A reação de redução de oxigênio (RRO) e a reação de oxidação do borohidreto (ROB) foram estudadas em eletrólito alcalino em eletrodos formados por diversos tipos de óxidos de manganês dopados com Ni (II) dispersos sobre carbonos Monarch 1000, MM225 e E350. As técnicas de caracterização físico-química foram difração de raios X (DRX), microscopia eletrônica de transmissão de alta resolução (HR-TEM) equipado com espectrômetro de energia dispersiva de raios X (EDX). Já os estudos eletroquímicos compreenderam voltametria cíclica, curvas de polarização de estado quase estacionário além das técnicas de Espectroscopia de massas diferencial on line (DEMS) e Infravermelho com transformada de Fourier in situ (FTIR). Foi observada pequena inserção dos átomos de Ni na estrutura dos MnOx. A fase correspondente a NiMnOx/C está presente na forma de aglomerados nanocristalinos ou em forma de agulhas com tamanhos da ordem de 1,5 a 6,7 nm dependendo do tipo de carbono utilizado como substrato. Manganita (MnOOH) apresentou-se como fase preponderante para óxido de manganês disperso sobre carbono Monarch 1000 enquanto que para materiais dispersos sobre carbono MM225 e E350G a fase MnO2 esta presente em maior quantidade. Estudos eletroquímicos em camada fina utilizando eletrodo disco rotatório revelaram melhores atividade para a RRO e estabilidade para MnOx dopados com níquel. A RRO procede segundo o mecanismo peróxido, seguida pela reação de desproporcionação do íon HO2- formado. A extensão da reação de desproporcionação do íon HO2- aumenta com o aumento da quantidade de Ni. Sobre eletrocatalisadores suportados em carbonos MM225 e E350 a reação de desproporcionação é mais rápida e envolve um total de 4 e- por oxigênio molecular. As curvas de polarização para RRO obtidas na presença do íon BH4- mostraram que todos os materiais são tolerantes à presença do borohidreto. Resultados de DEMS on line e FTIR in situ mostraram que óxidos de manganês dopados com Ni além de serem ativos para RRO são também ativos para a ROB, porém há uma grande influência da composição e da morfologia dos materiais uma vez que, quando fases segregadas de Ni estão presentes nas amostras, a reação compete com a hidrólise heterogênea do BH4- levando a uma diminuição da eficiência faradaica. / The oxygen reduction reaction (ORR) and the borohydride oxidation reaction (BOR) were studied in alkaline medium on Ni (II) doped MnOx catalysts supported on different carbon powder substrates. Characterizations of physico chemical properties were made by X ray diffraction (XRD), high resolution transmition electronic microscopy (HR-TEM) equipped with X ray dispersive energy spectroscopy (EDS). Electrochemical studies involved cyclic voltammetry and oxygen reduction voltammograms. Also it was used Differential Electrochemical Mass Spectrometry on line (DEMS) and Fourier Transform Infra Red Spectrometry (FTIR) in situ. A small insertion of Ni atoms in the MnOx lattice was observed, this consisting of a true doping of the manganese oxide phase. The corresponding NiMnOx phase is present in the form of needles or agglomerates, with crystallite sizes in the order of 1.5-6.7 nm. Layered manganite (MnOOH) phase has been detected for the Monarch1000 supported NiMnOx material, while different species of MnOx phases are present at the E350G and MM225 carbons. Electrochemical studies in thin porous coating active layers in the rotating ring-disk electrode setup revealed that the MnOx catalysts present better ORR kinetics and electrochemical stability upon Ni doping. The ORR follows the so-called peroxide mechanism on MnOx/C catalysts, with the occurrence of minority HO2- disproportionation reaction. The HO2- disproportionation reaction progressively increases with the Ni content in NiMnOx materials. The catalysts supported on the MM225 and E350G carbons promote faster disproportionation reaction, thus leading to an overall four-electron ORR pathway. The results towards ORR in presence of sodium borohydride showed that all materials are tolerant to the presence of BH4- ion into some extent. DEMS on line and FTIR in situ showed that NiMnOx/C are also active toward the BOR, but there is a strong influence of the nature of the electrocatalysts with respect to the morphology, composition, the nature of the carbon substrate and the Ni load. Results indicate that the electrocatalysts containing segregate Ni phases, the bohohydride oxidation occurs together with the heterogeneous hydrolysis of the BH4- ion resulting in a decrease of the faradaic efficiency.

