Hierarchical TiN Nanostructured Thin Film Electrode for Highly Stable PEM Fuel Cells

Abstract

Fuel cells are, to date, on the verge of large-scale commercialization. Still, long-term stability is of concern, especially in the automotive field, mainly because of the cathodic catalyst support. In fact, carbonaceous materials, the state of the art to date, suffer from severe corrosion phenomena during discontinuous operation. In the effort to replace carbon as Pt support and develop a nanoengineered architecture for the fuel cell electrodes, we report here the concept of a hierarchical TiN nanostructured thin film (HTNTF) electrode, in which Pt is deposited on an array of quasi-1D TiN nanostructures with good conductivity, high roughness factor, tunable porosity, and outstanding chemical stability. The HTNTF is grown by self-assembly from the gas phase by means of a one-step, template-free, room-temperature process, namely, pulsed laser–scattered ballistic deposition, PL–SBD. The activity of the nanostructured thin film electrode is assessed toward the oxygen reduction reaction and its stability evaluated according to DOE accelerated stress test (AST) standard protocols, revealing an electrochemical surface area (ECSA) loss as low as 7% with respect to the 40% goal. Moreover, a proof-of-concept cell has been realized to demonstrate the applicability of our supports to the device scale. Despite the fact that further optimization is needed to achieve high performances, this new class of electrodes has clear potential in terms of stability with respect to the state of the art, overcoming carbon corrosion by simply removing it from direct contact with the Pt electrocatalyst.

Hierarchical TiN‐Supported TsFDH Nanobiocatalyst for CO2 Reduction to Formate

“…Lastly, for the highest gas pressure of 60 Pa (TiN 60 Pa, Figure 1c) the very low kinetic energy of the plasma and the growth of the nanostructure by the successive ‘soft’ landing of weakly‐energetic particles leads to the formation of nanostructured aggregates with a ‘nanotree’ character, with higher porosity and pronounced internal spacing. In our previous work on the characterization of TiN hierarchical nanostructures, we confirmed this evolution in mesoscale morphology by BET (Brunauer‐Emmett‐Teller) surface area analysis and BJH (Barrett, Joyner, and Halenda) method for pore size distribution calculation [47] . Since in this study the same fabrication conditions for the TiN support are employed, we extend here our previous considerations on morphology.…”

“…Here we focus solely on the conditions leading to arrays of hierarchical nanostructures with a characteristic “nanotree” morphology, already demonstrated to be promising as catalyst support [47] . The evolution of the morphology and porosity features of the TiN support is visualized by the scanning electron microscope (SEM) micrographs shown in Figure 1a–c.…”

“…This is key to optimize the binding of the enzyme and to retain its activity. TiN is chosen as a support material thanks to its remarkable stability [47] in the electrochemical environment and high electrical conductivity [48–53] . Additionally, enzyme coupling is favored on TiN thanks to the transfer of s‐valency and d‐valency electrons of the Ti atoms to the 2p‐states of N atoms, [52] therefore it is a promising material for enzyme immobilization.…”

Non‐Equilibrium Synthesis of Highly Active Nanostructured, Oxygen‐Incorporated Amorphous Molybdenum Sulfide HER Electrocatalyst

“…Typically, below 10 −2 mbar dense thin films are obtained, while above 100 mbar nanoparticles nucleate and assemble into aerogels. [ 57,61–64 ] Within this range nanoparticles in the 5–20 nm range self‐assemble in arrays of quasi‐1D hierarchical nanostructures, which proved to be highly advantageous for liquid [ 65 ] and solid state dye‐sensitized solar cells, [ 62 ] photochromic, [ 66 ] photonic, [ 67 ] photoelectrochemical, [ 68–70 ] and fuel cell applications [ 64 ] thanks to the high surface area, tuneable pore size distribution, high electrical conductivity, and efficient mass transport. In this work, we used PLD to disrupt the structural order of MoS 2 , forcing its constituent atoms to condense in highly defective clusters, which in turn self‐assemble in arrays of quasi‐1D hierarchical nanostructures with different geometries onto a substrate placed at 5 cm from the target, for Ar pressures in the range 5–30 Pa. Out of this range, the same trend reported in previous studies is observed with dense films obtained for lower pressures and poorly connected nanostructures for higher ones.…”

“…Such successive landing of the nanoparticles would form hyperbranched “nanotrees” with high porosity, similar to what described elsewhere. [ 61,64,67 ] However, the heavier clusters of the fourth component mix with the as‐deposited nanoparticles, densifying them and growing hierarchical elements composed of weakly‐attached aggregates of nanoparticles—where the weak attachment is a consequence of the smaller kinetic energy—resembling the “nanotree” morphology with high interelement porosity. The Brunauer–Emmett–Teller (BET) surface area values in Figure 2d quantify the evolution in porosity and surface area of the MoS x nanostructured films: as the Ar pressure is incremented, the BET surface area (black trace) increases from 4.7 m 2 g −1 for MoS x 5 Pa to 17.5 m 2 g −1 for MoS x 10 Pa and 65.2 m 2 g −1 for MoS x 30 Pa. Concomitantly, the density (red trace) diminishes, starting from a value of 2.93 g cm −3 for MoS x 5 Pa to 2.09 g cm −3 for MoS x 30 Pa. As a comparison, density for bulk MoS 2 is 5.06 g cm −3 .…”