Spectroelectrochemical Investigation Research Articles

Time-resolved Spectroelectrochemical Investigation of Organic Mixed Conductors.

Organic mixed ionic and electronic conductors (OMIECs) are an emerging class of materials that have been applied for a wide range of electrochemical applications. Due to the complexity inherent to the ionic-electroniccoupling, understanding the underlying mechanisms involved in the OMIEC operation is an exciting and very lively research field. In this work, we highlight the use of time-resolved Vis-NIR spectroelectrochemistry tocharacterize these materials. We discuss an example, where we show that by combining this tool with spectraldecomposition, we are able to understand fundamental aspects of the doping in an OMIEC film. The methodswe present here can be generalized and used to characterize any electrochromic material.

Copper(II)-Assisted Degradation of Pheophytin a by Reactive Oxygen Species.

The central ion Mg2+ is responsible for the differences between chlorophyll a and its free base in their reactivity toward metal ions and thus their resistance to oxidation. We present here the results of spectroscopic (electronic absorption and emission, circular dichroism, and electron paramagnetic resonance), spectroelectrochemical, and computational (based on density functional theory) investigations into the mechanism of pheophytin, a degradation that occurs in the presence of Cu ions and O2. The processes leading to the formation of the linear form of tetrapyrrole are very complex and involve the weakening of the methine bridge due to an electron withdrawal by Cu(II) and the activation of O2, which provides protection to the free ends of the opening macrocycle. These mechanistic insights are related to the naturally occurring damage to the photosynthetic apparatus of plants growing on metal-contaminated soils.

Monitoring SEIRAS on a Graphitic Electrode for Surface-Sensitive Electrochemistry: Real-Time Electrografting.

The ubiquity of graphitic materials in electrochemistry makes it highly desirable to probe their interfacial behavior under electrochemical control. Probing the dynamics of molecules at the electrode/electrolyte interface is possible through spectroelectrochemical approaches involving surface-enhanced infrared absorption spectroscopy (SEIRAS). Usually, this technique can only be done on plasmonic metals such as gold or carbon nanoribbons, but a more convenient substrate for carbon electrochemical studies is needed. Here, we expanded the scope of SEIRAS by introducing a robust hybrid graphene-on-gold substrate, where we monitored electrografting processes occurring at the graphene/electrolyte interface. These electrodes consist of graphene deposited onto a roughened gold-sputtered internal reflection element (IRE) for attenuated total reflectance (ATR) SEIRAS. The capabilities of the graphene-gold IRE were demonstrated by successfully monitoring the electrografting of 4-amino-2,2,6,6-tetramethyl-1-piperidine N-oxyl (4-amino-TEMPO) and 4-nitrobenzene diazonium (4-NBD) in real time. These grafts were characterized using cyclic voltammetry and ATR-SEIRAS, clearly showing the 1520 and 1350 cm-1 NO2 stretches for 4-NBD and the 1240 cm-1 C-C, C-C-H, and N-Ȯ stretch for 4-amino-TEMPO. Successful grafts on graphene did not show the SEIRAS effect, while grafting on gold was not stable for TEMPO and had poorer resolution than on graphene-gold for 4-NBD, highlighting the uniqueness of our approach. The graphene-gold IRE is proficient at resolving the spectral responses of redox transformations, unambiguously demonstrating the real-time detection of surface processes on a graphitic electrode. This work provides ample future directions for real-time spectroelectrochemical investigations of carbon electrodes used for sensing, energy storage, electrocatalysis, and environmental applications.

Probing the Electronic Structure of Dinuclear Carbon-Rich Complexes Containing an Octa-3,5-diene-1,7-diyndiyl Bridging Ligand

