High temperature and pressure alkaline electrolysis

This paper describes experimental work involving the direct-current electrolysis of highly concentrated potassium hydroxide solutions at high temperatures (up to 400 °C) and under various pressures. A high-temperature alkaline electrolysis cell resistant to chemical attack from the highly corrosive electrolyte solution and capable of high-pressure operation was designed and tested. The cell was constructed with a Monel^® alloy housing and cathode, while various anode materials were compared. The anode materials tested included nickel, Monel alloy, lithiated nickel, and cobalt-plated nickel. The advantages of operating an alkaline electrolysis cell at high temperatures include increasing the ionic conductivity of the electrolyte and enhancing the rates of electrochemical reactions at the electrode surfaces. Cell operation with increasing steam partial pressure over the solution is also shown to enhance cell performance. The prudent selection of anode material also impacts the required terminal potential for a given current density, and consequently the cell's electric power efficiency. The best cell performance was achieved using a cobalt-plated nickel anode at a temperature of 400 °C and a steam partial pressure of 8.7 MPa.     

A modular design approach for PEM electrolyser systems with homogeneous operation conditions and highly efficient heat management

Because of its advantages, which are fast response times, high power densities and, therefore, compact system design, using polymer electrolyte membrane electrolyser (PEMEL) technology is a promising way to store the excess energy generated by renewable energy sources. With the approach of hydraulic cell compression homogeneous pressure and current distribution is guaranteed, and waste heat management as well as high-pressure operation are improved compared to conventional mechanically compressed PEMEL stacks. The new design approach presented in this work brings the concept of hydraulic cell compression close to an industrial design while preserving the mentioned advantages and providing a high level of modularity. The concept was experimentally validated for hydrogen production using a laboratory scale stack consisting of five single cells having an active cell area of 25 cm² each. A study on process water independent stack temperature control was performed using water as a hydraulic medium. Furthermore, the capability of high-pressure operation was investigated up to a process media pressure of 30 bar.     

Performance improvement of proton exchange membrane fuel cells with compressed nickel foam as flow field structure

In order to improve the performance of proton exchange membrane fuel cell (PEMFC), the compressed nickel foam as flow field structure was applied to the fuel cell. The fuel cell test system was built and the performance of fuel cells with nickel foam flow field with different thicknesses were tested and analyzed by electrochemical active surface area (EASA), electrochemical impedance and polarization curve. And its operating parameters were optimized to improve the performance of PEMFC. Our results show that the membrane electrode assembly (MEA) can show a larger catalytic active area and uniformity of gas diffusion can be improved by using the nickel foam flow field instead of the conventional graphite serpentine flow field, and the impedance characteristic of 110PPI nickel foam can be improved by increasing the compression ratio of the original material. What's more, the polarization characteristic and power output performance of PEMFC with nickel foam flow field were improved by optimizing the operating parameters. Using the optimized operating parameters (cell temperature = 80 °C; humidification temperature = 75 °C; stoichiometric ratio = 2; back pressure = 0.23 Map), a peak power density of 1.89 W cm⁻² was obtained with an output voltage of 0.46 V.     

Impact of clamping pressure and stress relaxation on the performance of different polymer electrolyte membrane water electrolysis cell designs

One promising option for storing surplus electricity from renewable energy sources is the conversion of electricity to hydrogen by polymer electrolyte membrane (PEM) electrolysis and the subsequent storage of the hydrogen produced. In order to obtain good contact, the components of an electrolysis cell are compressed at a certain clamping pressure. However, too high of a pressure can have a negative effect on cell performance. This work discusses how clamping pressure affects the cell performance of different PEM electrolysis cell designs. A special test cell is designed that makes it possible to apply pressure directly onto the active area of the cell. Polarization curves are measured at different clamping pressures, while electrochemical impedance spectroscopy (EIS) is used to show the effect of pressure on performance losses. Above a critical clamping pressure of 2.5 MPa ohmic losses are found to rise. In addition, it is tested as to whether the clamping pressure remains constant over time. The results show that stress relaxation of the catalyst coated membrane (CCM) leads to a pressure loss and thus to a decline in performance. Therefore, not only is it shown that pressure is crucial for cell performance but also, for the first time, a mechanical effect is described as an element of the cell's degradation.     

