Evidence of Mars-Van-Krevelen Mechanism in the Electrochemical Oxygen Evolution on Ni-Based Catalysts

Article abstract of DOI:10.1002/anie.202101698

Water oxidation is a crucial reaction for renewable energy conversion and storage. Among the alkaline oxygen evolution reaction (OER) catalysts, NiFe based oxyhydroxides show the highest catalytic activity. However, the details of their OER mechanism are still unclear, due to the elusive nature of the OER intermediates. Here, using a novel differential electrochemical mass spectrometry (DEMS) cell interface, we performed isotope-labelling experiments in ¹⁸O-labelled aqueous alkaline electrolyte on Ni(OH)₂ and NiFe layered double hydroxide nanocatalysts. Our experiments confirm the occurrence of Mars-van-Krevelen lattice oxygen evolution reaction mechanism in both catalysts to various degrees, which involves the coupling of oxygen atoms from the catalyst and the electrolyte. The quantitative charge analysis suggests that the participating lattice oxygen atoms belong exclusively to the catalyst surface, confirming DFT computational hypotheses. Also, DEMS data suggest a fundamental correlation between the magnitude of the lattice oxygen mechanism and the faradaic efficiency of oxygen controlled by pseudocapacitive oxidative metal redox charges.

Full text of DOI:10.1002/anie.202101698

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Evidence of Mars-Van-Krevelen Mechanism in the
Electrochemical Oxygen Evolution on Ni Based Catalysts
Authors: Jorge Ferreira de Araújo, Fabio Dionigi, Thomas Merzdorf,
Hyung-suk Oh, and Peter Strasser
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10.1002/anie.202101698
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Evidence of Mars-Van-Krevelen Mechanism in the Electrochemical Oxygen
Evolution on Ni Based Catalysts
Jorge Ferreira de Araújo, Fabio Dionigi, Thomas Merzdorf, Hyung-suk Oh and Peter
Strasser *
Department of Chemistry, Chemical Engineering Division, Technical University of
Berlin, Straße des 17. June 124, 10623, Berlin, Germany
*e-mail: pstrasser@tu-berlin.de
RESEARCH ARTICLE
Abstract
Water oxidation is a crucial reaction for renewable energy conversion and storage.
Among the alkaline oxygen evolution reaction (OER) catalysts, NiFe based
oxyhydroxides show the highest catalytic activity. However, the details of their OER
mechanism are still unclear, due to the elusive nature of the OER intermediates. Here by
using a novel differential electrochemical mass spectrometry (DEMS) cell interface, we
performed water isotope-labelled experiments in 18O-labelled alkaline electrolyte on
Ni(OH)2 and NiFe layered double hydroxide nanocatalysts. Our experiments confirm the
occurrence of Mars-van-Krevelen lattice oxygen evolution reaction mechanism in both
catalysts to various degrees, which involves the coupling of oxygen atoms from the
catalyst and the electrolyte. The quantitative charge analysis suggests that the
participating lattice oxygen atoms belong exclusively to the catalyst surface, confirming
DFT computational hypotheses. Also, DEMS data suggest a fundamental correlation
between the magnitude of the lattice oxygen mechanism and the faradaic efficiency of
oxygen controlled by pseudocapacitive oxidative metal redox charges.
Keywords: Alkaline OER Catalyst, Differential Electrochemical Mass Spectrometry,
Isotope 18O, Lattice Oxygen Evolution
1
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Introduction
Future large-scale electrochemical generation of fuels and chemicals at
electrolyzer cathodes will require reactions and catalysts at the counter anode that
facilitate the release of protons (H+) and electrons (e–) with maximum efficiency. The
electrocatalytic oxygen evolution reaction (OER) or electrochemical water oxidation
reaction leading to the formation of O2, electrons, and protons is such a key counter
reaction. The OER catalysts activity is assumed to be kinetically controlled by surface
binding energies between catalytic active surface sites and reactive oxygenated
intermediate species [1]. A molecular understanding of energy- and cost efficient catalysts
[2] [3]
for the OER is vital for the design of advanced anodes for water electrolyzers
or
CO2-water co-electrolyzers for the generation of hydrogen or carbonaceous products,
respectively. [4] [5]
Ni -based materials are among the most active and energy-efficient OER catalysts
for alkaline electrolyzer anodes. The catalytic active state of Ni‐based catalysts is their -
phase that forms from the inactive - or -phase. The activity of -NiOOH was shown to
[6]
increase drastically after the addition of Fe under anodic conditions
with reported
[7]
optimal molar Fe:Ni ratios ranging between 0.1 and 0.5 . The catalytic active -NiFe
OER catalyst presents a layered crystalline structure with intercalated cations between
brucite-type metal oxide layers, in which the metal atoms occupy the center of edge-
connected octahedral. In contrast, the non-active -phase is characterized by intercalated
anions, and it is referred to as - NiFe layered double hydroxide (LDH). By structural
analogy with -NiOOH, the OER active deprotonated phase that forms by oxidation of
-NiFe LDH is referred to as -NiFe LDH. Over the years, distinct hypotheses about the
nature of the catalytically active site on the brucite layers were put forward [1b, 8]. Older
[8c]
[9]
hypotheses include single Fe active sites
or single Ni sites . Later, there emerged
heightened interest in the mechanistic role of the oxygen ligands of the edge-connected
M-O octahedra of the 2D brucite layers. [10]
Recently, a new model of the bulk structure of the catalytically active -NiFe LDH
[1b]
phase and of its active surface sites was brought to the fore.
