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Book chapterHankin A, Bedoya Lora F, 2024,
Development of Reactors for Direct Solar Water Splitting
, Chemical Technologies in the Energy Transition, Editors: Costa Figueiredo, White, Publisher: Royal Society of Chemistry, Pages: 44-90, ISBN: 9781839165825In this book, readers are introduced to selected concepts, challenges, steps forward and necessities relating to the technologies required to deepen the integration between the energy and chemical sectors.
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Journal articleJang I, Hankin A, Xie Z, et al., 2024,
Structural effects of 3D inkjet-printed Ni(O)-YSZ pillared electrodes on performances of solid oxide electrochemical reactors
, Small, Vol: 20, ISSN: 1613-6810Increasing densities of reaction sites for gaseous reactants in solid oxide electrochemical reactors (SOERs), is a key strategy for achieving enhanced performance in either fuel cell or electrolysis modes. Fabrication of 3D structured components in SOERs can enhance those densities of reaction sites, which is achieved by 3D inkjet printing with high reproducibility, having developed inks with appropriate properties. First, the effects of pillar geometries on SOER performances are predicted through numerical simulations, enabling subsequent 3D printing to focus on the more effective geometries. Herein, the study reports the results of experimental validation of those predictions by evaluating the electrochemical performances of cells with various heights of 3D inkjet-printed Ni(O)- yttria stabilized zirconia (YSZ) pillars and YSZ pillars. Those measurements prove that increasing pillar heights generally increases SOER peak power densities in fuel cell mode and increased current densities at the thermoneutral potential (1.285 V) in steam electrolysis mode, as predicted by simulations. With increasing pillar heights, more limitations in performance enhancement are found with YSZ electrolyte pillars than with Ni-YSZ pillars, again as predicted by simulations. The subsequent microstructural analysis of Ni-YSZ pillars proves the suitability of the Ni(O)-YSZ composite particle ink formulation and the reliability of 3D printing.
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Journal articleReddick C, Sotelo-Vazquez C, Tam B, et al., 2024,
Photoelectrochemical disinfection efficiency of WO3-based photoanodes: development of multifunctional photoelectrocatalytic materials
, Catalysis Today, Vol: 437, ISSN: 0920-5861Access to safe water is a growing global concern, with millions lacking acceptable water sources. Photocatalysis offers eco-friendly water remediation, yet its combination with electrocatalysis for both water treatment and hydrogen production remain underexplored. This study investigates UVA LED photoelectrocatalysis using WO3-based photoanodes, alone or in heterojunction with BiVO4, to purify wastewater and co-produce hydrogen. Tests on polluted water streams containing 105 PFU mL−1 of MS2 bacteriophage virus and 106 CFU mL−1 of E. coli reveal that nanostructured WO3 achieves rapid MS2 disinfection within 5 min. (k= 0.80 min−1), with enhanced efficiency over flat counterparts. However, nanostructuring does not improve E. coli inactivation due to bacterium size constraints. These findings advance the design of tandem photoreactors for dual wastewater purification and energy generation.
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Journal articleWei X, Guo Z, Zhao Y, et al., 2024,
Recovery of graphite from industrial lithium-ion battery black mass
, RSC SustainabilityThe escalating production of commercial lithium-ion batteries (LIBs) is anticipated to result in a substantial accumulation of waste upon end-of-life disposal of LIBs, which however also represents a secondary source of raw materials. Among the components of LIBs, graphite anode is a critical material and its production via high-temperature carbonisation is highly energy- and cost-intensive. One of the major challenges regarding recycling of graphite materials from spent LIBs is the presence of residual metal and organic species that are difficult to eliminate, preventing direct reuse as anodes. Here, we propose a recycling workflow to eliminate the various impurities and regenerate the graphite materials from industrially sourced black mass, composed of mixed cathode materials, anode materials, aluminium and copper current collectors, Li salts, and polyvinylidene fluoride binders. After selective extraction of high-value transition metal ions, such as Li, Ni, and Co, from the black mass, the proposed workflow for graphite recovery involves a second step of acid leaching for the removal of Al, Cu, and other residual metal species, and mild-temperature pyrolysis for the removal of polyvinylidene fluoride (PVDF). The regenerated graphite (AG-2.0M-800) demonstrates an initial specific charge capacity of 387.44 mA h g−1 at 0.1C (35 mA g−1) in lithium half cells, on par with commercial battery-grade graphite. This workflow provides a promising approach to the recycling of spent graphite that could be integrated with existing cathode materials' recycling processes developed in the industry.
