Search

In this study, the mass transfer on a structured surface composed of a lattice of conical nanoelectrodes is modelled, and a variety of geometrical setups are simulated. The optimal size and distance between cones are sought in terms of dimensionless groups. In the calculations, we look at three different definitions of current density based on three different reference surfaces: the total surface, the surface of the cones, and the local infinitesimal area. Optimisation based on different definitions results in different optimal configurations. This implies that, in designing the structured surface, a choice must be made between optimising the electrode at the nanoscale or at the macroscale based on the actual cost of manufacturing and the return expected by running the electrochemical system.

Green hydrogen production can only be realized via water electrolysis using renewable energy sources. Proton exchange membrane water electrolyzers have been demonstrated as the technology of choice for mass production of green hydrogen due to their scalability and potential high efficiency. However, the technology is still relatively expensive due to the catalyst materials cost and operational limitations due to mass transfer and activation polarizations. During the oxygen evolution reaction, oxygen bubbles stick to the electrode surface and this causes a low reaction rate and high mass transfer losses. In this study, the commonly used electrocatalyst for oxygen evolution reactions; IrO2, is modified by intro-ducing magnetic Fe3O4 to achieve greater bubble separation at the anode during operation. The prepared composite catalysts were characterized using Scanning Electron Microscope, Energy Dispersive X-Ray Analysis, X-Ray Powder Diffraction, X-ray photoelectron spec-troscopy and Brunauer-Emmett-Teller characterization methods. The modified composite electrocatalyst samples are magnetized to investigate the magnetic field effect on oxygen evolution reaction performance in proton exchange membrane water electrolyzers. 90% IrO2-10% Fe3O4 and 80% IrO2-20% Fe3O4 samples are tested via linear sweep voltammetry both ex-situ and in-situ in a proton exchange membrane water electrolyzer single cell. According to the linear sweep voltammetry tests, the magnetization of the 80% IrO2-20% Fe3O4 sample resulted in 15% increase in the maximum current density. Moreover, the single cell electrolyzer test showed a four-fold increase in current density by employing the magnetized 80% IrO2-20% Fe3O4 catalyst. (C) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Iridium-based oxides, currently the state-of-the-art oxygen evolution reaction (OER) electrocatalysts in acidic electrolytes, are cost-intensive materials which undergo significant corrosion under long-term OER operation. Thus, numerous researchers have devoted their efforts to mitigate iridium corrosion by decoration with corrosion-resistant metal oxides and/or supports to maximize OER catalyst durability whilst retaining high activity. Herein a one-step, facile electrochemical route to obtain improved IrOx thin film OER stability in acid by decorating with amorphous tungsten sulphide (WS3−x) upon electrochemical decomposition of a [WS4]2− aqueous precursor is proposed. The rationale behind applying such WS3−x decoration stems from the generation of a tungsten oxide phase, a well-known corrosion-resistant photoactive OER catalyst. The study demonstrates the viability of the proposed WS3−x decoration, allowing the tailoring of experimental parameters responsible for WS3−x nanoparticle size and surface coverage. OER stability tests coupled by ex situ SEM and XPS corroborate the beneficial effect of WS3−x decoration, yielding improved OER specific activity metrics along with minimized Ir surface roughening, a characteristic of electrodissolution. Iridium decoration with electrodeposited, corrosion-resistant oxides is consequently shown to be a promising route to maximize OER stabilities.

Recent developments in the use of nanoimpacts as an investigative electrochemical technique are discussed. Highlights include literature on the imaging of nanoimpacts themselves and developments in the application of nanoimpacts such as, surface analysis of single particles including surface coverage and sizing. Also included are factors to consider which may affect the outcome of nanoimpact experiments.

We report the fabrication and electrochemical study of edge-abundant transition metal dichalcogenide (TMD) nanocone arrays. Time-dependent etching by sequential use of isotropic O2 and anisotropic SF6/C4F8 plasmas on nanosphere monolayer-modified TMD crystals results in very high coverage nanocone array structures with tunable aspect ratios and interspacings. Electrochemical characterization of these arrays via the hydrogen evolution reaction (HER), using a low proton concentration electrolyte (2 mM HClO4, 0.1 M NaClO4) to reveal morphology-dependent mass transport features at the proton diffusion-controlled region, show significant changes in electrocatalytic behaviour at both WS2 and MoS2: notably onset potential shifts of 100 and 200 mV, and Tafel slope decreases of 50 and 120 mV dec−1 respectively. These improvements vary according to the geometry of the arrays and the availability of catalytic edge sites, and thus the observed electrochemical behaviour can be rationalized via kinetic and mass transport effects.

