A numerical (finite element) model has been developed to simulate the proton coupled electron transfer (PCET) reaction for surface bound quinone species. It is easily adaptable to a wide range of PCET systems. Employing a user-friendly interface we show how factors such as scan rate, pH, buffer capacity, quinone pKa and surface coverage, along with protonation and electron transfer rate constant(s) can impact simulated voltammograms. It is further demonstrated how the model can be used to assess the validity of Laviron plots when applied to PCET reactions for the determination of apparent electron transfer (ET) rate constants. The extracted ET rate constants from simulated voltammograms of peak potential versus log scan rate are compared against those obtained from analytical expressions derived for use with PCET reactions. For the (1-electron, 1-proton) 1e-1H+ system, when a fast rate of proton transfer is employed, typical of aqueous systems, there is very good agreement between the ET rate constants extracted using the two methods. In contrast, for the case of 2e-2H+ PCET, good agreement is not seen and highlights for the more complex system that even when the rate of protonation is high, slower rates of deprotonation can result (and vice versa), moving the system away from being ET rate-limited. Hence Laviron analysis should not be applied quantitatively to 2e-2H+ PCET systems. The simulation model is also employed to offer practical solutions for use of quinone functionalised electrodes in the voltammetric sensing of solution pH. Experimentally, deviations from 2e-2H+ PCET Nernstian behaviour are typically seen in unbuffered solutions with monolayer coverage electrodes due to local depletion (or accumulation) of protons during voltammetry. By simulating the interfacial proton concentrations at the electrode surface as a function of quinone surface coverage, it is possible to determine the required reduction in surface coverage (ca. two orders of magnitude) to negate such effects.

Gaining insight into the early stages of the electrochemical dissolution of metal nanoparticles (NPs) provides crucial insights into mechanisms that control this important process. Being able to do this under conditions where atom loss from individual NPs can be quantified is especially challenging. Here, we use identical-location, annular dark field, scanning transmission electron microscopy (IL-ADF-STEM) to provide "time-stamped" snapshots of the dissolution of gold NPs on electron-transparent carbon electrodes. Dissolution is carried out in aqueous chloride (mM) solutions, at anodic potentials, over millisecond timescales. IL-ADF-STEM analysis of the integrated image intensities is employed to estimate the number of atoms within each NP, allowing atom loss (and gain) to be tracked for the same NP, over time, on a particle-by-particle basis. 3D reconstruction of NPs enables changes in 3D morphology to be visualized. Hemispherical-shaped gold NPs ≤4 nm in diameter are interrogated, with the smallest showing the largest atom loss. NPs are revealed to flatten during dissolution, as opposed to a gradual reduction in diameter, and the number of isolated gold atoms on the surface increases. Considerable interactions between NPs also occur, including the formation of single atom bridges and coalescence events. The vertical growth of NPs is also observed.

Electrochemical dealloying is a promising technique to tune the activity and material utilization of electrocatalysts. Studying this process in bimetallic nanoparticles remains challenging since their synthesis yields ensembles with varied properties. To understand the dealloying behavior of individual nanoparticles, we present a combined characterization method that provides statistical distributions in composition, morphology, and catalytic activity. AgxAuy alloy nanoparticles synthesized via reverse micelles were dealloyed, by potential cycling, to tune their activity for the hydrogen evolution reaction. In a multiscale approach, we performed macroscale measurements on a glassy carbon electrode and scanning electrochemical cell microscopy (SECCM) on small groups of particles/aggregates within the confines of the scanned droplet. The SECCM pipet was used to create different states of dealloying within a single sample, followed by higher-resolution SECCM activity mapping. The faster mass transport in the SECCM tip, compared to the macroelectrode configuration, led to accelerated Ag dissolution from the AgxAuy nanoparticles. While the particles showed the highest activity and largest Ag content decrease after only 0.5 cycles, heterogeneous behavior was still evident among individual particle groups. Further, using a boron-doped diamond electrode, identical-location scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy studies allowed Ag leaching and particle shrinking to be followed at the single-particle level, as a function of cycle number, for a statistically relevant sample number (n > 70). Increased cycling induced particle deactivation, shrinking, and coalescence. Overall, our results showcase an efficient and versatile route to combine systematic tuning and high-throughput screening of nanocatalyst property-activity relationships.

