Stability and Catalytic Performance of Reconstructed Fe3O4(001) and Fe3O4(110) Surfaces during Oxygen Evolution Reaction Authors: Matthias Müllner, Michele Riva, Florian Kraushofer, Michael Schmid, Gareth S. Parkinson*, Stijn F.L. Mertens* and Ulrike Diebold
Institute of Applied Physics, TU Wien, Wiedner Hauptstrasse 8-10/134, 1040 Vienna, Austria
Abstract
Earth-abundant oxides are promising candidates as effective and low-cost catalysts for the oxygen evolution reaction (OER) in alkaline media, which remains one of the bottlenecks
in
electrolysis
and
artificial
photosynthesis.
A
fundamental
understanding of the atomic-scale reaction mechanism during OER could drive further progress, but a stable model system has yet to be provided. Here we show that Fe3O4 single crystal surfaces, prepared in ultra high vacuum (UHV) are stable in alkaline electrolyte in the range pH 7–14 and under OER conditions in 1 M NaOH. Fe3O4(001) and (110) surfaces where studied with X-ray photoelectron spectroscopy, low-energy electron diffraction and scanning tunneling microscopy in UHV, and atomic force microscopy in air. Fe3O4(110) is found to be more reactive for oxidative water splitting than (001)-oriented magnetite samples. Magnetite is electrically conductive, the structure and properties of its major facets are well understood in UHV. With these newly obtained results we propose magnetite (Fe3O4) as a promising model system for further mechanistic studies of electrochemical reactions in alkaline media and highly oxidizing conditions. Corresponding authors:
[email protected],
[email protected] 1
1. Introduction Renewable electricity resources such as solar or wind power are a promising alternative to decreasing reserves of fossil fuels 1. The fluctuating nature of these energy sources poses great challenges however, because energy storage solutions are required to compensate for downtimes in production and give relief to increasingly strained electricity grids. Water electrolysis is a promising technology for the storage of surplus electricity in the form of hydrogen. An alternative to electricity-driven electrolysis would be direct, photo-driven water splitting or artificial photosynthesis, where solar energy is stored in the form of hydrogen or more complex, carbon-based energy-rich chemicals. In all these electrochemical processes the oxygen evolution reaction (OER), i.e., 2 H2O ⟶ 4 H+ + O2 + 4 e− in acidic media or 4 OH− ⟶ 2 H2O + O2 + 4 e− in alkaline media, is one of the bottlenecks due to slow kinetics at the anode surface.
In acidic media, only noble metals such as Ru, Pt or Ir show promising OER activity and stability. In contrast, in alkaline solution many inexpensive, abundant metals and their alloys show comparable or even better catalytic performance than noble metals2. Nonetheless, alkaline water electrolysis has long been considered inefficient compared to acidic electrolysis, where the electrodes with Ir and Pt catalysts are separated by proton conducting polymer electrolyte membranes (PEM). Recent results however suggest that alkaline cells with Ni-based catalysts could be more efficient with respect to acidic systems if thinner hydroxide conducting PEMs than those typically used in current systems would be used 3, 4. The search for the best OER catalyst in alkaline media is ongoing, and transition metal oxides containing Ni, Co and Fe have been proposed as suitable candidates. Recently, Ni0.9Fe0.1Ox was
2
demonstrated to be the most active water oxidation catalyst in basic media, with OER activity roughly one order of magnitude higher than IrOx control films 5. A recent review6 focused on NiFe-based (oxy)hydroxide catalysts, and determined the optimum range of Fe concentration in Ni (oxy)hydroxides to be 10-50%. The reason for the excellent activity of these compounds is still debated however, and strategies to identify the catalytically active sites have yet to be provided. The synergistic role of Fe and Ni cations on the OER mechanism is unexplained, and it is not yet possible to assess to which extent factors such as the Fe and Ni oxidation states are important. To obtain a detailed understanding of the mechanisms governing chemical reactions on surfaces, a useful approach has been to reduce the level of complexity using model systems
7-9.
Studies on noble-metal single-crystal samples have laid the
foundation for the field of physical electrochemistry and electrochemical surface science at the atomic level. Atomically well-defined metallic surfaces can be reproducibly prepared either in UHV, by flame annealing in air, or by annealing in controlled atmospheres; early studies have investigated surface phenomena within the stability region of water. Experiments focused on adsorption of molecules underpotential deposition of metals 14,
12
10-11,
and electro-oxidation of metal electrodes
13-
as well as the activity and stability of bimetallic systems 15-16.