What distinguished this
latest model from earlier ones was the primary catalytic role of specific 2-oxygen bridges
between neighboring Ni and Fe sites, while explicitly accounting for i) a reversible cation
and water intercalation in the interlayer space and ii) non-covalent interactions between
the interlayer species and the Ni/FeOx brucite layers. [1b] The reactivity of this structural
and mechanistic model relied on a direct involvement of surface lattice oxygen ligands in
the elementary catalytic reaction mechanisms, also referred to as a surface lattice oxygen
evolution reaction (LOER) mechanism. For NiFe LDH, a particular surface LOER
mechanism has been proposed where, following a Mars-van-Krevelen-type mechanism,
the surface lattice oxygen atom combines with oxygen from the electrolyte forming
molecular oxygen and leaving a vacancy, which is then filled by a hydroxide ion from the
[1b]
electrolyte.
The LOER character of state-of-art NiFe LDH anodes of alkaline water
electrolyzers has remained a contentious issue and calls for experimental verification or
dismissal using atomic-level analytics.
2
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To gain atomic-level experimental insights into operating NiFe-based OER
catalysts, most earlier studies employed a combination of in situ/operando X-ray
absorption spectroscopy (XAS)[8c, 11] Mössbauer spectroscopy [12], and voltammetric
techniques. However, this set of techniques is not suitable to discriminate whether or not
a LOER is present.[13] To achieve this, Differential Electrochemical Mass Spectrometry
(DEMS), where product sampling occurs directly from the electrified liquid-solid
interface is most suitable. Earlier DEMS studies on a bimetallic NiFe mass-selected
[14]
nanoparticle model catalyst
as well as a solvothermally prepared NiFe LDH OER
catalyst revealed important details on the stability, activity, and the faradaic efficiency [15]
[11a]. In almost all previous DEMS studies, the electrochemical flow cell architecture
involved a mono- or a dual thin-electrolyte layer type. In these DEMS cells, volatile
products generated inside a very thin electrolyte film between electrode and a
hydrophobic membrane transition into the differentially pumped vacuum system for
detection. Thin layer cells suffer from severe mass transport limitations and don’t sustain
large catalytic current densities. Moreover, they require large electrolyte volumes, which
is problematic for studies with expensive isotope labelled electrolytes available only at
micro- or milliliter scale. This is why new innovative bulk electrolyte DEMS cell as well
as small volume cell designs are needed.
In this contribution, we use a novel DEMS cell interface to experimentally test a
recently reported computational hypothesis regarding the participation of surface lattice
oxygen ligands, which is a LOER, in the OER catalysis on -/-NiFe LDH catalysts.
Isotope labelling results indeed suggest the presence of a LOER for both liquid precursor-
derived -NiFe LDH catalysts as well as a Fe-free -/-NiOxHy reference catalyst.
Crystalline Ir reference oxides showed no LOER. We further unravel and discuss a
previously overlooked fundamental correlation between the Faradaic efficiency and the
contribution of the LOER for various catalyst systems. This relation calls for chemical or
synthetic measures to minimize the LOER character of OER catalysts in order to maintain
high faradaic efficiency. Finally, we demonstrate the ability of the present millisecond
scale time-resolved DEMS technique to accurately deconvolute faradaic charge stored
in molecular oxygen from pseudocapacitive charge stored in the catalyst surface. This
previously inaccessible charge balance analysis quantifies the anodic Ni3+ and Ni4+redox
charges as a function of applied electrode potential under catalytic operating conditions.
Results and Discussion
Differential electrochemical mass spectrometry in a hanging droplet cell
A new differential electrochemical mass spectrometry (DEMS) setup (LIQUIDLOOP
GmbH) (Figure S1) was used in this work employing two distinct electrochemical
liquid/vacuum cell interfaces (Figure 1 and Figure S2). The dual thin-layer electrolyte
cell (“thin layer cell”) design is based on earlier similar approaches. [16] It is a robust and
reliable interface design that requires relatively large electrolyte volumes. It consists of
two horizontal parallel disk-shaped compartments with connecting liquid channels
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(Figure 1a). Computational details about the flow conditions and shear stress
distributions inside the dual thin film cell are provided in the Supporting Information and
Figure S3.
The “hanging droplet DEMS flow cell” (Figure 1b) is a new design (see Figure S2 and
S4). It was developed for isotope labelling experiments because it allows experiments
with microliter scale electrolyte volumes (typically 20-50 µl). This capability is useful
whenever expensive solvents or electrolytes are, for instance, isotope-labelled
compounds. The electrochemical measurements are performed inside a hanging
electrolyte droplet, the volume of which is maintained under constant in/out flow
conditions. The outlet flow ranged typically at 1 μl s–1. The inlet tube was placed at 2 mm
from the electrode. Reaction products were withdrawn through a capillary placed at 500
µm distance from electrode. The capillary is a concentric tube inside the inlet flow tube.
