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As demands for alternative sources of energy increase over the coming decades, water electrolysis will play a larger role in meeting our needs. The oxygen evolution reaction (OER) component of water electrolysis suffers from slow kinetics. An efficient, inexpensive, alternative electrocatalyst is needed. We present here high-activity, low onset potential, stable catalyst materials for OER based on a hierarchical network architecture consisting of Fe and Ni coated on carbon fiber paper (CFP). Several compositions of Fe-Ni electrodes were grown on CFP using a hydrothermal method, which produced an interconnected nanosheet network morphology. The materials were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Electrochemical performance of the catalyst was examined by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The best electrodes showed favorable activity (23 mA/cm², 60 mA/mg), onset potential (1.42 V vs. RHE), and cyclability. / text

Intermittent renewable energy sources, such as solar and wind, will only be viable if the electrical energy can be stored efficiently. It is possible to store electrical energy cleanly by splitting the water into oxygen (a clean byproduct) and hydrogen (an energy dense fuel) via water electrolysis. The efficiency of hydrogen production is limited, in part, by the high kinetic overpotential of the oxygen evolution reaction (OER). OER catalysts have been extensively studied for the last several decades. However, no new highly active catalyst has been developed in decades. One reason that breakthroughs in this research are limited is because there have been many conflicting activity trends. Without a clear understanding of intrinsic catalyst activity it is difficult to identify what makes catalysts active and design accordingly. To find commercially viable catalysts it is imperative that electrochemical activity studies consider and define the catalyst’s morphology, loading, conductivity, composition, and structure. The research goal of this dissertation is twofold and encompasses 1) fundamentally understanding how catalysis is occurring and 2) designing and developing a highly active, abundant, and stable OER catalyst to increase the efficiency of the OER. Specifically, this dissertation focuses on developing methods to compare catalyst materials (Chapter II), understanding the structure-compositional relationships that make Co-Fe (oxy)hydroxide materials active (Chapter III), re-defining activity trends of first row transition metal (oxy)hydroxide materials (Chapter IV), and studying the role of local geometric structure on active sites in Ni-Fe (oxy)hydroxides (Chapter V). As part of a collaboration with Proton OnSite, the catalysts studied are to be integrated into an anion exchange membrane water electrolyzer in the future. This dissertation includes previously published and unpublished co-authored material. / 10000-01-01

Metal oxides and (oxy)hydroxides, particularly those containing two or more metals have many uses as electronic materials and catalyst, especially in energy applications. In this dissertation, the structure-property relationships of these mixed-metal materials are explored in order to understand how these materials work and to guide design of materials with even higher efficiency for a given application. Chapter I introduces the materials and studies undertaken. Chapter II presents a fundamental analysis of the electronic and local atomic properties of mixed-transition-metal aluminum oxide thin films. The final three chapters focus on water electrolysis for hydrogen production, which is limited in part by the slow kinetics of the oxygen evolution reaction (OER). Nickel-iron and cobalt-iron (oxy)hydroxides have been shown to be the most active in alkaline conditions. Although it is evident that Fe is essential for high activity, its role is still unclear. Chapter III investigates the role of Fe in NiOOH by comparing the effects of Ti, Mn, La, and Ce incorporation on the OER activity of NiOOH in base. Chapter IV evaluates the OER activity and Tafel behavior of Fe3+ impurities on different noble metal substrates. Chapter V describes the results of in situ and in operando X-ray spectroscopy experiments, which shows that the local structure around Fe atoms in Co(Fe)OOH changes during OER while that of Co stays the same. This work adds to the growing body of literature that suggests Fe is essential to the catalytic active site for the OER on transition-metal (oxy)hydroxides. This dissertation contains previously published and un-published coauthored material. / 2020-01-11

