Desenvolvimento de multicamadas metálicas do tipo Pt/Ir/Pt eletrodepositadas em titânio para eletrocatálise

Regarding to the search for new energy sources, of renewable character, fuel cells fall into one of the most studied proposals on Electrochemistry. More specifically, the direct fuel cells (DAFCs) using short-chain alcohols as fuel, have attracted the attention of researchers over the past two decades. Problems related to the energy efficiency of Pt-based catalysts for use in fuel cells, drive several studies in the literature concerning to the modification of these catalytic systems. Thus, have been proposed in the literature, other materials than pure Pt, such as alloys in binary and ternary composition, and more recently, metallic multilayered structures(MM), the latter being the study object of this work. Platinized titanium electrodes were prepared by potentiostatic electrodeposition technique (0.05 V vs RHE), and times of 1000 sec were necessary to obtain an identical polycrystalline platinum voltammetric profile, with guaranteed reproducibility. On these substrates, Ir/Pt bilayered systems were electrodeposited controlling films thickness through the deposition charge. Calculations revealed mass loading of platinum between13.4 and 26.3 μg and maximum film thickness of 43.2 nm. AFM images confirmed the homogeneity of the coatings as well as the total covering of the substrate. A screening of variables with the aid of a 22 factorial design with central point showed better intrinsic catalytic activity (current density normalized by the electroactive area) for the Ti/Pt25mC/Ir6mC/Pt system, which answers were around 270% higher than those obtained for Ti/Pt systems. In addition to the methanol oxidation voltammetry, chronoamperometric tests of methanol oxidation and CO stripping voltammetry suggest the MM-like systems are less susceptible to the catalyst poisoning phenomenon compared to the Ti/Pt systems. Assuming that there are no area effects due the current normalization through the electroactive area, nor the possibility of a bifunctional mechanism occurring,considering that are just Pt catalytic centers exposed, an explanation for the purposes of promoting catalytic activity may be based on electronic effects due to the possible compressive stress generated by the bilayer Ir/Pt causing an energy enlargement of the d band, lowering the adsorbates energy of adsorption. EIS data indicated a lower resistance to charge transfer in the reactions performed on MM systems, which strengthens the argument regarding the positive electronic effects associated with MM-like structures.Universidade Federal de Sao CarlosNo que diz respeito à busca por novas fontes energéticas, de caráter renovável, as células combustíveis se enquadram em uma das propostas mais estudadas na Eletroquímica. Mais especificamente, as células à combustível direto (DAFCs) que utilizam alcoóis de cadeia curta como combustível, vêm atraindo a atenção dos pesquisadores nas últimas duas décadas. Problemas relativos à eficiência energética dos catalisadores baseados em Pt para aplicação em células combustíveis, impulsionam diversos estudos na literatura concernentes à modificação desses sistemas catalíticos. Desta forma, têm sido propostos na literatura, diferentes materiais além da Pt pura, como ligas de composição binária e ternária, e mais recentemente, as multicamadas metálicas(MM), tendo sido esta última proposta objeto de estudo do presente trabalho. Eletrodos de titânio platinizados foram confeccionados através da técnica de eletrodeposição potenciostática a 0,05V vs ERH, sendo que tempos de 1000s foram necessários para se obter um perfil voltamétrico idêntico ao da platina policristalina, com reprodutibilidade garantida. Sobre estes substratos, sistemas de bicamadas Ir/Pt foram eletrodepositados controlando-se a espessura através da carga de deposição. Os cálculos de massa revelaram cargas de platina entre 13,4 e 26,3μg e espessuras máximas de filme de 43,2nm. Imagens de AFM confirmaram a homogeneidade dos eletrodepósitos bem como o recobrimento total do substrato. Uma triagem de variáveis com o auxílio de um planejamento fatorial 22 com ponto central apontou melhores atividades catalíticas intrínsecas (densidade de corrente normalizada pela área eletroativa) para o sistema Ti/Pt25mC/Ir6mC/Pt, cujas respostas foram em torno de 270% maiores que em relação ao Ti/Pt. Além das voltametrias de oxidação do metanol, ensaios de cronoamperometrias de oxidação do metanol e voltametria do stripping de CO sugerem que os sistemas tipo-multicamada sejam menos suscetíveis ao fenômeno de envenenamento catalítico, em relação aos sistemas Ti/Pt somente. Partindo do pressuposto que não existam efeitos de área, devido a normalização das correntes obtidas pelas áreas eletroativas, e nem a possibilidade de um mecanismo bifuncional, assumindo que apenas Pt está exposta no sistema catalítico, uma explicação para os efeitos promotores da atividade catalítica pode estar baseado em efeitos eletrônicos devido a um possível estresse compressivo gerado pela bicamada Ir/Pt, causando alargamento em energia da banda d e logo, menores energias de adsorção de adsorbatos. Dados de EIS indicaram uma menor resistência à transferência de carga nas reações realizadas sobre as MM, o que fortalece a argumentação em relação aos efeitos eletrônicos positivos associados às estruturas tipo MM