Investigation of external compression in scaling up of planar solid oxide fuel cells

The effect of contact pressure on the performance of electrolyte supported planar solid oxide fuel cells (SOFCs) are experimentally investigated in this study by varying the pressure applied on the push rod. For this purpose, cells with 1 cm², 9 cm², 16 cm², 81 cm² and 150 cm² active areas are manufactured and tested under different external compression pressures. Maximum power densities of 0.486 W/cm², 0.308 W/cm² and 0.231 W/cm² are obtained from the cells with an active area of 1 cm², 9 cm² and 16 cm², respectively, under the same contact pressure. When the impedance results are considered, it is seen that under the same compression pressure, the cell resistance increases nonlinearly with the cell size. However, when the pressure is adjusted according to the active area, a similar power density of approximately 0.4 W/cm² is obtained from these three cells. Moreover, very similar performances are measured from all cells when a portion of cells with 1 cm² active is cut and tested under the same contact pressure of 0.2 MPa. The overall results indicate that the external load should be adjusted according to the cell size, but there is no linear relationship between the active area and the applied external pressure.     

Fabrication of titanium bipolar plates by rubber forming and performance of single cell using TiN-coated titanium bipolar plates

In this study, a rubber forming method is used to fabricate titanium bipolar plates for proton exchange membrane fuel cells. A titanium blank with a thickness of 0.1 mm is compressed using a stamping mold equipped with a 200-ton hydraulic press to fabricate bipolar plates. A forming experiment is carried out by changing different parameters such as the punch velocity, punch pressure, rubber thickness, rubber hardness, and draft angle of the channel. The optimum forming conditions are found to be a rubber thickness of 10 mm, rubber hardness equivalent to that of Shore A 20, punch velocity of 30 mm s⁻¹, punch pressure of 55 MPa, and punch draft angle of 30°. The fabricated titanium bipolar plate is coated with a TiN layer. A single cell with a TiN-coated titanium bipolar plate is examined and compared to those with uncoated titanium and graphite bipolar plates. The initial performances (in terms of current densities) of the single cells with the uncoated titanium, TiN-coated titanium, and graphite bipolar plates are 396, 799, and 1160 mA cm⁻², respectively, at a cell voltage of 0.6 V.     

Dependence of high-temperature PEM fuel cell performance on Nafion® content

Operating a proton exchange membrane (PEM) fuel cell at elevated temperatures (above 100 °C) has significant advantages, such as reduced CO poisoning, increased reaction rates, faster heat rejection, easier and more efficient water management and more useful waste heat. Catalyst materials and membrane electrode assembly (MEA) structure must be considered to improve PEM fuel cell performance. As one of the most important electrode design parameters, Nafion^® content was optimized in the high-temperature electrodes in order to achieve high performance. The effect of Nafion^® content on the electrode performance in H₂/air or H₂/O₂ operation was studied under three different operation conditions (cell temperature (°C)/anode (%RH)/cathode (%RH)): 80/100/75, 100/70/70 and 120/35/35, all at atmospheric pressure. Different Nafion^® contents in the cathode catalyst layers, 15–40 wt%, were evaluated. For electrodes with 0.5 mg cm⁻² Pt loading, cell voltages of 0.70, 0.68 and 0.60 V at a current density of 400 mA cm⁻² were obtained at 35 wt% Nafion^® ionomer loading, when the cells were operated at the three test conditions, respectively. Cyclic voltammetry was conducted to evaluate the electrochemical surface area. The experimental polarization curves were analyzed by Tafel slope, catalyst activity and diffusion capability to determine the influence of the Nafion^® loading, mainly associated with the cathode.     

