Combined electrochemical and spectroscopic investigations of carbonate-mediated water oxidation to peroxide.

The development of electrosynthetic technologies for H2O2 production is appealing from a sustainability perspective. The use of carbonate species as mediators in water oxidation to peroxide has emerged as a viable route to do so but still many questions remain about the mechanism that must be addressed. To this end, this work combines electrochemical and spectroscopic methods to investigate reaction pathways and factors influencing the efficiency of this reaction. Our results indicate that CO32- is the key species that undergoes electrochemical oxidation, prior to reacting with water away from the catalyst. Through spectroelectrochemical experiments, we noted that CO32- depletion is a factor that limits the selectivity of the process. In turn, we showed how the application of pulsed electrolysis can augment this, with an initial set of optimized parameters increasing the selectivity from 20% to 27%. In all, this work helps pave the way for future development of practical H2O2 electrosynthetic systems.

Electrocatalysis with molecules and molecular assemblies within gas diffusion electrodes.

Molecular catalysts and their assemblies are important model systems in electrocatalysis. This is largely because their active sites, secondary coordination spheres, and reaction environments can be rationally modulated. Such experiments yield important insights into the structure-activity relationships that can be used to design improved catalysts or translated to more technologically mature systems. However, in the context of electrocatalysis, molecular catalysts are often dissolved in an electrolyte or heterogenized on an electrode that is completely submersed in an electrolyte (e.g. H-cell) or reaction setups that are not used in practical systems and use poorly soluble gaseous reactants like CO2, CO, or O2. This is beginning to change, with a growing emphasis being placed on investigating molecular catalysts and catalytic assemblies (e.g. metal/covalent organic frameworks and polymers with molecular active sites) in gas-diffusion electrodes (GDEs) that feed the reactant directly from the gas phase to the catalytic sites and enable industrially viable current densities. Against this backdrop, this perspective first details the emerging set of molecular catalyst-embedded GDE-based systems and what the community has learned thus far from these efforts. We next identify the gaps in knowledge and performance that are yet to be closed and offer strategies for exploring in this direction. Finally, we conclude with a forward-looking discussion that highlights several new avenues to be pursued with molecule-based GDE platforms and how this can accelerate progress in the electrocatalysis field as a whole.

Highly Durable Nanoporous Cu2-xS Films for Efficient Hydrogen Evolution Electrocatalysis under Mild pH Conditions.

Copper-based hydrogen evolution electrocatalysts are promising materials to scale-up hydrogen production due to their reported high current densities; however, electrode durability remains a challenge. Here, we report a facile, cost-effective, and scalable synthetic route to produce Cu2-xS electrocatalysts, exhibiting hydrogen evolution rates that increase for ∼1 month of operation. Our Cu2-xS electrodes reach a state-of-the-art performance of ∼400 mA cm-2 at -1 V vs RHE under mild conditions (pH 8.6), with almost 100% Faradaic efficiency for hydrogen evolution. The rise in current density was found to scale with the electrode electrochemically active surface area. The increased performance of our Cu2-xS electrodes correlates with a decrease in the Tafel slope, while analyses by X-ray photoemission spectroscopy, operando X-ray diffraction, and in situ spectroelectrochemistry cooperatively revealed the Cu-centered nature of the catalytically active species. These results allowed us to increase fundamental understanding of heterogeneous electrocatalyst transformation and consequent structure-activity relationship. This facile synthesis of highly durable and efficient Cu2-xS electrocatalysts enables the development of competitive electrodes for hydrogen evolution under mild pH conditions.

Electrochemically driven C-N bond formation from CO2 and ammonia at the triple-phase boundary.