3D printed electrodes – carbon composite electrodes - are growing in popularity but characterization studies are typically limited to voltammetric measurements, with peak to peak separation, for fast electron transfer redox couples, being used as a key metric of performance and properties. To fully understand voltammetric responses investigation of the nano- to microscale origins is critical. In this study, carbon black in poly lactic acid (PLA) electrodes, printed using fused deposition modeling and mechanically polished prior to use are investigated using cyclic voltammetry (CV), conducting atomic force microscopy (C-AFM) and scanning electrochemical cell microscopy (SECCM). Thermal gravimetric analysis shows the commercial filament contains ~23% by weight carbon black. The CV response for ferrocenylmethyl trimethylammonium (FcTMA+) oxidation in 0.1 M potassium nitrate shows peak shaped responses which increase in magnitude with scan rate up to 5 V s-1 where the current response becomes more sigmoidal. C-AFM reveals a RMS surface roughness of 68 nm after polishing, and localized conducting regions covering 11-15 % of the surface (n = 3). These sites vary in size, with mean and largest conductive site areas of 0.045 µm2 and 0.797 µm2 respectively. The active sites lie mostly in small recesses in the electrode surface, with the majority spaced less than 400 nm apart and a minority spaced at distances greater than a μm. Voltammetric SECCM maps of the surface, using a double-channel tip ~ 580 nm diameter, support the C-AFM data, showing a significant fraction of the surface has no electrochemical activity. For those sites with electrochemically-active regions, different CV characteristics are observed, as determined by the change in mass transport-limited current and waveshape (half-wave potential and inter-quartile potentials). Capacitance values obtained from the foot of each CV are used as a proxy for estimating active surface area in the SECCM footprint (not otherwise visible) and the limiting current provides a measure of the mass transport rate to the site. At 0.5 V s-1, SECCM CV responses range from slightly peak shaped, associated with the largest area active sites, to steady-state responses with different limiting current and interquartile potentials. These data are analyzed to reveal a greater than two orders of magnitude distribution in heterogeneous electron transfer kinetics at carbon black sites. Moreover the SECCM data help explain the macroscopic scan rate voltammetric data where widely spaced active sites become increasingly diffusionally-isolated and the wide distribution of local kinetics becomes more apparent as scan rate increase. Both are reflected in the observed change in CV waveshape.

Probing platinum (Pt) nanoparticle (NP) stability, free from convolution with corrosion of the underlying carbon support, under the start-stop conditions of a proton exchange membrane fuel cell, is challenging. To address this problem with a focus on tracking NP morphology changes, we use identical-location transmission electron microscopy, in combination with corrosion-resistant, electron-transparent, boron-doped diamond electrodes. Automated image analysis is developed to enable faster and easier processing. Information concerning NP area, positional changes, and relationship to nearest neighbor distance is extracted on an NP-by-NP basis by tracking the same NPs, pre- and post-accelerated stress testing in perchloric acid under conditions which promote carbon corrosion for sp2 carbons. Measurements are made in several locations with ca. 200 NPs analyzed per image. A significant fraction of the NPs, 40%-50%, are found to be area stable, with only a small number growing and the remaining decreasing very slightly in area (less than a single atom layer) due to transient dissolution. The average distance of travel of an NP after accelerated stress testing is ∼0.36 nm (n = 545). This data highlights both the stability of the Pt-BDD interaction and the potential for BDD use as an electrocatalyst support, under high anodic potentials.