Influence of the bolt torque on PEFC performance with different gasket materials

In this work, different gasket materials (NBR, expanded PTFE and PTFE) with different thicknesses were investigated by evaluating the electrochemical performance as function of a torque moment applies to fasten the cell. Because the materials composing the gasket are subjected to a different deformation, depending on their mechanical properties, a different compression was obtained on the GDL as a function of the clamping force. These effects influence the cell performance, above all the diffusive region of the polarisation curves, where the problems related to the mass transport are more important. These problems are minimised when the cell is fed with gas pressurized at 3 bar_abs, in fact at higher pressure the gas concentration is higher and the diffusion is favoured despite the lowering of GDL porosity due to the compression.When the gas pressure is 1 bar_abs, the cell performance is more evidently affected by the GDL compression and contact resistance increase.In any case, an optimal clamping force was found to be as a function of the mechanical properties of materials composing the gasket. The NBR and Expanded PTFE reached the best performance with a torque moment of 11Nm while the PTFE reached similar performance at 9Nm. It was found that thinner PTFE is more stable than others during the time, with an average power density of 250 mWcm⁻² and the lowest standard deviation. The expected over-compression of the GDL is prevented by distortion of the clamping plates. This distortion results in unexpectedly good cell performance.     

Demonstration of hydrogen production in a hybrid lignite-assisted solid oxide electrolysis cell

A Ni-YSZ/YSZ/Ag–Pt cell was used to demonstrate the concept of high temperature lignite-assisted electrolysis in hybrid (combined solid oxide and molten carbonate) operation. To assess the performance of the hybrid concept, the same cell was also used for lignite-assisted electrolysis in absence of an anodic carbonate load, as well as for fuel cell measurements using H₂ and lignite as fuels, the latter both with and free of molten carbonates. In fuel cell operation, the hybrid direct lignite fuel cell obtained 45% higher maximum power density than the H₂ fed SOFC and 160% higher power density than the lignite fuel cell without carbonates. For high temperature electrolysis, the hybrid concept of admixed lignite and carbonates at the anode led to a 350% higher current density (up to 508 mA∙cm⁻², at 1.95 V), compared to the lignite-assisted operation in absence of carbonates (145 mA∙cm⁻², at 1.95 V). Thus, the anodic addition of carbonates within the same cell, increased H₂ production 3.5 times. This was accompanied by an equivalent increase of the anodic fuel consumption, and the cell's efficiency was essentially unaffected. Nonetheless, significant anodic and cathodic resistances at low overpotentials restricted electrolysis performance and efficiency, in either the absence or the presence of carbonates. These resistances, most likely due to both the cathodic steam activation and the anodic “shuttle” of the CO intermediate, were drastically alleviated at higher overpotentials. The presence of carbonates caused an earlier and more rapid decrease of the anodic area specific resistance, to much lower values at high overpotentials, resulting in the considerably higher performance of the hybrid mode.     

Material behavior of rubber sealing for proton exchange membrane fuel cells

Reliable sealing is necessary for the stable operation of proton exchange membrane fuel cell (PEMFC). In practical application, various materials have been tried in PEMFC sealing. However, the mechanical properties of these sealing materials, which play a key role in the sealing stability, have not been fully understood in PEMFC environment, especially after long-term operation. In this paper, according to the operating environment of PEMFC, sealing material experiments are carried out to explore the differences in mechanical behaviors of sealing materials, including silicone rubber (SR), fluororubber (FR), nitrile rubber (NBR) and ethylene-propylene-diene-terpolymer rubber (EPDM) and the variation of mechanical properties of these sealing materials is predicted as time goes on. The results indicate that compression rate has a great influence on sealing contact stress. SR and EPDM, with the variation of 0.15 MPa and 0.45 MPa in stress, show the best and worst mechanical stability at different compression rates, respectively. In terms of temperature, it is found that SR can adapt to different operating temperature of PEMFC and only 18% variation is found from 20 °C to 100 °C. Finally, based on Time-Temperature Superposition (TTS), high temperature experiments are conducted to predict long-term relaxation stress under PEMFC working condition. The analysis results are beneficial for choosing suitable sealing material, and it can also be applied to predict sealing ability in PEMFC.     

