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Activation of Oxygen Reduction Reaction on Well-Defined Pt Electrocatalysts in Alkaline Media Containing Hydrophobic Organic Cations Tomoaki Kumeda, Ryota Kubo, Nagahiro Hoshi, and Masashi Nakamura* Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 5.188.217.228 on 05/21/19. For personal use only.

S Supporting Information *

ABSTRACT: The activity of the oxygen reduction reaction (ORR) is significantly affected by the nonspecifically adsorbed ions at the electrode/ electrolyte interface as well as the surface structures of the catalytic substrate. In this study, we found that an organic cation activates the ORR on Pt single-crystal electrodes in alkaline media, and we evaluated ORR activity using shape-controlled Pt nanoparticles. The ORR activity on Pt(111) was significantly enhanced by a tetramethylammonium cation (TMA+), which correlates with its hydrophobicity as is the case for alkali metal cations. However, an alkylammonium cation with a long chain deactivated electrochemical reactions because these cations assemble at high concentrations on the surface through their hydrophobic interionic interactions with alkyl chains. The ORR activity on Pt nanoparticles was also enhanced by TMA+; cuboctahedral Pt nanoparticles with a (111) plane exhibit a markedly high rate of increase in ORR activity. KEYWORDS: alkaline fuel cell, single-crystal electrode, shape-controlled nanoparticles, electrocatalyst, noncovalent interaction

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INTRODUCTION The development of high-performance electrocatalysts of the oxygen reduction reaction (ORR) is an urgent issue for the widespread use of metal−air batteries and low-temperature fuel cells. The ORR activity strongly depends on the electronic and geometric structures of the electrocatalyst. Platinum (Pt) is commonly used as the ORR catalyst for low-temperature fuel cells; moreover, the introduction of heterogeneous metals, such as nickel and cobalt, significantly activates the ORR.1−4 Recently, it has been revealed that nonspecifically adsorbed ions in an electrical double layer (EDL) affect many electrochemical reactions. These nonspecifically adsorbed ions are located at the outer Helmholtz plane (OHP) of the EDL, which noncovalently interacts with the substrate via the hydration shell. 5−7 The strength of the noncovalent interactions in the EDL is one of the factors governing the ORR activity. The effect of noncovalent interactions on the electrocatalytic reactions has been supported by theoretical calculations.8 On single-crystal Pt electrodes in acidic solution, the ORR is enhanced by the addition of a small amount of tetrahexylammonium cations that have a strong hydrophobicity because of long alkyl chains, resulting in an ORR activity comparable to that on Pt alloy electrodes.9 The ORR is deactivated by a rigid hydrogen-bonding network of adsorbed oxygen species such as water and hydroxide. Hydration water of hydrophobic cations destabilizes the hydrogen-bonded adlayer. Thus, the structural © XXXX American Chemical Society

understanding of the EDL as well as the substrate will accelerate the development of high-performance and low-cost ORR electrocatalysts. Although the proton is the general cationic species in acidic solution, various cations are available in alkaline solution. The selection of the appropriate cationic species is necessary for alkaline electrochemical devices. Markovic et al. found an inverse correlation between the hydration energies of alkali metal cations and the ORR activities of Pt electrodes in alkaline media.10 They also proposed an interfacial model including noncovalent interactions, such as hydrogen-bonding and dipole−dipole interactions, between hydrated cations and adsorbed OH (OHad) species on the Pt electrode.10−15 OHad species are stabilized by hydrogen bonding with water molecules and form a coadsorption bilayer of OHad−H2O on the Pt(111) surface, which blocks active sites and inhibits the ORR.16,17 Infrared (IR) spectra revealed the presence of OHad on Pt(111) in LiOH and small cations with a high charge density, such as Li+, that strongly interact with OHad.6 Consequently, the ORR activity of a Pt(111) single-crystal electrode in LiOH solution was found to decrease by 1 order of magnitude compared with that in CsOH solution.10 Received: March 18, 2019 Accepted: May 9, 2019 Published: May 9, 2019 A

