Peer reviewed journal publications
The following papers were published with contributions from NEXTAEC
Abstract
With interest in renewable energy sources and in the decarbonisation of industry rapidly accelerating, alkaline water electrolysis can now be regarded as a key technology enabling efficient energy conversion and storage. Since an alkaline environment is suitable for a vast range of materials with satisfactory chemical stability under operating conditions, the topic of catalysts for the alkaline route could give rise to confusion in the community. Hence, the focus of this review is on analysing the current situation in the electrocatalysis of the hydrogen evolution reaction in an alkaline and a neutral environment, presenting the main group of materials studied, and discussing their potential to achieve industrial relevance. It addresses the main limitations of common parameters used for evaluating and comparing the catalytic activity and stability of selected catalysts. Furthermore, the review provides a comprehensive comparison of catalyst activities with respect to the individual groups depending on their composition as well as to the most used cations and cation-based materials. For the sake of clarity, the comparison is also presented in graphical form. Finally, based on the literature data, fundamental material characteristics to be evaluated in the development of new catalysts for electrochemical water splitting are proposed.
Abstract
Hydrogen can be produced in a clean way from water by its electrochemical splitting. Water electrolysis consists of two half-reactions, of which the oxygen evolution reaction (OER) is the major source of energy loss. In the case of the alkaline water electrolysis, the spectrum of materials that are stable under the conditions of the OER is significantly broader than in the case of an acidic route. This review compares systematically different materials classes based on reported overpotential at current density 10 mA cm 2. Plethora of studies was gathered to accomplish this task and the OER catalysts reported in the literature to date are summarized. In addition, we have provided an insight into the catalyst activity descriptors allowing theoretical identification of the most active materials. In order to assist the reader in gaining a better understanding of this complex subject, the catalysts are classified into three main groups in agreement with their chemical composition as transition metal oxides, phosphides and selenides. Less frequently used materials are reported in a separate group. Near-neutral or neutral pH conditions are considered as well.
Abstract
The separator is a critical component for the performance of alkaline water electrolysis as it ensures the ionic contact between the electrodes and prevents the product gases from mixing. While the ionic conductivity of the separator affects the cell voltage, the permeability of the dissolved product gases influences the product gas impurity. Currently, diaphragms are used as separators, the pore system of which is filled with the electrolyte solution to enable the exchange of ions. The breakthrough of the gas phase can be prevented up to a specific differential pressure. A drawback of diaphragms is the requirement of a highly concentrated electrolyte solution to maintain a high ionic conductivity. The usage of anion-exchange membranes could solve this problem. However, the long-term stability of such materials remains unproven. This study compares two pre-commercial diaphragms, an anion-exchange membrane, and an ion-solvating membrane with the state-of-the-art diaphragm ZirfonTM Perl UTP 500. Besides physical characterization, the material samples were evaluated electrochemically to determine the ohmic resistance and the product gas impurities. The results show that the thinner diaphragm outperforms the reference material and that polymer membranes can compete with the performance of the reference material.
Abstract
To further reduce the capital expenditure of alkaline water electrolyzers, an improvement in power density can still be achieved through process intensification. The change from traditional gap-cells to zero-gap cells has already been proven to be very promising in this respect. In the zero-gap design, macro-porous 3D structures are typically being used as electrodes. Here, pure nickel foams with different pore sizes in the range 450 – 3000 μm have been studied under well-controlled electrolyte flow conditions. To this end, a dedicated flow cell has first been constructed that allows for imposing the same relatively high flow rates on the order of 1 L/min that are typically encountered under industrial conditions. Our specific cell design was shown by computational fluid dynamic simulations to be able to homogenize the flow field before entering the electrodes. This is absolutely mandatory to reliably evaluate the intrinsic bubble removal efficiency of the differently sized foams. We then demonstrate that the effect of pore size and the electrochemical performance can be rationalized by considering the cell voltage as a function of electrochemical current density, i.e. the current divided by the theoretically available electrochemical surface area (ECSA) rather than by the projected electrode area. Based on this analysis, we were also able to quantify the bubble removal efficiency by estimating the effective fraction of the ECSA that is not impeded by bubble coverage.