One electron oxidation of the monometallic alkenylacetylide complexes [Ru{C≡CC(R)=CH2}(dppe)Cp*] (1) and [Ru{C≡CC(R)=CH2}Cl(dppe)2] (2) (R = Ph (a); R = 4-MeS-C6H4 (b)) generates in each case a dinuclear bis(allenylidene) complex [{Ru}2{μ-C=C=C(R)–CH2–H2C–(R)C=C=C}][PF6]2 ({Ru} = Ru(dppe)Cp* ([3a,b][PF6]2); {Ru} = RuCl(dppe)2 ([4a,b][PF6]2), containing an unsaturated ethane bridge between both allenylidene moieties. Deprotonation of this ethane bridge results in the formation of the previously reported octa-3,5-diene-1,7-diyndiyl-bridged bimetallic species [{Ru}2{μ-C≡CC(R)=CH–HC=(R)CC≡C}] ({Ru} = Ru(dppe)Cp* (5a,b); {Ru} = RuCl(dppe)2 (6a,b). The isolation of these complexes illustrates a general synthetic route to these conjugated bimetallic species from monomeric alkenylacetylide precursors. Electrochemical and spectroelectrochemical investigations evince the ready formation of the representative redox series [5a]n+, and TD-DFT calculations performed on optimised structures featuring the simplified {Ru(dmpe)Cp} coordination sphere [{Ru(dmpe)Cp}2{μ-C≡CC(Ph)=HC–CH(Ph)CC≡C}]n+ ([5a†]n+) (n = 0, 1, 2) reveal significant delocalisation of the unpaired charge in the formally mixed-valent species (n = 1), consistent with Class III assignment within the Robin–Day classification scheme.

In Situ Raman Spectroelectrochemical Investigation of Composite Si Nanoparticle-Based Anode for Li-Ion Batteries during (de)Lithiation Process

The key degradation processes in the composite anode for the Li-ion batteries (LIBs) prepared using Si nanoparticles (SiNPs) with two types of a conductive carbon-based matrix [carbon black (CB) and carbonized polypyrrole (CPPy)] were studied by in situ Raman spectroelectrochemistry (SEC). This combined technique provides non-destructive and real-time monitoring of the chemical and structural changes that occur during battery operation. These processes, such as the crystal lattice changes (expansion/contraction) and possible degradation/amorphization of silicon, the solid electrolyte interphase (SEI) layer formation on the electrode surface during the charge/discharge reactions, and the stability of the carbon-based matrix, significantly affect the performance of Si-based LIBs in many ways. Especially, the large silicon volume expansion (more than 300%) and associated stress cause mechanical instability resulting in rapid capacity fading. To overcome this challenge, various strategies have been proposed, including the use of CPPy as a conductive matrix to prevent large volumetric changes and also provide a pathway for electron transfer.This study investigates the initial degradation mechanisms such as the formation of the SEI layer, as well as possible structural changes caused by the volume expansion of SiNPs. The shift of the first-order Raman peak of Si at 521 cm-1 which is assigned to crystalline silicon is related to the stress evolution in nanoparticles caused by the (de)lithiation-induced stress (tensile-to-compressive transition) in SiNPs and the native oxide on their surface. Additionally, the detailed in situ Raman measurement of the first lithiation cycle allowed us to detect the decomposition of the electrolyte (LiPF6 in EC/DMC) close to the Si surface which is associated with SEI layer formation. A decrease in the intensity of the Raman vibrational modes (700–1050 cm−1) of EC/DMC corresponding to the decomposition of the electrolyte was observed at reduction potentials of 0.8 V and 0.3 V vs. Li/Li+ corresponding to the same potentials determined by cyclic voltammetry. The Si-based anode containing CPPy as active carbon showed better electrochemical properties (higher charge/discharge capacity and cycling stability) in Li-ion batteries than Si/CB despite the higher conductivity of the latter. The correlation between the chemical structure of the active carbon and the electrochemical properties of the resulting Si-based anodes will be discussed.Acknowledgment: This work was supported by the Czech National Foundation, Project No. 21–09830S.

Cobalt di-selenide embedded polymerised melamine paraformaldehyde resin: A robust and efficient electrocatalyst for OER with operando investigations

In past research metal selenides have reported versatile results in the field of water splitting. Cobalt di-selenide has received much attention in the oxygen evolution reaction (OER) due to its excellent results towards OER. To excel the reaction capability, herein we have reported CoSe2, pMF-R and CoSe2@pMF-R nanostructures in which the freshly prepared CoSe2 nanostructures were decorated on the surface of pMF-R (polymerised melamine paraformaldehyde resin). The study showed that the bonds between cobalt and nitrogen were well formed, which is maintaining the improved charge transfer channels to annihilate the barriers between the GC (glassy carbon) electrode surface and active sites of CoSe2. Because of the coaction of CoSe2 and pMF-R, the OER calibration of the CoSe2@pMF-R catalyst has shown a low onset potential 1.56 V and its electrochemical activity towards OER was found to be much higher as 10 mAcm−2 at 1.608 V Vs. RHE. Operando spectroelectrochemical investigations revealed that active sites of selenium, Co2+, Co3+ and pMF-R available for generation of intermediate oxygen species and responsible for exponential increase in rate of OER. The order of reaction is found to one. The compositional and structural characteristics of CoSe2@pMF-R composite over other electrocatalysts is responsible for outstanding electrochemical performance.

Interfacial Coupling of Graphene with Nickel Nanoparticles for Water Splitting and Urea Oxidation: A Spectroelectrochemical Investigation.

Nickel nanoparticle and graphene interfaces of various stoichiometries were created through electrodeposition techniques. The catalytic behavior of the electrodeposited films was investigated through spectro-electrochemical methodologies. UV-vis absorbance spectra of the electrodeposited films are significantly different in the air and alkaline medium. Furthermore, UV-vis and Raman spectroscopy confirmed the coupling of Ni nanoparticles (Ni-NP) with the graphene framework, along with NiO and Ni(OH)2 . A combination of Raman and impedance spectroscopy revealed that the surface adsorption and charge transfer properties of the electrodeposited films are entirely dependent on the defects on graphene structure as well as distribution of Ni-NP on graphene. The electrodeposited films possess heterogeneous catalytic properties with a low overpotential of 50 mV (10 mA/cm-2 ) for hydrogen evolution reaction, as well as 601 mV and 391 mV (at 50 mA/cm-2 ) for the oxygen evolution reaction and urea oxidation reaction, respectively. In addition, eelectrodeposited samples show extraordinary overall water splitting performance by achieving a current density of 10 mA/cm2 at a very low applied potential of 1.38 V. This synergistic coupling of Ni and graphene renders the electrodeposited samples promising candidates as electrodes for overall water splitting in alkaline and urea-supplemented solutions.

A Bimetallic Benzene-1,2,4,5-Tetrathiolate (btt) Molybdenocene Complex Cp2 Mo(btt)MoCp2 : Radical and Diradical States.

Benzene-1,2,4,5-tetrathiolate (btt) has been used as a bridging ligand to prepare a redox active (molybdenocene dithiolene)-based bimetallic complex Cp2 Mo(btt)MoCp2 , which exhibits four successive electron transfers up to the tetracation. Spectro-electrochemical investigations together with DFT and TD-DFT calculations evidence that the two electroactive MoS2 C2 metallacycles are electronically coupled in the monocationic as in the dicationic state. Two salts of the dication [Cp2 Mo(btt)MoCp2 ]2+ have been structurally characterized with PF6 - and HSO4 - counterions, showing different chair or boat conformations associated with variable folding angles of the two MoS2 C2 metallacycles along the S-S hinge. The bis-oxidized dicationic complex exhibits a diradical character, with both radicals essentially localized on the metallacycles and with antiferromagnetic coupling evidenced from magnetic susceptibility measurements.

Importance of Ligand Exchange in the Modulation of Molecular Catalysis: Mechanism of the Electrochemical Reduction of Nitrous Oxide with Rhenium Bipyridyl Carbonyl Complexes

Molecular catalysis of electrochemical reactions involving transition-metal complexes as catalysts requires getting a free metal coordination site to bind the substrate. It implies that the generation of a strong coordinating ligand as a product or coproduct of the reaction might be detrimental for an efficient catalysis because it can bind the metal center and block or slow down the catalytic process. This self-modulation phenomenon is revealed and illustrated via a thorough spectro-electrochemical investigation of the mechanism of the electrochemical reduction of nitrous oxide with rhenium bipyridyl triscarbonyl complexes [Re(bpy)(CO)3X]n+ (X = CH3CN, Cl–, n = 0 or 1) as a catalyst. We show that the bi-reduced [Re0(bpy•–)(CO)3]−, electrogenerated from [Re(bpy)(CO)3X]n+, readily reacts with N2O and produces the hydroxo complex [ReI(bpy)(CO)3(OH)]. Because hydroxide, a product of the reaction, is a stronger coordinating ligand than acetonitrile or chloride, catalysis does not occur significantly at a potential where [Re0(bpy•–)(CO)3]− is generated from [Re(bpy)(CO)3X]n+. Substantial catalysis is only triggered at a potential corresponding to the second reduction of [ReI(bpy)(CO)3(OH)]. Nonetheless, we show that a slower innersphere reduction of N2O by the mono-reduced [ReI(bpy•–)(CO)3X](n−1)+ (X = CH3CN, Cl–) occurs due to the lability of acetonitrile and chloride in these species. Because hydroxide is less labile and cannot be displaced to create an open coordination position for N2O, only an even slower outersphere reduction of N2O by the mono-reduced [ReI(bpy•–)(CO)3(OH)]− takes place. However, we finally show that an excess of free chlorides is able to displace hydroxide and then open the way for a faster innersphere process. This remarkable example emphasizes the critical role of ligand exchange in modulating molecular catalysis of electrochemical reactions with transition-metal complexes as catalysts, a likely general phenomenon.

Spectroelectrochemical Investigation of the Local Alkaline Environment on the Surface-Nanostructured Au for the Conversion of CO2 to CO

The electrocatalytic conversion of carbon dioxide (CO2) into fuels could potentially achieve a sustainable carbon-based economy. The engineering of nanostructured metal electrodes can enhance their activity and selectivity by controlling their local chemical environment; however, direct observation is challenging. In this study, we investigate the molecular-level reaction mechanism of a nanoporous-structured Au electrode for the conversion of CO2 to carbon monoxide (CO) using surface-enhanced infrared absorption spectroscopy (SEIRAS). We designed a well-structured nanoporous Au layer (with a depth distribution of 56.3 nm) on a Si prism using a high-temperature non-aqueous anodization process and characterized the nanoporous Au electrode using atomic force microscopy (AFM) and X-ray absorption spectroscopy (XAS). The in situ SEIRAS results demonstrated that the nanoporous Au electrode has a dominant active site, promoting the linear CO intermediate and suppressing the bridging CO intermediate when compared with the non-structured Au electrode. We also revealed a high local pH at a reaction potential of −0.9 V and the slow diffusion kinetics of local CO32– at an open-circuit potential. These findings provide deeper insights into the electrochemical kinetics and corresponding mechanisms occurring in the electric double layers and highlight the potential for the design of efficient electrocatalysts for CO2 reduction.

Synthesis and Spectroelectrochemical Investigation of Anodic Black TiOx Nanotubes

Anodic TiO2 nanotubes were transformed into anatase at 400 °C for 2 h in air and subjected to electrochemical reduction at different conditions. It revealed that the reduced black TiOx nanotubes were not stable in contact with air; however, their lifetime was considerably extended to even a few hours when isolated from the influence of atmospheric oxygen. The order of polarization-induced reduction and spontaneous reverse oxidation reactions were determined. Upon irradiation with simulated sunlight, the reduced black TiOx nanotubes generated lower photocurrents than non-reduced TiO2, but a lower rate of electron-hole recombination and better charge separation were observed. In addition, the conduction band edge and energy level (Fermi level), responsible for trapping electrons from the valence band during the reduction of TiO2 nanotubes, were determined. The methods presented in this paper can be used for determination of the spectroelectrochemical and photoelectrochemical properties of electrochromic materials.

Red Light Absorption of [ReI(CO)3(α-diimine)Cl] Complexes through Extension of the 4,4'-Bipyrimidine Ligand's π-System.

Rhenium(I) complexes of type [Re(CO)3(NN)Cl] (NN = α-diimine) with MLCT absorption in the orange-red region of the visible spectrum have been synthesized and fully characterized, including single crystal X-ray diffraction on two complexes. The strong bathochromic shift of MLCT absorption was achieved through extension of the π-system of the electron-poor bidiazine ligand 4,4'-bipyrimidine by the addition of fused phenyl rings, resulting in 4,4'-biquinazoline. Furthermore, upon anionic cyclization of the twisted bidiazine, a new 4N-doped perylene ligand, namely, 1,3,10,12-tetraazaperylene, was obtained. Electrochemical characterization revealed a significant stabilization of the LUMO in this series, with the first reduction of the azaperylene found at E1/2(0/-) = -1.131 V vs. Fc+/Fc, which is the most anodic half-wave potential observed for N-doped perylene derivatives so far. The low LUMO energies were directly correlated to the photophysical properties of the respective complexes, resulting in a strongly red-shifted MLCT absorption band in chloroform with a λmax = 586 nm and high extinction coefficients (ε586nm > 5000 M-1 cm-1) ranging above 700 nm in the case of the tetraazaperylene complex. Such low-energy MLCT absorption is highly unusual for Re(I) α-diimine complexes, for which these bands are typically found in the near UV. The reported 1,3,10,12-tetraazaperylene complex displayed the [Re(CO)3(α-diimine)Cl] complex with the strongest MLCT red shift ever reported. UV-Vis NIR spectroelectrochemical investigations gave further insights into the nature and stability of the reduced states. The electron-poor ligands explored herein open up a new path for designing metal complexes with strongly red-shifted absorption, thus enabling photocatalysis and photomedical applications with low-energy, tissue-penetrating red light in future.

Donor-Acceptor Dyads and Triads Employing Core-Substituted Naphthalene Diimides: A Synthetic and Spectro (Electrochemical) Study.

Donor-acceptor dyads and triads comprising core-substituted naphthalene diimide (NDI) chromophores and either phenothiazine or phenoxazine donors are described. Synthesis combined with electrochemical and spectroelectrochemical investigations facilitates characterisation of the various redox states of these molecules, confirming the ability to combine arrays of electron donating and accepting moieties into single species that retain the redox properties of these individual moieties.

Microfluidic devices for photo-and spectroelectrochemical applications

The review presents recent developments in electrochemical devices for photo- and spectroelectrochemical investigations, with the emphasis on miniaturization (i.e., nanointerdigitated complementary metal-oxide-semiconductor devices, micro- and nano-porous silicon membranes or microoptoelectromechanical systems), silica glass/microreactors (i.e., plasmonic, Raman spectroscopy or optical microcavities) or polymer-based devices (i.e., 3D-printed, laser-engraved channels). Furthermore, we have evaluated inter alia the efficiency of various fabrication approaches for bioelectrochemical systems, biocatalysis, photochemical synthesis, or single nanoparticle spectroelectrochemistry. We envisioned the miniaturization of applied techniques such as cathodoluminescence, surface plasmon resonance, surface-enhanced Raman spectroscopy, voltametric and amperometric methods in the spectroelectrochemical microdevices. The research challenges and development perspectives of microfluidic, and spectroelectrochemical devices were also elaborated on.

Multifunctional nanopore electrode array method for characterizing and manipulating single entities in attoliter-volume enclosures

Structurally regular nanopore arrays fabricated to contain independently controllable annular electrodes represent a new kind of architecture capable of electrochemically addressing small collections of matter—down to the single entity (molecule, particle, and biological cell) level. Furthermore, these nanopore electrode arrays (NEAs) can also be interrogated optically to achieve single entity spectroelectrochemistry. Larger entities such as nanoparticles and single bacterial cells are investigated by dark-field scattering and potential-controlled single-cell luminescence experiments, respectively, while NEA-confined molecules are probed by single molecule luminescence. By carrying out these experiments in arrays of identically constructed nanopores, massively parallel collections of single entities can be investigated simultaneously. The multilayer metal–insulator design of the NEAs enables highly efficient redox cycling experiments with large increases in analytical sensitivity for chemical sensing applications. NEAs may also be augmented with an additional orthogonally designed nanopore layer, such as a structured block copolymer, to achieve hierarchically organized multilayer structures with multiple stimulus-responsive transport control mechanisms. Finally, NEAs constructed with a transparent bottom layer permit optical access to the interior of the nanopore, which can result in the cutoff of far-field mode propagation, effectively trapping radiation in an ultrasmall volume inside the nanopore. The bottom metal layer may be used as both a working electrode and an optical cladding layer, thus, producing bifunctional electrochemical zero-mode waveguide architectures capable of carrying out spectroelectrochemical investigations down to the single molecule level.

Electrochemical SEIRAS Analysis of Imidazole-Ring-Functionalized Self-Assembled Monolayers.

An essential amino acid, histidine, has a vital role in the secondary structure and catalytic activity of proteins because of the diverse interactions its side chain imidazole (Im) ring can take part in. Among these interactions, hydrogen donating and accepting bonding are often found to operate at the charged interfaces. However, despite the great biological significance, hydrogen-bond interactions are difficult to investigate at electrochemical interfaces due to the lack of appropriate experimental methods. Here, we present a surface-enhanced infrared absorption spectroscopy (SEIRAS) and density functional theory (DFT) study addressing this issue. To probe the hydrogen-bond interactions of the Im at the electrified organic layer/water interface, we constructed Au-adsorbed self-assembled monolayers (SAMs) that are functionalized with the Im group. As the prerequisite for spectroelectrochemical investigations, we first analyzed the formation of the monolayer and the relationship between the chemical composition of SAM and its structure. Infrared absorption markers that are sensitive to hydrogen-bonding interactions were identified. We found that negative electrode polarization effectively reduced hydrogen-bonding strength at the Im ring at the organic layer–water interface. The possible mechanism governing such a decrease in hydrogen-bonding interaction strength is discussed.

Spectro-Electrochemical Investigations of Interfacial Phenomena in Concentrated Hydrogen-Bonded Electrolytes for Electrochemical Energy Storage

Spectro-electrochemistry combines spectroscopic and electrochemical techniques, enabling time-resolved and in-situ measurements of phenomena occurring at charged electrode surfaces. Surface-enhanced Raman spectroscopy (SERS) yields highly specific information about the structure and composition of molecules. Coupling SERS with electrochemical experiments gives insight into changes near the electrode surface under polarization. In-situ SERS and electrochemical measurements are demonstrated to provide information on the electrolyte-electrode interface in three different applications: (1) specific ion adsorption in hydrogen-bonded concentrated electrolytes under polarization, (2) adsorption and charge-transfer mechanisms of redox-active organic in aqueous electrolytes, and (3) charge-transfer reactions of redox-active species in hydrogen-bonded electrolytes.Differential capacitance measurements suggest that surface adsorption is prevalent in deep eutectic solvents (DESs) and more broadly concentrated hydrogen-bonded electrolytes, however, these measurements do not directly probe the surface layer. Therefore, in this study, SERS was employed to investigate potential-dependent changes in the surface layer of the electrode-electrolyte interface where surface adsorption was spectroscopically confirmed. However, this behavior was observed to depend on the charge density and the strength of hydrogen bonding.In situ SERS coupled with electrochemical measurements was also used to probe redox reactions of redox-active molecules at the electrode-electrolyte interface. One study elucidated an adsorption and charge-transfer mechanism for a potential redox-active organic molecule, 4–hydroxy–2,2,6,6–tetramethylpiperidine–1–oxyl (4–hydroxy–TEMPO).1 Nitroxide radicals, such as 4–hydroxy–TEMPO, are of interest for application in redox flow batteries due to their relative stability, and tunability through functionalization. In situ SERS combined with density functional theory (DFT) simulations confirmed the presence of surface–adsorbed species on an Au electrode during the oxidation of 4–hydroxy–TEMPO in an aqueous electrolyte. Direct spectroscopic evidence shows the oxidation of 4–hydroxy–TEMPO leads to the adsorption of the oxidation product, which then undergoes slow rate-limiting desorption from the electrode surface. This information is important for the design of redox flow battery systems that employ nitroxide radical redox-active species, as surface adsorption can affect battery performance.In an H-bonded electrolyte containing redox-active species (methyl viologen and ferrocenedimethanol in ChCl:EG), SERS, electrochemical, and spectrochemical UV-vis techniques were used to study their charge-transfer reactions. UV-vis measurements provide molecular information related to the electronic levels of molecules. UV-vis coupled with SERS allows for the study of changes in the oxidation state of redox-active molecules under electrode polarization.This work demonstrates the importance of spectrochemical measurements in determining phenomena at the electrode-electrolyte interface, particularly in a complex solvent system such as DESs. Emphasis is placed on the coupling spectroscopic techniques with other techniques to determine interfacial structuring and charge-transfer reactions relevant to energy storage applications.(1) Shaheen, N. A.; Ijjada, M.; Vukmirovic, M. B.; Akolkar, R. Mechanism of Electrochemical Oxidation of Nitroxide Radicals in Ethaline Deep Eutectic Solvent. Journal of The Electrochemical Society 2020, 167 (14), 143505–143505. https://doi.org/10.1149/1945-7111/abc439.

(Invited) Spectroelectrochemical Investigation of Oxygen Electrocatalysis on Metal Oxides

Improving the kinetics of oxygen electrocatalysis is key to increasing the efficiency of hydrogen production from renewable sources, production of carbon-neutral fuels such as ethylene and rechargeable metal-air batteries1. Metal oxides exhibit state-of-the-art activity, but fundamental atomic-level insights into the reaction mechanism are often unknown. Particularly, various differences between materials, including the differences in active surface area, chemical state of the metal cations, fractional coverage of oxidized species and the range of ordered structure, renders it difficult to identify the origin of the differing activity. Thus far, ambiguities in measuring the number of sites participating in the reaction have prevented the accurate measurement of intrinsic catalytic activity, or turnover frequency. Furthermore, while recent studies have indicated that the density of oxidized species can enhance reaction rates on metal oxides surfaces via cooperative effects between adjacent adsorbates2-4, extending these mechanistic implications to a range of oxides remains a challenge.In this talk, I will present developments in time-resolved optical spectroscopy to identify the density of different oxidized species as a function of potential and establish how this controls the reaction kinetics. These results will be combined with (i) X-ray absorption spectroscopy to measure the oxidation state and coordination of the active site (ii) time of flight secondary ion mass spectrometry to measure the depth of redox active states5 and (iii) on chip electrochemical mass spectrometry to measure the degree of lattice oxygen participation. As an example, a range of iridium-based catalysts – namely monolayers of molecular iridium dimers6, amorphous7 and crystalline oxides will be compared for the water oxidation reaction. For all the catalysts investigated, three redox transitions can be observed, and the physical origin of these redox processes can be assigned using density functional theory studies. Although similar oxidized species are found to accumulate at water oxidation potentials, the correlation between the density of oxidized species and water oxidation kinetics is very different. On molecular catalysts, there is limited interaction between isolated iridium centres, and thus the intrinsic activity per oxidized site is invariant with potential5. On the contrary, for heterogeneous oxide catalysts, a high degree of cooperative effects results in faster kinetics with increasing accumulation of oxidized species on the surface. The potential for accumulation for oxidized species and the degree of interaction of these oxidized species will be compared for the amorphous and crystalline oxides. Therefore, through this work, I will highlight the power of operando time-resolved spectroscopy in unravelling the critical role of oxidized species in facilitating water oxidation kinetics.The author would like to acknowledge the funding and technical support from BP through the BP International Centre for Advanced Materials (bp-ICAM), which made this research possible.

Tuning the main electrochromic features by polymer backbone variation of triphenylamine-based polyamides

A novel triphenylamine (TPA)-based diamine integrating structural elements to endow highly stable redox and electrochromic features was successfully synthesized and structurally characterized. Three new TPA-containing polyamides were obtained via polycondensation of this diamine with acid dichlorides, as confirmed by thorough NMR and FT–IR investigations. The main electronic transitions and specific oxidation sites were evaluated by photo-optical and (time-dependent) density functional theory ((TD)– DFT) analysis, respectively. Cyclic voltammetry and spectroelectrochemical investigations were used to assess the redox stability and to comprehend the electrochromic conduct in terms of electrochromic contrast, response time, and switching stability. Thus, the synthesized polyamides integrated high transmittance variation up to 67%, response time to coloring and blanching down to 6.52 s/2.01 s, a maximum of 219 cm2/C of the electrochromic efficiency (EC) and a decay up to 5% after 500 cycles. All these electrochromic parameters and their variation are contingent to the electronic effect of each acid dichloride segment from the polymers. Also, the presence of the ortho-catenated methyl groups lowered the overall oxidation potentials and therefore triggered long-term stability. Computational evaluations were used to endorse practical assessments. All experimental and theoretical observations were subjected to interconnected reasoning to determine the correlations between structural motifs and resulting electrochromic performances.

Electronic interaction between two fluorenyl-bridged molybdenocene dithiolene electroactive centers

A bimetallic metal complex involving a fluorene linker between two redox active bis(cyclopentadienyl)molybdenum dithiolene moieties has been synthesized and characterized by electrochemistry and spectroelectrochemistry. Cyclic voltammetry experiments show sequential oxidation of the two redox centers with small ΔE values between the first and the second redox processes and between the third and the fourth ones. Comparative electrochemical studies carried out using either [NBu4][PF6] or the poorly coordinating supporting electrolyte [Na][B(C6H4(CF3)2)4] in dichloromethane indicate strong electrostatic effects on consecutive electron transfers, especially for highly charged species. Spectroelectrochemical investigations together with DFT calculations evidence that the two molybdenocene dithiolene electrophores are weakly electronically coupled through the organic bridge.