Rational Design of TiC-Supported Single-Atom Electrocatalysts for

Nov 19, 2018 - ... Design of TiC-Supported Single-Atom Electrocatalysts for Hydrogen Evolution ... of a wide range of transition-metal single atoms on...
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Rational Design of TiC-Supported Single-Atom Electrocatalysts for Hydrogen Evolution and Selective Oxygen Reduction Reactions Suman Kalyan Sahoo,†,§ Youngjin Ye,†,§ Seonggyu Lee,†,§ Jinkyu Park,† Hyunjoo Lee,‡ Jinwoo Lee,*,‡ and Jeong Woo Han*,† Downloaded via UNIV OF WINNIPEG on December 20, 2018 at 08:24:24 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 37673, Republic of Korea ‡ Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea S Supporting Information *

ABSTRACT: We use a combination of density functional theory (DFT) calculations and experimental approaches to explore the stability and electrocatalytic activity of a wide range of transitionmetal single atoms on a TiC support. Our theoretical prediction that single atoms can be stabilized on the modified TiC surface is confirmed by experimental findings using them on a TiC support. The predicted activities where Pt and Au single atoms would be the best for hydrogen evolution and selective oxygen reduction reactions, respectively, agree well with experimental results. This rational strategy using computational modeling of materials enables effective design of highly active and stable single-atom catalysts.

for selective electrochemical reactions.15 Although the SAC has very good activity, it has the major drawback that these single atoms are not chemically stable in harsh, corrosive, and oxidizing electrocatalytic reaction conditions. Transition-metal carbides are among the finest support materials to anchor the single atom due to their chemical stability in an acid medium, resistance to poisoning, high electrical conductivity, and strong interaction with noble metals.16,17 Wang et al. found that the activities for hydrogen oxidation and evolution reactions on a low loading of Pt nanoparticles (NPs) supported over transition-metal carbides are comparable to those of commercial Pt/C catalysts.18 The monolayer of Pt on TiC as a low-cost and stable electrocatalyst also showed comparable hydrogen evolution reaction (HER) activity to that of bulk Pt.19 However, TiC has been relatively less investigated as a support material compared to other carbides such as tungsten carbide or molybdenum carbide, which are better as stand-alone catalysts than TiC.20−22 This study had two aims, with the goal of reducing production costs in the chemical industry. Our first aim was to find a suitable reaction condition to stabilize a single atom

R

educing the cost of production is one of the major goals in the chemical industry. Most of the chemical industry uses precious metals as catalysts; therefore, one strategy to decrease production costs is to replace the expensive metal by a suitable cheaper metal and to utilize the catalyst effectively, such that every atom of it is active. A singleatom catalyst (SAC) consists of isolated metal atoms singly dispersed on a substrate; this arrangement has the highest atom count, i.e., a maximized number of catalytic active sites.1,2 These single atoms have very high surface energy due to the low coordination; therefore, to stabilize the catalyst and to prevent its aggregation into large clusters, a suitable substrate is required. To synthesize a SAC, the main requirement is to find a suitable support and suitable reaction condition, in which the single atoms can anchor finely, densely, and tightly. SACs that have higher activity, selectivity, or both than their nanostructure or bulk counterparts have been designed using theoretical methods and synthesized using experimental methods.3−6 Recently, the use of SACs in electrocatalytic reactions have been actively studied because they can provide energy sources or chemicals in an environmentally benign way.7−12 Choi et al. stabilized the atomically dispersed Pt on zeolite-templated carbon that contains an extra large amount of sulfur, which selectively produced H2O2.13 Skomoski et al. used metal− organic chains to stabilize single-site Pt(II) on a Au(111) surface.14 Recently, Yang et al. synthesized a SAC of Pt on TiN © XXXX American Chemical Society

Received: October 10, 2018 Accepted: November 19, 2018 Published: November 19, 2018 126

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Figure 1. (a) Binding energies of single M atoms on the CT site of perfect TiC(001) and the diffusion barriers from the CT site to the nearest CT site. The lines are guides to the eye. (b) Binding energy of the M atom in the C-vacancy site of the TiC(001) surface as a function of the carbon chemical potential μc. Δμc is the thermodynamically allowed range of μc from −1.76 eV (Ti-rich) to 0.00 eV (C-rich).

Figure 2. HAADF-STEM images of (a) Pd/TiC (circles: Pd atoms) and (b) Pt/TiC (circles: Pt atoms). XANES spectra at the (c) Pd K edge and (d) Pt L3 edge. k3-weighted FT-EXAFS spectra at the (e) Pd K edge and (f) Pt L3 edge.

on the substrate to maximize the atom utilization of the catalyst. Using first-principles calculations followed by the corresponding experiment, we investigated the potential of TiC as a support to prepare a SAC of a transition metal (M = Cu, Ag, Au, Ni, Pd, or Pt). Because carbon vacancy is very common in experimental findings, we examined the effects of carbon vacancy on stabilization of single M atoms on the TiC substrate.23 After confirming the stability of the M atoms, we quantified their catalytic activities and compared them with those of commercial or pristine NP catalysts for the HER and the selective oxygen reduction reaction (ORR) to produce H2O2. We chose these two electrochemical reactions because, to now, scarce and high-cost noble metals are the most soughtafter catalysts for HER and ORR. Thus, our second aim was to find better SACs that can potentially replace commercial or pristine NP catalysts. M Single-Atom Adsorbed on Perfect and Defective TiC(001) Surfaces. To determine the most stable adsorption site for a single M atom on a perfect surface, we assessed three positions: carbon top (CT), titanium top (TT), and hollow (H) sites (Figure S1). We calculated the adsorption of all M

atoms on these sites and found that CT was the most favorable adsorption site for all cases (Table S1), as was determined earlier.24,25 The binding energies (Figure 1a) of single M atoms on a TiC surface were calculated with respect to the cohesive energy of bulk metal (Table S2) to evaluate the clustering tendency of the M atom on the surface (eq S2). The Pd atom has a negative binding energy of −0.78 eV, which implies that its metal−support interaction is higher than the Pd−Pd interaction in the bulk form. All of the other atoms assessed have positive binding energies, which means that they tend to form clusters rather than remain solitary on the surface. Using the climbing image nudged elastic band (CI-NEB) method, we calculated the diffusion barrier (Figure 1a) of a single M atom on the surface by selecting two neighboring CT sites as the initial and final states. The transition state was located at the bridge position. Compared to the binding energy, the energy barrier of a single M atom on this surface was relatively low; as a result, single M atoms were not stable over their clustering and migration tendencies on the perfect TiC(001). 127

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temperature reduction in this study, single Pt atoms were placed at the sites with surface ligands rather than the formation of uniform chemical bonding or alloying with supports.15,27 Yao et al. also synthesized atomic Au (0.9 wt %) using α-MoC as a support. The EXAFS data for their atomic Au on the support have a higher Au−Au coordination number (2) than that (0.763) of our Au single atoms on TiC (0.5 wt %), which implies more dispersed atomic Au on the TiC support than α-MoC.22 After establishing the stability of single atoms both theoretically and experimentally, we tested their activity in HER and ORR to evaluate their applicability as single-atom electrocatalysts. Our DFT calculations to predict the activity of SACs/TiC considered the ligand’s presence near the catalyst (Figure S9), which often arises from their metal precursor during synthesis. Previous studies reported from EXAFS data that there exists a chlorine ligand for Au,27 Pd,28 and Pt29 single atoms. In the case of Cu,30 Ag,31 and Ni32 single atoms, there is no evidence of chlorine ligands, but the oxygen atom may exist as a ligand. The presence of these ligands was also confirmed in our EXAFS data (Table S5). Hydrogen Evolution Reaction (HER). The HER reaction mechanism in acid solution has been described as a three-stage process: (1) H+ + e− generation, (2) H* adsorption, and (3) 1 production of 2 H 2(g).33 Under standard conditions (pH = 0, p(H2) = 1 bar, U = 0 VSHE), the overall HER pathway can be described as

Modification of the perfect surface is a possible approach to stabilize single atoms. Experimental findings commonly observe defects in TiC. To create a carbon vacancy defect (Cvac), we pulled one carbon atom out of the surface. The formation energy of Cvac was calculated using eq S1 under different chemical environments. It was energetically favorable (−0.95 eV) in a Ti-rich condition but was significantly disfavored (2.50 eV) in a C-rich environment. We then put a single M atom on a vacancy site, that was on top of the carbon vacancy (CvacT), TT, or H site. The placement of the M atom at the CvacT was thermodynamically favored (Table S3). The binding energies (Figure 1b) of single M atoms on the vacancy-defected surface were affected by the carbon chemical potential (Δμc) where the thermodynamically allowed range of Δμc was from −1.76 eV (Ti-rich) to 0.00 eV (C-rich). As the system changed from C-rich to Ti-rich, the formation of atomic M was stabilized on this surface. Of the single atoms that we considered, Au, Pd, and Pt had the highest tendencies to be stabilized in an atomic form. The single M atoms had a higher charge transfer and shorter surface−metal distance on the defective surface than on the perfect one (Table S4); this difference indicates that surface−metal bonding was stronger on the defective surface than on the perfect one. Density of state (DOS) analysis (Figure S2) also supports the strong interaction of single M atoms with the support because the spin magnetic moment of the adsorbed single atom on the TiC surface is fully quenched26 and M atoms have the same intensity and position of spin up and spin down on the carbon vacancy surface. Experimental Characterization of Single-Atom Catalysts. On the basis of the theoretical findings, we synthesized 0.2 wt % M/TiC powders by the incipient wetness method and reduction at desired temperature with 10% H2 gas (balanced with Ar). Due to high Z (atomic number) contrast between noble metals and the TiC support, single atoms can be identified as isolated bright dots in high-angle annular darkfield scanning transmission electron microscopy (HAADFSTEM) images (Figures 2a,b and S3a,b). X-ray absorption spectroscopy (XAS) analysis was conducted to examine the overall electronic and local structures of M single atoms. In the case of Pt, Au, Cu, and Ni, 0.5 wt % M/ TiC was used because the signal obtained using 0.2 wt % M/ TiC was too weak. In the XANES spectra, SACs showed slightly higher white line intensity compared to their metal foil (Figures 2c,d, S3c,d, and S4a,b). The white line corresponds to the electronic transition from the core level (1s for K edge, 2p2/3 for L3 edge) to the unoccupied valence state level. Thus, a higher white line intensity indicates more vacancies in the valence orbital, that is, more oxidized electronic structure. Bonding with surface oxidizing ligands (highly electronegative Cl or O from their precursors) in SACs induces electron transfer from the single-atom metal to surface oxidizing ligands. Therefore, the single-atom metal has increased the valence state compared to their metal foil.3,15 In EXAFS spectra, all of metal−metal bond peaks almost disappear in SACs, and a new bond with Cl or O ligands, which come from the precursors of metal, arises (Figures 2e,f, S3e,f, and S4c,d); this result is consistent with formation of a single atom on a TiC support. The coordination number (Table S5) for each bond was obtained by fitting of EXAFS spectra. Lin et al. also found that Pt is favorable for the formation of a single atom on α-MoC,21 but in this case, the Pt−Mo coordination number was higher than that of Pt−Ti on TiC here. Due to the low-

H + + e− →

1 H 2(g) 2

ΔG 0 = 0 eV

where the free energies of initial and final states are the same under the standard conditions. Thus, the Gibbs free energy ΔG H* = ΔE H + ΔZPE − T ΔS

for the adsorption of intermediate hydrogen on the catalyst is a key quantity to describe HER activity of the catalyst. ΔE H = E(H*) −

1 E(H 2) − E(M/TiC) 2

is the hydrogen adsorption energy, where E(H*) represents the total energy of a surface on which single atoms are deposited with adsorbed hydrogen and E(H2) and E(M/TiC) are the total energies of the gas-phase H2 molecule and the surface without the adsorbed hydrogen, respectively. ΔZPE = ZPE(H*) −

1 ZPE(H 2) 2

is the difference in zero point energy (ZPE), where ZPE(H*) is the ZPE of adsorbed hydrogen atom, which does not change significantly on the surfaces. ΔS is the difference between the entropy of adsorbed hydrogen and half of the entropy of the H2 in the gas phase under the standard condition. The entropy and ZPE contributions of the solid surface are typically very small and can be neglected. The vibrational frequencies, ZPE, and ΔS values used for our calculations are summarized in Table S6. We optimized the adsorbed structure of atomic H, placing it on the top of a single metal atom on the surface. Then we calculated the Gibbs free energies and hydrogen adsorption energies on our catalysts. For an ideal HER catalyst, ΔGH* should be close to zero, where the hydrogen atom does not 128

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Figure 3. (a) Free-energy diagram (at U = 0 V) for HER on the M/TiC surface. On the right-hand side, we plot the hydrogen adsorption energy (ΔEH). (b) HER polarization curves, (c) mass activities, and (d) TOF of SACs and commercial Pt/C, obtained in H2-purged 0.5 M H2SO4 solution with a rotating speed of 1600 rpm.

bind onto the catalyst too strongly nor too loosely.34 The free energy diagram (Figure 3a) of HER demonstrates that Pt is the best catalyst for HER. The following activity order of HER is Ni > Pd > Au > Cu > Ag. To understand the trend of HER activity, we calculated the d-band center (εd) to correlate with ΔGH* (Figure S5). It turns out that the enhanced HER performance of Pt, Pd, and Ni is attributed to their up-shifted εd toward the Fermi level. To validate our theoretical prediction that Pt is the best catalyst for HER, the electrochemical HER activities of the prepared SACs were measured in H2-purged 0.5 M H2SO4 solution (Figure 3b). For the purpose of comparison, HER activities of commercial Pt/C and 5 wt % M NPs supported on TiC (M NPs/TiC, Figure S6) were also measured (Figure S7a). However, Cu and Ni/TiC SACs were excluded due to the poor acid stability for this comparison (Figure S4e,f). At 10 mA/cm2, Au/TiC and Pd/TiC had overpotentials of 230 and 220 mV, respectively, whereas Pt/TiC had an overpotential of 156 mV; this order is consistent with our theoretical expectation based on ΔGH*. Considering that the M contents were fixed at 0.2 wt %, the results show that only a small amount of transition metal leads to a drastic change in the HER activities of the prepared catalysts. To further confirm the high activity of single atoms, the mass activity and turnover frequency (TOF) were calculated. As an example, Pt/TiC showed 6.5 times and 3.5 times higher mass activity at a 50 mV overpotential than commercial Pt/C and Pt NP/TiC, repectively (Figures 3c and S7b). Furthermore, Pt/TiC (1.52 s−1) showed a comparable TOF with commercial Pt/C (2.30 s−1) and a twice higher TOF than Pt NP/TiC (0.76 s−1) (Figures 3d and S7c), which indicates that each single Pt atom on the TiC support has high activity for HER. In addition, electrochemical impedance spectroscopy (EIS) analysis was carried out to analyze the kinetics of SACs during HER. In EIS, the charge transfer resistance (Rct) is

related to the kinetics of the electrocatalytic reaction, and a lower Rct corresponds to a faster reaction rate. During HER, it was confirmed that Rct of SACs decreased as the activity of SACs increased (Figure S8a,b). In Tafel plots of HER, Tafel slopes of SACs were 44 mV/dec for Pt/TiC, 88 mV/dec for Pd/TiC, and 76 mV/dec for Au/TiC (Figure S8c); these slopes suggest that SACs follow the Volmer−Heyrovsky mechanism in HER and that electrochemical desorption (Had + H+ + e− → H2) is a rate-limiting step. This mechanism occurs because an ensemble site does not exist in SACs, and therefore, the chemical desorption step (Had + Had → H2) is impossible in SACs. In contrast, Ag/TiC showed a 146 mV/ dec Tafel slope; this slope suggests that adsorption of a proton (H+ + e− → Had) is a rate-limiting step due to the too weak adsorption of the proton from its down-shifted εd (Figure S5). Finally, SACs showed the superior stability tested by chronopotentiometry at 10 mA cm−2 (Figure S8d). Oxygen Reduction Reaction (ORR). ORR in acidic solution is quite complex, where the final product can be H2O by the four-electron path (4eP) O2 + 4H+ + 4e− F 2H 2O

E 0 = 1.229 V vs SHE

or H2O2 by the two-electron path (2eP) O2 + 2H+ + 2e− F 2H 2O2

E 0 = 0.695 V vs SHE

In the reaction, H2O production is thermodynamically more favorable than H2O2 production. However, from recent experiments using a single-atom ORR catalyst, the particle morphology controls the degree of H2O2 production over the complete reduction to H2O.35 The results confirmed that H2O2 is a major reaction product of the electrocatalytic reaction on atomically dispersed catalyst. Because we consider the SAC, which cannot dissociate the O−O bond, H2O2 is considered as a main product. We consider the following elementary steps for H2O2 production by the 2eP 129

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Figure 4. (a) Free diagram for O2 reduction along the 2e− pathway to produce H2O2 on the M/TiC surface (considering the presence of ligand) in an acid medium. The effect of potential calculated by adding −eU to the free energy. (b) Volcano plot of ORR for the limiting potential of the catalyst (UL) with ΔGHOO*. ORR polarization curves of SACs and commercial Pt/C, obtained in O2-purged 0.1 M HClO4 solution with a rotating speed of 1600 rpm. (c) Ring current measured concurrently during ORR with the potential held at 1.2 V, calculated (d) H2O2 selectivity, and (e) ORR polarization curves with a scan rate of 10 mV s−1. (f) TOF of the catalysts.

* + O2 + [H+ + e−] → HOO*, ΔG1

In addition, the intermediate HOO* has a key role in H2O2 production, and therefore, the adsorption free energy of the ORR intermediate, HOO* (ΔGHOO*, eq S4) acts as a descriptor in controlling the catalytic activity; for optimal activity, ΔGHOO* ≈ 4.2 ± 0.2 eV.36 When the electrode potential is zero, all ORR steps for H2O2 production are thermodynamically favorable (i.e., downhill in free energy) on the surface except for Ni. As the ΔG2 value for Ni is positive, it is not able to produce H2O2; thus, we exclude this catalyst for further consideration. By increasing the potential, we identified the potential determining step (PDS) that is the first uphill step in the reaction diagram. The PDS is HOO* to H2O2 production for Cu, Ni, Pd, and Pt but O2 to HOO* for Ag and Au (Table S7). We also determined the limiting potential for the catalysts; this is a maximum potential at which all reaction steps are downhill in the free-energy diagram. In Figure 4b, we plot a volcano-type relation between the limiting potential (catalytic activity) and the ΔGHOO*. The best catalyst has a very low overpotential by sitting on top of the volcano with a ΔGHOO* value of 4.2 ± 0.2 eV. In our studied series, the Au is predicted to be the best catalyst with the lowest overpotential. The following activity order of ORR is Pd > Ag > Cu > Pt. We obtained the ORR polarization curves of the catalysts in O2-saturated 0.1 M HClO4 solution. As in HER, Cu and Ni were excluded from further study of ORR due to the poor acid

HOO* + [H+ + e−] → H 2O2 + *, ΔG2

where * denotes an unoccupied M/TiC surface. To calculate the reaction free energies, we first optimized the geometries (Figure S9) of the ORR intermediate, HOO* adsorbed on the M/TiC surfaces. To overcome the difficulty in obtaining the exact free energy of the HOO* intermediate in the electrolyte solution, ΔGHOO* is expressed relative to the stoichiometrically appropriate amounts of H2O(g) and H2(g). The reaction free energies ΔGi of ORR reaction steps i were calculated at zero potential with respect to the standard hydrogen electrode (SHE) and at pH = 0 as ΔGi = ΔEi + ΔZPEi − T ΔSi

where ΔE, ΔZPE, and ΔS are, respectively, the differences of the DFT-calculated total energy, ZPE, and entropy between reactants and products. The vibrational frequencies, ZPE, and ΔS used for ORR calculations are given in Table S6, while the free energy values for the different steps are given in Table S7. In Figure 4a, we plot the free energy diagram of oxygen reduction to H2O2 for all of our studied catalysts. For an ideal catalyst, the free energy of HOO* formation (ΔG1) should be zero at equilibrium potential and the free energy diagram becomes flat. We found that Au is closest to the ideal catalyst. 130