DOI: 10.1021/acsaem.9b00582 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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above 0.60 V. On the Pt(111) electrode, the hydrogen adsorption/desorption region is independent of electrolyte cations, indicating that cations do not inhibit hydrogen adsorption. The region above 0.6 V consists of multiple peaks that are assigned to the formation of the Pt oxide species, including PtOH, PtO, and PtOx. The first oxidation in region I and the second oxidation in region II in LiOH are significantly different from those in other solutions. In LiOH, peak I is spectroscopically assigned to the formation of OHad, which is stabilized by the strong oxygen affinity of Li+.6 Figure 1b shows the potential-dependent charge density for hydrogen desorption and the formation of the oxidized Pt species. The charge density was calculated by time integration of the anodic current from 0.5 V after correction for the double-layer charging current. In contrast to the hydrogen adsorption/ desorption, the surface oxidation on the Pt(111) electrode differs depending on the electrolyte solution. The charge density below 0.9 V is definitely reduced by the presence of TMA+. The dependence of the ORR on the cation was investigated on Pt(111) and Pt(100), whose activities were the highest and the lowest in the low-index planes of Pt, respectively.19,20 Figure 2a shows the linear sweep ORR voltammograms of the Pt(111) electrode in 0.1 M alkali metal hydroxide and TMAOH. The ORR activity estimated from the kinetic current density at 0.90 V increased in the order of Li+ < Na+

Figure 1. (a) Cyclic voltammograms and (b) charge density of the Pt(111) electrode in 0.1 M LiOH, NaOH, KOH, and tetramethylammonium hydroxide (TMAOH) saturated with Ar. The scanning rate was 0.050 V s−1.

Figure 2. (a) Linear sweep oxygen reduction reaction (ORR) voltammograms of the Pt(111) electrode in 0.1 M LiOH, NaOH, KOH, and TMAOH saturated with O2. Potential was scanned in the positive direction. The scanning rate is 0.010 V s−1, and the rotation rate of the electrode is 1600 rpm. (b) Correlation between the specific oxygen reduction reaction (ORR) activities of Pt(111) and Pt(100) at 0.9 V (RHE) and hydration energies of cations.

Conventional studies of the effect of cations on the ORR have focused on monatomic cations, such as alkali metals. Organic cations, such as tetraalkylammonium cations (TAA+), can control hydrophobicity and the structure of the hydration shell. The addition of ppm amounts of TAA+s with long alkyl chains to an acidic medium activates the ORR on the Pt(111) electrode.9 In alkaline media, dominant cations that interact with the electrode are added in large amounts as a support electrolyte; the interfacial behavior of these cations may be different from that in acidic media where protons are dominant. Nanomaterials with a high specific surface area are required as electrocatalysts when considering the practical applications of hydrophobic cations in electrochemical devices. Therefore, the effects of cations on the ORR have been studied on Pt nanoparticles (NPs) and on the model electrodes using single crystals.10,11,18 However, the cationic effects depend on the surface structure; therefore, it is important to estimate the ORR activity using shape-controlled particles with well-defined surfaces. In this study, we evaluated the ORR activities of single-crystal Pt electrodes and Pt NPs in alkaline media containing alkali metal and TAA+ cations. The structural effects on the ORR were examined using commercial Pt/C and cuboctahedral Pt NPs with (111) facets.

RESULTS AND DISCUSSION In alkaline media, the ORR activity on Pt(111) is higher than that on other low-index planes.19,20 First, the cation effects on Pt(111) were investigated herein in detail. Figure 1a shows the cyclic voltammograms (CVs) of the Pt(111) electrode in 0.1 M alkali metal hydroxide and tetramethylammonium hydroxide (TMAOH) solutions. The CVs indicate the hydrogen adsorption/desorption on Pt between 0.05 and 0.45 V, the double-layer charging/discharging between 0.45 and 0.60 V, and the formation/dissociation of the oxidized Pt species

B

DOI: 10.1021/acsaem.9b00582 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 3. (a) Potential dependence of the infrared spectra on the Pt(111) electrode in 0.1 M TMAOH saturated with Ar. The potential of the reference spectrum is 0.5 V (RHE). The potentials of the sample spectra were stepped in the positive direction. (b) Schematic model of the interfacial structure on the Pt electrode. The green, red, and blue balls represent the cation (M+: alkali metal; N+: tetraalkylammonium), water, and adsorbed oxygenated species, respectively. The arrows and wedges denote the direction of the water dipoles and the noncovalent interactions, respectively.

