- Email: [email protected]
Electrocatalyst design for promoting two-electron oxygen reduction reaction: Isolation of active site atoms on precious metal surfaces
Journal Pre-proof Electrocatalyst Design for Promoting Two-Electron Oxygen Reduction Reaction: Isolation of Active Site Atoms on Precious Metal Surfaces Jae Hyung Kim, Yong-Tae Kim, Sang Hoon Joo PII:
S2451-9103(20)30014-4
DOI:
https://doi.org/10.1016/j.coelec.2020.01.007
Reference:
COELEC 501
To appear in:
Current Opinion in Electrochemistry
Received Date: 30 December 2019 Revised Date:
9 January 2020
Accepted Date: 21 January 2020
Please cite this article as: Kim JH, Kim Y-T, Joo SH, Electrocatalyst Design for Promoting Two-Electron Oxygen Reduction Reaction: Isolation of Active Site Atoms on Precious Metal Surfaces, Current Opinion in Electrochemistry, https://doi.org/10.1016/j.coelec.2020.01.007. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Electrocatalyst Design for Promoting Two-Electron Oxygen Reduction Reaction: Isolation of Active Site Atoms on Precious Metal Surfaces Jae Hyung Kim1, Yong-Tae Kim*,2, and Sang Hoon Joo*,1
1
Department of Energy Engineering and School of Energy and Chemical Engineering, Ulsan
National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea. 2
Department of Materials Science and Engineering, Pohang University of Science and
Technology (POSTECH), 77 Cheongam-Ro, Pohang, Gyeongbuk 37673, Republic of Korea.
Corresponding authors Joo, Sang Hoon ([email protected]); Kim, Yong-Tae ([email protected])
1
Abstract Selective two-electron (2 e−) pathway oxygen reduction reaction (ORR) has gained prominence for enabling small-scale, on-site electrochemical H2O2 production, and has emerged as a promising alternative to the conventional anthraquinone process. Thus, the rational design of catalysts that can suppress the competing four-electron pathway ORR is critical. This review highlights catalyst design strategies for promoting the selective 2 e− pathway ORR, including alloying with inert metals, partial surface poisoning, and generating atomically dispersed sites. The major results and advances and unresolved challenges are summarized.
Keywords electrocatalysis, oxygen reduction reaction, H2O2 production, catalyst design, metal isolation.
Introduction The oxygen reduction reaction (ORR) is arguably one of the most important electrochemical reactions affecting the efficiency of energy conversion devices such as fuel cells [1] and metal-air batteries [2]. Two ORR pathways are possible: the four-electron (4 e−) pathway, where oxygen is converted to H2O by complete reduction, and the two-electron (2 e−) pathway involving partial reduction of O2 to generate hydrogen peroxide (H2O2) [3]. The 2 e− pathway ORR has been regarded as an adverse side reaction that impedes the efficient 4 e− pathway in polymer electrolyte membrane fuel cells (PEMFCs), as it degrades the performance of the PEMFC by destroying the Nafion membrane [4,5]. However, the selective 2 e− pathway has recently garnered a surge of interest for the electrochemical production of H2O2 [6–16].
2
H2O2 is strongly oxidizing yet environmentally benign, and is thus widely exploited in polymer and pharmaceutical syntheses, pulp and textile bleaching, and wastewater and ballast water treatment [6-9]. The annual global production of H2O2 is estimated to reach a value of ~ 6 billion US dollars by 2023 [8]. Ninety-five percent of the current H2O2 production utilizes the anthraquinone process [6], which undesirably requires high pressure H2 and expensive Pdbased catalysts, large infrastructures, and energy-intensive distillation processes. This process typically produces H2O2 in high concentration in a large volume, with attendant safety risks related to the storage and transportation of H2O2. Recently, electrochemical H2O2 production has emerged as a promising alternative to the anthraquinone process [6–9]. Electrochemical H2O2 production allows continuous, on-site H2O2 production with dilute H2O2, mitigating the drawbacks of the anthraquinone process. Pivotal to efficient H2O2 electrosynthesis is the design of electrode catalysts that can promote the selective 2 e− pathway ORR while suppressing the competing 4 e− pathway ORR [9]. In the ORR, oxygen can be adsorbed on the surface of metal catalysts in two different configurations: i) dissociative side-on adsorption which leads to the increased bond length and weakening of the oxygen double bond, yielding H2O as the product; ii) associative end-on adsorption where oxygen is adsorbed in the form of *OOH, which can produce both H2O2 and H2O [3,17–19]. To promote the 2 e− pathway ORR, ensemble or hollow sites that facilitate the side-on adsorption of O2 should be eliminated by isolating the surface metal atoms. In this short review, we present catalyst preparation strategies for isolating the active metal atoms to promote electrochemical H2O2 production. Three major catalyst design approaches are introduced: i) alloying the active metal with an inert metal to provide an isolated geometry of active metal atoms; ii) surface poisoning to block the exposed ensemble sites with inert species; iii) preparing atomically dispersed catalysts (Figure 1). We summarize the underpinning principles and present notable examples of these strategies. We 3
conclude this review by highlighting the outstanding challenges in this field. We note that this review focuses on precious metal-based catalysts for H2O2 production. Rapidly advancing carbon-based H2O2 production catalysts are comprehensively summarized elsewhere [8,9].
Figure 1
ORR pathways depending on geometric structure of catalysts and three major approaches for isolating active metal sites.
Alloying with inert metal The most straightforward technique for designing electrocatalysts with isolated metal sites is alloying an active metal with an inert metal. The inert metal species can induce geometric isolation of the active metal, which can alter the adsorption geometry of the O2 molecule. This approach was initially exploited in heterogeneous thermo-catalytic reactions and later adapted to electrocatalysis. One notable example is PdAu alloy catalysts for the gas-phase direct synthesis of H2O2 from H2 and O2, where high H2O2 selectivity was achieved by isolating the active Pd atoms with inactive Au atoms [20–23]. Inspired by this, Jirkovský and co-workers introduced the alloying concept in electrocatalytic H2O2 production [24**]. They first 4
screened alloy combinations to identify an optimum catalyst composition by density functional theory (DFT) calculations, showing that an isolated Pd, Pt, or Rh site on the surface of Au enhanced H2O2 production compared to that achieved with pure Au. Pd-contentcontrolled PdAu alloys were prepared to validate the DFT results. With 8% Pd, no surface segregation of Pd occurred, leading to 95% H2O2 selectivity (Figure 2a). Higher Pd loading led to a decline in the selectivity by forming Pd ensemble sites that preferentially dissociate the oxygen bond. They further investigated the potential-dependent structural deformation of PdAu alloys and the effect on the selectivity towards H2O2 production [25]. Changing the preconditioning potential of the electrode surface controlled the H2O2 selectivity from 10 to 60%. Oxidative preconditioning induced rapid surface rearrangement to generate a Pd-rich surface, yielding mainly H2O, whereas reductive preconditioning triggered reordering of the Au–Pd surface composition to produce isolated Pd atoms, generating H2O2 (Figure 2b). These results confirmed the critical effect of the surface site geometry on the ORR selectivity. Rossmeisl, Stephens, and co-workers screened new alloy catalysts for electrochemical H2O2 production using DFT calculations, demonstrating the optimal performance of the PtHg4 alloy comprising active Pt atoms surrounded by inert Hg atoms [26*]. Based on DFT calculations, core-shell nanoparticles (NPs) comprising a Pt core and a Pt−Hg shell were prepared via electrodeposition of Hg on Pt. The resulting Hg-modified Pt NPs provided over 90% selectivity at 0.3‒0.5 V. Importantly, the mass activity of this catalyst was 26 A gnoble −1 metal
at an overpotential of 50 mV, which was the best mass activity achieved for H2O2
production at that time. Later, the same group extended this approach to the Pd−Hg alloy, where the mass activity was five-fold that of the Pt−Hg alloy, with 95% H2O2 selectivity [27]. Wagner and co-workers later scrutinized the structure of the Pd−Hg alloy catalyst with highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDX) [28], revealing that the Pd−Hg particle 5
comprised a Pd core and Pd−Hg ordered alloy shell with isolated Pd atoms, clarifying the origin of the high catalytic activity for H2O2 production.
