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*Akpınar, Işıl ( Aksaray, Yazar ), Hydrogen is one of the most attractive renewables for future energy application, therefore it is vital to develop cost-effective and highly-efficient electrocatalysts for the hydrogen evolution reaction (HER) to promote the generation of hydrogen from mild methods. In this work, Co–Mo phosphide nanosheets with the adjustable ratio of Co and Mo were fabricated on carbon cloth by a facile hydrothermal-annealing method. Owing to the unique nanostructures, abundant active surfaces and small resistance were achieved. Excellent electrocatalytic performances are obtained, such as the small overpotential of ∼67.3 mV to realize a current density of 10 mA cm−2 and a Tafel slope of 69.9 mV dec−1. Rapid recovery of the current response under multistep chronoamperometry is realized and excellent stability retained after the CV test for 2000 cycles. The change of electronic states of different elements was carefully studied which suggested the optimal electrochemical performance can be realized by tuning phosphorous and metal interactions.

Operando microbeam Raman spectroscopy is used to map the changes in hard carbon during sodiation and desodiation in unprecedented detail, elucidating several important and unresolved aspects of the...

Redox flow batteries represent a remarkable alternative for grid-scale energy storage. They commonly employ carbon felts or carbon papers, which suffer from low activity towards the redox reactions involved, leading to poor performance. Here we propose the use of electrospun freestanding carbon materials derived from lignin as alternative sustainable electrodes for all-vanadium flow batteries. The lignin-derived carbon electrospun mats exhibited a higher activity towards the VO2+/VO2+ reaction than commercial carbon papers when tested in a three-electrode electrochemical cell (or half-cell), which we attribute to the higher surface area and higher amount of oxygen functional groups at the surface. The electrospun carbon electrodes also showed performance comparable to commercial carbon papers, when tested in a full cell configuration. The modification of the surface chemistry with the addition of phosphorous produced different effect in both samples, which needs further investigation. This work demonstrates for the first time the application of sustainably produced electrospun lignin-derived carbon electrodes in a redox flow cell, with comparable performance to commercial materials and establishes the great potential of biomass-derived carbons in energy devices.

Effective diagnostic techniques for Li-ion batteries are vital to ensure that they operate in the required voltage and temperature window to prevent premature degradation and failure. Ultrasonic analysis has been gaining significant attention as a low cost, fast, non-destructive, operando technique for assessing the state-of-charge and state-of-health of Li-ion batteries. Thus far, the majority of studies have focused on a single C-rate at relatively low charge and discharge currents, and as such the relationship between the changing acoustic signal and C-rate is not well understood. In this work, the effect of cell temperature on the acoustic signal is studied and shown to have a strong correlation with the signal’s time-of-flight. This correlation allows for the cell temperature to be inferred using ultrasound and to compensate for these effects to accurately predict the state-of-charge regardless of the C-rate at which the cell is being cycled. Ultrasonic state-of-charge monitoring of a cell during a drive cycle illustrates the suitability of this technique to be applied in real-world situations, an important step in the implementation of this technique in battery management systems with the potential to improve pack safety, performance, and efficiency

The porous transport layer (PTL) in polymer electrolyte membrane water electrolysers (PEMWEs) has the multiple roles of delivering water to the electro-catalyst, removal of product gas, and acts as a conduit for electronic and thermal transport. They are, thus, a critical component for optimized performance, especially at high current density operation. This study examines the relationship between the microstructure and corresponding electrochemical performance of commonly used titanium sinter PTLs. Four PTLs, with mean pore diameter (MPD) ranging from 16 μm to 90 μm, were characterized ex-situ using scanning electron microscopy and X-ray computed micro-tomography to determine key structural properties. The performance of these PTLs was studied operando using polarization and electrochemical impedance spectroscopy. Results showed that an increase in mean pore size of the PTLs correlates to an increase in the spread and multimodality of the pore size distribution and a reduction in homogeneity of porosity distribution. Electrochemical measurements reveal a strong correlation of mean pore size of the PTLs with performance. Smaller pore PTLs showed lower Ohmic resistance but higher mass transport resistance at high current density of 3.0 A cm−2. A non-monotonic trend of mass transport resistance was observed for different PTLs, which suggests an optimal pore size beyond which the advantageous influence of macroporosity for mass transport is diminished. The results indicate that maximizing contact points between the PTL and the catalyst layer is the overriding factor in determining the overall performance. These results guide PTL design and fabrication of PEMWEs.