Electrosynthetic techniques are gaining prominence across the fields of chemistry, engineering and energy science. However, most works within the direction of synthetic heterogeneous electrocatalysis focus on water electrolysis and CO2 reduction. In this work, we moved to expand the scope of small molecule electrosynthesis by developing a synthetic scheme which couples CO2 and NH3 at a gas-liquid-solid boundary to produce species with C-N bonds. Specifically, by bringing in CO2 from the gas phase and NH3 from the liquid phase together over solid copper catalysts, we have succeeded in forming formamide and acetamide products for the first time from these reactants. In a subsequent complementary step, we have combined electrochemical analysis and a newly developed operando spectroelectrochemical method, capable of probing the aforementioned gas-liquid-solid boundary, to extract an initial level of mechanistic analysis regarding the reaction pathways of these reactions and the current system's limitations. We believe that the development and understanding of this set of reaction pathways will play significant role in expanding the community's understanding of on-surface electrosynthetic reactions as well as push this set of inherently sustainable technologies towards widespread applicability.

Conductive Metal-Organic Frameworks Bearing M-O4 Active Sites as Highly Active Biomass Valorization Electrocatalysts.

The electrochemical oxidation of the biomass platform 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA), is an important reaction in the emerging area of renewable energy-powered biomass valorization. A key limitation in this field is the ill-defined nature of the catalytic sites of the highest-performing materials that limits the fundamental insights that can be extracted. To this end, a conductive metal-organic framework-based electrocatalytic model system with well-defined M-O4 active sites for electrochemical HMF oxidation was developed. These materials were found to be highly active towards FDCA generation, with product yields of over 95 %. In parallel, infrared spectroscopy was employed to capture a surface-bound aldehyde group as the key intermediate in the catalytic cycle, which forms once M(II/III) oxidation occurs. This work illustrates the advantage of utilizing molecularly defined active sites coupled with operando spectroscopy to provide fundamental insights into a variety of electrosynthetic reactions and thus paves the way for future catalyst design.

Construction of C-N bonds from small-molecule precursors through heterogeneous electrocatalysis.

Energy-intensive thermochemical processes within chemical manufacturing are a major contributor to global CO2 emissions. With the increasing push for sustainability, the scientific community is striving to develop renewable energy-powered electrochemical technologies in lieu of CO2-emitting fossil-fuel-driven methods. However, to fully electrify chemical manufacturing, it is imperative to expand the scope of electrosynthetic technologies, particularly through the innovation of reactions involving nitrogen-based reactants. This Review focuses on a rapidly emerging area, namely the formation of C-N bonds through heterogeneous electrocatalysis. The C-N bond motif is found in many fertilizers (such as urea) as well as commodity and fine chemicals (with functional groups such as amines and amides). The ability to generate C-N bonds from reactants such as CO2, NO3- or N2 would provide sustainable alternatives to the thermochemical routes used at present. We start by examining thermochemical, enzymatic and molecular catalytic systems for C-N bond formation, identifying how concepts from these can be translated to heterogeneous electrocatalysis. Next, we discuss successful heterogeneous electrocatalytic systems and highlight promising research directions. Finally, we discuss the remaining questions and knowledge gaps and thus set the trajectory for future advances in heterogeneous electrocatalytic formation of C-N bonds.

Pushing the methodological envelope in understanding the photo/electrosynthetic materials-microorganism interface.

Biohybrid photo/electrosynthetic systems synergize microbial metabolic pathways and inorganic materials to generate the fuels and chemicals to power our society. They aim to combine the strengths of product selectivity from biological cells and efficient charge generation and light absorption of inorganic materials. However crucial mechanistic questions still remain. In this review we address significant knowledge gaps that must be closed and recent efforts to do so to push biohybrid systems closer to applicability. In particular, we focus on noteworthy advances that have recently been made in applying state-of-the-art analytical spectroscopic, electrochemical, and microelectronic techniques to help pinpoint key complexities of the microbe-materials interface. We discuss the basic function of these techniques, how they have been translated over to study biohybrid systems, and which key insights and implications have been extracted. Finally, we delve into the key advances necessary for the design of next generation biohybrid energy conversion systems.

Rational incorporation of defects within metal-organic frameworks generates highly active electrocatalytic sites.