The reaction products are collected together with the electrolyte solution and introduced
into a disk-shaped compartment where a PTFE membrane acts as the interface between
the liquid and vacuum. A Ag/AgCl electrode served as reference electrode.
Figure 1. Electrochemical flow cells for differential electrochemical mass spectrometry (DEMS) setup. a)
the DEMS dual thin-layer electrolyte flow cell with close up of main components: working electrode, WE,
reversible hydrogen electrode (RHE) as reference electrode, RE, and the Pt counter electrode, CE; 1) Teflon
gasket between each cell stack, 2) the inlet channel, 3) the outlet channel, 4) the internal channel and 5) the
bulk flow channel stack. b) Illustration of the DEMS hanging droplet cell design showing the actual
electrolyte droplet and the main components working electrode (WE) and Teflon mask, the glass capillary,
the Ag/AgCl reference electrode, RE, and the Pt counter electrode, CE.
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Mass spectrometric and faradaic voltammetry in non-labelled conditions
The OER catalysts addressed here comprised a -/-NiFe LDH and a -Ni(OH)2 / -
NiO(OH)x powder thin film catalysts and were compared to a crystalline Ir oxide
electrocatalyst.
At the outset of this study, the DEMS thin film cell was used to characterize the surface
voltammetry and O2 faradaic efficiency (“FEO2”) of the catalysts in non isotope-enriched
(non-labelled) environments with natural 16O/18O abundances. After a cyclic
voltammetric (CV) activation protocol, faradic voltammograms and simultaneous mass
spectrometric cyclic voltammograms (MSCV) were recorded between +0.5 VRHE and the
point when 6 mAcm-2 was reached at a scan rate of 5 mVs-1 (Figure 2). MSCVs tracked
16
16
the three mass currents of m/z = 32 of O2 (16O16O) (green line), m/z = 34 of O18O
(grey line), and m/z = 36 of 18O2 (blue line in Figure 2).
Figure 2. Mass spectrometric cyclic voltammograms (MSCVs), iMS, of a) the NiFe LDH and b) the -
Ni(OH)2 OER electrocatalyst. The data was obtained in the dual thin-layer DEMS flow cell setup in 0.1 M
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K16OH at a scan rate 5 mVs-1 up to current density around 6 mAcm-2. Plotted are signals of m/z=32 of
16O16O (green line), and m/z = 34 and 36 corresponding to the naturally abundant isotopes 16O18O (grey
line, intensity x10) and 18O18O (blue line, intensity x10).
The faradaic CVs of the NiFe LDH and Ni(OH)2 catalysts are plotted together with their
associated MSCVs in Figure 2a and b. The faradaic CVs obtained in the DEMS cell
were nearly identical in shape to those reported previously in conventional three electrode
cell and electrode setups, which validates the cell and electrode design.[17] In the anodic
scan direction, the MSCVs closely traced the CVs, while on the cathodic scans mass
currents revealed characteristic tailings. Such tailings may originate from slow diffusional
O2 transport out of/across the porous catalyst film [11a, 18]. However, we observed some
tailing in non-porous thin Ni oxide layers, as well (see Supporting Note 1), implying that
the origin of the tailings might be at least in part related to the charge (hole) storage
mechanism and slow discharge in Ni-based OER catalysts. No other volatile products
than oxygen (e.g. CO2 at m/z 44) were detected.
For the Ni(OH)2 catalyst, the m/z=32 MSCV featured an unexpected quite cathodic
16
(“low”) onset potential of O2 formation near +1.41 VRHE (green line in Figure 2b)
tracing closely the well-documented Ni(II+)(OH)2 → Ni(III+)OOH redox wave (red
16
line). This O2 generation, however, appears transient in nature, which points to an
incomplete reduction of Ni(III+)OOH to Ni(II+)(OH)2 during an earlier cathodic scan
resulting in oxidative charge trapped by the formation of poorly conductive Ni(OH)2
[19]
domains.
Once the electrode potential was swept anodically again, the Ni(OH)2 →
NiOOH oxidation re-occurred restoring a conductive catalyst layer. As a result of this,
the trapped hole charges could now discharge by reacting with the electrolyte molecules,
resulting in the transient evolution of molecular oxygen.
In bimetallic NiFe LDH catalysts, the partial overlap of the anodic Ni(OH)2 → NiOOH
wave with the voltammetric OER onset makes an accurate estimate of the onset potential
of sustained O2 evolution during potential sweep measurements difficult. MSCVs,
16
however, are able to provide them accurately. Figure 2a shows that the O2 evolution
onset of the NiFe LDH catalyst occurred at +1.47 VRHE, hence slightly more cathodic
compared to Ni(OH)2 (+1.55 VRHE ) (Figure 2b).
