In Situ Deposition of Pd during Oxygen Reduction Yields Highly

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In Situ Deposition of Pd during Oxygen Reduction Yields Highly Selective and Active Electrocatalysts for Direct H2O2 Production Yu Lei Wang, Sadi Gurses, Noah Felvey, Alexey Boubnov, Samuel S. Mao, and Coleman Kronawitter ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01758 • Publication Date (Web): 31 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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In Situ Deposition of Pd during Oxygen Reduction Yields Highly Selective and Active Electrocatalysts for Direct H2O2 Production Yu Lei Wang1, Sadi Gurses1, Noah Felvey1, Alexey Boubnov2, Samuel S. Mao3, and Coleman X. Kronawitter1* 1Department 2SLAC

of Chemical Engineering, University of California, Davis, CA 95616, USA.

National Accelerator Laboratory, Menlo Park, CA, 94025, USA.

3Department

of Mechanical Engineering, University of California, Berkeley, CA 94720, USA.

*Corresponding author

KEYWORDS: electrocatalysis, hydrogen peroxide, oxygen reduction reaction, palladium, catalyst synthesis

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ABSTRACT

Hydrogen peroxide (H2O2) is a commodity chemical that serves as an oxidant and disinfectant for a number of historically important chemical end-use applications. Its synthesis can be made more sustainable, clean, and geographically distributed through technology based on the aqueous electrocatalytic two-electron reduction of O2, which produces H2O2 using only air, water, and electricity as inputs. Herein results are presented establishing that Pd, which is widely known to strictly catalyze the four-electron reduction of O2 to H2O, can be made highly selective toward H2O2 production when it is deposited in situ – that is, through electrochemical deposition from Pd ions during O2 reduction. The resultant cathodes are found to be comprised of sub-five nanometer amorphous Pd nanoparticles and are measured to yield H2O2 selectivities above 95% in the relevant potential range. In addition, the cathodes are highly active – they yield the second-highest partial kinetic current density for H2O2 production in acidic media reported in the known literature. It is observed that in situ synthesis of Pd catalysts enables dramatic gains in H2O2 yield for all inert, conductive supports studied (including glassy carbon, commercial activated carbon, graphene, and antimony-doped tin oxide). Further efforts to generalize these results to other systems establish that even Pt, the prototypical four-electron O2 reduction catalyst, can be engineered to be highly selective to H2O2 when synthesized in situ in relevant conditions. These results and the comprehensive electrochemical and physical characterization presented, including synchrotron-based X-ray absorption spectroscopy, suggest that in situ synthesis is a promising approach to engineer O2 reduction electrocatalysts with tunable product selectivity and activity.


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1. INTRODUCTION Recent advances in energy conversion technologies utilizing renewable and natural gas resources have led to the increased electrification of high energy-demand sectors that have historically relied on petroleum-based feedstocks and associated catalytic processes. The rise in electrification is most apparent in the transportation sector, where the market share of vehicles with electrified powertrains has been projected to increase through 2050.1 Chemical manufacturing, one of the fastest-growing energy-demand industries,2 is similarly positioned to transition toward electrification. An important driving force for this transition in the chemical industry is the development of new electrosynthesis technologies for production of both commodity chemicals and fine chemicals.3 Many of these technologies will be enabled by novel electrocatalytic processes, and are therefore particularly reliant on the discovery and development of new functional catalysts that facilitate high production rates with high chemical selectivities.

In this context, hydrogen peroxide (H2O2) is notable as a commodity chemical whose synthesis can be made more sustainable, efficient, and geographically distributed through electrocatalytic technology.4,5 It serves as an oxidant for number of historically important industries and is currently associated with significant market growth in new chemical end-use segments.6 Aqueous solutions of H2O2 of various concentrations can be produced by electrocatalytic reduction of O2, using only air and water as reactants.7 Electrocatalytic H2O2 production therefore presents an opportunity for delocalized or distributed production of an important commodity chemical with negligible negative environmental impact.

