Photoelectrochemical Performance of the Ag(III)-Based Oxygen-Evolving Catalyst

29 July 2021

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Photoelectrochemical Performance of the Ag(III)-Based Oxygen-Evolving Catalyst

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DOI:10.1021/acsami.7b05901 Terms of use:

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Supporting Information

Photoelectrochemical Performance of the

Ag(III)-based Oxygen Evolving Catalyst.

Fabrizio Sordello, Manuel Ghibaudo, and Claudio Minero*

Dipartimento di Chimica, Università degli Studi di Torino – via Pietro Giuria 5 -10125 Torino, Italy

*Corresponding author: e-mail: claudio.minero@unito.it. Tel.: +39 011 60 8449. Fax: +39 011 670 5242

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In the first part of this document we will provide supplementary experimental details, illustrating the pH-Eh diagrams for Ag in the presence of sulfate 10 mM and 100 mM (Figure S 1), to better understand the choice of electrosynthesis conditions, and the optimization of the current density as a function of electrosynthesis time and potential obtained through an experimental design approach (Figure S 2). We will then discuss the performance of TiO2 and hematite electrodes

with respect to water oxidation reaction. Figure S 3 and Figure S 4 report chronopotentiometry and cyclic voltammetry of a pristine TiO2 electrode, respectively. Anodic LSV of TiO2-Co-Pi is

reported in Figure S 5. We report linear sweep voltammetries of electrodeposited α-Fe2O3,

α-Fe2O3 (PRED), and α-Fe2O3 (PRED)-Co-Pi in the dark and under irradiation in Figure S 6,

Figure S 7, and Figure S 8, respectively, while the O2 production rates in the dark and under

irradiation are listed in Table S 1 with the corresponding faradaic efficiencies. Example of experimental current density and O2 evolution rate as functions of time for α-Fe2O3 (PRED)

electrode is reported in Figure S 9. The O2 production rate and faradaic efficiency in the dark and

under irradiation for bare and AgCat-modified α-Fe2O3 electrodes are reported in Table S 2.

Linear sweep voltammetries and ATR-FTIR spectra of α-Fe2O3 (PRED) electrode before and

after electrolysis experiments, which were carried out at positive applied bias and under irradiation, are reported in Figure S 10 and Figure S 11, respectively. An example of linear sweep voltammetry of AgCat on ITO is reported in Figure S 12, Overpotential values for water oxidation and photocurrent density for TiO2 (Table S 3) and hematite (Table S 4) electrodes are

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1.1 AgCat synthesis

Figure S 1. pH Eh diagrams for Silver in water in the presence of sulphate 10 mM (top)

and 100 mM (bottom). 0 2 4 6 8 1 0 1 2 1 4 - 0 . 5 0 . 0 0 . 5 1 . 0 1 . 5 ESH E / V p H A g + A g ( c ) A g 2 O ( c ) A g 2 O 3 ( c ) A g O ( c ) A g 2 S ( c ) A g ( c ) [ A g + _] T O T = 1 . 0 0 m M [ S O 4 2 − ] T O T = 1 0 . 0 0 m M t = 2 5 ° C 0 2 4 6 8 1 0 1 2 1 4 - 0 . 5 0 . 0 0 . 5 1 . 0 1 . 5 ESH E / V p H A g + A g S O 4 − A g ( c ) A g 2 O ( c ) A g 2 O 3 ( c ) A g O ( c ) A g 2 S ( c ) A g ( c ) [ A g + _] T O T = 1 . 0 0 m M [ S O 4 2 − ] T O T = 1 0 0 . 0 0 m M t = 2 5 ° C

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Figure S 2. Experimental design for the optimization of the current density as a function of electrosynthesis time and potential for AgCat synthesis.

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2.1 Bare TiO

electrodes

Figure S 3. Open circuit chronopotentiometry of TiO2 electrode obtained by means of

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Figure S 4. Cyclic voltammetry of TiO2 electrode in the dark and under irradiation in

KNO3 0.1 M at pH 7.