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Journal articleXie Z, Jang I, Ouyang M, et al., 2023,
High performance composite Pr4Ni3O10±δ-Ce0.75Gd0.1Pr0.15O2−δ solid oxide cell air electrode
, JPhys Energy, Vol: 5, Pages: 1-16, ISSN: 2515-7655A composite electrode composed of Pr4Ni3O10±δ - Ce0.75Gd0.1Pr0.15O2−δ (50 wt. % - 50 wt. %) was thoroughly investigated in terms of the electrochemical performance as a function of microstructure. The electrochemical performance was characterized by electrochemical impedance spectroscopy and the microstructures, characterized by focused ion beam-scanning electron microscopy and 3D reconstructions, were modified by changing the particle size of Pr4Ni3O10±δ and the electrode thickness. The distribution of relaxation time (DRT) method was applied to help resolve electrochemical processes occurring in the electrodes. It was found that an appropriate increase in electrode thickness and an appropriate decrease in particle size enhanced the oxygen reduction reaction kinetics. The
lowest area specific resistance obtained in this study at 670 °C under pO2 of 0.21 atm was 0.055 Ω cm2. Finally, a comparison to the Adler Lane Steele (ALS) model was made and the main active site for the oxygen reduction reaction was concluded to be triple phase boundaries. A fuel cell made of the composite material as the cathode was fabricated and tested. The peak power density was 1
Wcm−2 at 800 °C, which demonstrates this composite material is promising for SOFC cathodes.
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Journal articleYang M, Cui J, Daboczi M, et al., 2023,
Interplay between Collective and Localized Effects of Point Defects on Photoelectrochemical Performance of TiO<sub>2</sub> Photoanodes for Oxygen Evolution
, ADVANCED MATERIALS INTERFACES, ISSN: 2196-7350 -
Journal articleSadeek SA, Bedoya-Lora FE, Campbell KS, et al., 2022,
Formation of protective surface films on carbon steel in mildly alkaline aqueous alkanolamine CO2 solutions
, CORROSION SCIENCE, Vol: 211, ISSN: 0010-938X -
Journal articleHankin A, Bedoya-Lora FE, 2022,
Reply to the ‘Comment on “Flat band potential determination: avoiding the pitfalls”’ by M. I. Díez-García, D. Monllor-Satoca and R. Gómez, J. Mater. Chem. A, 2022, 10, DOI: 10.1039/D1TA06474F
, Journal of Materials Chemistry A, Vol: 10, Pages: 8594-8595, ISSN: 2050-7488The comment of Díez-García and co-workers on the article ‘Flat band potential determination: avoiding the pitfalls’ is a very valuable contribution to the discussion about the appropriateness of various models and techniques used for the determination of flat band potentials of semiconducting photoelectrodes, as well as other parameters. Such discussions will benefit the community and should improve the reliability of published parameters characterising photoelectrode interfaces with electrolytes. Herein we respond to the specific topics addressed in the comment: (i) the correction of the geometric photoelectrode surface area by surface roughness to enable more accurate characterisation of materials with nanotextured surfaces and (ii) the inclusion of photon flux limitation in the Gärtner–Butler model.
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Journal articleBedoya-Lora FE, Valencia-García ME, Hankin A, et al., 2022,
Determination of photon-driven charge transfer efficiency: drawbacks, accuracy and precision of different methods using Hematite as case of study
, Electrochimica Acta, Vol: 402, Pages: 139559-139559, ISSN: 0013-4686The electrochemical properties of photoelectrodes must be measured accurately and precisely to enable better comparisons between different materials. Along with the flat band potential, the interfacial charge transfer efficiency, which is the ratio between charge transfer rate at the photoelectrode surface and rate of charge carrier generation in the photoelectrode, can be used to predict the current density response at a given photon flux and electrode potential. The most widely used techniques for measuring charge transfer efficiencies are Photo-Electrochemical Impedance Spectroscopy (PEIS), current density ratios in the presence and absence of hole/electron scavengers, chrono-amperometry and Intensity Modulated Photocurrent Spectroscopy (IMPS). Charge transfer efficiencies can be estimated from PEIS and IMPS spectra either by using raw data (graphically), by fitting equivalent electrical circuits or by computing the Distribution of Relaxation Times (DRT). However, these techniques have their own drawbacks and impracticalities, that require researchers to make a choice between measuring accurately or pragmatically. Hitherto, the theoretical and experimental details of these techniques have not been summarised collectively and comprehensively. Here, we report the benefits and drawbacks, the accuracy, precision and best experimental recommendations when employing different techniques for photon-driven charge transfer efficiency determination.
