Jiwei Ma

Electronic interaction between platinum nanoparticles and nitrogen-doped reduced graphene oxide: effect on the oxygen reduction reaction

In this study, low-mass loadings (ca. 5 wt%) Pt/C catalysts were synthesized using the carbonyl chemical route allowing for the heterogeneous deposition of Pt nanoparticles on different carbon-based substrates. N-doped reduced graphene oxide, reduced graphene oxide, graphene oxide, graphite and Vulcan XC-72 were used for the heterogeneous deposition of Pt nanoparticles. The effect of the chemical nature of the carbon-based substrate on the Oxygen Reduction Reaction (ORR) kinetics at Pt nanoparticles surfaces was investigated. XPS results show that using N-doped reduced graphene oxide materials for the deposition of Pt nanoparticles leads to formation of Pt–N chemical bonds. This interaction between Pt and N allows for an electronic transfer from Pt to the carbon support. It is demonstrated that ca. 25% of the total amount of N atoms were bound to Pt ones. This chemical bond also revealed by the DFT analysis, induces changes in the oxygen adsorption energy at the platinum surface, engendering an enhancement of the catalyst activity towards ORR. In comparison with Vulcan XC-72, the mass activity at 0.9 V vs. RHE is 2.1 fold higher when N-doped reduced graphene oxide is used as substrate. In conjunction with the experimental results, DFT calculations describe the interaction between supported platinum clusters and oxygen where the support was modelled accordingly with the carbon-based materials used as substrate. It is demonstrated that the presence of N-species in the support although leading to a weaker O2 adsorption, induces elongated O–O distances suggesting facilitated dissociation. Additionally, it is revealed that the strong interaction between Pt clusters and N-containing substrates leads to very slight changes of the cluster–substrate distance even when oxygen is adsorbed at the interfacial region, thus leading to a lower resistance for electron charge transfer and enabling electrochemical reactions.

Toward high surface area TiO2 brookite with morphology control

TiO2 materials have many practical applications due to their intrinsic physico-chemical properties. Research activities on the properties of TiO2 brookite have been restrained by the difficulty of preparing such a phase. Here, we report on the synthesis of TiO2 brookite prepared by thermal decomposition of a titanium oxalate hydrate compound. Since the characteristics of the prepared TiO2 brookite are dictated by those of the precursor, the present work aims to understand the aqueous precipitation process of the oxalate hydrate phase. It was shown that the formation of the phase occurred via different steps that are affected by the synthesis conditions, i.e. the oxalate source and the duration time. At first, in agreement with the Ostwalds rule of stages, the formation of the oxalate phase implied a metastable intermediate that is a poorly crystallized TiO2 phase. The pH of the solution was shown to impact on the kinetic of transformation of this intermediate toward the final compound. In the presence of alkali ions, the oxalate phase was shown to undergo a dissolution/etching phenomenon that is dependent on the nature of the alkali ion used. The apparent difference in adsorption ability of the alkali ions on the different crystal planes of the titanium oxalate hydrate phase accounted for the variety of the obtained morphologies. Finally, it was suggested that the reaction was promoted by a coordination-assisted mechanism involving the complexing properties of the oxalate anions toward Ti4+ ions. The obtained TiO2 brookite materials exhibit unreached high specific surface area lying between 150 and 400 m2 g−1 while displaying high packing density around 1–1.2 g cm−3. The lithium insertion ability of the prepared material depends on the calcination temperature. Increasing the temperature led to a decrease of the lithium uptake properties but was shown to improve the kinetics of lithium insertion. This was due to an increase of the pore radius size that enabled a faster lithium diffusion transport to be achieved under high current density conditions.

