Stable Molecular Photocathode for Solar-Driven CO2

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Letter

A Stable Molecular Photocathode for SolarDriven CO2 Reduction in Aqueous Solutions Ting-Ting Li, Bing Shan, and Thomas J. Meyer ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b02512 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Energy Letters

A Stable Molecular Photocathode for Solar-Driven CO2 Reduction in Aqueous Solutions Ting-Ting Li1,2, Bing Shan1, Thomas J. Meyer*,1

1Department

of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States.

2Research

Center of Applied Solid State Chemistry, Ningbo University, Ningbo, Zhejiang 315211, China.

ABSTRACT The performance of dye-sensitized photoelectrodes for artificial photosynthesis is typically limited by instability in aqueous solutions. We describe here a new molecular-based photocathode that integrates functional chromophore-catalyst assemblies for long-term solar-driven CO2 reduction in stabilized polymeric film structures. The assemblies include a silane surface-anchoring bridge, a ruthenium polypyridyl chromophore and a rhenium-based molecular catalyst. They were prepared on nanocrystalline oxide films by silanization of the oxide and a two-step electropolymerization of vinyl-derivatized precursors. The integrated photocathode was stable toward CO2 reduction for over ten hours with a Faradaic efficiency of ~65%. The long-term stability arises from the silane surface-anchoring groups and the carbon-carbon bonds formed by electropolymerization between the three components. Transient absorption measurements on a nano-to-microsecond timescale show that the assemblies undergo rapid hole injection into the oxide electrode followed by relatively slow interfacial charge recombination.

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Table of Contents


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ACS Energy Letters

Inspired by natural photosynthesis, conversion of CO2 into energy-rich forms of carbon with solar light as an energy source is one of the primary goals of artificial photosynthesis.1-3 Photoelectrochemical CO2 reduction with hybrid photocathodes based on p-type semiconductors, visible light-absorbing chromophores and molecular catalysts have been explored.4-10 In the preparation of photocathodes with high catalytic efficiencies, key factors include: (1) a catalytic p-type semiconductor with a valence band allowing for hole injection by a photoexcited chromophore;11-16 (2) a light absorber with high extinction coefficients for capturing incident visible light, and a catalyst with an onset potential aligned with the reduction potential of the chromophore; (3) an assembly strategy for immobilizing the chromophore and catalyst on electrode surfaces for sustainable photoelectrocatalysis. In addition to surface co-loading,17,18 a variety of strategies have been explored for binding chromophores and catalysts to metal oxide surfaces, including metal ion bridging19-21 and atomic layer deposition (ALD) for the formation of layer-by-layer22-25 assemblies. These approaches have resulted in improved performance compared to conventional co-loading methods. However, they typically utilize phosphonate surface binding with stability limitations by desorbing from oxide electrodes due to hydrolysis in aqueous solutions at pH ≥ 5.26-28 We have initiated a series of studies designed to improve the stability of dye-sensitized photoanodes by introducing metal oxide protection layers via ALD29,30 and by stabilization with electro-polymerized overlayers.31-33 Here, we focus on a new strategy for integrating molecular assemblies on nanocrystalline oxide films for long-term solar-driven CO2 reduction. The assemblies, consisting of molecular chromophore and catalyst, were synthesized on electrode surfaces by silanization of the oxide, followed by a two-step electropolymerization procedure. Long-term photoelectrocatalytic experiments demonstrate that the integrated photocathode can reduce CO2 to CO from dissolved CO2 in neutral aqueous solutions over ten hours without a significant decrease in photoelectrocatalytic efficiency. The photocathode assembly includes a silane surface bridge, vinyltrimethoxysilane (Si), a ruthenium polypyridine chromophore, [Ru(dvb)2bpy]2+ (RuII; dvb = 5,5′-divinyl-2,2′-bipyridine), and a rhenium-based CO2 reduction catalyst, [Re(dvb)(CO)3Cl] (ReI), on nanocrystalline p-type NiO films.34,35 The X-ray diffraction pattern and scanning electron microscope images of the NiO electrode are shown in Figures S1 and S2. The thickness of the NiO film is 1.1 μm. The structures of the molecular precursors are shown in Figure 1a. The three molecular components were derivatized with vinyl functional groups as a way to assemble surface structures by electropolymerization. As the first step in the photoelectrode synthesis, the NiO films were silanized in a toluene solution of vinyltrimethoxysilane to form stable surface Si-O bonds.36 Electropolymerization of the silanized NiO (NiO|Si) was then performed by cycling the electrode through the potential window of −0.6 ~ −1.6 V vs Ag/AgCl at a scan rate of 100 mV s−1 in N2-degassed acetonitrile solution. As the scans were repeated, the current densities in Figure 2a were enhanced continuously, consistent with the in-situ formation of polymeric layers on the electrode surface. During the process, reduction of the vinyl groups induces radical-radical coupling and carbon-carbon bond formation.37 Figure S3 shows the CV for the electrode after the first electropolymerization scan to give the