Figure 2
(a) H2O2 selectivity of PdAu alloy as a function of Pd content, x, at the potentials of 0 V (black squares), −0.1 V (red circles), and −0.2 V (blue triangles), and models of adsorbed O2 molecules with PdxAu1-x model structures (x = 0, 0.08, and 0.3). Reprinted with permission from Ref. [24**]. Copyright (2011) American Chemical Society. (b) Changes in surface composition of PdAu alloys with preconditioning and effect on H2O2 selectivity. Reprinted with permission from Ref. [25]. Copyright (2013) American Chemical Society. (c) Schematic of PtHg4 and its ORR activity and selectivity in the form of nanoparticles. Reprinted with permission from Ref. [26*]. Copyright (2013) Nature Publishing Group.
Surface poisoning methods
6
Another notable strategy for modifying metal catalysts is partial poisoning of the active metal surfaces with catalytically inert molecules. Markovic and co-workers demonstrated this concept by adsorbing poisonous ions on the metal surfaces [29–31*], achieving a significant decline in the activity of Pt catalysts for the ORR in H2SO4 electrolyte due to the strong adsorption of sulfate ions, compared to that in HClO4. The addition of of Br− anions to the H2SO4 electrolyte further altered the ORR selectivity toward the 2 e− pathway, with reduced activity [29]. A similar phenomenon was observed with chloride and cyanide ions (Figure 3a) [30,31*], attributed to configurational variation of the adsorbed species on the Pt surfaces according to the type of anion in the electrolyte. While the adsorption of sulfate anions requires at least four contiguous Pt atoms, the hetero anions (e.g. bromide, chloride, and cyanide ions) are adsorbed on single Pt atom sites. Hence, with only sulfate in the electrolyte, desorption of the adsorbed sulfate anions leaves behind Pt ensemble sites, which promotes the 4 e− pathway ORR, albeit with decreased activity. However, in the presence of both sulfate and hetero anions, the hetero anions are preferentially adsorbed on the Pt surface due to their stronger absorption power than sulfate ions. Subsequent desorption of the adsorbed hetero anions generates vacant single Pt sites, which induce the 2 e− pathway ORR [31*]. On the basis of the above concept, Markovic and co-workers covered a Pt(111) electrode with a self-assembled monolayer of calix[4]arene molecules to lower the number of Pt ensemble sites [32]. As the coverage of the calix[4]arene molecules increased, the available Pt ensemble sites decreased, reducing the ORR activity, with concomitant formation of H2O2. Similarly, Choi and co-workers coated Pt nanoparticles on carbon (Pt/C) with amorphous carbon layers by acetylene chemical vapor deposition (CVD) [33*]. The carbon coating layer in the resulting amorphous carbon coated Pt/C catalysts promoted the end-on adsorption of O2 molecules while suppressing their side-on adsorption, thereby enhancing the 2 e− ORR pathway. The H2O2 selectivity was proportional to the thickness of the carbon layer, with a 7
maximal selectivity of 41%. The carbon coating layer further suppressed decomposition of the produced H2O2, which produces water via disproportionation or reduction reactions, by hindering access to the produced H2O2. Selenium, sulfur, or an ammonium ion has also been utilized as a modifier for Pt catalysts to promote the 2 e− pathway ORR [34–37]. Figure 3
(a) Disk and ring currents of cyanide-adsorbed Pt(111) electrode with variation of the cyanide coverage in 0.05 M H2SO4 electrolyte (solid lines). Brown lines indicate disk and ring current of Pt(111) electrode in 0.1 M HClO4. Reprinted with permission from Ref. [31*]. Copyright (2015) Elsevier B.V. (b) Schematic illustration of effect of carbon coating on Pt/C catalysts for ORR by CVD and H2O2 selectivity depending on the CVD operation time for Pt/C catalysts. Reprinted with permission from Ref. [33*]. Copyright (2014) American Chemical Society.
Atomically dispersed catalysts The most efficient method for eliminating metal ensemble sites is full dispersion of metal atoms into single sites on the support. The resulting catalysts are termed atomically dispersed catalysts [38,39] or single-atom catalysts [40,41]. Lee and co-workers [42**] prepared TiNsupported Pt nanoparticle (Pt/TiN) catalysts by an incipient wetness impregnation method. At low Pt loading (0.35 wt%), atomically dispersed Pt sites were exclusively obtained. The H2O2 8
selectivity of the resulting 0.35 wt% Pt/TiN catalyst reached 65%, which was much higher than that of the 2 wt% Pt/TiN (30%) comprising nanoparticles as well as atomically dispersed sites (Figure 4a). The Lee group further investigated the effects of supports on the H2O2 production selectivity with the TiN and TiC supports [43], in which 0.2 wt% Pt/TiC exhibited higher ORR activity and H2O2 selectivity than 0.2 wt% Pt/TiN. DFT calculations suggested that the more favorable adsorption energy and energy profiles of Pt/TiC toward the 2 e− pathway ORR led to its superior activity and selectivity for H2O2 production. Atomically dispersed sites inevitably have high surface energy and readily agglomerate during preparation, making their syntheses with high metal loading challenging. To circumvent this problem, Choi and co-workers utilized zeolite-templated carbon (ZTC) possessing a high concentration of sulfur (HSC) as a support, where the ultra-high surface area of HSC (2,770 m2g−1) and numerous S sites with strong anchoring ability facilitated the formation of atomically dispersed Pt sites up to a loading of 5 wt% [44*]. This is markedly higher than that of atomically dispersed catalysts reported at that time (less than 1 wt). The Pt/HSC catalyst showed 95% H2O2 selectivity in the potential range of 0.1‒0.7 V, which is a dramatic improvement compared to the H2O2 selectivity of 28% achieved with Pt NP-based catalysts (Pt/ZTC) (Figure 4b). Li and co-workers prepared ~15 wt% atomically dispersed Pt catalysts using a sulfur-containing support, CuSx; the resulting h-Pt1-CuSx catalyst afforded ≥90% H2O2 selectivity in the potential range of 0.05‒0.7 V (Figure 4c) [45]. Importantly, the h-Pt1-CuSx catalyst could generate H2O2 with a yield of 546 mol kgcat−1 h−1, which is among the highest values reported for atomically dispersed electro- and thermo-catalysts for H2O2 production. Other atomically dispersed metal catalysts comprising Ni, Au, or Fe as the metal center were prepared and exploited to facilitate selective electrochemical H2O2 production [46–50].
