ICP-MS coupled to electrochemistry: correlation of corrosion with catalysis at atomic level

O presente trabalho fundamenta-se na aplicação do método da sonda-estacionária acoplada a um eletrodo de disco rotatório (SPRDE) conectado a um inductively coupled plasma mass spectrometry (ICP-MS) para examinar: a dinâmica da dissolução de átomos na superfície de Pt(111) em eletrólito de HClO4 sob condições experimentais relevantes para a operação de células a combustível; e finalmente a dinâmica da dissolução eletroquímica de átomos na superfície de calcogéis amorfos e cristalinos em eletrólito de HClO4 sob condições experimentais relevantes para a operação de eletrolisadores (particularmente a reação de evolução de hidrogênio (HER)). Além disso, os detalhes experimentais do método SPRDE-ICPMS e sua validação são demonstrados. No geral, no Capítulo 2 examinamos a dinâmica da dissolução eletroquímica de átomos de superfície de Pt(111) em eletrólito HClO4 limpo e não adsorvente sob condições experimentais que são relevantes para a operação da célula de combustível. Nós nos concentramos nas superfícies de Pt(111) por duas razões: (I) a adsorção dependente potencial de espécies oxigenadas está bem estabelecida e razoavelmente bem compreendida; e (II) esta superfície contém a menor quantidade de defeitos superficiais - uma necessidade estrutural para explorar mudanças morfológicas induzidas por óxidos/dissolução em escala atômica. Usando estas superfícies bem definidas, nós sondamos a dinâmica da dissolução de Pt através do SPRDE-ICPMS, e rastreamos as mudanças morfológicas concomitantes usando microscopia de tunelamento (STM). Descobrimos que dois processos distintos de dissolução de Pt podem ocorrer nas regiões de varrimento positivo e negativo, respectivamente, durante uma voltametria cíclica (CV) com taxas de dissolução e alterações morfológicas fortemente dependentes das condições experimentais. Descobrimos também que a taxa de dissolução durante a formação de óxido (varrimento positivo) é pequena e pode ser considerada um processo faradáico, uma vez que não está intimamente relacionada com a cinética de formação de óxido. Além disso, a taxa de dissolução e seu depósito de Pt associado é um processo rápido que é controlado pelo limite de potencial positivo e também pela taxa de varredura usada para redução de óxido (varredura negativa). No geral, os resultados fornecem uma base sólida para entender como diferentes perfis de tempo e potencial têm impacto sobre a estabilidade das superfícies de Pt e sua correspondente transição de uma morfologia bem definida para \"rugosa\" que determina a durabilidade do eletrodo a longo prazo. No Capítulo 3, apresentamos evidências de processos de desativação e regeneração de sítios ativos que estão operacionais em calcogenetos baseados em Mo (por exemplo, MoS2) e calcogéis (por exemplo, MoSx). Primeiro, monitorando as taxas de dissolução in situ de ambos os átomos, Mo e S, juntamente com a atividade de HER, fomos capazes de estabelecer sua dinâmica única de dissolução em meio ácido em função do potencial do eletrodo. Surpreendentemente, encontramos uma perda seletiva de átomos de S com mínima dissolução de Mo durante o HER, enquanto nos potenciais positivos moderados a dissolução de Mo ocorre com perda mínima de S. Propomos que a liberação seletiva de enxofre concomitante à produção de H2 provenha de sua interação com íons hidrônio (H+) e sua força motriz termodinâmica para formar H2S, enquanto a interação Mo-S é enfraquecida como parte do ciclo catalítico HER a mais negativo potenciais do eletrodo. Como conseqüência, descobrimos que a remoção seletiva de S leva à conversão do sítio ativo de Mo-S em espécies Mo-Ox formadas por Mo \"livre\" (Mon+) que interage com H2O e H+ circunvizinhos, criando novos sítios de superfície que são menos ativo para a produção de hidrogênio (por exemplo, desativação). Por sua vez, também descobrimos que essa nova espécie de Mo-Ox pode ser removida seletivamente da superfície do catalisador, dando lugar à dissolução seletiva de Mo observada em maiores potenciais de eletrodos, limpando efetivamente a superfície e expondo locais virgens de Mo-S para a produção eficiente de hidrogênio (por exemplo, regeneração). Finalmente, nossos resultados demonstram que as interfaces eletroquímicas estão em constante evolução, e que a compreensão dos