Regenerative PEM fuel cells (RPEMFC)

Fuel cells represent one of the most promising technologies for the production of clean energy. They offer several advantages such as high efficiency and reliability, fuel flexibility, low maintenance cost and distributed power generation. Among other technologies, Polymer Exchange Membrane (PEM) fuel cells are the best option for low temperature applications (< 100oC), zero gas emissions and quick start up/shut down of the system. In recent years, worldwide research has focused in the development of PEM type water electrolysers for the production of high purity hydrogen and oxygen providing the required energy for electrolysis. Furthermore, development of self-sustaining systems producing and consuming energy according to the current energy demand is of great interest. An example of such a system is the Regenerative or Reversible PEM fuel cell, which combine a PEM electrolyser for hydrogen and oxygen production which are consequently stored, and a PEM fuel cell for electricity production from stored gases. Unitized Regenerative PEM fuel cells combine the two systems in one compact cell. Despite of the various advantages, this system faces major limitations, like: (a) efficiency and stability of the bifunctional oxygen electrode, (b) corrosion of carbon, GDL and bipolar plates. Additionally, the high anodic overpotentials needed for the oxygen evolution reaction limit the stability and life time of the supported catalyst. Materials usually used as bifunctional oxygen electrode electrocatalysts are Pt and Ir. The main objective of the present study was the development of nanostructured Ir-based oxide electrocatalysts with high surface area of the type IrxM1-xO2, M=Pt or Ru with high efficiency and stability for the oxygen evolution reaction in PEM electrolysers and for the oxygen reduction reaction in Unitized Regenerative PEM fuel cells. In particular, two families of oxides were synthesized: a. IrxPt1-xO2, x=1, 0.8, 0.5 and 0.2 and b. IryRu1-yO2, y=1, 0.9, 0.7 and 0.5 using the modified Adams fusion synthesis method. All oxides consisted of small particles with the desired structure and composition. The intrinsic electrocatalytic activity and stability during redox cycling were studied with cyclic and linear scan voltammetry. The most efficient and stable oxides for the oxygen evolution reaction were identified and developed as anode oxygen electrodes in membrane electrode assemblies (MEA) using electrolytic membrane Nafion®115 and commercial hydrogen electrode (Pt/C). Pure IrO2 was the most stable and efficient electrocatalyst for oxygen evolution reaction, while the mixed oxide Ir0.5Ru0.5O2 although presented lower performance it exhibits remarkable stability. Concerning bimetallic Ir-Pt oxides, the prospective bifunctional operation in both reduction and oxygen evolution was not observed. Among these oxides, the best performance for water splitting was achieved with Ir0.5Pt0.5O2, but IrO2 still has the highest efficiency.Οι κυψέλες καυσίμου αποτελούν μια πολλά υποσχόμενη τεχνολογία για την παραγωγή καθαρής ενέργειας. Μεταξύ των διαφόρων τεχνολογιών, οι Κυψέλες Καυσίμου τύπου Πολυμερικής Μεμβράνης αποτελούν ιδανική επιλογή για εφαρμογές όπου απαιτούνται χαμηλές θερμοκρασίες λειτουργίας (< 100οC), μηδενικοί αέριοι ρύποι και γρήγορη εκκίνηση/διακοπή λειτουργίας του συστήματος. Τα τελευταία χρόνια η παγκόσμια ερευνητική κοινότητα έχει στραφεί στην έρευνα και ανάπτυξη διατάξεων τύπου πολυμερικής μεμβράνης για την ηλεκτρόλυση του νερού, καθώς είναι δυνατό να παραχθεί υπερκαθαρό υδρογόνο και οξυγόνο από νερό παρέχοντας την απαιτούμενη ενέργεια. Επίσης, γίνεται λόγος για την ανάπτυξη αυτοσυντηρούμενων συστημάτων, δηλαδή συστημάτων τα οποία είναι δυνατό να παράγουν και να καταναλώνουν ενέργεια με βάση τις απαιτήσεις. Τέτοια συστήματα αποτελούν και οι Αναγεννούμενες Κυψέλες Καυσίμου τύπου Πολυμερικής Μεμβράνης, οι οποίες συνδυάζουν μια διάταξη ηλεκτρόλυσης νερού για την παραγωγή υδρογόνου και οξυγόνου, τα οποία αποθηκεύονται, και μια διάταξη κυψέλης καυσίμου για την παραγωγή ηλεκτρικής ενέργειας με χρήση των αποθηκευμένων αερίων. Οι Ενοποιημένες Αναγεννούμενες Κυψέλες Καυσίμου PEM (URFC) συνδυάζουν την κυψέλη καυσίμου και την διάταξη ηλεκτρόλυσης σε ένα σύστημα. Παρά τα πλεονεκτήματα, οι διατάξεις αυτές αντιμετωπίζουν σημαντικούς περιορισμούς, όπως: (α) απόδοση και σταθερότητα του διλειτουργικού ηλεκτροδίου οξυγόνου, (β) διάβρωση του φορέα από άνθρακα, της στιβάδας διάχυσης αερίων και των διπολικών πλακών. Επίσης τα υψηλά θετικά δυναμικά κατά την αντίδραση της έκλυσης οξυγόνου (OER) πρακτικά ορίζουν ένα όριο στη διάρκεια ζωής του υποστηριγμένου καταλύτη. Οι καταλύτες που συνήθως χρησιμοποιούνται στο διλειτουργικό ηλεκτρόδιο οξυγόνου είναι Pt και Ir. Κύριος στόχος της παρούσας διδακτορικής διατριβής ήταν η ανάπτυξη νανοδομημένων μεγάλης ειδικής επιφάνειας ηλεκτροκαταλυτών με βάση το ιρίδιο, του τύπου IrxM1-xΟ2, Μ = Pt και Ru με υψηλή απόδοση και σταθερότητα για την OER σε διατάξεις ηλεκτρόλυσης PEM αλλά και για την αναγωγή του οξυγόνου σε αναγεννούμενες διατάξεις URFC. Συγκεκριμένα αναπτύχθηκαν τα οξείδια του τύπου: α. IrxPt1-xO2, x = 1, 0.8, 0.5 και 0.2 και β. IryRu1-yO2, y = 1, 0.9, 0.7 και 0.5 χρησιμοποιώντας την τροποποιημένη μέθοδο Adams. Τα οξείδια αποτελούνταν από μικρά σωματίδια και είχαν την επιθυμητή δομή και σύσταση. Η εγγενής ηλεκτροκαταλυτική ενεργότητα και η σταθερότητα των ηλεκτροκαταλυτών κατά την οξειδοαναγωγική λειτουργία μελετήθηκε με κυκλική βολταμετρία και καμπύλες γραμμικής σάρωσης του δυναμικού. Βάσει των αποτελεσμάτων του ηλεκτροχημικού χαρακτηρισμού αναγνωρίστηκαν τα αποδοτικότερα και σταθερότερα οξείδια κατά την OER και αναπτύχθηκαν σε ανοδικά ηλεκτρόδια οξυγόνου διατάξεις ηλεκτροδίων-ηλεκτρολύτη (ΜΕΑ), χρησιμοποιώντας Nafion®115 ως ηλεκτρολυτική μεμβράνη και εμπορικό ηλεκτρόδιο Pt/C ως ηλεκτρόδιο υδρογόνου. Σε κάθε περίπτωση το καθαρό IrO2 αποτέλεσε τον σταθερότερο και αποδοτικότερο ηλεκτροκαταλύτη για την αντίδραση έκλυσης οξυγόνου, ενώ το μικτό οξείδιο Ir0.5Ru0.5O2 εμφάνισε χαμηλότερη απόδοση με εξίσου ικανοποιητική σταθερότητα. Όσον αφορά τα διμεταλλικά οξείδια Ir-Pt, δεν παρατηρήθηκε η αναμενόμενη διλειτουργική τους απόδοση τόσο κατά την αντίδραση έκλυσης οξυγόνου όσο και αναγωγής του. Μεταξύ αυτών, την καλύτερη απόδοση στην ηλεκτρόλυση του νερού παρουσίασε το Ir0.5Pt0.5O2, σημαντικά όμως μικρότερη από αυτή του καθαρού IrO2

