (≥0.8 V), yielding high current densities. As the pH of the solution is lowered, the glycerol oxidation becomes significantly more sluggish on both Au and Pt electrodes, which results in glyceraldehyde being the main oxidation product under neutral conditions, especially on gold. In acidic solutions, only the Pt electrode shows catalytic activity with a relatively low conversion rate, mainly to glyceraldehyde. At positive potentials corresponding to the formation of a Pt surface oxide, the PtOx surface oxide catalyzes the conversion of glyceraldehyde finally to formic acid and CO2, but only under acidic conditions. Gold catalyzes glycerol oxidation only under alkaline conditions, in contrast to a “real catalyst,” that is, platinum, which catalyzes glycerol oxidation over the entire pH range.
The oxidation of methanol, ethanol and iso-propanol and their respective product formation on platinum and palladium electrodes in alkaline solution are studied by voltammetry combined with high performance liquid chromatography. The oxidation products observed at platinum are formaldehyde and formate for methanol, acetaldehyde and acetate for ethanol and acetone for iso-propanol oxidation. On palladium, the same products (except formaldehyde) are detected. Palladium appears to be a better catalyst for the selective oxidation of the alcohol group in alkaline media, but as soon as poisoning by adsorbed carbon monoxide plays a significant role, such as in methanol oxidation, platinum is the preferred catalyst.
On the basis of a comparison of the oxidation activity of a series of similar alcohols with varying pKa on gold electrodes in alkaline solution, we find that the first deprotonation is base catalyzed, and the second deprotonation is fast but gold catalyzed. The base catalysis follows a Hammett-type correlation with pKa, and dominates overall reactivity for a series of similar alcohols. The high oxidation activity on gold compared to platinum for some of the alcohols is related to the high resistance of gold toward the formation of poisoning surface oxides. These results indicate that base catalysis is the main driver behind the high oxidation activity of many organic fuels on fuel cell anodes in alkaline media, and not the catalyst interaction with hydroxide.
We first established a process for the autonomous creation of PbO nanostructures consisting of a simple three-step procedure for both the formation of Pb nanoparticles and their oxidation. Oxygen contacting aqueous media results in an autonomous conversion from electrodeposited Pb particles to PbO nanostructures; i) flower-like PbO structures are placed at the interface of water and oxygen, ii) the growth/burst of PbO nanowires in various directions is observed in the middle of water media, and iii) ultra-thin PbO nano-platelets are dominantly placed onto the substrate. A new mechanistic origin was also proposed based on experimental observations and further suggests that major requirements are essential for the autonomous creation of PbO nanostructures.
We have investigated the reaction mechanism of the electrochemical reduction of carbon dioxide to hydrocarbons on copper electrodes. This reaction occurs via two pathways: a C1 pathway leading to methane, and a C2 pathway leading to ethylene. To identify possible intermediates in the reduction of carbon dioxide we have studied the reduction of small C1 and C2 organic molecules containing oxygen. We followed the formation and consumption of intermediates during the reaction as a function of potential, using online mass spectrometry. For the C1 pathway we show that it is very likely that CHOads is the key intermediate towards the breaking of the C–O bond and, therefore, the formation of methane. For the C2 pathway we suggest that the first step is the formation of a CO dimer, followed by the formation of a surface-bonded enediol or enediolate, or the formation of an oxametallacycle. Both the enediol(ate) and the oxametallacycle would explain the selectivity of the C2 pathway towards ethylene. This new mechanism is significantly different from existing mechanisms but it is the most consistent with the available experimental data.
Electrodeposited and sprayed Pt anode catalysts were electrochemically characterised by CO stripping voltammetry as well as their activity to CO tolerance in micro‐PEMFCs was demonstrated using polarisation measurements. While the onset and peak potentials of CO oxidation on the sprayed Pt/C varied with the CO coverage, these were lower (∼50 mV) with the electrodeposited Pt anode. This difference is attributed to the varying properties of the Pt–OH on either rough or smooth surface mainly created from different sizes of Pt particles. In fuel cell performance test, the electrodeposited Pt anode showed maximum power density of 360 mW cm–2 and it was markedly (∼110 mW cm–2) higher than the sprayed Pt/C anode. The enhanced activity of the electrodeposited Pt anode is also reflected by the fact that the entire amount of adsorbed CO becomes almost desorbed during the first three polarisation scans, while with the Pt/C anode at least five cycles are required.
Electrodeposited Pt nanoparticles on carbon substrate show various morphologies depending on the applied potentials. Dendritic, pyramidal, cauliflower-like, and hemi-spherical morphologies of Pt are formed at potential ranges between −0.2 and 0.3 V (vs. Ag/AgCl) and its particle sizes are distributed from 8 to 26 nm. Dendritic bulky particles over 20 nm are formed at an applied potential of −0.2 V, while low deposition potential of 0.2 V causes dense hemi-spherical structure of Pt less than 10 nm. The influence of different Pt shapes on an electrocatalytic oxidation of formic acid is represented. Consequently, homogeneous distribution of Pt nanoparticles with average particle of ca. 14 nm on carbon paper results in a high surface to volume ratio and the better power performance in a fuel cell application.
