Evaluation of complex multi-physics phenomena at gas diffusion electrodes during high-pressure water electrolysis with AEMs

doi:10.21203/rs.3.rs-4923261/v1

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Evaluation of complex multi-physics phenomena at gas diffusion electrodes during high-pressure water electrolysis with AEMs

https://doi.org/10.21203/rs.3.rs-4923261/v1

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The alkaline water electrolysis is a well-established process for producing green hydrogen from renewable energy sources. Hereby, electrochemical gas compression can be realized with water electrolysis and ion pumping membranes, to avoid costly mechanical compression.

In this experimental study we researched an electrolyzer cell with a strong metal structure, for internal pressure difference of up to 100 bar. Micro-porous gas diffusion electrodes containing non-precious nickel catalysts as well as different separators, alkaline membranes and AEMs have been investigated in the range of 300 to 800 mA cm^− 2. For one preferred AEM, characteristics are shown for hydrogen pressures between 20 and 80 bars, while the anode remains at ambient 1 bar.

Impedance spectroscopy diagrams are used to display the individual cell components: the ohmic resistance of the AEM and the complex impedances of both electrodes. Therewith we could visualize the complex multi-physics phenomena and show that the oxygen electrode works as a Wartburg-element, especially due to higher diffusion rates and therewith entropy production during bubble formation.

Physical sciences/Engineering/Chemical engineering

Physical sciences/Chemistry/Energy

alkaline membranes

AEM

gas diffusion electrodes

high pressure electrolyzer

impedance analysis

OER

HER

Wartburg element

By applying a voltage to two electrodes immersed in an aqueous solution, water can be split electrochemically, as was discovered at the beginning of the 19th century. Especially alkaline electrolyzer came up early, where the hydrogen evolution reaction (HER) takes place at the negative cathode

$$\:2\:{H}_{2}O\:+2\:{e}^{-}\to\:2\:O{H}^{-}+{H}_{2}$$

while the oxygen evolution reaction (OER) happens at the positive anode.

$$\:2\:O{H}^{-}\to\:\:{H}_{2}O\:+2\:{e}^{-}\:+\:\frac{1}{2}\:{O}_{2}$$

Hereby, simple Nickel mesh electrodes were separated by asbestos diaphragms which allow no pressure difference between the two gas sides.

Some 200 years later, water electrolysis is a promising method of using chemical energy to store excess capacity of intermittent electrical energy from renewable sources [1–5]. For example, hydrogen produced by electrolysis can be used in combustion processes [6], methanation of CO₂[5, 7, 8] and fuel cell regeneration [9, 10]. Fuel cells are an exciting new technology for vehicle drive applications that could provide zero-emission driving using green hydrogen [11]. Our future energy systems will most definitely include green hydrogen produced from water electrolysis using renewable electricity. Generally, it is recommended that electricity from wind and solar power should be used - whenever it is possible - in the most effective manner, like in electric drives, battery cars or heat pumps. However not all the operations are amenable to easy electrification. In the chemical industry there are various processes which requires gaseous energy carriers with high power density like natural gas or hydrogen. Along with substituting fossil fuels, hydrogen as a fuel is in use in the processes like the production of ammonia and fertilizer which uses up to 50% of the currently global hydrogen production[12]

For long distances vehicles, fuel cell drives with hydrogen are a promising alternative to batteries. Especially for storage capacities above 60 kWh, fuel cell drives have a lower impact on the environment than batteries, when the entire life cycle (including production, use and disposal) is considered[13] But, for long distance vehicles, high volumetric energy densities are required, and therewith hydrogen pressures of around 700 bar [14, 15]. Due to the thermodynamics of isothermal mechanical compression, a temperature increase affects the amount of energy required to compress the gas. In contrast to that, low temperature water electrolysis are advantageous [16, 17]. Electrochemical hydrogen pre-compression can prevent further mechanical compression and the associated maintenance problems and investment costs.

