Development and characterization of catalysts for electrolytic hydrogen production and chlor–alkali electrolysis cells

Gli argomenti di questa tesi hanno riguardato l’elettrolisi cloro-soda e l’elettrolisi dell’acqua mediante sistemi basati su membrane a scambio protonico (PEM). • Elettrolisi cloro-soda. Il cloro è oggi essenzialmente ottenuto mediante i processi industriali di elettrolisi di cloro-soda ed, in minore quantità, dall’elettrolisi dell’acido cloridrico. Il principale problema di questi processi è l’elevato consumo di energia elettrica che, solitamente, rappresenta una parte sostanziale del costo totale di produzione. Per l’ottimizzare di tale processo è necessario, quindi, ridurre il consumo energetico. La sostituzione del tradizionale catodo ad evoluzione di idrogeno, con un elettrodo a diffusione gassosa ad ossigeno, comporta una nuova reazione che riduce il potenziale termodinamico di cella e questo si traduce in un risparmio energetico del 30-40%. L’attività di ricerca è stata indirizzata verso lo studio di elettrodi a diffusione gassosa per la reazione di riduzione di ossigeno con particolare attenzione all’analisi superficiale e morfologica degli elettrocatalizzatori. In particolare l’attenzione è stata focalizzata sui fenomeni di deattivazione che coinvolgono questo tipo di elettrodi. Test di durata sono stati condotti sugli elettrodi in cella cloro-soda. Analisi di tipo comparativo sugli stessi sono state condotte, prima e dopo il loro funzionamento, nelle condizioni operative di interesse. La superficie degli elettrodi è stata analizzata mediante microscopio elettronico a scansione e spettroscopia fotoelettronica a raggi X. Analisi di bulk sono state effettuate mediante diffrattometria a raggi X ed analisi termogravimetrica. • Elettrolisi dell’acqua (PEM). L’idrogeno può essere prodotto a partire da sorgenti energetiche rinnovabili come fotovoltaico, eolico mediante l’elettrolisi dell’acqua. In particolare, l’elettrolisi, mediante l’utilizzo di un elettrolita polimerico (PEM), è considerata una promettente metodologia per la produzione di idrogeno, alternativa al convenzionale processo di elettrolisi il cui elettrolita è un liquido alcalino, altamente tossico e corrosivo. Un elettrolizzatore PEM possiede certamente dei vantaggi confrontato con il classico processo alcalino in termini di semplicità, sicurezza ed alta efficienza energetica. Questo sistema utilizza la già affermata tecnologia delle celle a combustibile ad elettrolita polimerico. Sfortunatamente il processo di scissione elettrochimica dell’acqua è associata ad un elevato consumo energetico, principalmente dovuto agli alti sovrapotenziali nella reazione anodica di evoluzione di ossigeno. Risulta quindi di fondamentale importanza trovare elettrocatalizzatori per l’evoluzione di ossigeno ottimali in modo da minimizzare le perdite. Il platino è utilizzato al catodo per la reazione di evoluzione di idrogeno (HER) e gli ossidi di iridio o rutenio sono usati all’anodo per la reazione di evoluzione di ossigeno (OER). Questi ossidi metallici sono richiesti perché, confrontati al platino metallico, offrono alta attività catalitica, una migliore stabilità a lungo termine ed una minore perdita di efficienza dovuta alla corrosione o all’inquinamento. Il lavoro è stato principalmente indirizzato verso: 1) la sintesi e caratterizzazione di anodi a base di RuO2 e IrO2; 2) la sintesi di supporti conduttori a base di subossidi di titanio con alta area superficiale. 1) Catalizzatori nanostrutturati a base di RuO2 e IrO2 sono stati preparati mediante un processo colloidale a 100°C; gli idrossidi così ottenuti sono stati calcinati a differenti temperature. L’attenzione è stata focalizzata sugli effetti che il trattamento termico produce sulla struttura cristallografica e sulla dimensione delle particelle di questi catalizzatori e come queste proprietà possono influenzare le performance degli elettrodi per la reazione di evoluzione di ossigeno. Caratterizzazioni elettrochimiche sono state fatte mediante curve di polarizzazioni, spettroscopia d’impedenza, e misure di crono-amperometria. 2) Una nuova metodologia di sintesi per la preparazione dei subossidi di titanio con fase Magneli (TinO2n-1) è stata sviluppata. Le caratteristiche di questi materiali sono state valutate sotto condizioni operative, in elettrolizzatori di tipo SPE, e confrontate con la polvere commerciale Ebonex. La stessa fase attiva a base di IrO2 è stata usata, come elettrocatalizzatore, per entrambi i sistemi.The topics of this PhD thesis are concerning with Chlor alkali electrolysis and PEM water electrolysis. • Chlor alkali electrolysis. The industrial production of chlorine is today essentially achieved through sodium chloride electrolysis, with only a minor quantity coming from hydrochloric acid electrolysis. The main problem of all these processes is the high electric energy consumption which usually represents a substantial part of the total production cost. Therefore, in order to improve the process, it is necessary to reduce the power consumption. The substitution of the traditional hydrogen-evolving cathodes with an oxygen-consuming gas diffusion electrode (GDE) involves a new reaction that reduces the thermodynamic cell voltage and leads to an energy savings of 30-40%. My research activity was addressed to the investigation of the oxygen reduction at gas-diffusion electrodes as well as to the surface and morphology analysis of the electrocatalysts. Specific attention was focused on deactivation phenomena involving this type of GDE configuration. The catalysts used in this study were based on a mixture of micronized silver particles and PTFE binder. In this study, fresh gas diffusion electrodes were compared with electrodes tested at different times in a chlor-alkali cell. Electrode stability was investigated by life-time tests. The surface of the gas diffusion electrodes was analyzed for both fresh and used cathodes by scanning electron microscopy and X-ray photoelectron spectroscopy. The bulk of gas diffusion electrodes was investigated by X-ray diffraction and thermogravimetric analysis. • PEM water electrolysis. Water electrolysis is one of the few processes where hydrogen can be produced from renewable energy sources such as photovoltaic or wind energy without evolution of CO2. In particular, an SPE electrolyser is considered as a promising methodology for producing hydrogen as an alternative to the conventional alkaline water electrolysis. A PEM electrolyser possesses certain advantages compared with the classical alkaline process in terms of simplicity, high energy efficiency and specific production capacity. This system utilizes the well know technology of fuel cells based on proton conducting solid electrolytes. Unfortunately, electrochemical water splitting is associated with substantial energy loss, mainly due to the high over-potentials at the oxygen-evolving anode. It is therefore important to find the optimal oxygen-evolving electro-catalyst in order to minimize the energy loss. Typically, platinum is used at the cathode for the hydrogen evolution reaction (HER) and Ir or Ru oxides are used at the anode for the oxygen evolution reaction (OER). These metal oxides are required, compared to the metallic platinum, because they offer a high activity, a better long-term stability and less efficiency losses due to corrosion or poisoning. My work was mainly addressed to a) the synthesis and characterisation of IrO2 and RuO2 anodes; b) conducting Ti-suboxides support based on a high surface area. a) Nanosized IrO2 and RuO2 catalysts were prepared by using a colloidal process at 100°C; the resulting hydroxides were then calcined at various temperatures. The attention was focused on the effect of thermal treatments on the crystallographic structure and particle size of these catalysts and how these properties may influence the performance of oxygen evolution electrode. Electrochemical characterizations were carried out by polarization curves, impedance spectroscopy and chrono-amperometric measurements. b) A novel chemical route for the preparation of titanium suboxides (TinO2n−1) with Magneli phase was developed. The relevant characteristics of the materials were evaluated under operating conditions, in a solid polymer electrolyte (SPE) electrolyser, and compared to those of the commercial Ebonex®. The same IrO2 active phase was used in both systems as electrocatalyst

