Investigation of Water Dissociation and Surface Hydroxyl Stability on

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Investigation of Water Dissociation and Surface Hydroxyl Stability on Pure and Ni-Modified CoOOH by Ambient Pressure Photoelectron Spectroscopy Zhu Chen, Coleman X. Kronawitter, Iradwikanari Waluyo, and Bruce E. Koel J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b06960 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 17, 2017

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The Journal of Physical Chemistry

Investigation of Water Dissociation and Surface Hydroxyl Stability on Pure and Ni-Modified CoOOH by Ambient Pressure Photoelectron Spectroscopy Zhu Chen1, Coleman X. Kronawitter2, Iradwikanari Waluyo3 and Bruce E. Koel1*

1

Department of Chemical and Biological Engineering, Princeton University, Princeton, NJ 08544 2

Department of Chemical Engineering, University of California at Davis, Davis, CA 95616 3

Photon Science Division, National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY 11973

1

ACS Paragon Plus Environment


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Abstract

Water adsorption and reaction on pure and Ni-modified CoOOH nanowires was investigated using ambient pressure photoemission spectroscopy (APPES). The unique capabilities of APPES enable us to observe water dissociation and monitor formation of surface species on pure and Nimodified CoOOH under elevated pressures and temperatures for the first time. Over a large range of pressures (UHV to 1 Torr), water dissociates readily on the pure and Ni-modified CoOOH surfaces at 27 °C. With an increase in H2O pressure, a greater degree of surface hydroxylation was observed for all samples. At 1 Torr H2O, ratios of different oxygen species indicate a transformation of CoOOH to CoOxHy in pure and Ni-modified CoOOH. In temperature dependent studies, desorption of weakly bound water and surface dehydroxylation were observed with increasing temperature. Larger percentages of surface hydroxyl groups at higher temperatures were observed on Ni-modified CoOOH compared to pure CoOOH, which indicates an increased stability of surface hydroxyl groups on these Ni-modified surfaces.

2


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conversion devices, including water splitting

INTRODUCTION The interaction of water with surfaces

cells12-14 and metal-air batteries.5, 15-16

plays a key role in many applications including

heterogeneous

environmental

Metal oxide and (oxy)hydroxide catalysts

catalysis,

science,

are commonly utilized to effect low

and

overpotentials

for

OER

in

alkaline

electrochemistry. This interaction, which

conditions. A volcano-type relationship is

can involve H2O molecules, hydronium

frequently reported for these catalysts,

(H3O+) ions, and hydroxide ions (OH-), is

where optimal activity is observed when the

especially

water

interaction between the catalyst surface site

electrolysis half reactions—the hydrogen

and a key surface-bound intermediate is

evolution reaction (HER)1-3 and the oxygen

neither too strong nor too weak.17-19 As a

evolution reaction(OER)—which must be

result, scaling relationships based on the

driven

emerging

calculated free energy of formation of

renewable energy technologies.4-7 In the

surface intermediates have been established

case of OER, a complex reaction mechanism

and are used as descriptors for predicting

that involves four electron transfer steps and

and explaining trends in the OER activity of

multiple surface intermediates is believed to

different catalysts.20 Cobalt-based oxides

be the reason for the observed slow kinetics

and (oxy)hydroxides represent a promising

associated with most electrocatalysts.8-11 As

and interesting group of OER catalysts.

a result, a large overpotential is required to

These catalysts have demonstrated excellent

obtain high rates of reaction, which makes

OER activities that are comparable to the

OER the bottleneck reaction in many energy

most active precious metal catalysts in

important

efficiently

for

for

the

many

alkaline conditions.21-24 A typical reaction 3



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Page 4 of 28

mechanism proposed for the OER half

discovered that OER on Co3O4 is facilitated

reaction at oxide catalysts is shown in Eq.

by a dual site mechanism.28 There are two

(1-4), where * denotes a surface site.11, 25-26

types of surface active sites on Co3O4: a

H2O(l) + * HO* + H+ + e-

(1)

kinetically fast site consisting of two CoIII-

HO* O* + H+ + e-

(2)

OH surface species bridged by an O atom,

O* + H2O(l) HOO* + H+ + e-

(3)

and a slow site associated with formation of

HOO* O2 (g) + * + H+ + e-

(4)

isolated CoIV=O surface species.28 These

mechanism,

results illustrate the strong influence that

different rate-determining steps are proposed

different surface groups (i.e., OH* and O*)

for

have on OER activity. Recent studies using

Based

on

cobalt

this

oxide

general

and

(oxy)hydroxide

spectroscopy29

catalysts of different compositions and

Raman

structures. For example, Chen et al. has

absorption30 techniques have also revealed a

predicted

strong correlation between the structure of

that

Eq.