For electrochemical compression, the hydrogen gas evolution by water electrolysis can be conducted under pressure [18–21]. Generally, the methods of water electrolysis involves the use of liquid or solid electrolytes, whose conductivity is provided by protons or hydroxide ions [11]. The solid acids known as polymer electrolyte membranes (PEMs) [22–27] are essentially positive charged protons that are movable in the aqueous phase, while the negative charged acid residue anion is fixed to the polymer structure of the membrane. The power density of water electrolysis using solid polymer electrolytes is superior to that of conventional liquid alkaline electrolytes across a wider dynamic load range [1, 28–31]. Oxygen pressurization is no longer necessary since solid polymer electrolytes are mechanically robust and can withstand higher cathodic than anodic pressure [5, 16, 17, 19]. Commercial PEM electrolyzes are working with a hydrogen outlet pressure of typically 30–40 bar while the oxygen is released at ambient pressure [32]. Some special electrolysis prototypes can produce even 200 bars and more [33, 34]. However, precious metals like platinum and iridium must be used as catalysts in PEM electrolyzers, because they are acidic, where nearly all metal catalysts are unstable due to a high oxidative potential [35].

In alkaline environment, a variety of nonprecious metals such as nickel, carbon, iron and cobalt can be used as catalysts, while the electrochemical efficiency is generally higher than in acidic cells. These advantages have led to an impressive development in recent decades, as shown in Fig. 1. In conventional alkaline electrolyzers, simple separators are used which have a stable support structure (asbestos or high performance plastics), with some hydrophilic filling material (e.g. titanium oxide TiO₂). They are highly conductible for hydroxide ions when soaked with concentrated KOH, but they cannot hold any pressure difference. Alkaline membranes have a closed structure like a foil and can withstand high differences of gas pressure. On the other hand, the resistance for ion flows is relatively high and the carbonate formation with free potassium ions (K⁺) can be hindering. Modern anion exchange membranes (AEMs) combine two advantages: they are highly conductive for hydroxide ions and they are tight enough to build a remarkable pressure difference [36].

The chemical structure of AEMs is comparable to PEMs, but with opposite charge conditions. Here, the positive functional group (e.g. quaternary ammonium group NH₃⁺) is fixed to the polymer backbone of the membrane, while the hydroxide ions (OH⁻) are mobile. But, the long-term stability of these chemical compounds of all anion exchange membranes (AEMs) needs improvement and is still a challenge[37–41]. In [42] for example, the degradation of quaternary ammonium groups due to Hofmann elimination in concentrated KOH is described. Especially, functional groups based on imidazole, which are connected to aromatic polymer backbones (e.g. the AEM Sustainion® from Dioxide Materials) seem to be stable for a longer duration at temperatures above 60°C [43].

Another crucial point of water electrolysis is the inefficient evolution of gas bubbles in aqueous electrolyte solution or pure water. We have shown before that the formation of gas bubbles at the surface of simple nickel mesh has a high significance for the overvoltage at the oxygen electrode, in contrast to efficient gas diffusion electrodes with no bubble formation [44].

Using a reversible alkaline fuel cell in the classic design with an electrolyte gap at ambient pressure, detailed measurements with highly-efficient gas diffusion electrodes, different catalysts (Raney-nickel, graphite, platinum and Raney-silver) were conducted. Herewith, very high electrochemical efficiencies up to 100% for the hydrogen production could be obtained [45].

But, for the development of electrochemical compressors with ion-pumping membranes, a robust electrolyzer cell in zero-gap design must be realized. It is important to find stable alkaline membranes or AEMs that that can withstand internal pressure difference up to 100 bars. Under this extreme pressure conditions, especially the diffusive mass transport within the membrane and the gas diffusion electrodes should be analyzed in detail using impedance spectroscopy.

In this experimental work, we studied alkaline membranes and AEMs which are sandwiched between porous gas diffusion electrodes (GDE) using a high contact pressure. The design concept of the electrochemical cell allows the evolution of hydrogen by diffusion into dry gas spaces, while the oxygen evolution takes place in flooded flow structures with aqueous electrolyte.