9
Figure 4
(a) HAADF-STEM image of 0.35 wt% Pt/TiN and ORR activity and selectivity. Reprinted with permission from Ref. [42**]. Copyright (2016) Wiley-Vch Verlag GmbH & Co. (b) HAADF-STEM image of 5 wt% Pt supported on zeolite-templated carbon with high Scontent (Pt/HSC) and its ORR activity and selectivity. Scale bar is 2 nm. Reprinted with permission from Ref. [44*]. Copyright (2016) Nature Publishing Group. (c) HAADF-STEM image of single atomic Pt site catalysts embedded in hollow CuSx support (h-Pt1-CuSx) and its ORR activity and selectivity. Reprinted with permission from Ref. [45]. Copyright (2019) from Cell Press.
Concluding remarks In this review, the strategies for designing electrocatalysts for electrochemical H2O2 production via the 2 e− pathway ORR were summarized. The key for H2O2 production is to promote end-on adsorption of O2 molecules on the catalyst surfaces via isolation of the active
10
atomic sites during the ORR. Toward this goal, three prominent strategies have been developed, i.e., alloying with inert metals, partial surface poisoning, and fabricating atomically dispersed catalysts. Although the developed strategies have attempted to exploit geometric effect of the catalyst for selective H2O2 production, the mode of O2 adsorption has been largely inferred from DFT calculations. The direct spectroscopic detection of intermediate species and defining the relationship between the O2 adsorption mode and reactivity under real reaction conditions remain elusive. Thus, whether controlling the geometry of the catalyst only affects the adsorption configuration of the reactants (geometric effect) and/or also influences the electronic structure of the catalyst (electronic effect) remain unclear and should be explored in the future. Practically, electrocatalysts for highly active and selective H2O2 production should be integrated with currently available H2O2 production cells to facilitate potential commercialization.
Conflicts of interest statement The authors declare no conflict of interest.
Acknowledgements This work was supported by the National Research Foundation (NRF) of Korea funded by the Ministry of Science and ICT (NRF-2019M3E6A1064521, and NRF-2019M3D1A1079306).
References Papers of particular interest, published within the period of review, have been highlighted as: * of special interest ** of outstanding interest
11
1.
Debe MK: Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012, 486:43–51.
2.
Bruce PG, Freunberger SA, Hardwick LJ, Tarascon J-M: Li–O2 and Li–S batteries with high energy storage. Nat Mater 2012, 11:19–29.
3.
Kulkarni A, Siahrostami S, Patel A, Nørskov JK: Understanding catalytic activity trends in the oxygen reduction reaction. Chem Rev 2018, 118:2302–2312.
4.
Shao M, Chang Q, Dodelet J-P, Chenitz: Recent advances in electrocatalysts for oxygen reduction reaction. Chem Rev 2016, 116:3594–3657.
5.
Borghei M, Lehtonen J, Liu L, Rojas OJ: Advanced biomass-derived electrocatalysts for the oxygen reduction reaction. Adv Mater 2018, 30:1703691.
6.
Yang S, Verdaguer-Casadevall A, Arnarson L, Silvioli L, Čolić V. Frydendal R, Rossmeisl J, Chorkendorff I, Stephens IEL: Toward the decentralized electrochemical production of H2O2: a focus on the catalysis. ACS Catal 2018, 8:4064–4081.
7.
Perry SC, Panhotra D, Vieira L, Csepei L-I, Sieber V, Wang L, Ponce de León C, Walsh FC: Electrochemical synthesis of hydrogen peroxide from water and oxygen. Nat Rev Chem 2019, 3:442–458.
8.
Melchinonna M, Fornasiero P, Prato M: The rise of hydrogen peroxide as the main product by metal-free catalysis in oxygen reductions. Adv Mater 2019, 31:1802920.
9.
Zhang J, Zhang H, Cheng M-J, Lu Q: Tailoring the electrochemical production of H2O2: strategies for the rational design of high-performance electrocatalysts. Small 2019, 1902845.
10.
Otsuka K, Yamanaka I: One step synthesis of hydrogen peroxide through fuel cell reaction. Electrochim Acta 1990, 35:319–322.
11.
Yamanaka I, Onizawa T, Takenaka S, Otsuka K: Direct and continuous production of hydrogen peroxide with 93% selectivity using a fuel-cell system. Angew Chem Int Ed 2003, 42:3653–3655.
12.
Fellinger T-P, Hasché F, Strasser P, Antonietti M: Mesoporous nitrogen-doped carbon for the electrocatalytic synthesis of hydrogen peroxide. J Am Chem Soc 2012, 134:4072–4075.
13.
Park J, Nabae Y, Hayakawa T, Kakimoto M: Highly selective two-electron oxygen reduction catalyzed by mesoporous nitrogen-doped carbon. ACS Catal 2014, 4:3749–3754.
14.
Lu Z, Chen G, Siahrostami S, Chen Z, Liu K, Xie J, Liao L, Wu T, Lin D, Liu Y, Jaramillo TF, Nørskov JK, Cui Y: High-efficiency oxygen reduction to hydrogen peroxide catalyzed by oxidized carbon materials. Nat Catal 2018, 1:156–162.
15.
Kim HW, Ross MB, Kornienko N, Zhang L, Guo J, Yang P, McCloskey BD: Efficient hydrogen peroxide generation using reduced graphene oxide-based oxygen reduction electrocatalysts. Nat Catal 2018 1:282–290.
16.
Sa YJ, Kim JH, Joo SH: Active edge-site-rich carbon nanocatalysts with enhanced electron transfer for efficient electrochemical hydrogen peroxide production. Angew Chem Int Ed 2019, 58:1100–1105.
12
17.
Yeager E: Disoxygen electrocatalysis: mechanisms in relation to catalyst structure. J Mol Catal 1986, 38:5–25.
18.
Chan AWE, Hoffmann R, Ho W: Theoretical aspects of photoinitiated chemisorption, dissociation, and desorption of O2 on Pt(111). Langmuir 1992, 8:1111–1119.
19.
Viswanathan V, Hansen HA, Rossmeisl J, Nørskov JK: Unifying the 2e− and 4e− reduction of oxygen on metal surfaces. J Phys Chem Lett 2012, 3:2948–2951.
20.
Edwards JK, Solsona B, N EN, Carley AF, Herzing AA, Kiely CJ, Hutchings GJ: Switching off hydrogen peroxide hydrogenation in the direct synthesis process. Science 2009, 323:1037–1041.
21.
Gao F, Wang Y, Goodman DW: CO oxidation over AuPd(100) from ultrahigh vacuum to near-atmospheric pressures: the critical role of contiguous Pd atoms. J Am Chm Soc 2009, 131:5734–5735.
22.
Baber AE, Tierney HL, Sykes ECH: Atomic-scale geometry and electronic structure of catalytically important Pd/Au alloys. ACS Nano 2010, 4:1637–1645.
23.
Ham HC, Hwang GS, Han J, Nam SW, Lim TH: On the role of Pd ensembles in selective H2O2 formation on PdAu alloys. J Phys Chem C 2009, 113:12943–12945.
24.
Jirkovský JS, Panas I, Ahlberg E, Halasa M, Romani S, Schiffrin DJ: Single atom hotspots at Au–Pd nanoalloys for electrocatalytic H2O2 production. J Am Chem Soc 2011, 133:19432–19441. ** The first study reporting the use of metal alloys to eliminate metal ensemble sites for promoting electrochemical H2O2 production. 25.