Formic acid is electrochemically generated from carbon dioxide (CO2) on nanolayered lead (Pb) electrode. Stepwise potential deposition method is applied to prepare nanostructured Pb, composed of particles and platelets with hexagonal and cubic crystallinities. Their electrocatalytic activities in an electroreduction of CO2 are compared. We observed higher faradaic efficiencies of 94.1% on a cubic Pb surface than that of polycrystalline Pb smooth films (52.3%) at 278 K. Analyzing the mass changes of the electrodes by electrochemical quartz crystal microbalance, the mechanistic origin of CO2 reduction is studied, and the indirect reduction of CO2 via Had atoms might be more reasonable than the direct electron transfer of CO2 molecules.
The combination of cyclic voltammetry and “online” chromatographic techniques for product detection is limited by the typically long analysis times in chromatographic columns. Therefore, traditionally, product analysis is performed offline after long bulk electrolysis experiments. To overcome the limitation of the inherently different time scales of voltammetry and high-performance liquid chromatography (HPLC), we suggest here to adopt rapid online sample collection with a micrometer-sized sampling tip placed close to the working electrode, followed by offline analysis of the sample fractions in an HPLC system. To demonstrate this concept, we applied online fraction collection and offline HPLC analysis to the glycerol electro-oxidation on Au and Pt electrodes in alkaline media and show that we can successfully follow the concentration changes of glycerol and its reaction products in good correspondence with the current profile obtained simultaneously with voltammetry. Moreover, the method allows for a detailed discrimination of the different mechanisms of glycerol oxidation on both electrodes. Therefore, this simple approach enables the monitoring of soluble reaction products during voltammetry with an HPLC system and thereby allows for new insights into the mechanisms of complex multistep electrode reactions.
As global warming directly affects the ecosystems and humankind in the 21st century, attention and efforts are continuously being made to reduce the emission of greenhouse gases, especially carbon dioxide (CO2). In addition, there have been numerous efforts to electrochemically convert CO2 gas to small organic molecules (SOMs) and vice versa. Herein, we highlight recent advances made in the electrocatalytic recycling of CO2 and SOMs including (i) the overall trend of research activities made in this area, (ii) the relations between reduction conditions and products in the aqueous phase, (iii) the challenges in the use of gas diffusion electrodes for the continuous gas phase CO2 reduction, as well as (iv) the development of state of the art hybrid techniques for industrial applications. Perspectives geared to fully exploit the potential of zero‐gap cells for CO2 reduction in the gaseous phase and the high applicability on a large scale are also presented. We envision that the hybrid system for CO2 reduction supported by sustainable solar, wind, and geothermal energies and waste heat will provide a long term reduction of greenhouse gas emissions and will allow for continued use of the abundant fossil fuels by industries and/or power plants but with zero emissions.
The effect of physico-electrochemical properties of carbon bipolar plate (BPP) on hydrogen and formic acid fuel cell performance has been investigated. BPP made of conventional graphite and carbon fiber composite were compared with the factors of interfacial contact resistance (ICR), corrosion behaviours, and hydrophobicity. Among them, the ICR of carbon fiber composite BPP has 50% higher than conventional graphite and the surface of carbon fiber composite BPP became rougher due to weaker corrosion resistance. Fuel cell performance was strongly dependent of ICR value of carbon bipolar plate.
Cheap and stable: A PtBi catalyst was fabricated in three consecutive electrochemical steps (see picture): electrochemical oxidation of carbon paper to form an adequate catalyst support (1), Pt electrodeposition (2), and underpotential deposition of Bi onto the as‐prepared Pt (3). This process resulted in a well‐dispersed and thin catalyst layer as well as a significantly enhanced power performance with a Pt loading of only 0.5 mg cm−2.
We report the mass transport characteristics of formic acid and performance enhancement in a direct formic acid fuel cell in terms of the property of anode components. The effect of hydrophobicity of anode diffusion media as well as catalyst layer was investigated applying different cell temperature and fuel concentration. The operation over 80 °C and concentrated formic acid is of great advantage to the enhancement of catalytic activity and better water management. On the other hand, the conductivity of formic acid decreases by means of the formation of more complex chains of formic acid and the fuel cell resistance increases by membrane dehydration effect due to the hygroscopic property of formic acid, resulting in overall decrease of cell performance and long-term stability. Optimizing operating conditions, the use of 60% PtRu/C with only 1 mg/cm2 on plain carbon paper can be one of the good choice to achieve both sustainable power performance and higher utilization of anode catalysts keeping cell resistance.
The underpotential deposited Bi on Pt(Biupd /Pt) anode for formic acid fuel cells (FAFCs) was developed using multi-layered preparation method for better electrocatalytic utilization of Pt. The electron probe microanalysis (EPMA) result indicated that Biupd remains through the catalyst layer during stability test. In performance test, the multilayered Biupd on Pt black showed superior performance by approximately 200 mV at current density of 150 mA/cm2 compared with PtRu black anode catalyst. Based on preparation condition of Biupd /Pt black, carbon supported Biupd /Pt/C electrode was prepared and it showed enhanced performance and stability.