As shown in Fig. 2, there are three main regions which must be considered: the AEM in the center is highly conductive for hydroxide ions (OH⁻) and allows the diffusion of water molecules. Furthermore, the membrane is gas-tight and can withstand a high differential pressure between the oxygen and the hydrogen side. Modelling of the membrane is done using a simple electrical resistance R_AEM for the ion conductivity that is essentially temperature dependent. The behavior of the gas diffusion electrodes (GDE) is fundamentally different. Here, the complex physical phenomena of the phase transition from the liquid to the gaseous state takes places, while exergy is dissipated, and entropy is being produced. Here, all these complex mechanisms are summarized for the respective electrode using linear electrical resistances (R_OER / R_HER), which are connected in parallel with the respective capacitors (C_OER / C_HER). The combination of a resistor with a capacitor leads to an impedance in the complex level, represented by a pointer with amplitude and phase. Since the impedance is strongly dependent on the excitation frequency, it can be viewed as a PT1 element, typically represented by a semicircle in the Nyquist diagram.

For the experimental evaluation of gas diffusion electrodes and their underlying physical mechanisms, the impedance of the OER and HER are carefully characterized. Hereby, the research question must be answered, if the higher entropy production due to inefficient bubble formation will be measurable by an impedance with stronger parts of a Wartburg element. The influence of increasing hydrogen gas pressure on the OER and HER should be also analyzed and discussed.

Alkaline pressure electrolyzer design

For realizing the above introduced concept, an electrolyzer cell was designed with strong metal plates to withstand high internal pressure of up to 100 bar. It has a special sealing to allow a high differential pressure over alkaline membranes and anion exchange membranes (AEM) in combination with gas diffusion electrodes (GDE).

The Frankfurt University of Applied Sciences improved an early design from the ACTA Company to create the zero-gap design for anion exchange membrane electrolyzer. It was set up to test various electrodes and membranes in the Lab of Fuel Cell Technology at Frankfurt University of Applied Sciences. To achieve pure hydrogen and provide the opportunity to manufacture pre-compressed hydrogen gas, the design is similar to a PEM electrolyze. Figure 3 provides a schematic illustration to demonstrate the zero-gap operating concept (please do not scale from this picture). A separator or alkaline membrane is the one that guarantees the movement of water and hydroxide ions (OH-). Gas diffusion electrodes are squeezed on each side of this membrane with the assistance of heavy steel endplates and metallic support structures. The support structures were made from welded-together stainless-steel meshwork of various sizes. To provide a solid electric connection, the side that faces the electrode is polished. A synthetic material framework constructed of polypropylene (PP) encloses the support structures, which include the electrodes and membrane, and is sealed with several gaskets. Comparing this AEM cell design to other popular ones, the ohmic resistance of the cell is significantly reduced, but highly depending on the tolerances to achieve a proper electrical and chemical transport.

Figure 2 shows that the hydrogen side of the electrolyzer is nearly dry whereas the oxygen side is completely submerged in liquid electrolyte. In terms of mass transfer in the electrochemical process, the generated gas at the HER can permeate freely into the dry support structure, but is constrained by the amount of Liquid passing through the membrane. Gas bubbles need to develop also at the oxygen side, which is a complex and energy-intensive process. On the other hand, water needs to diffuse across the membrane against the current of the hydroxide ions because water splitting occurs at the HER. The negative impact of mass transportation could reduce the benefits of a thin membrane and the resulting decrease in electrolyte resistance.

Experimental setup

The DC power source for the electrolyzer in this setup is a four-quadrant NL series power supply from Höcherl & Hackl GmbH (model NL1V10C20 Standard), which provides a constant current in the range − 20A to 20A or a constant voltage from − 1V to 10V. The accuracy of the H&H power supply is ± 0.2% of the set value and ± 0.05% of the total range. This means a maximum of ± 0.0048V and ± 0.005V. The maximum deviation is therefore ± 0.0098V. The losses via cables and terminals are compensated by additionally attached sense cables and can therefore be neglected. The impedance of the cells was measured using an electrochemical workstation from the German company ZAHNER (model Zennium Pro). It allows a frequency range from 10 µHz to 8 MHz, with a current from − 3 to + 3A, in a voltage range from − 15 to + 15 V. Connected directly to the Höcherl & Hackl four-quadrant supply, the power values are raised to the same level as the four-quadrant supply, without sacrificing precision. The pressure on the hydrogen side is controlled and measured by a pressure maintenance valve from Bronkhost. The pump used is a KNF diaphragm liquid pump, model FP 1.150, for the KOH solution with a pumping capacity of up to 1.1 l/h. The temperature of the KOH is controlled by a Lauda Bar thermostat (model eco silver) and measured by a PT100 sensor, which is added to the KOH stream immediately before and after the electrolyser. Heating mats with Omron temperature controllers (ESCC-TQX3A5M-003) are also installed at the end plates. Table 1 lists the testing circumstances.