The electrocatalytic production of H2O2 through oxygen reduction electrocatalysis is a wellestablished research field; the state of this technology is comprehensively described in a recent


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review article on the subject by Stephens and coworkers.4 The goal of electrocatalytic O2 reduction to H2O2 is to generate stable, high production rates while minimizing the complete reduction to H2O. Production technology based on this process has the additional requirement of limiting further decomposition of H2O2. Findings in the research into efficient catalysts for this process often closely align with those associated with the direct synthesis route (𝐻2 + 𝑂2→𝐻2𝑂2) by thermal catalysis, efforts for which have also been reviewed extensively.8-10 In acidic media, the overall reactions of interest for oxygen reduction reaction (ORR) and their associated standard thermodynamic potentials are:

𝑂2 + 4𝐻 + + 4𝑒 ― ⇄ 2𝐻2𝑂

(𝑈𝑂0 /𝐻 𝑂 = 1.23 𝑉 𝑣𝑠. 𝑅𝐻𝐸)

𝑂2 + 2𝐻 + + 2𝑒 ― ⇄2𝐻2𝑂2

(𝑈𝑂0 /𝐻 𝑂

2

2

2

2 2

= 0.68 𝑉 𝑣𝑠. 𝑅𝐻𝐸)

(1) (2)

The charge transfer steps associated with the four-electron, four-proton reduction of O2 to H2O in acidic electrolytes can be written as:11 𝑂2 + ∗ + 4𝐻 + + 4𝑒 ― → ∗ 𝑂𝑂𝐻 + 3𝐻 + + 3𝑒 ―

(3)

∗ 𝑂𝑂𝐻 + 3𝐻 + + 3𝑒 ― → ∗ 𝑂 + 𝐻2𝑂 + 2𝐻 + + 2𝑒 ― (4) ∗ 𝑂 + 𝐻2𝑂 + 2𝐻 + + 2𝑒 ― → ∗ 𝑂𝐻 + 𝐻2𝑂 + 𝐻 + + 𝑒 ―

(5)

∗ 𝑂𝐻 + 𝐻2𝑂 + 𝐻 + + 𝑒 ― → ∗ + 2𝐻2𝑂

(6)

Where ∗ indicates a surface site on the catalyst. In contrast, the two-electron, two-proton reduction of O2 to H2O2 is typically considered to occur through the following associative mechanism: 𝑂2 + ∗ + 2𝐻 + + 2𝑒 ― → ∗ 𝑂𝑂𝐻 + 𝐻 + + 𝑒 ― (7)


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∗ 𝑂𝑂𝐻 + 𝐻 + + 𝑒 ― → ∗ + 𝐻2𝑂2 Catalytic

activities

for

(8)

these

reactions

are

determined

by

the

stabilities

of

∗ 𝑂𝑂𝐻, ∗ 𝑂𝐻, 𝑎𝑛𝑑 ∗ 𝑂 intermediates, whose surface binding energies follow established scaling laws.12 A critical determining factor is whether the catalyst is capable of dissociating the 𝑂 ― 𝑂 bond. Weak adsorption of the intermediates will lead to high H2O2 selectivity but low activity.4 Platinum group metals are known for their especially high activity for the four-electron reduction of O2 to H2O; they are non-selective for H2O2 production.13 This study demonstrates that the ORR activity and selectivity of elemental Pd, as well as Pt, can be completely altered and tuned through in situ catalyst synthesis – that is, the electrochemical deposition of catalysts during O2 reduction.

2. RESULTS AND DISCUSSION 2.1 In situ deposition of Pd electrocatalysts during ORR. In this study, Pd was deposited by reduction of aqueous dissolved PdCl2 at various concentrations during simultaneous reduction of O2 onto a number of electrically conductive and electrochemically inert supports/substrates. For all results reported, prior to deposition, the ORR activities and selectivities of all bare supports/substrates were measured in O2-saturated 0.1 M HClO4 solution to ensure that the substrate (blank) electrode yielded no ORR activity (Figure S1). Throughout, in situ-deposited Pd samples are labeled according to their corresponding PdCl2 deposition concentrations (Pd[PdCl2]). Two electrochemical procedures for Pd deposition in the O2-saturated electrolyte were utilized: cyclic voltammetry (CV), whereby the potential of the electrode was cycled multiple times between 0 and 1.0 V vs. RHE until an unchanging steady-state CV was obtained; and chronoamperometry, whereby the electrode was held at constant potential for various times. In both cases, H2O2 generation was quantified during in situ deposition through rotating ring disk