In Figure 2, the mean O2 faradic efficiency, FEO2, of NiFe LDH over the entire cyclic
voltammetric potential range (from and up to +0.5 VRHE) was 94%, while that of Ni(OH)2
was 60% ( see also Supporting Note 2).
The signals of the other two oxygen isotopes (16O18O and 18O2) were at least 2 orders of
magnitude smaller than that of 16O2. While the 16O18O signal displayed a weak rise at the
most anodic potentials, the signal of 18O2 was too low to discern a detailed
current/potential response.
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18O isotope-enriched mass spectrometric and faradaic voltammetry
To obtain deeper insight into the character of the OER reaction mechanism on the two
Ni-based OER catalysts, we performed DEMS experiments, during which the non-
labelled 16O-NiFe LDH and Ni(16OH)2 catalysts were catalytically operated in 18O
18
isotope-labelled electrolyte, prepared from solid K16OH dissolved in H2 O (99.3%
18
abundance O) to obtain a 0.1 M solution. These experiments were conducted in the
hanging droplet DEMS flow cell. The effective number of dissociated 16OH- ions deriving
from the 0.1 M K16OH contributed with an additional 0.18 % (ca. 500x excess) abundance
to the isotope mix (see Table S1). The electrode potential was swept three times from
+0.8 VRHE to +1.6 VRHE (inside the catalytic OER regime) and back at a scan rate of 10
mVs-1 and corresponding MSCVs were recorded (Figure 3).
Figure 3. a-d) MSCV and faradic CV curves in time domain of a) 16O-NiFe LDH (MSCV top, CV bottom)
and b) Ni(16OH)2 (MSCV top, CV bottom) recorded in 18O -enriched 0.1 M KOH electrolyte prepared using
H218O (99.3% 18O). a) and b) show the mass ion currents, iMS , of m/z = 36 18O2 (blue line), m/z = 34 18O16O
(grey line) and m/z = 32 16O2 (green line) related to the faradaic current iF (red line) and the applied potential
E (black line). The experimental m/z = 34 MSCVs, inside the dashed square boxes in a) and b), are shown
enlarged in panels c) and d). Beside the experimental m/z = 34 18O16O (grey line) traces, panels c) and d)
show the experimental mass current of m/z = 36 normalized to the maximum m/z = 34 mass current (dotted
blue line), as well as the theoretically expected m/z = 34 18O16O ion current based on the 16O atomic
abundance of 0.78% in the 0.1 M K16OH/H218O electrolyte (dotted black line). e) Comparison of the ratios
of mass spectrometric charges of evolved 16O18O (m/z = 34) and 18O18O (m/z = 36) over the first three
potential cycles for the Ni(OH)2 (cyan) and NiFe LDH (orange) (see details in Figure S5 and S6 and
corresponding 16O abundance in Figure S7). The dashed areas corresponds to the expected 16O18O/18O18O
ratio, 1.58%, based on the 16O atomic abundance of 0.78%. Error bars represent standard deviation from
the average of two measurements of the same catalysts on two different electrodes.
Figures 3a and 3b show the first MSCV and CV scans of the two catalysts. The prevalent
18
18
molecular oxygen isotope was O2 at m/z=36, originating from a O-18O coupling of
two electrolyte-derived intermediates. No m/z=32 mass signal of 16O16O was detectable.
The mass signal m/z =34 (18O16O) is the most relevant for our discussions. It formed in
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the coupling of two distinct oxygen isotopes. The 16O isotope may originate from residual
16
solvent H2 O or K16OH with a combined abundance of 0.78% (balance is 17O); or else,
and that is the key point of our discussion, from surface lattice or bulk lattice oxygen
ligands present in the catalyst at the beginning of the experiment. A comparison of the
theoretically expected m/z=34 (18O16O) ion current profiles derived from supporting
equation S4 (Figures 3c and 3d dotted black line) with the experimentally observed
18
18O16O profile (Figures 3c and 3d, grey line) revealed a significant O16O ion current
excess for both Ni based catalysts. For better comparison of the two experimental m/z=34
and m/z=36 MSCVs, the m/z=36 trace was normalized to the maximum intensity of
m/z=34 and is shown as blue dotted line in Figures 3c, and 3d. Figure 3e displays the
ratio of the experimental mass ion charges of 16O18O and 18O18O, i.e. their integrated mass
currents, 푖푀푆, for the NiFe LDH catalyst (orange bars) and the Ni(16OH)2 catalyst (cyan
bars) over each cycle. The significant excess in the mixed isotope 18O16O oxygen
evidences the transient participation of lattice oxygen atoms at the surface or/and the bulk
of the catalyst in the oxygen evolution reaction mechanism. While the ratios in Figure 3e
is expected to decline by the gradual replacement of 16O by 18O on the catalyst surface,
the low amount of lattice oxygen participating to the LOER probably requires more cycles
to show the expected declining behavior. Kinetically further relevant is the cathodic shift
of the onset potential of the cross-coupled 18O16O isotope compared to 18O18O (Figure 3c
and 3d inset). The coupling of lattice oxygen atoms with solvent oxygen atoms appears
kinetically, which may have to do with the ready initial availability of 16O ligands on the
catalyst surface, while the 18O18O product requires the adsorption of two solvent
molecules. The formation of m/z= 32 16O16O remained at noise level at all times, showing
that the direct coupling of surface or bulk lattice oxygen atoms is unlikely.