Effect of Steam to Carbon Dioxide Ratio on the Performance of a Solid Oxide Cell for H₂O/CO₂ Co-Electrolysis

The mixture of H2 and CO, the so-called syngas, is the value-added product of H2O and CO2 co-electrolysis and the feedstock for the production of value-added chemicals (mainly through Fischer-Tropsch). The H2/CO ratio determines the process in which syngas will be utilized and the type of chemicals it will produce. In the present work, we investigate the effect of H2O/CO2 (steam/carbon dioxide, S/C) ratio of 0.5, 1 and 2 in the feed, on the electrochemical performance of an 8YSZ electrolyte-supported solid oxide cell and the H2/CO ratio in the outlet, under co-electrolysis at 900 °C. The B-site iron doped lanthanum strontium chromite La0.75Sr0.25Cr0.9Fe0.1O3-δ (LSCF) is used as fuel electrode material while as oxygen electrode the state-of-the art LSM perovskite is employed. LSCF is a mixed ionic-electronic conductor (MIEC) operating both under a reducing and oxidizing atmosphere. The cell is electrochemically characterized under co-electrolysis conditions both in the presence and absence of hydrogen in the feed of the steam and carbon dioxide mixtures. The results indicate that under the same concentration of hydrogen and different S/C ratios, the same electrochemical performance with a maximum current density of approximately 400 mA cm−2 is observed. However, increasing p(H2) in the feed results in higher OCV, smaller iV slope and Rp values. Furthermore, the maximum current density obtained from the cell does not seem to be affected by whether H2 is present or absent from the fuel electrode feed but has a significant effect on the H2/CO ratio in the analyzed outlet stream. Moreover, the H2/CO ratio seems to be identical under polarization at different current density values. Remarkably, the performance of the LSCF perovskite fuel electrode is not compromised by the exposure to oxidizing conditions, showcasing that this class of electrocatalysts retains their reactivity in oxidizing, reducing, and humid environments