(2),

the

second

deprotonation step prior to the formation of

and

X-ray

Co-based catalysts and their OER activities.

O*, is the rate determining step, and

In a recent study, we showed based on

stabilization of such product species reduces

electrochemical impedance analysis that Ni

Co3O4.26

incorporation into CoOOH reduces the

However, either Eq. (1) or (3), the formation

interfacial charge transfer resistance and

of either OH* or OOH*, respectively, has

improves the stability of surface groups.31

been proposed to be the potential-limiting

Our results were in excellent agreement with

step depending on the crystal surface

results from DFT calculations by the

orientation of CoOOH.27 Using operando

NØrskov

FTIR spectroscopy, Frei and coworkers

influence of Ni doping in CoOOH towards

the

OER

overpotential

for

group

4


regarding the positive

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its OER activity27. However, there was no

adsorbed

direct evidence correlating Ni doping with

oxide34-36 and semiconductor37-39 surfaces,

the

A

which was only observed under near-

this

ambient pressure (1 to 10 TorrH2O). As a

information would be to monitor and

result, studying the water/solid interface of

compare the proportion of surface oxygen

pure and Ni-modified CoOOH under such

species at different temperatures on pure and

relatively high H2O pressures can provide a

Ni-modified CoOOH. To this end, ambient

better

pressure

spectroscopy

especially with respect to the surface

(APPES) is an excellent technique because it

composition and concentrations of surface-

provides information on the water/solid

bound OH and H2O species at conditions

interface at a molecular level, with high

closer to the environment encountered

surface and chemical sensitivity, but does

during practical catalysis. Herein, we report

not rely on an ultrahigh vacuum (UHV)

the use of APPES to investigate the

measurement environment. Previous X-ray

interactions of adsorbed water and hydroxyl

photoelectron spectroscopy (XPS) studies of

species with pure and Ni-modified CoOOH

H2O adsorption on Co(0001)32 and oxidized

catalyst surfaces at different temperatures

cobalt33 surfaces under UHV conditions

and H2O pressures to provide direct

have been reported, however, the adsorption

comparisons of water and hydroxyl group

and interaction of adsorbates at near-

stability

ambient pressure can differ greatly. One

oxyhydroxides, a type of oxide catalyst

example is the appearance of interactions

shown previously to be active for OER.22, 24,

between surface hydroxyl groups (OHad) and

31, 40-44

stability

straightforward

of

surface

way

to

photoemission

species. obtain

water

molecules

understanding

on

5


the

of

(H2Oad)

this

surface

on

interface,

of

these


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followed by chemical oxidation to form Ni-

EXPERIMENTAL SECTION Synthesis

of

pure

and

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Ni-modified

modified CoOOH nanowires on the stainless

CoOOH

steel mesh. Prior to APPES experiments, Ni

Pure and Ni-modified CoOOH materials

concentration was determined by XPS

were synthesized according to a previously

(Thermo

reported procedure.31 Briefly, 5 mmol of

monochromatic Al K-α source operated at

Co(NO3)2•6H2O (Aldrich, >99.99%) and 2.5

150

mmol of NH4NO3 (Aldrich, >99.0%) were

electrochemical characterizations of the pure

added to a mixture of de-ionized water (35

and Ni-modified CoOOH were performed

mL) and 30 wt.% ammonia (5 mL, Aldrich)

prior to the APPES experiments and the

and pre-heated to 85 °C for 1 h. A degreased

details of these experiments are outlined in

stainless steel mesh was added to the pre-

the Supporting Information. A

heated Co precursor solution and incubated

K-Alpha

W.