Significant performance variation exists across the lateral extent of a fuel cell electrode as a consequence of interconnected factors including reactant pressure drop, membrane humidification and catalyst utilisation. This can result in heterogeneous degradation rates, diminishing its lifetime. Here, current and temperature mapping are coupled with localised cyclic voltammetry (electrochemical surface area (ECSA)) and linear sweep voltammetry (hydrogen crossover) to study performance degradation in a 100 cm2 fuel cell, subjected to voltage cycling simulating repetitive start-up/shutdown procedures. With increased cycling, the current distribution below mid-currents become more heterogeneous, whilst their temperature maps remain relatively uniform. At higher currents, rising temperatures homogenises the current profiles. ECSA loss declines non-linearly, with no discernible changes in hydrogen crossover. Degradation rates vary across the active area; with differing areas of highest ECSA and performance loss. Results suggests that deteriorating water management, over ECSA loss from the carbon support corrosion, is the main contribution to the performance deterioration.

This work considers the relationship between the morphology of porous carbon materials used for supercapacitors and the electrochemical impedance spectroscopy (EIS) response. EIS is a powerful tool that can be used to study the porous 3-dimensional electrode behavior in different electrochemical systems. Porous carbons prepared by treatment of cellulose with different compositions of potassium hydroxide (KOH) were used as model systems to investigate the form vs. electrochemical function relationship. A simple equivalent circuit that represents the electrochemical impedance behavior over a wide range of frequencies was designed. The associated impedances with the bulk electrolyte, Faradaic electrode processes and different pore size ranges were investigated using a truncated version of the standard transmission line model. The analysis considers the requirements of porous materials as electrodes in supercapacitor applications, reasons for their non-ideal performance and the concept of ‘best capacitance’ behavior in different frequency ranges. Graphical Abstract: The electrolyte ions penetrate the macropores, into the mesopores and finally reach the micropores. The capacitance measured by impedance spectroscopy is a function of frequency and porous structure.

Redox flow batteries offer a potential solution to an increase in renewable energy generation on the grid by offering long-term, large-scale storage and regulation of power. However, they are currently underutilised due to cost and performance issues, many of which are linked to the microstructure of the porous carbon electrodes used. Here, for the first time, we offer a detailed study of the in situ effects of compression on a commercially available carbon felt electrode. Visualisation of electrode structure using X-ray computed tomography shows the non-linear way that these materials compress and various metrics are used to elucidate the changes in porosity, pore size distribution and tortuosity factor under compressions from 0%–90%.

In-depth understanding of the dynamics of water formation, accumulation and removal is important for flow-field design optimization to ensure robust performance and durability of polymer electrolyte fuel cells (PEFCs). Here, in-operando neutron radiography is used to display and quantify liquid water distribution across the entire active area of single-, double- and quad-channel serpentine flow-fields. The results revealed that the water management and performance of PEFCs is strongly affected by the number of serpentine channels in the cathode flow-field. The single-channel serpentine-based PEFC exhibits both a better cell performance and uniformity in the local water distribution. The quad-channel based PEFC exhibits the largest voltage fluctuations caused by severe water flooding in the gas channels. However, the single-channel design leads to significantly larger pressure drop than the multiple-channel counterparts, which requires much higher parasitic power to pressurize and recirculate the reactants. Three different regimes of operation can be defined based on the current density: gradually increasing hydration ( > 600 mA cm−2). The reduced overall quantity of water in the channels with an increase in current density can be attributed to faster gas velocity and higher cell temperature.

The relative vast abundance and more equitable global distribution of terrestrial sodium makes sodium-ion batteries (NIBs) potentially cheaper and more sustainable alternatives to commercial lithium-ion batteries (LIBs). However, the practical capacities and cycle lives of NIBs at present do not match those of LIBs and have therefore hindered their progress to commercialisation. The present drawback of NIB technology stems largely from the electrode materials and their associated Na+ ion storage mechanisms. Increased understanding of the electrochemical storage mechanisms and kinetics is therefore vital for the development of current and novel materials to realise the commercial NIB. In contrast to x-ray techniques, the non-dependency of neutron scattering on the atomic number of elements (Z) can substantially increase the scattering contrast of small elements such as sodium and carbon, making neutron techniques powerful for the investigation of NIB electrode materials. Moreover, neutrons are far more penetrating which enables more complex sample environments including in situ and operando studies. Here, we introduce the theory of, and review the use of, neutron diffraction and quasi-elastic neutron scattering, to investigate the structural and dynamic properties of electrode and electrolyte materials for NIBs. To improve our understanding of the actual sodium storage mechanisms and identify intermediate stages during charge/discharge, ex situ, in situ, and operando neutron experiments are required. However, to date there are few studies where operando experiments are conducted during electrochemical cycling. This highlights an opportunity for research to elucidate the operating mechanisms within NIB materials that are under much debate at present.