Células a combustível regenerativas unitizadas (URFCs) são dispositivos eletroquímicos capazes de atuar como um eletrolisador de água ou como uma célula a combustível. Contudo, para que o potencial de uma URFC seja plenamente alcançado é essencial o desenvolvimento de componentes ativos e estáveis nos dois modos de operação, em especial em relação ao catalisador a ser utilizado no eletrodo de oxigênio. Em meio ácido, catalisadores obtidos pela combinação de platina e óxido de irídio têm apresentado desempenho satisfatório para as reações de redução (RRO) e evolução de oxigênio (REO), mas a estabilidade desses materiais ainda é relativamente pouco explorada. Neste trabalho, catalisadores bifuncionais foram sintetizados pela deposição de nanopartículas de platina sobre óxido irídio amorfo (Pt/IrOx) e cristalino (Pt/IrO2), e caracterizados físico-quimicamente através de diferentes técnicas, como EDX, XRD, XPS, XAS e XPS. A caracterização eletroquímica e a avaliação da atividade catalítica foi realizada em célula eletroquímica de três eletrodos, no qual é mostrado que, enquanto catalisadores Pt/IrO2 possuem maior atividade para a RRO, materiais Pt/IrOx são mais ativos para a REO. A estabilidade dos catalisadores bifuncionais foi avaliada empregando-se diferentes protocolos de envelhecimento. Uma investigação detalhada dos processos de degradação foi feita através da técnica de microscopia eletrônica de transmissão de localização idêntica (IL-TEM), enquanto que a dissolução eletroquímica dos catalisadores foi monitorada online utilizando-se uma célula eletroquímica de fluxo hifenada a um espectrômetro de massas com plasma indutivamente acoplado (SFCICP-MS). / Unitized regenerative fuel cells (URFCs) are electrochemical devices that can operate as a water electrolyzer or as a fuel cell. However, for the potential of an URFC to be fully achieved, it is essential to develop components that are active and stable in both operation modes, especially in relation to the catalyst to be used in the oxygen electrode. In acidic media, catalysts obtained by the combination of platinum and iridium oxide have shown satisfactory performance for the oxygen reduction (ORR) and evolution (OER) reactions, but the stability of these materials is still relatively little explored. In this work, bifunctional catalysts were synthesized by the deposition of platinum nanoparticles on hydrous (Pt/IrOx) and crystalline (Pt/IrO2) iridium oxide, and physicochemically characterized by different techniques such as EDX, XRD, TEM, XPS and XAS. The electrochemical characterization and the evaluation of the catalytic activity were performed in a three-electrodes electrochemical cell, in which it is shown that, while Pt/IrO2 catalysts have higher activity for the ORR, Pt/IrOx materials are more active for the OER. The stability of the bifunctional catalysts was evaluated using different aging protocols. A detailed investigation of the degradation processes was done using the identical location transmission electron microscopy (IL-TEM) technique, while the electrochemical dissolution of the catalysts was monitored online using a scanning flow cell inductively coupled to a plasma mass spectrometer (SFC-ICP-MS) setup.

京都大学 / 新制・課程博士 / 博士(人間・環境学) / 甲第23996号 / 人博第1048号 / 新制||人||246(附属図書館) / 2022||人博||1048(吉田南総合図書館) / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 内本 喜晴, 教授 高木 紀明, 教授 白井 理, 教授 光島 重徳 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DFAM

<p>In the chlorate process, natural convection arises thanks to the hydrogen evolving cathode. This increases the mass transport of the different species in the chlorate electrolyte. There is a strong connection between mass transport and the kinetics of the electrode reactions. A better knowledge about these phenomena and their interactions is desirable in order to understand e.g. the reasons for deactivation of anode coatings and what process conditions give the longest lifetime and the highest efficiency.</p><p>One of the aims of his work was to understand how the chlorate process has to be run to avoid exceeding the critical anode potential (<em>E</em>_cr) in order to keep the potential losses low and to achieve a long lifetime of the DSAs. At <em>E</em>_cr anodic polarisation curves in chlorate electrolyte bend to higher Tafel slopes, causing increasing potential losses and accelerated ageing of the anode. Therefore the impact on the anode potential and on <em>E</em>_cr of different electrolyte parameters and electrolyte impurities was investigated. Additionally, the work aimed to investigate the impact of an addition of chromate on oxygen evolution and concentration profiles under conditions reminiscent of those in the chlorate process (high ionic strength, 70 °C, ruthenium based DSA, neutral pH), but without chloride in order to avoid hypochlorite formation. For this purpose a model, taking into account mass transport as well as potential- and concentration-dependent electrode reactions and homogeneous reactions was developed. Water oxidation is one of the side reactions considered to decrease the current efficiency in chlorate production. The results from the study increase the understanding of how a buffer/weak base affects a pH dependent electrode reaction in a pH neutral electrolyte in general. This could also throw light on the link between electrode reactions and homogeneous reactions in the chlorate process.</p><p>It was found that the mechanism for chloride oxidation is likely to be the same for potentials below <em>E</em>_cr as well as for potentials above <em>E</em>_cr. This was based on the fact that the apparent reaction order as well as α_a seem to be of the same values even if the anode potential exceeds<em> E</em>_cr. The reason for the higher slope of the polarisation curve above <em>E</em>_cr could then be a potential dependent deactivation of the active sites. Deactivation of active ruthenium sites could occur if ruthenium in a higher oxidation state were inactive for chloride oxidation.</p><p>Concentration gradients of H⁺, OH^-, CrO₄ ^2- and HCrO₄ ^-during oxygen evolution on a rotating disk electrode (RDE) were predicted by simulations. The pH dependent currents at varying potentials calculated by the model were verified in experiments. It was found that an important part of the chromate buffering effect at high current densities occurs in a thin (in the order of nanometers) reaction layer at the anode. From comparisons between the model and experiments a reaction for the chromate buffering has been proposed. Under conditions with bulk pH and chromate concentration similar to those in the chlorate process, the simulations show that the current density for oxygen evolution from OH^- would be approximately 0.1 kA m^-2, which corresponds to about 3% of the total current in chlorate production.</p>

Oxygenic photosynthesis in plants, algae and cyanobacteria converts sunlight into chemical energy. In this process electrons are transferred from water molecules to CO2 leading to the assembly of carbohydrates, the building blocks of life. A cluster of four manganese ions and one calcium ion, linked together by five oxygen bridges, constitutes the catalyst for water oxidation in photosystem II (Mn4CaO5 cluster). This cluster stores up to four oxidizing equivalents (S0,..,S4 states), which are then used in a concerted reaction to convert two substrate water molecules into molecular oxygen. The reaction mechanism of this four-electron four-proton reaction is not settled yet and several hypotheses have been put forward. The work presented in this thesis aims at clarifying several aspects of the water oxidation reaction by analyzing the mode of substrate water binding to the Mn4CaO5 cluster. Time-resolved membrane-inlet mass spectrometric detection of flash-induced O2 production after fast H218O labelling was employed to study the exchange rates between substrate waters bound to the Mn4CaO5 cluster and the surrounding bulk water. By employing this approach to dimeric photosystem II core complexes of the red alga Cyanidoschyzon merolae it was demonstrated that both substrate water molecules are already bound in the S2 state of the Mn4CaO5 cluster. This was confirmed with samples from the thermophilic cyanobacterium Thermosynechococcus elongatus. Addition of the water analogue ammonia, that is shown to bind to the Mn4CaO5 cluster by replacing the crystallographic water W1, did not significantly affect the exchange rates of the two substrate waters. Thus, these experiments exclude that W1 is a substrate water molecule. The mechanism of O-O bond formation was studied by characterizing the substrate exchange in the S3YZ● state. For this the half-life time of this transient state into S0 was extended from 1.1 ms to 45 ms by replacing the native cofactors Ca2+ and Cl- by Sr2+ and I-. The data show that both substrate waters exchange significantly slower in the S3YZ● state than in the S3 state. A detailed discussion of this finding lead to the conclusions that (i) the calcium ion in the Mn4CaO5 cluster is not a substrate binding site and (ii) O-O bond formation occurs via the direct coupling between two Mn-bound water-derived oxygens, which were assigned to be the terminal water/hydroxy ligand W2 and the central oxo-bridging O5. The driving force for the O2 producing S4→S0 transition was studied by comparing the effects of N2 and O2 pressures of about 20 bar on the flash-induced O2 production of photosystem II samples containing either the native cofactors Ca2+ and Cl- or the surrogates Sr2+ and Br-. While for the Ca/Cl-PSII samples no product inhibition was observed, a kinetic limitation of O2 production was found for the Sr/Br-PSII samples under O2 pressure. This was tentatively assigned to a significant slowdown of the O2 release in the Sr/Br-PSII samples. In addition, the equilibrium between the S0 state and the early intermediates of the S4 state family was studied under 18O2 atmosphere in photosystem II centers devoid of tyrosine YD. Water-exchange in the transiently formed early S4 states would have led to 16,18O2 release, but none was observed during a three day incubation time. Both experiments thus indicate that the S4→S0 transition has a large driving force. Thus, photosynthesis is not limited by the O2 partial pressure in the atmosphere.

El hidrógeno es un portador de energía que ya ha demostrado su capacidad para reemplazar el petróleo como combustible. Sin embargo, los medios de producción actualmente en uso siguen siendo altamente emisores de gases de efecto invernadero. La foto-electrólisis del agua es un proceso que, a partir de la energía solar, separa los compuestos elementales del agua como el hidrógeno y el oxígeno utilizando un semiconductor con propiedades físicas adecuadas. La hematita (¿-Fe2O3) es un material prometedor para esta aplicación debido a su estabilidad química y su capacidad para absorber una porción significativa de la luz (con una banda prohibida entre 2.0 - 2.2 eV). A pesar de estas propiedades ventajosas, existen limitaciones intrínsecas al uso de óxido de hierro para la descomposición fotoelectroquímica del agua. La primera restricción es la posición de su banda de conducción que es menor que el potencial de reducción de agua. Esta limitación se puede superar mediante la adición en serie de un segundo material, en tándem, que absorberá una parte complementaria del espectro solar y llevar a los electrones a un nivel de energía más alto que el potencial para la liberación de hidrógeno. El segundo obstáculo proviene del desacuerdo entre la corta longitud de difusión de los portadores de carga y la profundidad de penetración larga de la luz. Por lo tanto, es necesario controlar la morfología de los electrodos de hematita en una escala de tamaño similar a la longitud de transporte del orificio. En esta tesis, se introduce un nuevo concepto para mejorar el rendimiento fotoelectroquímico de la hematita. Usando el método hidrotermal depositamos capas delgadas de hematita dopada con Cr en sustratos de vidrio conductivo. También se ha preparado por medios electroquímicos una heterounión del tipo p-CuSCN/n-Fe2O3 depositando secuencialmente una capa de ¿-Fe2O3 y una película de CuSCNsobre sustratos de FTO (SnO2: F).Finalmente, se ha preparado células solares de perovskitas y óxido de hierro. Para ello se depositó una capa delgada, densa y uniformede óxido de hierro (¿-Fe2O3) como capa de transporte de electrones (ETL) en lugar de dióxido de titanio (TiO2) que se utiliza convencionalmente en las células fotovoltaicas perovskitastipoCH3NH3PbI3 (SGP). Este último dispositivo mostró un aumento en la fotocorriente del 20% y un IPCE30 veces mayor que la hematita simple, lo que sugiere una mejor conversión de las longitudes de onda por encima de 500 nm. Palabras clave: Fotoelectroquímica, división de agua, producción de hidrógeno, evolución de oxígeno, semiconductores de óxido de metal, hematita, óxido de hierro, nanoestructuras / Hydrogen is an energy carrier that has already demonstrated its ability to replace oil as a fuel. However, the means of production currently used remain highly emitting greenhouse gases. Photo-electrolysis of water is a process that uses solar energy to separate the elemental compounds of water such as hydrogen and oxygen using a semiconductor with adequate physical properties. Hematite (¿-Fe2O3) is a promising material for this application because of its chemical stability and ability to absorb a significant portion of light (with a band-gap between 2.0 - 2.2 eV). Despite these advantageous properties, there are intrinsic limitations to the use of iron oxide for the photoelectrochemical cracking of water. The first constraint is the position of its conduction band, which is lower than the water reduction potential. This constraint can be overcome by the addition in series of a second material, in tandem, which will absorb a complementary part of the solar spectrum and bring the electrons to a higher energy level than the potential of hydrogen release. The second obstacle comes from the disagreement between the short diffusion length of the charge carriers and the long light penetration depth. It is therefore necessary to control the morphology of the hematite electrodes on a scale of similar size to the transport length of the hole. In this thesis a new concept is introduced to improve the photoelectrochemical performances. Using the hydrothermal method we deposited thin layers of Cr-doped hematite on conductive glass substrates. We also electrochemically prepared a p-CuSCN / n-Fe2O3 heterojunction by sequentially depositing ¿-Fe2O3 and CuSCN films on FTO (SnO2: F) substrates. Finally, we have used uniform and dense thin layers of iron oxide (¿-Fe2O3) as an electron transport layer (ETL) in place of titanium dioxide (TiO2) conventionally used in photovoltaic cells based on perovskites CH3NH3PbI3 (PSC). This latter concept showed a 20% increase of the photocurrent and an IPCE 30 times greater than the simple hematite, suggesting better conversion of high wavelengths (> 500 nm). Keywords: Photoelectrochemistry, Water Splitting, Hydrogen Production, Oxygen Evolution, MetalOxide Semiconductors, Hematite, Iron Oxide, Nanostructures, Surface. / L'hidrogen és un proveïdor d'energia que ja ha