Structurally ordered
intermetallic compounds possess unique chemical
and physical properties, making them an interesting class of materials
for application in electrocatalytic reactions. This Review comprises
the work on intermetallic compounds used for energy relevant electrocatalysis
and is structured by the reactions in scope, which are the hydrogen
evolution reaction (HER), electrochemical carbondioxide reduction
reaction (eCO2RR), oxygen reduction reaction (ORR), hydrogen
oxidation reaction (HOR) as well as the oxidation reactions of formic
acid (FAOR), methanol (MOR), and ethanol (EtOR). Optimization pathways
for electrocatalysts, based on the adjustability of the intermetallic
materials, are highlighted, and experimental data are provided in
a comparative manner, to provide an overview, foundation, and reference
for further development.
Efficient
development of catalytic materials requires knowledge
of the decisive parameters defining the catalytic properties. In multicomponent
metallic catalysts, these are categorized as electronic and geometric
effects, yet they are strongly interrelated. A systematic disentanglement
can be achieved by fixing one parameter while altering the other,
which becomes possible through the substitution in isostructural intermetallic
compounds. This approach enables the evaluation of electronic or geometric
contributions both individually and combined. Herein, this is achieved
by substitution of indium (three valence electrons) with tin (four
valence electrons) in the series In1–x
Sn
x
Pd2, which allows
for a systematic variation of the total number of electrons per unit
cell with only a minor variation of the unit cell parameters and thus
the evaluation of the electronic effect. Geometric effects were evaluated
by substitution of indium with gallium in the Ga1–x
In
x
Pd2 series,
which allows for a systematic variation of the interatomic distances
while maintaining the same number of valence electrons per unit cell
and close atomic coordinates. By substituting gallium with tin in
the Ga1–x
Sn
x
Pd2 series, both effects are combined and addressed
simultaneously. The activity enhancement of the methanol oxidation
reaction on the Ga1–x
Sn
x
Pd2 series is attributed to the synergy
of the combined effects.
Molybdenum–nickel
materials are catalysts of industrial
interest for the hydrogen evolution reaction (HER). Well-characterized
surfaces of the single-phase intermetallic compounds Ni7Mo7, Ni3Mo, and Ni4Mo were subjected
to accelerated durability tests (ADTs) and thorough characterization
to unravel whether crystallographic ordering affects the activity.
Their intrinsic instability leads to molybdenum leaching, resulting
in higher specific surface areas and nickel-enriched surfaces. These
are more prone to form Ni(OH)2 layers, which leads to deactivation
of the Mo–Ni materials. The crystal structure of the intermetallic
compounds has, due to the intrinsic instability of the materials in
alkaline media, no effect on the activity. Ni7Mo7, identified earlier as durable, proves to be highly unstable in
the applied ADTs. The results show that the enhanced activity of unsupported
bulk Mo–Ni electrodes can solely be ascribed to increased specific
surface areas.
Besides activity and selectivity, the stability is of great importance in the development of catalysts for long-term applications. However, the lack of standardized stability protocols in electrocatalysis remains a fundamental hurdle hindering progress in the field by the lack of quantitative comparability of data in different studies. Herein, an electrochemical protocol to address the stability of bulk electrodes is developed. The protocol tests in situ and operando stability in the electrochemical methanol oxidation in alkaline media using the intermetallic compounds SnPd 2 and ZnPd 2 as test materials. Stability tests resulted in an equimolar mixture of 0.5 M methanol and 0.5 M KOH being the optimum composition of the electrolyte to obtain low corrosion rates and in ZnPd 2 being less stable than SnPd 2 . The reported protocol is a first, easy-to-do step to investigate the stability of electrode materials which is a prerequisite for application as well as a knowledge-based development of new electrode materials.
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