2.2 Photoelectrochemical performance of reference catalysts

In the following the photocurrent and oxygen evolution on TiO2 and hematite, synthetized with

different methods, alone and coupled with Co-Pi are reported for comparison with the Ag-based catalyst deposited on the same substrates. Since both TiO2 and hematite are in the form of thin

porous films, they cannot develop significant band bending, and these experiments are ideal to test the co-catalyst performance under photoelectrochemical conditions, since the creation of a rectifying interface allowing water oxidation and avoiding recombination must solely rely on the properties of the catalysts and on its interface with the semiconductor. Conversely, in the presence of significant band bending, the semiconductor alone would be responsible for charge separation, giving no further information on the properties of the co-catalysts, as their activity as electrocatalysts has already been demonstrated.

2.2.1 TIO₂ ELECTRODES

TiO2 electrodes used in this work behave as n-type semiconductors, as witnessed by the shape of

the chronopotentiometry (CP) and cyclic voltammetry (CV) (

Figure S 3 and Figure S 4, respectively), comparable to literature reports.1 Indeed, the open circuit potential (OCP) upon illumination is more negative than its dark value, revealing accumulation of electrons, which, therefore, constitute the majority carriers. The entity of the photovoltage (350 mV) highlights the marked photoactivity of the material, which can be evidenced also in linear sweep voltammetry (LSV, Figure S 5), where we observed photocurrent of 1.5∙10-5 A cm-2 between 0.5 V and 1.4 V. At 1.5 V we observed the onset of anodic current which can be attributed to water oxidation; its value reaches 0.10 mA cm-2 at 1.8 V upon illumination and 0.12 mA cm-2 in the dark. Considering the different radiant intensity on the electrodes, the i-V curves are in agreement with those reported in literature.2

S-6 When we coupled Co-Pi with TiO2 we observed large dark catalytic current density, up to 2 mA

cm-2_{, with current onset at 0.9 V vs Ag/AgCl with an evident shifting to cathodic potential,}

corresponding to 300 mV overpotential in accordance with previous reports.3 _{Nonetheless we did}

not observe photocurrent (Figure S 5) as occasionally reported.2 _{The photocurrent quenching}

caused by Co-Pi indicates that it acts as a recombination center. In this experiment the starting potential is 0.5 V vs Ag/AgCl, implying that in the present case cobalt is mainly present as Co(III) (for the couple Co2O3/Co2+ E = 0.505 V vs NHE, pH 7)3.

Figure S 5. Anodic LSV of TiO2-Co-Pi electrode in phosphate buffer 0.1 M at pH 7 in

dark conditions and under irradiation, LSV of TiO2 electrode is reported for comparison.

Conversely, photocurrent is usually observed starting from more cathodic potential (-0.4V), where Co-Pi is richer in Co(II) that cannot be further reduced. In Co-Pi catalytic cycle Co(II) reacts with valence band holes to form Co(III), and from Co(III) to Co(IV),4 _{both able to react}

with conduction band electrons. Therefore, when Co-Pi is present in its active form, it can catalyze electron hole recombination on TiO2 surface, suppressing photocurrent.

Figure S 6: Anodic LSV on electrodeposited α-Fe2O3 in NaOH 0.1 M (pH 13) in the dark

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2.2.2 HEMATITE (α-FE₂O₃) ELECTRODES

To achieve better directionality in electron transfer we decided to couple Co-Pi to hematite, owing to its capability to oxidize water5 _{and to its less positive valence band potential compared}

to TiO2.6 Hematite has an indirect band gap of 2.1 eV that could utilize about 40% of the

incident solar spectrum, which is better than TiO2 (Eg 3.0 eV) and WO3 (Eg 2.7 eV). Hematite

valence band potential is sufficient for oxygen evolution reaction.7

In the case of electrodeposited hematite we observed large anodic current density (2 mA cm-2

above 1.2 V vs Ag/AgCl), which can be ascribed to oxygen evolution. The current onset can be estimated at 0.7 V vs Ag/AgCl, corresponding to 400 mV of overpotential. However, the photocurrent is negligible in the potential window explored (Figure S 6) because α-Fe2O3 has

very short hole diffusion-length (2-4 nm), low electron mobility (10-1 _cm2 _V-1 _s-1_{), high}

recombination rate of photo-generated charge carriers, and poor electrical conductivity.8