We conclude the existence of a kinetically favored lattice oxygen evolution reaction
(LOER) process on the two Ni-based OER electrocatalysts. Note we use the term LOER
regardless whether the oxygenated ligand (OH, O) from the catalyst lattice belonged to
the surface or to the bulk.
To get further insight in the gradual exchange of oxygen atoms between the catalyst
surface and electrolyte, we kept tracing the oxygen isotope ratios in non-labelled
electrolyte after an electrolyte exchange. Non-labelled electrolyte was continuously flown
over the catalyst surface to ensure a complete exchange of the electrolyte.
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Figure 4. The evolution in the atomic fraction of 18O of the total DEMS charge of evolved oxygen
measured in non-enriched H216O-based electrolyte for a) NiFe LDH and b) Ni(OH)2. The catalysts were
previously cycled into the OER range in 18O isotope-enriched electrolyte. A rinsing step with non-enriched
H216O was conducted before the experiments shown here. Shown are data obtained from MSCVs over the
first four cycles (see also Figure S8 and S9). The bars represent the experimental 18O fraction (full blue
bars) and the expected 18O fraction based on the natural isotope abundance (hashed bars).
Figure 4 shows the evolution of the experimental (blue bars) vs. the natural (hashed bars)
18
atomic abundance of O during four potential cycles for both Ni-based catalysts. The
data revealed a nearly 3-fold and more than 4-fold higher 18O abundance on the first cycle
for Ni(OH)2 and NiFe LDH, respectively, which now reflects the opposite oxygen isotope
16
18
exchange between H2 O electrolyte and catalyst, following the measurements in H2 O-
based electrolyte. The absolute 18O isotope excess remained lower than that of 16O before,
which indicates that only a fraction of the catalyst surface is actually contributing to the
oxygen exchange processes. The faster depletion in 18O of NiFe LDH vs 18O of Ni(OH)2
is fully consistent with its higher OER catalytic activity (Figure 2).
From the isotope labelling experiments, we derived the percentage of catalyst oxygen
atoms participated in the LOER mechanism. Over one potential cycle, this ratio ranged
at 2.9 % for NiFe LDH and at 3.6 % for Ni(OH)2 (see Supporting Note 3).These numbers
suggest a minute contribution of the lattice oxygens of the catalysts, conceivably due to
a limited accessibility of metal/oxygen moieties at the surface of the catalyst. For
comparison, the ratio of electrochemically reactive Nickel atoms was evaluated from the
pre-catalytic anodic voltammetric charge under the assumption of a 1 electron transfer
per Nickel center (see Supporting Note 4). For the NiFe LDH catalyst, the estimate of
the electrochemically redox active Ni amounted to 1.7% of the total Nickel atoms
evidencing limited accessibility. In the case of Ni(OH)2, the ratio exceeded unity under
the 1 electron/Ni atom assumption, evidencing redox transition from Ni2+ to Ni3+ and Ni4+.
This analysis suggests a quite distinct electrochemical Ni accessibility between the two
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electrocatalysts. A role of the distinctly difference morphologies in terms of nanoplatelet
size combined with a role of Fe appears plausible. This analysis is consistent with the
existence of a LOER process occurring only at the surface of the nanoplatelets (and at a
limited number of internal sites accessible through cracks, defects and edges of their
polycrystalline domain structure [1b]), in agreement with the absence of bulk lattice
involvement in the OER mechanism demonstrated by Roy et al. [14] A bulk LOER process,
involving sites deep inside the LDH interlayer regions in the center of the crystalline
domains, would result in a much larger lattice oxygen contribution than that observed
here.
To compare the contribution of the LOER mechanism of the Ni based catalysts with
another benchmark catalyst, we performed similar isotope-labelling DEMS OER
experiments on an Iridium oxide catalyst in HCl electrolyte. [18]
Figure 5. a) Mass spectrometric cyclic voltammograms (MSCVs) of the IrOx catalyst during OER in 0.5
M HCl in 18O isotope labelled electrolyte (97% abundance). Plotted are the experimental mass currents iMS
of m/z = 36 18O2 (blue line, top), experimental iMS of m/z = 34 18O16O (grey line, center), the expected iMS
of m/z = 34 18O16O (dotted black line, center) based on the 18O abundance in the electrolyte, and the applied
electrode potential sweep (black line, bottom). b) Trend in mean faradaic efficiency of O2 (FEO2), evaluated
over the entire potential cycle shown in Figure 2 plotted versus 18O16O/18O2 percentage ratio of IrOx, NiFe
LDH and Ni(OH)2. FEO2 data of IrOx catalyst taken from reference [18]
.