The oscillatory electro-oxidation of small organic molecules

The electrooxidation of small organic molecules such as formaldehyde, formic acid, methanol, ethanol, ethylene glycol, glycerol, and so on is relevant to interconversion between chemical and electrical energies. Although these have considerably low thermodynamic potentials compared to hydrogen, the oxidation process generally demands high overpotentials because of the ubiquitous formation of surface‐blocking carbonaceous species. The occurrence of parallel pathways and the formation of stable soluble by‐products also contribute to the poor utilization of all electrons involved in the oxidation process. Thecomplex kinetics found in these systems can also result in nonlinear manifestations such as autocatalysis and oscillatory dynamics. Besides the considerable amount of earlier experimental reports, only recently has some understanding of the chemistry underlying the dynamics been achieved. Moreover, a number of interesting and unexpected behaviors have been observed under oscillatory regime. In this chapter, we briefly review the recent advances on the oscillatory electrooxidation of small organic molecules, with emphasis on (a) the general phenomenology, (b) the use ofin situ andonline approaches, (c) the effect of temperature, and (d) the oscillations on modified surfaces. Moreover, some implications of nonlinearities in low temperature fuel cells are also discussed

Communication—Why High-Precision Coulometry and Lithium Plating Studies on Commercial Lithium-Ion Cells Require Thermal Baths

We demonstrate how insufficient heat transport in environmental chambers compromises the meaningfulness of high-precision charge counting and leads to an underestimation of lithium plating in fast-charged lithium-ion batteries. Direct-contact liquid cooling of cylindrical cells excludes temperature fluctuations observed in thermal chambers and restricts self-heating to 2 K in comparison to the 14 K in thermal chambers, under 1.5 C-rate cycling. Our thermal-electrochemical model replicated well the experimental results. For high-precision coulometric studies to be meaningful and more comparable across different laboratories, especially for large-format and high-power cells, direct-contact cooling in thermal baths must become the new standard

Temperature dependence of the evolving oscillations along the electrocatalytic oxidation of methanol

Despite the constancy of all controllable experimental parameters, most natural and artificial oscillators are known to slowly evolve over time. In electrochemical systems, this spontaneous evolution has been ascribed to the surface deactivation that gently drives the system and acts a bifurcation parameter. We investigate the effect of temperature on the electro-oxidation of methanol on platinum, with focus on the potential oscillations and its spontaneous temporal evolution. The study was performed at comparable applied currents (normalized with respect to the oscillatory window) at nine temperatures between 10 °C and 50 °C, in acidic media, and oscillations were not observed at 50 °C. The main results were discussed in connection with voltammetric data, which were deconvoluted into three regions according to the electrode potential. While the frequency of potential oscillations followed a regular Arrhenius-like dependence, the size of the oscillatory window was found to remain nearly unaffected by temperature. From the mechanistic point-of-view, these dependencies were attributed to the existence of more than one Langmuir–Hinshelwood (LH) step that consumes adsorbed oxygenated species. This fact was corroborated by voltammetric data. The relative magnitude of the activation energies of the LH processes were estimated as higher than that of the deactivation process, as previously suggested