To improve this feature we electrodeposited α-Fe2O3 with the PRED method. When the PRED

method is applied, several cycles of dissolution and precipitation take place, promoting the dissolution of the least stable particles, which are likely to be the least crystalline, the most soluble and with the highest energy surface sites. Because on these sites the charge carrier recombination is more likely to occur, PRED cycles of dissolution/ precipitation favour the growth of the most crystalline particles, which could be more active and behave as less efficient recombination centres. The material obtained shows 100 mV larger overpotential, but with larger current density at high overpotential (3 mA cm-2 _{above 1.5 V vs Ag/AgCl, Figure S 7). We also}

observed photocurrent density of 4∙10-5 _{A cm}-2 _{between 0.75 and 0.95 V vs Ag/AgCl. To}

confirm that the current density observed in the LSV experiment is responsible for O2 evolution,

and that the photocurrent measured can contribute to water oxidation, we performed electrolyses at 0.7 V, 0.8 V and 0.9 V vs Ag/AgCl (overpotentials 450 mV, 550 mV and 650 mV, respectively), in the dark and upon irradiation, combined with gas chromatographic determination of the O2 produced.

Figure S 7. Anodic LSV on α-Fe2O3 electrodeposited with PRED method in NaOH 0.1 M

S-8 We measured O2 evolution on α-Fe2O3 electrodes above 450 mV overpotential (Table S 1). At

550 mV overpotential we recorded 0.15 µL min-1 _{in the dark and 0.30 µL min}-1 _{under irradiation}

with an increase under irradiation of 0.15 µL min-1_{, while at 650 mV overpotential we observed}

larger production, especially under irradiation, with an increase under irradiation of 3.2 µL min-1_.

With increasing overpotential we also observed larger faradaic efficiency up to 90 % at 650 mV overpotential (Table S 1).

Table S 1. Faradaic efficiency and O2 production rate in the dark and upon irradiation for

α-Fe2O3(PRED) electrodes in NaOH 0.1 M pH 13 at different overpotential values in the

presence of Co-Pi co-catalyst compared to the values reported in its absence (in the main text already in Table 3).

Co-Pi Overpotential _(mV) Orate in the dark 2 production

(µL min-1) Faradaic efficiency % O2 production rate upon irradiation (µL min-1) Faradaic efficiency % - 450 < 0.05 - < 0.05 - - 550 0.15 ± 0.02 45 ± 5 0.30 ± 0.02 45 ± 5 - 650 0.8 ± 0.2 90 ± 5 4.0 ± 0.1 90 ± 5 Yes 400 0.12 ± 0.02 25 ± 5 < 0.05 - Yes 650 0.33 ± 0.04 75 ± 10 < 0.05 -

The coupling with Co-Pi resulted in lower overpotential for O2 evolution, as witnessed by the

current onset at 0.5 V (η=240 mV) and in appreciable photocurrents in the potential range 0.7– 1.0 V vs Ag/AgCl (Figure S 8). Nevertheless, the photocurrents observed cannot be related to O2

evolution as reported in Table S 1. Even though in dark conditions at 400 mV overpotential O2

evolution is already measurable (0.12 µL min-1_{), under irradiation O}

2 production stops, even at

larger overpotential (650 mV, Table S 1).

Figure S 8. Anodic LSV on α-Fe2O3 (PRED)-Co-Pi in NaOH 0.1 M (pH 13) in the dark

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Figure S 9. Experimental current density and O2 evolution rate vs electrolysis time.

Conditions: α-Fe2O3(PRED) at overpotential 650mV NaOH 0.1 M at pH 13.

Table S 2. O2 production rate and faradaic efficiency in the dark and upon irradiation for

α-Fe2O3(PRED) electrodes without and with AgCat in NaOH 0.1 M at different

overpotential values. AgCat Overpotential, mV Dark O2 rate, µmol h-1 Irradiation O2 rate, µmol h-1 Faradaic efficiency, % - 450 < 0.12 < 0.12 - - 550 0.37 ± 0.05 0.74 ± 0.05 45 ± 5 - 650 2.0 ± 0.50 9.8 ± 0.20 90 ± 5 PEDa _{450 < 0.12 < 0.12 -} PEDa _{650 33 ± 5.0 65 ± 6.0 95 ± 5} PREDb _{400 0.22 ± 0.02 0.39 ± 0.05 50 ± 5} PREDb _{500 0.49 ± 0.05 0.98 ± 0.12 50 ± 5} a) PED photoelectrodeposition, b) PRED pulse reverse electrodeposition

Coherently with our observations in the case of α-Fe2O3(PRED)-AgCat, also in the case of

α-Fe2O3(PRED) we observed increased water oxidation activity after irradiation of the electrode

with applied positive bias. As discussed in the main body of the paper, this behavior is supposed to be caused by the presence of oxidized Fe species, namely Fe(IV) and Fe(V), or by surface reconstruction able to increase the stability of those species. These modifications can be observed during bulk electrolysis (Figure S 9), as a slow (60-80 minutes) increase of the current under irradiation, and, later, in dark conditions, as a stabilization of the current density at a higher value (approximately 0.1 mA cm-2 _{at minute 400) compared with the beginning of the}

experiment (0.02mA cm-2 _{at minute 100). Nevertheless these modifications can be revealed also}

S-10 The current density in LSV measurements tends to decrease slightly with increasing duration of an applied positive bias in the dark, but, on the other hand, it significantly increases if the bias is applied for a sufficient time under irradiation (Figure S 10).

Figure S 10. Anodic LSVs in the dark for a α-Fe2O3(PRED) electrode after

chronoamperometries of different duration at 650mV overpotential in NaOH 0.1 M at pH 13 in the dark and under irradiation.

2.3 ATR-FTIR spectrum

The features responsible for this behavior, even though not yet clarified, are responsible for an appreciable modification of the IR spectrum of the electrode: after irradiation with positive applied bias three new peaks can be detected at 990 cm-1_{, 1400 cm}-1_{, and 1680 cm}-1 _{(Figure S}

11).

Figure S 11. ATR-FTIR spectra of pristine α-Fe2O3(PRED) and α-Fe2O3(PRED)

irradiated for 2 h at 650mV overpotential in NaOH 0.1 M at pH 13. The peaks signaled with arrows are significantly more intense in the irradiated sample.

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2.4 The Ag-based catalyst

Figure S 12. LSV of the AgCat deposited at 1.9 V vs Ag/AgCl for 1200 s, compared with the pristine ITO substrate.

2.4.1 PHOTOELECTROCATALYTIC PERFORMANCE ON TIO₂ ELECTRODES

Table S 3. Overpotential for water oxidation and photocurrent density for TiO2 electrodes

in KNO3 0.1 M at pH 7.

Sample Water oxidation _{overpotential} Photocurrent density

TiO2 + 1 V 15 µA cm -2

between 0.5 V and 1.3 V vs Ag/AgCl TiO2-Co-Pi + 0.3V No photocurrent

TiO2-AgCat + 0.8 V 7 µA cm -2

between 0.5 and 1 V vs Ag/AgCl TiO2-AgCat(PRED) + 0.8 V 25 µA cm

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2.4.2 PHOTOELECTROCATALYTIC PERFORMANCE ON α-FE2O3 ELECTRODES

Table S 4. Overpotential for water oxidation and photocurrent density for α-Fe2O3

electrodes in NaOH 0.1 M at pH 13.

Sample Water oxidation _{overpotential} Photocurrent density

α-Fe2O3 + 0.4 V No photocurrent

α-Fe2O3(PRED) + 0.4 V 40 µA cm -2

between 0.75 and 0.95 V vs Ag/AgCl α-Fe2O3 - Co-Pi + 0.2 V 150 µA cm

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between 0.7 V and 1.1 V vs Ag/AgCl α-Fe2O3(PRED)-CoPi + 0.2 V 300 µA cm

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between 0.7 V and 1 V vs Ag/AgCl α-Fe2O3(PRED) – AgCat(PED) + 0.4 V 500 µA cm

-2 _{at 0.9 V vs Ag/AgCl}

800 µA cm-2 _{above 1V vs Ag/AgCl}

α-Fe2O3(PRED) – AgCat(PRED) + 0.4 V

2 µA cm-2 _{at 0.4 V vs Ag/AgCl}

5 µA cm-2 _{at 0.7 V vs Ag/AgCl}

20 µA cm-2 _{at 0.8 V vs Ag/AgCl}

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