< K+ < TMA+. The ORR activities in alkali metal hydroxide solutions are identical to those in the previous report10 and show a linear correlation with the hydration energies of alkali metal cations (K+ < Na+ < Li+). The correlation in TMA+ is located on a line with alkali metal cations, as shown in Figure 2b. The activity of Pt(111) in TMAOH is higher than in CsOH reported previously.10 The kinetic control region of the ORR corresponds to the first oxidation between 0.6 and 0.9 V in the CVs (region I). In all alkali metal hydroxide solutions, the charge densities up to 0.9 V are almost the same despite the different ORR activities. The alkali metal cations at the OHP affect the stabilization of OHad, which deactivates the ORR. Infrared reflection absorption spectroscopy (IRAS) can detect the OHad formation on the Pt electrode.21 IRAS spectra were measured on Pt(111) in LiOH and CsOH of which ORR activity is the lowest and the highest, respectively.6 Although the band for the PtOH bending mode (δPtOH) appeared in LiOH, no band for δPtOH was observed in CsOH, suggesting that the hydrophobic cation at the OHP inhibits OHad formation.6 The lower charge density in TMAOH up to 0.9 V indicates a decrease in the total coverage of Pt oxides, which synergistically enhances the ORR. The ORR activities on Pt(100) in the same alkaline solutions were evaluated. Although the activities on Pt(100) in all the solutions were lower than those on Pt(111), they correlated to the hydration energies of cations with a slope different from that on Pt(111). The activities of Pt(111) and Pt(100) in TMAOH were enhanced by 140 and 4 times greater than those in LiOH, respectively. The OH blocking the ORR is also adsorbed on Pt(100) as well as Pt(111). On Pt(111), the hydrogen-bonded OHad−H2O layer is stabilized because the symmetry and the OH−O bond length match well with the Pt−Pt distance on the (111) surface. The hydrophobic cation destabilizes this OHad−H2O hydrogen-bonding network and activates the ORR on Pt(111). Cation effects on the destabilization of OHad− H2O layer cause a huge difference in the cation dependence between Pt(111) and Pt(100). In an acidic solution, the ORR activity was enhanced by TAA+ with longer alkyl chains, such as tetra-n-butylammonium (TBA+) and tetra-n-hexylammonium (THA+), at low concentrations (10−5−10−6 M). We examined the CVs and the ORR voltammograms in 0.1 M TBA hydroxide (TBAOH)

(Supporting Information, Figure S1). TBA+ added as a supporting electrolyte at a high concentration significantly inhibited hydrogen adsorption and the ORR because these cations at high concentrations assemble on the surface through their hydrophobic interionic interactions via alkyl chains. The coverage of OHad also decreases significantly, suggesting that OHad may not be a site blocker in 0.1 M TBAOH. A recent theoretical calculation predicted the multiple roles of adsorbed oxygen species during the ORR.22 The high interfacial coverage of hydrophobic TBA+ excludes water and O2 molecules from the interface. In-situ IRAS was performed to identify the adsorbed species in TMA+. Figure 3a shows the potential-dependent IR spectra of Pt(111) in 0.1 M TMAOH. The negative-going bands at 1420 and 1490 cm−1 are assigned to the symmetric and asymmetric CH3 bending modes (δCH3) of TMA+, respectively.23,24 The monopolar band corresponding to the bulk solution (Figure S2a) suggests that TMA+ is a nonspecifically adsorbed species. The intensity of δCH3 increases at positive potentials. This observation indicates a decrease in the interfacial coverage of TMA+ because of the weakening of the electrostatic interactions.25 However, the surface concentration and the interfacial structure of nonspecifically adsorbed species are difficult to determine from the IRAS band quantitatively. The positive and negative-going bands at 1650 and 1610 cm−1 above 0.7 V are assigned to the HOH bending mode (δHOH) from nonadsorbed and adsorbed water on the Pt(111) electrode, respectively.6,9,26 The onset potential of δHOH corresponds to the first anodic peak of the voltammogram (Figure 1a), indicating that the coverage of adsorbed water decreases because of the Pt oxidization. In TMAOH, no band of δPtOH was observed near 1100 cm−1 as is the case in CsOH, which showed a higher ORR activity.6 On Pt(111), a rigid hydrogen-bonding network composed of the OHad + H2O layer inhibits the ORR.27−29 The hydrated water molecules with hydrophilic cations orient with their dipole moments pointed outward, as shown in Figure 3b, which stabilizes the hydrogen-bonding network of the OHad + H2O coadsorbed layer.9 However, the hydration water around the hydrophobic cations is restricted to coordination with the outer oxygen species because of the complete hydrogenC

