A Generalized Method for High‐Speed Fluorination of Metal Oxides by Spark Plasma Sintering Yields Ta₃O₇F and TaO₂F with High Photocatalytic Activity for Oxygen Evolution from Water

Abstract

The search for clean, low-cost, and renewable energy sources is one important challenge of modern industrial societies. [1] Hydrogen generated by photochemistry has been identified as a promising energy carrier with high energy density and zero CO 2 emission while being environmentally clean. [2,3] To set up a light-driven and hydrogen based economy an exploration of new materials for eco-friendly, economically viable, stable, and efficient photocatalysts is needed. [4] Noble metals like platinum, iridium, and ruthenium are efficient catalysts for the electrolysis of water, but their scarcity and high-costs limit large-scale technological use. [5] The development of cheap and active catalysts with long-term stability for the hydrogen or oxygen evolution reaction in standard electrolytes is an important goal. A general method to carry out the fluorination of metal oxides with poly(tetrafluoroethylene) (PTFE, Teflon) waste by spark plasma sintering (SPS) on a minute scale with Teflon is reported. The potential of this new approach is highlighted by the following results. i) The tantalum oxyfluorides Ta 3 O 7 F and TaO 2 F are obtained from plastic scrap without using toxic or caustic chemicals for fluorination. ii) Short reaction times (minutes rather than days) reduce the process time the energy costs by almost three orders of magnitude. iii) The oxyfluorides Ta 3 O 7 F and TaO 2 F are produced in gram amounts of nanoparticles. Their synthesis can be upscaled to the kg range with industrial sintering equipment. iv) SPS processing changes the catalytic properties: while conventionally prepared Ta 3 O 7 F and TaO 2 F show little catalytic activity, SPS-prepared Ta 3 O 7 F and TaO 2 F exhibit high activity for photocatalytic oxygen evolution, reaching photoconversion efficiencies up to 24.7% and applied bias to photoconversion values of 0.86%. This study shows that the materials properties are dictated by the processing which poses new challenges to understand and predict the underlying factors.

Fluorinated MAX Phases for Photoelectrochemical Hydrogen Evolution

“…The lower photoelectrocatalytic HER enhancement by TaO 2 F than TiOF 2 can be related to their optical band gap. A recent study reported that TaO 2 F presents an energy band value of ≈4.10 eV, so it is expected to be less photoactive than TiOF 2 when both are exposed to the light source (660 nm) in this work. Overall, Cr 2 AlC and F-Cr 2 AlC presented the lowest overpotential in their respective category.…”

“…As a confirmation, the cubic-like structures observed on the surface of F-MAX in Figure 1a,c,d can be attributed to TaO 2 F and TiOF 2 since both possess a simple cubic crystal structure and the cubic morphology agrees with the reported works. 44,45 Despite fluorine being detected in F-Cr 2 AlC, the etching effect on the Al layer is least prominent, with the smallest separation between layers, the lowest amount of F within the sample, and no Cr-based oxyfluoride structures were detected. The different fluorination effects can be related to the properties of individual MAX and its corresponding metal oxides.…”

Photocatalytic Water‐Splitting by Organic Conjugated Polymers: Opportunities and Challenges

“…These include metal oxides such as TiO 2 , hematite, ZnO, BiVO 4 , rare earth metals, coordinate complexes, (oxy)sulfides, (oxy)fluorides, and (oxy)nitrides with d 0 and d 10 electronic configuration. [10][11][12][13] Nevertheless, most of these suffer from intrinsic limitations of large bandgap (> 3.0 eV), poor solar light-harvesting ability and photostability, and low conversion efficiency. [14] Most recently, organic conjugated polymers have established their potential to be a versatile platform for water splitting owing to their low cost, superior photo-/thermostability, and tunable structure and properties.…”

Advances in Interfacial Engineering and Their Role in Heterostructure Formation for HER Applications in Wider pH

“…In this regard, water splitting with the lowest carbon footprint as the most promising method to generate hydrogen has been extensively researched with a rich reserve of electrocatalysts for hydrogen evolution (HER). With the practical use of high cost noble metal Pt as the state of the art catalyst, a pool of non-noble metal catalysts have been explored, yet it still leaves behind unfilled gaps. − A clear understanding of the HER mechanism described in acidic/basic medium will provide education about the challenges and possibilities (Figure ). Briefly, H 2 evolution takes place at the cathode of a water electrolyzer and follows two elementary steps. , …”