processos cinéticos subjacentes é necessária para projetar com sucesso interfaces dinâmicas que podem permanecer ativas após extensas excursões de eletrocatálise.The present work relies on the application of the stationary probe rotating disk electrode inductively coupled plasma mass spectrometry (SPRDEICPMS) method approach to examine: the dynamics of the electrochemical dissolution of Pt(111) surface atoms in clean, non-adsorbing HClO4 electrolyte under experimental conditions that are relevant to fuel cell operation; and finally the dynamics of the electrochemical dissolution of crystalline chalcogenide and amorphous chalcogels surface atoms in clean, non-adsorbing HClO4 electrolyte under experimental conditions that are relevant to electrolyzers operation (particularly the hydrogen evolution reaction (HER)). Furthermore, the experimental details of the SPRDEICPMS method and its validation are demonstrated. Overall, in Chapter 2 we examine the dynamics of the electrochemical dissolution of Pt(111) surface atoms in clean, non-adsorbing HClO4 electrolyte under experimental conditions that are relevant to fuel cell operation. We focus on Pt(111) surfaces for two reasons: (I) the potential dependent adsorption of oxygenated species are well-established and reasonably well understood; and (II) this surface contains the least amount of surface defects -- a structural necessity for exploring oxide/dissolution induced morphological changes at atomic scale. Using these well-defined surfaces, we probed the dynamics of Pt dissolution via the SPRDEICPMS, and tracked the concomitant morphological changes using scanning tunneling microscopy (STM). We found that two distinct Pt dissolution processes can take place on the positive going sweep and negative going sweep regions, respectively, during a cyclic voltammetry (CV) with dissolution rates and morphological changes strongly dependent on the experimental conditions. We also found that the rate of dissolution during oxide formation (positive going sweep) is small and can be considered a faradaic process, as it is not closely related to the kinetics of oxide formation. Furthermore, the rate of dissolution and its associated Pt re-deposition is a fast process that is controlled by the positive potential limit and also by the scan rate used for oxide reduction (negative going sweep). Overall, the results provide a strong foundation for understanding how different potential and time profiles have an impact on the stability of Pt surfaces and their corresponding transition from a well-defined to \"rough\" morphology that ultimately determines the long term electrode durability. In Chapter 3 we present evidence for active site deactivation and regeneration processes that are operational on Mo based chalcogenides (e.g., MoS2) and chalcogels (e.g., MoSx) materials. First, by monitoring the in situ dissolution rates of both Mo and S atoms together with HER activity we were able to establish their unique dissolution dynamics in acid media as a function of the electrode potential. Surprisingly, we found a selective S atom loss with minimal Mo dissolution during HER, whilst at mild positive potentials Mo dissolution happens with minimum S loss. We propose that the selective sulfur release concomitant to H2 production arise from its interaction to hydronium ions (H+) and its thermodynamic driving force to form H2S, while the MoS interaction is weakened as part of the HER catalytic cycle at more negative electrode potentials. As a consequence, we found that the selective removal of S leads to MoS active site conversion to MoOx species formed by \"free\" Mon+ that interacts with surrounding H2O and H+, creating new surface sites that are less active for hydrogen production (e.g., deactivation). In turn, we also found that this newly MoOx species can be selectively removed from the catalyst surface, giving way for the selective Mo dissolution observed at higher electrode potentials, effectively cleaning the surface and exposing fresh MoS sites for the efficient hydrogen production (e.g., regeneration). Finally, our results demonstrate that electrochemical interfaces are constantly evolving, and that understanding the underlying kinetic processes is necessary to successfully design dynamic interfaces that can remain active after extensive electrocatalysis excursions