Table 1

testing Parameters of the different Separators, Membranes and AEMs
company Membrane	Sustainion X37	FuMATech FAS50 / 30	AGFA UTP 220	Celgard 5550	FuMATech FAAM 20	Tokuyama A201
Type	AEM	AEM	Separator	Separator	Membrane	AEM
KOH	1 mol l-1	1 mol l-1	6mol l-1	6mol l-1	6mol l-1	1 mol l-1
Temperature used	70°C	70°C	70°C	70°C	70°C	70°C
Temperature recommended	60°C	40°C	130°C	70°C	60°C	50°C
Differential Pressure	Up to 10 bar	Up to 100 bar	Up to 10 bar	Up to 4 bar	Up to 60 bar	Up to 60 bar
Pump speed	0,5 l/min	0,5 l/min	0,5 l/min	0,5 l/min	0,5 l/min	0,5 l/min

Gas diffusion electrodes

The gas diffusion electrodes created and manufactured by the German company GASKATEL are utilized across all experiments. They are created using a unique mixture of RANEY-Nickel (NI) with polytetrafluoroethylene (PTFE) with a rolling process. The final product is a 0.3mm thick flexible electrode band. Figure 4 depicts the microscopic electrode's structural layout. There are shown the Materials, Nickel, PTFE a together and the wholes with a diameter between 5 and 20µm.

The adjustment of the pore size is decisive for effective mass transfer, since larger micropores result in higher gas flow, particularly for oxygen. The control of water transportation and gas bubble formation become more challenging as the active area shrinks. The use of PTFE prevents the Nickel from flooding and also supports the structure. This type of gas diffusion electrode has been developed to achieve a very high number of three-phase-contact lines, between solid catalyst, liquid electrolyte and gas, which are the regions with very high activity[45, 46] [25, 32, 33]. The electrode has a hydrophobic part and a hydrophilic part. To create the optimum hydrodynamic equilibrium, the electrode could be used in different ways by integrating electrodes appropriately under the conditions of the change in Electrolyze the NI. Depending on the PH value and the voltage, the oxidation state from the NI is changing. This has a high influence on the open circuit voltage of the cell and therefore the performance. During operation, the oxidation of the electrodes changes from $\:{Ni}_{3}{O}_{4}$ to a higher state $\:{Ni}_{2}{O}_{3}\:$and raise the potential with them shown in Formula 1. Those is the idle potential in case that there is nothing applied which causes a reduction of the $\:{Ni}_{2}{O}_{3}$.

$$\:2{Ni}_{3}{O}_{4}+{H}_{2}O={3Ni}_{2}{O}_{3}+2H+2{e}^{-}\:\:\to\:\:\:{E}_{0}=1.305-0.0591pH$$

1:Limit of the domains of relative stability of the substances from Ni in alkane milieu as oxygen reaction according to [47]

In case of this oxidation the voltage increase from every membrane should be the same. If the voltage in idle conditions is below the 1.305V it indicates that the membrane is not gas stable or that the oxidation of the NIH33 is less.

To produce hydrogen under high pressure using non-noble materials as electrodes, we tested various separators, alkaline membranes, and AEMs. Our focus was on the most effective way to produce pure hydrogen with the gas-diffusion electrodes described above. The cell design posed challenges for the separators, especially the no-gap design with the dry hydrogen side, which is atypical for alkane technology. The process efficiency is known to be highly dependent on the operating temperature [45]. Therefore, all membranes were tested at 70°C to keep uniform operating condition, despite some AEM membranes starting to degrade at this temperature unlike others that were capable of withstanding higher temperatures. Figure 5 compares the performance when two different separators, the UTP220 from AGFA and the Celgard 5550, were used.

