Figure 1. a) Electrochemical oxide pathway. [13] Copyright 2016, John Wiley and Sons. b) Cationic redox mechanism proposed by Kçtz et al. [21] Copyright 1984, IOP Publishing. c) Scheme of the OER, including the formation of an OOH intermediate, as detected by Sivasankar et al. [24] Copyright 2011, AmericanC hemical Society.d )Schematic representation of O2pbands penetrating into Ir dorbitals and triggering an anionic redox process. [27] Copyright 2016, Springer Nature. e) OER scheme showing the formation of oxyl species, as aresult of hybridization of Ir and O orbitals, which are prone to nucleophilic attack by water and the formation of an OÀObond. [33] Copyright2 019, Elsevier.
The production of
hydrogen via a proton-exchange membrane water
electrolyzer (PEM-WE) is directly dependent on the rational design
of electrocatalysts for the anodic oxygen evolution reaction (OER),
which is the bottleneck of the process. Here, we present a smart design
strategy for enhancing Ir utilization and stabilization. We showcase
it on a catalyst, where Ir nanoparticles are efficiently anchored
on a conductive support titanium oxynitride (TiON
x
) dispersed over carbon-based Ketjen Black and covered by
a thin layer of copper (Ir/CuTiON
x
/C),
which gets removed in the preconditioning step. Electrochemical OER
activity, stability, and structural changes were compared to the Ir-based
catalyst, where Ir nanoparticles without Cu are deposited on the same
support (Ir/TiON
x
/C). To study the effect
of the sacrificial less-noble metal layer on the catalytic performance
of the synthesized material, characterization methods, namely X-ray
powder diffraction, X-ray photoemission spectroscopy, and identical
location transmission electron microscopy were employed and complemented
with scanning flow cell coupled to an inductively coupled plasma mass
spectrometer, which allowed studying the online dissolution during
the catalytic reaction. Utilization of these advanced methods revealed
that the sacrificial Cu layer positively affects both Ir OER mass
activity and its durability, which was assessed via S-number, a recently
reported stability metric. Improved activity of Cu analogue was ascribed
to the higher surface area of smaller Ir nanoparticles, which are
better stabilized through a strong metal–support interaction
(SMSI) effect.
The commercialization of acidic proton exchange membrane water electrolyzers (PEMWE) is heavily hindered by the price and scarcity of oxygen evolution reaction (OER) catalyst, i. e. iridium and its oxides. One of the solutions to enhance the utilization of this precious metal is to use a support to distribute well dispersed Ir nanoparticles. In addition, adequately chosen support can also impact the activity and stability of the catalyst. However, not many materials can sustain the oxidative and acidic conditions of OER in PEMWE. Hereby, we critically and extensively review the different materials proposed as possible supports for OER in acidic media and the effect they have on iridium performances.
More efficient utilization of iridium is of immense importance for the future development of proton exchange membrane electrolyzers. In this study, we introduce a new facile and scalable synthesis of an Ir-based high-performance oxygen evolution reaction (OER) electrocatalytic nanocomposite. The composite consists of Ir nanoparticles with an average size of 3-4 nm, which are effectively anchored on a titanium oxynitride support (TiON x ), which is distributed across high-surface-area Ketjen Black carbon (Ir/TiON x /C). We provide complete structural, morphological and compositional characterization (x-ray diffraction, scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy) and propose a proper benchmark protocol to measure true electrochemical performance. Compared to the state-of-the-art Ir Black electrocatalyst, Ir/TiON x /C exhibits approximately three times higher OER performance.
Decreasing iridium loading in the electrocatalyst presents
a crucial
challenge in the implementation of proton exchange membrane (PEM)
electrolyzers. In this respect, fine dispersion of Ir on electrically
conductive ceramic supports is a promising strategy. However, the
supporting material needs to meet the demanding requirements such
as structural stability and electrical conductivity under harsh oxygen
evolution reaction (OER) conditions. Herein, nanotubular titanium
oxynitride (TiON) is studied as a support for iridium nanoparticles.
Atomically resolved structural and compositional transformations of
TiON during OER were followed using a task-specific advanced characterization
platform. This combined the electrochemical treatment under floating
electrode configuration and identical location transmission electron
microscopy (IL-TEM) analysis of an in-house-prepared Ir-TiON TEM grid.
Exhaustive characterization, supported by density functional theory
(DFT) calculations, demonstrates and confirms that both the Ir nanoparticles
and single atoms induce a stabilizing effect on the ceramic support
via marked suppression of the oxidation tendency of TiON under OER
conditions.
A versatile approach
to the production of cluster- and single atom-based
thin-film electrode composites is presented. The developed TiO
x
N
y
–Ir
catalyst was prepared from sputtered Ti–Ir alloy constituted
of 0.8 ± 0.2 at % Ir in α-Ti solid solution. The Ti–Ir
solid solution on the Ti metal foil substrate was anodically oxidized
to form amorphous TiO2–Ir and later subjected to
heat treatment in air and in ammonia to prepare the final catalyst.
Detailed morphological, structural, compositional, and electrochemical
characterization revealed a nanoporous film with Ir single atoms and
clusters that are present throughout the entire film thickness and
concentrated at the Ti/TiO
x
N
y
–Ir interface as a result of the anodic oxidation
mechanism. The developed TiO
x
N
y
–Ir catalyst exhibits very high oxygen evolution
reaction activity in 0.1 M HClO4, reaching 1460 A g–1
Ir at 1.6 V vs reference hydrogen electrode.
The new preparation concept of single atom- and cluster-based thin-film
catalysts has wide potential applications in electrocatalysis and
beyond. In the present paper, a detailed description of the new and
unique method and a high-performance thin film catalyst are provided
along with directions for the future development of high-performance
cluster and single-atom catalysts prepared from solid solutions.
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