Carbon Tolerant Fuel Electrodes for Reversible Sofc Operating on Carbon Dioxide

A challenging barrier for the broad, successful implementation of Reversible Solid Oxide Fuel Cell (RSOFC) technology for Mars application utilizing CO2 from the Martian atmosphere as primary reactant, remains the long term stability by the effective control and minimization of degradation resulting from carbon built up. The perovskitic type oxide material La0.75Sr0.25Cr0.9Fe0.1O3-δ (LSCF) has been developed and studied for its performance and tolerance to carbon deposition, employed as bi-functional fuel electrode in a Reversible SOFC operating on the CO2 cycle (Solid Oxide Electrolysis Cell/SOEC: CO2 electrolysis, Solid Oxide Fuel Cell/SOFC: power generation through the electrochemical reaction of CO and oxygen). A commercial state-of-the-art NiO-YSZ (8% mol Y2O3 stabilized ZrO2) cermet was used as reference material. CO2 electrolysis and fuel cell operation in 70% CO/CO2 were studied in the temperature range of 900-1000°C. YSZ was used as electrolyte while LSM-YSZ/LSM (La0.2Sr0.8MnO3) as oxygen electrode. Results showed that LSCF had high and stable performance under RSOFC operation

Carbon Tolerant Fuel Electrodes for Reversible Sofc Operating on Carbon Dioxide

A challenging barrier for the broad, successful implementation of Reversible Solid Oxide Fuel Cell (RSOFC) technology for Mars application utilizing CO2 from the Martian atmosphere as primary reactant, remains the long term stability by the effective control and minimization of degradation resulting from carbon built up. The perovskitic type oxide material La0.75Sr0.25Cr0.9Fe0.1O3-δ (LSCF) has been developed and studied for its performance and tolerance to carbon deposition, employed as bi-functional fuel electrode in a Reversible SOFC operating on the CO2 cycle (Solid Oxide Electrolysis Cell/SOEC: CO2 electrolysis, Solid Oxide Fuel Cell/SOFC: power generation through the electrochemical reaction of CO and oxygen). A commercial state-of-the-art NiO-YSZ (8% mol Y2O3 stabilized ZrO2) cermet was used as reference material. CO2 electrolysis and fuel cell operation in 70% CO/CO2 were studied in the temperature range of 900-1000°C. YSZ was used as electrolyte while LSM-YSZ/LSM (La0.2Sr0.8MnO3) as oxygen electrode. Results showed that LSCF had high and stable performance under RSOFC operation

Investigation of Advanced Components in a High Pressure Single-Cell Electrolyser for the Development of a HP-PEM-ELY Stack as Part of a Regenerative Fuel Cell System

The objective of the presented work, done under current ESA activity (Contract No. 4000109578/13/NL/SC), is the performance and tolerance evaluation of selected components and materials for the development of a High Pressure, Polymer Electrolyte Membrane (PEM) Electrolyser (HP-PEM-ELY) Stack, aiming to operate at 80 bar with a performance output of 0.3 A/cm2 at 1.6 V. An extensive study was performed on a single-cell high pressure PEM electrolyser manifold, leading to a list of materials with suitable properties and engineering solutions towards operation in space environment. This investigation provided the necessary feedback for the design of a HP-PEM-ELY stack, which is also discussed. The ultimate target of the current ESA activity is to implement research findings, develop and operate a complete regenerative fuel cell system, comprising of a High Temperature Fuel Cell Stack and the HP-PEM-ELY stack. System aspects are briefly discussed

Investigation of Advanced Components in a High Pressure Single-Cell Electrolyser for the Development of a HP-PEM-ELY Stack as Part of a Regenerative Fuel Cell System

The objective of the presented work, done under current ESA activity (Contract No. 4000109578/13/NL/SC), is the performance and tolerance evaluation of selected components and materials for the development of a High Pressure, Polymer Electrolyte Membrane (PEM) Electrolyser (HP-PEM-ELY) Stack, aiming to operate at 80 bar with a performance output of 0.3 A/cm2 at 1.6 V. An extensive study was performed on a single-cell high pressure PEM electrolyser manifold, leading to a list of materials with suitable properties and engineering solutions towards operation in space environment. This investigation provided the necessary feedback for the design of a HP-PEM-ELY stack, which is also discussed. The ultimate target of the current ESA activity is to implement research findings, develop and operate a complete regenerative fuel cell system, comprising of a High Temperature Fuel Cell Stack and the HP-PEM-ELY stack. System aspects are briefly discussed