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Journal articleEisner F, Tam B, Belova V, et al., 2021,
Color-tunable hybrid heterojunctions as semi-transparent photovoltaic windows for photoelectrochemical water splitting
, Cell Reports Physical Science, Vol: 2, Pages: 1-16, ISSN: 2666-3864The strong but narrow-bandwidth absorption spectra of organic semiconductors make them excellent candidates for semi-transparent solar cell applications in which color specificity is important. In this study, using a hybrid heterojunction combining the transparent inorganic semiconductor copper thiocyanate (CuSCN) with organic semiconductors (C70, PC70BM, C60, ITIC, IT-4F, or Y6), we show that simple color-tunable solar cells can be fabricated in which the transmission spectrum is determined solely by choice of the organic semiconductor. Using a joint electrical-optical model, we show that it is possible to combine the unique attributes of high photovoltage and color tunability to use these heterojunctions as photovoltaic windows in tandem photoelectrochemical (PEC)-photovoltaic (PV) cells. We demonstrate that this configuration can lead to a reduction in the parasitic absorption losses in the PEC-PV cells and, thus, to solar-to-hydrogen efficiencies (>3%) that are higher than that predicted using the traditionally used architecture in which the PV is placed behind the PEC.
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Journal articleBedoya-Lora FE, Hankin A, Kelsall GH, 2021,
En route to a unified model for photoelectrochemical reactor optimization. II–geometric optimization of perforated photoelectrodes
, Frontiers in Chemical Engineering, Vol: 3, Pages: 1-16, ISSN: 2673-2718Results have been reported previously of a model describing the performance of photoelectrochemical reactors, which utilize semiconductor | liquid junctions. This model was developed and verified using SnIV-doped α-Fe2O3 as photoanodes. Hematite films were fully characterized to obtain parameter inputs to a model predicting photocurrent densities. Thus, measured photocurrents were described and validated by the model in terms of measurable quantities. The complete reactor model, developed in COMSOL Multiphysics, accounted for gas evolution and desorption in the system. Hydrogen fluxes, charge yields and gas collection efficiencies in a photoelectrochemical reactor were estimated, revealing a critical need for geometric optimization to minimize H2-O2 product recombination as well as undesirable spatial distributions of current densities and “overpotentials” across the electrodes. Herein, the model was implemented in a 3D geometry and validated using solid and perforated 0.1 × 0.1 m2 planar photoanodes in an up-scaled photoelectrochemical reactor of 2 dm3. The same model was then applied to a set of simulated electrode geometries and electrode configurations to identify the electrode design that would maximize current densities and H2 fluxes. The electrode geometry was modified by introducing circular perforations of different sizes, relative separations and arrangements into an otherwise solid planar sheet for the purpose of providing ionic shortcuts. We report the simulated effects of electrode thickness and the presence or absence of a membrane to separate oxygen and hydrogen gases. In a reactor incorporating a membrane and a photoanode at 1.51 V vs RHE and pH 13.6, an optimized hydrogen flux was predicted for a perforation geometry with a separation-to-diameter ratio of 4.5 ± 0.5; the optimal perforation diameter was 50 µm. For reactors without a membrane, this ratio was 6.5 and 8.5 for a photoanode in a “wired” (mo
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Journal articleMoss B, Babacan O, Kafizas A, et al., 2021,
A review of inorganic photoelectrode developments and reactor scale-up challenges for solar hydrogen production
, Advanced Energy Materials, Vol: 11, Pages: 1-43, ISSN: 1614-6832Green hydrogen, produced using solar energy, is a promising means of reducing greenhouse gas emissions. Photoelectrochemical (PEC) water splitting devices can produce hydrogen using sunlight and integrate the distinct functions of photovoltaics and electrolyzers in a single device. There is flexibility in the degree of integration between these electrical and chemical energy generating components, and so a plethora of archetypal PEC device designs has emerged. Although some materials have effectively been ruled out for use in commercial PEC devices, many principles of material design and synthesis have been learned. Here, the fundamental requirements of PEC materials, the top performances of the most widely studied inorganic photoelectrode materials, and reactor structures reported for unassisted solar water splitting are revisited. The main phenomena limiting the performance of up‐scaled PEC devices are discussed, showing that engineering must be considered in parallel with material development for the future piloting of PEC water splitting systems. To establish the future commercial viability of this technology, more accurate techno‐economic analyses should be carried out using data from larger scale demonstrations, and hence more durable and efficient PEC systems need to be developed that meet the challenges imposed from both material and engineering perspectives.