Optimization of contact resistance with better gasketing for a unitized regenerative fuel cell

Unitized regenerative fuel cells, as being able to use and regenerate the hydrogen, seem to be compact solution for the standalone systems. The cells with smaller active areas (<50 cm²) have better contact between electrode and bipolar plate due to their smaller sizes. It therefore results in very low resistance at the interface owing to high performance. However, the power produced by the cells is not generally observed to linearly follow the changes in the size of electrodes. Such losses in the performance for scaled up cells is due to the high interfacial contact resistance incurred at the interface. Such resistance could be lowered using optimized gaskets as well as applied torque. The present study evaluates different gaskets for a scaled-up version of the cell (300 cm²) as a measure of increased contact resistance when operated at high pressures during electrolysis mode of operation. The cell is modelled structurally and simulated for most available gasket materials i.e. silicon and Teflon. Average contact pressure at the interface of electrode and bipolar plate is considered as the parameter to estimate the interfacial contact resistance. Silicon is evaluated better material than Teflon and is observed to hold almost 4.5 bar of gas in electrolysis mode when clamped with 8 Nm of torque. The cell is observed to perform close to state of art system and delivered 125 A at 0.5 V during fuel cell mode and generate 500 and 250 mL/min of hydrogen and oxygen during electrolysis mode of operation.     

Hydrogen production by advanced proton exchange membrane (PEM) water electrolysers—Reduced energy consumption by improved electrocatalysis

Proton exchange membrane (PEM) water electrolysis systems offers several advantages over traditional technologies including greater energy efficiency, higher production rates, and more compact design. Normally in these systems, the anode has the largest overpotential at typical operating current densities. By development of the electrocatalytic material used for the oxygen evolving electrode, great improvements in efficiency can be made. We find that using cyclic voltammetry and steady state polarisation analysis, enables us to separate the effects of true specific electrocatalytic activity and active surface area. Understanding these two factors is critical in developing better electrocatalytic materials in order to further improve the performance of PEM water electrolysis cells. The high current performance of a PEM water electrolysis cell using these oxides as the anode electrocatalyst has also been examined by steady state polarisation measurements and electrochemical impedance spectroscopy. Overall the best cell voltage obtained is 1.567 V at 1 A cm⁻² and 80 °C was achieved when using Nafion 115 as the electrolyte membrane.     

Improving microbial electrolysis stability using flow-through brush electrodes and monitoring anode potentials relative to thermodynamic minima

Graphite fiber brush electrodes are commonly used in microbial electrolysis cells (MECs) for simultaneous wastewater treatment and electrochemical hydrogen production. Previous brush anode designs for continuous flow systems were configured to have flow over an array of brush electrodes. Here we compared the performance of two systems, one with flow through a single smaller or larger brush anode to an MEC with multiple brushes. The single brush MECs had only a single large brush that had a diameter larger than the chamber height, so that the brush fibers were compressed to nearly (4.5 cm diameter) or completely (5.5 cm diameter) fill the 1.3 cm high anode chamber. To evaluate the time needed for acclimation of the anode potentials were continuously monitored for 138 d (4.5 cm brush) or 143 d (5.5 cm brush). The best performance was obtained using the 5.5 cm brush fibers with a volumetric current density of 554 ± 26 A/m³, compared to <400 A/m³ when using the smaller 4.5 cm brush or the multiple brush reactor. Full acclimation was shown by a consistent and low anode potential, for example by ?248 ± 8 mV (vs. a standard hydrogen electrode) for the 5.5 cm brush, which was only 31 ± 8 mV above the minimum estimated for acetate oxidation under standard biological conditions. These results show that brush compression into a smaller chamber can enhance MEC performance and produce anode potentials close the thermodynamic minima.     