Electrocatalytic activity and stability of platinum nanoparticles supported on molybdenum oxides and carbon towards oxygen reduction reaction

O envelhecimento dos eletrocatalisadores utilizados em cátodos de células a combustível de eletrólito polimérico (PEMFCs) é um dos principais fatores que restringem sua aplicação como conversores de energia em larga escala. Esse trabalho visa contribuir para o aprimoramento da estabilidade de nanopartículas de platina (NPs de Pt) por meio da modificação do suporte catalítico aos quais encontram-se impregnadas. Desse modo, foram realizadas sínteses de suportes catalíticos baseados em óxidos de molibdênio (MoO3 e MoO2) ancorados em carbono Vulcan® XC72-R, sendo os materiais produzidos caracterizados física, estrutural e eletroquimicamente antes e após a impregnação de NPs de Pt. Para investigar a estabilidade dos eletrocatalisadores, foi realizado um teste de degradação eletroquímico acelerado, o qual consistiu em aplicar os ciclos de potenciais entre 0,6 e 1,0 V vs. ERH por curto período de tempo. Os resultados mostraram que os métodos de síntese utilizados foram satisfatórios, levando a formação dos catalisadores com as proporções bem próximas das requeridas. O catalisador Pt/MoO3-C apresentou a maior atividade específica frente a reação de redução de oxigênio (RRO), atribuída a efeitos sinérgicos metal/suporte. Quando investigada a estabilidade dos materiais frente ao teste de degradação eletroquímico acelerado, observou-se que, a princípio, nenhum dos óxidos de molibdênio diminui a extensão da degradação da platina. Analisando-se as atividades específicas frente à RRO para cada catalisador antes e após o envelhecimento eletroquímico, foi observado que Pt/MoO2-C apresentou-se como o material mais estável dentre os demais.The aging of Pt based electrocatalysts used in the polymer electrolyte fuel cell (PEMFC) cathodes is one of the main issues that restrict its wide application as energy converters. This work aims to contribute to the improvement of the stability of platinum nanoparticles (Pt NPs) by modification of the catalyst support at which they are impregnated. Thus, syntheses of catalyst supports based on molybdenum oxide (MoO3 and MoO2) anchored on Vulcan® XC72-R carbon were carried out and the produced materials were characterized physically, structurally and electrochemically prior and after impregnation of the Pt NPs. To investigate their stability, an electrochemical accelerated degradation test was performed, which consisted of applying a large number of short duration potential cycling steps between 0.6 and 1.0 V vs. RHE. The results showed that the synthetic methods used here were satisfactory, leading to the formation of catalysts with compositions near to those expected. The Pt/MoO3-C catalyst showed the highest specific activity toward the oxygen reduction reaction (ORR), and this was attributed to metal/support synergistic effects. When the stability against electrochemical accelerated degradation test of the materials was investigated, it was observed that, in principle, none of the molybdenum oxides really decreases the extent of platinum degradation. However, comparing the specific activities towards the ORR for each catalyst, before and after electrochemical aging, it is concluded that Pt/MoO2-C is the most stable material among all others

Prospect of microfluidic devices for on-site electrochemical production of hydrogen peroxide

Hydrogen peroxide can be produced electrochemically by selective oxygen reduction reaction or selective water oxidation reaction in various electrolytes. A very promising, sustainable, and efficient method to produce hydrogen peroxide is the so-called electrochemical “lab-on-a-chip” technology, where microfluidic electrochemical flow cells can be used. The main advantage of such a system is “on demand” and “on site” production. If these systems are to be commercialized, suitable electrocatalysts (anode and cathode), sensors, and a device design must be developed and interconnected. Such technology could then eventually be deployed in an industrial environment with internal/external numbering-up approach

Design of Advanced Thin-Film Catalysts for Electrooxidation of Formic Acid

Successful development of catalysts for electrochemical formic acid oxidation (FAO) requires fnding an optimal balance between catalytic performance (activity, stability, and selectivity) and catalyst cost. While platinum is one of the most active catalyst materials for FAO, it suffers from performance loss at low overpotentials due to poisoning with CO, which is one of the intermediates formed in the so-called indirect path of FAO. In this work, we explored the synergistic effects of the supporting material and annealing temperature on the performance of Pt thin flms for FAO in acidic media. Compared to the as-prepared Pt flms, the annealed flms show up to 5-fold and 15-fold improvement for FAO on Pt@Ni and Pt@Cr, respectively. While the most active Pt@Ni thin flm shows the lowest stability, the most active Pt@Cr thin flm is also the most stable, challenging conventional tradeoffs in electrocatalysis and providing a promising candidate for FAO nanocatalyst synthesis