The allure of metal-organic frameworks (MOFs) in heterogeneous electrocatalysis is that catalytically active sites may be designed a priori with an unparalleled degree of control. An emerging strategy to generate coordinatively-unsaturated active sites is through the use of organic linkers that lack a functional group that would usually bind with the metal nodes. To execute this strategy, we synthesize a model MOF, Ni-MOF-74 and incorporate a fraction of 2-hydroxyterephthalic acid in place of 2,5-dihydroxyterephthalic acid. The defective MOF, Ni-MOF-74D, is evaluated vs. the nominally defect-free Ni-MOF-74 with a host of ex situ and in situ spectroscopic and electroanalytical techniques, using the oxidation of hydroxymethylfurtural (HMF) as a model reaction. The data indicates that Ni-MOF-74D features a set of 4-coordinate Ni-O4 sites that exhibit unique vibrational signatures, redox potentials, binding motifs to HMF, and consequently superior electrocatalytic activity relative to the original Ni-MOF-74 MOF, being able to convert HMF to the desired 2,5-furandicarboxylic acid at 95% yield and 80% faradaic efficiency. Furthermore, having such rationally well-defined catalytic sites coupled with in situ Raman and infrared spectroelectrochemical measurements enabled the deduction of the reaction mechanism in which co-adsorbed *OH functions as a proton acceptor in the alcohol oxidation step and carries implications for catalyst design for heterogeneous electrosynthetic reactions en route to the electrification of the chemical industry.

Probing electrosynthetic reactions with furfural on copper surfaces.

This work entails the integrated use of electrochemistry and operando Raman spectroscopy to probe the reduction of a biomass platform, furfural, to value-added chemicals on Cu electrodes. The results reveal key structural differences of the Cu that dictate selectivity for furfuryl alcohol or 2-methylfuran.

Towards atomic precision in HMF and methane oxidation electrocatalysts.

With the increasing emphasis on transitioning to a sustainable society, electrosynthetic routes to generate fuels and chemicals are rapidly gaining traction. While the electrolysis of water and CO2 has been heavily investigated over the last decade, electrocatalysis of other abundant resources such as biomass and methane is now increasingly coming into focus. As this area is relatively less mature, much work remains to be done. In particular, efforts to decipher reaction mechanisms and extract the fundamental insights are necessary to develop economically competitive electrosynthetic routes using biomass and methane. Against this backdrop, this feature article focuses on the recent developments within the community using atomically precise catalysts, both homogeneous and heterogeneous, as model systems to understand these reactions.

Operando spectroscopy of nanoscopic metal/covalent organic framework electrocatalysts.

Metal and covalent organic frameworks (MOFs and COFs) are increasingly finding exceptional utility in electrocatalytic systems. Their chemically defined porous nature grants them key functions that may enhance their electrocatalytic performance relative to conventional molecular or heterogeneous materials. In order to obtain insights into their function, mechanism, and dynamics under electrocatalytic conditions, operando spectroscopy, that which is performed as the catalyst is functioning, has been increasingly applied. This mini review highlights several key works emerging in recent years that have used various operando spectroscopic techniques, namely UV-vis absorption, Raman, Infrared, and X-ray absorption spectroscopy, to investigate electrocatalytic MOFs and COFs. A brief introduction to each technique and how it was applied to investigate MOF/COF-based electrolytic systems is detailed. The unique set of data obtained, interpretations made, and progress attained all point to the power of operando spectroscopy in truly opening the functionality of MOFs and COFs across many aspects of catalysis.

Speeding up Nanoscience and Nanotechnology with Ultrafast Plasmonics.

Surface plasmons are collective oscillations of free electrons at the interface between a conducting material and the dielectric environment. These excitations support the formation of strongly enhanced and confined electromagnetic fields. As well, they display fast dynamics lasting tens of femtoseconds and can lead to a strong nonlinear optical response at the nanoscale. Thus, they represent the perfect tool to drive and control fast optical processes, such as ultrafast optical switching, single photon emission, as well as strong coupling interactions to explore and tailor photochemical reactions. In this Virtual Issue, we gather several important papers published in Nano Letters in the past decade reporting studies on the ultrafast dynamics of surface plasmons.

