Thesis advisor: Michael J. Naughton / The nanoscale morphology of metals can enable special functionality in plasmonic and electrochemical devices, with applications in energy conversion and storage, sensors, and computers. In particular, sharp edges on metal nano and microstructures are understood to affect the density of electrons on the metal surface. The associated concentration of electric field can concentrate surface plasmon polaritons (SPPs) and enable waveguiding of the SPPs, as we show in this thesis for sharp ridges along aluminum nanowires. Also important is the presence of facets on the metal structures, which determines the orbitals that electrons occupy on the metal surface. Changes in both the electron density and orbitals can affect the binding of molecules to the metal, which can improve reaction kinetics in catalysis. We demonstrate this on gold dendrite and plate electrocatalysts for CO2 electrolysis. Regarding metal nanostructure fabrication, electrochemical deposition and corrosion have demonstrated promising control over the morphology, including the topography, crystallinity, grain boundaries, and crystal faceting. This is important, because existing methods for metal nanostructure fabrication can only produce a circumscribed assortment of morphologies. In contrast, semiconductors and insulators have many new deposition techniques that produce a wide range of controlled morphologies. Of further appeal, electrochemical techniques are solution-based and typically operate at room temperature and pressure, allowing facile scale-up to industrial production. Here we demonstrate and discuss the mechanisms of two new techniques, which produce the aluminum nanowires and gold dendrites and plates discussed above. / Thesis (PhD) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Foram preparados eletrocatalisadores Pt-Sn/C a partir da deposição de Pt sobre Sn/C por diferentes metodologias. Os suportes Sn/C foram preparados pela redução com boroidreto de sódio (BH) e pelo método da redução por álcool (MRA). A deposição da Pt foi efetuada pelo processo de troca galvânica e utilizando agentes redutores pelos métodos BH e MRA. Os materiais obtidos foram caracterizados por energia dispersiva de raios X (EDX), difração de raios X (DRX), microscopia eletrônica de transmissão (TEM), energia dispersiva de raios X de varredura linear (EDX- line scan), voltametria cíclica (VC) e stripping de CO e testados na oxidação eletroquímica do etanol. O difratograma de raios X do Sn/C preparado pelo método BH mostrou a presença da fase Sn metálico, enquanto que, o Sn/C preparado pelo método MRA levou a formação da fase SnO2. Apesar de o material obtido por deposição espontânea apresentar-se mais ativo que o suporte Sn(BH)/C, sua atividade foi inferior ao do catalisador comercial PtSn/C BASF. Os materiais obtidos apresentaram-se mais ativos que o catalisador comercial. A deposição da Pt pelo método BH sobre o suporte Sn(BH)/C levaram a catalisadores com desempenhos inferiores ao catalisador comercial, isto devido a deposição da Pt ocorrer preferencialmente sobre as nanopartículas de Sn metálico, tornando a superfície do catalisador rica em Pt. Os estudos realizados por microscopia eletrônica de transmissão EDX scan-line mostraram que este material apresentou a distribuição mais homogênea dos sítios de Pt e Sn na superfície do catalisador. Assim, nas condições estudadas, a deposição de Pt na superfície do Sn/C, apesar de alguns casos, os materiais obtidos apresentarem melhor atividade que o catalisador PtSn/C comercial, estes apresentaram-se menos ativos que os materiais preparados em uma única etapa por co-redução. / We present a study of the achievement of Pt-Sn/C electrocatalysts from the deposition of Pt on Sn/C surface using different methodologies. The Sn/C support were obtained by reduction method with sodium borohydride (BH) and by the alcohol reduction method (MRA). Pt deposition was carried out by the galvanic exchange process and by using reducing BH and MRA methods. The materials were characterized using energy dispersive X-ray (EDX), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray linear scan (EDX- line scan), cyclic voltammetry (VC) and CO stripping and tested by electrochemical oxidation of ethanol. A comparison between MRA and BH method showed the presence of Sn metal phase in the X-ray diffractogram Sn/C using the first method while the second lead to the formation of the SnO2 phase. The obtained material resulted by deposition process more active than the Sn holder (BH)/C but less active than commercial catalyst PtSn/C BASF. The materials result to be more active than the commercial catalyst. The Pt deposition process using BH method on Sn (BH)/C support led to catalysts with lower performance to the commercial catalyst because, in this case, the deposition occurs of Pt preferably on Sn metal nanoparticles, making the surface of the catalyst rich in Pt. Studies by transmission electron microscopy EDX-line - scan showed that this material had the most even distribution of Pt and Sn sites on the catalyst surface. In this work, we obtained electrocatalyst by Pt deposition on Sn/C\'s surface, with higher activity than the Commercial PtSn/C catalyst and verified that the materials obtained in a single step process by co-reduction are the ones with higher activity.