The solid polymer electrolyte (SPE) alkaline anion exchange membrane (AAEM) fuel cell exhibits facile oxygen reduction reaction (ORR) kinetics and has the ability to utilise non-precious metal electrocatalysts. However, the AAEM is reported to suffer from increased instability within the alkaline media (degradation) via a number of routes, including nucleophilic elimination when operated at temperatures above 60 °C, somewhat eliminating the kinetic advantage of operating at higher temperatures. Nonetheless, modelling studies have indicated that the membrane hydration could show improved resistance to alkaline instability and subsequent degradation when operated at elevated temperatures. This investigation uses the quartz crystal microbalance (QCM) to examine the thermal stability of a commercial AAEM as a function of humidity. The results show that hydration improves ionomer resistance to degradation, as the ions within the system (namely the OH− nucleophile and cationic headgroups) become less reactive. In-line mass spectrometry data confirms that the ionomer degrades during the elevated temperature excursions used in this study.

The tortuosity of a porous structure is commonly used to relate the bulk transport property to an effective transport property. This approach is applied for a wide range of phenomena including charge transport and gas diffusion alike. However, due to the inherent differences between ionic and diffusive transport, the effect of the microstructure on either effective transport property differs. Here, we calculate the tortuosity of a porous membrane via gas diffusion experiments and ionic resistance measurements and compare the results of both techniques. The ionic resistance measurement arrives at lower tortuosity value compared to the gas diffusion measurements. The reason for this might stem from Knudsen diffusion effects, which only affect diffusive mass transport in the gas phase and is affected by the local variation in pore diameter. Hence, Knudsen diffusion effects cannot be neglected in microstructures featuring nano-scale pores.

Many electrochemical studies exist using the acidic ionomer Nafion as a binder in the ink formulation when operating in high pH systems. However, Nafion acts as an ionic insulator for OH−, and for reactions such as the hydrogen oxidation reaction, the transport of OH− to the catalyst surface is of utmost importance when elucidating the performance of a catalyst. This work demonstrates that when using an alkaline polymer binder in the ink, the apparent activity of a commercially synthesized Pt/C catalyst is increased due to a lower diffusion resistance for the reaction. In order to obtain accurate values for kinetic data in alkaline media, the use of the acidic binder should be avoided.

The compression of carbon felt electrodes for redox flow batteries leads to changes in the electrochemical performance and has a large effect on the pressure drop of electrolyte flow through the system. In this investigation, the authors have characterised the electrochemical performance of all-vanadium redox flow batteries by studying the effect of compression on the contact resistance, polarisation behaviour and efficiency. Contact resistance was seen to reduce from ca. 2.0 Ω cm2 to 1.2 Ω cm2 and an energy efficiency of 85% was obtained from a felt compressed to 75%. Moreover, X-ray computed tomography (CT) has been employed to study the microstructure of felt electrodes at compressions up to 70%, showing a linear decrease in porosity and a constant fibre surface area-to-volume ratio. The pressure drop was modelled using computational fluid dynamics and employing the 3D structure of the felts obtained from CT, revealing that a 60% increase in compression related to a 44.5% increase in pressure drop.

The need to understand the effects of degradation in polymer electrolyte fuel cells (PEFCs) has led to the development of in-situ and operando imaging studies of failure mechanisms occurring in their constituent materials, but studies using X-ray computed tomography (X-ray CT) for imaging have focussed primarily on a single region of the PEFC flow channel. Whilst studies have shown local variation in degradation rates using electrochemical techniques, this work employs identical-location X-ray CT imaging to elucidate the local degradation of a membrane electrode assembly (MEA) at various locations along-the-channel of a serpentine flow field. Using a carbon corrosion specific accelerated stress test (AST), in-depth analyses of the catalyst layers (CLs) and crack network allow for quantification of the material’s degradation at the inlet, middle and outlet regions of the flow channel on the cathode side. Imaging of the regions was done after 0, 2000 and 5000 cycles and results show significant regional variation in the extent of degradation of the cathode CL. The order of degradation was found to be outlet > middle > inlet, with the outlet CL found to be thinner than the middle and inlet regions, with a greater extent of cracking. Furthermore, additional land and channel degradation effects were probed, with the land regions found to be less degraded than channel regions. This work further highlights the need to understand and develop ASTs that can promote a more uniform degradation rate across an MEA.