Additional

Plus)

using

physical

a

and

Ambient pressure X-ray photoelectron

at 85 °C for 12 h. The resultant material

spectroscopy

consisted of Co(OH)2 nanowires attached to

The APPES experiments were performed

the stainless steel mesh and this was then

at the undulator beamline 23-ID-2 (CSX-2)

subjected to chemical oxidation in 6 M

of the National Synchrotron Light Source II

NaOH (Aldrich, 99.99%) and 30 wt.% H2O2

at Brookhaven National Laboratory. The

at 45 °C for 8 h to form CoOOH nanowires

endstation consist of a load-lock connected

on

Nickel

to a chamber for XPS measurements at near-

incorporation was performed through wet-

ambient pressures. The base pressure of the

impregnation of the as-prepared Co(OH)2

APPES chamber was 2 x 10-9 Torr. The

sample in 1 mM Ni(NO3)2•6H2O solution

sample was heated by a button heater that

the

stainless

steel

mesh.

6


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was

capable

temperature

of

using a photon energy of 1100 eV. This was

thermocouple attached to a Ta plate that was

chosen to include the Co and Ni 2p regions

placed between the sample and the button

as well as to avoid overlapping of the XPS

heater. Briefly, X-rays were incident on the

core level emission lines with the Auger

sample through a 100 nm thick SiNx

emission from Ni and Co. Water vapor, after

window, which separated the XPS chamber

being degassed by multiple freeze-pump-

at high pressure from the synchrotron

thaw

beamline in UHV. The photoelectrons

introduced into the XPS chamber to achieve

emitted from the sample were collected by a

different pressures (PH2O) using a high

small aperture (0.3-mm dia.), which is the

precision variable leak valve. The binding

entrance

differentially-pumped

energy scale was calibrated by aligning the

hemispherical electron analyzer (Phoibos

Fermi energy level of the APPES survey

150, Specs). The sample was placed in close

spectrum of the pure and Ni-modified

(ca. 500 µm) proximity to the analyzer

CoOOH catalysts in isothermal or isobaric

entrance aperture in order to minimize

conditions. Co 2p, Ni 2p, and O 1s XPS

elastic

of

spectra were decomposed into components

photoelectrons in the gas phase. The angle

having a convoluted Gaussian (70%)—

between the incident X-rays and the emitted

Lorentzian (30%) lineshape after a Shirley

photoelectrons was set at 70°. The incident

background subtraction. The Co 2p3/2 and Ni

angle of the X-ray beam and the emission

2p3/2

angle of photoelectrons were 50° and 20°,

principle

inelastic

which

respectively, with respect to the surface

monitored by a chromel-alumel (K-type)

the

°C,

sample

normal. All APPES spectra were collected

and

900

a

was

to

of

reaching

scattering

cycles

where

7


of

fitted

peaks,

Milli-Q

with

water,

was

corresponding

multiplet

splitting


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components, and shake-up satellites. The O

peak fitting procedure, four parameters were

1s XPS spectra were fitted with four peaks

considered: peak position, full-width-at-half-

corresponding to oxide (O2-) and hydroxide

maximum

(OHL) that are part of the crystal structure,

(L/G) -ratio, and intensity. Three of these

adsorbed hydroxyl (OHad), and adsorbed

parameters (position, fwhm, and L/G ratio)

water (H2Oad). Two additional O 1s peaks

for each component were constrained to be

were required for fitting spectra obtained at

constant for all spectra of the same element,

water pressures above 0.1 Torr and these

unless clear changes were discernible in the

correspond to species involved in OHad-

raw XPS spectra.

(fwhm),

Lorentzian-Gaussian

H2Oad hydrogen bonding interactions and gas-phase water molecules (H2Ovap). In the

Figure 1. Physical and electrochemical characterizations of pure and Ni-modified CoOOH nanowires. Left: scanning electron micrographs. Middle: Raman spectroscopy. Right: linear sweep voltammetry. The inset in the right panel displays Nyquist plots from electrochemical impedance analysis. 8


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current density of Ni-modified CoOOH

RESULTS AND DISCUSSION Our objective was to understand how

(11.5 mAcm-2) measured at 1.53 V vs. RHE

hydroxylation and H2O adsorption proceed

is 2.2 times greater than that of pure

on pure and Ni-modified CoOOH surfaces

CoOOH (5.3 mAcm-2). Additionally, the

by measurements as a function of PH2O at

impedance analysis shown in the inset in the

constant temperatures (isotherms) and as a

right panel of Figure 1 indicates that Ni-

function of temperature at constant PH2O

modified

(isobars). All oxygens in pure and Ni-

transfer resistance and can better stabilize

modified CoOOH materials are oxides.