A reação de redução de oxigênio (RRO) e a reação de oxidação do borohidreto (ROB) foram estudadas em eletrólito alcalino em eletrodos formados por diversos tipos de óxidos de manganês dopados com Ni (II) dispersos sobre carbonos Monarch 1000, MM225 e E350. As técnicas de caracterização físico-química foram difração de raios X (DRX), microscopia eletrônica de transmissão de alta resolução (HR-TEM) equipado com espectrômetro de energia dispersiva de raios X (EDX). Já os estudos eletroquímicos compreenderam voltametria cíclica, curvas de polarização de estado quase estacionário além das técnicas de Espectroscopia de massas diferencial on line (DEMS) e Infravermelho com transformada de Fourier in situ (FTIR). Foi observada pequena inserção dos átomos de Ni na estrutura dos MnOx. A fase correspondente a NiMnOx/C está presente na forma de aglomerados nanocristalinos ou em forma de agulhas com tamanhos da ordem de 1,5 a 6,7 nm dependendo do tipo de carbono utilizado como substrato. Manganita (MnOOH) apresentou-se como fase preponderante para óxido de manganês disperso sobre carbono Monarch 1000 enquanto que para materiais dispersos sobre carbono MM225 e E350G a fase MnO2 esta presente em maior quantidade. Estudos eletroquímicos em camada fina utilizando eletrodo disco rotatório revelaram melhores atividade para a RRO e estabilidade para MnOx dopados com níquel. A RRO procede segundo o mecanismo peróxido, seguida pela reação de desproporcionação do íon HO2- formado. A extensão da reação de desproporcionação do íon HO2- aumenta com o aumento da quantidade de Ni. Sobre eletrocatalisadores suportados em carbonos MM225 e E350 a reação de desproporcionação é mais rápida e envolve um total de 4 e- por oxigênio molecular. As curvas de polarização para RRO obtidas na presença do íon BH4- mostraram que todos os materiais são tolerantes à presença do borohidreto. Resultados de DEMS on line e FTIR in situ mostraram que óxidos de manganês dopados com Ni além de serem ativos para RRO são também ativos para a ROB, porém há uma grande influência da composição e da morfologia dos materiais uma vez que, quando fases segregadas de Ni estão presentes nas amostras, a reação compete com a hidrólise heterogênea do BH4- levando a uma diminuição da eficiência faradaica. / The oxygen reduction reaction (ORR) and the borohydride oxidation reaction (BOR) were studied in alkaline medium on Ni (II) doped MnOx catalysts supported on different carbon powder substrates. Characterizations of physico chemical properties were made by X ray diffraction (XRD), high resolution transmition electronic microscopy (HR-TEM) equipped with X ray dispersive energy spectroscopy (EDS). Electrochemical studies involved cyclic voltammetry and oxygen reduction voltammograms. Also it was used Differential Electrochemical Mass Spectrometry on line (DEMS) and Fourier Transform Infra Red Spectrometry (FTIR) in situ. A small insertion of Ni atoms in the MnOx lattice was observed, this consisting of a true doping of the manganese oxide phase. The corresponding NiMnOx phase is present in the form of needles or agglomerates, with crystallite sizes in the order of 1.5-6.7 nm. Layered manganite (MnOOH) phase has been detected for the Monarch1000 supported NiMnOx material, while different species of MnOx phases are present at the E350G and MM225 carbons. Electrochemical studies in thin porous coating active layers in the rotating ring-disk electrode setup revealed that the MnOx catalysts present better ORR kinetics and electrochemical stability upon Ni doping. The ORR follows the so-called peroxide mechanism on MnOx/C catalysts, with the occurrence of minority HO2- disproportionation reaction. The HO2- disproportionation reaction progressively increases with the Ni content in NiMnOx materials. The catalysts supported on the MM225 and E350G carbons promote faster disproportionation reaction, thus leading to an overall four-electron ORR pathway. The results towards ORR in presence of sodium borohydride showed that all materials are tolerant to the presence of BH4- ion into some extent. DEMS on line and FTIR in situ showed that NiMnOx/C are also active toward the BOR, but there is a strong influence of the nature of the electrocatalysts with respect to the morphology, composition, the nature of the carbon substrate and the Ni load. Results indicate that the electrocatalysts containing segregate Ni phases, the bohohydride oxidation occurs together with the heterogeneous hydrolysis of the BH4- ion resulting in a decrease of the faradaic efficiency.

Este trabalho teve por objetivo a pesquisa e o desenvolvimento de catalisadores formados por ligas formadoras de hidreto metálico (LaNi4,7Sn0,2Cu0,1, LaNi4,78Al0,22, LaNi4,78Mn0,22 e LaNi4,7Sn0,2Co0,1), com e sem depósitos de metais nobres (Pt, Au, Pd), para uso no ânodo de células a combustível alcalinas. Buscou-se o entendimento do mecanismo da reação de oxidação de boroidreto de sódio em meio alcalino nestes diferentes materiais em diferentes configurações de eletrodos e em diversas concentrações de boroidreto. As ligas foram preparadas pelo método de fusão em forno a arco e os metais nobres foram depositados por troca galvânica. A caracterização físico-composicional foi feita por energia dispersiva de raios X (EDS), análise elementar, difração de raios X (DRX) e microscopia eletrônica de transmissão (MET). Já a caracterização eletroquímica foi feita por voltametria cíclica, curvas de capacidade de descarga, curvas de polarização e espectrometria de massas eletroquímica online (OLEMS). Os resultados mostraram que a quantidade de material nobre presente nas ligas foi da ordem de 0,1 a 0,3 % e suas composições e organizações estruturais foram investigadas por EDS, DRX e MET. Os resultados eletroquímicos mostraram que o estado de hidretação da liga base é um fator importante na definição da cinética da reação de oxidação de boroidreto e que a extensão da formação de hidreto é dependente da velocidade do processo de hidrólise dos íons boroidreto, da capacidade da liga em estocar hidrogênio e da cinética de difusão dos átomos de hidrogênio atômico no interior das ligas. A atividade para a reação direta de oxidação de boroidreto foi baixa para as ligas que não continham em suas composições os metais nobres, entretanto a taxa de hidrólise para estes foi menor e a capacidade de armazenamento de hidrogênio foi maior. Os resultados também mostraram, para todas as diferentes composições, que as ligas podem ser carregadas com hidrogênio atômico tanto eletroquimicamente quanto pela exposição à solução de boroidreto. Foi observado que houve contínua hidretação de todas as ligas durante ensaios cronoamperométricos na presença de boroidreto. Esses fenômenos indicaram que a liga LaNi4.7Sn0.2Cu0.1 sem a deposição de metais nobres é a mais indicada para uma futura aplicação em um sistema DBFC/MHB (célula a combustível de boroidreto direto/bateria de hidreto metálico). / This work provides insights into the processes involved in the borohydride oxidation reaction (BOR) and hydrogen oxidation reaction in alkaline media on metal hydride alloys formed by LaNi4.7Sn0.2Cu0.1 and LaNi4.78Al0.22 with and without deposited Pt, Pd and Au, LaNi4,78Mn0,22 and LaNi4,7Sn0,2Co0,1. Measurements of BET, TEM, in situ XRD, DRIFTS, RRDE (gold ring) and OLEMS were made with the aim to characterize all the materials by different methodologies. The present investigation showed that the state of hydriding of LaNi4.7Sn0.2Cu0.1 and LaNi4.78Al0.22 is important in defining the kinetics and outcome of the BOR at these materials. The extent of hydride formation has been shown to depend on the rate of the BH4- hydrolysis, the hydrogen storage capacity of the alloy, and kinetics of H atom diffusion inside the alloy. The activity for the direct BOR is low in both bare metal hydride alloys, but the rate of the BH4- hydrolysis and the hydrogen storage capacity are higher, while the rate of H diffusion is slower in the bare LaNi4.78Al0.22. Results have shown that both the bare and all the noble metal modified alloys can be hydrided either electrochemically or by exposure to BH4- solutions, although for the LaNi4.78Al0.22-based materials the extent of this phenomenon is smaller. A continuous hydriding of all LaNi4.7Sn0.2Cu0.1-based and the bare LaNi4.78Al0.22 alloys are observed in the chronoamperometric BH4- oxidation measurements at low current densities. Addition of Pt on both alloys resulted in an increase of the BH4- hydrolysis, but the H2 formed is rapidly oxidized, confirming the initial predictions for this noble metal. In addition, the rates of the alloy hydriding/de-hydriding were not significantly affected by the presence of Pt, but this was not the case for Pd and Au, for which there was a drastic reduction of the rate of these processes. In the case of gold some increase of the BH4- hydrolysis is observed, although its presence does not change significantly the performances of the bare alloys. It was possible to confirm the formation of BH3OH- for all the samples by RRDE measurements, however for LaNi4.7Sn0.2Cu0.1 the formation of this product was lower compared to the other samples. Results showed that the first step of the electrode process is the hydriding of alloy by the hydrogen formed in the BH4- hydrolysis. The in situ XRD results have shown that the method of charging of the alloy, electrochemical or chemical, leads to different phase predominance and different site occupancies, but both methods lead to almost the same discharge capacity. In the electrolysis process, the α-β phase transition is predominant while in the chemical charging by the exposure to borohydride solution the α-β phase transition is more important. The results have also shown that the alloys with and without Pt lead essentially to the same phenomena, either with respect to the alloy structure and the electrochemical characteristics. All these phenomena point to the bare LaNi4.7Sn0.2Cu0.1 as a more adequate alloy system for applications in DBFC/MHB batteries.

Este trabalho teve por objetivo a pesquisa e o desenvolvimento de catalisadores formados por ligas formadoras de hidreto metálico (LaNi4,7Sn0,2Cu0,1, LaNi4,78Al0,22, LaNi4,78Mn0,22 e LaNi4,7Sn0,2Co0,1), com e sem depósitos de metais nobres (Pt, Au, Pd), para uso no ânodo de células a combustível alcalinas. Buscou-se o entendimento do mecanismo da reação de oxidação de boroidreto de sódio em meio alcalino nestes diferentes materiais em diferentes configurações de eletrodos e em diversas concentrações de boroidreto. As ligas foram preparadas pelo método de fusão em forno a arco e os metais nobres foram depositados por troca galvânica. A caracterização físico-composicional foi feita por energia dispersiva de raios X (EDS), análise elementar, difração de raios X (DRX) e microscopia eletrônica de transmissão (MET). Já a caracterização eletroquímica foi feita por voltametria cíclica, curvas de capacidade de descarga, curvas de polarização e espectrometria de massas eletroquímica online (OLEMS). Os resultados mostraram que a quantidade de material nobre presente nas ligas foi da ordem de 0,1 a 0,3 % e suas composições e organizações estruturais foram investigadas por EDS, DRX e MET. Os resultados eletroquímicos mostraram que o estado de hidretação da liga base é um fator importante na definição da cinética da reação de oxidação de boroidreto e que a extensão da formação de hidreto é dependente da velocidade do processo de hidrólise dos íons boroidreto, da capacidade da liga em estocar hidrogênio e da cinética de difusão dos átomos de hidrogênio atômico no interior das ligas. A atividade para a reação direta de oxidação de boroidreto foi baixa para as ligas que não continham em suas composições os metais nobres, entretanto a taxa de hidrólise para estes foi menor e a capacidade de armazenamento de hidrogênio foi maior. Os resultados também mostraram, para todas as diferentes composições, que as ligas podem ser carregadas com hidrogênio atômico tanto eletroquimicamente quanto pela exposição à solução de boroidreto. Foi observado que houve contínua hidretação de todas as ligas durante ensaios cronoamperométricos na presença de boroidreto. Esses fenômenos indicaram que a liga LaNi4.7Sn0.2Cu0.1 sem a deposição de metais nobres é a mais indicada para uma futura aplicação em um sistema DBFC/MHB (célula a combustível de boroidreto direto/bateria de hidreto metálico). / This work provides insights into the processes involved in the borohydride oxidation reaction (BOR) and hydrogen oxidation reaction in alkaline media on metal hydride alloys formed by LaNi4.7Sn0.2Cu0.1 and LaNi4.78Al0.22 with and without deposited Pt, Pd and Au, LaNi4,78Mn0,22 and LaNi4,7Sn0,2Co0,1. Measurements of BET, TEM, in situ XRD, DRIFTS, RRDE (gold ring) and OLEMS were made with the aim to characterize all the materials by different methodologies. The present investigation showed that the state of hydriding of LaNi4.7Sn0.2Cu0.1 and LaNi4.78Al0.22 is important in defining the kinetics and outcome of the BOR at these materials. The extent of hydride formation has been shown to depend on the rate of the BH4- hydrolysis, the hydrogen storage capacity of the alloy, and kinetics of H atom diffusion inside the alloy. The activity for the direct BOR is low in both bare metal hydride alloys, but the rate of the BH4- hydrolysis and the hydrogen storage capacity are higher, while the rate of H diffusion is slower in the bare LaNi4.78Al0.22. Results have shown that both the bare and all the noble metal modified alloys can be hydrided either electrochemically or by exposure to BH4- solutions, although for the LaNi4.78Al0.22-based materials the extent of this phenomenon is smaller. A continuous hydriding of all LaNi4.7Sn0.2Cu0.1-based and the bare LaNi4.78Al0.22 alloys are observed in the chronoamperometric BH4- oxidation measurements at low current densities. Addition of Pt on both alloys resulted in an increase of the BH4- hydrolysis, but the H2 formed is rapidly oxidized, confirming the initial predictions for this noble metal. In addition, the rates of the alloy hydriding/de-hydriding were not significantly affected by the presence of Pt, but this was not the case for Pd and Au, for which there was a drastic reduction of the rate of these processes. In the case of gold some increase of the BH4- hydrolysis is observed, although its presence does not change significantly the performances of the bare alloys. It was possible to confirm the formation of BH3OH- for all the samples by RRDE measurements, however for LaNi4.7Sn0.2Cu0.1 the formation of this product was lower compared to the other samples. Results showed that the first step of the electrode process is the hydriding of alloy by the hydrogen formed in the BH4- hydrolysis. The in situ XRD results have shown that the method of charging of the alloy, electrochemical or chemical, leads to different phase predominance and different site occupancies, but both methods lead to almost the same discharge capacity. In the electrolysis process, the α-β phase transition is predominant while in the chemical charging by the exposure to borohydride solution the α-β phase transition is more important. The results have also shown that the alloys with and without Pt lead essentially to the same phenomena, either with respect to the alloy structure and the electrochemical characteristics. All these phenomena point to the bare LaNi4.7Sn0.2Cu0.1 as a more adequate alloy system for applications in DBFC/MHB batteries.

Le travail présenté dans cette thèse porte sur l'anode de la pile à combustible directe aux borohydrures (DBFC, selon l'acronyme anglais). Une première approche pour développer l'anode de la DBFC est d'étudier cette anode à l'intérieur du système global de la DBFC. Dans cette optique, des anodes composées des catalyseurs Pt/C et Pd/C ont été caractérisée en banc de test DBFC. D'autres facteurs ont aussi été étudiés, tels que la morphologie de l'anode et la stabilité des nanoparticules des catalyseurs.Le catalyseur d'anode de la DBFC doit idéalement exhiber une activité catalytique suffisante pour la réaction d'oxydation des borohydrures (BOR), tout en minimisant la production et l'échappement d'hydrogène gazeux durant la BOR. Ces aspects sont relativement difficiles à étudier en raison des nombreuses variables ne dépendant pas de l'anode dans un système DBFC réel. Une solution à ce problème consiste à isoler l'anode de la DBFC et de l'étudier en configuration demi-pile, avec un environnement d'étude mieux contrôlé. Les différentes méthodes pour évaluer un catalyseur d'anode de DBFC en demi-pile sont discutées, et des marqueurs sont proposés pour l'évaluation pertinente d'un catalyseur d'anode de DBFC par rapport aux résultats de la littérature.Une autre stratégie possible pour développer des catalyseurs adéquats d'anode de DBFC est de mieux comprendre le mécanisme de la BOR. Dans cette optique, la BOR est étudiée sur des électrodes modèles à base de platine. Chaque type d'électrode modèle permet de contrôler un paramètre précis de la surface catalytique, menant ainsi à différentes études de la BOR. La sensibilité de la BOR à la structure de surface catalytique est étudiée sur des électrodes massives de platine (polycristallin et monocristallin). L'empoisonnement de la surface active de Pt durant la BOR est étudié sur nanoparticules de Pt déposées sur substrat carbone vitreux plan. Des électrodes à trois dimensions ont également été réalisées : nanoparticules de Pt déposées sur nanofibres de carbone verticalement alignées. Le dépôt de différentes quantité de Pt a permis d'étudier l'influence de la densité en sites actifs de Pt sur la BOR. Les résultats obtenus sur ces électrodes modèles sont discutés avec ceux de la littérature, et un mécanisme pour la BOR sur Pt est proposé. Ce mécanisme est simulé en utilisant une modélisation de micro-cinétiques de type champs moyens. Les courbes simulées reproduisent les caractéristiques majeures des résultats expérimentaux. / The present work focuses on direct borohydride fuel cell (DBFC) anodes. A first approach to develop a suitable anode design for the DBFC consists in the study of the anode within the real DBFC system. In that frame, carbon-supported platinum and palladium nanoparticles are characterized and compared as anode electrocatalyst in DBFC configuration. Other variables such as the morphology of the anode and the stability of the catalyst nanoparticles are considered.The ideal DBFC anode catalyst should show a suitable electrocatalytic activity towards the borohydride oxidation reaction (BOR), without quantitative production/escape of gaseous hydrogen during the reaction. Studying these aspects is not straightforward using a real DBFC system, as the global behavior of the DBFC depends on numerous experimental variables external to the anode. In order to overcome this issue, a prospective anode catalyst can be isolated and specifically studied in half-cell configuration in a more controlled environment. The different methods possible for the evaluation of an electrocatalyst for the anode of the DBFC are discussed in this work, and benchmarks are proposed to compare a given material with the DBFC literature.Another strategy to develop suitable DBFC anode catalysts is to further understand the BOR mechanism. In that frame, the BOR is studied on model platinum-based electrodes with different levels of complexity. Bulk polycrystalline and single-crystals Pt flat electrodes enable to study the structure sensitivity of the BOR. The poisoning of the Pt active surface is investigated using Pt nanoparticles supported on flat glassy carbon substrate. Three-dimensional electrodes are also surveyed: Pt nanoparticles supported on vertically-aligned carbon nanofiber electrodes. The deposition of various amounts of Pt nanoparticles on the VACNF substrate enables to study the influence of the density of Pt active sites towards the BOR. The findings obtained using these model electrodes are gathered with previous results from the literature in order to propose a BOR mechanism on Pt. This mechanism is used in a mean-field microkinetics model. The simulated curves of this mechanism reproduce the main experimental features.

