Alkaline water electrolysis powered by renewable energy: a review

Alkaline water electrolysis is a key technology for large-scale hydrogen production powered by renewable energy. As conventional electrolyzers are designed for operation at fixed process conditions, the implementation of fluctuating and highly intermittent renewable energy is challenging. This contribution shows the recent state of system descriptions for alkaline water electrolysis and renewable energies, such as solar and wind power. Each component of a hydrogen energy system needs to be optimized to increase the operation time and system efficiency. Only in this way can hydrogen produced by electrolysis processes be competitive with the conventional path based on fossil energy sources. Conventional alkaline water electrolyzers show a limited part-load range due to an increased gas impurity at low power availability. As explosive mixtures of hydrogen and oxygen must be prevented, a safety shutdown is performed when reaching specific gas contamination. Furthermore, the cell voltage should be optimized to maintain a high efficiency. While photovoltaic panels can be directly coupled to alkaline water electrolyzers, wind turbines require suitable converters with additional losses. By combining alkaline water electrolysis with hydrogen storage tanks and fuel cells, power grid stabilization can be performed. As a consequence, the conventional spinning reserve can be reduced, which additionally lowers the carbon dioxide emissions

Experimental and model-based analysis of electrolyte intrusion depth in silver-based gas diffusion electrodes

The electrolyte distribution is a central point of discussion for understanding the processes inside gas-diffusion electrodes (GDE) for the oxygen reduction reaction in highly alkaline media. During first radiographic operando synchrotron experiments, the liquid electrolyte was located, however, the through-plane distribution remains unclear. Therefore, model electrodes consisting of nickel and silver layers are developed to determine the electrolyte intrusion depth. Nickel-based GDEs are modified to achieve a pore system morphology suitable for the oxygen reduction reaction and subsequently coated with silver-PTFE catalyst layers. These graded electrodes form gas-diffusion (nickel) and reaction (silver) layers. The electrodes performance is determined under industrial conditions (80 °C, 30 wt % NaOH electrolyte) as a function of the silver layer thickness and thus of the effective intrusion depth of the electrolyte. The model-based analysis confirms the experimental determined intrusion depths. Nevertheless, additional operando tomography measurements would help to further improve the understanding of the processes inside GDE

Battery-buffered alkaline water electrolysis powered by photovoltaics

The combination of an alkaline water electrolyzer (AWE) with a battery system powered by photovoltaics (PV) for the production of green hydrogen is investigated. A model describes the power distribution between these three subsystems (AWE, battery and PV). Variation of AWE and battery power and capacity is carried out for two locations, to identify the most appropriate setup, where the highest energy usage and operating time can be reached. The battery helps to reduce the power level of the AWE. However, an estimation of the costs indicates that further optimization is necessary

Capacity balancing for vanadium redox flow batteries through continuous and dynamic electrolyte overflow

The vanadium crossover through the membrane can have a significant impact on the capacity of the vanadium redox flow battery (VFB) over long-term charge–discharge cycling. The different vanadium ions move unsymmetrically through the membrane and this leads to a build-up of vanadium ions in one half-cell with a corresponding decrease in the other. In this paper, a dynamic model is developed based on different crossover mechanisms (diffusion, migration and electro osmosis) for each of the four vanadium ions, water and protons in the electrolytes. With a simple to use approach, basic mass transport theory is used to simulate the transfer of vanadium ions in the battery. The model is validated with own measurements and can therefore predict the battery capacity as a function of time. This is used to analyse the battery performance by applying an overflow from one half-cell to the other. Different constant overflow rates were analysed with regard to an impact of the performance and electrolyte stability. It was observed that a continuous overflow increases the capacity significantly but that the electrolyte stability plays an essential role using a membrane with a big vanadium crossover. Even with a good performance, a complete remixing of the tanks is necessary to prevent electrolyte precipitations. Therefore, a dynamic overflow was determined in such a way that the capacity of the battery is maximised while the electrolytes remain stable for 200 cycles

Determination of rate constants and reaction orders of vanadium-ion kinetics on carbon fiber electrodes

In the present work, the kinetic behavior of vanadium-ion reactions on novel single carbon-fiber electrodes is investigated. The theory of reaction orders and charge-transfer coefficients is reviewed and typical sources of error due to incorrectly determined electrochemically active surface area, inhomogeneous current density distributions, mass transfer resistances, and aging are highlighted. The measured rate constants are in a range of 2.5 ⋅ 10−7–1.1 ⋅ 10−5 s−1 for the positive and 7.0 ⋅ 10−8–2.6 ⋅ 10−6 s−1 for the negative electrolyte. Despite these different activities of individual fibers, the reaction orders of V2+, V3+, VO2+ and VO2+ species are precisely determined as a function of the concentration and the state of charge. Moreover, charge-transfer coefficients are calculated with two different approaches based on Tafel slopes and through adjustment of the Butler-Volmer equation

Residual stresses couple microscopic and macroscopic scales

We show how residual stresses emerge in a visco-elastic material as a signature of its past flow history, through an interplay between flow-modified microscopic relaxation and macroscopic features of the flow. Long-lasting temporal-history dependence of the microscopic dynamics and nonlinear rheology are incorporated through the mode-coupling theory of the glass transition (MCT). The theory's integral constitutive equation (ICE) is coupled to continuum mechanics in a finite-element method (FEM) scheme that tracks the flow history through the Finger tensor. The method is suitable for a calculation of residual stresses from a "first-principles" starting point following well-understood approximations. As an example, we calculate within a schematic version of MCT the stress-induced optical birefringence pattern of an amorphous solid cast into the shape of a slab with a cylindrical obstacle and demonstrate how FEM-MCT can predict the dependence of material properties on the material's processing history.Comment: 5 pages, 3 figure

Selective hydrogenolysis of biomass-derived xylitol to glycols: reaction network and kinetics

The conversion of bio-based xylitol to ethylene glycol (EG) and propylene glycol (PG) was studied to replace the petrochemical production route and achieve a sustainable process. The reaction network for aqueous-phase catalytic hydrogenolysis of xylitol over a supported Pt catalyst with Ca(OH) 2 as promotor was identified and the reaction kinetics was determined. The effects of reaction conditions such as educt concentration, H2 pressure, and temperature were investigated. With the developed kinetic model, the composition of the product mixture regarding the desired products (EG, PG) and by-products can be described. The maximum EG yield was achieved at high pressure and low temperature, while high pressure and temperature favored PG production

Modeling the dynamic power-to-gas process: coupling electrolysis with CO2 methanation

The dynamic operation of a power-to-gas plant powered by wind energy is theoretically studied by coupling an empirical model of an alkaline water electrolyzer with a 1D heterogeneous model of a methanation reactor. H 2 produced by the electrolyzer follows the wind power profile, but operation in the part-load range can raise safety concerns. The dynamically generated methane quality comes close to the required value for injection into the gas grid, if the stoichiometric ratio is controlled. To satisfy the gas quality at all times, it is necessary to design a more tolerant reactor