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electrochemistry (RRDE). In the RRDE configuration, the Pt ring potential was held potentiostatically at 1.28 V vs. RHE, a potential where H2O2 is fully oxidized and the process is mass transport-limited.14 After deposition, the samples were removed from the PdCl2-containing 0.1 M HClO4 electrolyte and placed directly into pure 0.1 M HClO4 for evaluation of ORR activity using RRDE. Figure 1 shows, through linear sweep voltammograms in the RRDE configuration, the performance of a representative in situ-deposited Pd electrocatalyst for O2 reduction. For comparison, crystalline PdO nanoparticles and commercial crystalline Pd nanoparticles supported on Vulcan activated carbon (Pd/C) were also evaluated. X-ray diffractograms (XRD) image of PdO nanoparticles is provided in Figure S2. As has been observed previously in the literature,14,15 results show Pd/C (crystalline Pd) primarily facilitates the four-electron reduction of O2 to H2O. It is associated with low activity for H2O2 production; the potential-averaged selectivity was 17.8% and the highest selectivity recorded over the potential range of interest was 24%. PdO shows similar activity to Pd/C, yielding a potential-averaged selectivity of 22.6% and at high overpotential a maximum H2O2 selectivity of 32%. In contrast, the in situ-deposited Pd electrocatalyst yielded ORR activity with extremely high H2O2 selectivity - greater than 90% for the entire potential range of interest, with a potential-averaged selectivity of 95.1%. Nearly 100% selectivity was observed at low overpotential (at 0.55 V vs. RHE, or 0.13 V overpotential with respect to 𝐸0𝑂2/𝐻2𝑂2= 0.68 V). Additionally, in situ-deposited Pd electrocatalyst yields the secondhighest reported kinetic current density for H2O2 production in acidic conditions (the accepted performance metric) in the literature (Figure S3). To quantify the deposited quantity of Pd, inductively coupled plasma mass spectrometry (ICPMS) measurements were recorded. As shown in Table S1, 1.391 μg of Pd was deposited onto the


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glassy carbon (GC) electrode. The turnover frequency (TOF) was estimated based on the ICP-MS results and electrochemically active surface area measurements, following established procedures (Figure S4). These results indicated that TOF is 0.0134 H2O2 s-1 for this reaction – at 100 mV overpotential. For completeness this value is reported here, but to the authors' best knowledge, there are no reports in the literature on H2O2 TOF for the two-electron reduction of O2.

Figure 1. RRDE ORR results in pure 0.1 M HClO4. (a) Linear sweep voltammograms of a representative in situ-deposited Pd sample (Pd1μM), PdO, and commercial crystalline Pd/C. (b) H2O2 selectivity calculated from the Pt ring current in (a). 0.1 M HClO4; 10 mV s-1; 1600 rpm; Pt ring held at 1.28 V vs. RHE for H2O2 detection.

The high H2O2 selectivity associated with in situ-deposited Pd was also observed when other electrochemically inert support materials were utilized. In these experiments, commercial Vulcan XC 72R carbon (C), graphene, and antimony-doped tin oxide (ATO) nanoparticles were drop cast onto the GC electrode and verified to be electrochemically inert for oxygen reduction prior to their use as conductive supports for Pd electrocatalyst synthesis and evaluation (see Methods in the Supporting Information). H2O2 production efficiencies were observed to be similar for all carbon