surface species compared to the pure

These catalysts do not contain peroxide

CoOOH sample based on the smaller

moieties and CoOOH does not refer to

Nyquist-plot diameter. A more detailed

hydroperoxide groups bound to a Co cation.

analysis of the physical and electrochemical

The morphology, crystal structure, and

characterization is given in the Supporting

electrochemical characterizations of pure

Information.

and Ni-modified CoOOH samples are

electrochemical results for the promoting

summarized in Figure 1. The pure and Ni-

effect of Ni incorporation for OER activity

modified CoOOH samples clearly exhibit

provides

nanowire morphologies and their Raman

APPES studies to better understand water

spectra are consistent with crystalline β-

chemistry on the surface of pure and Ni-

CoOOH31. These nanowires are highly

modified CoOOH.

active OER catalysts and the Ni-modified

Isothermal Experiments. First, we consider

CoOOH

greater

APPES isothermal experiments at 300K.

activity compared to pure CoOOH. The

The Co 2p APPES spectra (Figure 2A) are

clearly

demonstrates

CoOOH

excellent

9


has

smaller

These

motivation

charge

promising

for

our


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decomposed

into

multiple

Page 10 of 28

(Figure 2B). Under UHV conditions (2 x 10-

components

showing a principle peak located at 780.1

9

eV binding energy (BE) as well as multiplet

which

splitting and shake-up satellite peaks at

components. We attribute the peaks at 529.6

higher binding energies.45 The position of

and 530.6 eV to O2- and OHL, respectively.

the satellite peaks and the overall lineshape

These binding energies for these species

of the Co 2p spectra confirm trivalent Co

agree well with the values from previous

(Co3+) as the primary Co species in pure

investigations.31, 46-48 The concentration ratio

CoOOH, and also Ni-modified CoOOH (as

of O2- to OHL is 1:0.75, which is lower than

shown below). With increasing PH2O, no

the 1:1 ratio expected for stoichiometric

significant change to the Co 2p lineshape

CoOOH and indicates the presence of proton

was observed indicating a similar oxidation

vacancies in the near-surface region (ca. 5

state of Co at all levels of PH2O examined.

nm). A third O 1s component at 531.6 eV

Based on the kinetic energy of the Co 2p

BE is attributed to OHad groups bound at

photoelectrons,

is

Co3+ sites, and a fourth component at 532.6

estimated to be 3 nm. The lattice spacing of

eV BE is assigned to H2Oad. The presence of

β-CoOOH in the (0001) direction is 0.44

OHad and H2Oad observed under UHV

nm, which contains two layers of Co ions.

conditions prior to explicit H2O dosing we

Thus, the XPS signal from the top layer of

assume is due to retention of moisture from

Co ions (0.22 nm) is estimated to constitute

synthesis and exposure of the sample to

20% of the total XPS signal intensity.

moisture in the air prior to and during

the

probing

depth

Torr), a broad O 1s peak was observed, can

be decomposed

into

four

The O 1s APPES spectra illustrate a clear

sample transfer into the UHV chamber. As

picture of H2O chemistry on CoOOH

the water pressure increased from 2x10-9 to 10


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1 Torr, the proportion of OHL and OHad

the Co 2p and O 1s XPS spectra at PH2O=1

increased from 38% to 47% and 10% to

Torr is enlarged by a factor of 2 for clarity.

22%,

respectively,

accompanied

by

a

decreased in the proportion of O2- from 52%

All results for Ni-modified CoOOH (5.2

to 31%. At PH2O>1 mTorr, two new peaks

at% Ni) at 27 °C at different PH2O are shown

appeared at 533.7 and 535.3-535.8 eV BE.

in Figure 3. Co 2p spectra of Ni-modified

These two peaks are due to the interactions

CoOOH are similar to those of pure CoOOH

of H2Oad and OHad through hydrogen

under different PH2O (Figure S3). An

bonding, i.e. (OHad—H2Oad),34-36,

additional satellite peak at 6.4 eV above the

38-39

and

primary peak indicates the presence of small

from H2Ovap, respectively.