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ACS Energy Letters stability. In addition, Ag/TiC and Ag NP/TiC were also excluded due to the oxidative dissolution of Ag in ORR conditions (Figure S10). The yields of H2O2 production on SACs calculated from the disk and ring current were >65% in the entire potential range (Figure 4c−f). Production of H2O generally requires two adjacent active sites (ensemble sites) to break the O−O bond; therefore, predominant production of H2O2 implies that the active sites on SACs are mainly atomically dispersed. The onset potential and oxygen reduction current density of Au/TiC were higher than those of Pd/TiC and Pt/TiC; this trend is the same as the theoretical results. H2O2 selectivity at 0.2 V was 87% for Au/TiC, 80% for Pd/TiC, and 71% for Pt/TiC but only 1.4% for commercial Pt/C and 52% for Au NP/TiC (Figures 4d and S11b). The mass activity of Au/TiC for H2O2 production at 0.2 V was 4.16 A mgAu−1, which was 22.7 times higher than that of Au NP/TiC (0.183 A mgAu−1) (Figure S12a). Furthermore, the TOF of Au/ TiC (1.57 s−1 at 0.2 V) was also 2.93 times higher than that of Au NP/TiC (0.536 s−1 at 0.2 V) (Figures 4f and S11d). Through EIS analysis, it was confirmed that Rct of Au/TiC was much smaller than that of Pd/TiC and Pt/TiC (Figure S12b), which further supports the superior catalytic activity of Au/ TiC for H2O2 production. To test the stability of SACs, ORR polarization curves were measured after 1000 cycles of cyclic voltammetry between 0.2 and 0.7 V. After the test, all SACs retained >67% of initial mass activity (Figure S12c). This reasonable stability originated from stabilization of the single atom by strong interaction with TiC and from the stable nature of TiC. In summary, the stability and electrocatalytic activity of single-atom M (M = Cu, Ag, Au, Ni, Pd, Pt)/TiC catalysts were investigated by theoretical calculation followed by experimental implementation. Carbon vacancies in the TiC substrate help to stabilize these SACs on TiC surface. SACs are successfully synthesized by incipient wetness impregnation. The overpotential of these catalyst for HER at 10 mA cm−2 was 230 mV in Au/TiC, 220 mV in Pd/TiC, and 156 mV in Pt/TiC; these agree well with our DFT predictions. Pt/TiC showed 6.5 times and 3.5 times higher mass activity at a 50 mV overpotential than commercial Pt/C and Pt NP/TiC, respectively. In the ORR, Au and Pd SACs showed more potential for H2O2 production than Pt SAC. Especially, Au/ TiC showed 1240 times and 22.7 times higher mass activity at 0.2 V than commercial Pt/C and Au NP/TiC, respectively. By combining our theoretical predictions with the corresponding experiments, we could successfully pinpoint Pt for HER and Au for selective H2O2 production as the best SACs on TiC, which is the first report as far as we know. Especially, Au/TiC and Pd/TiC have never been experimentally reported for any kind of electrochemical reactions until now. The strategy developed in this study may have wide applications to various heterogeneous catalysis systems, including SAC for other applications.

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FT-EXAFS data, TEM images of M nanoparticles on a TiC surface, and their HER and ORR activity measurements (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.W.H.). *E-mail: [email protected] (J.L.). ORCID

Hyunjoo Lee: 0000-0002-4538-9086 Jinwoo Lee: 0000-0001-6347-0446 Jeong Woo Han: 0000-0001-5676-5844 Author Contributions §

S.K.S., Y.Y, and S.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1601-05. This research was also supported by the R&D program of the Global Frontier Center for Multiscale Energy System (NRF-2014M3A6A7074784) and Basic Science Research Program (NRF-2017R1A2B3004648) through the National Research Foundation of Korea (NRF) funded by the Korea government (MSIT).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b01942. Computational and experimental details, adsorption energetics, adsorption distances, atomic charges of M atoms on a TiC surface, HAADF-STEM, XANES, and 131

DOI: 10.1021/acsenergylett.8b01942 ACS Energy Lett. 2019, 4, 126−132

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DOI: 10.1021/acsenergylett.8b01942 ACS Energy Lett. 2019, 4, 126−132