However, in highly oxidizing conditions, such as during OER, all metals are oxidized. This can lead to electrochemical roughening or dissolution of well-defined singlecrystal surfaces, and thus to a loss of atomic-level control 17-18. It is therefore reasonable to expand such investigations to well-defined metal-oxide model surfaces, which are expected to be intrinsically more stable than metals in an 3
oxidizing environment. The choice of a suitable model system is, however, crucial since it should: 1) mimic the system on which the reaction under investigation occurs as closely as possible; 2) be electrically conductive; 3) have a well-understood surface structure; and 4) be stable under reaction conditions. Upon introduction of a new model system, fulfillment of these criteria should be explicitly evaluated: in this respect, UHV-based surface science provides guidelines on how to address 1) – 3). One approach is to grow well-defined oxide thin films on metal substrates. This is particularly useful when the oxides under investigation are non-conductive or when bulk single crystals (of sufficient size) are not available. Fe3O4(111)
19
and FeO(111)
thin films grown on Pt(111) have been shown to be stable systems for CO and H2 reduction, and show good stability in liquid water
20.
Cobalt oxide is another
promising material for OER in alkaline media. Interestingly, Co3O4 nanorods and nanocubes exposing (001) and (110) surfaces, respectively, differed in reactivity
21.
Whether this is due to different surface reactivity or other parameters remains to be seen. Pioneering electrochemical studies of UHV-prepared Co3O4 thin films
22
have
demonstrated a stability region in phosphate buffer at pH 10 at potentials between 0.33 to 1.33 VRHE was reported 22, but focused on the (111) facet. The alternative to thin film growth is to study bulk single crystals. TiO2, a widebandgap semiconductor, is a popular model system in UHV surface science since it can be made conductive by annealing in UHV 23, and recipes exist for the preparation of atomically well-defined surfaces. TiO2 is stable in a wide potential and pH region, and the surface-science approach has been recently used to investigate its photoelectrochemical properties. 24-26.
4
Another popular single-crystalline model system is the wide-bandgap semiconductor ZnO(0001)
27.
The stability region in electrolyte has been identified as a pH range
from 5.5 to 11, and slow dissolution below pH 5.5 has been shown directly by in situ by EC-AFM
28.
The search for a stable model system in realistic OER conditions,
namely highly alkaline media (pH > 13) is ongoing. This work investigates magnetite (Fe3O4) as a promising model system for the OER in alkaline media. Fe3O4 belongs to the family of spinels, which are of great interest for OER
29.
It is conductive at room temperature
30
and methods to prepare
homogeneous surface terminations on various low-index facets of single crystals are well established in UHV
31.
The Fe3O4(110) surface reconstructs with a (1 × 3)
periodicity due to regularly spaced (111) nano-facets, which have a lower surface energy than a (110) termination 32-33. The Fe3O4(001) surface exhibits a (√2x√2)R45° reconstruction originating from the ordering of subsurface iron vacancies and interstitials
34.
Interestingly, the reconstructed Fe3O4(001) surface provides well-
defined sites for the adsorption or incorporation of various metal atoms including Ni or Co
35-36,
offering an ideal template for the investigation of the OER on bimetallic
model Ni/Fe oxides and hydroxides with atomic-scale control. With this in mind, it is imperative to first test the stability and reactivity of the Fe3O4 oxide substrate itself under reaction conditions. Pourbaix diagrams for the iron-water system at room temperature indicate the tendency of Fe3O4 to be oxidized to Fe2O3 in solutions with pH > 12.5, and at potentials required for water oxidation 37. However, all cations in the four outermost layers of the reconstructed Fe3O4(001)-(√2x√2)R45° surface already have an Fe3+ like character 34, which may aid its stability in alkaline media.
5
In the current paper, we examine whether UHV-prepared (001) and (110) surfaces are stable in electrolyte in the pH range 7–14, whether the stability is affected when the samples are subjected to cyclic voltammetry under OER conditions, and whether the atomic structure affects reactivity. We show that the surface morphology remains unchanged after electrolyte exposure, and that the nanostructure of the surfaces also appear unaffected under OER conditions in 1 M NaOH, pH 14, as judged by ambient AFM. Comparing the onset for OER, the (110) facet appears more reactive than (001). We conclude that Fe3O4 is a suitable substrate for future studies into fundamental aspects of the OER.
6
2. Experimental Methods Two mineral Fe3O4(001) single crystals (top-hat-shaped samples from Surface Preparation Laboratory; base = 10 x 6 x 1 mm3, top = 6 x 6 x 2 mm3, miscut