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chromophore-derivatized electrode, NiO|Si-Poly(RuII). The bridging -(CH2)4- spacer formed between the vinyl groups at Si and RuII is omitted in NiO|Si-Poly(RuII). The appearance of the reversible RuIII/II couple at ~1.10 V vs Ag/AgCl reveals that the molecular structure of RuII was maintained during the electropolymerization process. Growth of the polymer on the electrode surface was investigated as a function of the number of CV scans. The resulting changes in the absorption spectra of -Poly(RuII)n on NiO|Si are given in Figure 2b. The surface coverage (Γ) of RuII was determined from spectroscopic measurements with the molar absorptivity of ε455 nm ~ 13,400 M−1 cm−1 at 455 nm.38 During the first 30 scans, Γ(RuII) increased linearly with the number of scan cycles at a rate of 0.63 nmol cm−2 per cycle (Figure 2c). At the maximum Γ(RuII) (~26.9 nmol cm−2), the electropolymerized RuII forms a layer at the electrode interface with no evidence for further growth by additional scans.

b

a

Cl N OC Re N OC CO

H3CO Si OCH3 OCH3

Cl N CO N Re OC CO

Si 2+

N

N

N N

N

N

Ru

N

N

CO Re Cl

N N N Ru N N N

CO CO

Si OO O

ReI

RuII

N N N Ru N N N

c

Si O OO

d Cl

CO CO Re CO N N

N N N Ru N N N

COCO Re CO N N

Cl

N N N N Ru N N

Si O

OO

N

N N

N Ru N N N

N

Si O O O

N N N N Ru N N N O

Ru N N N

Si O OO

Si OO

OC CO OC Re Cl N N

N N OC Re Cl OC CO

N N N Ru N N N

Si OO O

Figure 1. a, Molecular structures for the surface bridge (Si), the chromophore (RuII) and the catalyst (ReI). b-d, Possible surface assembly structures on NiO|Si-Poly(RuII)-Poly(ReI).


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ACS Energy Letters

Figure 2. a, Reductive electropolymerization of RuII on the NiO|Si electrode showing 20 CV scans in N2-degassed acetonitrile solution with 0.5 mM in RuII and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) at a scan rate of 100 mV s−1. b, Absorptive changes of NiO|Si-Poly(RuII)n as a function of the CV scan numbers. The molar absorptivity (ε) of RuII is shown in the inset. c, Surface coverage (Γ) of RuII as a function of the CV scan numbers. Γ(RuII) was determined from Γ = A(λ)/[103ε(λ)], where A(λ) and ε(λ) are the absorbance and molar absorptivity of RuII at wavelength λ, respectively. d, Reductive electropolymerization of ReI on NiO|Si-Poly(RuII) showing 15 CV scans in N2-degassed acetonitrile solution with 0.5 mM ReI and 0.1 M TBAPF6 at a scan rate of 100 mV s−1.

The electrochemical behavior of NiO|Si-Poly(RuII) as a photocathode was first examined with the added sacrificial electron acceptor, 4,4’-dithiodipyridine (DTDP), with one-electron reduction potential at −0.66 V vs Ag/AgCl.17 The photocurrent responses for NiO|Si-Poly(RuII)n with DTDP (5 mM in H2O with 0.1 M Na2SO4) prepared following different number of CV scans are shown in Figure S4. The data clearly show that the magnitudes of photocurrent densities depend on the extent of RuII surface loading. As noted above, the optimized photoelectrode in these measurements was the one prepared from 25 reductive scan cycles with a Γ(RuII) of ~21.2 nmol cm−2. The maximum photocurrent density at ~72 μA cm−2 was obtained from the optimized photoelectrode under −0.6 V vs Ag/AgCl. The chromophore-catalyst assembly, -Poly(RuII)-Poly(ReI), on the silanized NiO film was