The measurements with the separators were conducted with a 6 mol/l-1 KOH solution, but the results between the separators were significantly different. The Agfa UTP220 separator registered the highest efficiency, performed well and is temperature stable. Its power density of up to 800 mA cm^− 2 at less than 1.9 V is remarkable, but this is only possible because the membrane has satisfactory water transport and a low resistance of only 16 mOhm. However, the Celgard 5550 separator is registering limited power output due to issues with water transport and higher resistance. Both separators, despite the promising power density of the UTP foil, are unable to generate a higher differential pressure as in this case differential pressure as in the range of 2 bar-4 bar. This is due to the general open pore structure of a separator compared to a membrane [36]. To overcome the problem, the separators were replaced with the alkane membrane FAAM20 from the Fumatech company. As mentioned in the previous paper [45], the performance of the cell is strongly dependent on the electrode structure, which directly affects the microscopic water and gas management. The membrane’s performance, along with the performance of the two separators, was measured using a 6 mol/L KOH solution. The total power output of the cell is rather low in the case of the membrane, at 300 mA cm^− 2 and 1.85 V. The initial voltage drops below 1.23 V, indicating that the membrane is not gas-stable in this configuration. So, further testing was carried out with AEM membranes Sustainion X37, Tokuyama A201 and the Fumatech FAS 50, which were chosen as they are all suitable for use in a light alkaline medium with sub 1 mol/L KOH or even with pure water, so the testing parameters where changed here to 1 mol/L KOH. Another test with the Sustainion membrane resulted in a high-power density of up to 750 mA cm^− 2 with only 2 V, but its lifespan and power density are limited by water transport. This AEM shows unstable behaviour during initial operation: thus, it is important to avoid fast power changes or membrane dry out, as this can cause damage. The initial voltage also drops below 1.23 V indicates that the membrane operation is not stable in this configuration at least with higher pressure.

The high gas mixing leads to the formation of an explosive gas, making the operation of this membrane too dangerous for further applications. The A201 from Tokuyama has an output of 350 mA cm^− 2 at 2 V and 2 bars. It is pressure-stable up to 60 bars, even though the power is limited. The open circuit voltage of 1.3 V also indicates a high level of tightness. The FAS 50 is the only membrane capable of building up a high differential pressure without any special treatment and with high power densities. It is possible to achieve 500 mA cm^− 2 with 2 V. These power density, combined with light KOH and stable, reproducible power, show great promise

The measured characteristics in Fig. 5 indicate that the FAS 50 exhibits highly stable pressure behaviour, even at high differential pressures. The idle voltage is above 1.3 V, which is quite high in comparison to the reversible cell of 1.23V at standard conditions. This is caused by the potential from the Nickel electrode, as described initially. Figure 6 shows the voltage offset between the curves, which is linked to increased pressure. The mean offset value is between 2 bar and 60 bar, with a value of 0.0547V. At 85 mA cm^− 2, the curve progression is linear, and the voltage offset is small. From 2 bar to 20 bars, the voltage offset exhibits slight variability. Above 300 mA cm^− 2, the offset decreases from 0.03V to 0.02V due to improved mass transport. Smaller gas bubbles can easier tunnel through the small pores of the electrodes. At higher pressures, this effect is less significant than the differential pressure, as seen in the voltage offset from the 60- bar curve. After reaching 600 mA cm^− 2, the voltage offset is slightly increasing from 0.05V to 0.08V.

The voltage offset is also observed in the impedance spectra for the different pressures presented in Fig. 7. The impedance was measured at 125 mA cm⁻², at 70°C, with 1 mol/l KOH. The resistance, measured with the Impedance measured Resistance is 31 mOhm at 20 bar, 30 mOhm at 40 bar, and 31.7 mOhm at 60 bars. For all measurements, the variables that can influence the impedance as in[48] were kept the same to avoid measurement inaccuracies.