Traffic Engineering with Segment Routing: SDN-based Architectural Design and Open Source Implementation

Traffic Engineering (TE) in IP carrier networks is one of the functions that can benefit from the Software Defined Networking paradigm. By logically centralizing the control of the network, it is possible to "program" per-flow routing based on TE goals. Traditional per-flow routing requires a direct interaction between the SDN controller and each node that is involved in the traffic paths. Depending on the granularity and on the temporal properties of the flows, this can lead to scalability issues for the amount of routing state that needs to be maintained in core network nodes and for the required configuration traffic. On the other hand, Segment Routing (SR) is an emerging approach to routing that may simplify the route enforcement delegating all the configuration and per-flow state at the border of the network. In this work we propose an architecture that integrates the SDN paradigm with SR-based TE, for which we have provided an open source reference implementation. We have designed and implemented a simple TE/SR heuristic for flow allocation and we show and discuss experimental results.Comment: Extended version of poster paper accepted for EWSDN 2015 (version v4 - December 2015

OSHI - Open Source Hybrid IP/SDN networking (and its emulation on Mininet and on distributed SDN testbeds)

The introduction of SDN in IP backbones requires the coexistence of regular IP forwarding and SDN based forwarding. The former is typically applied to best effort Internet traffic, the latter can be used for different types of advanced services (VPNs, Virtual Leased Lines, Traffic Engineering...). In this paper we first introduce the architecture and the services of an "hybrid" IP/SDN networking scenario. Then we describe the design and implementation of an Open Source Hybrid IP/SDN (OSHI) node. It combines Quagga for OSPF routing and Open vSwitch for OpenFlow based switching on Linux. The availability of tools for experimental validation and performance evaluation of SDN solutions is fundamental for the evolution of SDN. We provide a set of open source tools that allow to facilitate the design of hybrid IP/SDN experimental networks, their deployment on Mininet or on distributed SDN research testbeds and their test. Finally, using the provided tools, we evaluate key performance aspects of the proposed solutions. The OSHI development and test environment is available in a VirtualBox VM image that can be downloaded.Comment: Final version (Last updated August, 2014

Micromagnetic simulations of persistent oscillatory modes excited by spin-polarized current in nanoscale exchange-biased spin valves

We perform 3D micromagnetic simulations of current-driven magnetization dynamics in nanoscale exchange biased spin-valves that take account of (i) back action of spin-transfer torque on the pinned layer, (ii) non-linear damping and (iii) random thermal torques. Our simulations demonstrate that all these factors significantly impact the current-driven dynamics and lead to a better agreement between theoretical predictions and experimental results. In particular, we observe that, at a non-zero temperature and a sub-critical current, the magnetization dynamics exhibits nonstationary behaviour in which two independent persistent oscillatory modes are excited which compete for the angular momentum supplied by spin-polarized current. Our results show that this multi-mode behaviour can be induced by combined action of thermal and spin transfer torques.Comment: 7pages, 2 figures, submitted JAP via MMM 200