prepared by electropolymerization of the catalyst, ReI, on the surface of NiO|Si-Poly(RuII)25 (Γ(RuII) ~21.2 nmol cm−2). The corresponding CV scans are shown in Figure 2d. The chromophore/catalyst loading ratio was controlled by varying the number of scan cycles. At a scan rate of 100 mV s−1, 15 CV scans from 0 to −1.6 V vs Ag/AgCl in the presence of ReI (0.5 mM) in acetonitrile resulted in the photocathode, NiO|Si-Poly(RuII)-Poly(ReI), with an elemental ratio of RuII and ReI of ~1:1. The ratio of RuII to ReI was established by X-ray photoelectron spectroscopic (XPS) measurements based on the peak area ratios of the core level of Ru and Re elements. These values are in consistent with the elemental analysis data for RuII and ReI on the electrode by inductively coupled plasma-optical emission spectrometry(ICP-OES) measurements. The XPS spectrum in Figure S5a for NiO|Si-Poly(RuII)-Poly(ReI) in the Re 4f region includes resonances at 42.8 and 40.5 eV which are assigned as the binding energies of Re 4f5/2 and Re 4f7/2, respectively (Figure S5a).9,39 The Ru 3d spectrum (Figure S5b) has an overlap with the C 1s peak with its main band located at 284.6 eV. The latter can be further deconvolved into three bands, with one at 284.2 eV that corresponds to Ru 3d3/2. The band at 280.4 eV matches Ru 3d5/2, consistent with previous studies40. The presence of Poly(ReI) was further confirmed by Fourier transform infrared (FTIR) measurements, as shown in Figure S6, with intense features appearing at 1900 and 2025 cm−1 that are characteristic for the three CO groups in ReI with its fac-configuration.41 Given the nature of the process, the final layer-by-layer structure shown in Figure 1b was anticipated. However, because of the multiple vinyl groups on the RuII and ReI precursors, interconnected -RuII-RuII- and -ReI-ReI- polymer layers can form. Molecular assemblies on metal oxide electrodes are known to undergo cross-surface charge percolation due to the highly compact nature of the molecular layers.26 The percolation rate depends on the distance between neighboring molecules and on whether they are chemically linked. In order to gain insights on the surface structure of NiO|Si-Poly(RuII), we utilized the results of kinetic studies on cross-surface electron hopping in NiO|Si-Poly(RuII) and in a control sample, NiO|RuCP ([RuII(4,4’-(CH2PO3H2)2-2,2’-bipyridine) (2,2’-bipyridine)2]Cl2), where -RuCP molecules are separated from each other at the interface. The results are shown in Figure S7. For both samples, under a forward bias at −1.5 V vs Ag/AgCl, RuII is reduced to RuI as shown by the appearance of a positive absorptive feature at 500 nm. Anson plots extracted from the data in Figure S7a are shown in Figure S7b. Linear fits of the plots gave apparent diffusion coefficients (Dapp) values of 9.9×10−11 cm2 s−1 for NiO|Si-Poly(RuII) and 2.1×10−11 cm2 s−1 for NiO|RuCP. Since RuCP was loaded on NiO at the same surface coverage (21.2 nmol cm−2) as for RuII in NiO|Si-Poly(RuII), the higher Dapp value for electron hopping in NiO|Si-Poly(RuII) is presumably a result of the chemical structures shown in Figures 1c and 1d, with the presence of interconnected RuII. Reduction of CO2 to CO by the photocathode, NiO|Si-Poly(RuII)-Poly(ReI), under solar irradiation was investigated in a two-compartment electrochemical cell with a platinum mesh as the counter electrode. The surface coverages of RuII and ReI were maintained at roughly equal in the assembly, -Poly(RuII)-Poly(ReI). As shown by the photocurrent results in Figures 3a and 3b, the presence of CO2 clearly enhances the magnitudes of photocurrents. The photocurrent density is bias-dependent, reaching 25 μA cm−2 under a bias at −0.8 V Ag/AgCl. The long-term stability of the


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photocathode was investigated by extending the irradiation period to 10 hours during the controlled potential photoelectrocatalysis in the presence of CO2. The photoelectrocatalytic products were analyzed by gas chromatograph (GC, Figure S8). Table 1 summarizes the results for the photocathodes at applied biases from −0.7 to −0.5 V vs Ag/AgCl. From the data, the production of CO is accompanied by evolution of H2 as a minor byproduct (entries 1-3 in Table 1). Control experiments in the absence of CO2, Poly(ReI) or light irradiation did not result in measurable CO as the photoelectrocatalytic product (entries 4-7 in Table 1). Photoelectrochemical experiment with just Poly(ReI) bound in the same way as Poly(RuII) on the silanized NiO resulted in no detectable CO under the same conditions as those for the full assembly (irradiation source, pH 6.8 in CO2-satuarated buffer). The best-performing photocathode gave a turnover number for CO production (TONCO) of 58 and a Faradaic efficiency of 65% after 10 hours of photoelectrocatalysis. The incident photon-to-current efficiency (IPCE) profile of the photocathode in Figure 3d agrees with the absorption features of RuII, with the highest value of 1.0% reached at the absorption maximum (~455 nm). From the long-term photoelectrocatalysis data in Figure 3c, more than 80% of the initial current density was maintained for ~10 hours of photoelectrocatalysis. In comparison, the photocurrents showed initial rapid drop for the system with phosphonate surface binding under the same conditions42, which confirms the high photochemical stability of the assembly and the sustainability of the catalytic reaction. In order to explore the origin of the slow loss in photocurrent over time, the electrode after 10 hours of photoelectrocatalysis was evaluated by CV and XPS measurements. The CV plots in Figure S9 show that the RuIII/II couple was maintained after the long-term photoelectrocatalysis, probably due to the surface protection by the polymeric -Poly(ReI) overlayer. The XPS results in Figure S10 for the Re 4f show an apparent loss in peak intensities, indicating the slow decomposition of the catalyst under the photoelectrocatalytic conditions.