The Role of Interface in Stabilizing Reaction Intermediates for Hydrogen Evolution in Aprotic Li-Ion Battery Electrolyte

p { margin-bottom: 0.1in; direction: ltr; color: rgb(0, 0, 10); line-height: 120%; text-align: left; }p.western { font-size: 12pt; }p.cjk { font-size: 12pt; }a:link { color: rgb(5, 99, 193); } By combining idealized experiments with realistic quantum mechanical simulations of the interface, we investigate electro-reduction reactions of HF and water impurities on the single crystal (111) facets of Au, Pt, Ir and Cu in an organic aprotic electrolyte, 1M LiPF6 in EC/EMC 3:7w (LP57), which are common reactions happening during the formation of the SEI on graphite. In our previous work, we have established that the LiF formation, accompanied with H2 evolution, is caused by a reduction of HF impurities and requires the presence of Li at the interface, which catalyzes the HF dissociation. In the present paper, we find that the measured potential of the electrochemical response for these reduction reactions correlates with the work function of the electrode surfaces and that the work function determines the potential for Li+ adsorption. The reaction path is investigated further by electrochemical simulations suggesting that the overpotential of the reaction is related to stabilizing the active structure of the interface having Li+ adsorbed. The Li+ is needed to facilitate the dissociation of HF which is the source of proton. Further experiments on the other proton sources, water and methanesulfonic acid, show that if the hydrogen evolution involves negatively charged intermediates, F- or HO-, a cation at the interface can stabilize them and facilitate the reaction kinetics. When the proton source is already significantly dissociated (in the case of a strong acid), there is no negatively charged intermediate and thus the hydrogen evolution can proceed at much lower overpotentials. This reveals a situation where the overpotential for electrocatalysis is related to stabilizing the active structure of the interface, facilitating the reaction rather than providing the reaction energy. This has implications for the SEI layer formation in Li-ion batteries and for reduction reactions in alkaline environment as well as for design principles for better electrodes.</p

Dynamic stability of active sites in hydr(oxy)oxides for the oxygen evolution reaction

The poor activity and stability of electrode materials for the oxygen evolution reaction are the main bottlenecks in the water-splitting reaction for H2 production. Here, by studying the activity–stability trends for the oxygen evolution reaction on conductive M1OxHy, Fe–M1OxHy and Fe–M1M2OxHy hydr(oxy)oxide clusters (M1 = Ni, Co, Fe; M2 = Mn, Co, Cu), we show that balancing the rates of Fe dissolution and redeposition over a MOxHy host establishes dynamically stable Fe active sites. Together with tuning the Fe content of the electrolyte, the strong interaction of Fe with the MOxHy host is the key to controlling the average number of Fe active sites present at the solid/liquid interface. We suggest that the Fe–M adsorption energy can therefore serve as a reaction descriptor that unifies oxygen evolution reaction catalysis on 3d transition-metal hydr(oxy)oxides in alkaline media. Thus, the introduction of dynamically stable active sites extends the design rules for creating active and stable interfaces.The peer-reviewed version: [https://cer.ihtm.bg.ac.rs/handle/123456789/3700

Role of catalytic conversions of ethylene carbonate, water, and HF in forming the solid-electrolyte interphase of Li-ion batteries

Compared to aqueous electrolytes, fundamental understanding of the chemical and electrochemical processes occurring in non-aqueous electrolytes is far less developed. This is no different for Li-ion battery (LiB) electrolytes, where many questions regarding the solid-electrolyte interphase (SEI) on the anode side remain unanswered, including its chemical composition, the mechanism of formation, and its impact on LiB performance. Here, we present a detailed experimental and theoretical study of the electrochemistry of ethylene carbonate (EC) and its chemical relationship with trace amounts of water and HF across a vast range of electrode materials, from well-ordered single crystals to realistic graphite electrodes. We reveal the electrocatalytic nature of EC, HF, and water electroreductions at all interfaces, and unveil the catalytic role of water in EC electroreduction. Moreover, we show that these reactions are connected in a closed cycle by chemical reactions that take place either at the electrode/electrolyte interface or in the bulk of the electrolyte and demonstrate that the composition of the SEI depends predominantly on the balance between the (electro)chemistry of EC, water, and HF