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Journal articleBedoya-Lora FE, Holmes-Gentle I, Hankin A, 2021,
Electrochemical techniques for photoelectrode characterisation
, Current Opinion in Green and Sustainable Chemistry, Vol: 29, Pages: 1-8, ISSN: 2452-2236Photoelectrodes enable simultaneous light absorption and catalysis of water splitting reactions. Their performance is established using electrochemical characterisation methods. Besides basic characterisation techniques such as voltammetry and chronoamperometry, employed in the dark or under illumination, more advanced techniques, including (photo-)electrochemical impedance spectroscopy, intensity-modulated impedance spectroscopy and transient absorption spectroscopy, can be used to evaluate key parameters and processes. For some of these techniques, data is often interpreted using over-simplified models, leading to the calculation of unreliable parameters. The values of the flat band potential and charge transfer efficiency depend heavily on the methods used to determine them, and it is recommended that the values are corroborated using multiple techniques. Lastly, certain ‘efficiencies’ defined in the literature for electrically biased systems should be revised.
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Journal articleHankin A, Bedoya-Lora FE, Alexander JC, et al., 2019,
Flat band potential determination: avoiding the pitfalls
, Journal of Materials Chemistry A, Vol: 7, Pages: 26162-26176, ISSN: 2050-7488The flat band potential is one of the key characteristics of photoelectrode performance. However, its determination on nanostructured materials is associated with considerable uncertainty. The complexity, applicability and pitfalls associated with the four most common experimental techniques used for evaluating flat band potentials, are illustrated using nanostructured synthetic hematite (α-Fe2O3) in strongly alkaline solutions as a case study. The motivation for this study was the large variance in flat band potential values reported for synthetic hematite electrodes that could not be justified by differences in experimental conditions, or by differences in their charge carrier densities. We demonstrate through theory and experiments that different flat band potential determination methods can yield widely different results, so could mislead the analysis of the photoelectrode performance. We have examined: (a) application of the Mott–Schottky (MS) equation to the interfacial capacitance, determined by electrochemical impedance spectroscopy as a function of electrode potential and potential perturbation frequency; (b) Gärtner–Butler (GB) analysis of the square of the photocurrent as a function of electrode potential; (c) determination of the potential of transition between cathodic and anodic photocurrents during slow potentiodynamic scans under chopped illumination (CI); (d) open circuit electrode potential (OCP) under high irradiance. Methods GB, CI and OCP were explored in absence and presence of H2O2 as hole scavenger. The CI method was found to give reproducible and the most accurate results on hematite but our overall conclusion and recommendation is that multiple methods should be employed for verifying a reported flat band potential.
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Journal articleBedoya-Lora FE, Hankin A, Kelsall GH, 2019,
Hydrogen sulfide splitting using solar energy and hematite photo-anodes
, Electrochimica Acta, Vol: 317, Pages: 384-397, ISSN: 0013-4686The mechanism was investigated of hydrogen sulfide splitting in alkaline aqueous solutions using spray-pyrolysed SnIV-doped α-Fe2O3 photo-anodes in a photo-electrochemical cell. In principle, hydrogen sulfide splitting can be used to treat hydrogen sulfide in natural and process gases and simultaneously to produce hydrogen using solar energy. A comparison with conventional water splitting demonstrated the lower energy requirements to achieve the same photocurrent densities, while producing soluble polysulfide ions and elemental sulfur from hydrogen sulfide oxidation. However, neither splitting process was spontaneous using SnIV-doped α-Fe2O3 photo-anodes without inputs of electrical energy; two judiciously chosen photo-electrodes are required to achieve that objective. The effects were also studied of stirring, hydrogen sulfide ion concentration, electrode potential and annealing of SnIV-doped α-Fe2O3 films on titanium substrates. Under potentiostatic conditions during photo-assisted electrolysis, the photo-anodes exhibited no compositional or morphological changes after 18 h. In a bench-scale reactor (0.1 dm3), stable photocurrent densities of ca. 12.5 A m−2 were recorded over 12 h at an electrode potential of 1.17 V vs. RHE and an effective irradiance of 2670 W m−2. Similarly, photocurrent densities corresponding to ca. 4.3 A m−2 were achieved in an up-scaled reactor under an effective irradiance of 457 W m−2. Charge yields for formation of polysulfide ions were close to unity when operating at optimised potentials and hydrogen sulfide ion concentrations. The shift towards lower electrode potentials of photocurrent densities for hydrogen sulfide splitting compared with those for water splitting was associated with increased charge transfer rates due to decreased interfacial electron-hole recombination rates. The potential dependences of sulfur coverage and oxygen evolution rates were also estimated.
