Novel porous coatings for performance benefits and cost reduction of polymer exchange membrane water electrolyzer

Kurzfassung

To produce hydrogen from renewable energy (green hydrogen), polymer electrolyte membrane water electrolysis is the most promising technology due to the highly dynamic operation, the safety of the overall system as well as the scalability to relevant plant sizes at least in the MW range. However, due to the use of titanium for manufacturing stack components such as bipolar plates and porous transport layers as well as the requirement of precious metal based protective coatings, polymer electrolyte membrane water electrolysis is still too expensive to compete with current state of the art technologies for hydrogen production. Since the bipolar plates and porous transport layers made of titanium account for about 2/3 of the overall stack cost depending on the cell configuration, a significant cost reduction could be achieved in this technology if they could be manufactured from low-cost materials. Additionally, a cost reduction is also conceivable by establishing new operating parameters such as high current densities, temperature and pressure, which result in an increased hydrogen production rate. With this the operational cost of polymer electrolyte membrane water electrolysis can be reduced. The main objective of this dissertation is to develop high-performance, durable and cost-effective polymer electrolyte membrane water electrolysis cell components as well as investigating extreme operating conditions for polymer electrolyte membrane water electrolysis. For this purpose, previous research work was followed up and coated stainless steel bipolar plates were investigated for their stability on the anode side for polymer electrolyte membrane water electrolysis use. Therefore, a polymer electrolyte membrane water electrolysis stack with niobium and niobium/titanium coated stainless steel and uncoated stainless steel bipolar plates for the anode and cathode side, respectively, was operated for 14,000 hours. The niobium/titanium coated bipolar plate was manufactured using two different coating procedures: first on top of plasma-sprayed titanium, niobium was applied via magnetron sputtered physical vapor deposition. In contrast, the niobium coated bipolar plate only required a single coating process via plasma spraying. In this long-term test, the aforementioned coatings were investigated in comparison to commercially available bipolar plates. The cells were analyzed regularly using polarization curves and post-test characterizations were performed by measuring ICR, SEM/EDX, AFM and XPS. These analysis methods showed that both applied coatings are suitable for use in polymer electrolyte membrane water electrolysis. A remarkable degradation rate of only 5.5 µV h-1 for all tested cells was achieved. These results indicate that stainless steel is a real alternative to pure titanium bipolar plates for commercial use and contributes to a significant cost reduction of the polymer electrolyte membrane water electrolysis technology. To achieve further cost reduction for polymer electrolyte membrane water electrolysis, the also very expensive porous transport layers was optimized by material substitution to compete with current state-of-the-art technologies in hydrogen production. So far, the use of uncoated stainless steel components in the polymer electrolyte membrane water electrolysis cell is not possible due to strongly increasing cell potentials. Therefore, the approach was pursued to apply non-precious metal coatings of titanium and niobium/titanium to both the stainless steel bipolar plates and a stainless steel porous transport layers, so that the current density could be increased by a factor of 13 for the same cell potential. Once again, the components used were comprehensively characterized physically and electrochemically, which, supported by modelling of the pore network, shows that the niobium/titanium coating on the stainless steel porous transport layers leads to efficient water and gas transport. In addition, the polymer electrolyte membrane water electrolysis cell with coated stainless steel components was tested in an accelerated stress test for more than 1000 hours. After the test, no signs of iron contamination were detected in either the membrane or the electrodes. Finally, the polymer electrolyte membrane water electrolysis cell with stainless steel-based components was investigated up to 6 A cm-2 at 80°C to cope with future operating strategies. A remarkable performance of 4 A cm-2 at 1.91 V was achieved, which is comparable to the highest performances reported in the literature by reknown institutes. Likewise, the cost of green hydrogen can be reduced by changing the operating parameters of the polymer electrolyte membrane water electrolysis. For this purpose, the current density, the temperature and the hydrogen pressure of the polymer electrolyte membrane water electrolysis have been significantly increased. However, this requires the use of components optimized with regard to media transport. A porous transport layer for polymer electrolyte membrane water electrolysis was developed that allows operation up to 6 A cm-2, 90 °C and 90 bar hydrogen outlet pressure. For this purpose, a porous sintered layer of titanium was deposited on a low-cost titanium mesh by diffusion bonding. This novel approach eliminates the need for a flow field in the bipolar plate. Comparatively, the mesh porous transport layer without porous sintered layer was tested, but its cell potential increases significantly due to mass transport losses, reaching about 2.5 V at 2 A cm-2 and 90 °C. In contrast, the polymer electrolyte membrane water electrolysis with the porous sintered layer/mesh porous transport layers has shown 6 A cm-2 at the same cell potential, which significantly extends the operating range of the electrolyser. The behaviour of the porous transport layer during cell operation was extensively investigated by means of physical characterization and pore network simulations, which showed that the porous sintered layer/mesh porous transport layer leads to efficient gas/water management in the polymer electrolyte membrane water electrolysis. Finally, the porous sintered layer/mesh porous transport layer was validated in an industrial-sized and containerized polymer electrolyte membrane water electrolysis stack operating at 90 bar hydrogen outlet pressure. Therefore, key findings of my research were that the most expensive components, namely the bipolar plates and porous transport layer, can be made of stainless steel by coating them with non-precious metals to fully protect against corrosion and brings an efficiency increase of 12 % for the coated stainless steel porous transport layer in comparison to the reference material. The conduction of the, to date longest polymer electrolyte membrane water electrolysis test in academy, of a novel plasma sprayed coating revealed neglectable degradation rates. This test demonstrated that uncoated stainless steel can be used on the cathode side resulting in further cost reduction. In conclusion, it can be said that titanium as a base material can be replaced by stainless steel as the new standard material. Furthermore, a novel porous transport layer was developed which achieves a 31 % higher efficiency compared to the reference material during operation under extreme conditions. High permeabilities for the media ensures effective two-phase transport what makes the flow-field structure of the bipolar plates obsolete. Thereby new operating parameters suggested can be established as future state-of-the-art.