The commercialisation of polymer electrolyte fuel cells (PEFCs) has been hampered by the high cost of platinum metal. Due to its high durability and catalytic activity, platinum is widely used to catalyse the oxygen reduction and hydrogen oxidation reactions essential to the operation of these cells. Core@shell nanoparticles with thin layers of platinum deposited on cores composed of cheaper materials have offered an attractive route towards the reduction of overall loading of platinum, with the retention of active catalyst surface area. This review surveys approaches taken to prepare idealised active and durable core@shell nanocatalysts by tweaking core compositions. A critical reflection on the current status of the field, as well as predictions as to likely directions for future developments, are discussed as a conclusion to the review.

A hydrogen economy powered by fuel cells is emerging as an alternative to the current fossil-fuel based energy system where hydrogen, produced through renewable sources, is used to generate electricity via fuel cells. A commonly investigated fuel cell, the Polymer Electrolyte Fuel Cell (PEMFC), usually uses platinum and other platinum group metal nanomaterials to catalyze the rate limiting Oxygen Reduction Reaction (ORR) of this process. The high prices and durability limitations of these catalysts have prevented their mass commercialization. Biosynthesis of nanomaterials has emerged as a potentially attractive “eco-friendly” alternative to conventional chemical synthesis methods. Various attempts have been made to biosynthesize nanoparticles for use in fuel cells. However, the processing methods used during and post synthesis increase their costs and limit their overall efficacies. We report bimetallic Pt/Pd nanoparticles (NPs) biosynthesized by E. coli [E. coli-Pt/Pd (10 wt%:10 wt%)] that shows promise for direct use as a PEMFC catalyst. This catalyst outperformed single metal versions of the same, i.e., E. coli-Pt (20 wt%) and E. coli-Pd (20 wt%) when tested as an electrocatalyst ex-situ. Direct use of E. coli-synthesized nanoparticles in PEMFCs is limited by the inherent resistances of the bacteria and the internal localization of nanoparticles. Transmission electron microscopy images and impedances (resistivity) tests showed that by initially synthesizing Pd nanoparticles on the E. coli cells, followed by Pt, gave a cell surface-localized metallic shell that improved conductivity of the catalyst. Catalyst Pt synthesis is likely mediated by initially-formed Pd-NPs reducing Pt (IV) under H2 resulting in alloying; this was evidenced by XRD data that showed XRD peaks for E. coli-Pt/Pd (10%:10%) which lie in between XRD peaks for Pt-only and Pd-only nanoparticles on the same planes. Protrusions of agglomerated nanoparticles were seen on the cell surface to form sites for catalytic activity. The catalyst, used in the ORR without optimization, performed significantly worse (~100 times) than a commercial catalyst (extensively developed for purpose) but which contained 5 times as much Pt. This serves as a starting point for a more engineered approach to bio-synthesizing nanoparticles for PEMFC catalysts.

This paper demonstrates thesignificant and positive effect of applying a magnetic field on the performanceof Proton Exchange Membrane Water Electrolysers (PEMWE). A magnetizer and a transparentPEMWE cell are used to observe the effect of the magnetic field at variablewater flow rates on the PEMWE performance. The presence of the magnetic fieldintroduces Lorentz force which results in a significant improvement in theelectrolyser performance. The magnetic flux density is varied between 0 T and0.5 T, while the water flow rate is varied from 100 ml min-1 to 300ml min-1 to study the effect and relationship between the twoparameters and the performance of the PEMWE. Under a 0.5 T magnetic field and300 ml min-1 flow rate, a 33% increase in the cell performance isachieved compared to the conventional operation at the same flow rate. Thepositive effect is explained by the introduction of Lorentz force from themagnetic field to the operating PEMWE. The improvement here is due to therelaxation and pumping effect of the magnetic field on the electrode surfacewhich results in enhancing oxygen bubbles removal and lowering mass transportpolarisation. Moreover, the enhanced oxygen bubbles removal is expected toincrease the lifetime of the electrolyser as a result of the reduced contact betweenthe produced oxygen and the anode materials.

Herein, we demonstrate strongly enhanced visible light photoelectrocatalytic hydrogen evolution reaction (HER) in few-layer MoS2 grown on a mesoporous TiO2(B) nanobelt (NB) by selective decoration of platinum (Pt) nanoparticles (NPs) on the edge/defect sites of the MoS2 layer. Three catalytically active components are anchored together to increase the photoelectrocatalytic HER activity synergistically, beyond that of commercial Pt/C electrodes (20 wt% Pt). An extremely low concentration of Pt NPs (1.4 wt%) with average size ∼3.8 nm was decorated over the preferentially edge-site-exposed few-layer MoS2, with lateral sizes 130–350 nm, as evidenced from high-angle annular dark-field STEM imaging. During the heterojunction formation, S is doped in the TiO2 layer causing a high density of electrons in TiO2 that migrate to the MoS2 layer inducing n-type doping in it and thus TiO2 acts as an efficient photocathode in photoelectrocatalysis. Quantitative XPS analysis reveals that the catalytically active bridging S22−/apical S2− increases up to ∼72% after the formation of the ternary system Pt@MoS2/TiO2(B). S-enriched MoS2/TiO2(B) selectively loaded with Pt NPs on the edge sites of MoS2 exhibits a giant enhancement in the HER activity in an acidic medium under light. We record a nearly 16 fold higher exchange current density (0.296 mA cm−2) for the ternary system as compared to that of the MoS2/TiO2 binary system under visible light excitation. The marginally Pt loaded ternary system exhibits an extremely low charge transfer resistance (14 Ω) and a low overpotential as well as Tafel slope (−74 mV and 30 mV dec−1, respectively) boosting the overall HER performance under visible light. Chronopotentiometric measurements reveal the high stability of binary and ternary systems to sustain a 10 mA cm−2 cathodic current up to 12 hours. The results show that the marginally loaded Pt NPs activate the inert basal plane, edge sites of MoS2 and porous sites of TiO2, forming an integrated network where the photogenerated electrons can easily be injected from the TiO2 to MoS2 and then to Pt NPs, presenting a feasible approach to boost the HER activity under visible light.

Intermediate temperature (100–120 °C) polymer electrolyte fuel cells (IT-PEFCs) offer simplified water and thermal management compared to conventional PEFCs, since any water should exist in the vapour phase, allowing for easier removal. The higher operating temperature also facilitates greater temperature differentials between the fuel cell and the surrounding atmosphere, thus easing the thermal management of an IT-PEFC stack. However, the study of IT-PEFC is still a relatively poorly covered field within the literature and thus little information is available on performance characteristics. We therefore present a simple multiphysics model as a quantitative tool for describing the IT-PEFC. This tool is then used to optimise different materials and parameters within an IT-PEFC. Experimental data is presented as a test of the model, and excellent quantitative agreement is demonstrated. Having validated this model, we present a detailed study of the GDL materials in order to understand the influence of different parameters, namely: (i) porosity, (ii) permeability and (iii) electrical conductivity. We report that the optimal porosity for IT-PEFC operation is 40–50%, whereas that the GDL permeability was found to have little impact on the cell performance. Further, we used the model as a design tool: proposing a novel cell design, taking into account the considerable advantages when using a metallic GDL which yielded potential significant improvements in the system efficiency.

The gas diffusion layer (GDL) has a vital role in the operation of a polymer electrolyte fuel cell (PEFC). Therefore, studying GDL characteristics and their effect on the cell performance is fundamental for the development of more efficient PEFCs. The work presented covers a selection of commercially available GDL types used in fuel cell development. It highlights some key GDL properties and their influence on PEFC performance. The results show that GDL materials have a direct effect on the ohmic and mass transport losses in the membrane electrode assembly (MEA). They also show that studying the effect of GDL properties on the performance is rather complex, due to the many interrelated properties. The study shows that GDL thickness has a significant effect on the mass transport properties of MEA, but has minimal effect on the ohmic losses. The bulk density of the substrate has a significant effect on the water transport properties and the maximum current density achieved. It is also found that woven and non-woven GDLs achieve comparable performance at optimized operating conditions. Moreover, the felt fiber structure has higher ohmic resistance but achieves better performance than that of straight fiber carbon paper due to its enhanced water transport ability.