ACS Energy Letters

b -2

Current density (μA cm )

0

-2

Current density (μA cm )

a

-10

-20

N2

0

-10

-20

N2 -30

CO2

CO2

0

c

200

400

600 800 Time (s)

-0.1

1000 1200

0

d

Applied bias: -0.7 V vs Ag/AgCl -5

-0.2

-0.3 -0.4 -0.5 -0.6 E (V vs Ag/AgCl)

-0.7

-0.8

1.0 0.8

-10

IPCE

Current density (μA cm-2)

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

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Turnover number (CO): 58 Faradaic efficiency (CO): 64.8%

-15

0.6 0.4 0.2 0.0

-20 0

2

4 6 Irradiation time (hour)

8

10

400

450

500

550

Wavelength (nm)

600

Figure 3. a, Current densities under chopped light irradiation in N2-degassed (gray line) or CO2-saturated (blue line) NaHCO3 (50 mM) aqueous buffer at an applied bias of −0.7 V vs Ag/AgCl. b, Linear sweep voltammetry plots for NiO|Si-Poly(RuII)-Poly(ReI) at a scan rate of 5 mV s−1 in N2-degassed (gray line) or CO2-saturated (blue line) NaHCO3 (50 mM) aqueous buffer under chopped light irradiation (solar simulator, 100 mW cm−2). c, Long-term photoelectrocatalytic performance of NiO|Si-Poly(RuII)-Poly(ReI) at −0.7 V vs Ag/AgCl. d, IPCE for NiO|Si-Poly(RuII)-Poly(ReI) at different wavelengths at −0.7 V vs Ag/AgCl in CO2 saturated 50 mM NaHCO3 buffer.


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ACS Energy Letters

Table 1 Photoelectrochemical CO2 reduction with variations in the photocathode and reaction conditions a. Entry

Sample

Γ(ReI) /nmol

a

CO /nmol

Potential /V b

(TONCO)

cm-2

H2 /nmol

Fred c

1

NiO|Si-Poly(RuII)-Poly(ReI)

21.4

−0.7

1248 (58)

11

0.65

2


20.7

−0.6

903 (44)

28

0.63

3


21.2

−0.5

627 (30)

35

0.72

4d


20.3

−0.7

n.d f

110

0.082

5e


21.8

−0.7

n.d

n.d

0

6

NiO|Si-Poly(RuII)

−

−0.7

n.d

n.d

0

7

NiO

−

−0.7

n.d

n.d

0

Reaction conditions including continuous visible light irradiation (100 mW cm−2, λ ≥ 400 nm) for 10 hours in

CO2 saturated 50 mM NaHCO3 solution unless otherwise noted.

b

Potential values vs Ag/AgCl.

c

Faradaic

efficiencies of the photoelectrocatalytic products. Samples in N2-degassed 50 mM NaHCO3 solutions without d

CO2 present. e Without irradiation. f Product not detected.

To explore electron transfer dynamics within the molecular assembly, transient absorption (TA) measurements were carried out on the nano-to-microsecond timescale with and without the catalyst or CO2 present.6 Acetonitrile was used as the solvent43, as a way to avoid competitive side reactions (e.g., water reduction) under reductive potentials. Based on the laser flash photolysis results in Figure 4a, excitation of NiO|Si-Poly(RuII) at 488 nm is followed by reductive quenching of the excited state, Poly(RuII*), through hole injection into NiO and formation of the reduced chromophore, Poly(RuI). The appearance of Poly(RuI) is shown by the positive absorptive feature at ~520 nm based on the molar absorptivity data in Figure 4e.43 The TA spectra for the photocathode under an applied bias at −0.5 V vs normal hydrogen electrode (NHE) (Figure 4a) include enhanced absorptive features from Poly(RuI) relative to the spectrum obtained under open circuit conditions (Figure S11). The significantly enhanced absorption magnitude of Poly(RuI) under the negative potential is due to the bias-dependent depletion region for p-type NiO which has been known to inhibit interfacial recombination between the electron at surface molecules and the hole at NiO(h+).44 Based on the timescale for the Poly(RuI) decay monitored at 520 nm in Figure 4b, the lifetime for back electron transfer is ~18 μs from fitting of the decay with the Kohlrausch-Williams-Watts stretched exponential model (see details in the Supplementary Information). With Poly(ReI) added as the CO2 reduction catalyst in NiO|Si-Poly(RuII)-Poly(ReI), hole injection from Poly(RuII*) into NiO, eq. 1, is followed by intra-assembly electron transfer from



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Poly(RuI) to Poly(ReI), eq. 2. The latter is illustrated by the TA spectrum in Figure 4c which shows the initial rapid decay of the Poy(RuI) signal at 510-550 nm accompanied by a hypsochromic shift to 450-520 nm. Based on the molar absorptivities of Poy(RuI) and Poy(Re0)

41

in Figures 4e and 4f, the

hypsochromic shift of the absorption changes is consistent with the appearance of Poly(Re0). Since the chromophore and catalyst are connected by the electronically insulating carbon-carbon bond, the photogenerated electron at the reduced chromophore appear by electron tunneling between sites. The transient decay at 520 nm in Figure 4d arises from both rapid decay of Poly(RuI) and slower delay of Poly(Re0). It is worth noting that the lifetime for loss of the transient intermediate is kinetically complex. A fit of the data to biphasic Kohlrausch-Williams-Watts kinetics45 gave lifetimes of 6.4 μs and 1.7 ms (see details in Experimental Section). From the analysis above, the longer lifetime should arise from back electron transfer in the redox-separated state, NiO(h+)|Si-Poly(RuII)-Poly(Re0), to give the initial state of the photocathode, as illustrated by eq. 3. NiO|Si-Poly(RuII)-Poly(ReI)