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Journal articleBedoya-Lora FE, Hankin A, Kelsall GH, 2019,
In situ determination of polysulfides in alkaline hydrogen sulfide solutions
, Electrochimica Acta, Vol: 314, Pages: 40-48, ISSN: 0013-4686A method was developed to determine low concentrations of polysulfide ions (Sn2- expressed as zero-valent sulfur) in situ and in the presence of high concentrations (0.5 mol dm-3) of hydrogen sulfide ions, HS-, at pH 14. UV-visible spectrophotometry was used to determine absorbances at 295 and 420 nm using an immersion probe, designed for highly corrosive environments. Three absorbance trends were found, corresponding to three concentration ranges of zero-valent sulfur: low (0 – 1.2 10-3 mol dm-3), medium (1.2 – 3.6 10-3 mol dm-3) and high (3.6 – 10 10-3 mol dm-3). The non-linear dependence of absorbance on concentration over the range studied was due to disproportionation of polysulfides. Determination of these species is well known to be problematic at low concentrations due to the effects of adventitious oxygen in solution, meta-stability and speciation of polysulfide species: S22- – S82-. Oxygen concentrations must be minimised in the inert gas used to de-oxygenate sulfide solutions and for the same reason, their contact with atmospheric oxygen should be minimised. During potentiostatic oxidation of alkaline solutions containing HS- ions in the anolyte of electrochemical reactors incorporating cation-permeable membranes, temporal changes in anolyte absorbance and charge were used to estimate polysulfide concentrations. Charge yields for sulfide to polysulfide oxidation were close to unity, confirming the utility of the technique developed. Molar attenuation coefficients of the predominant polysulfide ions S32- at 420 nm and S42- at 295 nm were also estimated as 289 and 3609 dm3 mol-1 cm-1, respectively, and comparable to values of (190, 206) and (3420, 3690) dm3 mol-1 cm-1 reported previously.
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Journal articleAitchison CM, Andrei V, Antón-García D, et al., 2019,
Synthetic approaches to artificial photosynthesis: general discussion.
, Faraday Discuss, Vol: 215, Pages: 242-281 -
ReportHankin A, Guillen Gosalbez G, Kelsall G, et al., 2019,
Assessing the economic and environmental value of carbon capture and utilisation in the UK
, Briefing Note – summary of Briefing Paper No 3• As a signatory to the 2015 Paris Climate Change Agreement, the UK has committed to an ambitious transformation of its economy.• Decarbonisation of the UK’s economy must be a priority, but carbon-based fuels and platform chemicals will remain important to the global economy; their production from captured carbon dioxide and renewable energy can support this industrial need.• In this Briefing Paper, we report on results of a systematic procedure developed to assess the viability of different carbon capture and utilisation (CCU) pathways.• Our findings on three CCU pathways show that proposed CCU projects should always be assessed on a case-by-case basis, using detailed, UK centric, cradle-to-grave life cycle analyses.• CCU cannot provide the emission mitigation rate of carbon capture and storage (CCS), but as the UK’s entire geological storage capacity is offshore, CCU could mitigate emissions from inland point sources.• Of the considered CCU pathways, presently the production of polyurethane is the most promising for the UK and could provide an immediate short-term mitigation solution for greenhouse gas (GHG) emissions. Currently, methanol production does not appear to be a viable solution.