High-energy NCA cells on idle:anode versus cathode driven side reactions

We report on the first year of calendar ageing of commercial high‐energy 21700 lithium‐ion cells, varying over eight state of charge (SoC) and three temperature values. Lithium‐nickel‐cobalt‐aluminium oxide (NCA) and graphite with silicon suboxide (Gr‐SiOx) form cathodes and anodes of those cells, respectively. Degradation is fastest for cells at 70–80 % SoC according to monthly electrochemical check‐up tests. Cells kept at 100 % SoC do not show the fastest capacity fade but develop internal short circuits for temperatures T≥40 °C. Degradation is slowest for cells stored close to 0 % SoC at all temperatures. Rates of capacity fade and their temperature dependencies are distinctly different for SoC values below and above 60 %, respectively. Differential voltage analyses, apparent activation energy analysis, and endpoint slippage tracking provide useful insights into the degradation mechanisms and the respective roles of anode and cathode potential. We discuss how reversible losses of lithium might play a role in alleviating the rate of irreversible losses on commercial cells

Entropy Profiling for the Diagnosis of NCA/Gr-SiOx Li-Ion Battery Health

Graphite-silicon (Gr-Si) blends have become common in commercial Li-ion battery negative electrodes, offering increased capacity over pure graphite. Lithiation/delithiation of the silicon particles results in volume changes, which may be associated with increased hysteresis of the open circuit potential (OCP). The OCP is a function of both concentration and temperature. Entropy change measurement, which probes the response of the OCP to temperature, offers a unique battery diagnostics tool. While entropy change measurements have previously been applied to study degradation, the implications of Si additives on the entropy profiles of commercial cells have not been explored. Here, we use entropy profiling to track aging markers in the same way as differential voltage analysis. In addition to lithiation/delithiation hysteresis in the OCP of Gr-Si blends, cells with Gr-Si anodes also exhibit differences in entropy profile depending on cycling direction, reflecting degradation-related morphological changes. For cycled cells, entropy change decreased during discharge, likely corresponding to graphite particles breaking and cracking. However, entropy change during charge increased with cycling, likely due to the volume change of silicon. Over a broad voltage range, these combined effects led to the observed rise in entropy hysteresis with age. Conversely, for calendar aged cells entropy hysteresis remained stable

Entropy profiling for the diagnosis of NCA/Gr-SiOx Li-Ion battery health

Graphite-silicon (Gr-Si) blends have become common in commercial Li-ion battery negative electrodes, offering increased capacity over pure graphite. Lithiation/delithiation of the silicon particles results in volume changes, which may be associated with increased hysteresis of the open circuit potential (OCP). The OCP is a function of both concentration and temperature. Entropy change measurement, which probes the response of the OCP to temperature, offers a unique battery diagnostics tool. While entropy change measurements have previously been applied to study degradation, the implications of Si additives on the entropy profiles of commercial cells have not been explored. Here, we use entropy profiling to track aging markers in the same way as differential voltage analysis. In addition to lithiation/delithiation hysteresis in the OCP of Gr-Si blends, cells with Gr-Si anodes also exhibit differences in entropy profile depending on cycling direction, reflecting degradation-related morphological changes. For cycled cells, entropy change decreased during discharge, likely corresponding to graphite particles breaking and cracking. However, entropy change during charge increased with cycling, likely due to the volume change of silicon. Over a broad voltage range, these combined effects led to the observed rise in entropy hysteresis with age. Conversely, for calendar aged cells entropy hysteresis remained stable

Sodiation of hard carbon:how separating enthalpy and entropy contributions can find transitions hidden in the voltage profile

Sodium-ion batteries (NIBs) utilise cheaper materials than lithium-ion batteries (LIBs), and can thus be used in larger scale applications. The preferred anode material is hard carbon, because sodium cannot be inserted into graphite. We apply experimental entropy profiling (EP), where the cell temperature is changed under open circuit conditions. EP has been used to characterise LIBs; here, we demonstrate the first application of EP to any NIB material. The voltage versus sodiation fraction curves (voltage profiles) of hard carbon lack clear features, consisting only of a slope and a plateau, making it difficult to clarify the structural features of hard carbon that could optimise cell performance. We find additional features through EP that are masked in the voltage profiles. We fit lattice gas models of hard carbon sodiation to experimental EP and system enthalpy, obtaining: 1. a theoretical maximum capacity, 2. interlayer versus pore filled sodium with state of charge