ℎ

NiO|Si-Poly(RuII*)-Poly(ReI) → NiO(h+)|Si-Poly(RuI)-Poly(ReI)

eq. 1

NiO(h+)|Si-Poly(RuI)-Poly(ReI) → NiO(h+)|Si-Poly(RuII)-Poly(Re0)

eq. 2

NiO(h+)|Si-Poly(RuII)-Poly(Re0) → NiO|Si-Poly(RuII)-Poly(ReI)

eq. 3

In the presence of dissolved CO2, the transient absorptive features in Figure S12 for the reduced catalyst, Poly(Re0), rapidly diminish within the first 200 ns after the laser pulse. The disappearance of the reduced catalyst should result both from back electron transfer to NiO(h+) and from the series of reactions leading to CO2 reduction46. The latter involves recycling of the catalyst to its initial form through reaction with CO2, followed by a second reduction of the CO2(e−) intermediate with release of CO.46,47 The overall reaction is shown in eq. 4. NiO|Si-Poly(RuII)-Poly(ReI) + CO2 + 2H+

2hν

NiO|Si-Poly(RuII)-Poly(ReI) + CO + H2O

eq. 4

To conclude, we report here a mechanically robust, dye-sensitized photocathode based on a p-type silanized NiO film modified with an electropolymerized chromophore-catalyst assembly. The photocathode was prepared by electrodeposition of vinyl-derivatized ruthenium polypyridyl chromophore

and

rhenium-based

CO2

reduction

catalyst

through

stepwise

reductive

electropolymerization on the silanized oxide films. The resulting hybrid photocathode is stable during long-term photoelectrocatalytic reduction of CO2 to CO in a neutral pH buffer, yielding a Faradaic efficiency of ~0.65 and a turnover number of 58 after 10 hours of photoelectrocatalysis. Results from transient absorption measurements show that excitation of the chromophore is followed by hole injection into NiO and catalyst reduction by the one-electron reduced chromophore. The spectral data also reveal the relatively slow interfacial back electron transfer from the reduced catalyst to NiO which in turn facilitates further electron transfer toward CO2 reduction.


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ΔAbsorbance /10-3

12

b

NiO|Si-Poly(RuII) 35 ns; 50 ns; 2.0 μs; 5.0 μs; 9.0 μs

8

NiO|Si-Poly(RuII) Monitored at 520 nm

7

ΔAbsorbance /10-3

a

4

0

6 τb(RuI) ~ 18 ms

5 4 3

450

ΔAbsorbance /10-3

c

500 550 600 Wavelength (nm)

650

NiO|Si-Poly(RuII)-Poly(ReI) 35 ns; 55 ns; 1.0 μs; 10 μs; 0.1 ms

6

2

700

4 2

10-7

12

450

500 550 600 Wavelength (nm) II

Monitored at 520 nm

5

τb(ReI) ~ 1.7 ms

4 3

700

Time-resolved TA at 520 nm; Monophasic KWW fit

10-7

f I

Poly(Ru )

Δε / 103 M-1cm-2

Poly(Ru )

650

8 4 0

400


800

10-5


2

400

10-6 Time (sec)

6

0

e

Time-resolved TA at 520 nm; Monophasic KWW fit

d ΔAbsorbance /10-3

400

Δε / 103 M-1cm-2

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

ACS Energy Letters

8

10-6

10-5 Time (sec)

Poly(ReI)

10-4

10-3

Poly(Re0)

6 4 2 0

400


800

Figure 4. Transient absorption spectra for the redox-separated intermediates. TA changes (a) and time-resolved trace (b) monitored at 520 nm (blue points: data; gray trace: monophasic KWW) following photo-excitation of NiO|Si-Poly(RuII) at 488 nm under −0.5 V vs NHE. TA changes (c) and time-resolved trace (d) monitored at 520 nm (red points: data; gray trace: biphasic KWW) following photo-excitation of NiO|Si-Poly(RuII)-Poly(ReI) at 488 nm under −0.5 V vs NHE. e, f, Extinction coefficient changes for the reduced chromophore, Poly(RuI), and the reduced catalyst, Poly(Re0), respectively.



ASSOCIATED CONTENT Supporting Information Supporting Information contains experimental details, XRD, XPS, FTIR, SEM images, additional LSV and CV results, representative GC data, and additional transient absorption spectra.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0015739. The experiments with the nanosecond TA spectrometer, fluorimeter, profilometer, and solid-state light source were performed with the instruments within the AMPED EFRC Instrumentation Facility established by the Alliance for Molecular PhotoElectrode Design for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy under Award DE-SC0001011. T.L. acknowledges the support from the National Natural Science Foundation of China (Grant No. 21603110) and State Scholarship Fund from China Scholarship Council.