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ReportHankin A, Guillen Gosalbez G, Kelsall G, et al., 2019,
Assessing the economic and environmental value of carbon capture and utilisation in the UK
, Briefing paper, 3• As a signatory to the 2015 Paris Climate Change Agreement, the UK has committed to an ambitious transformation of its economy.• Decarbonisation of the UK’s economy must be a priority, but carbon-based fuels and platform chemicals will remain important to the global economy; their production from captured carbon dioxide and renewable energy can support this industrial need.• In this Briefing Paper, we report on results of a systematic procedure developed to assess the viability of different carbon capture and utilisation (CCU) pathways.• Our findings on three CCU pathways show that proposed CCU projects should always be assessed on a case-by-case basis, using detailed, UK centric, cradle-to-grave life cycle analyses.• CCU cannot provide the emission mitigation rate of carbon capture and storage (CCS), but as the UK’s entire geological storage capacity is offshore, CCU could mitigate emissions from inland point sources.• Of the considered CCU pathways, presently the production of polyurethane is the most promising for the UK and could provide an immediate short-term mitigation solution for greenhouse gas (GHG) emissions. Currently, methanol production does not appear to be a viable solution.
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Journal articleBedoya Lora FE, Hankin A, Kelsall G, 2017,
En route to a unified model for photo-electrochemical reactor optimization. I - Photocurrent and H₂ yield predictions
, Journal of Materials Chemistry A, Vol: 5, Pages: 22683-22696, ISSN: 2050-7496A semi-empirical model was developed for prediction of photocurrent densities and implemented to predict the performance of a photo-electrochemical reactor for water splitting in alkaline solutions, using Sn-doped α-Fe₂O₃ photo-anodes produced by spray pyrolysis. Photo-anodes annealed at different temperatures were characterized using photo-electrochemical impedance spectroscopy, cyclic voltammetry in the presence and absence of a hole scavenger and also the open circuit potential under high intensity illumination. Mott-Schottky analysis was used cautiously to estimate charge carrier concentration and the flat band potential. In addition to overpotential/current distribution and ohmic potential losses, the model also accounts for absorbed photon flux, surface and bulk electron-hole recombination rates, gas desorption, bubble formation and (H₂-O₂) cross-over losses. This allows the model to estimate the total yield of hydrogen, charge and gas collection efficiencies. A methodology is presented here in order to evaluate parameters required to assess the performance of a photo-electrochemical reactor in 1D and 2D geometries. The importance of taking into account bubble generation and gas desorption is discussed, together with the difficulties of measuring charge carrier concentration and electron-hole recombination in the bulk of the semiconductor, which are of major importance in the prediction of photocurrent densities.
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Journal articleBedoya-Lora FE, Hankin A, Holmes-Gentle I, et al., 2017,
Effects of low temperature annealing on the photo-electrochemical performance o tin-doped hematite photo-anodes
, Electrochimica Acta, Vol: 251, Pages: 1-11, ISSN: 0013-4686The effects of post-deposition annealing at 400 and 500 °C on the photo-electrochemical performance of SnIV-doped α-Fe2O3 photo-anodes are reported. Samples were fabricated by spray pyrolysis on fluorine-doped tin oxide (FTO) and on titanium substrates. Photo-electrochemical, morphological and optical properties were determined to explain the shift in photocurrent densities to lower electrode potentials and the decrease of maximum photocurrent densities for alkaline water oxidation after annealing. Annealing at 400 and 500 °C in air did not affect significantly the morphology, crystallinity, optical absorption or spatial distributions of oxygen vacancy concentrations. However, XPS data showed a redistribution of SnIV near SnIV-doped α-Fe2O3 | 1 M NaOH interfaces after annealing. Thus, electron-hole recombination rates at photo-anode surfaces decreased after annealing, shifting photocurrents to lower electrode potentials. Conversely, depletion of SnIV in the α-Fe2O3 bulk could increase recombination rates therein and decrease photon absorption near 550 nm, due to an increased dopant concentration in the semiconductor depletion layer. This accounted for the decrease of maximum photocurrents when electron-hole recombination rates were suppressed using HO2− ions as a hole scavenger. The flat band potential of SnIV-doped α-Fe2O3 remained relatively constant at ca. 0.7 V vs. RHE, irrespective of annealing conditions.