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Figure 4. (a) Cyclic voltammograms of carbon-unsupported cuboctahedral Pt NPs supported on the GC electrode in 0.1 M LiOH, NaOH, KOH, and tetramethylammonium hydroxide (TMAOH) saturated with Ar. The scanning rate was 0.050 V s−1. (b) Specific oxygen reduction reaction (ORR) activities of commercial Pt/C and cuboctahedral Pt NPs in 0.1 M LiOH, NaOH, KOH, CsOH, and TMAOH saturated with O2 at 0.9 V (RHE). (c) Potential cycle dependence of the specific ORR activities of commercial Pt/C and cuboctahedral Pt NPs in 0.1 M TMAOH saturated with O2 at 0.9 V (RHE) after the accelerated durability test between 0.6 and 1.0 V (RHE).

bonding network within the shell. 30,31 Therefore, the interaction between the hydration water and the hydrogenbonding network is weakened, and the ORR active sites remain. The promotive effect of TAA+ on the ORR was remarkable on the (111) surface of Pt;9 therefore, shape-controlled NPs with (111) facets are appropriate as the electrocatalysts of the ORR. We evaluated the cation effect on the ORR using carbon-unsupported cuboctahedral Pt NPs that have the (111) and (100) nanofacets with sizes of 13.8 ± 1.2 nm, as shown in a TEM image (Figure S3a). The characteristic CV of cuboctahedral Pt NPs in H2SO4 is shown in Figure S3d. Figure 4a shows the CVs of cuboctahedral Pt NPs supported on a glassy carbon (GC) electrode in 0.1 M alkali metal hydroxide and TMAOH. The peaks at approximately 0.25 and 0.4 V are assigned to the adsorption/desorption of hydrogen + OH on the (110) and (100) surfaces, respectively,32,33 which depends on the electrolyte cations. A similar dependence also appears on the Pt(100) electrode as well as on commercial Pt/ C as shown in the literature and in Figure S4a.11,24 The region above 0.6 V consists of multiple peaks that are assigned to the formation of oxidized Pt species. The cation dependence of the onset potentials for the first oxidation between 0.6 and 0.9 V and the second oxidation above 0.9 V corresponds to that on the Pt(111) electrode. Figure 4b shows the ORR activities of the cuboctahedral Pt NPs and commercial Pt/C supported on the GC electrode in alkaline solutions at 0.9 V (the CVs and the linear sweep ORR voltammograms are shown in Figure S4). On these NPs, the dependence of the ORR activity on cations is similar to that on the single-crystal electrodes. Furthermore, the activities of cuboctahedral Pt NPs are higher than those of the commercial Pt/C because of the active (111) facets. An accelerated durability test (ADT) of cuboctahedral Pt NPs and commercial Pt/C in TMAOH was performed using the potential cycles between 0.6 and 1.0 V. The CVs after ADT are shown in Figure S5. Although the ORR activity of cuboctahedral Pt NPs and commercial Pt/C at 0.9 V decreases with the same ratio as shown in Figure 4c, the activity of cuboctahedral Pt NPs is 7 times as high as that of the commercial Pt/C after 10000 cycles. The oxidation/reduction cycles of the Pt electrode cause the restructuring of the surface

atoms. The faceted structure of cuboctahedral Pt NP is destroyed by this ADT, resulting in the ORR deactivation.