Jirkovský JS, Panas I, Romani S, Ahlberg E, Schiffrin DJ: Potential-dependent structural memory effects in Au–Pd nanoalloys. J Phys Chem Lett 2013, 3:315–321.
26.
Siahrostami S, Verdaguer-Casadevall A, Karamad M, Deiana D, Malacrida P, Wickman B, Escudero-Escribano M, Paoli EA, Frydendal R, Hansen TW, Chorkendorff I, Stephens IEL, Rossmeisl J: Enabling direct H2O2 production through rational electrocatalyst design. Nat Mater 2013, 12:1137–1143. * This study identified a new composition of metal alloy, Pt–Hg, as a promising catalyst for H2O2 production based on DFT calculations and experimentally validated its high activity and selectivity for electrochemical H2O2 production. 27.
Verdaguer-Casadevall A, Deiana D, Karamad M, Siahrostami S, Malacrida P, Hansen TW, Rossmeisl J, Chorkendorff I, Stephens IEL: Trends in the electrochemical synthesis of H2O2: enhancing activity and selectivity by electrocatalytic site engineering. Nano Lett 2014, 14:1603–1608.
28.
Deiana D, Verdaguer-Casadevall A, Malacrida P, Stephens IEL, Chorkendorff I, Wagner JB, Hansen, TW: Determination of core-shell structures in Pd–Hg nanoparticles by STEM–EDX. ChemCatChem 2015, 7:3748–3752.
29.
Marković NM, Gasteiger HA, Grgur BN, Ross PN: Oxygen reduction reaction on Pt(111): effect of bromide. J. Electroanal Chem 1999, 467:157–163.
30.
Stamenkovic V, Marković NM, Ross PNJr: Structure-relationships in electrocatalysis: oxygen reduction and hydrogen oxidation reactions on Pt(111) and Pt(100) in solutions containing chloride ions. J Electroanal Chem 2001, 500:44– 51. 13
31.
Ciapina EG, Lopes PP, Subbaraman R, Ticianelli EA, Stamenkovic V, Strmcnik D, Markovic NM: Surface spectators and their role in relationships between activity and selectivity of the oxygen reduction reaction in acid environments. Electrochem Commun 2015, 60:30–33. * This study described the catalytic effect of the spectator ions on the Pt (111) surface on the ORR activity and selectivity, enhancing understanding of the geometric effect under electrochemical conditions. 32.
Genorio B, Strmcnik D, Subbaraman Ram, Tripkovic D, Karapetrov G, Stamenkovic VR, Pejovnik S, Markovic NM: Selective catalysts for the hydrogen oxidation and oxygen reduction reactions by patterning of platinum with calix[4]arene molecules. Nat Mater 2010, 9:998–1003.
33.
Choi CH, Kwon HC, Yook S, Shin H, Kim H, Choi M: Hydrogen peroxide synthesis via enhanced two-electron oxygen reduction pathway on carbon-coated Pt surface. J Phys Chem C 2014, 118:30063–30070. * This study presents enhancement of the selectivity for the 2 e− pathway ORR by blocking accessible Pt ensemble sites with amorphous carbon coating, demonstrating the geometric effect in the ORR selectivity. 34.
Schmidt TJ, Paulus UA, Gasteiger HA, Behm RJ: The oxygen reduction reaction on a Pt/carbon fuel cell catalyst in the presence of chloride anions. J Electroanal Chem 2001, 508:41–47.
35.
Mo Y, Scherson DA: Platinum-based electrocatalysts for generation of hydrogen peroxide in aqueous acidic electrolytes. J Electrochem Soc 2003, 150:E39–E46.
36.
Lopes T, Chlistunoff J, Sansiñena, J-M, Garzon FH: Oxygen reduction reaction on a Pt/carbon fuel cell catalyst in the presence of trace quantities of ammonium ions: an RRDE study. Int J Hydrogen Energy 2012, 37:5202–5207.
37.
Verdaguer-Casadevall A, Hernandez-Fernandez P, Stephens IEL, Chorkendorff I, Dahl S: The effect of ammonia upon the electrocatalysis of hydrogen oxidation and oxygen reduction on polycrystalline platinum. J. Power Sources 2013, 220:205–210.
38.
Flytzani-stephanopoulos M: Gold atoms stabilized on various supports catalyze the water–gas shift reaction. Acc Chem Res 2014, 47:783–792.
39.
Gates BC, Flytzani-stephanopoulos M, Dixon DA, Katz A: Atomically dispersed supported metal catalysts: perspectives and suggestions for future research. Catal Sci Technol 2017, 7:4259–4257.
40.
Wang A, Li J, Zhang T: Heterogeneous single-atom catalysis. Nat Rev Chem 2018, 2:65–81.
41.
Mitchell S, Vorobyeva E, Pérez-Ramírez: The multifaceted reactivity of single-atom heterogeneous catalysts. Angew Chem Int Ed 2018, 57:15316–15329.
42.
Yang S, Kim J, Tak YJ, Soon A, Lee H: Single-atom catalyst of platinum supported on titanium nitride for selective electrochemical reactions. Angew Chem Int Ed 2016, 55:2058–2062. ** The first study verifying the effectiveness of atomically dispersed catalysts for the electrosynthesis of H2O2. 43.
Yang S, Tak YJ, Kim J, Soon A, Lee H: Support effects in single-atom platinum catalysts for electrochemical oxygen reduction. ACS Catal 2017, 7:1301–1307. 14
44.
Choi CH, Kim M, Kwon HC, Cho SJ, Yun S, Kim H-T, Mayrhofer KJJ, Kim H, Choi M: Tuning selectivity of electrochemical reactions by atomically dispersed platinum catalyst. Nat Commun 2016, 7:10922. * By greatly increasing the number of atomically dispersed Pt sites, almost 100% H2O2 selectivity could be achieved. 45.
Shen R, Chen W, Peng Q, Lu S, Zheng L, Cao X, Wang Y, Zhu W, Zhang J, Zhuang Z, Chen C, Wang D, Li Y: High-concentration single atomic Pt sites on hollow CuSx for selective O2 reduction to H2O2 in acid solution. Chem 2019, 5:2099–2110.
46.
Luo J, Tang H, Tian X, Hou S, Li X, Du L, Liao S: Highly selective TiN-supported highly dispersed Pt catalyst: ultra active toward hydrogen oxidation and inactive toward oxygen reduction. ACS Appl Mater Interfaces 2018, 10:3530–3537.
47.
Wang T, Zeng Z, Cao L, Li Z, Hu X, An B, Wang C, Lin W: A dynamically stabilized single-nickel electrocatalyst for selective reduction of oxygen to hydrogen peroxide. Chem Eur J 2018, 24:17011–17018.
48.
Shin S, Kim J, Park S, Kim H-E, Sung Y-E, Lee H: Changes in the oxidation state of Pt single-atom catalysts upon removal of chloride ligands and their effect for electrochemical reactions. Chem Commun 2019, 55:6389–6392.
49.
Sahoo SK, Ye Y, Lee S, Park J, Lee H, Lee J, Han JW: Rational design of TiCsupported single-atom electrocatalysts for hydrogen evolution and selective oxygen reduction reactions. ACS Energy Lett 2019, 4:126–132.
50.
Jiang K, Back S, Akey AJ, Xia C, Hu Y, Liang W, Schaak D, Stavitski E, Nørskov JK, Siahrostami S, Wang H: Highly selective oxygen reduction to hydrogen peroxide on transition metal single atom coordination. Nat Commun 2019, 10:3997.
15
S2451-9103(20)30014-4
DOI:
https://doi.org/10.1016/j.coelec.2020.01.007
Reference:
COELEC 501
To appear in:
Current Opinion in Electrochemistry
Received Date: 30 December 2019 Revised Date:
9 January 2020
Accepted Date: 21 January 2020
Please cite this article as: Kim JH, Kim Y-T, Joo SH, Electrocatalyst Design for Promoting Two-Electron Oxygen Reduction Reaction: Isolation of Active Site Atoms on Precious Metal Surfaces, Current Opinion in Electrochemistry, https://doi.org/10.1016/j.coelec.2020.01.007. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Electrocatalyst Design for Promoting Two-Electron Oxygen Reduction Reaction: Isolation of Active Site Atoms on Precious Metal Surfaces Jae Hyung Kim1, Yong-Tae Kim*,2, and Sang Hoon Joo*,1
1
Department of Energy Engineering and School of Energy and Chemical Engineering, Ulsan
National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea. 2
Department of Materials Science and Engineering, Pohang University of Science and
Technology (POSTECH), 77 Cheongam-Ro, Pohang, Gyeongbuk 37673, Republic of Korea.
Corresponding authors Joo, Sang Hoon ([email protected]); Kim, Yong-Tae ([email protected])
1
Abstract Selective two-electron (2 e−) pathway oxygen reduction reaction (ORR) has gained prominence for enabling small-scale, on-site electrochemical H2O2 production, and has emerged as a promising alternative to the conventional anthraquinone process. Thus, the rational design of catalysts that can suppress the competing four-electron pathway ORR is critical. This review highlights catalyst design strategies for promoting the selective 2 e− pathway ORR, including alloying with inert metals, partial surface poisoning, and generating atomically dispersed sites. The major results and advances and unresolved challenges are summarized.
Keywords electrocatalysis, oxygen reduction reaction, H2O2 production, catalyst design, metal isolation.
Introduction The oxygen reduction reaction (ORR) is arguably one of the most important electrochemical reactions affecting the efficiency of energy conversion devices such as fuel cells [1] and metal-air batteries [2]. Two ORR pathways are possible: the four-electron (4 e−) pathway, where oxygen is converted to H2O by complete reduction, and the two-electron (2 e−) pathway involving partial reduction of O2 to generate hydrogen peroxide (H2O2) [3]. The 2 e− pathway ORR has been regarded as an adverse side reaction that impedes the efficient 4 e− pathway in polymer electrolyte membrane fuel cells (PEMFCs), as it degrades the performance of the PEMFC by destroying the Nafion membrane [4,5]. However, the selective 2 e− pathway has recently garnered a surge of interest for the electrochemical production of H2O2 [6–16].
2
H2O2 is strongly oxidizing yet environmentally benign, and is thus widely exploited in polymer and pharmaceutical syntheses, pulp and textile bleaching, and wastewater and ballast water treatment [6-9]. The annual global production of H2O2 is estimated to reach a value of ~ 6 billion US dollars by 2023 [8]. Ninety-five percent of the current H2O2 production utilizes the anthraquinone process [6], which undesirably requires high pressure H2 and expensive Pdbased catalysts, large infrastructures, and energy-intensive distillation processes. This process typically produces H2O2 in high concentration in a large volume, with attendant safety risks related to the storage and transportation of H2O2. Recently, electrochemical H2O2 production has emerged as a promising alternative to the anthraquinone process [6–9]. Electrochemical H2O2 production allows continuous, on-site H2O2 production with dilute H2O2, mitigating the drawbacks of the anthraquinone process. Pivotal to efficient H2O2 electrosynthesis is the design of electrode catalysts that can promote the selective 2 e− pathway ORR while suppressing the competing 4 e− pathway ORR [9]. In the ORR, oxygen can be adsorbed on the surface of metal catalysts in two different configurations: i) dissociative side-on adsorption which leads to the increased bond length and weakening of the oxygen double bond, yielding H2O as the product; ii) associative end-on adsorption where oxygen is adsorbed in the form of *OOH, which can produce both H2O2 and H2O [3,17–19]. To promote the 2 e− pathway ORR, ensemble or hollow sites that facilitate the side-on adsorption of O2 should be eliminated by isolating the surface metal atoms. In this short review, we present catalyst preparation strategies for isolating the active metal atoms to promote electrochemical H2O2 production. Three major catalyst design approaches are introduced: i) alloying the active metal with an inert metal to provide an isolated geometry of active metal atoms; ii) surface poisoning to block the exposed ensemble sites with inert species; iii) preparing atomically dispersed catalysts (Figure 1). We summarize the underpinning principles and present notable examples of these strategies. We 3
conclude this review by highlighting the outstanding challenges in this field. We note that this review focuses on precious metal-based catalysts for H2O2 production. Rapidly advancing carbon-based H2O2 production catalysts are comprehensively summarized elsewhere [8,9].
Figure 1
ORR pathways depending on geometric structure of catalysts and three major approaches for isolating active metal sites.
Alloying with inert metal The most straightforward technique for designing electrocatalysts with isolated metal sites is alloying an active metal with an inert metal. The inert metal species can induce geometric isolation of the active metal, which can alter the adsorption geometry of the O2 molecule. This approach was initially exploited in heterogeneous thermo-catalytic reactions and later adapted to electrocatalysis. One notable example is PdAu alloy catalysts for the gas-phase direct synthesis of H2O2 from H2 and O2, where high H2O2 selectivity was achieved by isolating the active Pd atoms with inactive Au atoms [20–23]. Inspired by this, Jirkovský and co-workers introduced the alloying concept in electrocatalytic H2O2 production [24**]. They first 4
screened alloy combinations to identify an optimum catalyst composition by density functional theory (DFT) calculations, showing that an isolated Pd, Pt, or Rh site on the surface of Au enhanced H2O2 production compared to that achieved with pure Au. Pd-contentcontrolled PdAu alloys were prepared to validate the DFT results. With 8% Pd, no surface segregation of Pd occurred, leading to 95% H2O2 selectivity (Figure 2a). Higher Pd loading led to a decline in the selectivity by forming Pd ensemble sites that preferentially dissociate the oxygen bond. They further investigated the potential-dependent structural deformation of PdAu alloys and the effect on the selectivity towards H2O2 production [25]. Changing the preconditioning potential of the electrode surface controlled the H2O2 selectivity from 10 to 60%. Oxidative preconditioning induced rapid surface rearrangement to generate a Pd-rich surface, yielding mainly H2O, whereas reductive preconditioning triggered reordering of the Au–Pd surface composition to produce isolated Pd atoms, generating H2O2 (Figure 2b). These results confirmed the critical effect of the surface site geometry on the ORR selectivity. Rossmeisl, Stephens, and co-workers screened new alloy catalysts for electrochemical H2O2 production using DFT calculations, demonstrating the optimal performance of the PtHg4 alloy comprising active Pt atoms surrounded by inert Hg atoms [26*]. Based on DFT calculations, core-shell nanoparticles (NPs) comprising a Pt core and a Pt−Hg shell were prepared via electrodeposition of Hg on Pt. The resulting Hg-modified Pt NPs provided over 90% selectivity at 0.3‒0.5 V. Importantly, the mass activity of this catalyst was 26 A gnoble −1 metal
at an overpotential of 50 mV, which was the best mass activity achieved for H2O2
production at that time. Later, the same group extended this approach to the Pd−Hg alloy, where the mass activity was five-fold that of the Pt−Hg alloy, with 95% H2O2 selectivity [27]. Wagner and co-workers later scrutinized the structure of the Pd−Hg alloy catalyst with highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDX) [28], revealing that the Pd−Hg particle 5
comprised a Pd core and Pd−Hg ordered alloy shell with isolated Pd atoms, clarifying the origin of the high catalytic activity for H2O2 production.
Figure 2
(a) H2O2 selectivity of PdAu alloy as a function of Pd content, x, at the potentials of 0 V (black squares), −0.1 V (red circles), and −0.2 V (blue triangles), and models of adsorbed O2 molecules with PdxAu1-x model structures (x = 0, 0.08, and 0.3). Reprinted with permission from Ref. [24**]. Copyright (2011) American Chemical Society. (b) Changes in surface composition of PdAu alloys with preconditioning and effect on H2O2 selectivity. Reprinted with permission from Ref. [25]. Copyright (2013) American Chemical Society. (c) Schematic of PtHg4 and its ORR activity and selectivity in the form of nanoparticles. Reprinted with permission from Ref. [26*]. Copyright (2013) Nature Publishing Group.
Surface poisoning methods
6
Another notable strategy for modifying metal catalysts is partial poisoning of the active metal surfaces with catalytically inert molecules. Markovic and co-workers demonstrated this concept by adsorbing poisonous ions on the metal surfaces [29–31*], achieving a significant decline in the activity of Pt catalysts for the ORR in H2SO4 electrolyte due to the strong adsorption of sulfate ions, compared to that in HClO4. The addition of of Br− anions to the H2SO4 electrolyte further altered the ORR selectivity toward the 2 e− pathway, with reduced activity [29]. A similar phenomenon was observed with chloride and cyanide ions (Figure 3a) [30,31*], attributed to configurational variation of the adsorbed species on the Pt surfaces according to the type of anion in the electrolyte. While the adsorption of sulfate anions requires at least four contiguous Pt atoms, the hetero anions (e.g. bromide, chloride, and cyanide ions) are adsorbed on single Pt atom sites. Hence, with only sulfate in the electrolyte, desorption of the adsorbed sulfate anions leaves behind Pt ensemble sites, which promotes the 4 e− pathway ORR, albeit with decreased activity. However, in the presence of both sulfate and hetero anions, the hetero anions are preferentially adsorbed on the Pt surface due to their stronger absorption power than sulfate ions. Subsequent desorption of the adsorbed hetero anions generates vacant single Pt sites, which induce the 2 e− pathway ORR [31*]. On the basis of the above concept, Markovic and co-workers covered a Pt(111) electrode with a self-assembled monolayer of calix[4]arene molecules to lower the number of Pt ensemble sites [32]. As the coverage of the calix[4]arene molecules increased, the available Pt ensemble sites decreased, reducing the ORR activity, with concomitant formation of H2O2. Similarly, Choi and co-workers coated Pt nanoparticles on carbon (Pt/C) with amorphous carbon layers by acetylene chemical vapor deposition (CVD) [33*]. The carbon coating layer in the resulting amorphous carbon coated Pt/C catalysts promoted the end-on adsorption of O2 molecules while suppressing their side-on adsorption, thereby enhancing the 2 e− ORR pathway. The H2O2 selectivity was proportional to the thickness of the carbon layer, with a 7
maximal selectivity of 41%. The carbon coating layer further suppressed decomposition of the produced H2O2, which produces water via disproportionation or reduction reactions, by hindering access to the produced H2O2. Selenium, sulfur, or an ammonium ion has also been utilized as a modifier for Pt catalysts to promote the 2 e− pathway ORR [34–37]. Figure 3
(a) Disk and ring currents of cyanide-adsorbed Pt(111) electrode with variation of the cyanide coverage in 0.05 M H2SO4 electrolyte (solid lines). Brown lines indicate disk and ring current of Pt(111) electrode in 0.1 M HClO4. Reprinted with permission from Ref. [31*]. Copyright (2015) Elsevier B.V. (b) Schematic illustration of effect of carbon coating on Pt/C catalysts for ORR by CVD and H2O2 selectivity depending on the CVD operation time for Pt/C catalysts. Reprinted with permission from Ref. [33*]. Copyright (2014) American Chemical Society.
Atomically dispersed catalysts The most efficient method for eliminating metal ensemble sites is full dispersion of metal atoms into single sites on the support. The resulting catalysts are termed atomically dispersed catalysts [38,39] or single-atom catalysts [40,41]. Lee and co-workers [42**] prepared TiNsupported Pt nanoparticle (Pt/TiN) catalysts by an incipient wetness impregnation method. At low Pt loading (0.35 wt%), atomically dispersed Pt sites were exclusively obtained. The H2O2 8
selectivity of the resulting 0.35 wt% Pt/TiN catalyst reached 65%, which was much higher than that of the 2 wt% Pt/TiN (30%) comprising nanoparticles as well as atomically dispersed sites (Figure 4a). The Lee group further investigated the effects of supports on the H2O2 production selectivity with the TiN and TiC supports [43], in which 0.2 wt% Pt/TiC exhibited higher ORR activity and H2O2 selectivity than 0.2 wt% Pt/TiN. DFT calculations suggested that the more favorable adsorption energy and energy profiles of Pt/TiC toward the 2 e− pathway ORR led to its superior activity and selectivity for H2O2 production. Atomically dispersed sites inevitably have high surface energy and readily agglomerate during preparation, making their syntheses with high metal loading challenging. To circumvent this problem, Choi and co-workers utilized zeolite-templated carbon (ZTC) possessing a high concentration of sulfur (HSC) as a support, where the ultra-high surface area of HSC (2,770 m2g−1) and numerous S sites with strong anchoring ability facilitated the formation of atomically dispersed Pt sites up to a loading of 5 wt% [44*]. This is markedly higher than that of atomically dispersed catalysts reported at that time (less than 1 wt). The Pt/HSC catalyst showed 95% H2O2 selectivity in the potential range of 0.1‒0.7 V, which is a dramatic improvement compared to the H2O2 selectivity of 28% achieved with Pt NP-based catalysts (Pt/ZTC) (Figure 4b). Li and co-workers prepared ~15 wt% atomically dispersed Pt catalysts using a sulfur-containing support, CuSx; the resulting h-Pt1-CuSx catalyst afforded ≥90% H2O2 selectivity in the potential range of 0.05‒0.7 V (Figure 4c) [45]. Importantly, the h-Pt1-CuSx catalyst could generate H2O2 with a yield of 546 mol kgcat−1 h−1, which is among the highest values reported for atomically dispersed electro- and thermo-catalysts for H2O2 production. Other atomically dispersed metal catalysts comprising Ni, Au, or Fe as the metal center were prepared and exploited to facilitate selective electrochemical H2O2 production [46–50].
9
Figure 4
(a) HAADF-STEM image of 0.35 wt% Pt/TiN and ORR activity and selectivity. Reprinted with permission from Ref. [42**]. Copyright (2016) Wiley-Vch Verlag GmbH & Co. (b) HAADF-STEM image of 5 wt% Pt supported on zeolite-templated carbon with high Scontent (Pt/HSC) and its ORR activity and selectivity. Scale bar is 2 nm. Reprinted with permission from Ref. [44*]. Copyright (2016) Nature Publishing Group. (c) HAADF-STEM image of single atomic Pt site catalysts embedded in hollow CuSx support (h-Pt1-CuSx) and its ORR activity and selectivity. Reprinted with permission from Ref. [45]. Copyright (2019) from Cell Press.
Concluding remarks In this review, the strategies for designing electrocatalysts for electrochemical H2O2 production via the 2 e− pathway ORR were summarized. The key for H2O2 production is to promote end-on adsorption of O2 molecules on the catalyst surfaces via isolation of the active
10
atomic sites during the ORR. Toward this goal, three prominent strategies have been developed, i.e., alloying with inert metals, partial surface poisoning, and fabricating atomically dispersed catalysts. Although the developed strategies have attempted to exploit geometric effect of the catalyst for selective H2O2 production, the mode of O2 adsorption has been largely inferred from DFT calculations. The direct spectroscopic detection of intermediate species and defining the relationship between the O2 adsorption mode and reactivity under real reaction conditions remain elusive. Thus, whether controlling the geometry of the catalyst only affects the adsorption configuration of the reactants (geometric effect) and/or also influences the electronic structure of the catalyst (electronic effect) remain unclear and should be explored in the future. Practically, electrocatalysts for highly active and selective H2O2 production should be integrated with currently available H2O2 production cells to facilitate potential commercialization.
Conflicts of interest statement The authors declare no conflict of interest.
Acknowledgements This work was supported by the National Research Foundation (NRF) of Korea funded by the Ministry of Science and ICT (NRF-2019M3E6A1064521, and NRF-2019M3D1A1079306).
References Papers of particular interest, published within the period of review, have been highlighted as: * of special interest ** of outstanding interest
11
1.
Debe MK: Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012, 486:43–51.
2.
Bruce PG, Freunberger SA, Hardwick LJ, Tarascon J-M: Li–O2 and Li–S batteries with high energy storage. Nat Mater 2012, 11:19–29.
3.
Kulkarni A, Siahrostami S, Patel A, Nørskov JK: Understanding catalytic activity trends in the oxygen reduction reaction. Chem Rev 2018, 118:2302–2312.
4.
Shao M, Chang Q, Dodelet J-P, Chenitz: Recent advances in electrocatalysts for oxygen reduction reaction. Chem Rev 2016, 116:3594–3657.
5.
Borghei M, Lehtonen J, Liu L, Rojas OJ: Advanced biomass-derived electrocatalysts for the oxygen reduction reaction. Adv Mater 2018, 30:1703691.
6.
Yang S, Verdaguer-Casadevall A, Arnarson L, Silvioli L, Čolić V. Frydendal R, Rossmeisl J, Chorkendorff I, Stephens IEL: Toward the decentralized electrochemical production of H2O2: a focus on the catalysis. ACS Catal 2018, 8:4064–4081.
7.
Perry SC, Panhotra D, Vieira L, Csepei L-I, Sieber V, Wang L, Ponce de León C, Walsh FC: Electrochemical synthesis of hydrogen peroxide from water and oxygen. Nat Rev Chem 2019, 3:442–458.
8.
Melchinonna M, Fornasiero P, Prato M: The rise of hydrogen peroxide as the main product by metal-free catalysis in oxygen reductions. Adv Mater 2019, 31:1802920.
9.
Zhang J, Zhang H, Cheng M-J, Lu Q: Tailoring the electrochemical production of H2O2: strategies for the rational design of high-performance electrocatalysts. Small 2019, 1902845.
10.
Otsuka K, Yamanaka I: One step synthesis of hydrogen peroxide through fuel cell reaction. Electrochim Acta 1990, 35:319–322.
11.
Yamanaka I, Onizawa T, Takenaka S, Otsuka K: Direct and continuous production of hydrogen peroxide with 93% selectivity using a fuel-cell system. Angew Chem Int Ed 2003, 42:3653–3655.
12.
Fellinger T-P, Hasché F, Strasser P, Antonietti M: Mesoporous nitrogen-doped carbon for the electrocatalytic synthesis of hydrogen peroxide. J Am Chem Soc 2012, 134:4072–4075.
13.
Park J, Nabae Y, Hayakawa T, Kakimoto M: Highly selective two-electron oxygen reduction catalyzed by mesoporous nitrogen-doped carbon. ACS Catal 2014, 4:3749–3754.
14.
Lu Z, Chen G, Siahrostami S, Chen Z, Liu K, Xie J, Liao L, Wu T, Lin D, Liu Y, Jaramillo TF, Nørskov JK, Cui Y: High-efficiency oxygen reduction to hydrogen peroxide catalyzed by oxidized carbon materials. Nat Catal 2018, 1:156–162.
15.
Kim HW, Ross MB, Kornienko N, Zhang L, Guo J, Yang P, McCloskey BD: Efficient hydrogen peroxide generation using reduced graphene oxide-based oxygen reduction electrocatalysts. Nat Catal 2018 1:282–290.
16.
Sa YJ, Kim JH, Joo SH: Active edge-site-rich carbon nanocatalysts with enhanced electron transfer for efficient electrochemical hydrogen peroxide production. Angew Chem Int Ed 2019, 58:1100–1105.
12
17.
Yeager E: Disoxygen electrocatalysis: mechanisms in relation to catalyst structure. J Mol Catal 1986, 38:5–25.
18.
Chan AWE, Hoffmann R, Ho W: Theoretical aspects of photoinitiated chemisorption, dissociation, and desorption of O2 on Pt(111). Langmuir 1992, 8:1111–1119.
19.
Viswanathan V, Hansen HA, Rossmeisl J, Nørskov JK: Unifying the 2e− and 4e− reduction of oxygen on metal surfaces. J Phys Chem Lett 2012, 3:2948–2951.
20.
Edwards JK, Solsona B, N EN, Carley AF, Herzing AA, Kiely CJ, Hutchings GJ: Switching off hydrogen peroxide hydrogenation in the direct synthesis process. Science 2009, 323:1037–1041.
21.
Gao F, Wang Y, Goodman DW: CO oxidation over AuPd(100) from ultrahigh vacuum to near-atmospheric pressures: the critical role of contiguous Pd atoms. J Am Chm Soc 2009, 131:5734–5735.
22.
Baber AE, Tierney HL, Sykes ECH: Atomic-scale geometry and electronic structure of catalytically important Pd/Au alloys. ACS Nano 2010, 4:1637–1645.
23.
Ham HC, Hwang GS, Han J, Nam SW, Lim TH: On the role of Pd ensembles in selective H2O2 formation on PdAu alloys. J Phys Chem C 2009, 113:12943–12945.
24.
Jirkovský JS, Panas I, Ahlberg E, Halasa M, Romani S, Schiffrin DJ: Single atom hotspots at Au–Pd nanoalloys for electrocatalytic H2O2 production. J Am Chem Soc 2011, 133:19432–19441. ** The first study reporting the use of metal alloys to eliminate metal ensemble sites for promoting electrochemical H2O2 production. 25.
Jirkovský JS, Panas I, Romani S, Ahlberg E, Schiffrin DJ: Potential-dependent structural memory effects in Au–Pd nanoalloys. J Phys Chem Lett 2013, 3:315–321.
26.
Siahrostami S, Verdaguer-Casadevall A, Karamad M, Deiana D, Malacrida P, Wickman B, Escudero-Escribano M, Paoli EA, Frydendal R, Hansen TW, Chorkendorff I, Stephens IEL, Rossmeisl J: Enabling direct H2O2 production through rational electrocatalyst design. Nat Mater 2013, 12:1137–1143. * This study identified a new composition of metal alloy, Pt–Hg, as a promising catalyst for H2O2 production based on DFT calculations and experimentally validated its high activity and selectivity for electrochemical H2O2 production. 27.
Verdaguer-Casadevall A, Deiana D, Karamad M, Siahrostami S, Malacrida P, Hansen TW, Rossmeisl J, Chorkendorff I, Stephens IEL: Trends in the electrochemical synthesis of H2O2: enhancing activity and selectivity by electrocatalytic site engineering. Nano Lett 2014, 14:1603–1608.
28.
Deiana D, Verdaguer-Casadevall A, Malacrida P, Stephens IEL, Chorkendorff I, Wagner JB, Hansen, TW: Determination of core-shell structures in Pd–Hg nanoparticles by STEM–EDX. ChemCatChem 2015, 7:3748–3752.
29.
Marković NM, Gasteiger HA, Grgur BN, Ross PN: Oxygen reduction reaction on Pt(111): effect of bromide. J. Electroanal Chem 1999, 467:157–163.
30.
Stamenkovic V, Marković NM, Ross PNJr: Structure-relationships in electrocatalysis: oxygen reduction and hydrogen oxidation reactions on Pt(111) and Pt(100) in solutions containing chloride ions. J Electroanal Chem 2001, 500:44– 51. 13
31.
Ciapina EG, Lopes PP, Subbaraman R, Ticianelli EA, Stamenkovic V, Strmcnik D, Markovic NM: Surface spectators and their role in relationships between activity and selectivity of the oxygen reduction reaction in acid environments. Electrochem Commun 2015, 60:30–33. * This study described the catalytic effect of the spectator ions on the Pt (111) surface on the ORR activity and selectivity, enhancing understanding of the geometric effect under electrochemical conditions. 32.
Genorio B, Strmcnik D, Subbaraman Ram, Tripkovic D, Karapetrov G, Stamenkovic VR, Pejovnik S, Markovic NM: Selective catalysts for the hydrogen oxidation and oxygen reduction reactions by patterning of platinum with calix[4]arene molecules. Nat Mater 2010, 9:998–1003.
33.
Choi CH, Kwon HC, Yook S, Shin H, Kim H, Choi M: Hydrogen peroxide synthesis via enhanced two-electron oxygen reduction pathway on carbon-coated Pt surface. J Phys Chem C 2014, 118:30063–30070. * This study presents enhancement of the selectivity for the 2 e− pathway ORR by blocking accessible Pt ensemble sites with amorphous carbon coating, demonstrating the geometric effect in the ORR selectivity. 34.
Schmidt TJ, Paulus UA, Gasteiger HA, Behm RJ: The oxygen reduction reaction on a Pt/carbon fuel cell catalyst in the presence of chloride anions. J Electroanal Chem 2001, 508:41–47.
35.
Mo Y, Scherson DA: Platinum-based electrocatalysts for generation of hydrogen peroxide in aqueous acidic electrolytes. J Electrochem Soc 2003, 150:E39–E46.
36.
Lopes T, Chlistunoff J, Sansiñena, J-M, Garzon FH: Oxygen reduction reaction on a Pt/carbon fuel cell catalyst in the presence of trace quantities of ammonium ions: an RRDE study. Int J Hydrogen Energy 2012, 37:5202–5207.
37.
Verdaguer-Casadevall A, Hernandez-Fernandez P, Stephens IEL, Chorkendorff I, Dahl S: The effect of ammonia upon the electrocatalysis of hydrogen oxidation and oxygen reduction on polycrystalline platinum. J. Power Sources 2013, 220:205–210.
38.
Flytzani-stephanopoulos M: Gold atoms stabilized on various supports catalyze the water–gas shift reaction. Acc Chem Res 2014, 47:783–792.
39.
Gates BC, Flytzani-stephanopoulos M, Dixon DA, Katz A: Atomically dispersed supported metal catalysts: perspectives and suggestions for future research. Catal Sci Technol 2017, 7:4259–4257.
40.
Wang A, Li J, Zhang T: Heterogeneous single-atom catalysis. Nat Rev Chem 2018, 2:65–81.
41.
Mitchell S, Vorobyeva E, Pérez-Ramírez: The multifaceted reactivity of single-atom heterogeneous catalysts. Angew Chem Int Ed 2018, 57:15316–15329.
42.
Yang S, Kim J, Tak YJ, Soon A, Lee H: Single-atom catalyst of platinum supported on titanium nitride for selective electrochemical reactions. Angew Chem Int Ed 2016, 55:2058–2062. ** The first study verifying the effectiveness of atomically dispersed catalysts for the electrosynthesis of H2O2. 43.
Yang S, Tak YJ, Kim J, Soon A, Lee H: Support effects in single-atom platinum catalysts for electrochemical oxygen reduction. ACS Catal 2017, 7:1301–1307. 14
44.
Choi CH, Kim M, Kwon HC, Cho SJ, Yun S, Kim H-T, Mayrhofer KJJ, Kim H, Choi M: Tuning selectivity of electrochemical reactions by atomically dispersed platinum catalyst. Nat Commun 2016, 7:10922. * By greatly increasing the number of atomically dispersed Pt sites, almost 100% H2O2 selectivity could be achieved. 45.
Shen R, Chen W, Peng Q, Lu S, Zheng L, Cao X, Wang Y, Zhu W, Zhang J, Zhuang Z, Chen C, Wang D, Li Y: High-concentration single atomic Pt sites on hollow CuSx for selective O2 reduction to H2O2 in acid solution. Chem 2019, 5:2099–2110.
46.
Luo J, Tang H, Tian X, Hou S, Li X, Du L, Liao S: Highly selective TiN-supported highly dispersed Pt catalyst: ultra active toward hydrogen oxidation and inactive toward oxygen reduction. ACS Appl Mater Interfaces 2018, 10:3530–3537.
47.
Wang T, Zeng Z, Cao L, Li Z, Hu X, An B, Wang C, Lin W: A dynamically stabilized single-nickel electrocatalyst for selective reduction of oxygen to hydrogen peroxide. Chem Eur J 2018, 24:17011–17018.
48.
Shin S, Kim J, Park S, Kim H-E, Sung Y-E, Lee H: Changes in the oxidation state of Pt single-atom catalysts upon removal of chloride ligands and their effect for electrochemical reactions. Chem Commun 2019, 55:6389–6392.
49.
Sahoo SK, Ye Y, Lee S, Park J, Lee H, Lee J, Han JW: Rational design of TiCsupported single-atom electrocatalysts for hydrogen evolution and selective oxygen reduction reactions. ACS Energy Lett 2019, 4:126–132.
50.
Jiang K, Back S, Akey AJ, Xia C, Hu Y, Liang W, Schaak D, Stavitski E, Nørskov JK, Siahrostami S, Wang H: Highly selective oxygen reduction to hydrogen peroxide on transition metal single atom coordination. Nat Commun 2019, 10:3997.
15