Reversible magnesium and aluminium ions insertion in cation-deficient anatase TiO2

In contrast to monovalent lithium or sodium ions, the reversible insertion of multivalent ions such as Mg2+ and Al3+ into electrode materials remains an elusive goal. Here, we demonstrate a new strategy to achieve reversible Mg2+ and Al3+ insertion in anatase TiO2, achieved through aliovalent doping, to introduce a large number of titanium vacancies that act as intercalation sites. We present a broad range of experimental and theoretical characterizations that show a preferential insertion of multivalent ions into titanium vacancies, allowing a much greater capacity to be obtained compared to pure TiO2. This result highlights the possibility to use the chemistry of defects to unlock the electrochemical activity of known materials, providing a new strategy for the chemical design of materials for practical multivalent batteries.

Enhanced HER and ORR behavior on photodeposited Pt nanoparticles onto oxide-carbon composite

The photodeposition process under ultraviolet domain for platinum nanoparticles was explored. The concomitant presence of different mechanisms during the photodeposition of Pt nanoparticles onto TiO2 in the presence of water and alcohol is evidenced. According to the process, one can devise various complex mechanisms. The presence of nanoparticulated oxide anatase phase enhances the photodeposition process of metal nanoparticles via the so-called heterogeneous photocatalysis. A description and the effect of mixing of various chemicals in the reactor reveal interesting information, which allows controlling the size of nanoparticles by the photodeposition process. This study also paves the way to decrease the amount of precious metals used in material composition used as catalysts towards hydrogen evolution reaction and oxygen reduction reaction for fuel cell technologies.

Electronic modification of Pt via Ti and Se as tolerant cathodes in air-breathing methanol microfluidic fuel cells

We reported herein on the use of tolerant cathode catalysts such as carbon supported Pt(x)Ti(y) and/or Pt(x)Se(y) nanomaterials in an air-breathing methanol microfluidic fuel cell. In order to show the improvement of mixed-reactant fuel cell (MRFC) performances obtained with the developed tolerant catalysts, a classical Pt/C nanomaterial was used for comparison. Using 5 M methanol concentration in a situation where the fuel crossover is 100% (MRFC-mixed reactant fuel cell application), the maximum power density of the fuel cell with a Pt/C cathodic catalyst decreased by 80% in comparison with what is observed in the laminar flow fuel cell (LFFC) configuration. With Pt(x)Ti(y)/C and Pt(x)Se(y)/C cathode nanomaterials, the performance loss was only 55% and 20%, respectively. The evaluation of the tolerant cathode catalysts in an air-breathing microfluidic fuel cell suggests the development of a novel nanometric system that will not be size restricted. These interesting results are the consequence of the high methanol tolerance of these advanced electrocatalysts via surface electronic modification of Pt. Herein we used X-ray photoelectron and in situ FTIR spectroscopies to investigate the origin of the high methanol tolerance on modified Pt catalysts.

Layered Lepidocrocite Type Structure Isolated by Revisiting the Sol-Gel Chemistry of Anatase TiO2: a New Anode Material for Batteries

Searches for new electrode materials for batteries must take into account financial and environmental costs to be useful in practical devices. The sol–gel chemistry has been widely used to design and implement new concepts for the emergence of advanced materials such as hydride organic–inorganic composites. Here, we show that the simple reaction system including titanium alkoxide and water can be used to stabilize a new class of electrode materials. By investigating the crystallization path of anatase TiO2, an X-ray amorphous intermediate phase has been identified whose local structure probed by the pair distribution function, 1H solid-state NMR and density functional theory (DFT) calculations, consists of a layered type structure as found in the lepidocrocite. This phase presents the following general formula Ti2–x□xO4–4x(OH)4x·nH2O (x ∼ 0.5) where the substitution of oxide by hydroxide anions leads to the formation of titanium vacancies (□) and H2O molecules are located in interlayers. Solid-state 1H NM...

Tailoring and Tuning the Tolerance of a Pt Chalcogenide Cathode Electrocatalyst to Methanol

Name that tune: A simple and new method to produce a methanol-tolerant Pt chalcogenide catalyst for the oxygen reduction reaction (ORR) is developed. The catalyst is tuned by electrochemical stripping of the Se atoms on the surface of the catalyst. The resulting electrode nanomaterial is PtSe0.2, and it shows the highest activity ever reported for the ORR in solutions containing methanol.