A successful transition from fossil fuels to renewable energies such as wind and solar will require the implementation of high-energy-density storage technologies. Promising energy storage technologies include lithium-ion batteries, metal-air batteries, and hydrogen production via photoelectrochemical water splitting. While these technologies differ substantially in their mode of operation, they often involve transition-metal oxides as a component. Thus, fundamental materials research on metal oxides will continue to provide much needed advances in these technologies. In this thesis, the electrochemical and electrocatalytic properties of Fe- and Mn-substituted layered LiNiO₂ materials were investigated. These materials were prepared by heating mixed nitrate precursors in O₂ atmosphere at 700-850 °C for 12 h with intermediate grindings. The products were chemically delithiated with NO₂BF₄, and the delithiated samples were annealed at moderate temperatures in order to transform them to a spinel-like phase. Samples were characterized by inductively coupled plasma analysis and Rietveld refinement of the X-ray diffraction patterns, which were found to be in reasonably close agreement regarding lithium stoichiometry. Spinel-like materials were found to possess an imperfect spinel structure when heated at lower temperatures and a significant amount of NiO impurity was formed when heated to higher temperatures. This structural disorder was manifested during electrochemical cycling -- only Mn-rich compositions showed reversible capacities at a voltage of around 4.5 V. The layered materials exhibited significant capacity loss upon cycling, and this effect was magnified with increasing Fe content. These materials were further investigated as catalysts for the oxygen evolution reaction (OER). All samples containing Mn exhibited low OER activity. In addition, delithiation degraded catalyst performance and moderate temperature annealing resulted in further degradation. Because delithiation significantly increased surface area, activities were compared to the relative to BET surface area. Li₀.₉₂Ni₀.₉Fe₀.₁O₂ exhibited significantly higher catalytic activity than Li₀.₈₉Ni₀.₇Fe₀.₃O₂. This prompted testing of Li[subscript x]Ni₁₋[subscript y]Fe[subscript y]O₂ (y = 0, 0.05, 0.1, 0.2, and 0.3) samples. It was found that a Fe content of approximately 10% resulted in the highest OER activity, with decreased activities for both larger and smaller Fe contents. These results were found to be consistent with studies of Fe substituted nickel oxides and oxyhydroxides, suggesting a similar activation mechanism. / text

The goal of this project is the design of heterogeneous catalysts to facilitate the oxygen evolution reaction (OER). Considering the industrial feasibility for this reaction, first-row transition-metal-based materials are good candidates since they are cheap, abundant and possess variable oxidation states. However, most of them give only moderate catalytic activities, compared with noble-metal-based materials. To achieve efficient catalysts while maintaining low cost, it is important to discover and modify new systems based on the study of existing materials.In chapter 3 we present a study of the effect of surface reduction of birnessite on catalytic activity. A sample of birnessite was reduced by stirring with sodium dithionite, in which case the oxidation states of surface Mn decreased faster than those of inside Mn. We characterized the difference between the oxidation states of Mn of surface and inside (ΔAOS) and further investigate the effect of ΔAOS on catalysis. The catalytic activity was examined by reaction of birnessite samples with ceric ammonium nitrate, and O2 evolution was monitored using a dissolved oxygen probe with respect to time. The most reduced samples with ΔAOS of 0.15 was found to possess a turnover number (TON) of 36 mmol O2 per mol Mn, a value 10-fold higher than the unmodified sample. This result suggests oxidation state differential across layers aids the catalysis. In chapter 4, a more rigorous study is conducted by the examination of few-layer catalysts constructed by manganese oxide sheets with different oxidation states. We stacked low-AOS manganese oxide sheets with high-AOS manganese oxide sheets in various ordered combinations to obtain few-layer birnessite samples with non-uniform distribution of Mn(III). We found samples with more variation in AOS had a lower overpotential (~510 mV) in electrochemical OER catalysis than uniform stacks of the parent manganese oxide sheets (~750 mV for low-AOS sheets, >1000 mV for high-AOS sheets. The result indicates that the distribution of Mn(III) in stacking direction was the dominant factor for OER catalysis in birnessite and is more important than the overall Mn(III) content. We also found the band structures via scanning tunneling microscopy (STM) and provide an electronic-structure-based explanation of the observed activity. In chapter 5 an analogous strategy to that used in chapter 4 is applied to optimize lithium cobalt oxide (LCO) and lithium nickel oxide (LNO) layered catalysts. LCO and LNO contains various oxidation states (or spin states) of cobalt and nickel atoms. With alternatively stacking a high-AOS and a low-AOS cobalt (or nickel) oxide sheets one by one, the electrochemical OER catalytic activity of the obtained few layer LCO (or LNO) sample was enhanced. The results indicated that the structural feature of the alternating distribution of oxidation states affected not only the birnessite catalysts but also both cobalt and nickel oxide materials. In chapter 6 we incorporated both cobalt and nickel oxide sheets into layered heterostructured catalysts. We present findings that mixed transition metal oxide material K-CoxNiyO2 with alternating distribution of cobalt and nickel oxide layers showed enhanced activity mixed Ni-Co metal oxides with homogeneously distributed transition metals. The overpotential of the sample K-Co0.5Ni0.5O2 with alternating distribution of Co and Ni is 460 mV, 190 mV smaller than that of the sample with homogeneously distributed Co and Ni, even though they had a similar elemental composition. / Chemistry

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

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

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.

<div><br></div><div><p>Noble metal nanoparticles have been studied extensively as heterogeneous catalysts for electrocatalytic and thermal reactions. In particular, the support material for the catalytic species is known to play a role in influencing the geometric and electronic properties of the active site as well as its catalytic performance. Polycrystalline gold electrodes have been used as a support to modify the electrocatalytic behavior of adsorbed molecular species. Here, we have studied two electrocatalytic processes- the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR), using Au nanoparticle-based catalysts.</p> <p>Transition metal dichalcogenides are well-known HER catalysts that show structure-sensitive catalytic activity. In particular, undercoordinated sulfur sites at the edges of bulk materials as well as amorphous clusters and oligomers tend to show the highest reactivity. The hydrogen adsorption energy of MoS_x nanoclusters can be further tuned through the metallic support. Here, we synthesize colloidal Au@MoS₄^2-, Au@WS₄^2-and Au@MoS₄^2--WS₄^2- using a biphasic ligand-exchange method. The MoS₄^2- and WS₄^2- complexes show higher HER activity when supported on Au nanoparticles than on to a carbon control, illustrating the electronic role played by the support material.</p> <p>In the second project, Au nanoparticle cores are utilized as supports for Pd submonolayer and monolayer surfaces in order to catalyze the two-electron reduction of O₂ to generate hydrogen peroxide. Bulk surfaces of Pt and Pd are excellent catalysts for the four-electron reduction of O₂ to H₂O. In order to achieve high selectivity for H₂O₂, we postulate that the ensemble geometry of the Pd surface must be reduced to small islands or single atoms based on literature studies that have shown that large Pd ensembles are required for O–O bond cleavage. In this study, we synthesize several submonolayers surface coverages of Au@Pd core-shell nanoparticles using a biphasic ligand-exchange method. As the Pd coverage decreases from monolayer to submonolayer, the peroxide selectivity rises but is accompanied by an increase in catalytic overpotential. The highest peroxide selectivity was observed for 0.1 layers of Pd on Au, which likely exhibits the highest fraction of isolated atom and small cluster geometric ensembles of Pd.</p><br></div>

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20397号 / 工博第4334号 / 新制||工||1672(附属図書館) / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 安部 武志, 教授 作花 哲夫, 教授 陰山 洋 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

The growing interest of renewable fuel and energy sources has steadily increased over time due to climate changes. Research is being made around the world to find solutions for the different problems; one possible solution is to produce hydrogen gas to help phase out the usage of fossil fuels. So far, the technology for the hydrogen gas production is expensive for various reasons, one of the challenges is to minimize the energy usage for the production. Hydrogen could be used in fuel cells which can be used to fuel an electric car. In a fuel cell, hydrogen and oxygen gas are mixed to produce electrical energy as the main product, but it also forms thermal energy and water. Hydrogen gas can be produced from the reversed reaction; by electrolysis of water. This reaction requires energy and one way to minimize the energy usage for this is by using acatalyst. The goal with this master thesis was to see how the reaction rate of the oxygen evolution reaction can be affected by different single atom catalyst systems. The main structure for this catalyst in this thesis is aporphyrin molecule where different transition metals were tried as the active site. Different modifications on the structure were also made by exchanging some of the structures atoms and by adding different ligands.The purpose of this is to see how these modifications change the activity of the catalyst. The catalysts were optimized and calculated in a computational chemistry program called Gaussian 16. The calculations was made by using the DFT functional PBE0 and the basis sets Def2svp and Def2tzvpp. The results show that different modifications do affect the activity of the catalyst. The biggest variations in activity are from placing ligands under the active site while exchanging hydrogens to other substituents on the outer radial position can fine tune the results. The best active sites for this system came by using iridium, rhodium and cobalt which are all elements in group 9 of the periodic table. The lowest overpotential of 0.513 V was given by an iridium based system with four hydrogens exchanged by fluorides. / Runt om i världen finns ett ökat intresse för förnyelsebara energi och bränslekällor för att tackla klimat förändringarna. Stor del av forskningen som görs idag har i syfte att hitta nya lösningar för att minska klimatpåverkan i olika områden. Ett av forskningsområderna är hitta vägar till en miljövänligare vätgasproduktion där vätgasen skulle kunna användas i bränsleceller. Dessa celler kan sättas i elbilar och på så sätt fasa ut användingen av fossila bränslen. En av utmaningarna för vätgasproduktionen är att den idag är kostsam och kräver mycket energi. Forskare försöker hitta olika katalysatorer som kan minska energiåtgången som krävs vid elektrolys av vatten där syrgas och vätgas produceras. Målet med det här examensarbetet är att se hur en single atom catalyst kan påverka reaktionskinitiken för den syrgasbildande reaktionen vid elektrolys av vatten. Huvudstrukturen för katalysatorn som beräkningarna är gjorda på är en porphyrinmolekyl där olika övergångsmetaller kommer testas som det aktiva sätet i katalysatorn. Olika ligander kommer även tillsättas systemet samt utbyte av några väteatomer till olika substituenter i porfyrinstrukturen. Katalysatorn optimerades i det kvantkemiska beräkningsprogrammet Gaussian 16 med funktionalen PBE0 med basset Def2svp och Def2tzvpp. Resultaten visade att olika modifikationer på systemet hade en påverkan på katalysatorns aktivitet. Den största påverkan hade de olika liganderna som placerades under det aktiva sätet jämfört med de olika substituenterna. De bästa metallerna för katalysatorn var iridium, rhodium och kobolt vilket alla ligger i grupp nio i det periodiska systemet. Den lägsta överpotentialen på 0.513 V gavs av iridium systemet med fyra utbyta väten till fluor.

