The stability number as a metric for electrocatalyst stability benchmarking

TLDR: In this paper, the authors introduce a metric to explore the dissolution processes of various iridium-based oxides, defined as the ratio between the amounts of evolved oxygen and dissolved iridium.

Abstract: Reducing the noble metal loading and increasing the specific activity of the oxygen evolution catalysts are omnipresent challenges in proton-exchange-membrane water electrolysis, which have recently been tackled by utilizing mixed oxides of noble and non-noble elements. However, proper verification of the stability of these materials is still pending. Here we introduce a metric to explore the dissolution processes of various iridium-based oxides, defined as the ratio between the amounts of evolved oxygen and dissolved iridium. The so-called stability number is independent of loading, surface area or involved active sites and provides a reasonable comparison of diverse materials with respect to stability. The case study on iridium-based perovskites shows that leaching of the non-noble elements in mixed oxides leads to the formation of highly active amorphous iridium oxide, the instability of which is explained by the generation of short-lived vacancies that favour dissolution. These insights are meant to guide further research, which should be devoted to increasing the utilization of highly durable pure crystalline iridium oxide and finding solutions to stabilize amorphous iridium oxides.

Figures

Figure 1. Crystal structure of the investigated materials. (a) rutile IrO2; (b) double perovskite (e.g. 138 Ba2PrIrO6); (c) single perovskite (SrIrO3); (d) assumed structure of amorphous iridium oxide, gaps are filled 139 with intercalated water molecules (not shown); (e) leached double perovskite showing isolated IrO6 octahedra, 140 which will collapse into an amorphous structure; (f) leached SrIrO3 resulting in an “anatase” iridium oxide 141 structure. 142

Figure 3. Investigation of iridium dissolution during OER. (a) Detected iridium concentration in the 209 electrolyte during the 2 nd

Figure 5. Online observation of lattice oxygen evolution on (a) rutile Ir 18 O2 / Ir 16 O2 and (b) hydrous Ir 18

Figure 7. Investigation of S-Number and lifetime depending on the current load. (a, c) Stability-number (S-426 Number) plotted versus mass specific current density for powders (a) and geometric current density for sputtered 427 films (c). Ba2PrIrO6 was leached for 5 days in advance. (b, d) Calculation of the catalyst’s lifetime, based on 428 equation 1 for powders (b) and films (d). This approach assumes “steady state” dissolution and neglects 429 increased dissolution towards the end of life due to loss of surface area. In case of powder materials m equals the 430

Figure 4. Comparison of the investigated materials in terms of activity. (a) OER-activity of the powder 252 materials recorded with a linear scan of potential at 5 mV s -1

Figure 2. XPS results of pristine Ba2PrIrO6, amorphous IrOx and crystalline IrO2. (a) Ir 4f and (b) O 1s 189 spectra. Additional results of the leached Ba2PrIrO6 are presented in the supporting information (Fig. S2). 190 191 192 193 194 195

Full text

HAL Id: hal-02390894
https://hal.archives-ouvertes.fr/hal-02390894
Submitted on 3 Dec 2019
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entic research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diusion de documents
scientiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
The stability number as a metric for electrocatalyst
stability benchmarking
Simon Geiger, Olga Kasian, Marc Ledendecker, Enrico Pizzutilo, Andrea M
Mingers, Wen Tian Fu, Oscar Diaz-Morales, Zhizhong Li, Tobias Oellers, Luc
Fruchter, et al.
To cite this version:
Simon Geiger, Olga Kasian, Marc Ledendecker, Enrico Pizzutilo, Andrea M Mingers, et al.. The
stability number as a metric for electrocatalyst stability benchmarking. Nature Catalysis, Nature
Publishing Group, 2018, �10.1038/s41929-018-0085-6�. �hal-02390894�

HAL Id: hal-02390894
https://hal.archives-ouvertes.fr/hal-02390894
Submitted on 3 Dec 2019
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entic research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diusion de documents
scientiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
The stability number as a metric for electrocatalyst
stability benchmarking
Simon Geiger, Olga Kasian, Marc Ledendecker, Enrico Pizzutilo, Andrea
Mingers, Wen Fu, Oscar Diaz-Morales, Zhizhong Li, Tobias Oellers, Luc
Fruchter, et al.
To cite this version:
Simon Geiger, Olga Kasian, Marc Ledendecker, Enrico Pizzutilo, Andrea Mingers, et al.. The stability
number as a metric for electrocatalyst stability benchmarking. Nature Catalysis, Nature Publishing
Group, 2018. �hal-02390894�

Page 1 of 26
The Stability-number as new metric for electrocatalyst stability benchmarking – 1
2
3
, 4
5
Simon Geiger
a,†,*
, Olga Kasian
a,†
, Marc Ledendecker
a
, Enrico Pizzutilo
a
, Andrea M. Mingers
a
Wen Ti
an Fu
b
, Oscar Diaz-Morales
b
, Zhizhong Li
c
, Tobias Oellers
d
, Luc Fruchter
c
, Alfred
Ludwig
d
, Karl J. J. Mayrhofer
a,e,f
, Marc T. M. Koper
b
, Serhiy Cherevko
a,e,*
6
7
a
Department of Interface Chemistry and Surface Engineering, 8
Max-Planck-Institut für Eisenforschung GmbH, 40237 Düsseldorf, Germany 9
b
Leiden Institute of Chemistry, Leiden University, Leiden 2300 RA, The Netherlands. 10
c
Laboratoire de Physique des Solides, C.N.R.S., Université Paris-Sud, 91405 Orsay, France 11
d
Institute for Materials, Ruhr-Universität Bochum, 44801 Bochum, Germany12
e
Helmholtz-Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), 13
Forschungszentrum Jülich, 91058 Erlangen, Germany 14
f
Department of Chemical and Biological Engineering, Friedrich-Alexander-Universität 15
Erlangen-Nürnberg, 91058 Erlangen, Germany 16
17
*
Corresponding authors: geiger@mpie.de, s.cherevko@fz-juelich.de
18
†
These authors contributed equally to this work 19
20
21
22
23
24
25
26
27
28
29

Page 2 of 26
Abstract 30
Reducing noble metal loading and increasing specific activity of oxygen evolution catalysts 31
are omnipresent challenges in proton exchange membrane (PEM) water electrolysis, which 32
have recently been tackled by utilizing mixed oxides of noble and non-noble elements (e.g. 33
perovskites, IrNiO
x
,
etc.). However, proper verification of the stability of these materials is 34
still pending. In this work dissolution processes of various iridium-based oxides are explored 35
by introducing a new metric, defined as the ratio between amount of evolved oxygen and 36
dissolved iridium. The so called Stability-number is independent of loading, surface area or 37
involved active sites and thus, provides a reasonable comparison of diverse materials with 38
respect to stability. Furthermore it can support the clarification of dissolution mechanisms and 39
the estimation of a catalyst’s lifetime. The case study on iridium-based perovskites shows that 40
leaching of the non-noble elements in mixed oxides leads to formation of highly active 41
amorphous iridium oxide, the instability of which is explained by participation of activated 42
oxygen atoms, generating short-lived vacancies that favour dissolution. These insights are 43
considered to guide further research which should be devoted to increasing utilization of pure 44
crystalline iridium oxide, as it is the only known structure that guarantees a high durability in 45
acidic conditions. In case amorphous iridium oxides are used, solutions for stabilization are 46
needed. 47
48
49
Graphical abstract 50
51
Keywords: oxygen evolution reaction, iridium, perovskites, stability-number, energy 52
conversion 53
54

Page 3 of 26
1. Introduction 55
Electrochemical water splitting is considered to play a key role in the new energy scenario 56
for the production of hydrogen, which can act as central energy carrier and as raw material for 57
the chemical industry. Still, the persistent challenges of this concept are (i) slow kinetics of 58
the oxygen evolution reaction (OER) and (ii) need of expensive materials as catalysts or 59
related components. Especially for proton exchange membrane (PEM) electrolysis, the acidic 60
environment caused by the membrane itself together with high anodic potentials limits the 61
choice of catalyst materials to expensive noble metals. The best known catalysts for OER 62
contain high amounts of scarce iridium that hampers large scale implementation of this 63
technology. Smart catalyst design is needed to decrease noble metal loadings and increase 64
specific activity and stability. 65
Various iridium-based mixed oxides
1-8
have been investigated as potential catalyst 66
material to tackle the mentioned challenges by increased specific activity and lower 67
percentage of expensive noble metals. Enhanced activity and apparently decent stability was 68
demonstrated in comparison to IrO
2
, Ir-black, or other benchmark materials. However, the 69
stability aspect needs more rigorous investigation. Especially non noble alkali or rare earth 70
elements are expected to be thermodynamically unstable in acidic electrolytes,
9
favouring the 71
formation of amorphous iridium oxide structures after leaching. The latter have been shown to 72
degrade significantly in acidic electrolyte during OER,
10-13
accentuating the need for further 73
understanding of degradation processes. 74
Most prominent examples are iridium-based perovskites recently investigated in acidic 75
electrolyte.
1,2
Initial studies on the usage of this material class in electrocatalysis originate 76
from Bockris and Otagawa,
14,15
who used alkaline electrolytes. Since then numerous studies 77
on the usage of perovskites for alkaline water splitting have been published.
16-25
Exceptionally 78
high OER activities were achieved for example by varying the occupancy of 3d orbitals of 79
surface transition metals
18
or tuning oxygen vacancies by means of straining.
21
However, 80
several groups brought up the important aspect of surface amorphization during OER.
26-29
81
May et al.
26
indicated, that especially those materials with high amorphization are the ones 82
that show high activity, expressing the need of further investigations on the number of 83
involved active sites. Even more in acid environment catalyst stability and amorphization is 84
an issue. Therefore a thorough investigation of specific activity and dissolution processes of 85
iridium-based perovskites in 0.1 M HClO
4
is presented in this work. 86

Frequently asked questions

Q1. What have the authors contributed in "The stability number as a metric for electrocatalyst stability benchmarking" ?

HAL this paper is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not.