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CONCLUSION In summary, we evaluated the ORR activities of the Pt(111) electrode and cuboctahedral Pt NPs with (111) facets in alkaline solutions containing alkali metal and tetraalkylammonium cations. The ORR activities increased in the order of Li+ < Na+ < K+ < TMA+, which correlates with the hydration energies of cations. The hydration water around the hydrophilic cations such as alkali metal cations interacts with OHad and inhibits the ORR. Conversely, hydrophobic cations, such as tetramethylammonium cation, weaken the interaction between the hydration water and the surface oxygen species and activates the ORR.

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EXPERIMENTAL SECTION

Single-crystal Pt beads for voltammetry and infrared spectroscopy were prepared by Clavilier’s method.34,35 The samples were annealed in an H2 + O2 flame and then cooled to room temperature in an Ar atmosphere. The annealed surfaces were protected with ultrapure water (Milli-Q Advantage A10, Millipore). Carbon-unsupported cuboctahedral Pt NPs were synthesized as described in a previous report (Supporting Information).36 The electrolyte solutions were prepared using LiOH·H2O (Sigma-Aldrich), NaOH (Sigma-Aldrich), KOH (Sigma-Aldrich), CsOH·H2O (Sigma-Aldrich), TMAOH·5H2O (Sigma-Aldrich, Wako Pure Chemical Industries), TMAOH solution (Alfa Aesar), TBAOH solution (Sigma-Aldrich, Wako Pure Chemical Industries), and ultrapure water (Milli-Q Advantage A10, Millipore). The reference electrode used in all measurements was the RHE. The iR compensated linear sweep voltammograms for the ORR were measured using an electrochemical analyzer (ALS 701DH, BAS) and a rotating ring−disk electrode (RRDE-3A, BAS). The ORR activities were estimated from the kinetic current density (specific activity, jk) at 0.90 V versus RHE according to the Koutecky−Levich equation: 1/j = 1/jk + 1/jL, where j and jL are the total current density and limiting current density, respectively. The current densities of the voltammograms of Pt(111) and Pt NPs were normalized to the geometrical and electrochemical surface areas (ECSAs), respectively. ECSAs of Pt NPs were calculated using the charge density of hydrogen desorption. The accelerated durability test was performed by applying a square potential wave between 0.6 and 1.0 V with 0.17 Hz. In-situ IRAS measurements were performed using a Fourier transform IR (FTIR) spectrometer (Vertex 70v, Bruker). An electrochemical IR cell with a trapezoid CaF2 window beveled at 60° was attached to the spectrometer with a narrow band mercury− D

DOI: 10.1021/acsaem.9b00582 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials cadmium−telluride (MCT) detector. The interferograms obtained were averaged over 640 scans by using subtractively normalized interfacial FTIR (SNIFTIRS) with p-polarized light at a resolution of 4 cm−1.