amount of divalent Co (Co2+) in addition to Co3+. Ni 2p XPS spectra shown in Figure 3A have a principle peak at 855.5 eV BE and multiplet splitting components that are fitted according to Grosvenor et al.49 Based on a lineshape analysis, Ni2+ and Ni3+ are the dominant Ni species in this sample49. The changes in the O 1s APPES spectra of Nimodified CoOOH under different PH2O Figure 2. A) Co 2p and B) O 1s APPES

displayed similar trends as those observed

spectra of CoOOH at 27 °C in the presence

for pure CoOOH (Figure 3B). As the water

of different water pressures. The y-scale for

pressure increased from 2 x 10-9 to 1 Torr, the proportion of OHL and OHad increased 11



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from 34% to 41% and 15% to 26%,

at different PH2O are shown in Figure 4. We

respectively, accompanied by a decrease in

are most interested in these surface species

the proportion of O2- from 51% to 33%. For

because they are intimately involved with

PH2O=1 mTorr, XPS peaks due to H2Ovap and

the surface chemistry of H2O. Additionally,

H2Oad involved in OHad—H2Oad interactions

the O 1s peaks due to H2Ovap and H2Oad

were also observed.

associated with OHad—H2Oad were omitted from these percentage calculations because the

concentrations

of

these

weakly

physisorbed species are not dependent upon the surface chemistry of H2O but scales mainly with PH2O. As a result, excluding these species presents more accurate trends regarding changes in O2-, OHL, and OHad as a function of PH2O. A clear increase in the OHL and OHad surface coverage is observed Figure 3. A) Ni 2p and B) O 1s XPS spectra

at higher water pressures, which we attribute

of Ni-CoOOH at 27 °C for several water

to dissociation of water on the surface of

pressures. The y-scale for the Co 2p and O

CoOOH. We believe H2O dissociation on

1s XPS spectra at PH2O = 1 Torr is enlarged

pure and Ni-modified CoOOH takes place at

by a factor of 2 for clarity.

oxygen vacancies based on the following reasons. First, the crystal structure of β-

The relative proportions of O2-, OHL, and

CoOOH consists of layers of edge-sharing

OHad for CoOOH and Ni-modified CoOOH

Co3+ oxyhydroxyl octahedra,30, 12


50-51

and a

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fully coordinated Co3+ cation in this

UHV surface science investigations are

octahedral geometry is unlikely to form

limited. An investigation of H2O chemistry

additional chemical bonds with H2O or with

on the Co3O4(111) surface has reported

components

H2O.

molecular adsorption of water at 110 K and

Alternatively, under-coordinated Co3+ that

hydroxylation in UHV was only observed

can form due to presence of an oxygen

when heating the sample to 650 K after an

from

dissociated

or

exposure of 150 L H2O.33 We observed

components from dissociated H2O, such as

extensive H2O dissociation on pure CoOOH

OH. In addition, the presence of oxygen

nanowires at PH2O≥10-5 Torr and at 27 °C

vacancies is confirmed by the Co:O ratio for

indicating that surface oxygen vacancies are

pure (1:1.8) and Ni-modified (1:1.83)

active towards H2O dissociation on CoOOH

CoOOH. Furthermore, higher reactivities of

at relatively low temperature. A crude

oxygen vacancies toward water dissociation

estimation of the oxygen vacancies (using

has been observed on several important

Co:O ratio) and the amount of dissociated

oxide catalysts.52-55 As a result, we propose

H2O (OHL and OHad percentages) showed

that a H2O molecule can dissociate at an

reasonable agreement within an order of

oxygen vacancy to form OHad on Co3+ and

magnitude.

vacancy

can

interact

with

H2O,

form OHL with a nearby lattice oxygen Based on DFT calculations by Bajdich et atom. A schematic representing the H2O al., the formation of O2 on the (011ത2) and dissociation process at an oxygen vacancy is (0001)

surfaces

of

CoOOH

is

shown in Figure S4. H2O interaction with thermodynamically limited by the formation cobalt oxyhydroxide has not been studied of surface OH groups (OH*).27 Given this previously by APPES and fundamental finding, achieving a high density of surface 13



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Page 14 of 28

hydroxyl groups should be important for

dominant oxygen species constituting the

OER on pure and Ni-modified CoOOH

material structure. The exact mechanism for

catalysts. On the basis of our observation of

the structural change is currently unclear

OHad formation on the catalyst surfaces in

based solely on the APPES results.