A major contribution to global warming is CO2 emitted from the combustion of fossil fuels. Electrochemical processes can help to mitigate the elevated CO2 emissions through either the conversion of CO2 into value-added chemicals or the replacement of fossil fuels with clean fuels such as hydrogen produced from water oxidation. The present dissertation focuses on the mechanistic aspects of electrochemical processes. Electrochemical water oxidation is hindered by the low efficiency of oxygen evolution reaction (OER) at the anode whereas electrochemical reduction of CO2 (ERCO2) is hampered by high overpotentials and poor product selectivity. In this dissertation, we studied the catalytic activity of transition metal-based catalysts, including FeNi spinels, metal-oxide/copper, and d metal cyclam complexes, for both OER and ERCO2 using the density functional theory (DFT) computational approach.We report a combined effort of fabricating FeNi oxide catalysts and identifying the active component of the catalyst for OER. Our collaborators at the University of California, Santa Cruze fabricated a series of FeNi spinels-based materials including Ni(OH)Fe2O4(Cl), Ni(OH)Fe2O4, Fe(OH)Fe2O4(Cl), Fe(OH)Fe2O4, Ni(OH)O(Cl), Ni(OH)O and some show exceptional activity for OER. Combined experimental characterization and computational mechanistic study based on the computational hydrogen electrode (CHE) model revealed that Ni(OH)Fe2O4(Cl) is the active ensemble for exceptional OER performance. We also investigated CO2 reduction to C1 products at the metal-oxide/copper interfaces ((MO)4/Cu(100), M = Fe, Co and Ni) based on the CHE model. The effect of tuning metal-oxide/copper interfaces on product selectivity and limiting potential was clearly demonstrated. This study showed that the catalyst/electrode interface and solvent can be regulated to optimize product selectivity and lower the limiting potential for ERCO2. Applied potential affects the stability of species on the surface of the electrode. The proton-coupled electron transfer (PCET) equilibrium assumed in the CHE model does not capture the change in free energy under the influence of the applied potential. In contrast, the constant electrode potential (CEP) model captures changes in free energy due to applied potential, we applied the CEP model to ERCO2 and OER on (MO)4/Cu(100) and compared the results with those from the CHE model. The results demonstrate that the CHE and the CEP models predict different limiting potentials and product selectivity for ERCO2, but they predict similar limiting potentials for OER. The results demonstrate the importance of accounting for the applied potential effect in the study of more complex multi-step electrochemical processes. We also studied transition metal-based homogeneous catalysts for ERCO2. We examined the performance of transition metal(M) - cyclam(L) complexes as molecular catalysts for the reduction of CO2 to HCOO- and CO, focusing on the effect of changing the metal ions in cyclam on product selectivity (either HCOO- or CO), limiting potential and competitive hydrogen evolution reaction. Our results show that among the complexes, [LNi]2+ and [LPd]2+ can catalyze CO2 reduction to CO, and [LMo]2+ and [LW]3+ can reduce CO2 to HCOO-. Notably, [LMo]2+, [LW]3+, [LW]2+ and [LCo]2+ have a limiting potential less negative than -1.6 V and are based on earth-abundant elements, making them attractive for practical application. In summary, the dissertation demonstrates high-performance catalysts can be designed from earth-abundant transition metals for electrochemical processes that would alleviate the high CO2 level in the environment. On the other hand, completely reversing the increasing trend of CO2 level in the atmosphere requires a collective human effort.