Filmes de politiofeno foram eletroquimicamente depositados em diversos substratos eletródicos em meio aquoso fortemente ácido e as características dos materiais formados foram estudadas através de técnicas voltamétricas, espectroscópicas (FTIR e UV-Vis. e raios X) e por microscopia eletrônica de varredura. As propriedades foram comparadas com as de filmes sintetizados em meio não-aquoso concluindo-se que o material produzido em meio aquoso mostra-se mais adequado para a aplicação como suporte de partículas eletrocatalisadoras, por sua maior atividade eletroquímica, homogeneidade e estabilidade. Foram então incorporadas partículas de Pd e Pt que foram investigadas quanto a ação catalítica frente as reações de oxidação de hidrogênio (ROH) e redução de oxigênio (RRO) em ácido sulfúrico 2,0 M. Os materiais produzidos foram primeiramente caracterizados através de técnica de absorção de raios X, tendo sido observada a formação de aglomerados de partículas bastante pequenas que não alteram seu estado de oxidação com a mudança do potencial eletródico. Verificou-se que ambos os catalisadores apresentam uma certa atividade inicial frente a ROH, mas o desempenho não é estável devido à ocorrência de processos degradativos da cadeia polimérica. Por outro lado, observou-se que os filmes contendo Pt apresentam elevada atividade catalítica frente a RRO, substancialmente maior que a de filmes contendo Pd. O mecanismo da reação mostrou-se dependente da natureza do catalisador, sendo também influenciado pelo contato com o filme polimérico. Para a Pt em contato direto com o eletrólito, a reação ocorre com envolvimento de 4 elétrons e com formação de água como produto final. Para o caso em que as partículas de Pt são recobertas por politiofeno, verifica-se a participação do peróxido de hidrogênio como intermediário, sendo que somente uma certa fração do mesmo reduz-se para formar água. No caso do Pd, o processo leva à formação de peróxido de hidrogênio em baixos sobrepotenciais, que depois é reduzido, com formação de água, em sobrepotenciais mais elevados. / Polythiophene films were electrochemically grown on several electrode substrates from strong acid aqueous solutions, and the characteristics of the materials investigated using cyclic voltammetry, FTIR and UV-Vis spectroscopies, and scanning electron microscopy. Compared to the materials prepared in non-aqueous media, the polymer synthesized in aqueous media shows more adequate characteristics for using as support for electrocatalytic particles, because of its higher electrochemical activity and stability, and bulk homogeneity. Particles of Pd and Pt were electrochemically incorporated on these polymer films, and the electrocatalytic properties of such composites investigated with respect to the hydrogen oxidation (HOR) and oxygen reduction (ORR) reactions in 2.0 M sulfuric acid solutions. Previously to these kinetic studies, the composites were characterized using X-ray absorption spectroscopy from which it is seen that the catalysts are deposited as agglomerates composed of very small particles whose oxidation states are not changed by changing the electrode potential. Both catalysts present some initial activity for the HOR, but the performance is not stable due to the occurrence of a degradation process involving the polymer chain. On the other hand, it is observed that the films containing Pt show an enhanced catalytic activity for the ORR which is considerably higher than of that containing Pd. The reaction mechanism is dependent on the catalyst nature and also influenced by the contact with a polymer film coating. For Pt in direct contact with the supporting electrolyte, the reaction occurs involving 4 electrons leading to water as final product. For Pt particles covered with a polymer layer, participation of hydrogen peroxide seems to be important with only a fraction of this specie being reduced to water. In the case of Pd, the process involves formation of hydrogen peroxide at low overpotentials, which is then reduced to water at higher overpotentials.

Os eletrocatalisadores PtRh (50:50)/85% C + 15% CeO2, PtRh (90:10)/ 85% C + 15% CeO2, PtSn (50:50)/ 85% C + 15% CeO2 e PtSnRh/85% C + 15% CeO2 Todos os eletrocatalisadores foram caracterizados físico-quimicamente pelas técnicas de difração de raios X (DRX) e microscopia eletrônica de transmissão (MET). Por meio das análises de DRX foi observada a presença a estrutura cúbica de face centrada típica de platina e suas ligas, além de uma fase de SnO, em diferentes composições atômicas, foram preparados pelo método da redução por álcool para serem testados nos estudos de oxidação eletroquímica de etanol. 2 para os eletrocatalisadores de PtSn/85% C + 15% CeO2 e PtSnRh/85% C + 15% CeO2. A MET permitiu a observação de uma distribuição relativamente homogênea das partículas, cujos tamanhos médios variaram de 2,5 a 3 nm, no suporte de C + CeO2 As caracterizações eletroquímicas foram realizadas por meio das técnicas de voltametria cíclica (VC) e cronoamperometria (CA). As medições foram feitas à temperatura ambiente e à 50°C, e a técnica de preparo do eletrodo de trabalho foi a de camada fina porosa. . O eletrocatalisador PtSnRh (50:45:5)/ 85% C + 15% CeO2 Na síntese do eletrocatalisador de PtSnRh (50:45:5)/ 85% C + 15% CeO apresentou o melhor desempenho para a oxidação do etanol de acordo com as técnicas eletroquímicas utilizadas, bem como pelos estudos em condições reais de operação em células a combustível alimentadas diretamente por etanol. 2 Estudos com a técnica de infravermelho confirmaram que a oxidação do etanol ocorreu de forma incompleta para os eletrocatalisadores ternários de PtSnRh (50:45:5)/ 85% C + 15% CeOa ordem de adição dos metais foi feita de duas maneiras distintas. O estanho adicionado numa primeira etapa seguida pela adição de PtRh mostrou ser a forma que resultou em maior atividade para a oxidação do etanol. 2 e PtSnRh (50:45:5)/C. / PtRh (50:50)/ 85% C + 15% CeO2, PtRh (90:10)/ 85% C + 15% CeO2, PtSn (50:50)/ 85% C + 15% CeO2 and PtSnRh/85% C + 15% CeO2 All catalysts were characterized by the physical chemical techniques of X-ray diffraction (XRD) and transmission electron microscopy (TEM). By means of XRD it could be observed the presence of the face-centered cubic structure, which is typical of platinum and its alloys. As well, it was noticed a phase of SnOelectrocatalysts were prepared, with different atomic compositions, by the reduction by alcohol method in order to be tested in the ethanol electrochemical oxidation studies. 2 for the diffractograms of PtSn/85% C + 15% CeO2 and PtSnRh/85% C + 15% CeO2 electrocatalysts. TEM allowed the observation of a relatively homogeneous distribution of particles whose average size ranged from 2.5 to 3 nm in C + CeO2 The electrochemical characterizations were carried out by means of cyclic voltammetry (CV) and chronoamperometry (CA) both at room temperature and 50°C. Thin porous coating technique was used for all ethanol electrochemical oxidation studies. PtSnRh (50:45:5)/ 85% C + 15% CeO support. 2 For the synthesis proceeding of PtSnRh (50:45:5)/ 85% C + 15% CeO electrocatalyst showed the best performance for the ethanol oxidation according to the electrochemical techniques used, as well as by direct ethanol fuel cell tests. 2 FTIR studies confirmed that ethanol oxidation was incomplete for the ternary catalysts of PtSnRh (50:45:5)/ 85% C + 15% CeOelectrocatalyst, the metal addition order was made in two different ways. When tin was added in the first step followed by addition of PtRh to prepare the catalyst, the highest current values for the ethanol oxidation were noticed. 2 and PtSnRh (50:45:5)/C

Neste trabalho os diferentes sistemas eletrocatalíticos PtIn/C, PtRu/C, PtRuIn/C e suas diferentes proporções mássicas foram sintetizados pelo método de redução por Borohidreto de Sódio, a fim de serem utilizados como ânodo na célula a combustível alcalina de metanol direto (DMFC). Os materiais obtidos foram caracterizados pelas técnicas de EDX, DRX e MET. O método de redução aplicado na síntese se mostrou efetivo, uma vez que as partículas apresentaram boa dispersão no suporte de carbono Vulcan XC72, de acordo com as analises de EDX e MET. Os resultados obtidos por DRX evidenciaram em todos os difratogramas apresentados a estrutura CFC da platina e um relativo deslocamento do pico equivalente ao plano (220) para valores maiores e menores que 2θ. O tamanho médio do cristalito e os parâmetros de rede calculados indicaram a inserção de átomos de Índio e Rutênio à estrutura da Platina, supondo a formação de ligas. A oxidação eletroquímica do metanol foi estudada por voltametria cíclica, cronoamperometria e curvas de polarização. Os experimentos eletroquímicos demonstraram que nos eletrocatalisadores binários com maior composição de Índio e Rutênio a eficiência catalítica frente a oxidação do combustível foi melhor e para os ternários, o eletrocatalisador que possuía maior composição de Rutênio se mostrou mais eficiente. Nos experimentos práticos em células a combustível, as curvas de polarização mostraram divergências de resultados com os obtidos por voltametria cíclica e cronoamperometria, justificadas por problemas de prensagem e descolamento de MEA\'s em sistemas que apresentaram maior quantidade de cocatalisador como Índio e Rutênio. / In this work the different electrocatalytic systems PtIn/C, PtRu/C, PtRuIn/C and their different mass proportions were synthesized by the sodium borohydride reduction method, in order to be used as an anode in the alkaline direct methanol fuel cell (DMFC). The obtained materials were characterized by EDX, DRX and MET techniques. The reduction method applied in the synthesis was effective, since the particles showed good dispersion in the carbon support Vulcan XC72, according to the EDX and MET analyzes. The results obtained by XRD showed in all the diffractograms presented the CFC structure of platinum and a relative displacement of the equivalent peak to the plane (220) for values greater than and less than 2θ. The mean crystallite size and the calculated net parameters indicated the insertion of Indium and Ruthenium atoms to the Platinum structure, assuming the formation of alloys. The electrochemical oxidation of methanol was studied by cyclic voltammetry, chronoamperometry and polarization curves. The electrochemical experiments showed that in the binary electrocatalysts with higher composition of Indium and Ruthenium, the catalytic efficiency against the oxidation of the fuel demonstrated a better result. Meanwhile the ternary, the electrocatalyst with the higher Ruthenium composition was more efficient. In terms of the practical experiments in fuel cells, the polarization curves showed divergences of results with those obtained by cyclic voltammetry and chronoamperometry, which can be justified by issues of pressing and detachment of MEA\'s in systems that presented higher amounts of cocatalysts such as Indium and Ruthenium.

Os eletrocatalisadores PtRh (50:50)/85% C + 15% CeO2, PtRh (90:10)/ 85% C + 15% CeO2, PtSn (50:50)/ 85% C + 15% CeO2 e PtSnRh/85% C + 15% CeO2 Todos os eletrocatalisadores foram caracterizados físico-quimicamente pelas técnicas de difração de raios X (DRX) e microscopia eletrônica de transmissão (MET). Por meio das análises de DRX foi observada a presença a estrutura cúbica de face centrada típica de platina e suas ligas, além de uma fase de SnO, em diferentes composições atômicas, foram preparados pelo método da redução por álcool para serem testados nos estudos de oxidação eletroquímica de etanol. 2 para os eletrocatalisadores de PtSn/85% C + 15% CeO2 e PtSnRh/85% C + 15% CeO2. A MET permitiu a observação de uma distribuição relativamente homogênea das partículas, cujos tamanhos médios variaram de 2,5 a 3 nm, no suporte de C + CeO2 As caracterizações eletroquímicas foram realizadas por meio das técnicas de voltametria cíclica (VC) e cronoamperometria (CA). As medições foram feitas à temperatura ambiente e à 50°C, e a técnica de preparo do eletrodo de trabalho foi a de camada fina porosa. . O eletrocatalisador PtSnRh (50:45:5)/ 85% C + 15% CeO2 Na síntese do eletrocatalisador de PtSnRh (50:45:5)/ 85% C + 15% CeO apresentou o melhor desempenho para a oxidação do etanol de acordo com as técnicas eletroquímicas utilizadas, bem como pelos estudos em condições reais de operação em células a combustível alimentadas diretamente por etanol. 2 Estudos com a técnica de infravermelho confirmaram que a oxidação do etanol ocorreu de forma incompleta para os eletrocatalisadores ternários de PtSnRh (50:45:5)/ 85% C + 15% CeOa ordem de adição dos metais foi feita de duas maneiras distintas. O estanho adicionado numa primeira etapa seguida pela adição de PtRh mostrou ser a forma que resultou em maior atividade para a oxidação do etanol. 2 e PtSnRh (50:45:5)/C. / PtRh (50:50)/ 85% C + 15% CeO2, PtRh (90:10)/ 85% C + 15% CeO2, PtSn (50:50)/ 85% C + 15% CeO2 and PtSnRh/85% C + 15% CeO2 All catalysts were characterized by the physical chemical techniques of X-ray diffraction (XRD) and transmission electron microscopy (TEM). By means of XRD it could be observed the presence of the face-centered cubic structure, which is typical of platinum and its alloys. As well, it was noticed a phase of SnOelectrocatalysts were prepared, with different atomic compositions, by the reduction by alcohol method in order to be tested in the ethanol electrochemical oxidation studies. 2 for the diffractograms of PtSn/85% C + 15% CeO2 and PtSnRh/85% C + 15% CeO2 electrocatalysts. TEM allowed the observation of a relatively homogeneous distribution of particles whose average size ranged from 2.5 to 3 nm in C + CeO2 The electrochemical characterizations were carried out by means of cyclic voltammetry (CV) and chronoamperometry (CA) both at room temperature and 50°C. Thin porous coating technique was used for all ethanol electrochemical oxidation studies. PtSnRh (50:45:5)/ 85% C + 15% CeO support. 2 For the synthesis proceeding of PtSnRh (50:45:5)/ 85% C + 15% CeO electrocatalyst showed the best performance for the ethanol oxidation according to the electrochemical techniques used, as well as by direct ethanol fuel cell tests. 2 FTIR studies confirmed that ethanol oxidation was incomplete for the ternary catalysts of PtSnRh (50:45:5)/ 85% C + 15% CeOelectrocatalyst, the metal addition order was made in two different ways. When tin was added in the first step followed by addition of PtRh to prepare the catalyst, the highest current values for the ethanol oxidation were noticed. 2 and PtSnRh (50:45:5)/C

No description available.

The sun is a vast source of renewable energy, which can potentially be used to satisfy the world's increasing energy demand. Yet many material challenges need to be overcome before solar energy conversion can be implemented on a larger scale. This dissertation focuses on materials used for solar energy conversion through photo-electrochemical (PEC) processes. It discusses methods for improving PEC materials, namely mixed metal oxide semiconductors, by nanostructuring, incorporation of additional elements, and application surface electrocatalysts. In this dissertation several material synthesis techniques are detailed. A high vacuum synthesis process known as reactive ballistic deposition (RBD) is used to synthesize nanostructured bismuth vanadate (BiVO₄), which is studied for PEC water oxidation. Additionally, ballistic deposition (BD) is used to incorporate Mo and W into nanostructured BiVO₄ to improve the PEC activity. An array dispenser and scanner system is used to incorporate metals into copper oxide (CuO) and copper bismuth oxide (CuBi₂O₄) and over 3,000 unique material compositions are tested for cathodic photoactivity. The system is also used to test 35 elements as single component metal oxides, mixed metal oxides, and dopants for titanium dioxide (TiO₂) for use in dye-sensitized solar cells (DSCs). Lastly, RBD is used to deposit tungsten semicarbide (W₂C) onto p-type silicon (p-type) substrates as an electrocatalyst for PEC proton reduction. In many cases, the synthesis techniques and new material combinations presented in this dissertation result in improved PEC performance. The materials are thoroughly assessed and characterized to gain insights into their nanostructure, chemical composition, light absorption, charge transport properties, catalytic activity, and stability. / text

<p>Energy storage systems are critically important for many areas in modern society including consumer electronics, transportation and renewable energy production. This dissertation summarizes our efforts on improving the performance metrics of energy storage and conversion devices through rational design and fabrication of hybrid nanomaterial systems. </p><p>This dissertation is divided into five sections. The first section (chapter 2) describes comparison of graphene and carbon nanotubes (CNTs) on improving the specific capacitance of MnO2. We show that CNTs provided better performance when used as ultrathin electrodes but they both show similar performance with rapid MnO2 specific capacitance decrease as electrodes become thicker. We further designed ternary composite electrodes consisting of CNTs, graphene and MnO2 to improve thick electrode performance (chapter 3). We demonstrate that these electrodes were flexible and mechanically strong, had high electrical conductivity and delivered much higher capacity than electrodes made without CNTs. </p><p>Chapter 4 describes assembly of flexible asymmetric supercapacitors using a graphene/MnO2/CNTs flexible film as the positive electrode and an activated carbon/CNTs flexible film as the negative electrode. The devices were assembled using roll-up approach and can operate safely with 2 V in aqueous electrolytes. The major advantage of these devices is that they can deliver much higher energy under high power conditions compared with those designed by previous studies, reaching a specific energy of 24 Wh/kg at a power density of 7.8 kW/kg. </p><p>Chapter 5 describes our approach to improve the energy and power densities of nickel hydroxides for supercapacitors. This was done by assembling CNTs with Co-Ni hydroxides/graphene nanohybrids as freestanding electrodes. The assembled electrodes have dramatically improved performance metrics under practically relevant mass loading densities (~6 mg/cm2), reaching a specific capacitance of 2360 F/g at 0.5 A/g and 2030 F/g even at 20 A/g (~86% retention). </p><p>Finally, we discuss our efforts on designing highly active electrocatalysts based on winged nanotubes for oxygen reduction reactions (ORR). The winged nanotubes were prepared through controlled oxidization and exfoliation of stacked-cup nanotubes. When doped with nitrogen, they exhibited strong activity toward catalyzing ORR through the four-electron pathway with excellent stability and methanol/carbon monoxide tolerance owning to their unique carbon structure.</p> / Dissertation

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