REFERENCES (1) Qiao, J.; Liu, Y.; Hong, F.; Zhang, J. A Review of Catalysts for the Electroreduction of Carbon Dioxide to Produce Low-Carbon Fuels. Chem. Soc. Rev. 2014, 43 (2), 631-675. (2) Schreier, M.; Gao, P.; Mayer, M. T.; Luo, J.; Moehl, T.; Nazeeruddin, M. K.; Tilley, S. D.; Grätzel, M. Efficient and Selective Carbon Dioxide Reduction on Low Cost Protected Cu2O Photocathodes Using a Molecular Catalyst. Energy Environ. Sci. 2015, 8 (3), 855-861. (3) Schreier, M.; Curvat, L.; Giordano, F.; Steier, L.; Abate, A.; Zakeeruddin, S. M.; Luo, J.; Mayer, M. T.; Grätzel, M. Efficient Photosynthesis of Carbon Monoxide from CO2 Using Perovskite Photovoltaics. Nat. Commun. 2015, 6, 7326. (4) Windle, C. D.; Perutz, R. N. Advances in Molecular Photocatalytic and Electrocatalytic CO2 Reduction. Coord. Chem. Rev. 2012, 256 (21), 2562-2570.


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ACS Energy Letters

(5) Kou, Y.; Nakatani, S.; Sunagawa, G.; Tachikawa, Y.; Masui, D.; Shimada, T.; Takagi, S.; Tryk, D. A.; Nabetani, Y.; Tachibana, H.; et al. Visible Light-Induced Reduction of Carbon Dioxide Sensitized by a Porphyrin–Rhenium Dyad Metal Complex on p-Type Semiconducting NiO as the Reduction Terminal End of an Artificial Photosynthetic System. J. Catal. 2014, 310, 57-66. (6) Tian, H. Molecular Catalyst Immobilized Photocathodes for Water/Proton and Carbon Dioxide Reduction. ChemSusChem 2015, 8 (22), 3746-3759. (7) Sahara, G.; Abe, R.; Higashi, M.; Morikawa, T.; Maeda, K.; Ueda, Y.; Ishitani, O. Photoelectrochemical CO2 Reduction Using a Ru(II)-Re(I) Multinuclear Metal Complex on a p-Type Semiconducting NiO Electrode. Chem. Commun. 2015, 51 (53), 10722-10725. (8) Kamata, R.; Kumagai, H.; Yamazaki, Y.; Sahara, G.; Ishitani, O. Photoelectrochemical CO2 Reduction Using a Ru(II)–Re(I) Supramolecular Photocatalyst Connected to a Vinyl Polymer on a NiO Electrode. ACS Appl. Mater. Interfaces 2018, DOI: 10.1021/acsami.8b05495. (9) Schreier, M.; Luo, J.; Gao, P.; Moehl, T.; Mayer, M. T.; Grätzel, M. Covalent Immobilization of a Molecular Catalyst on Cu2O Photocathodes for CO2 Reduction. J. Am. Chem. Soc. 2016, 138 (6), 1938-1946. (10) Kuehnel, M. F.; Orchard, K. L.; Dalle, K. E.; Reisner, E. Selective Photocatalytic CO2 Reduction in Water through Anchoring of a Molecular Ni Catalyst on CdS Nanocrystals. J. Am. Chem. Soc. 2017, 139 (21), 7217-7223. (11) Yu, Z.; Li, F.; Sun, L. Recent Advances in Dye-Sensitized Photoelectrochemical Cells for Solar Hydrogen Production Based on Molecular Components. Energy Environ. Sci. 2015, 8 (3), 760-775. (12) Gibson, E. A. Dye-Sensitized Photocathodes for H2 Evolution. Chem. Soc. Rev. 2017, 46 (20), 6194-6209. (13) Prévot, M. S.; Sivula, K. Photoelectrochemical Tandem Cells for Solar Water Splitting. J. Phys. Chem. C 2013, 117 (35), 17879-17893. (14) Shan, B.; Sherman, B. D.; Klug, C. M.; Nayak, A.; Marquard, S. L.; Liu, Q.; Bullock, R. M.; Meyer, T. J. Modulating Hole Transport in Multilayered Photocathodes with Derivatized p-Type Nickel Oxide and Molecular Assemblies for Solar-Driven Water Splitting. J. Phys. Chem. Lett. 2017, 8 (18), 4374-4379. (15) Shan, B.; Nayak, A.; N. Sampaio, R.; Eberhart, M. S.; Troian-Gautier, L.; Brennaman, M. K.; Meyer, G. J.; Meyer, T. J. Direct Photoactivation of a Nickel-Based, Water-Reduction Photocathode by a Highly Conjugated Supramolecular Chromophore. Energy Environ. Sci. 2018, 11 (2), 447-455. (16) Odobel, F.; Pellegrin, Y.; Gibson, E. A.; Hagfeldt, A.; Smeigh, A. L.; Hammarström, L. Recent Advances and Future Directions to Optimize the Performances of p-Type Dye-Sensitized Solar Cells. Coord. Chem. Rev. 2012, 256 (21-22), 2414-2423. (17) Gross, M. A.; Creissen, C. E.; Orchard, K. L.; Reisner, E. Photoelectrochemical Hydrogen Production in Water Using a Layer-by-Layer Assembly of a Ru Dye and Ni Catalyst on NiO. Chem. Sci. 2016, 7 (8), 5537-5546. (18) Ashford, D. L.; Gish, M. K.; Vannucci, A. K.; Brennaman, M. K.; Templeton, J. L.; Papanikolas, J. M.; Meyer, T. J. Molecular Chromophore-Catalyst Assemblies for Solar Fuel Applications. Chem. Rev. 2015, 115 (23), 13006-13049. (19) Farnum, B. H.; Wee, K.-R.; Meyer, T. J. Self-Assembled Molecular p/n Junctions for Applications in