We report on the synthesis and characterization of the square planar rhodium-(II) alkynyl complex [Rh-(PNP-tBu)-(CCtBu)]+ and its transformation into the vinylidene derivative [Rh-(PNP-tBu)-(CCHtBu)]+ by reaction with 9,10-dihydroanthracene. Computational analysis supports a mechanism involving carbon-to-metal hydrogen atom transfer followed by 1,3-hydride migration, and intermediate formation of the associated rhodium-(III) alkynyl hydride [Rh-(PNP-tBu)-H-(CCtBu)]+ has been substantiated experimentally. Study of [Rh-(PNP-tBu)-(CCtBu)]+ in the solid state by EPR spectroscopy, supplemented by multireference CAS-(9,6)/NEVPT2 calculations, enabled assignment as a metal-centered radical to be corroborated, while analysis of frozen glass solutions revealed the presence of square pyramidal solvent adducts of tetrahydrofuran, 2-methyltetrahydrofuran, 1,2-difluorobenzene, fluorobenzene, and α,α,α-trifluorotoluene. Consistent with a carbon-to-metal hydrogen atom transfer mechanism, the extent of solvent coordination measured by EPR spectroscopy inversely correlates with the rate at which the metalloradical reacts with 9,10-dihydroanthracene.

Boron doped diamond (BDD) has numerous advantages as an electrode material such as having a wide aqueous solvent window, water oxidation, which is thought to produce weakly adsorbed hydroxyl radicals, low background currents, and high electrochemical stability. While BDD has received interest as an optically transparent electrode for combined UV-Vis electrochemical measurements, there are no studies which use it in applications which capitalize significantly on the properties of BDD. In this paper, we describe the use of a BDD spectroelectrochemical (SEC) electrode, BDDSEC, fabricated from free-standing BDD (400 μm thickness) and containing laser-micromachined slot-shaped holes (360 μm wide). The electrode shows an optical transmittance of 63% within the wavelength range of 200 to 800 nm, which is the highest reported transmittance for a BDD SEC. UV-Vis electrochemical characterization measurements are made using the redox couple Ru-(bipy)3 2+/3+ over a wavelength range that indium tin oxide electrodes struggle to access due to high background absorption in the UV region. Time scales for Ru-(bipy)3 2+ conversion to Ru-(bipy)3 3+ in this setup are ascertained. We demonstrate the first operando measurements for removal of a UV-Vis active molecule (brilliant blue) using BDDSEC electrodes under advanced oxidation conditions. From the change in the UV-Vis absorption signal with time, comparative measurements of the removal rate as a function of applied potential can be obtained; specifically rate constants of 0.10 min-1 (1.04 V), 0.24 min-1 (at 1.39 V), and 0.68 min-1 (at 2.22 V) vs Ag|AgCl (3 M Cl-) are determined for this experimental arrangement. At the highest potential, we propose both direct and indirect oxidation (via production of hydroxyl radicals from water) are possible. As a second application, we demonstrate the viability of the BDDSEC electrode for quantifying metal ion removal rates (via electroreduction) from different solvent systems. Specifically, we consider electrochemical removal of Pd from Pd-acetate in aqueous acid and in a mixed water:acetonitrile solution.

Reference electrode (RE) drift is a common problem when electrodes are used for pH determination, especially over extended periods of time or in complex media. For voltammetric pH measurements, one method to mitigate against RE drift is to add a second pH insensitive redox species (the internal reference, IREF) and measure the difference in peak potential, E diff, between the signal associated with the pH sensitive species, E pH, and IREF, E IREF. This work strategically explores how to choose the correct IREF species. For these studies, a quinone-functionalized boron doped diamond (BDD-Q) electrode is employed as the pH sensing electrode over the pH range of 4-9. To avoid errors in reporting of the real E pH and E IREF values, there must be a minimum separation between the two peaks. Moreover, the distance on the potential axis between a peak and that of the current response pertaining to water electrolysis must also be considered. For the BDD-Q pH electrode, an operable potential window for IREF is established and the IREF redox species, hexachloroiridate (IrCl6 2-/3-), is determined to be most appropriate, showing an ∼0.08 pH error over the pH range of 4-9 and reducing to ∼0.02 pH error over the pH range of 6-8. The use of E diff is further assessed via the voltammetric measurement of dissolved carbon dioxide (CO2) in a Stow-Severinghaus arrangement over the partial pressure range of 30.4-152.0 mmHg. The R 2 linearity of the calibration line (=0.998) is shown to be equivalent and in agreement with theory when plotting either E diff or E pH versus CO2 partial pressure. This data bodes well for the use of E diff as a measurement signal in Stow-Severinghaus dissolved CO2 transcutaneous sensors, where continuous measurement of pH over several days is required.