The field of particle impact electrochemistry has grown rapidly in the few years since its development, with discoveries including: detection, characterisation, and quantification of nanoparticles (NPs); solution and surface aggregation of NPs; kinetics of redox reactions of NPs and of species at NPs; and information on surface chemistry of NPs. This mini-review aims to briefly cover the many advances made since the end of 2011. Keywords: Nanoparticles, Impacts, Collisions, Electrochemistry

We report proof-of-concept measurements relating to the impact of nanoparticles with an electrode potentiostatted at a value corresponding to the diffusion controlled oxidation of silver nanoparticles in authentic seawater media. The charge associated with the oxidation reveals the number of atoms in the nanoparticle and thus its size and state of aggregation.

Gold nanoparticles (AuNPs) in aqueous 0.10 M HCl are shown to be electroactive at oxidising potentials greater than 1.0 V (vs. Ag/AgCl) by means of voltammetric monitoring of AuNP-electrode collisions. The method promises the use of anodic particle coulometry for the detection and characterisation of the AuNPs.

We present experimental results to determine the proportion of nanoparticle (NP) impacts that result in adsorbed NPs, using gold and nickel nanoparticles (AuNPs/NiNPs) in collision with a glassy carbon electrode. Results are given for NP radii of 10 nm (Au) and 26 nm (Ni), as well as a range of electrode potentials. No significant systematic trends were found in either case, and the sticking coefficients were found to be s = 0.19 ± 0.03 for Au and s < 0.01 for Ni. © 2012 Elsevier B.V. All rights reserved.

The use of anodic particle coulometry (APC) for the sizing and detection of oxidisable metal nanoparticles such as gold or silver have previously been shown to be reliable, albeit destructive. For the first time, the voltammetric sizing and detection of nanoparticles has been accomplished non-destructively, via the reduction of electroactively-tagged silver nanoparticles during particle impacts. Tag-redox coulometry (TRC) thus significantly expands the scope of nanoparticle sizing by particle-impact methods. © 2012 Elsevier B.V. All rights reserved.

We present an experimental comparative evaluation of the Marcus-Hush (MH) and Butler-Volmer (BV) kinetic formalisms. Numerical simulations using both kinetic models are used to fit experimental voltammetry of the one-electron oxidation of 9,10-diphenylanthracene (DPA) and the one-electron reduction of 2-nitropropane (NP) at a high-speed channel microband electrode. For DPA the Butler-Volmer and Marcus-Hush formalisms yield indistinguishable fits, as expected for a system with fast electrode kinetics. For the BV formalism best fits were obtained using k0 = 0.83 cm s-1 and α = 0.49; for MH the best fit parameters were k0 = 0.85 cm s-1 and λ = 0.58 eV. For NP neither Butler-Volmer nor Marcus-Hush models are able to obtain very accurate fits to experiment, although it was possible for the Butler-Volmer model to yield more accurate fits if the transfer coefficients, α and β, are not required to sum to unity, which is possibly justifiable due to the very large difference between oxidative and reductive peak potentials. The best fit obtained using MH kinetics used k 0 = 7.0 × 10-6 cm s-1 and λ = 1.0 eV, while BV kinetics was able to fit using k0 = 9.5 × 10 -4 cm s-1 and α = β = 0.24. © 2011 American Chemical Society.

"Bamboo-like" multiwalled (b-MWCNT), "hollow-tube" multiwalled (h-MWCNT) and single-walled carbon nanotubes (SWCNT), chemically modified with 1-anthraquinonyl (AQ) or 4-nitrophenyl (NP) groups, are characterized using voltammetric, electron microscopic and Raman spectroscopic techniques. The pKa values of the AQ-modified CNTs are found to be shifted by greater than three units when compared to the pKa values of anthrahydroquinone (AHQ, the reduced form of AQ) in aqueous solution to beyond pH 14. These large changes in the surface pKa values of the modified CNTs are explored further by comparing the pKa values of CNTs modified with an anthraquinonyl-2-carboxylic acid group. These groups are attached to the CNT surface via the formation of an amide bond with an aminophenyl "spacer" unit derived from the chemical reduction of NP modified CNTs. The location of reactive sites on the CNT surface is investigated and their influence on the pKa of the modified materials is discussed. Comparison with modified pyrolytic graphite electrodes exposing pure edge-plane or pure basal-plane crystal faces indicates that the modifying aryl groups are predominantly located on edge-plane like defects at the tube ends of MWCNTs. The effect of polymer formation on electron transfer kinetics of b-MWCNTs and h-MWCNTs is also discussed. In contrast SWCNTs show both significant side-wall functionalisation and fast electron transfer kinetics which is attributed to their different electronic structure. This journal is © The Royal Society of Chemistry.