Water uptake, sorption mechanics and swelling characteristics of thin-film Nafion and a commercially available Tokuyama alkaline anion exchange membrane ionomer from the vapour phase is explored using a quartz crystal microbalance (QCM). The water uptake measures the number of water molecules adsorbed by the ionomer per functional group and is determined in-situ using the QCM frequency responses allowing for comparison with nanogram precision. Crystal admittance spectroscopy, along with equivalent circuit fitting, is applied to both thin films for the first time and is used to investigate the ionomer's viscoelastic changes during hydration; to elucidate the mechanisms at play during low, medium and high relative humidities.

The encapsulation of caesium (Cs) and strontium (Sr) contaminated clinoptilolite in Na and K based metakaolin geopolymers is reported. When Cs or Sr loaded clinoptilolite is mixed with a metakaolin geopolymer paste, the high pH of the activating solution and the high concentration of ions in solution cause ion exchange reactions and dissolution of clinoptilolite with release of Cs and Sr into the geopolymer matrix. The leaching of Cs and Sr from metakaolin-based geopolymer has therefore been investigated. It was found that Na-based geopolymers reduce leaching of Cs compared to K-based geopolymers and the results are in agreement with the hard and soft acids and bases (HSAB) theory. Cs ions are weak Lewis acids and aluminates are a weak Lewis base. During the formation of the geopolymer matrix Cs ions are preferentially bound to aluminate phases and replace Na in the geopolymer structure. Sr uptake by Na-geopolymers is limited to 0.4 mol Sr per mole of Al and any additional Sr is immobilised by the high pH which causes precipitation of Sr as low solubility hydroxide and carbonate phases. There was no evidence of any other phases being formed when Sr or Cs are added to metakaolin geopolymers.

Magnox swarf contaminated with trace levels of Al metal is an important UK legacy waste originated from the fuel rod cladding system used in Magnox nuclear power stations. Composite cements made from Portland cement and blast furnace slag form a potential encapsulation matrix. However the high pH of this system causes the Al metal to corrode causing durability issues. Geopolymers derived from metakaolin are being investigated as an alternative encapsulation matrix for Magnox swarf waste and the corrosion kinetics and surface interactions of Al with metakaolin geopolymer are reported in this paper. It is shown that the pH of the geopolymer paste can be controlled by the selection of metakaolin and the sodium silicate solution used to form the geopolymer. A decrease in pH of the activation solution reduces corrosion of the Al metal and increases the stability of bayerite and gibbsite layers formed on the Al surface. The bayerite and gibbsite act as a passivation layer which inhibits further corrosion and mitigates H 2 generation. The research shows that optimised metakaolin geopolymers have potential to be used to encapsulate legacy Magnox swarf wastes.

Flow-field plates are essential components for polymer electrolyte fuel cells (PEFCs), the most common types incorporating parallel or serpentine channels. A novel flow-field, the ‘through-plane array’ (TPA), is presented which provides significantly improved performance at higher current density with the advantage of lower backpressure. The concept modifies a conventional flow-field operating in dead-ended mode and incorporates an array of holes (

Water management and flooding play an important role in the performance and durability of polymer electrolyte fuel cells (PEFCs). In this study, a dynamic electro-mechanical analysis is performed to examine the performance of a working PEFC during hydration transients and flooding events. Cell resistance is measured using electrochemical impedance spectroscopy (EIS), and the stress/strain characteristics – cell compression and membrane electrode assembly (MEA) dimensional change – are studied using a controlled compression unit (CCU).Ex-situ measurements of membrane thickness as a function of hydration level provide a direct correlation between ionic conductivity and thickness. During initial hydration of Nafion membranes there is a direct relationship between membrane conductivity and dimensional change (swelling) of MEAs. Electrode flooding is found to result in membrane hydration and an increase in stress or strain, depending on the compression mode of the fuel cell. Results suggest that hydration cycles and flooding events can lead to cell degradation due to the stresses imposed.

The relationship between the properties of geopolymers and the characteristics of metakaolin samples used in their preparation has been investigated. Three commercial metakaolin samples have been characterised using 27Al-NMR to determine the coordination number of Al (IV, V and VI), and by acid and alkali dissolution to determine the reactive Si and Al content. The setting and mechanical properties of geopolymers formed from the metakaolin samples under identical conditions are reported, using Weibull statistics to analyse strength data. Although the metakaolin samples contained different levels of five coordinated aluminium (Al (V)) the mechanical properties of the geopolymers formed were very similar. The reactive fraction of metakaolinite determined by dissolution in 8 M NaOH provides the most relevant long-term indicator of geopolymer performance.