The ability to synthesize and structurally interrogate tailored nanostructured materials is a major theme in nanoscience. It requires multimodal methods and platforms, with high resolution microscopy being a central technique. Here a nanofabrication procedure is demonstrated to enable the production of free-standing, electron and optically transparent conductive polycrystalline boron doped diamond (BDD) membranes. These serve as both electrodes onto which nanomaterials can be produced using electrodeposition strategies and platforms for correlative microscopy. The membrane fabrication method involves ion implantation into free-standing BDD to create a damage layer ca. a micron below the surface, high-temperature annealing, electrochemical etching to lift-off the ca. micron thick layer of BDD and optimized reactive ion etching to thin the layer down. The methodology results in ∼50 nm thin BDD membranes of area > 5 mm2 with low surface roughness (∼1.7 nm RMS) comprising grains, tens of μm in size. Going thinner, and producing larger areas, is also possible using this approach. Electron diffraction of the BDD membrane reveals a dominant (110) crystallographic orientation and annular dark field (ADF) scanning TEM (STEM) shows atom resolution of supported metallic (gold) nanostructures and isolated single atoms is achievable. Experimental optical transmissions > 50% are demonstrated. The performance of the BDD membrane for correlative microscopies is assessed against the commonly used carbon film TEM substrate and shown to be superior. Finally, using combined scanning electrochemical cell microscopy and ADF-STEM, the impact of electrode potential on the size, number density and crystal quality of electrodeposited gold nanoparticles on the BDD membrane is examined. The pivotal role electrochemical deposition potential plays in controlling nanoparticle crystal quality i.e. defect free single crystal versus polycrystalline NP is demonstrated.

Short chain PFAS is known to be more challenging to destructively remove than its longer chain counterparts. Electrochemical oxidation at boron doped diamond (BDD) electrodes is one promising way forward. The majority of investigations are carried out using thin film, high grain density BDD electrodes (attached to the growth substrate) at low current densities of <50 mA cm-2. In this work, the impact of high current density on short chain (C4), perfluorobutanoic acid (PFBA), and perfluorobutanesulfonic acid (PFBS) removal rates and defluorination efficiency is investigated in a recirculating flow system. These studies are carried out using free-standing BDD electrodes, which are grown thick enough so that the BDD can be removed from the non-diamond growth substrate and thus contain a much lower grain density compared to thin-film BDD. The cell utilizes four BDD electrodes, where only the two outer electrodes are directly connected to a potential supply, the two inner electrodes are electrically unconnected, and driven in a bipolar arrangement. Solutions contain saturated potassium sulfate as the electrolyte. A current density of ≥390 mA cm-2 (after correcting for surface roughness) is employed for times up to 9 h. PFBS/PFBA concentrations in the range range ∼1-60 mg L-1 are investigated. Importantly when comparing rate constant data to literature for similar concentrations (after normalization of the rate constants to treatment volume/anode area), the values suggest removal rates approximately an order of magnitude higher than those at lower current density (in stirred solutions). Defluorination efficiency is also found to be higher with (close to) complete defluorination indicated at higher concentrations/longer times for short chain PFAS. Microscopy analysis of the free-standing electrodes after deployment for >90 h of advanced oxidation reveals no obvious signs of corrosion. This likely reflects both the reduced grain boundary density and lower sp2 carbon content in thick free-standing BDD. The data highlight the potential for this electrode material in long term electrochemical treatment of short chain PFAS solutions in recirculating flow systems.