The kinetics of proton reduction are reported for Ni and NiO surfaces and compared to that measured at Ni@NiO nanoparticles. Kinetic acceleration is found to occur by virtue of oxide overlayers and not due to size effects on the nanoscale.

The electro-oxidation of silver and nickel nanoparticles in aqueous solution was studied via their collisions with a carbon electrode. The average charge passed per impact varies with electrode potential and was analysed to determine that AgNPs display an electrochemically fast (“reversible”) one-electron oxidation, whilst the NiNPs exhibit slow (“irreversible”) 2-electron kinetics. Kinetic parameters are reported.

The chapter will cover the fundamentals of electrochemistry of specific reference to fuel cells, and nanomaterials evaluation. Whilst these may be familiar to the trained electrochemist, a significant amount of fuel cells research is conducted by non-specialists who often use electrochemical methods as a tool without an appreciation of its nuances. The article covers electron transfer and mass transport, issues of the nanoscale (as compared to macroscale), the evaluation of electrocatalytic behaviour, introduction to mechanism, and the electrochemical characterisation of catalyst materials.

We review the joint use of ionic liquids and electrochemical methods in the separation of carbon dioxide from other gases and transformation into useful organic feedstocks. © 2011 The Royal Society of Chemistry.

The oxidative electrochemistry of nicotine is measured in aqueous solution at a multi-walled carbon nanotube modified basal plane pyrolytic graphite (MWCNT-BPPG) electrode. Quantitative detection of nicotine is obtained with a limit of detection of 1.5 μM (based on 3σ) and a linear range of at least up to 1 mM. Evidence is found for a mass transport regime that includes thin-layer (within the MWCNT layers) as well as semi-infinite (from bulk solution) diffusional signatures, adding to the growing picture that the fundamental source of the 'electrocatalytic' properties claimed of many porous and multi-walled carbon nanotube-based modified electrodes may, at least in some cases, be due to mass transport effects rather than electronic or structural peculiarities of the modifying layers. © 2009 Elsevier B.V. All rights reserved.

“Nanosecond” electrochemistry is used to investigate the impacts of various solid particles (alumina and graphite suspended in aqueous perchloric acid and copper suspended in aqueous sodium perchlorate) at a polycrystalline gold electrode under insonation. Current spikes of microsecond duration, attributed to individual impacts, are observed under potentiostatic conditions, with a polarity that inverts at the potential of zero charge of the electrode/electrolyte system. It is found that there is no significant change in the magnitude of these transients as the sizes of alumina particles range from 0.3 to 25 Im. However, appreciable changes are observed in the transients when using electroactive particles, specifically graphite powder surface modified with adsorbed N,N′-diphenyl-1,4-phenylenediamine, and evidence is presented for the occurrence of electron transfer during the impact event which occurs on the sub-microsecond time scale.

Anodic particle coulometry (APC) is a recently established method of sizing individual metal nanoparticles by oxidising them during their impact on a micro electrode. Here it is demonstrated that the application of APC can be extended to sizing of metal oxide nanoparticles, such as FeO magnetite nanoparticles. Additionally, a new route to electrochemical nanoparticle sizing is introduced-cathodic particle coulometry (CPC). This method uses the reduction of impacting nanoparticles, e.g., metal oxide nanoparticles, and is demonstrated to yield correct size information for FeO nanoparticles. The combination of these two independent electrochemical methods of nanoparticle sizing, allows for purely electrochemical sizing of single nanoparticles and simultaneous verification of the obtained results. [Figure not available: see fulltext.] © 2013 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

We report a simple, rapid and cost effective fabrication of a vibrating platinum foil electrode whereby significant mass transport enhancement can be seen and applied for electroanalytical work. The vibrating and stationary nature of the platinum foil electrode was characterised by cyclic voltammetry using a Ru(NH 3) 62+/3+ redox system, and a four-fold enhancement in sensitivity could be reproducibly seen. Randles- Ševčik analysis gave a diffusion coefficient, D value, of 9.4 × 10 -6 cm 2 s -1. © 2012 The Royal Society of Chemistry.