Dye-Sensitized Solar Energy Conversion. Nat. Chem. 2016, 8 (9), 845-852. (20) Shan, B.; Nayak, A.; Brennaman, M. K.; Liu, M.; Marquard, S. L.; Eberhart, M. S.; Meyer, T. J. Controlling Vertical and Lateral Electron Migration Using a Bifunctional Chromophore Assembly in Dye-Sensitized Photoelectrosynthesis Cells. J. Am. Chem. Soc. 2018, 140 (20), 6493-6500. (21) Wang, J. C.; Ogunsolu, O. O.; Sykora, M.; Hanson, K. Elucidating the Role of the Metal Linking Ion on the Excited State Dynamics of Self-Assembled Bilayers. J. Phys. Chem. C 2018, 122 (18), 9835-9842. (22) Lapides, A. M.; Sherman, B. D.; Brennaman, M. K.; Dares, C. J.; Skinner, K. R.; Templeton, J. L.; Meyer, T. J. Synthesis, Characterization, and Water Oxidation by a Molecular Chromophore-Catalyst Assembly Prepared by Atomic Layer Deposition. The “Mummy” Strategy. Chem. Sci. 2015, 6 (11), 6398-6406. (23) Alibabaei, L.; Dillon, R. J.; Reilly, C. E.; Brennaman, M. K.; Wee, K.-R.; Marquard, S. L.; Papanikolas, J. M.; Meyer, T. J. Chromophore-Catalyst Assembly for Water Oxidation Prepared by Atomic Layer Deposition. ACS Appl. Mater. Interfaces 2017, 9 (44), 39018-39026. (24) Wang, D.; Sheridan, M. V.; Shan, B.; Farnum, B. H.; Marquard, S. L.; Sherman, B. D.; Eberhart, M. S.; Nayak, A.; Dares, C. J.; Das, A. K.; Bullock, R. M.; Meyer, T. J. Layer-by-Layer Molecular Assemblies for Dye-Sensitized Photoelectrosynthesis Cells Prepared by Atomic Layer Deposition. J. Am. Chem. Soc. 2017, 139 (41), 14518-14525. (25) Eberhart, M. S.; Wang, D.; Sampaio, R. N.; Marquard, S. L.; Shan, B.; Brennaman, M. K.; Meyer, G. J.; Dares, C.; Meyer, T. J. Water Photo-Oxidation Initiated by Surface-Bound Organic Chromophores. J. Am. Chem. Soc. 2017, 139 (45), 16248-16255. (26) Hanson, K.; Brennaman, M. K.; Ito, A.; Luo, H.; Song, W.; Parker, K. A.; Ghosh, R.; Norris, M. R.; Glasson, C. R. K.; Concepcion, J. J.; et al. Structure-Property Relationships in Phosphonate-Derivatized, RuII Polypyridyl Dyes on Metal Oxide Surfaces in an Aqueous Environment. J. Phys. Chem. C 2012, 116 (28), 14837-14847. (27) Hanson, K.; Brennaman, M. K.; Luo, H.; Glasson, C. R. K.; Concepcion, J. J.; Song, W.; Meyer, T. J. Photostability of Phosphonate-Derivatized, RuII Polypyridyl Complexes on Metal Oxide Surfaces. ACS Appl. Mater. Interfaces 2012, 4 (3), 1462-1469. (28) Brown, D. G.; Schauer, P. A.; Borau-Garcia, J.; Fancy, B. R.; Berlinguette, C. P. Stabilization of Ruthenium Sensitizers to TiO2 Surfaces through Cooperative Anchoring Groups. J. Am. Chem. Soc. 2013, 135 (5), 1692-1695. (29) Hanson, K.; Losego, M. D.; Kalanyan, B.; Ashford, D. L.; Parsons, G. N.; Meyer, T. J. Stabilization of [Ru(bpy)2(4,4′-(PO3H2)bpy)]2+ on Mesoporous TiO2 with Atomic Layer Deposition of Al2O3. Chem. Mater. 2013, 25 (1), 3-5. (30) Vannucci, A. K.; Alibabaei, L.; Losego, M. D.; Concepcion, J. J.; Kalanyan, B.; Parsons, G. N.; Meyer, T. J. Crossing the Divide between Homogeneous and Heterogeneous Catalysis in Water Oxidation. Proc. Natl. Acad. Sci. 2013, 110 (52), 20918-20922. (31) Lapides, A. M.; Ashford, D. L.; Hanson, K.; Torelli, D. A.; Templeton, J. L.; Meyer, T. J. Stabilization of a Ruthenium(II) Polypyridyl Dye on Nanocrystalline TiO2 by an Electropolymerized Overlayer. J. Am. Chem. Soc. 2013, 135 (41), 15450-15458. (32) Ashford, D. L.; Lapides, A. M.; Vannucci, A. K.; Hanson, K.; Torelli, D. A.; Harrison, D. P.;