Abstract
A new membrane fabrication process is presented, in which PBI is cast from a phosphoric acid-based solution and transformed into a KOH doped membrane by immersion in KOH solution. At room
temperature, such membranes show a remarkably high conductivity of 300 mS cm1 in 25 wt% KOH solution, 3 times higher than that of conventional PBI membranes cast from DMAc. While swelling of PBI in KOH solution is highly anisotropic, the phosphoric acid-cast membrane shrinks isotropically when immersed in KOH solution, indicating that the ordered, presumably lamellar structures, which are
suspected to hinder transfer of ions through the membrane, are transferred into a gel-state. This is further supported by WAXS analysis. DFT calculations suggest that the broad signal in WAXS is related to
the distance between hydrogen bonded imidazolides. The soft nature of the KOH doped PBI was mitigated by reinforcement with porous supports. By using a PTFE support, the tensile strength
increased from 2 to 32 MPa, and the Young’s modulus from 19 to 80 MPa. Immersion in 25 wt% KOH at 80 1C indicated high stability. In the electrolyzer, a conventional PBI membrane failed within 200 hours. In contrast to this, a PTFE-supported separator reached 1.8 A cm2 at 1.8 V and was operated for 1000 hours without failure, which is the longest operation time for ion-solvating membranes to date.
Abstract
A zero-gap cell with porous electrodes is a promising configuration for alkaline water electrolysis. However, gas evacuation becomes a challenge in that case, as bubbles can get trapped within the electrode’s 3D structure. This work considers a number of 3D printed electrode geometries with so-called triply periodic minimal surfaces (TPMS). The latter is a mathematically defined structure that repeats itself in three dimensions with zero mean curvature, and can therefore be expected to be particularly well-suited to enhance gas evacuation. Another advantage as compared to other state-of-the-art 3D electrodes like foams or felts lies in the fact that their porosity, which determines the available surface area, and their pore size or flow channel dimensions, which determines the degree of bubble entrapment, can be varied independently. By a combined experimental and modeling approach, this work then identifies the structural parameters that direct the performance of such 3D printed TPMS geometries toward enhanced gas evacuation. It is demonstrated that an optimal combination of these parameters allows, under a forced electrolyte flow, for a reduction in cell overpotential of more than 20%. This indicates that efforts in optimizing the electrode’s geometry can give a similar electrochemical performance enhancement as optimizing its electro-catalytic composition.
Abstract
Oxygen evolution reaction (OER) is a rate-determining process in alkaline water electrolysis (AWE). Herein, we report a novel one-step oxidation–electrodeposition (OSOE) approach to generate core@shell nanoarrays-based AWE electrode with outstanding OER performances: an overpotential of 245 mV at 10 mA cm−2 (Tafel slope: 37 mV dec−1), and excellent stability under huge current densities. Moreover, the alkaline (AEL) cell equipped with NM-OSOE-23 anode recorded significant performance improvement of 200 mV lower voltage (2 A cm−1) compared with a similar cell used bare Ni mesh as an anode, which was contributed by notable enhancements of interface contact, anodic charge transfer, and mass transfer. These promising results are attributed to the constructed specific core@shell Ni@Fe-doped Ni(oxy)hydroxide nanoarray architecture on commercial nickel mesh. This study demonstrates this first reported OSOE can be commercialized to make highly efficient anodes enabling next-generation AWE.
Abstract
Porous carbon nano/microfibres (PCFs) modified by NiCoP nanoparticles (NiCoP/CF) were tested as gas diffusion cathodes in a zero-gap alkaline water electrolyser. Two types of precursors were used for the preparation of carbon fibres by needle-less spinning: polyacrylonitrile (PAN) and a combination of PAN and polyvinylpyrrolidone (PVP). The influence of the selected precursors on the morphology, porosity, and surface area of the carbon fibres was examined. Both samples NiCoP/CF_PAN and NiCoP/CF_PAN-PVP were characterised by means of (i) structural and phase composition, (ii) catalytic activity for the hydrogen evolution reaction (HER), and (iii) performance and stability under membrane alkaline water electrolysis (MAWE) conditions. The more porous, yet stable fibres of the NiCoP/ CF_PAN outperformed the NiCoP/CF_PAN-PVP cathode. Nevertheless, both cathodes achieved high activity comparable to the Ni electrode modified by Pt/C catalyst, as well as exceptional stability for long-term intermittent electrolysis under MAWE conditions.