Figure 5 shows the data analysis of the Ir oxide DEMS experiments. In comparison to
the theoretical m/z=34 mass current that is expected based on the 18O isotope enrichment
of the electrolyte (dotted black line in Figure 5a, center), the experimental m/z=34 mass
current (grey line in Figure 5a, center) was essentially a match. This evidenced a
negligible contribution of lattice oxygen atoms and thus, unlike the Ni-based catalysts,
suggested no significant contribution of a LOER mechanism for the Ir catalyst.
While studies of LOER mechanisms on Ir catalysts are sparse, there are a number of
[20]
LOER studies for hydrous and crystalline Ru oxides.
A presence of LOER was
reported on porous RuOx [20c] and nanocrystalline RuOx [20b], but not on crystalline rutile
RuO2 nanoparticles of ~50 nm, and neither on well-defined (100), (110), (101), and (111)-
oriented rutile RuO2 surfaces [21]. For Ru catalysts, structural arguments were put forward,
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i.e. LOER mechanisms are likely to occur on amorphous or hydrous phases with their
large number of lattice defects, their undercoordinated sites and their high degree of
[21]
redox-active surface hydroxylation giving rise to large pseudo capacitance
.
Supporting this hypothesis further, a more pronounced presence of LOER was observed
for Ru0.9Ni0.1O2-δ than for RuO2 [20b]: Ni leaching is known to result in a lattice-defective,
redox-active hydroxylated surface with an elevated ratio of undercoordinated sites. In
view of the Ru results, the crystalline nature of our IrOx catalyst may explain the absence
of a LOER mechanism.
Similar structure-mechanism relations can be invoked to account for the presence of a
LOER mechanism on NiFe LDH and Ni(OH)2. Even though the prepared NiFe LDH and
Ni(OH)2 catalysts displayed bulk crystallinity (Figure S10), their catalytic active γ-NiFe
LDH and γ-NiOOH structures feature water intercalation and hydroxylated surfaces,[1b]
the nanoplate morphology of which favored undercoordinated edge sites. Similar to our
results, Shao-Horn and coworkers demonstrated LOER mechanisms in highly covalent
perovskites that show pH-dependent activity (La0.5Sr0.5CoO3−δ, Pr0.5Ba0.5CoO3−δ and
SrCoO3−δ), while less covalent and pH-independent LaCoO3 lacked LOER [20a]. Those
perovskites contain alkaline earth metals (Ba and Sr), which easily dissolve in the
electrolyte and result in the formation of surface oxyhydroxides of amorphous and,
possibly, hydrous nature, with high number of undercoordinated sites, as it has been
shown for BaSrCoFe perovskite [22]. Finally, LOER was also observed for spinel Co3O4
[23], the surface of which reconstructed under OER in amorphous CoOOH [24]. Along these
lines, Doyle et al. [25] suggested that hydrous transition metal oxides show pH-dependent
[17, 26]
activity. Indeed, NiFe LDH follows a (super-Nernstian) pH-dependence activity
and so does γ-NiOOH [27] and both are characterized by a LOER.
,
Following the most recent structural hypothesis as to the active surface site and ligand on
γ-NiFe LDH and γ-NiOOH [1b], the successful observation of a LOER mechanism
requires the presence of catalytic active surface lattice 2-OH ligands that perform a O-O
coupling with incoming electrolyte molecules. An initial, very rapid exchange of lattice
OH with OH- or H2O from the electrolyte, however, at the outset of voltammetric scans
and outside the OER potential range may prevent the experimental observation of the
LOER. To ensure the analytical detection of an existing LOER, the following conditions
need to apply: i) the catalyst has to continuously expose pristine surface facets with not-
yet exchanged oxygen ligands due to a morphological decomposition, iii) the time of the
voltammetric pretreatment should be kept at a minimum, and iii) the time resolution of
the DEMS analysis has to be sufficiently high. Otherwise, failure to detect a LOER
remains inconclusive.
Based on these arguments, we tend to attribute the lack of a LOER mechanism on the
[14]
surface of electrochemically activated NiFe alloy nanoparticles
to their stable bulk
structure combined with rapid ligand exchange prior to DEMS detection, even though
differences in pretreatment protocols may have a role, as well. Indeed, differences in the
XAS-determined local structure between NiFe LDH and an electrochemically Fe-
activated Ni (hydr)oxide have been recently reported by Hu and coworkers [28], suggesting
11
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10.1002/anie.202101698
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that local structural differences might exist which will have implications in the OER
mechanism.
Apart from quantitative estimates of the contributions of LOER mechanisms, derived
from the ratios of QMS / QMS in Figure 3e, our DEMS analysis revealed another
34
36
previously overlooked kinetic-mechanistic correlation, as shown in Figure 5b. The
contribution of the LOER mechanisms of the catalysts scaled very closely with their
faradaic efficiency of O2, FEO2. The catalyst with larger LOER contribution suffered from
lower efficiency, that is, more holes injected in the catalyst were stored as oxidative
pseudocapacitive charge in redox-active metal centers, rather than being used to generate
oxygen. Excess pseudocapacitive anodic charge, however, is known to promote undesired
[29]
catalyst corrosion pathways.