To lower the resistance of the membrane also the FAS 30 was tested, with the same membrane structure, but thinner. This membrane was also able to generate up to 80 bar differential pressure between the hydrogen and oxygen side. The measured data is shown in Fig. 8.

The membrane in this design offers lower resistance compared to the FAS 50, resulting in higher power output at less cell voltage. For optimizing the connection between electrodes and membrane, the cell design must be adapted to achieve a reliable contact pressure. The 2 bar measurement shows insufficient contact at higher densities, while at 20 bar and 40 bar, the pressure at the membrane-electrode assembly is higher, resulting in better contact and increased power output. At 60 bar and 80 bar, the curve progression shifts towards higher power above 300 mA cm^-2. This is due to mass transport issues caused by water and oxygen, which is evident in the impedance measurement shown in Fig. 9.

With the measured data it is possible to simulate the behaviour inside the electrolyser cell. Depending on the frequency of the impedance the behaviour of the membrane, the electrodes and the mass transport could be estimated. The model is used to describe the electrochemical process including both electrodes and the membrane and is shown in Fig. 2.

The trend was generated from the model outputs as shown in Fig. 10. It shows the comparison between the simulation data with the measured data. The rectangle points represent measured impedance and the curve represents simulation trend. The simulation was carried out for all the measured impedances ranging from 20 bar to 80bar. It is important to note that the 20- bar measurement was used solely as an example. In the model a simple ohmic resistance for the membrane and complex resistances for each electrode are used, as shown in Fig. 2. To explain the complex multi-physics behaviour of the porous Nickel structure, a constant phase element is required for the electrode. The constant phase element (CPE) is an important electrochemical modelling element used in the characterization of electrode systems [49–51]. In contrast to an ideal capacitor, the CPE represents a non-ideal, frequency-dependent capacitance and enables a more precise description of diffusion processes and non-linearity`s on electrode surfaces. Its complex impedance behaviour is represented by a phase angle (α) that can better represent the dynamic electrode processes. The value α = 1 gives an ordinary capacitor, α = 0 is a resistor, and α = 0.5 corresponds to the Warburg element used to model diffusion processes. Table 2 presents the results of the simulation model for impedance measurements conducted at 20, 40, 60, and 80 bars.

Table 2

Simulation results of the EIS measurement for the FAS30 from 20 to 80 bar
Pressure	HER electrode	Membrane	OER electrode	Error of Simulation
20 bar	V = 291 mF α = 675 m R = 5.29 mOhm	R = 27 mOhm	V = 1330 mF α = 669 m R = 18.5 mOhm	1,8%
40 bar	V = 392 mF α = 772 m R = 6 mOhm	R = 29 mOhm	V = 934 mF α = 664 m R = 21.6 mOhm	2,13%
60 bar	V = 262 mF α = 716 m R = 5,99 mOhm	R = 31 mOhm	V = 555 mF α = 638 m R = 23 mOhm	2,43%
80 bar	V = 335 mF α = 865 m R = 3,38 mOhm	R = 40.9 mOhm	V = 231 mF α = 586 m R = 28.3 mOhm	1,15%

The results are divided into columns for the three main simulation groups: membrane, HER and OER. The specified error indicates the deviation of the simulated values from the measured points. The values of the hydrogen electrode exhibit only slight changes with increasing pressure. The membrane resistance increases slightly with pressure, from 27 mOhm to 31 mOhm at 60 bar and up to 41 mOhm at 80 bar. The values of the HER change continuously with increasing pressure. α decreases from 669 mOhm to 586 mOhm, resistance increases from 18.5 mOhm to 28.3 mOhm, and V decreases from 1.33 F to 231 mF. The maximum failure in the simulation is 2.43% for the 60- bar characteristic curve. The increase in membrane resistance is linked to water transport limitations and could also be caused by membrane aging due to high temperature mixed with KOH. Some uncertainties due to aging must be taken into account, since there was a long run before the 80 bar measurement. A more detailed study of the aging effect will be presented soon in another papers.