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ACS Energy Letters

Templeton, J. L.; Meyer, T. J. Water Oxidation by an Electropolymerized Catalyst on Derivatized Mesoporous Metal Oxide Electrodes. J. Am. Chem. Soc. 2014, 136 (18), 6578-6581. (33) Ashford, D. L.; Sherman, B. D.; Binstead, R. A.; Templeton, J. L.; Meyer, T. J. Electro-Assembly of a Chromophore-Catalyst Bilayer for Water Oxidation and Photocatalytic Water Splitting. Angew. Chem. Int. Ed. 2015, 54 (16), 4778-4781. (34) Sumikura, S.; Mori, S.; Shimizu, S.; Usami, H.; Suzuki, E. Syntheses of NiO Nanoporous Films Using Nonionic Triblock Co-Polymer Templates and Their Application to Photo-Cathodes of p-Type Dye-Sensitized Solar Cells. J. Photochem. Photobiol. A: Chemistry 2008, 199 (1), 1-7. (35) Li, L.; Gibson, E. A.; Qin, P.; Boschloo, G.; Gorlov, M.; Hagfeldt, A.; Sun, L. Double-Layered NiO Photocathodes for p-Type DSSCs with Record IPCE. Adv. Mater. 2010, 22 (15), 1759-1762. (36) Yoshida, W.; Castro, R. P.; Jou, J.-D.; Cohen, Y. Multilayer Alkoxysilane Silylation of Oxide Surfaces. Langmuir 2001, 17 (19), 5882-5888. (37) Zhong, Y.-W.; Yao, C.-J.; Nie, H.-J. Electropolymerized Films of Vinyl-Substituted Polypyridine Complexes: Synthesis, Characterization, and Applications. Coord. Chem. Rev. 2013, 257 (7), 1357-1372. (38) Trammell, S. A.; Meyer, T. J. Diffusional Mediation of Surface Electron Transfer on TiO2. J. Phys. Chem. B 1999, 103 (1), 104-107. (39) Cui, S.-C.; Sun, X.-Z.; Liu, J.-G. Photo-Reduction of CO2 Using a Rhenium Complex Covalently Supported on a Graphene/TiO2 Composite. ChemSusChem 2016, 9 (13), 1698-1703. (40) Denisevich, P.; Abruna, H. D.; Leidner, C. R.; Meyer, T. J.; Murray, R. W. Electropolymerization of Vinylpyridine and Vinylbipyridine Complexes of Iron and Ruthenium: Homopolymers, Copolymers, Reactive Polymers. Inorg. Chem. 1982, 21 (6), 2153-2161. (41) O'Toole, T. R.; Sullivan, B. P.; Bruce, M. R. M.; Margerum, L. D.; Murray, R. W.; Meyer, T. J. Electrocatalytic Reduction of CO2 by a Complex of Rhenium in Thin Polymeric Films. J. Electroanal. Chem. Interfacial Electrochem. 1989, 259 (1), 217-239. (42) Sahara, G.; Kumagai, H.; Maeda, K.; Kaeffer, N.; Artero, V.; Higashi, M.; Abe, R.; Ishitani, O. Photoelectrochemical Reduction of CO2 Coupled to Water Oxidation Using a Photocathode with a Ru(II)-Re(I) Complex Photocatalyst and a CoOx/TaON Photoanode. J. Am. Chem. Soc. 2016, 138 (42), 14152-14158. (43) Farnum, B. H.; Nakada, A.; Ishitani, O.; Meyer, T. J. Bias-Dependent Oxidative or Reductive Quenching of a Molecular Excited-State Assembly Bound to a Transparent Conductive Oxide. J. Phys. Chem. C 2015, 119 (45), 25180-25187. (44) Nikolaou, V.; Charisiadis, A.; Charalambidis, G.; Coutsolelos, A. G.; Odobel, F. Recent Advances and Insights in Dye-Sensitized NiO Photocathodes for Photovoltaic Devices. J. Mater. Chem. A 2017, 5 (40), 21077-21113. (45) Shan, B.; Farnum, B. H.; Wee, K.; Meyer, T. J. Generation of Long-Lived Redox Equivalents in Self-Assembled Bilayer Structures on Metal Oxide Electrodes. J. Phys. Chem. C 2017, 121 (11), 5882-5890. (46) Hayashi, Y.; Kita, S.; Brunschwig, B. S.; Fujita, E. Involvement of a Binuclear Species with the Re-C(O)O-Re Moiety in CO2 Reduction Catalyzed by Tricarbonyl Rhenium(I) Complexes with Diimine Ligands: Strikingly Slow Formation of the Re-Re and Re-C(O)O-Re Species from



Re(dmb)(CO)3S (dmb = 4,4’-Dimethyl-2,2’-bipyridine, S = Solvent). J. Am. Chem. Soc. 2003, 125 (39), 11976-11987. (47) Kumar, B.; Llorente, M.; Froehlich, J.; Dang, T.; Sathrum, A.; Kubiak, C. P. Photochemical and Photoelectrochemical Reduction of CO2. Annu. Rev. Phys. Chem. 2012, 63 (1), 541-569.


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Stable Molecular Photocathode for Solar-Driven CO2

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