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supports for a given PdCl2 concentration (Figures S5). When graphene was used as a support, a slightly higher ORR current density and a more anodic ORR (lower reduction overpotential) onset potential was observed. 2.2 Disordered structure of Pd nanoparticles. To investigate the microscopic nature of the in situ-deposited Pd species yielding high H2O2 selectivity, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) experiments were performed. A Pd0.5µM/graphene sample with potential-averaged H2O2 production selectivity of 94.1% was prepared by CV in O2-saturated 0.5 µM PdCl2/0.1 M HClO4 solution (Figure S5). For comparison and to elucidate the microscopic characteristics associated with H2O2 selectivity, Pd5µM/graphene was synthesized by chronoamperometry at 0.1 V vs. RHE for 20 minutes in 5 µM PdCl2/0.1 M HClO4; this sample yielded H2O2 selectivity varying between 14% and 57% across the potential range of interest (Figure S6). STEM images show the existence of Pd clusters in both Pd0.5µM/graphene and Pd5µM/graphene; no such particles are observed on blank graphene (Figure S7). The Pd0.5µM/graphene sample (high H2O2 selectivity) was found (Figure 2a,d) to consist of ultra-small Pd clusters, with diameters measured between 0.8 and 2.5 nm, co-existing with a small number of larger nanoparticles with diameters ranging from 2.5 to 6.2 nm. In contrast, the Pd5µM/graphene sample (low H2O2 selectivity) possessed no ultra-small Pd clusters and a large number of Pd nanoparticles, with diameters measured between 3.9 and 7.4 nm (Figure 2b,e). The STEM images of commercial Pd/C (potential-averaged selectivity of 17.8%; Figure 1b) in Figures 2c,f and Figure S8 showed the existence of 2-3 nm Pd clusters with lattice fringes evident, indicating a well-developed crystal structure. Notably and in contrast, careful evaluation of STEM images of Pd0.5µM/graphene revealed that the Pd clusters are amorphous, as implied by the complete absence of lattice fringes (Figure S9). The Pd5µM/graphene sample exhibited features


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characteristic of an intermediate degree of crystallinity - primarily amorphous Pd particles were observed, with small amount of structural order evident with close inspection (Figure S10). The PdO synthesized for comparative ORR experiments (Figure 1) showed expected high crystallinity and possessed an average particle size of approximately 20 nm (Figure S11). These observations on the dissimilar structures of the examined Pd-based electrocatalysts prompted additional experiments to probe the electrochemical activity of the surfaces. As a simple qualitative probe reaction for electrochemical activity of surface species, carbon monoxide electro-oxidation (CO stripping) was performed for both in situ-deposited Pd and commercial Pd/C (Figure S12). The CO oxidation onset differs considerably between the samples, consistent with their observed dissimilar structural properties. A detailed account of the line shapes of the resultant CO oxidation waves are beyond the scope of this first report on the catalyst system.

These observations on particle sizes and crystallinity were supplemented by further characterization through powder XRD and X-ray photoelectron spectroscopy (XPS) measurements (Figure S13). The broad peak at 25° in all XRD patterns is consistent with the graphite (002) peak of the Vulcan support.16 For commercial Pd/C, the peaks at 40.1°, 46.7°, 68.1° and 82.1° are attributed to the (111), (200), (220), and (311) planes, respectively, of cubic crystal structure of Pd (PDF No. 46-1043). For Pd5µM/C, no peaks were observed except graphite (002). XPS measurements were performed to confirm the chemical signature of Pd and to attempt to investigate the chemical states of Pd in commercial Pd/C, Pd0.5µM/C, and Pd5µM/C. As shown in Figure S13, small but evident Pd 3d peaks can be observed for Pd0.5µM/C and Pd5µM/C; the weak signal is expected in the context of the sensitivity of XPS and the extremely small quantity of Pd observed through microscopy. Given the weak signal intensities, definitive conclusions regarding the oxidation states of in situ-deposited Pd could not be made through XPS.


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Figure 2. Representative STEM images: (a,d) Pd0.5µM/graphene (potential-averaged H2O2 selectivity of 94.1%); (b,e) Pd5µM/graphene (potential-averaged H2O2 selectivity of 38.1%); (c,f) Commercial Pd/C (potential-averaged H2O2 selectivity of 17.8%). (g) XANES spectra at the Pd K-edge of PdO, Pd foil, commercial crystalline Pd/C, and an in situ-deposited Pd sample (Pd/graphene). (h) The magnitudes (solid) and the imaginary (dashed) parts of the Fourier transforms of the EXAFS data for in situ-deposited Pd (Pd/graphene) and reference samples;