Water transport through the cell is a critical component. The rise in resistance and decrease in Farad indicate a change in the behaviour on the OER. The decrease in parameter α suggests a change in transport behaviour from a capacitor more to a Wartburg Element. It means that the transport and oxygen evolution issues by higher pressure rising. The slight decrease of resistance and changes of α from Wartburg to a capacitor behaviour shows the improvement on the HER electrode which is linked to a smaller bubble size of the hydrogen due to higher pressure.

For a clearer representation the values of Table 1 are also shown in Fig. 11 (in a dimensionless way).

For the HER, the changes are not very pronounced, in case that the influence is very slight for the overall performance. At the HER, at 60 bar some decreases of phase and amplitude are noticeable, which could be a sign for instabilities. The most noticeable thing in Fig. 11 is the significant drop in the phase of the OER. This change towards a Warburg element can be explained with distinctive diffusion problems at the oxygen electrode where gas bubbles are formed in liquid electrolyte. At higher pressure difference across the membrane could lead to strong back diffusion of hydrogen gas which disturbs especially the mass transport at the OER.

Additionally, in Fig. 12s the significant results of the simulation as a function of the excitation frequency are shown. As given in literature [52], it can be seen that the dynamic operational conditions are at play[52]. This significance describes the influence of the individual simulation constants on the resulting graph at various frequencies of the impedance measurement. The hydrogen electrode is described by HER [α; V; R], which have a smaller influence on the curve and which are mostly focused on high frequencies. The OER [α; V; R] describe the oxygen electrode and the mass transport, respectively. The simulation correlates perfect to the Measured data. Starting by 60 bar in Fig. 8 the mass transport limitations are shown in the measurement above 300mA, this also could be seen over the changes in the OER behaviour in the simulation. The changes occur surly before 60 bar also, the impact is at 60 bar the first time high enough to see it measured. They are significant for the overall reaction and change during the rise of the hydrogen pressure, as shown in Table 2. The resistance R of the membrane has a significant impact especially at high frequencies.

The simulation confirms that the membrane and OER have the greatest influence on the electrolyzer's performance. It also shows that increasing pressure difference negatively affects the behaviour of the mass transport system at the OER and membrane, reducing performance. However, it is important to note that the achieved pressures at this power density are noteworthy. As a differential pressure with a 'dry' HER electrode, these research results are unique and offer great potential for future applications.

One pathway to reduce the overall costs of renewable hydrogen and increase the reliability of electrolyzer plants is to increase the operating pressure of water electrolyzers, which limits the need for external mechanical compression. Another significant cost driver are the manufacturing costs of the electrolysis plant. Anion exchange electrolysis is a less expensive alternative to PEM technology for producing pre-compressed hydrogen.

This work presents a potential design that was experimentally investigated. The results of the experimental investigation of the high-pressure anion exchange electrolyzers are very promising. It is possible to produce dry hydrogen under high differential pressure using an anion exchange membrane. The use of a non-gap design and the AEM FAS30 allow to produce pure hydrogen at pressures up to 80 bars, with a power density of only 2.2 volts at 500 mA cm^− 2. The high pressure, combined with a slight voltage offset of 0.08V, results in high efficiency and improved total efficiency of hydrogen storage systems. The simulation Model clearly shows the way for future improvements on this technology which will have a huge impact on performance end efficiency. Future work aims to achieve greater power output while using the same area and lower cell voltage. The simulation makes it possible to gain further insights into the processes that take place in the electrolyser and to identify and understand the optimization potential. This will be achieved through the optimization of electrodes, resulting in a larger active area and lower resistance.

Author Contribution

E.D did the experimental part and the simulation, E.W did some basic research and adjustment for the material and methods part and did the supervision for E.D, R.M did the supervision for E.D and corrections of the paper.

Data Availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request

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No competing interests reported.

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Evaluation of complex multi-physics phenomena at gas diffusion electrodes during high-pressure water electrolysis with AEMs

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Abstract

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Introduction

Materials and Methods

Alkaline pressure electrolyzer design

Experimental setup

Gas diffusion electrodes

Results and discussion

Conclusion

Declarations

Author Contribution

Data Availability

References

Additional Declarations

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