PH2O≥10-5 Torr, it is likely that surface OHad groups in aqueous medium will similarly be formed by a chemical process via water dissociation, prior to anodic polarization. As a result, two mechanisms exist for the formation of OHad on pure and Ni-modified CoOOH—a

chemical

route

by

water

dissociation and an electrochemical route by the discharge of hydroxyl anions. However, without

experimental

observations

on

individual mechanistic steps during reaction, we are unable to determine the exact role of OHad formation by water dissociation in the oxygen

evolution

Interestingly,

for

reaction PH2O>1

Figure 4. Ratios of surface oxygen species

mechanism. mTorr,

associated with A) CoOOH and B) Ni-

the

CoOOH at 27 °C as a function of water

OHL:O2- ratio is greater than unity for both

pressure. Insets show individual species

samples studied. This indicates a low barrier

concentrations.

for surface hydroxylation of pure and Nimodified CoOOH, where OHL becomes the 14


Page 15 of 28

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thermal

are in good agreement with the changes

stabilities of surface oxygen species and the

observed in the Co 2p spectra. As the

structural evolution of pure and Ni-modified

sample temperature increased to 220 °C,

CoOOH were investigated as a function of

significant intensity decreases occurred in

temperature in isobaric experiments at

the OHL and OHad components, while the

PH2O=1 Torr. All APPES spectra were

proportion

collected by heating the sample in 1 Torr

observations

H2O and not at room temperature after

dehydroxylation

cooling down from heating. Based on the Co

following the reaction below,

Isobaric

Experiments.

The

of

O2-

increased.

These

point

to

surface

of

CoOOH,

likely

OHad+OHLVO + O2- + H2O (5)

2p spectra (Figure 5A), no significant change to the chemical state of Co cations

The H2O molecules from this reaction are

was observed below 220 °C. At higher

expected to desorb from the surface at

temperatures, a 0.5 eV shift of the primary

T≥120 °C due to the decrease in the H2Oad

Co 2p peak to lower binding energy was

peak intensity (Figure 5B). With further

observed. Simultaneously, a new satellite

increases in temperature, the O2- component

peak at 6.5 eV higher BE than the primary

became

peak also appeared. Both observations can

species, and this is consistent with the

be attributed to the formation of Co2+.56-58 At

transformation of CoOOH to Co3O4. Upon

320 °C, the transformation of CoOOH to

cooling the sample back to 25 °C, O2-

spinel Co3O4 took place, as indicated by the

species remained the dominant components

change in the Co 2p lineshape, and this is

with relatively lower proportions of OHL

consistent with previous reports.59 The

and OHad indicating irreversible structural

the

changes in the O 1s XPS spectra (Figure 5B) 15


dominant

surface

oxygen


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

changes occur due to sample heating in 1

temperature was increased to 120 °C. With

Torr H2O to 220 °C.

further increase in temperature, the intensity of this satellite peak continues to increase, which indicates a significant portion of Co3+ cations were converted to Co2+ and causes a much different lineshape. Upon cooling to 27 °C, the 786.5 eV satellite peak persists, which indicates an irreversible structural change from CoOOH to CoOxHy has occurred, where Co2+ cations constitute a significant proportion of Co near the surface. This is in stark contrast with the

Figure 5. A) Co 2p and B) O 1s XPS

behavior of pure CoOOH after heating to

spectra of CoOOH taken at different sample

320 °C, where we observed that Co3O4 was

temperatures in 1 Torr H2O.

the dominant phase formed. Thus, the

The temperature dependence of Co 2p

incorporation of Ni promotes the formation

XPS spectra (Figure 6A) from Ni-CoOOH

of Co2+ after annealing in 1 Torr H2O to

in 1 Torr H2O is similar to that of CoOOH at

T≥220 °C. Ni 2p spectra (Figure S5) show

lower temperatures, but some important

little change to the Ni concentration or

distinctions

chemical state in the temperature range and

are

observed

at

higher

temperatures. For Ni-CoOOH, a small

H2O pressure explored in this study.

satellite peak at 786.5 eV that represents the formation of Co2+ appeared when the 16


Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and OHad on the Ni-modified CoOOH surface compared to the CoOOH surface. The relative proportions of oxygen species for CoOOH and Ni-CoOOH at different temperatures

are

given

in

Figure

7.