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Journal articleHankin A, Shah N, 2017,
Process exploration and assessment for the production of methanol and dimethyl ether from carbon dioxide and water
, Sustainable Energy & Fuels, Vol: 1, Pages: 1541-1556, ISSN: 2398-4902A thermodynamic, model-based, study was carried out to assess the relative performance of methanol and dimethyl ether (DME) synthesis systems using CO- and CO2-based syngas feeds. The upstream production of a range of syngas feed compositions was simulated using CO2 and H2O as the sole chemical building blocks, a requirement motivated by the increasing constraints on permissible CO2 emissions and the successful adaptation by some industrial methanol plants to the direct utilisation of CO2. The objective was to establish whether the energy requirements and CO2 emissions associated with upstream conversion of CO2 to CO were justified by increased productivity in the methanol/DME systems. In the first part of the study, the performance of four systems was evaluated and compared in terms of energy efficiency and CO2 conversion: (1) methanol synthesis system, (2) direct DME synthesis system, (3) two-step DME synthesis system with an interposed syngas separation step between the methanol production reactor and methanol dehydration reactor and (4) two-step DME synthesis system with no separation step between the two reactors. Based on equilibrium yields at 250 °C and 50 bar, the direct DME synthesis system was found to exhibit the highest energy conversion efficiencies with both CO2- and CO-based syngas. Although this system demonstrated the lowest CO2 emissions per methanol equivalent product with a CO-based feed, the benefits were offset by emissions associated with the upstream conversion of H2O and CO2 to H2 and CO, evaluated in the second part of the study. It was determined that CO2 could be utilised directly in the direct DME synthesis route, whereas upstream conversion of CO2 to CO was necessary to achieve effective yields in the methanol/two-step DME systems. CO-based syngas production via high temperature co-electrolysis of H2O and CO2, or alternatively high temperature CO2 electrolysis coupled with the water–gas shift process, was identified as the bes
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Journal articleHankin A, Bedoya-Lora FE, Ong CK, et al., 2017,
From millimetres to metres: the critical role of current density distributions in photo-electrochemical reactor design
, Energy and Environmental Science, Vol: 10, Pages: 346-360, ISSN: 1754-56920.1×0.1 m2 tin-doped hematite photo-anodes were fabricated on titanium substrates by spray pyrolysis and deployed in a photo-electrochemical reactor for photo-assisted splitting of water into hydrogen and oxygen. Hitherto, photo-electrochemical research focussed largely on the fabrication, properties and behaviour of photo-electrodes, whereas both experimental and modelling results reported here address reactor scale-up issues of minimising inhomogeneities in spatial distributions of potentials, current densities and the resultant hydrogen evolution rates. Such information is essential for optimising the design and photon energy-to-hydrogen conversion efficiencies of photo-electrochemical reactors to progress their industrial deployment. The 2D and 3D reactor models presented here are coupled with a modified micro-kinetic model of oxygen evolution on hematite thin films both in the dark and when illuminated. For the first time, such a model is applied to a scaled-up photo-electrochemical reactor and validated against experimental data.
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Journal articleKleiminger L, Farandos N, Li T, et al., 2016,
Three-dimensional Inkjet Printed Solid Oxide Electrochemical Reactors.I. Yttria-stabilized zirconia Electrolyte
, Electrochimica Acta, Vol: 213, Pages: 324-331, ISSN: 0013-4686Solid oxide fuel cell (SOFC) and electrolyser (SOE) performances can be enhanced significantly by increasing the densities of (electrode | electrolyte | pore) triple phase boundaries and improving geometric reproducibility and control over composite electrode | electrolyte microstructures, thereby also aiding predictive performance modelling. We developed stable aqueous colloidal dispersions of yttria-stabilized zirconia (YSZ), a common SOFC electrolyte material, and used them to fabricate 2D planar and highly-customisable 3D microstructures by inkjet printing. The effects of solids fraction, particle size, and binder concentration on structures were investigated, and crack-free, non-porous electrolyte planes were obtained by tailoring particle size and minimising binder concentration. Micro-pillar arrays and square lattices were printed with the optimised ink composition, and a minimum feature size of 35 μm was achieved in sintered structures, the smallest published to-date. YSZ particles were printed and sintered to a 23 μm thick planar electrolyte in a Ni-YSZ|YSZ|YSZ-LSM|LSM electrolyser for CO2 splitting; a feed of 9:1 CO2:CO mixture at 1.5 V and 809 °C produced a current density of −0.78 A cm−2 even without more complex 3D electrode | electrolyte geometries.