In conclusion, from a charge efficiency point of view,
significant LOER contributions appear undesirable as they appear to be linked to low
FEO2.
DEMS based deconvolution of pseudocapacitive charge and the effective chemical
state of Ni under OER
To learn more from the DEMS data about the chemical state of the Ni catalyst during
OER, we conducted a more detailed charge analysis of the CV, 푖퐹 , and the faradaic
MSCV, 푖퐹퐷,퐸푂푀2 푆, of -Ni(OH)2 / -NiO(OH). From the faradaic current density 푖퐹 the total
anodic oxidative charge 푄퐹푡표푡 injected into the catalyst can be estimated. 푄퐹푡표푡can be
deconvoluted into three different components: 1) the faradic charge 푄퐹,푂퐸푅
퐷퐸푀푆 , (grey area)
associated with O2 evolution from solvent molecules, 2) the faradic charge 푄퐹퐷,퐸퐿푂푀퐸푆푅 (cyan
area) associated with evolution of mixed isotope O2 due to LOER, 3) the Ni oxidation
charge, 푄퐹,푁푖, consumed for redox state changes of the Ni centers.
12
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10.1002/anie.202101698
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Figure 6. Deconvolution of the total faradaic charge under 푖퐹 (red line) into faradaic charge and
pseudocapacitive oxidative metal charge: The charge under the faradic mass spectrometric cyclic
voltammogram (MSCVs) of Ni(OH)2 , 푄퐹퐷,퐸푂푀2 푆, splits into 푄퐹퐷,퐸푂푀퐸푅푆 (grey area, 86% of 푄퐹퐷,퐸푂푀2 푆) and the LOER-
퐷퐸푀푆
퐹,퐿푂퐸푅
퐷퐸푀푆
퐹,퐿푂퐸푅
derived 푄
(cyan color, 14% of 푄퐹퐷,퐸푂푀2 푆. The percentage of 푄
in respect to the total 푄퐹퐷,퐸푂푀2 푆 might
represent a lower limit due to the exchange of surface lattice hydroxides with electrolyte). The faradaic O2
efficiency, FEO2, is dependent on the potential window considered: FEO2= 82% for purple potential range,
i.e. without the Ni redox wave 1; FEO2 = 62% for pink potential range including the Ni redox waves. Only
anodic faradaic currents were included in the analysis to exclusively account for anodic processes
(molecular O2).
Figure 6 shows the deconvolution of the faradaic current 푖퐹 and the faradic mass
퐷퐸푀푆
퐹,푂2
spectrometric current 푖
for the first potential cycle (cf. Figure 3e) and their
퐷퐸푀푆
퐷퐸푀푆
respective charges 푄퐹푡표푡 and 푄퐹퐷,퐸푂푀2 푆. 푄
splits into the charge 푄
associated with
associated with
퐹,푂2
퐹,퐿푂퐸푅
퐷퐸푀푆
퐹,푂퐸푅
the lattice oxygen mechanism (cyan area), and into the charge 푄
oxygen formed from the electrolyte (grey area). The following relations for the mean
faradic efficiency hold:
푡표푡
퐷퐸푀푆
퐷퐸푀푆
퐹퐸푂2 = 푄퐹퐷,퐸푂푀2 푆 푄퐹 = 푄퐹,푂2
(푄퐹,푂2 + 푄퐹,푁푖
)
(1)
(2)
⁄
⁄
퐷퐸푀푆
퐷퐸푀푆
퐹,푂퐸푅
퐷퐸푀푆
푄
= 푄
+ 푄
퐹,퐿푂퐸푅
= ((1 − 푥퐿푂퐸푅) ∙ 푄퐹퐷,퐸푂푀2 푆 +푥퐿푂퐸푅 ∙ 푄퐹퐷,퐸푂푀2 푆
)
퐹,푂2
Where 푥퐿푂퐸푅 is the percentage ratio of the lattice oxygen with respect to all oxygen. From
the number ratio = Q34/Q36 (cf. Figure 3e) we estimate 푥퐿푂퐸푅 to be 14% (Supporting
Note 3). Mean FEO2 values were calculated to 62% (pink potential window in Figure 6).
In other words, 38% of 푄퐹푡표푡 is oxidative charge 푄퐹,푁푖 that was injected into Ni atoms and
served to increase the Ni redox state. If the Ni+2/+3 redox charge of peak “1” was excluded
13
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10.1002/anie.202101698
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from 푄퐹푡표푡 by narrowing the integrated potential window, a FEO2 of 82 % ensued (purple
potential window in Figure 6). However, this still left 18% of 푄퐹푡표푡 unaccounted for,
which was evidently used for the further oxidation of Ni3+ under peak “2”. We split the
total metal charge 푄퐹,푁푖 into the Ni2+/3+ transition (charge under peak 1) and the
subsequent Ni3+/4+ transition (convoluted with the OER charge under peak 2 of 푖퐹 )
according:
푄퐹,푁푖 = 푄퐹,푁푖2+/3+ + 푄퐹,푁푖3+/4+
.