Highly diastereoselective self-assembly reactions give both enantiomers (Λ and Δ) of anti-parallel triple-stranded bimetallic Co(ii) and Co(iii) cationic helices, without the need for resolution; the first such reaction for Co. The complexes are water soluble and stable, even in the case of Co(ii). Studies in a range of cancer and healthy cell lines indicate high activity and selectivity, and substantial differences between enantiomers. The oxidation state has little effect, and correspondingly, Co(iii) compounds are reduced to Co(ii) e.g. by glutathione. In HCT116 colon cancer cells the Λ enantiomer induces dose-dependent G2-M arrest in the cell cycle and disrupts microtubule architectures. This Co(ii) Λ enantiomer is ca. five times more potent than the isostructural Fe(ii) compound. Since the measured cellular uptakes are similar this implies a higher affinity of the Co system for the intracellular target(s); while the two systems are isostructural they have substantially different charge distributions as shown by calculated hydrophobicity maps. In contrast to the Λ enantiomer, Δ-Co(ii) induces G1 arrest in HCT116 cells, efficiently inhibits the topoisomerase I-catalyzed relaxation of supercoiled plasmid DNA, and, unlike the isostructural Fe(ii) system, causes DNA damage. It thus seems very likely that redox chemistry plays a role in the latter.

Electrochemical advanced oxidation (EAO) systems are of significant interest due to their ability to treat a wide range of organic contaminants in water. Despite their popularity, all studies to date that examine anodic corrosion of BDD electrodes are qualitative, using techniques such as scanning electron microscopy, electrochemistry, and spectroscopy. In this work we present a new method to quantify anodic corrosion and determine average corrosion rates as a function of solution composition, current density and BDD material. The method uses white light interferometry (WLI), in conjunction with BDD electrodes integrated into 3D-printed flow cells, to measure three-dimensional changes in surface structure pre- and post-anodic corrosion over a 72-hr period. It is equally applicable to both thin film BDD and much thicker, freestanding BDD. As WLI lends itself to large area measurements, data are collected over geometric areas of 0.5 cm2. In particular it is shown that the addition of 1 M acetic acid to a 0.5 M potassium sulfate solution results in an increase in the average corrosion rate of ~×60 (for freestanding polished BDD). In the same solution, thin film BDD is found to corrode ~×2 faster than freestanding polished BDD. This methodology also represents an important step forward for the prediction of BDD electrode lifetimes in a laboratory setting for a wide range of EAO applications.

The role and bonding arrangement of deliberately added sp2 carbon in maximising the current efficiency, output and longevity of boron doped diamond (BDD) electrodes for electrochemical dissolved ozone generation is elucidated. We show, using a zero-gap cell (ZGC) arrangement, how systematically increasing sp2 carbon results in increased ozone concentration and current efficiency. sp2 carbon addition is made using nanosecond pulse laser micromachining which converts BDD to sp2 carbon. Two ZGC geometries are investigated which incorporate a Nafion membrane sandwiched between two BDD electrodes. Through-holes are integrated into either the membrane or the BDD, the latter using laser micromachining which also converts the hole walls to sp2 carbon. Increasing the number of through-holes (or changing hole geometry) increases the sp2 carbon content of the electrode (from 5-100%). For the planar electrode, the proportion of the surface which is laser micromachined controls the sp2 carbon content (from 4-100%). sp2 carbon content %’s are significantly higher than is possible using diamond growth. sp2 carbon contents >40% and >60% for the planar and perforated BDD electrodes, respectively, are found to be particularly effective, allowing electrode designs to be proposed for optimised ZGC ozone generation. Importantly, the sp2 carbon introduced via laser micromachining is shown to be extremely stable over 20 hr (anode potential ∼ 10 V) in contrast to glassy carbon, which corrodes within 10 mins. Whilst both are 100% sp2 carbon, the laser-machined surface is amorphous whereas the glassy carbon contains disorganised graphitic layers. This work also highlights the intriguing stability of amorphous sp2 carbon towards high oxidative potentials.

Proton transfer at solid/liquid interfaces is a fundamental step in many complex biological and electrocatalytic processes. Previous model studies using electrodes modified with self-assembled monolayers (SAMs) of carboxylic acid-terminated alkanethiols have demonstrated that interfacial proton transfer is controlled by the local electrochemical microenvironment. The thermodynamic driving force for electrochemically driven protonation/deprotonation of acid/base SAMs is governed by a combination of the electric potential at the SAM/solvent interface, the pK(a) of the acid group, and the solution pH. Here, we develop a kinetic model to describe electric potential-driven protonation/deprotonation as a two-step process. This comprises a reversible proton transfer step at the SAM/electrolyte interface (i.e., (de)protonation) and a proton transport step describing the motion of protons as they traverse the diffuse electrical double layer to and from the solution bulk. The kinetics of the transport step are investigated using finite element simulations, providing numerical estimates for the transport rate constants under combined diffusional and migrational transport modes. Using the dependence of these rate constants on the electric potential at the SAM/electrolyte interface, we define situations where the overall rate expression is limited by either (de)protonation, proton transport, or a combination of both. From this analysis, we determine a lower limit for the acid group pK(a) of approximate to 3, above which proton transfer at the plane of acid dissociation is generally the rate-determining step. The electric potential-driven proton transfer/transport kinetic model developed herein provides a general approach to treat electric potential-driven coupled ion transfer and transport phenomena, with potential applications including proton-coupled electron transfer processes, ion intercalation in alkali metal batteries, and ion transport across biological membranes.

In this work we elucidate the role and bonding arrangement of deliberately added sp2 carbon in maximising the current efficiency, output and longevity of boron doped diamond (BDD) electrodes for electrochemical dissolved ozone generation. In particular we show, using a zero-gap cell (ZGC) arrangement, how systematically increasing sp2 carbon results in an increase in ozone concentration and current efficiency. sp2 carbon addition is made using nanosecond pulse laser micromachining which converts the BDD to sp2 carbon. Two ZGC geometries are investigated which incorporate a Nafion membrane sandwiched between two BDD electrodes, with through-holes integrated into either the membrane or the BDD. Holes in the BDD are generated using laser micromachining which also converts the hole walls to sp2 carbon. Increasing the number of through-holes (or changing hole geometry) increases the sp2 carbon content of the electrode (from 5-100%). For the planar electrode, the proportion of the surface which is laser micromachined controls the sp2 carbon content (from 4-100%). This approach enables significantly higher sp2 carbon contents than is possible using diamond growth. sp2 carbon contents >40% and >60% for the planar and perforated BDD electrodes, respectively, are found to be particularly effective, allowing electrode designs to be proposed for optimised ZGC ozone generation. The sp2 carbon introduced during laser micromachining is shown to be extremely stable over 20 hr (anode potential ~ 10 V) in contrast to glassy carbon, which corrodes within 10 mins. Whilst both are 100% sp2 bonded carbon, the sp2 carbon in the laser-machined surface is fully amorphous whereas in glassy carbon it contains disorganised graphitic layers. This work also highlights the intriguing stability of amorphous sp2 carbon towards high oxidative potentials.

Square wave voltammetry (SWV) is commonly used in electroanalytical applications to enhance analyte faradaic signals and minimize nonfaradaic processes. However, little attention is given as to how best use SWV to minimize faradaic interference signals that arise from redox species present in solution that have redox potentials that convolute with that of the analyte. In conventional SWV, a series of current-time (i-t) transients are collected, and i is averaged over a specified window of each transient (potentiostat dependent). This average i is reported against the electrode potential, E. As the i-t response is governed by the type of electron transfer reaction under investigation, we show how by collecting all i-t data and through judicious choice of the current averaging window, it is possible to enhance the analyte response while at the same time reducing the interferent signal. We look at three different electron transfer reactions, fast electron transfer outer sphere, metal electrodeposition/stripping, and surface-confined proton-coupled electron transfer (PCET) and demonstrate different i-t behaviors in SWV, visually aided by the use of 3D i-t-E plots. In the case of PCET quinone-based voltammetric sensing of pH in the presence of a heavy metal (here Cu2+), we show that the use of a much earlier current averaging window (2-10% of the i-t response) results in the pH signal being clearly distinguished from that of the overlapping heavy metal.

Eumelanin is a natural pigment found in many organisms that provides photoprotection from harmful UV radiation. As a redox-active biopolymer, the structure of eumelanin is thought to contain different redox states of quinone, including catechol subunits. To further explore the excited state properties of eumelanin, we have investigated the catechol/o-quinone redox couple by spectroelectrochemical means, in a pH 7.4 aqueous buffered solution, and using a boron doped diamond mesh electrode. At pH 7.4, the two proton, two electron oxidation of catechol is promoted, which facilitates continuous formation of the unstable o-quinone product in solution. Ultrafast transient absorption (femtosecond to nanosecond) measurements of o-quinone species involve initial formation of an excited singlet state followed by triplet state formation within 24 ps. In contrast, catechol in aqueous buffer leads to formation of the semiquinone radical Delta t>500 ps. Our results demonstrate the rich photochemistry of the catechol/o-quinone redox couple and provides further insight into the excited state processes of these key building blocks of eumelanin.

Proton exchange membrane fuel cell (PEMFC) electrocatalysts are typically Pt or Pt alloy nanoparticles (NPs) supported on high surface area carbon black powders, the latter which contain a significant sp 2 carbon component. One of the barriers to PEMFC commercialisation is the lifetime of the catalyst-support system, which should be at least 5,000 hours for automotive applications. [1] During the start-up and shut-down of the PEMFC, the cathode is subjected to significant oxidative potentials (greater than 1 V vs RHE) which can lead to corrosion of the predominantly sp2 bonded carbon support, a key factor which can limit the lifetime of PEMFCs. [2] High quality boron doped diamond (BDD) is an sp 3 bonded network with above 1:1000 B:C ratio, which is required for metal-like electronic conductivity. BDD has many favourable properties for electrochemical experiments, most notably: higher electrochemical corrosion resistance with respect to sp 2 carbons, low background currents, and a wide aqueous solvent window. [3] In this work we first explore the electrochemical corrosion stability of BDD when undergoing electrochemical cycling in acid solutions. To achieve this BDD substrates (BDD TEM grid) suitable for both electrochemical experiments and high magnification (single atom resolution) transmission electron microscopy (TEM) experiments were produced using precision ion polishing (Gatan PIPS-II). The surface was characterised using X-ray photoelectron spectroscopy to ensure the polishing process did not significantly alter the diamond substrate. Under aggressive electrochemical potential cycling in perchloric acid and sulfuric acid solutions, both TEM morphological analysis and Electron Energy Loss Spectroscopy (EELS) thickness measurements showed no evidence of BDD corrosion. [4] The BDD-TEM substrate was then used as a platform for investigating the degradation of a PEMFC electrocatalyst (Pt NPs) under accelerated stress testing (AST) [1] conditions on an atomic level, using the corrosion free carbon support. This enables the influence of other degradation pathways [2] (such as aggregation, Ostwald Ripening & direct dissolution) to be explored in more detail, free from issues associated with corrosion of the support. Pt NPs, in the size range 1 – 4 nm, were sputter coated onto the BDD TEM grid. Identical location ex-situ TEM (IL-TEM) [5] in combination with image analysis was used to probe NP changes e.g. size, shape, position, on an individual basis before, and after, AST (Fig. 1, which contains c.a. 200 NPs for analysis). Such measurements were complemented by Inductively Coupled Plasma – Optical Emission Spectroscopy analysis for any dissolved Pt, and electrochemical cyclic voltammetry measurements of the Pt to highlight changes in electrocatalytic behaviour (hydrogen evolution reaction and oxygen reduction reaction) due to AST cycling. Figure 1 – High magnification annular dark field (ADF) identical location images of Pt NPs, a) Pt/BDD before AST, b) Pt NPs after AST, c) Cyclic voltammogram of the 1000 cycle AST from 0.8 to 1.4 V vs Ag|AgCl (1.6 to 1.6 V vs RHE). References: U.S. D.O.E.; Multi-Year Research, Development, and Demonstration Plan; 2016; 3.4 Fuel Cells. Mayrhofer, K.J.J.; et al.; Beilstein J. Nanotechnol.; 2014; 5; 44–67. Macpherson, J.V.; Phys. Chem. Chem. Phys.; 2015;17; 2935-2949 Hussein, H.E.M.; Wood. G.; Houghton. D.; Walker, M.; Han, Y.; Zhao, P.; Beanland, R.; Macpherson, J.V.; ACS Meas. Sci. Au; 2022; 2; 5; 439–448 Feliu, J.M.; Abruña, H.D.; J. Am. Chem. Soc. ; 2015; 137; 47; 14992–14998 Figure 1