Abstract
It is a great challenge to develop membranes based on polyaromatic backbone chemistries that combine high alkaline resistance with high ionic conductivity and low gas crossover for alkaline water electrolysis. Hence, a new alkaline stable aromatic monomer containing side ion-solvating poly(ethylene oxide) (PEO) groups was synthesized and polymerized with isatin and biphenyl via super acid catalyzed hydroxyalkylation to yield ionsolvating copolymers. The prepared aryl-ether free backbone aromatic copolymers (P(IB-PEO)-y) have excellent film-forming properties, high thermal stability, but moderate KOH electrolyte uptake and relatively low conductivity. Therefore, to further enhance the electrolyte uptake and ionic conductivity, P(IB-PEO)-20 copolymers were blended with polybenzimidazole in ratios 80/20, 70/30, 60/40 and 50/50. The prepared blend membranes exhibit high electrolyte uptakes (up to 97 wt%) while the highest ionic conductivity of 110 mS cm 1 at 80 ◦C was observed for PBI80/P(IB-PEO) blend. The KOH doped PBI70/P(IB-PEO) membrane shows a tensile strength of 20 MPa and a significant increase in Young’s modulus (131%) compared to that of PBI80/P(IB-PEO). The alkaline stability test demonstrated that PBI80/P(IB-PEO) membrane exhibits a substantially higher Young’s modulus (144% increase) than non-aged analogue, after 1 month in 20 wt% KOH solution at 80 ◦C. Further, PBI80/P(IB-PEO) and PBI70/P(IB-PEO) membranes retained 96–98% of their original conductivity after aging, indicating their excellent alkaline resistance. Selected membranes were tested in a single cell electrolyzer to probe feasibility and crossover behaviour.
Abstract
ABSTRACT: Multi-gigawatt-scale hydrogen production by water electrolysis is central in the green transition when it comes to storage of energy and forming the basis for sustainable fuels and materials. Alkaline water electrolysis plays a key role in this context, as the scale of implementation is not limited by the availability of scarce and expensive raw materials. Even though it is a mature technology, the new technological context of the renewable energy system demands more from the systems in terms of higher energy efficiency, enhanced rate capability, as well as dynamic, part-load, and differential pressure operation capability. New electrode separators that can support high currents at small ohmic losses, while effectively suppressing gas crossover, are essential to achieving this. This Focus Review compares the three main development paths that are currently being pursued in the field with the aim to identify the advantages and drawbacks of the different approaches in order to illuminate rational ways forward.
Abstract
A series of poly(vinyl alcohol-co-vinyl acetal) gel electrolytes was synthesized, characterized and assessed as electrode separators in alkaline water electrolysis. The copolymers were prepared by reacting poly(vinyl alcohol) with benzaldehyde or 4-formylbenzoic acid under acidic conditions at different ratios, and visually homogenous and water-insoluble membranes were subsequently obtained by solution casting. The physicochemical characteristics in terms of electrolyte uptake, swelling behavior, and ion conductivity could be tuned by varying the degree of functionalization. At a moderate vinyl acetal content of 5%, the membrane combined mechanical robustness with ion conductivity reaching 36 mS cm 1 in 30 wt% aqueous KOH at room temperature. Current densities of up to 1000 mA cm 2 were reached with uncatalyzed Ni-foam electrodes at a cell voltage of less than 2.6 V in alkaline water electrolysis tests, while the membrane effectively prevented hydrogen crossover. Although apparent membrane degradation was observed after a few days of electrolysis operation, the strategies presented in this work to tune membrane properties are of general relevance to the field towards the development of new ion-solvating membrane systems based on more alkaline stable and robust backbone chemistries.
Abstract
research. They are connected with current efforts to increase the flexibility of alkaline water electrolysis and improve its efficiency with regard to renewable energy sources. This study reports on the impact of reduced KOH solution concentration on mass and charge transport in a laboratory alkaline water electrolysis stack with two separator types. The separators used are a homogeneous anion-selective polymer-electrolyte membrane based on chloromethylated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene copolymer functionalised by 1,4–diazabicyclo[2.2.2]octane and a commercial composite porous Zirfon™ Perl UTP 500 diaphragm. The stack was assembled in zero-gap mode and a bipolar connection of the electrodes was used. Load curves were recorded for different KOH concentrations and operational temperatures to assess the performance of the stack. Surprisingly, mass transfer limiting behaviour of the stack was observed at KOH concentrations below 2 wt% KOH for the Zirfon™ Perl UTP 500 separator. This was not the case for the stack utilising an anion-selective membrane as a separator. A 1-dimensional, single cell, stationary mathematical model was developed and implemented to clarify this phenomenon and to understand the details of the mass transport across the different types of separators in this process. This information is crucial for understanding obstacles faced once the concentration of KOH approaches zero, i.e., the final target of research in this field.
Abstract
While anion exchange membrane (AEM) water electrolysis has many advantages, its commercialization is impeded by the low alkaline stability of most AEM, due to the fragility of quaternary ammonium groups. Ion solvating membranes (ISM) can be an alternative, but so far require high alkaline concentrations. Here, it is shown that sulfonation of polybenzimidazole results in ISM which swell strongly in 1 m KOH. Crosslinking with dibromoxylene controls the swelling, and after activation conductivities of >100 mS cm−1 can be reached in 1 m KOH. Stability in 1 m KOH at 80 °C is excellent: Conductivity remains unchanged and tensile strength and Young’s modulus remains high over the test period of a half year. In an electrolyzer operating with 1 m KOH feed solution at 80 °C, a stable performance is achieved for over 500 h test without failure, suggesting that the high alkaline stability observed in ex situ tests is also achieved in the electrolyzer.
Abstract
Alkaline ion-solvating polymer electrolyte membranes derived from poly(isatin biphenyl) and polybenzimidazole (m-PBI) blends with different ratios were prepared, characterized, and evaluated as electrode separators in alkaline water electrolysis. The physicochemical properties could be tuned by varying the composition of the blend, and the membrane with a poly(isatin biphenyl) content of 25 % showed a suitable balance between conductivity and mechanical robustness. The polarization behavior was comparable to that of state-of-the-art separators, with significantly lower H2 permeability. No signs of degradation could be observed after 70 h of electrolysis testing in 30 % aqueous KOH at 80 ◦C, supporting that macromolecular reinforcement is a promising way forward in the development of high-performing and durable alkaline ion-solvating membranes for water electrolysis.
Abstract
Mechanically robust anion-exchange membranes (AEMs) with high conductivity and long-term alkali resistance are needed for water electrolysis application. In this work, aryl-ether free polyaromatics containing isatin moieties were prepared via super acid-catalyzed copolymerization, followed by functionalization with alkaline stable cyclic quaternary ammonium (QA) cationic groups, to afford high performance AEMs for application in water electrolysis. The incorporation of side functional cationic groups (pyrrolidinium and piperidinium) onto a polymer backbone via a flexible alkyl spacer aimed at conductivity and alkaline stability improvement. The effect of cation structure on the properties of prepared AEMs was thoroughly studied. Pyrrolidinium- and piperidinium-based AEMs showed similar electrolyte uptakes and no obvious phase separation, as revealed by SAXS and further supported by AFM and TEM data. In addition, these AEMs displayed high conductivity values (81. 5 and 120 mS cm−1 for pyrrolidinium- and piperidinium-based AEM, respectively, at 80 ◦C) and excellent alkaline stability after 1 month aging in 2M KOH at 80 ◦C. Especially, a pyrrolidinium-based AEM membrane preserved 87% of its initial conductivity value, while at the same time retaining its flexibility and mechanical robustness after storage in alkaline media (2M KOH) for 1 month at 80 ◦C. Based on 1H NMR data, the conductivity loss observed after the aging test is mainly related to the piperidinium degradation that took place, probably via ring-opening Hofmann elimination, alkyl spacer scission and nucleophilic substitution reactions as well. The synthesized AEMs were also tested in an alkaline water electrolysis cell. Piperidinium-based AEM showed superior performance compared to its pyrrolidinium analogue, owing to its higher conductivity as revealed by EIS data, further confirming the ex situ conductivity measurements.