Study of catalyst sprayed membrane under irradiation method to prepare high performance membrane electrode assemblies for solid polymer electrolyte water electrolysis

In this work, a catalyst sprayed membrane under irradiation (CSMUI) method was investigated to develop high performance membrane electrode assembly (MEA) for solid polymer electrolyte (SPE) water electrolysis. The water electrolysis performance and properties of the prepared MEA were evaluated and analyzed by polarization curves, electrochemistry impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The characterizations revealed that the CSMUI method is very effective for preparing high performance MEA for SPE water electrolysis: the cell voltage can be as low as 1.564 V at 1 A cm⁻² and the terminal voltage is only 1.669 V at 2 A cm⁻², which are among the best results yet reported for SPE water electrolysis with IrO₂ catalyst. Also, it is found that the noble metal catalysts loadings of the MEA prepared by this method can be greatly decreased without significant performance degradation. At a current density of 1 A cm⁻², the MEA showed good stability for water electrolysis operating: the cell voltage remained at 1.60 V without obvious deterioration after 105 h operation under atmosphere pressure and 80 °C.     

Application of CuNi–CeO2 fuel electrode in oxygen electrode supported reversible solid oxide cell

The oxygen electrode-supported reversible solid oxide cell (RSOC) has demonstrated distinguishing advantages of fuel flexibility, shorter gas diffusion path and more choices for fuel electrode materials. However, there are serious drawbacks including the difficulty of co-firing the oxygen electrode and electrolyte, and the inefficient electrochemical performance. In this study, a (La_0.8Sr_0.2)_0.95MnO_3-δ (LSM) supported RSOC with the configuration of La_0.6Sr_0.4Fe_0.9Sc_0.1O_3-δ (LSFSc)-YSZ/YSZ/CuNi–CeO₂-YSZ is fabricated by tape casting, co-sintering and impregnation technologies. The single cell is evaluated at both fuel cell (FC) and electrolysis cell (EC) mode. Significant maximum power density of 436.0 and 377 mW cm^?2 is obtained at 750 °C in H₂ and CH₄ fuel atmospheres, respectively. At electrolysis voltage of 1.3 V and 50% steam content, current density of ?0.718, ?0.397, ?0.198 and ?0.081 A cm^?2 is obtained at 750, 700, 650 and 600 °C respectively. Much higher electrolysis performance than FC mode is exhibited probably due to the optimized electrodes with increased triple phase boundary (TPB) area and faster gas diffusion (oxygen and steam) and electrochemical reactions for water splitting. Additionally, the short-term stability of single cell in H₂ and CH₄ are also studied.     

Reversible operation performance of microtubular solid oxide cells with a nickelate-based oxygen electrode

This paper describes the reversible operation of a highly efficient microtubular solid oxide cell (SOC) with a nickelate-based oxygen electrode. The fuel cell was composed of a microtubular support of nickel and yttria stabilized zirconia (Ni-YSZ), an YSZ dense electrolyte, and a double oxygen electrode formed by a first composite layer of praseodymium nickelate (PNO) and gadolinium-doped ceria (CGO) and a second one of PNO. A good performance of the cell was obtained at temperatures up to 800 °C for both fuel cell (SOFC) and electrolysis (SOEC) operation modes, specially promising in electrolysis mode. The current density in SOEC mode at 800 °C is about −980 mA cm⁻² at 1.2V with 50% steam. Current density versus voltage curves (j-V) present a linear behavior in the electrolysis mode, with a specific cell area resistance (ASR) of 0.32 Ω cm⁻². Durability experiments were carried out switching the voltage from 0.7V to 1.2V. No apparent degradation was observed in fuel cell mode and SOEC mode up to a period of about 100 h. However, after this period especially in electrolysis mode there is an accumulated degradation associated to nickel coarsening, as confirmed by SEM and EIS experiments. Those results confirm that nickelate based oxygen electrodes are excellent candidates for reversible SOCs.     

Hydrogen production performance of 3-cell flat-tubular solid oxide electrolysis stack

Flat-tubular solid oxide electrolysis cells have been manufactured with a ceramic interconnector in a body to minimize the stack volume and eliminate metallic components. The NiO-YSZ supports are prepared by the extrusion method, and the other cell components, which included the electrolyte, air electrode, and ceramic interconnector, are fabricated by slurry coating methods. The active area of a single cell is 30 cm², and the gas tightness of the stack is checked in the range below 2 bars. The effects of the operating conditions on the performance of a solid oxide electrolysis stack are investigated using electrochemical impedance spectroscopy and the I-V test. Consequently, it is confirmed that sufficient steam content stimulates the electrochemical splitting of water and decreases the activation energy for water electrolysis at a high temperature. In our 3-cell stack test, the hydrogen production rate is 4.1 lh⁻¹, and the total hydrogen production was 144.32 l during 37.1 h of operation.     