A finite-element model has been developed to simulate the cyclic voltammetric (CV) response of a planar electrode for a 1e outer-sphere redox process, which fully accounts for cell electrostatics, including ohmic potential drop, ion migration, and the structure of the potential-dependent electric double layer. Both reversible and quasi-reversible redox reactions are treated. The simulations compute the time-dependent electric potential and ion distributions across the entire cell during a voltammetric scan. In this way, it is possible to obtain the interdependent faradaic and non-faradaic contributions to a CV and rigorously include all effects of the electric potential distribution on the rate of electron transfer and the local concentrations of the redox species Oz and Rz-1. Importantly, we demonstrate that the driving force for electron transfer can be different to the applied potential when electrostatic interactions are included. We also show that the concentrations of Oz and Rz-1 at the plane of electron transfer (PET) significantly depart from those predicted by the Nernst equation, even when the system is characterised by fast electron transfer/diffusion control. A mechanistic rationalisation is also presented as to why the electric double layer has a negligible effect on the CV response of such reversible systems. In contrast, for quasi-reversible electron transfer the concentrations of redox species at the PET are shown to play an important role in determining CV wave shape, an effect also dependant on the charge of the redox species and the formal electrode potential of the redox couple. Failure to consider electrostatic effects could lead to incorrect interpretation of electron-transfer kinetics from the CV response. Simulated CVs at scan rates between 0.1 and 1000 V s-1 are found to be in good agreement with experimental data for the reduction of 1.0 mM Ru(NH3)63+ at a 2 mm diameter gold disk electrode in 1.0 M potassium nitrate.

Many photoactivated processes involve a change in oxidation state during the reaction pathway and formation of highly reactive photoactivated species. Isolating these reactive species and studying their early-stage femtosecond to nanosecond (fs-ns) photodynamics can be challenging. Here we introduce a combined ultrafast transient absorption-spectroelectrochemistry (TA-SEC) approach using freestanding boron doped diamond (BDD) mesh electrodes, which also extends the time domain of conventional spectrochemical measurements. The BDD electrodes offer a wide solvent window, low background currents, and a tuneable mesh size which minimises light scattering from the electrode itself. Importantly, reactive intermediates are generated electrochemically, via oxidation/reduction of the starting stable species, enabling their dynamic interrogation using ultrafast TA-SEC, through which the early stages of the photoinduced relaxation mechanisms are elucidated. As a model system, we investigate the ultrafast spectroscopy of both anthraquinone-2-sulfonate (AQS) and its less stable counterpart, anthrahydroquinone-2-sulfonate (AH(2)QS). This is achieved by generating AH(2)QS in situ from AQS via electrochemical means, whilst simultaneously probing the associated early-stage photoinduced dynamical processes. Using this approach we unravel the relaxation mechanisms occurring in the first 2.5 ns, following absorption of ultraviolet radiation; for AQS as an extension to previous studies, and for the first time for AH(2)QS. AQS relaxation occurs via formation of triplet states, with some of these states interacting with the buffered solution to form a transient species within approximately 600 ps. In contrast, all AH(2)QS undergoes excited-state single proton transfer with the buffered solution, resulting in formation of ground state AHQS(-) within approximately 150 ps.