Mechanochemical synthesis of cobalt/copper fluorophosphate generates a multifunctional electrocatalyst.

The utilisation of inductive effects is emerging as a powerful tool to enhance material properties. Within the context of electrocatalysis, such effects may alter an active site's electronic structure and consequently, its catalytic activity. To this end, we introduce catalytically active cobalt species within an electron-withdrawing copper fluorophosphate host via a mechanochemical synthetic method. The resulting mixed-metal material features exceptional performance towards electrochemical water oxidation (η of ∼300 mV for 100 mA cm-2) and biomass valorisation (95% selectivity for 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid conversion), thus opening avenues for the rational design of heterogeneous catalysts.

Electrochemically Triggered Dynamics within a Hybrid Metal-Organic Electrocatalyst.

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

Operando vibrational spectroscopy for electrochemical biomass valorization.

Electrocatalysis is a promising route to generate fuels and value-added chemicals from abundant feedstocks powered by renewable electricity. The field of electrocatalysis research has made great progress in supplementing electrocatalyst development with operando vibrational spectroscopic techniques, those carried out simultaneously as the reaction is occurring. Such experiments unveil reaction mechanisms, structure-activity relationships and consequently, accelerate the development of next generation electrocatalytic systems. While operando techniques have now been extensively applied to water electrolysis and CO2 reduction, their application to the emerging area of biomass valorization is rather nascent. The electrocatalytic conversion of biomass can provide an alternate, environmentally friendly route to the chemicals which power our society, but this field still requires much growth before the envisioned technologies are economically competetive with thermochemical routes. Within this context, a growing body of work has begun to translate the methodology and concepts from water/CO2 electrolysis to biomass valorization to elucidate links between catalyst structure, adsorbed surface intermediates, and the resultant catalytic performance. The reactions of interest here include the upgrading of biomass platforms such a 5-hydroxymethylfurfural or glycerol to value-added chemicals. In this feature article we highlight these efforts and provide a critical view on the steps necessary to take to further progress the field. We further show how the knowledge derived from these studies can be translated to a plethora of other organic transformations to forge new avenues in renewable energy electrocatalysis.

Electrochemical biomass valorization on gold-metal oxide nanoscale heterojunctions enables investigation of both catalyst and reaction dynamics with operando surface-enhanced Raman spectroscopy.

The electrochemical oxidation of biomass platforms such as 5-hydroxymethylfurfural (HMF) to value-added chemicals is an emerging clean energy technology. However, mechanistic knowledge of this reaction in an electrochemical context is still lacking and operando studies are even more rare. In this work, we utilize core-shell gold-metal oxide nanostructures which enable operando surface-enhanced Raman spectroelectrochemical studies to simultaneously visualize catalyst material transformation and surface reaction intermediates under an applied voltage. As a case study, we show how the transformation of NiOOH from ∼1-2 nm amorphous Ni layers facilitates the onset of HMF oxidation to 2,5-furandicarboxylic acid (FDCA), which is attained with 99% faradaic efficiency in 1 M KOH. In contrast to the case in 1 M KOH, NiOOH formation is suppressed, and consequently HMF oxidation is sluggish in 10 mM KOH, even at highly oxidizing potentials. Operando Raman experiments elucidate how surface adsorption and interaction dictates product selectivity and how the surface intermediates evolve with applied potential. We further extend our methodology to investigate NiFe, Co, Fe, and CoFe catalysts and demonstrate that high water oxidation activity is not necessarily correlated with excellent HMF oxidation performance and highlight catalytic factors important for this reaction such as reactant-surface interactions and the catalysts' physical and electronic structure. The insights extracted are expected to pave the way for a deepened understanding of a wide array of electrochemical systems such as for organic transformations and CO2 fixation.

Disparity of Cytochrome Utilization in Anodic and Cathodic Extracellular Electron Transfer Pathways of Geobacter sulfurreducens Biofilms.