A novel bulk acoustic wave (BAW) microgravimetric sensor based on gallium orthophosphate is demonstrated that it is capable of operation at high temperature (up to 900 °C). The sensor is applied to the detection of carbon deposition onto electrodeposited nickel from dry methane at 600 °C and used as an analogue for studying the coking of solid oxide fuel cell (SOFC) anodes. The degradation of electrochemical performance due to deposition of carbon onto symmetrical SOFCs with nickel/gadolinium doped ceria electrodes is measured using electrochemical impedance spectroscopy (EIS). Direct correlation is observed between the frequency shift of the sensor and the change in resistance to charge transfer of the SOFC anode. An induction period (∼2 h) following exposure to methane is observed where no significant carbon deposition occurs.

This paper describes the use of an in situ analytical technique based on simultaneous displacement and resistance measurement of gas diffusion layers (GDLs) used in polymer electrolyte fuel cells (PEFCs), when exposed to varying compaction pressure. In terms of the losses within fuel cells, the ohmic loss makes up a significant portion. Of this loss, the contact resistance between the GDL and the bipolar plate (BPP) is an important constituent. By analysing the change in thickness and ohmic resistance of GDLs under compression, important mechanical and electrical properties are obtained. Derived parameters such as the ‘displacement factor’ are used to characterise a representative range of commercial GDLs. Increasing compaction pressure leads to a non-linear decrease in resistance for all GDLs. For Toray paper, compaction becomes more irreversible with pressure with no elastic region observed. Different GDLs have different intrinsic resistance; however, all GDLs of the same class share a common compaction profile (change in resistance with pressure). Cyclic compression of Toray GDL leads to progressive improvement in resistance and reduction in thickness that stabilises after ∼10 cycles.

Conventional polymer electrolyte fuel cells (PEFCs) require a means of placing the series of laminar components that make up cells under mechanical compression so as to ensure effective electrical conduction, mass transport and gas-tight operation. This review describes the effect of mechanical compression and dimensional change on the components of PEFCs and reviews the range of methods used to achieve desired stack compression. The case is made for improved understanding of the mechanisms of fuel cell component compression and greater attention to the development of technological approaches for stack compression. � 2015 Elsevier B.V. All rights reserved.

A combined X-ray diffraction and thermal imaging technique is described to investigate the effect of thermal gradients on high-temperature composite materials., A new technique combining in situ X-ray diffraction using synchrotron radiation and infrared thermal imaging is reported. The technique enables the application, generation and measurement of significant thermal gradients, and furthermore allows the direct spatial correlation of thermal and crystallographic measurements. The design and implementation of a novel furnace enabling the simultaneous thermal and X-ray measurements is described. The technique is expected to have wide applicability in material science and engineering; here it has been applied to the study of solid oxide fuel cells at high temperature.

Developing a suitable separation and purification procedure for multi-component gas mixtures is a ubiquitous problem. Very little is known experimentally about multi-component adsorption isotherms, yet these are crucially important for designing separation processes. Multi-component adsorption isotherms over a wide range of composition, pressure and temperature are virtually unknown. Adsorption onto zeolites is widely used in adsorptive separation processes due to the unique characteristics of its structural pores which may be “engineered” to permit adsorption to take place on the basis of molecular shape and size. In this article, we discuss the methodological development of rapidly obtaining multi-component adsorption isotherms using the interplay between the essentially exact (in the statistical mechanical sense) lattice model and configurational bias Monte Carlo simulations. The latter technique has the known capability of being able to reproduce experimental isotherms reliably, but can be very time consuming and expensive. In contrast, the lattice model adsorption isotherms can be parameterized by comparing with those obtained by Monte Carlo simulation and thereafter used to cheaply and rapidly explore a very wide range of mixture concentrations and conditions. The interactions required in the lattice model are effective interaction-free energies. This approach has been applied to the adsorption isotherms of selective binary and ternary mixtures of small alkanes (methane, ethane) and carbon dioxide adsorbed in silicalite. The lattice model results are shown to have the potential, when suitably parameterized, to rapidly and cheaply reproduce the main features of the much more expensive Monte Carlo isotherms over a very wide range of mixture compositions, temperatures and pressures.