Comparing the two samples, the amount of OHL and OHad retained by Ni-modified CoOOH at all temperatures is greater than that for pure CoOOH. These results from Figure 6. A) Co 2p and B) O 1s XPS

our isobaric experiments are in excellent

spectra of Ni-CoOOH taken at different

agreement with the DFT calculations by

sample temperatures in 1 Torr H2O.

Bajdich et al. who predicted improved stability

Analysis of the O 1s spectra (Figure 6B)

of

surface

species

with

Ni

incorporation into β-CoOOH.

at increasing temperatures clearly illustrates dehydroxylation of Ni-modified CoOOH,

The greater stability of OHad on Ni-

consistent with the observations made from

modified CoOOH has profound implications

the Co 2p spectra. For Ni-CoOOH at 220

for water oxidation reactions. Based on the

°C, the proportion of OHL and OHad is 36%

DFT calculations by Selloni and coworkers,

and 26%, respectively, which are much

H2O deprotonation to form a surface OH

greater than the proportions of these species

species is energetically costly, and that

found for CoOOH at the same temperature.

increasing the surface stability of OHad is

This observation provides additional strong

critical in lowering the barrier for its

evidence for the greater stabilities of OHL

formation.26 As a result, the greater stability 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

of OHad on Ni-modified CoOOH, as shown

Figure 7. Ratios of the concentrations of

by our isothermal experiments, should be

surface oxygen species associated with A)

beneficial for OHad formation. Indeed, our

CoOOH and B) Ni-CoOOH as functions of

previous

temperature at 1 Torr H2O. Insets show

characterization

of

the

OER

performance of pure and Ni-modified

individual species compositions.

CoOOH showed a better activity for Nimodified

CoOOH.

Therefore,

CONCLUSION

our

In conclusion, we have observed extensive

experimental results support the theoretical mechanistic

proposal

that

water dissociation on pure and Ni-modified

increased

CoOOH catalyst surfaces at 27 °C using

adsorbed hydroxyl stability promotes OER

APPES in all pressure range from 10-7 to 1

activity.

Torr. For both catalysts, the transformation of CoOOH to CoOxHy at 27 °C in 1 Torr H2O indicates only a small activation barrier for phase change. Based on APPES isobaric results, Ni incorporation was found to improve the stability of OHad and promote the

formation

of

Co2+

at

elevated

temperatures. The greater thermal stabilities of OHad observed for Ni-modified CoOOH compare to the pure CoOOH is consistent with

previous

DFT

calculations.

This

APPES study has revealed that oxygen vacancies are highly reactive towards H2O 18


Page 19 of 28

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dissociation and that Ni incorporation can improve OHad stability. This additional

Corresponding Author

experimental information on the chemistry

*E-mail: [email protected] (B.E.K.)

of water at CoOOH and Ni-CoOOH surfaces Notes provides key insights that contribute to the The authors declare no competing financial continued rational design of the structure interest. and chemical composition of catalysts ACKNOWLEDGEMENTS

optimized for OER activity.

This ASSOCIATED CONTENT

research

supported

by

is the

based

upon

National

work

Science

Supporting Information The Supporting

Foundation under Grant No. CHE-1465082.

Information is available free of charge on

This research used resources of the 23-ID-2

the ACS Publications website at DOI:

(CSX-2)

beamline

Synchrotron

of

the

Light Source

National

II,

a U.S.

Detailed procedures for electrochemical Department of Energy (DOE) Office of characterizations of nanowire catalysts; Science User Facility operated for the DOE detailed analysis of Raman spectroscopy; Office of Science by Brookhaven National detailed analysis of impedance data; Laboratory APPES Co 2p spectra of Ni-modified SC0012704. CoOOH at different PH2O; APPES spectra of Ni 2p of Ni-modified CoOOH at different temperatures. AUTHOR INFORMATION 19


under

Contract

No.

DE-


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Batteries: from Aqueous to Nonaqueous

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by Annealing Atmosphere. Appl. Surf. Sci.

(56) Wei, W. F.; Chen, W. X.; Ivey, D. G.

2011, 257, 3358-3362.

Rock Salt-Spinel Structural Transformation in Anodically Electrodeposited Mn-Co-O

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TOC Graphic

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Investigation of Water Dissociation and Surface Hydroxyl Stability on

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