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Journal articleBedoya-Lora F, Hankin A, Kelsall GH, 2016,
Photo-electrochemical Hydrogen Sulfide Splitting using SnIV-doped Hematite Photo-anodes
, Electrochemistry Communications, Vol: 68, Pages: 19-22, ISSN: 1388-2481Spray-pyrolysed SnIV-doped α-Fe2O3 photo-anodes were used for photo-assisted splitting of HS- ions in alkaline aqueous solutions, producing polysulfide (Sn2-) ions together with hydrogen at the cathode. Subsequent aerial oxidation of polysulfide could be used to produce elemental sulfur. At an applied electrode potential of 1.07 V (RHE) and an irradiance of 5.6 kW m-2, stable photocurrents of ca. 11 A m-2 (210-3 A W-1) were recorded over 75 hours, polysulfide concentrations increasing linearly with time. Despite being predicted thermodynamically to form iron sulfide(s) in sulfide solutions, such photo-anodes appeared to be stable. In comparison with conventional water splitting under alkaline conditions, the coupled processes of hydrogen sulfide ion oxidation and water reduction had a lower energy requirement.
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Conference paperVideira JJH, Barnham KWJ, Hankin A, et al., 2015,
Introducing novel light management to design a hybrid high concentration photovoltaic/water splitting system
, 2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC), Publisher: IEEEWe present a novel way to utilize high-concentrationphotovoltaic (HCPV) radiative losses and diffuse light, otherwiseunused in conventional HCPV systems, to power an ImperialCollege designed photoelectrochemical reactor (PECR) producingH2 fuel through water splitting. A high efficiency photovoltaic(HEPV) is embedded inside a Luminescent Solar Concentrator(LSC). Edge emission from the radiative recombination lossmechanism in the HEPV is guided within the LSC to the PECRphotocathode, whilst the LSC emitted light is guided to thephotoanode. The photon streams can be independently optimisedin intensity and wavelength. We demonstrate how photon streamswith balanced intensity can be achieved.
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Journal articleHankin A, Kelsall GH, Ong CK, et al., 2014,
Photo-electrochemical production of H2 using solar energy
, Chemical Engineering Transactions, Vol: 41, Pages: 199-204, ISSN: 2283-9216Ti | SnIV-Fe2O3 photo-anodes are implemented in a photo-electrochemical reactor for photo-assistedsplitting of water into hydrogen and oxygen. Attention is focused on the issues concerning electrode scaleupwith the aim of addressing the present need for the design, optimisation and demonstration of thecommercial feasibility of photo-electrochemical reactors.
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Journal articleHankin A, Alexander JC, Kelsall GH, 2014,
Constraints to the flat band potential of hematite photo-electrodes
, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 16, Pages: 16176-16186, ISSN: 1463-9076- Author Web Link
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Journal articleDroushiotis N, Hankin A, Rozain C, et al., 2014,
Phase Inversion and Electrophoretic Deposition Processes for Fabrication of Micro-Tubular Hollow Fiber SOFCs
, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, Vol: 161, Pages: F271-F279, ISSN: 0013-4651- Author Web Link
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Journal articleDroushiotis N, Hankin A, Kelsall GH, 2013,
New fabrication techniques for micro-tubular hollow finer solid oxide fuel cells
, ECS Transactions, Vol: 50, ISSN: 1938-6737A novel combination of phase inversion and electrophoretic deposition was used in the fabrication of anode supported micro tubular (hollow fiber) solid oxide fuel cells (MT-HF-SOFCs). The phase inversion process was used to produce ca. 240 μm thick, highly porous 60 wt. % NiO-40 wt. % yttria-stabilised zirconia (YSZ) hollow fiber anode precursors. The electrophoretic deposition process was then used to apply ca. 40 μm thick, particulate YSZ electrolyte layers onto the unsintered NiO-YSZ HFs from an ethanol suspension at an applied electric field of ca. 0.22 kV cm-1. The YSZ-coated NiO-YSZ HFs were sintered at 1500 oC for twelve hours. Dispersions of YSZ-LSM particles were then painted on top of the electrolyte layer, as ‘graded’ YSZ-LSM porous cathode precursors that were sintered at 1200 oC for three hours. The fabrication process was completed by winding silver wire current collectors spirally round the cathodes and through the lumen of the fibers to enable current collection from the anodes. Single MT-HF-SOFCs delivered peak power densities of 0.20, 0.18 and 0.14 W cm-2 at 800, 750 and 700 oC, respectively, with flow rates of 15 cm3 min-1 H2 (97% H2-3% H2O) and 30 cm3 min-1 of air.
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