The transport of nanoparticles to a substrate electrode for collision can be quantitatively described by a Fickian (i.e. diffusional) model both where the nanoparticles are oxidatively electrolysed at the electrode and where electrolysis of solution-phase species occurs on the surface of the impacting nanoparticles. © 2012 The Royal Society of Chemistry.

The developing field of particle-impact electrochemistry is rapidly establishing itself as a powerful tool for the interpretation of a wide range of phenomena associated with nanoparticles. In this article we present results showing the characterisation of nickel nanoparticles and, for the first time, a mixture of nanoparticles: in this case nickel and silver. The former are shown not to aggregate in aqueous solution whereas the latter do. We report a novel method for the determination on unknown concentrations of nanoparticles in a sample and demonstrate its use for both aggregating and non-aggregating nanoparticles. © 2012 The Royal Society of Chemistry.

In this communication, we combine anodic particle coulometry (APC) with anodic stripping voltammetry, to find the proportion of NP impacts that result in adsorbed NPs, using AgNPs in collision with glassy carbon electrode. Sticking coefficients are reported for AgNP radii of 14, 29, and 45 nm, measured at electrode biases ranging from OCV to -0.2 to -1.2 V (vs. Ag/AgCl). No significant systematic trends were found in either case. We suggest that this methodology may be widely applicable to measuring the sticking coefficient of any oxidisable metal nanoparticle on an electrode surface in solution. © 2011 Elsevier B.V. All rights reserved.

We report, for the first time, the bulk deposition (electroplating) of a metal onto nanoparticles during collisions with an inert electrode surface. Experiments show that for silver nanoparticles, multiple layers of Cd atoms can be electroplated onto the AgNPs from aqueous Cd2+ during collisions with a glassy carbon electrode held at a suitably reducing potential, and an average of 19 atomic layers of cadmium are found to be deposited in the few milliseconds that the NP is in contact with the electrode. For comparison, results are also presented for the underpotential deposition of Cd onto AgNPs under similar conditions. © 2011 Elsevier B.V. All rights reserved.

Current spikes after collision: Analysis of current transients arising from impacts between the nanoparticles and an electrode surface (see picture) can be used to detect, identify, and determine the size of silver nanoparticles. This provides an exciting new strategy for the characterization of metal nanoparticles for analytical and environmental monitoring applications.

Informative collisions: The use of quantitative oxidative collisions between silver nanoparticles (AgNPs) and an electrode surface to monitor the aggregation of single AgNPs into larger clusters is reported (see graph). Copyright © 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

This Letter highlights recent work carried out in our laboratory on methods of fabricating and characterising arrays of nanoelectrodes. We review the template fabrication of random arrays of nanoband electrodes on highly ordered graphite surfaces and on carbon nanotubes, acoustic methods of fabricating random arrays of metal nanoparticles on electrode surfaces, and numerical methods of simulating and modelling the electrochemical processes that occur at such arrays. Finally, examples of electroanalytical applications of nanoelectrode arrays are given, including multi-metal nanoparticle arrays for combinatorial electrochemistry. © 2008 Elsevier B.V. All rights reserved.

The use of particle-impact coulometry in identifying and quantifying nanoparticles tagged (or labelled) with electroactive molecules is demonstrated via the detection of 1,4-nitrothiophenol-tagged silver nanoparticles in aqueous dispersion at potentials more negative than -0.17 V (vs. Ag/AgCl, the reduction potential of nitrothiophenol) via monitoring of particle-electrode collisions. This journal is © The Royal Society of Chemistry 2012.

We present the novel use of photoelectrochemistry to detect and monitor the motion of a single Daphnia magna swimming in a confined volume of water. Using an array of individually-addressable electrodes under illumination and potentiostatted so that a photocurrent is generated, the motion of the daphnid is detected by means of measuring "dark" transients as the shadow cast by the moving sphere passes over each electrode. The method can be used to determine the size and speed of a single daphnid.