Gadolinium doped ceria-impregnated nickel-yttria stabilised zirconia cathode for solid oxide electrolysis cell

Steam electrolysis (H₂O → H₂ + 0.5O₂) was investigated in solid oxide electrolysis cells (SOECs). The electrochemical performance of GDC-impregnated Ni-YSZ and 0.5% wt Rh-GDC-impregnated Ni-YSZ was compared to a composite Ni-YSZ and Ni-GDC electrode using a three-electrode set-up. The electrocatalytic activity in electrolysis mode of the Ni-YSZ electrode was enhanced by GDC impregnation. The Rh-GDC-impregnated Ni-YSZ exhibited significantly improved performance, and the electrode exhibited comparable performance between the SOEC and SOFC modes, close to the performance of the composite Ni-GDC electrode. The performance and durability of a single cell GDC-impregnated Ni-YSZ/YSZ/LSM-YSZ with an H₂ electrode support were investigated. The cell performance increased with increasing temperature (700 °C-800 °C) and exhibited comparable performance with variation of the steam-to-hydrogen ratio (50/50 to 90/10). The durability in the electrolysis mode of the Ni-YSZ/YSZ/LSM-YSZ cell was also significantly improved by the GDC impregnation (200 h, 0.1 A/cm², 800 °C, H₂O/H₂ = 70/30).     

Proton exchange membrane water electrolysis at high current densities: Investigation of thermal limitations

In this work the thermal limitations of high current density proton exchange membrane water electrolysis are investigated by the use of a one dimensional model. The model encompasses in-cell heat transport from the membrane electrode assembly to the flow field channels. It is validated by in-situ temperature measurements using thin bare wire thermocouples integrated into the membrane electrode assemblies based on Nafion® 117 membranes in a 5 cm² cell setup. Heat conductivities of the porous transport layers, titanium sinter metal and carbon paper, between membrane electrode assembly and flow fields are measured in the relevant operating temperature range of 40 °C – 90 °C for application in the model. Additionally, high current density experiments up to 25 A/cm² are conducted with Nafion® 117, Nafion® 212 and Nafion® XL based membrane electrode assemblies. Experimental results are in agreement with the heat transport model. It is shown that for anode-only water circulation, water flows around 25 ml/(min cm²) are necessary for an effective heat removal in steady state operation at 10 A/cm², 80 °C water inlet temperature and 90 °C maximum membrane electrode assembly temperature. The measured cell voltage at this current density is 2,05 V which corresponds to a cell efficiency of 61 % based on lower heating value. Operation at these high current densities results in three to ten-fold higher power density compared to current state of the art proton exchange membrane water electrolysers. This would drastically lower the material usage and the capital expenditures for the electrolysis cell stack.     

Evaluation of MEA manufacturing parameters using EIS for SO2 electrolysis

Membrane electrode assembly (MEA) manufacturing parameters such as hot pressing pressure and pressing time were investigated for the use in a SO₂ electrolyser. The SO₂ electrolysis was optimised in terms of cell temperature, membrane thickness and catalyst loading. The electrolysis efficiency was evaluated using polarisation curves while electrochemical impedance spectroscopy (EIS) was used to determine the membrane resistance, activation energy and mass transport limitations. An electrical circuit, which included inductance, ohmic resistance, charge transfer, constant phase and Warburg elements, was used to fit the experimental data. The optimum hot pressing conditions were 50 kg cm⁻² for 5 min at 120 °C. Increased cell temperature (80 °C) resulted in a reduction of mass transport, while thicker membranes resulted in an increased mass transport due to lower water transport through the membrane. Increased catalyst loading (from 0.3 to 1 mgPtC.cm⁻²) improved the cell performance due to improved kinetics confirmed by the EIS data.