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corresponding EXAFS spectra in k-space are shown in Figure S14 in the Supporting Information. X-ray absorption spectroscopy (XAS) was performed to investigate the oxidation state and local atomic structure of the in situ-deposited Pd species, using a Pd foil, PdO nanoparticles, and commercial crystalline Pd/C as references for comparison. Figure 2g displays the normalized Xray absorption near-edge structure (XANES) spectra of in situ-deposited Pd and reference samples. For the in situ-deposited Pd, the intensity of the white line is between that of the Pd foil and PdO, suggesting that in situ-deposited Pd exists with a net positive charge. Figure 2h shows Fouriertransformed extended X-ray absorption fine structure (EXAFS) spectra of in situ-deposited Pd and reference samples. The peaks at 1.0-2.0 Å, 2.0-3.0 Å, and 2.7-3.6 Å can be attributed to Pd-O, PdPd (Pd metal), and Pd-(O)-Pd (PdO) coordination shells, respectively.17 By comparison of the imaginary components of the spectra, it is clear that the spectrum of the commercial Pd sample is similar to that of the PdO reference in the range 1-2 Å (Pd-O) and similar to that of Pd foil between 2-3 Å (Pd-Pd). Thus, the commercial sample consists of metallic Pd with a small amount of oxidic Pd. For the in situ-deposited Pd sample, the imaginary component of the spectrum also resembles the PdO reference in the range 1-2 Å, suggesting that in situ-deposited Pd contains a similar bonding environment to that of PdO. At higher radial distance, however, the imaginary part of the in situ-deposited Pd spectrum does not resemble PdO and resembles a minor Pd-Pd component of Pd metal (1.7-3.0 Å), suggesting that perhaps in situ-deposited Pd does not share the long-range structural arrangements of the reference materials.18 The above observations are consistent with the presence of small clusters in the in situ-deposited Pd sample. More comprehensive analysis of EXAFS data is beyond the scope of this article presenting the discovery of high H2O2 selectivity for in situ-deposited Pd ORR electrocatalysts; such analysis will be conducted in a future study.


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Given the unique disordered character of in situ-deposited Pd particles, additional experiments were performed to assess the possible effect of a greater degree of surface oxidation in these electrocatalysts. To this end, a series of electrochemical oxidation and reduction experiments were performed. For reductive treatment, in situ-deposited Pd2.8µM was synthesized and transferred to pure N2-purged 0.1 M HClO4 and the electrode potential was held at 0 V vs. RHE for 10 minutes. For oxidative treatment, the sample was transferred to pure O2-purged 0.1 M HClO4 and held at 1.6 V vs. RHE for 10 minutes. After treatments, ORR activity and H2O2 selectivities were recorded (Figure S15). After electrochemical reduction or oxidation, a slight increase or significant decrease in H2O2 selectivity was observed, indicating that the surface Pd could be reduced or oxidized to some extent, consistent with the XANES results presented above.

2.3 The effect of PdCl2 concentration on ORR performance. To further understand the process of in situ synthesis of highly H2O2-selective Pd ORR electrocatalysts, the influence of the concentration of PdCl2 in the synthesis solution on ORR performance was investigated through a series of systematic experiments. Figure 3a and Figure S16 show selected ORR cycles during continuous CV of a clean GC electrode in O2-saturated 0.1 M HClO4 containing 0.5, 1.0, 2.8, 5.0, and 10.0 μM PdCl2, respectively. During cycles, the ring current density accurately reports the H2O2 produced; only in the presence of O2 reduction to H2O2 did the ring measure oxidation current (control experiments in N2-saturated PdCl2 solutions confirmed that the Pt ring at 1.28 V vs. RHE cannot oxidize dissolved Pd ions or ionic complexes). Results from the deposition CV experiments in Figure S16 show that in the presence of 0.5 or 1.0 μM PdCl2, both the cathodic current density of the disk (ORR rate) and anodic current density of the ring (detection rate of H2O2 produced) increase with increasing cycle number, with a steady-state rate and H2O2 selectivity obtained after the tenth cycle. In contrast, for higher precursor concentrations (2.8, 5.0, and 10.0 μM PdCl2), with


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increasing cycle number the current density of the disk increases, but the current density of ring decreases as well, resulting in decreasing selectivity toward H2O2. After this CV-based synthesis, the ORR performance of the in situ-deposited Pd electrocatalysts from each of these precursor concentrations were measured in pure O2-saturated 0.1 M HClO4. As shown in Figure 3b,c, with increasing PdCl2 concentration, the measured H2O2 selectivity decreases, while the disk current density derived from the four-electron reduction of O2 to H2O increases.

Figure 3. (a) Selected RRDE cyclic voltammograms during continuous in situ-deposition of Pd on GC in O2-saturated 5 μM PdCl2/0.1 M HClO4. 10 mV s-1; 1600 rpm. (b) RRDE cyclic voltammograms showing ORR performance in O2-saturated pure 0.1 M HClO4 for five deposition PdCl2 concentrations. 10 mV s-1; 1600 rpm. (c) Calculated values of H2O2 selectivity from data in (b). (d) H2O2 partial current density for five deposition PdCl2 concentrations at 0.45 V vs. RHE (red) and 0.1 V vs. RHE (blue). (e) Semi-log plot of potential-dependent H2O2 kinetic current density for five deposition PdCl2 concentrations as well as for Au and AuPd nanoparticles for comparison. (f) Corresponding cyclic voltammograms in N2-saturated 0.1 M HClO4. 200 mV s-1.


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The total ORR rate (disk current) includes contributions from both two-electron reductions to H2O2 and four-electron reductions to H2O. A useful measure of the direct H2O2 production rate is the partial current density for H2O2, which is the product of the total current density and the H2O2 selectivity. The H2O2 current density was calculated for all PdCl2 concentrations studied; Figure 3d provides this measure of H2O2 production rate, shown for 0.45 V vs. RHE (0.23 V overpotential) and 0.1 V vs. RHE (0.58 V overpotential). These plots indicate that the highest measured values of H2O2 current density are 1.9 mA cm-2 for 0.45 V vs. RHE and 2.6 mA cm-2 for 0.1 V vs. RHE, which correspond respectively to in situ-deposited Pd5.0μM and Pd1.0μM. The kinetic current density, derived from the Koutecky-Levich (K-L) equation (see Supporting Information) provides a quantifiable measure of the catalytic rate in the potential regime of kinetic control and in the absence of transport effects, and therefore facilitates comparison to catalysts in prior studies in the literature.4 Figure 3e provides a semi-log plot of the kinetic partial (H2O2) current density as a function of potential for five deposition PdCl2 concentrations as well as for Au and AuPd nanoparticles for comparison. In acidic conditions, Au has been established to be associated with a high overpotential for ORR and high H2O2 selectivity, and AuPd is frequently studied for its efficacy as a H2O2 production electrocatalyst.15,16 The kinetic H2O2 current density for in situ-deposited Pd derived from a range of PdCl2 concentrations is significantly higher than those of these two reference materials. In fact, the kinetic H2O2 current density of Pd1μM is equivalent or greater than that of reported PtHg nanoparticles.4,19,20 This is remarkable, considering that the active sites are comprised only of Pd. The use of only Pd presents practical advantages over the use of expensive and toxic metals. CV of as-deposited samples performed in N2-saturated 0.1 M HClO4 facilitates further examination of the deposited Pd species at various precursor concentrations, because metallic Pd


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is associated with a well-studied characteristic CV lineshape exhibiting reversible surface Pd oxidation and reduction waves.14 Figure 3f shows CVs recorded in N2-saturated pure HClO4 solution for electrodes deposited in situ in O2-saturated PdCl2/HClO4 solutions. The magnitudes of the reversible waves for both reduction and oxidation of Pd increase with increasing concentration of PdCl2, indicating deposition of more Pd and larger nanoparticles.21 Data resulting from measurement of electrochemically active surface area (ECSA) of in situ-deposited Pd samples (Table S2) are in agreement with the CV results, and the measured H2O2 current was normalized by ECSA (Figure S17). The quantity of Pd deposited therefore correlates negatively with H2O2 selectivity.