(3)
From data in Figure 6 and the relations in Supporting Note 2 we obtain
푄퐹,푁푖3+/4+ = 0.6 푄퐹,푁푖2+/3+
(4)
Our charge balance analysis implies the formation of Ni4+; more importantly, it suggests
2
that more than half and almost ⁄ of all Ni centers of the Ni(OH)2 catalyst have reached
3
the Ni4+ state inside the OER range. This is excellent agreement with independent
measurements of the mean Ni oxidation state of +3.6 for γ-NiOOH by previous XAS
[6c, 25, 30]
studies
and fully consistent with the classical Bode redox model of Ni
oxyhydroxides. [30]
For the NiFe LDH catalyst, the faradaic contribution of the evolved O2 was distinctly
different (Figure S11). The DEMS-based evaluation of 푄퐹,Ni2+/3+ was no longer
possible, since the Ni2+/3+ redox process had merged with the OER voltammetric profile.
Indeed, the oxidation states of Ni and Fe during catalytic OER are still being debated.
Earlier XAS measurements on unsupported NiFe LDH suggested a large portion of Ni to
remain in a +II state during OER, while Ni4+ remained below 4% [31]. By contrast, higher
pH, supported catalysts or very thin catalyst films showed increased ratios of Ni4+. In our
present study of unsupported NiFe LDH, the catalyst exhibited a large mean FEO2=90%,
which implied little metal redox charge, which is consistent with the low Ni4+ ratios
reported by Görlin et al. for unsupported NiFe- based catalyst in 0.1 M KOH. [11a, 26, 31]
In summary, the DEMS-based faradaic oxygen efficiency and charge analysis is able to
deconvolute faradaic molecular oxygen charge from pseudocapacitive redox metal
charge. It can be used to extract independent estimates of the chemical state of the catalyst
under catalytic reaction conditions. The metal centers of the NiFe LDH catalyst appear to
be in a less oxidized state compared to the Ni centers of the Fe-free Ni(OH)2; however,
NiFe LDH outperforms the Fe-free catalyst in catalytic reactivity (Figure 2), which
speaks to the high intrinsic activity of the active NiFe LDH active sites.
Conclusions
The present work has revealed new mechanistic aspects of the oxygen evolution process
on the surface of Ni-based OER electrocatalysts in alkaline environments.
14
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10.1002/anie.202101698
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To achieve these insights, we have first presented a versatile new differential
electrochemical mass spectrometry (DEMS) liquid/vacuum cell interface, referred to as
“hanging droplet cell”. The cell design addresses the need of minimum electrolyte flows
where expensive isotope-labelled reagents or solvents are involved. The usefulness of the
new DEMS cell was demonstrated in the study of the OER mechanism of a Ni(OH)2 and
a NiFe LDH catalyst in 18O-labelled electrolyte.
Characteristic 16O-18O isotope DEMS data suggested that the mechanism of catalytic O-
O bond formation involves, to a small portion, lattice oxygen atoms at the catalyst surface.
This observation was more evident in Ni(OH)2 than in NiFe LDH, even though valid for
both catalysts. During this so-called lattice OER (LOER) mechanism, oxygen atoms from
the catalyst lattice are continuously consumed. In the present case of a LOER Mars-Van-
Krevelen mechanism, the lattice oxygen atoms are continuously substituted by oxygen
atoms from the electrolyte. In parallel to the LOER mechanism, oxygen evolves from
18
H2 O resulting in 18O-18O. A relation between LOER, faradic efficiency, the
amorphous/hydrous catalyst structure, and its pH-dependent activity is hypothesized and
discussed. The case of a Mars-Van-Krevelen LOER mechanism has important
implications for future designs or models of OER electrocatalysts that now have to
consider the role and the binding of lattice atoms ligands, as well. This study highlights
the importance of understanding the surface atomic structure of oxides to tune their
catalytic activity.
Acknowledgement
Financial support by the German Research Foundation (DFG) through Grant Reference
Number STR 596/12-1 (Paketantrag – Trägerwechselwirkungen) is gratefully
acknowledged. Authors acknowledge partial funding by the DFG under Germany’s
Excellence Strategy – EXC 2008/1 – 390540038 (zum Teil gefördert durch die Deutsche
Forschungsgemeinschaft (DFG) im Rahmen der Exzellenzstrategie des Bundes und der
Länder – EXC 2008/1 – 390540038). Federal ministry for economic affairs and energy
(Bundes-ministerium für Wirtschaft und Energie, BMWi) under grant number
03EIV041F in the collaborative research project "MethQuest” in the group “MethFuel”
and the German Re-search Foundation (DFG) STR 596/8-1 is grateful acknowl-edged.
15
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Graphical Abstract: Differential electrochemical mass spectrometry using a hanging
droplet cell confirmed the occurrence of Mars-van-Krevelen lattice oxygen evolution
reaction mechanism in both Ni(OH)2 and NiFe layered double hydroxide nanocatalysts.
This mechanism involves the coupling of oxygen atoms from the catalyst surface and the
electrolyte.
17
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