Abstract
Polybenzimidazole (PBI) is currently considered as a membrane material for alkaline water electrolyzers (AWEs), and has to be fed with highly concentrated aqueous KOH electrolytes in order to ensure
sufficient electrolyte uptake and conductivity. However, the harsh operating conditions significantly limit the lifetime of PBI membranes. In response, we here report on the synthesis and performance of a series of PBI membranes tethered with alkali-stable mono-piperidinium (monoPip) and bis-piperidinium (bisPip) side groups, respectively, which allows the use of more dilute KOH concentrations. The electrolyte uptake of these membranes was found to be inversely proportional to the electrolyte concentration, which was in stark contrast to pristine PBI membranes. The high electrolyte uptake at low
concentrations by the present membranes enables operation of AEMWE systems fed with dilute electrolytes, which significantly decrease membrane degradation. After immersion in 2 M aqueous KOH
at 80 °C for up to 6 months, no degradation was detected by 1H NMR spectroscopy in the monoPip series of AEMs, and a mere 7% ionic loss by Hofmann elimination in the bisPip series. Membranes
tethered with bisPip groups produced the best AEMWE performance, and a sample with a hydroxide ion exchange capacity of 2.4 meq. g−1 reached a high current density of 358 mA cm−2 at 2 V with
demonstrated stability over 100 h, using 2 M aqueous KOH and only simple nickel foam electrodes. This is comparable to the performance reported for Zirfon diaphragms and pristine PBI membranes operating with much higher concentrations of KOH in the range of 5–7 M. The low KOH concentration of the present membranes brings important advantages for the material stability in the cell, as well as for the balance of plant, and the results provide useful insights into the molecular design of AEMs for dilute electrolyte-fed AEMWE systems.
Abstract
Traditionally, alkaline water electrolysis (AWE) uses diaphragms to separate anode and cathode and is operated with 5−7 M KOH feed solutions. The ban of asbestos diaphragms led to the development of polymeric diaphragms, which are now the state of the art material. A promising alternative is the ion solvating membrane. Recent developments show that high conductivities can also be obtained in 1 M KOH. A third technology is based on anion exchange membranes (AEM); because these systems use 0−1 M KOH feed solutions to balance the trade-off between conductivity and the AEM’s lifetime in alkaline environment, it makes sense to treat them separately as AEM WE. However, the lifetime of AEM increased strongly over the last 10 years, and some electrode-related issues like oxidation of the ionomer binder at the anode can be mitigated by using KOH feed solutions. Therefore, AWE and AEM WE may get more similar in the future, and this review focuses on the developments in polymeric diaphragms, ion solvating membranes, and AEM.
Abstract
Alkaline ion-solvating membranes derived from a tetrazole functionalized poly(arylene alkylene) are prepared, characterized and evaluated as electrode separators in alkaline water electrolysis. The base polymer, poly[[1,1’-biphenyl]-4,4’-diyl-(1,1,1-trifluoropropan-2-yl)], is synthesized by superacid catalyzed polyhydroxyalkylation and subsequently functionalized with tetrazole pendants. After equilibration in aqueous KOH, the relatively acidic tetrazole pendants are deprotonated to form the corresponding potassium tetrazolides. The room temperature ion conductivity is found to peak at 19 mScm 1 in 5 wt.% KOH, and slightly declines with increasing KOH concentration to 13 mScm 1 in 30 wt.% KOH. Based on an overall assessment of the mechanical properties, conductivity and electrode activity, 30 wt.% KOH is applied for alkaline electrolysis cell tests. Current densities of up to 1000 mAcm 2 were reached with uncatalyzed Ni-foam electrodes at a cell voltage of less than 2.6 V, with improved gas barrier characteristics compared to that of the several times thicker Zirfon separator.
Abstract
For high rate water electrolysers, minimising Ohmic losses through efficient gas bubble evacuation away from the active electrode is as important as minimising activation losses by improving the electrode’s electrocatalytic properties. In this work, by a combined experimental and computational fluid dynamics (CFD) approach, we identify the topological parameters of flow-engineered 3-D electrodes that direct their performance towards enhanced bubble evacuation. In particular, we show that integrating Ni-based foam electrodes into a laterally-graded bi-layer zero-gap cell configuration allows for alkaline water electrolysis to become Proton Exchange Membrane (PEM)-like, even when keeping a state-of-the-art Zirfon diaphragm. Detailed CFD simulations, explicitly taking into account the entire 3-D electrode and cell topology, show that under a forced uniform upstream electrolyte flow, such a graded structure induces a high lateral velocity component in the direction normal to and away from the diaphragm. This work is therefore an invitation to start considering PEM-like cell designs for alkaline water electrolysis as well, in particular the use of square or rectangular electrodes in flowthrough type electrochemical cells.
Abstract
Ion-solvating polymer electrolytes have the potential to drastically lower the ohmic resistance in alkaline water electrolyzers without compromising gas purity, but the stability remains to be improved. This work shows that the tetrazole scaffold is remarkably stable in alkaline environment and demonstrates an operationally simple one-step procedure to convert a commercially available vinyl polymer into an
ion-solvating polymer electrolyte.