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(12) Lopes, P. P.; Strmcnik, D.; Jirkovsky, J. S.; Connell, J. G.; Stamenkovic, V.; Markovic, N. Double layer effects in electrocatalysis: The oxygen reduction reaction and ethanol oxidation reaction on Au(111), Pt(111) and Ir(111) in alkaline media containing Na and Li cations. Catal. Today 2016, 262, 41−47. (13) Stamenkovic, V. R.; Strmcnik, D.; Lopes, P. P.; Markovic, N. M. Energy and fuels from electrochemical interfaces. Nat. Mater. 2017, 16, 57−69. (14) Colic, V.; Pohl, M. D.; Scieszka, D.; Bandarenka, A. S. Influence of the electrolyte composition on the activity and selectivity of electrocatalytic centers. Catal. Today 2016, 262, 24−35. (15) Jensen, K. D.; Tymoczko, J.; Rossmeisl, J.; Bandarenka, A. S.; Chorkendorff, I.; Escudero-Escribano, M.; Stephens, E. L. Elucidation of the oxygen reduction volcano in alkaline media using a copper− platinum(111) alloy. Angew. Chem., Int. Ed. 2018, 57, 2800−2805. (16) Wakisaka, M.; Suzuki, H.; Mitsui, S.; Uchida, H.; Watanabe, M. Identification and quantification of oxygen species adsorbed on Pt(111) single-crystal and polycrystalline Pt electrodes by photoelectron spectroscopy. Langmuir 2009, 25, 1897−1900. (17) Chen, J.; Fang, L.; Luo, S.; Liu, Y.; Chen, S. Electrocatalytic O2 reduction on Pt: multiple roles of oxygenated adsorbates, nature of active sites, and origin of overpotential. J. Phys. Chem. C 2017, 121, 6209−6217. (18) Zhu, S.; Hu, X.; Zhang, L.; Shao, M. Impacts of perchloric acid, nafion, and alkali metal ions on oxygen reduction reaction kinetics in acidic and alkaline solutions. J. Phys. Chem. C 2016, 120, 27452− 27461. (19) Markovic, N. M.; Gasteiger, H. A.; Ross, P. N. Oxygen reduction on platinum low-index single-crystal surfaces in alkaline solution: rotating ring diskPt(hkl) studies. J. Phys. Chem. 1996, 100, 6715−6721. (20) Briega-Martos, V.; Herrero, E.; Feliu, J. M. Effect of pH and water structure on the oxygen reduction reaction on platinum electrodes. Electrochim. Acta 2017, 241, 497−509. (21) Clay, C.; Haq, S.; Hodgson, A. Hydrogen bonding in mixed OH + H2O overlayers on Pt(111). Phys. Rev. Lett. 2004, 92, 046102. (22) Huang, J.; Zhang, J.; Eikerling, M. Unifying theoretical framework for deciphering the oxygen reduction reaction on platinum. Phys. Chem. Chem. Phys. 2018, 20, 11776−11786. (23) Chung, H. T.; Choe, Y.-K.; Martinez, U.; Dumont, J. H.; Mohanty, A.; Bae, C.; Matanovic, I.; Kim, Y. S. Effect of organic cations on hydrogen oxidation reaction of carbon supported platinum. J. Electrochem. Soc. 2016, 163, 1503−1509. (24) Chung, H. T.; Martinez, U.; Matanovic, I.; Kim, Y. S. Cation− hydroxide−water coadsorption inhibits the alkaline hydrogen oxidation reaction. J. Phys. Chem. Lett. 2016, 7, 4464−4469. (25) Dunwell, M.; Wang, J.; Yan, Y.; Xu, B. Surface enhanced spectroscopic investigations of adsorption of cations on electrochemical interfaces. Phys. Chem. Chem. Phys. 2017, 19, 971−975. (26) Nakamura, M.; Kato, H.; Hoshi, N. Infrared spectroscopy of water adsorbed on M(111) (M = Pt, Pd, Rh, Au, Cu) electrodes in sulfuric acid solution. J. Phys. Chem. C 2008, 112, 9458−9463. (27) Jinnouchi, R.; Kodama, K.; Morimoto, Y. DFT calculations on H, OH and O adsorbate formations on Pt(111) and Pt(332) electrodes. J. Electroanal. Chem. 2014, 716, 31−44. (28) Jinnouchi, R.; Kodama, K.; Nagoya, A.; Morimoto, Y. Simulated volcano plot of oxygen reduction reaction on stepped Pt surfaces. Electrochim. Acta 2017, 230, 470−478. (29) Gomez-Marin, A. M.; Feliu, J. M. Oxygen reduction at platinum electrodes: The interplay between surface and surroundings properties. Curr. Opin. Electrochem. 2018, 9, 166−172. (30) Yamakata, A.; Osawa, M. Destruction of the water layer on a hydrophobic surface induced by the forced approach of hydrophilic and hydrophobic cations. J. Phys. Chem. Lett. 2010, 1, 1487−1491. (31) Yamakata, A.; Osawa, M. Cation-dependent restructure of the electric double layer on CO-covered Pt electrodes: Difference between hydrophilic and hydrophobic cations. J. Electroanal. Chem. 2017, 800, 19−24.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00582.

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Supporting methods and Figures S1−S5 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. ORCID

Nagahiro Hoshi: 0000-0001-5808-580X Masashi Nakamura: 0000-0002-2986-1133 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Kurita Water and Environment Foundation, Research Foundation for the Electrotechnology of Chubu, JSPS KAKENHI Grants 2435001, 15H03763, and 18J11901, and NEDO.

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DOI: 10.1021/acsaem.9b00582 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX