Auswahl der wissenschaftlichen Literatur zum Thema „Borohydride oxidation reaction (BOR)“

Researchers are exploring direct liquid fuel cells (DLFCs) as alternatives to proton-exchange membrane fuel cells because of their higher energy density and ease of storing and transporting the fuel. Direct borohydride fuel cells (DBFCs) are of particular interest as they offer a sustainable energy source with their high-power density output and the use of a highly alkaline NaBH4 medium [1]. Ensuring efficient and cost-effective catalysts for DBFCs is crucial for their commercial viability. Metal-organic frameworks (MOFs) have demonstrated significant potential as anodic electrocatalysts for BOR in DBFCs [2]. However, research should explore various modifications to MOFs, such as the incorporation of alternative metal ions or functional groups, to improve their catalytic efficiency and reduce cost. This study evaluated the performance of newly developed MOF-based electrocatalysts for DBFCs. Specifically, six MOF-based materials were synthesized and analyzed for their ability to facilitate borohydride oxidation (BOR) using cyclic voltammetry and chronoamperometry in alkaline media. MIL-101_Au@NH2 and MOF-808_Au@NH2 were found to be highly effective for BOR. The kinetic parameters for BOR with MOF-based electrocatalysts, including activation energy, reaction order, exchanged electrons, and anodic charge transfer coefficient, were determined. The activation energy for BOR was found to be 13.6 kJ mol−1 and 15.3 kJ mol−1 for MIL-101_Au@NH2 and MOF-808_Au@NH2, respectively. The number of transferred electrons, n, was found to be 7.0 and 3.1 for MIL-101_Au@NH2 and MOF-808_Au@NH2, respectively. This study demonstrates that MOF-based electrocatalysts can enhance DBFCs' performance, while offering insight into the potential usage of MOFs in other fuel cell technologies. [1] B. Šljukić, D.M.F. Santos, "Direct borohydride fuel cells", in: "Direct Liquid Fuel Cells: Fundamentals, Advances, and Future", 1st ed., R.G. Akay, A.B. Yurtcan (eds.), Academic Press, USA, 203-232 (2021) [2] G. Backovic, B. Šljukić, G.S. Kanberoglu, M. Yurderi, A. Bulut, M. Zahmakiran, D.M.F. Santos, Ruthenium (0) nanoparticles stabilized by the metal-organic framework as an efficient electrocatalyst for borohydride oxidation reaction, International Journal of Hydrogen Energy, 45, 27056-27066 (2020).

Three different carbon-supported metal (gold, platinum, nickel) nanoparticle (M/c-IL) electrocatalysts are prepared by template-free carbonization of the corresponding ionic liquids, namely [Hmim][AuCl4], [Hmim]2[PtCl4], and [C16mim]2[NiCl4], as confirmed by X-ray diffraction analysis, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and Raman spectroscopy. The electrochemical investigation of borohydride oxidation reaction (BOR) at the three electrocatalysts by cyclic voltammetry reveals different behavior for each material. BOR is found to be a first-order reaction at the three electrocatalysts, with an apparent activation energy of 10.6 and 13.8 kJ mol−1 for Pt/c-IL and Au/c-IL electrocatalysts, respectively. A number of exchanged electrons of 5.0, 2.4, and 2.0 is obtained for BOR at Pt/c-IL, Au/c-IL, and Ni/c-IL electrodes, respectively. Direct borohydride-peroxide fuel cell (DBPFC) tests done at temperatures in the 25–65 °C range show ca. four times higher power density when using a Pt/c-IL anode than with an Au/c-IL anode. Peak power densities of 40.6 and 120.5 mW cm−2 are achieved at 25 and 65 °C, respectively, for DBPFC with a Pt/c-IL anode electrocatalyst.

Gold and silver nanoparticles in different atomic ratios deposited on reduced graphene oxide (AuAg/rGO-1, AuAg/rGO-2, and AuAg/rGO-3) were tested for the borohydride oxidation reaction in an alkaline medium. The morphology, structure, and composition of AuAg/rGO electrodes were investigated by transmission electron microscopy (TEM) and scanning electron microscopy with integrated energy dispersive spectroscopy (SEM-EDS). The TEM analysis showed that the most of gold and silver particles (more than 80%) are up to 9 nm in size, while slightly larger particles represent less than 5%. SEM-EDS showed a similar morphology, and the composition of three AuAg/rGO electrodes was determined. AuAg/rGO-1 and AuAg/rGO-2 electrodes showed similar, while AuAg/rGO-3 electrodes demonstrated slightly lower electrocatalytic activity for borohydride oxidation. The studied electrodes were observed to be active for the borohydride hydrolysis as well, which is expected based on literature data. AuAg/rGO certainly represent potential anode materials for application in direct borohydride fuel cells.

The synthesis of palladium-based trimetallic catalysts via a facile and scalable synthesis procedure was shown to yield highly promising materials for borohydride-based fuel cells, which are attractive for use in compact environments. This, thereby, provides a route to more environmentally friendly energy storage and generation systems. Carbon-supported trimetallic catalysts were herein prepared by three different routes: using a NaBH4-ethylene glycol complex (PdAuNi/CSBEG), a NaBH4-2-propanol complex (PdAuNi/CSBIPA), and a three-step route (PdAuNi/C3-step). Notably, PdAuNi/CSBIPA yielded highly dispersed trimetallic alloy particles, as determined by XRD, EDX, ICP-OES, XPS, and TEM. The activity of the catalysts for borohydride oxidation reaction was assessed by cyclic voltammetry and RDE-based procedures, with results referenced to a Pd/C catalyst. A number of exchanged electrons close to eight was obtained for PdAuNi/C3-step and PdAuNi/CSBIPA (7.4 and 7.1, respectively), while the others, PdAuNi/CSBEG and Pd/CSBIPA, presented lower values, 2.8 and 1.2, respectively. A direct borohydride-peroxide fuel cell employing PdAuNi/CSBIPA catalyst in the anode attained a power density of 47.5 mW cm−2 at room temperature, while the elevation of temperature to 75 °C led to an approximately four-fold increase in power density to 175 mW cm−2. Trimetallic catalysts prepared via this synthesis route have significant potential for future development.

The electrochemical behavior of BH4- on Cu electrode in 1M NaOH was investigated by cyclic voltammetry(CV) in the potential range of -1.2V to 0.4V versus Hg/HgO. The CV results show that Cu electrode has obvious catalytic activities to the BH4- hydrolysis which belongs to ‘catalytic’ electrode materials. The BH4- electro-oxidation process on Cu is complex and it could associate with the BH4- hydrolysis reaction, followed by oxidation of the intermediate H, then, the intermediate product (e.g. BH3OH−) oxidized, and direct oxidation of BH4- at more positive potentials.

Three-dimensional Pd nanochain networks (Pd-NCNs) were prepared by an arginine-assisted self-assembly process, exhibiting excellent electrocatalytic performance towards the borohydride oxidation reaction.

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

La production d'énergie électrochimique à partir de combustibles sans carbone a récemment suscité un grand intérêt. Ce manuscrit se concentre sur l'étude du mécanisme de la réaction d'oxydation de l'hydrazine (HHOR) sur des électrodes de Ni et le compare avec ceux des réactions d'oxydation du borohydrure et de l’ammoniac-borane (BOR, ABOR). Les sites métalliques de Ni ont été identifiés comme les sites catalytiques pour la HHOR, la BOR et l'ABOR, tandis que la présence de sites de Ni (hydr)oxydés a un effet négatif sur l'activité sans influencer clairement le mécanisme réactionnel. Sur la base des résultats de calculs DFT, de la modélisation microcinétique et de mesures DEMS en ligne, un mécanisme de la HHOR sur Ni a été proposé. Celui-ci implique la réaction directe de l'hydrazine dissoute avec des espèces Ni-OH adsorbées, formant un intermédiaire N2Hx,ad (x<4), qui est ensuite oxydé électrochimiquement, conduisant à la formation de N2 et d’eau
Electrochemical energy production from carbon-free fuels has recently attracted much attention. This manuscript focuses on studying the mechanism of the hydrazine oxidation reaction (HHOR) on Ni electrodes and comparing it with the ones of the borohydride and ammonia-borane oxidation reactions (BOR, ABOR). Metallic Ni sites were identified as the catalytic sites for the HHOR, BOR, and ABOR, while the presence of Ni (hydr)oxide sites was found to negatively affect activity without a clear influence on the reaction mechanism. Based on the results of DFT calculations, microkinetic modelling, and online DEMS measurements, a mechanism for HHOR on Ni was proposed. It involves the direct reaction of dissolved hydrazine with adsorbed Ni-OH species forming N2Hx,ad (x<4) intermediate, which is subsequently electrochemically oxidized, leading to the formation of N2 and water

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.

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

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.

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.