Extracellular electron transfer (EET) in microorganisms is prevalent in nature and has been utilized in functional bioelectrochemical systems. EET of Geobacter sulfurreducens has been extensively studied and has been revealed to be facilitated through c-type cytochromes, which mediate charge between the electrode and G. sulfurreducens in anodic mode. However, the EET pathway of cathodic conversion of fumarate to succinate is still under debate. Here, we apply a variety of analytical methods, including electrochemistry, UV-vis absorption and resonance Raman spectroscopy, quartz crystal microbalance with dissipation, and electron microscopy, to understand the involvement of cytochromes and other possible electron-mediating species in the switching between anodic and cathodic reaction modes. By switching the applied bias for a G. sulfurreducens biofilm coupled to investigating the quantity and function of cytochromes, as well as the emergence of Fe-containing particles on the cell membrane, we provide evidence of a diminished role of cytochromes in cathodic EET. This work sheds light on the mechanisms of G. sulfurreducens biofilm growth and suggests the possible existence of a nonheme, iron-involving EET process in cathodic mode.

Host-Guest Chemistry Meets Electrocatalysis: Cucurbit[6]uril on a Au Surface as a Hybrid System in CO2 Reduction.

The rational control of forming and stabilizing reaction intermediates to guide specific reaction pathways remains to be a major challenge in electrocatalysis. In this work, we report a surface active-site engineering approach for modulating electrocatalytic CO2 reduction using the macrocycle cucurbit[6]uril (CB[6]). A pristine gold surface functionalized with CB[6] nanocavities was studied as a hybrid organic-inorganic model system that utilizes host-guest chemistry to influence the heterogeneous electrocatalytic reaction. The combination of surface-enhanced infrared absorption (SEIRA) spectroscopy and electrocatalytic experiments in conjunction with theoretical calculations supports capture and reduction of CO2 inside the hydrophobic cavity of CB[6] on the gold surface in aqueous KHCO3 at negative potentials. SEIRA spectroscopic experiments show that the decoration of gold with the supramolecular host CB[6] leads to an increased local CO2 concentration close to the metal interface. Electrocatalytic CO2 reduction on a CB[6]-coated gold electrode indicates differences in the specific interactions between CO2 reduction intermediates within and outside the CB[6] molecular cavity, illustrated by a decrease in current density from CO generation, but almost invariant H2 production compared to unfunctionalized gold. The presented methodology and mechanistic insight can guide future design of molecularly engineered catalytic environments through interfacial host-guest chemistry.

Operando Raman probing of electrocatalytic biomass oxidation on gold nanoparticle surfaces.

Electrocatalytic conversion of biomass-derived intermediates is a green route to value-added chemicals. However, this technology is just emerging and the mechanisms of this process are not fully resolved. Here, we present the first operando Raman spectroscopic investigation of 5-hydroxymethylfurfural oxidation on gold nanoparticle surfaces, opening up avenues for understanding such reactivity and for rational systems design.

Probing CO2 Conversion Chemistry on Nanostructured Surfaces with Operando Vibrational Spectroscopy.

With the rising emphasis on renewable energy research, the field of electrocatalytic CO2 conversion to fuels has grown tremendously in recent years. Advances in nanomaterial synthesis and characterization have enabled researchers to screen effects of elemental composition, size, and surface chemistry on catalyst performance. However, direct links from structure and active state to catalytic function are difficult to establish. To this end, operando spectroscopic techniques, those conducted simultaneously as catalysts operate, can provide key complementary information by investigating electrocatalysis under turnover conditions. In particular, Raman and infrared spectroscopy have the potential to reveal the identity of surface-bound intermediates, catalyst active state, and possible reaction sites to supplement the insights extracted from conventional electrochemistry. Such research aims to work in tandem synthetic and catalytic efforts to guide the development of next-generation CO2 electrocatalytic systems through rational design. In this Mini Review, we examine the latest developments in the operando probing of electrochemical CO2 reduction on nanostructured electrocatalysts and detail how this research accelerates the advancement of this field.