demostrat la seva capacitat per reemplaçar el petroli com a combustible, però els mitjans de producció actuals continuen essent fortament emissors dels gasos responsables d'efecte hivernacle. La fotoelectròlisi de l'aigua és un procés que, a partir de l'energia solar, separa els compostos elementals d'aigua com l'hidrogen i l'oxigen utilitzant un semiconductor amb propietats físiques adequades. La hematita (¿-Fe2O3) és un material prometedor per a aquesta aplicació a causa de la seva estabilitat química i capacitat d'absorbir una porció significativa de la llum (amb un gap entre 2,0 i 2,2 eV). Malgrat aquestes propietats avantatjoses, hi ha limitacions intrínseques per a l'ús d'òxid de ferro per a la descomposició fotoelectroquímica de l'aigua. La primera restricció és la posició de la seva banda de conducció que és inferior al potencial de reducció d'aigua. Aquesta limitació es pot superar mitjançant l'addició en sèrie d'un segon material, en tàndem, que absorbirà una part complementària de l'espectre solar i portar els electrons a un nivell d'energia més alt que el potencial per a l'alliberament d'hidrogen. El segon obstacle prové del desacord entre la curta durada de la difusió dels portadors de càrrega i la llarga profunditat de penetració de la llum. Per tant, és necessari controlar la morfologia dels elèctrodes d'hematita en una escala de mida similar a la longitud del forat del transport. En aquesta tesi, es presenta un nou concepte per millorar el rendiment fotoelectroquímic. Mitjançant el mètode hidrotermal es van dipositar capes primes de hematita Cr-doped sobre substrats de vidre conductor. També s'han preparat electroquímicamentheterounions de tipus p-CuSCN/n-Fe2O3 dipositant seqüencialment una capa de ¿-Fe2O3 i altra de CuSCN sobre substrats FTO (SnO2: F).Finalment, s'han produït cél·lules solars de perovskitesi óxid de ferro. Per això es va depositaruna capa prima,densai uniforme d'òxid de ferro (¿-Fe2O3) com a capa de transport d'electrons (ETL) en lloc de diòxid de titani (TiO2) que s'utilitza convencionalment en les cèl·lules fotovoltaiques de perovskita híbrida del tipus CH3NH3PbI3 (SGP). Aquest últim dispositiu va mostrar un augment del fotocorrent del 20% i una IPCE30 vegades superior a la hematita simple, la qual cosa suggereix una millor conversió a longitud d'ones per sobre de 500 nm. Paraules clau:Fotoelectroquímica, divisió d'aigua, producció d'hidrogen, evolució d'oxigen, semiconductors d'òxids metàl·lics, hematita, òxid de ferro, nanoestructures. / Bouhjar, F. (2018). Preparation et performance d'une cellule photocatalytique à base d'hématite pour la génération d'hydrogène [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/106345 / TESIS

The development and design of efficient catalysts are essential for catalytic energy technologies, accompanied with the fundamental understanding of structure-property relationships of these catalysts. Metal-organic frameworks (MOFs), as the new class of promising catalysts, have been intensively investigated primarily in their fundamental electrochemistry and the broad spectrum of catalytic applications due to their structural flexibility, tailorable crystalline, and multi-functionality. In this work, we combine experiments and mechanism investigation to gain a fundamental understanding of how the surface property and the structure of MOFs affect their catalytic performance. With the aim of material design for MOFs catalysts, we developed two novel superhydrophilic and aerophobic metal-organic frameworks (AlFFIVE-1-Ni MOFs and FeFFIVE-1-Ni MOFs) used as electrocatalysts for the first time during oxygen evolution reactions (OER). Under the facilitation of hydrophilicity and aerophobicity, developed FeFFIVE-1-Ni MOFs electrocatalysts deliver optimal OER performance, better than that of the state-of-art RuO2 and referred NiFe-BDC MOFs electrocatalysts. Most importantly, the practical strategy demonstrated that the hydrophilic and aerophobic structure of MOFs does indeed deliver the optimal electrocatalytic performance. With the aim of investigating the structural transformation process of metal-organic framework, we used a series of advanced characterization techniques to monitor the structure evolution and defects presence for post-heating treated UiO-66 MOFs. The structural and electronic features of UiO-66 MOFs were intensely studied in their hydroxylated, dehydroxylated, defected, and pyrolytic forms. Meanwhile, one concept about the framework situation, quasi-MOF (like a transition state, defined high activation along the structure evolution corresponding to the presence of many defects), was presented and demonstrated. Compared with pristine UiO-66 MOF, the Quasi-MOF with the presence of active defects showed enhanced catalytic activity on the Meerwein-Ponndorf-Verley reduction reaction, which offers an opportunity to understand the structure-property relationship along with the structure evolution process of UiO-66 MOFs.