Made available in DSpace on 2016-08-19T12:56:39Z (GMT). No. of bitstreams: 1 dissertacao Alielson.pdf: 883958 bytes, checksum: f6ca1ed8e1b56766025ce616aa4101fb (MD5) Previous issue date: 2012-06-10 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The oxygen reduction reaction (ORR) in 1.0 mol/L KOH solutions catalyzed by FeTMPP/C and CoTMPP/C was investigated in absence and presence of BH4- anions with concentrations varying from 10-7 to 1.0 mol/L. Similar cyclic voltammetry and rotating disk electrode experiments have been also performed the commercial E-TEK Pt/C electrocatalysts. In absence of BH4- anions, the ORR proceeds via a direct 4 electron pathway on FeTMPP/C and Pt/C and via a peroxide pathway involving (2-electron process) on CoTMPP/C and Au/C catalysts. In addition, Pt/C presented the most positive onset potential for the reaction. In presence of BH4- anions, a drastic loss in the catalytic activity of the Pt/C electrode was observed, even for concentrations as low as 10-7 mol/L. The effect of temperature on the ORR was studied and from 20ºC up to 60ºC the diffusion limited current densities remained nearly constants. At 80ºC, a decrease in the current of density was observed, probably due to a decrease in the oxygen concentration and viscosity of the solution. Arrhenius plots [ln (k) vs. T-1] were used to evaluate the activation s energy (Ea) of the ORR in absence of borohydride and resulted in 47.6 kJ/mol and 50.3 kJ/mol for Pt/C and FeTMPP/C, respectively. In presence of borohydride in solution, the Ea value for Pt/C showed a pronounced decrease to 38.7 kJ/mol, due to borohydride oxidation as a parallel reaction, while for FeTMPP/C the value calculate was 52.7 kJ/mol, i.e., remained practically constant. Such results indicates that FeTMPP is an efficient catalyst for ORR in alkaline solutions and tolerant to BH4 anions. / A reação de redução de oxigênio (RRO) em solução KOH 1,0 mol/L catalisada por FeTMPP/C e CoTMPP/C foi investigada na ausência e presença de anions BH4- com concentrações variando de 10-7 a 1 mol/L. Experimentos similares com voltametria cíclica e eletrodo disco-anel e disco rotatório foram feitos em eletrocatalisadores Pt/C comercial da E-TEK e Au/C. Na ausência de BH4-, a RRO se processa via mecanismo 4 elétrons para eletrodos FeTMPP/C e Pt/C e via peróxido, ou seja, 2 elétrons sobre eletrodos CoTMPP/C e Au/C. Diante disto, o eletrodo Pt/C apresenta melhor potencial para a reação. Na presença de anions BH4-, foi observada uma drástica perda na atividade catalítica do eletrodo Pt/C até mesmo em baixas concentrações (10-7 mol/L). O efeito da temperatura sobre a RRO foi estudado na faixa de 20ºC a 60ºC manteve-se constante em relação às densidades de corrente. A 80ºC foi observada uma diminuição na densidade de corrente, muito provavelmente, devido à diminuição na concentração de oxigênio e a viscosidade da solução. Foi usada a relação de Arrhenius (ln K vs. T-1) para o calculo da energia de ativação da RRO na ausência de boroidreto resultando em 47.6 kJ/mol e 50.3 kJ/mol para Pt/C e FeTMPP/C, respectivamente. Na presença de boroidreto em solução, a energia de ativação da Pt/C mostrou uma diminuição considerável, devido a reação de oxidação dos anions BH4- acorrer paralelamente, enquanto que para o eletrodo FeTMPP/C aumentou para 52.7 kJ/mol, mantendo-se praticamente constante. Tais resultados indicam que o FeTMPP/C é um eficiente catalisador para a RRO em meio alcalino e tolerante a anions BH4-.

Synopsis of the thesis entitled “Titanium Nitride-Based Electrode Materials for Oxidation of Small Molecules: Applications in Electrochemical Energy Systems” submitted by Muhammed Musthafa O. T under the supervision of Prof. S. Sampath at the Department of Inorganic and Physical Chemistry of the Indian Institute of Science for the Ph.D degree in the faculty of science. Fuel cells have been the focus of interest for many decades because of the ever increasing demands in energy. Towards this direction, there have been considerable efforts to find efficient electrocatalysts to oxidize small organic molecules (SOMs) such as methanol, ethanol, glycerol, hydrazine and borohydride that are of potential interest in direct fuel cells. Most studies revolve around platinum which is the best electrocatalyst known for the oxidation of many SOMs. However, platinum is extremely susceptible to carbon monoxide (CO) poisoning which is an intermediate in the electrooxidation of aliphatic alcohols. The best known catalyst, platinum-ruthenium alloy (PtRu), suffers from leaching of Ru during cycling resulting in decrease in efficiency in addition to loss of precious metal. Another important aspect of fuel cell catalyst degradation is corrosion of widely-used carbon support, under fuel cell conditions. Corrosion of carbon support weakens the adherence of catalyst particles on the support and in turn results in loss of catalyst and also in its easy oxidation. Carbon corrosion is also reported to decrease the electronic continuity of the catalyst layer. Hence, replacement of carbon support with durable material is required. The present research explores the use of non-carbonaceous, transition metal nitride for anchoring catalytic particles. The favorable physicochemical properties of titanium nitride (TiN) such as extreme hardness, excellent corrosion resistance in aggressive electrolytes, resistance to nearly all chemicals, salt and humidity, very good support for the adherence of fuel cell catalysts and excellent electronic conductivity motivated us to use this material for anchoring fuel cell catalysts such as Pt, PtRu and Pd. In the present studies, TiN coated on stainless steel (SS 304) surface is used as an electrode material. Catalysts such as Pt, Pd and PtRu are anchored on to TiN and used for the oxidation of methanol and ethanol in acidic as well as in alkaline media. Use of bare TiN is explored for the oxidation of sodium borohydride. The efficiency of TiN supported catalysts are compared with carbon supported ones. Preliminary studies on the use of TiN supported catalysts in fuel cells have been conducted as well. Figure 1 shows the topographic atomic force microscopic (AFM) image in combination with scanning Kelvin probe (SKP) image of platinized TiN (Pt-TiN) surface. Since Pt particles are metallic, they are expected to show lower work function values than that of TiN domains which is indeed observed in figure 1B where the location of Pt particles is shown as dip in the work function. Very interestingly, the interface of Pt-TiN possesses very different work function values confirming the existence of metal-support interaction and this is expected to have positive implications in fuel cell catalysis. Figure 1. Contact mode AFM (A) and the corresponding scanning Kelvin probe image (B) of Pt-TiN surface. Figure 2. Cyclic voltammograms of Pt-TiN and Pt-C electrodes in 0.5 M H2SO4 containing 0.5 M methanol at a scan rate of 10 mV/s. Loading of the catalyst used is 1 mg of Pt/cm2. The performance of Pt-TiN and PtRu-TiN are compared with the corresponding carbon supported catalysts (Pt-C, PtRu-C) for the electrooxidation of methanol. Figure 2 shows the voltammograms obtained on Pt-TiN and Pt-C in presence of acidified methanol. TiN supported catalyst performs better than carbon supported catalyst in terms of high currents at low over voltages (based on I-t measurements), long term stability and high exchange current densities (based on Tafel studies). The electrochemical characteristics of methanol oxidation on Pt-TiN and Pt-C catalysts are given in table 1. The current densities observed on TiN supported catalyst are almost three times higher than that of carbon supported catalyst confirming the promoting effect of TiN support towards methanol oxidation reaction. The performance of Pt-TiN electrocatalyst under fuel cell conditions reveals peak power densities close to 396 mW/cm2 at a current density of 375 mA/cm2, at 90C. Table 1. Characteristics of methanol oxidation on TiN and carbon supported catalysts in acidic medium. Material Onset Ep (mV) Ip EAA Ip Ip/Ib E=Ep-Eb potential (mA/mg (cm2/mg)b (mA/cm2 (mV) of Pt)a of Pt)c (mV) Pt-TiN 170 720 56 78.4 0.714 1.24 82 Pt-C 250 700 18 68.6 0.262 0.98 106 a Mass activity; Ip is the forward peak current and Ib is the reverse peak current; Ep and Eb are forward and reverse peak potentials. b Electrochemically active area (EAA) c Current density normalized for EAA Figure 3. In-situ FTIR spectra on bare TiN surface as a function of applied DC bias vs.SCE. The spectra are shown in regions of 1000 to 2000 cm-1 (A) and 2500 to 4000 cm-1 (B). Electrolyte used is 0.5 M methanol in 0.5 M H2SO4. Reference spectrum is obtained at 0 V. In-situ FTIR spectroelectrochemical measurements have been carried out to understand the intermediates and products formed during methanol oxidation. TiN surface is highly reflective and is quite amenable for reflectance IR studies. Figure 3 shows the potential dependant spectral characteristics of TiN in methanolic sulphuric acid. The bands observed at 1600 and 3600 cm-1 correspond to –OH bending and stretching vibrations of adsorbed water molecules. Interestingly, bands corresponding to adsorbed water are observed even at remarkably low over potentials of around 0.1 V vs. SCE where CO poisoning of Pt can be very severe. This experiment confirms the ability of inexpensive TiN to function like expensive Ru in fuel cell catalysis. Similar studies have been carried out for ethanol electrooxidation on TiN supported catalysts such as Pd, Pt and PtRu in acidic as well as alkaline conditions. Adherence of fuel cell catalyst on to TiN and carbon support is followed by cycling the electrode potential continuously as shown in figure 4. The adherence of Pd on TiN surface is very good and the stability tests reveal that Pd adheres and remains on TiN for a long time as compared to carbon support. Figure 4. Cyclic voltammograms of Pd-C (A) and Pd-TiN (B) in 1 M KOH at 100 mV/s. Pd loading used is 83 µg/cm2. In the chapter on borohydride oxidation, bare TiN electrode is used for the electrochemical oxidation of sodium borohydride. In direct borohydride fuel cells (DBFC), H2 evolution that occurs at low over voltages decreases the apparent number of electrons transferred and consequently the fuel cell efficiency. TiN has been shown to be a relatively H2 evolution-free electrocatalyst for borohydride oxidation (figure 5A). As shown in figure 5A, no H2 oxidation is observed (below -0.5 V) on TiN surface with increase in concentration of borohydride. This point to the fact that direct oxidation of borohydride is very favourable on TiN electrode and is confirmed by fuel cell measurements as shown in figure 5B. Non-platinum DBFCs using TiN as the anode (borohydride oxidation) and prussian blue supported carbon (PB-C) as the cathode (oxygen or hydrogen peroxide) electrocatalysts (figure 5B) reveal peak power density of 107 mW/cm2 for a current density 130 mA/cm2, at 80C. Figure 5. Cyclic voltammograms of TiN in 1 M NaOH containing varying concentrations of borohydride at a scan rate of 20 mV/s (A). Polarization studies of DBFC with TiN anode catalyst and PB-C (prussian blue supported on carbon) cathode catalyst (B). Anolyte is 0.79 M borohydride in 5 M NaOH and catholyte is 2.2 M acidified H2O2. The second aspect of the thesis is related to the use of TiN to prepare visible light active, nitrogen doped TiO2 (N-TiO2). This is carried out by electrochemical anodization of TiN in 0.5 M HNO3 at 1.4 V. The X-ray photoelectron spectroscopy (XPS) suggests the formation of oxide phase on anodized TiN surface (figure 6A) and is confirmed by reflectance UV-Visible spectroscopy. The visible light activity is used for the sunlight induced reduction of graphene oxide to reduced graphene oxide. As shown in the Raman spectra (figure 6B), a negative shift of the D and G band positions by about 20 cm-1 and the intensity ratio reversal after reduction confirms the formation of reduced graphene oxide on N-TiO2. Figure 6. (A) Ti (2p) region of XPS of fresh TiN and anodized TiN. Anodization has been carried out at 1.4 V vs. SCE in 0.5 M HNO3. (B) Raman spectra of exfoliated graphene oxide on anodized TiN before and after sunlight induced reduction. In summary, TiN has been shown to be an active support material for fuel cell catalysts in the present studies. The appendix details the basic electrochemical studies on TiN using various redox couples, electroploymerization of aniline and the formation of nanostructures on TiN surface. (For figures pl refer the abstract pdf file)

Materials for studies related to nanoscience and nanotechnology have gained tremendous attention owing to their unique physical, chemical and electronic properties. Among various anisotropic nanostructures, one dimensional (1D) materials have received immense interest in numerous fields ranging from catalysis to electronics. Imparting multi-functionality to nanostructures is one of the major areas of research in materials science. In this direction, use of nanosized materials in energy systems such as fuel cells has been the subject of focus to achieve improved performance. Tuning the morphology of nanostructures, alloying of catalysts, dispersing catalytic particles onto various supports (carbon nanotubes, carbon nanofibers, graphene, etc.) are some of the ways to address issues related to electrochemical energy systems. It is worth mentioning that highly stable and corrosion resistant electrodes are mandatory as electrochemical cells operate under aggressive environments. Additionally, carbon, which is often used as a support for catalysts, is prone to corrosion and is subsequently implicated in reduced performance due to poor adherence of catalyst particles and loss in electrochemically active area. Hence, there is a quest for the development of stable and durable electrocatalysts / supports for various studies including fuel cells. The present thesis is structured in exploring the multi-functional aspects of titanium carbide (TiC), an early transition metal carbide. TiC, a fascinating material, possesses many favorable properties such as extreme hardness, high melting point, good thermal and electrical conductivity. Its metal-like conductivity and extreme corrosion resistance prompted us to use this material for various electrical and electrochemical studies. The current study explores the versatility of TiC in bulk as well as nanostructured forms, in electrical and electrochemical studies towards sensing, electrocatalytic reactions and active supports. 1D TiC nanowires (TiC-NW) are prepared by simple solvothermal method without use of any template and are characterized using various physico-chemical techniques. The TiC-NW comprise of 1D nanostructures with several µm length and 40 ± 15 nm diameter (figure 1). Electrical properties of individual TiC-NW are probed by fabricating devices using focused ion beam deposition (FIB) technique. The results depict the metallic nature of TiC-NW (figure 2). Figure 1. (a) SEM, (b) TEM and (c) HRTEM images of TiC-NW prepared by solvothermal method. Figure 2. (a) SEM image and (b) I-V characteristics of TiC-NW - based device as a function of temperature. The contact pads are made of Pt. Subsequently, oxidized TiC nanowires are prepared by thermal annealing of TiC-NW, leading to carbon - doped TiO2 nanowires (C-TiO2-NW) (figure 3). Photodetectors are fabricated with isolated C-TiO2-NW and the device is found to respond to visible light (figure 3) radiation with very good responsivity (20.5 A/W) and external quantum efficiency (2.7 X 104). The characteristics are quite comparable with several reported visible light photodetectors based on chalcogenide semiconductors. Figure 3. (a) HRTEM, (b) EDAX, (c) Scanning TEM-DF images of C-TiO2-NW along with (d) Ti (e) O and (f) C mapping. (g) Current – voltage curves of single C-TiO2-NW recorded in dark (black) and in presence of visible light radiation (red) of intensity 57.7 mW/cm2 at 25oC. Inset of (g) shows the SEM image of the device (top) and schematic illustration of fabricated photodetector (bottom). The next chapter deals with the electrochemical performance of TiC demonstrated for studies involving oxygen reduction and borohydride oxidation reactions. Electrochemical oxygen reduction reaction (ORR) reveal that TiC-NW possess high activity for ORR and involves four electron process while it is a two electron reduction for bulk TiC particles (figure 4). The data has been substantiated by density functional theory (DFT) calculations that reveal different modes of adsorption of oxygen on bulk and nanowire morphologies. Stable performance is observed for several hundreds of cycles that confirm the robustness of TiC. The study also demonstrates excellent selectivity of TiC for ORR in presence of methanol and thus cross-over issue can be effectively addressed in direct methanol fuel cells. In the chapter on borohydride oxidation, bare TiC electrode is explored as a catalyst for the oxidation of borohydride. One of the major issues in direct borohydride fuel cells (DBFC) is the hydrolysis of borohydride that happens on almost all electrode materials leading to low efficiency. The present study reveals that TiC is a very good catalyst for borohydride oxidation with little or no hydrolysis of borohydride [figure 5 (a)] under the experimental conditions studied. Further, shape dependant activity of TiC has been studied and fuel cell performance is followed [figure 5 (b)]. Polarization data suggests that the performance of TiC is quite stable under fuel cell experimental conditions. Figure 4. (a) Linear sweep voltammograms for ORR recorded using (i) bulk TiC particles and (ii) TiC-NW in O2-saturated 0.5 M KOH at 1000 rpm. Scan rate used is 0.005 Vs-1. (b) Variation of number of electrons with DC bias. Black dots correspond to TiC bulk particles while red ones represent nanowires. Figure 5. (a) Cyclic voltammograms of borohydride oxidation on TiC coated GC electrode in 1 M NaOH containing 0.1 M NaBH4. Scan rate used is 0.05 Vs-1. (b) Fuel cell polarization data at 70oC for DBFC assembled with (i) bulk TiC particles and (ii) TiC-NW as anode catalysts and 40 wt% Pt/C as cathode. Anolyte is 2.1 M NaBH4 in 2.5 M NaOH, and catholyte is 2.2 M H2O2 in 1.5 M H2SO4. Anode loading is 1.5 mg cm-2 and cathode loading is 2 mg cm-2. The corrosion resistance nature of TiC lends itself amenable to be used as an active support for catalytic particles (Pt and Pd) for small molecules oxidation reactions. In the present study, electro-oxidation of methanol, ethanol and formic acid have been studied. As shown in figure 6 (a), the performance of Pd loaded TiC (Pd-TiC) is found to be higher than that of Pd loaded carbon (Pd-C) suggesting the active role of TiC. The catalytic activities of TiC-based supports are further improved by tuning their morphologies. Figure 6 (c) reveals that the activities are higher in case of Pd-TiC-NW than that of Pd-TiC. Figure 6. (a) Cyclic voltammograms of Pd-TiC and Pd-C for ethanol oxidation, (b) T EM image of Pd-TiC-NW and (c) voltammograms of Pd-TiC-NW in N2-saturated 1 M ethanol in 1 M KOH medium, scan rate used is 0.05 Vs-1. The next aspect explored, is based on the preparation of C-TiO2 and its use as a substrate for surface enhanced Raman spectroscopy (SERS). Carbon doped titanium dioxide is prepared by thermal annealing of TiC. It is observed that the amount of dopant (carbon content) is dependent on the experimental conditions used. SERS studies using 4¬mercaptobenzoic acid (4-MBA) as the analyte, indicates that C-TiO2 [figure 7 (a)] enhances Raman signals based on chemical interactions between the analyte and the substrate. Raman signal intensities can be tuned with the amount of carbon content in C¬TiO2. Enhancement factors are calculated to be (7.7 ± 1.2) x 103 (for 4-MBA) and (1.7 ± 1.2) x 103 (for 4-nitrothiophenol). The SERS substrates are found to be surface renewable using visible light, a simple strategy to re-use the substrate [figure 7 (b)]. The regeneration of SERS substrates is based on self cleaning action of TiO2 that produces highly reactive oxygen containing radicals known to degrade the molecules adsorbed on TiO2. Thus, the versatility of TiC has been demonstrated with various studies. In addition to using TiC-based materials, nanoparticles of Rh, Ir and Rh-Ir alloy structures have also been used for borohydride oxidation reaction. This is explained in the last section. In Appendix-I, preliminary studies on the preparation of TiC-polyaniline (PANI) composites using liquid-liquid interfacial polymerization is explained. Raman spectroscopy results suggest that the presence of TiC-NW makes PANI to assume preferential orientation in the polaronic (conducting) form. Appendix-II discusses the role of TiC-NW as a fluorescence quencher for CdS semiconductor nanoparticles.

La pile à combustible directe à borohydrures (DBFC en anglais), qui est une sous-catégorie des piles à combustible alcalines, bénéficie des avantages de son combustible, le borohydrure de sodium (NaBH4), qui confère à ce système des caractéristiques thermodynamiques et énergétiques très intéressantes. Cependant, la réaction d’électrooxydation de NaBH4 (BOR en anglais) est très complexe et reste à ce jour encore peu étudiée et mal comprise sur la majorité des électrocatalyseurs (la plupart étant sous forme de nanoparticules métalliques supportées sur des noirs de carbone). De plus, de récentes études ont montré l’agressivité du milieu alcalin sur la durabilité des électrocatalyseurs conventionnels, révélant une grande perte de surface catalytique active, due principalement à un détachement des nanoparticules du support carboné. Dans ce contexte, ces travaux de thèse se sont orientés vers trois axes d’étude : (i) l’étude de la BOR sur des électrocatalyseurs à base de palladium dans des conditions proches des conditions réelles de fonctionnement de la DBFC ; (ii) l’étude de l’impact de la structure de l’anode sur les performances globales de la DBFC, et (iii) l’étude du mécanisme de dégradation d’électrocatalyseurs à base de métaux nobles dans un environnement alcalin. Les expérimentations ont été réalisées en étroite collaboration avec le U.S. Naval Research Laboratory (Washington, USA).Les résultats obtenus ont montré qu’une grande concentration en NaBH4 entraine un ralentissement de la cinétique de la réaction, due en partie à un fort empoisonnement de la surface catalytique. Par ailleurs, des marqueurs d’activité pour la BOR ont été proposés. Ensuite, l’utilisation d’électrodes à gradient de catalyseurs s’est avérée être une solution prometteuse pour mieux valoriser l’hydrogène produit via des réactions secondaires à la BOR. Enfin, l’utilisation de la spectroscopie infrarouge à transformée de Fourier couplée à de la microscopie électronique en transmission à localisation identique a permis de détecter la formation de carbonates au cours d’un test de vieillissement accéléré d’électrocatalyseurs à base de métaux nobles en milieu alcalin. Ce mécanisme explique, en partie, le détachement des nanoparticules observé au cours du test. / The direct borohydride fuel cell (DBFC), a subclass of alkaline fuel cells, benefits from the advantages of its fuel, sodium borohydride (NaBH4), which exhibits very interesting thermodynamic and energetic characteristics. However, the NaBH4 electrooxidation reaction (BOR) is very complex; to date it remains poorly studied and understood on many electrocatalysts (most of them are in the form of metal nanoparticles supported on carbon black). In addition, recent studies reported the aggressiveness of the alkaline medium on the durability of conventional carbon-supported electrocatalysts, revealing a large loss of the active catalytic surface, mainly due to the detachment of nanoparticles from the carbon support. In this context, this thesis focused on three main areas of study: (i) the study of the BOR on palladium-based electrocatalysts in conditions close to the real operating conditions of the DBFC; (ii) the study of the impact of the anode structure on the overall performance of the DBFC, and (iii) the study of the degradation mechanism of noble metal electrocatalysts in alkaline environment. The experiments were carried out in close collaboration with the U.S. Naval Research Laboratory (Washington, USA).The results obtained showed that a high concentration of NaBH4 leads to a decrease of the reaction kinetics, due in part to poisoning of the catalytic surface. In addition, activity markers for the BOR have been proposed. Then, the use of catalysts-gradient electrodes proved to be a promising solution to better valorize the hydrogen produced via side reactions of the BOR. Finally, the use of Fourier transform infrared spectroscopy coupled with identical-location transmission electron microscopy enabled to detect the formation of carbonates during the accelerated stress test of carbon-supported noble metal electrocatalysts in alkaline medium, explaining, in part, the detachment of nanoparticles observed during the test.