2.4 The influence of electrode potential during in situ deposition. To develop a deeper understanding of the influence of electrode potential on the ORR performance of in situ-deposited Pd, a series of potentiostatic experiments were performed to synthesize the catalysts and simultaneously measure the evolving H2O2 production rate during synthesis. In these experiments, the disk (blank GC electrode) was held in O2-saturated 5.0 μM PdCl2/0.1 M HClO4 with potentiostatic control at 0.1, 0.45, and 0.7 V vs. RHE, respectively, and the ring potential was held at 1.28 V vs. RHE for detection of H2O2 (again, all ring current was confirmed to correspond only to oxidation of H2O2 produced). As shown in Figure 4a, when the disk was held at 0.1 V vs. RHE (high overpotential for both O2 reduction and Pd2+ reduction) both the disk and ring current densities increased rapidly within the first 20 seconds, which is attributed to the rapid electrodeposition of the Pd species and increasing O2 reduction rate and H2O2 production rate. After this initial period, the disk current density increased gradually, while the ring current density decreased. Since in these conditions the quantity of Pd deposited is increasing in time, these results provide additional evidence that increasing quantity or spatial density of Pd correlates with


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decreasing H2O2 selectivity. In contrast, when the disk potential was held at 0.45 V vs. RHE (Figure 4c), both the ring and disk current densities increased gradually over the 600 second period. When the disk was held at 0.7 V vs. RHE (Figure 4e), only a very small current density was recorded. After each of these potentiostatic experiments for synthesis, RRDE voltammograms were recorded in O2-saturated pure 0.1 M HClO4 to evaluate the associated ORR activities and H2O2 selectivities (Figures 4b,d,f). It was found that with increasingly anodic deposition potential, the total disk current density decreased and H2O2 selectivity increased. The Pd nanoparticles prepared at more anodic potentials preferentially execute the two-electron reduction of O2, but an optimum intermediate deposition potential exists for maximum H2O2 production rate.


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Figure 4. Potentiostatic in situ deposition of Pd on GC at various potentials. (a) 0.1 V vs. RHE, (c) 0.45 V vs. RHE, and (e) 0.7 V vs. RHE; inset: magnification of disk current. O2-saturated 5.0 μM PdCl2/0.1 M HClO4; 1600 rpm. (b, d, f) RRDE ORR results in O2-saturated pure 0.1 M HClO4


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after corresponding depositions in (a, c, e). 10 mV s-1. 1600 rpm. Ring potential held at 1.28 V vs. RHE.

2.5 Mechanistic studies. A series of additional studies were performed to elucidate further mechanistic details related to the electrochemical behavior of the in situ-deposited Pd.

2.5.1 Stability of in situ-deposited Pd electrocatalysts. To examine the electrochemical stability of in situ-deposited Pd during ORR, a series of chronoamperometry experiments were performed. It was found that the stability of the electrode was highly dependent on the catalyst support. In one set of experiments, after synthesis the time dependence of the ORR rate was measured in O2saturated pure 0.1 M HClO4. For Pd deposited on GC, the disk and ring current measurements indicated that the ORR rate decreased considerably within 1 hour (Figure S18a). This decline in current density is attributed to desorption of the Pd nanoparticles, characterized by STEM, because the carbon support cannot effectively stabilize them.22-25 The desorption of this class of in situdeposited Co-based,24 Cu-based23 and Ni-Fe-based22 electrocatalysts have been proposed for oxygen evolution in reported literatures. This finding contrasts with the stability observed when Pd was deposited onto ATO nanoparticle supports deposited on the GC electrode. As shown in Figure S18b, the stability of catalyst is improved considerably, which suggests that ATO provides a more stable support for Pd. Consistent with other studies and expectations, this experiment implies that the strength of the chemical interaction of Pd with the support material is critical for ORR stability.25

The ORR stability of the catalyst on carbon supports was further investigated in the presence of the dissolved Pd precursor. The time dependence of the ORR rate of Pd5.0μM/GC held at 0.45 V vs. RHE in O2-saturated 5.0 μM PdCl2/0.1 M HClO4 solution was measured. Figure S19a shows that


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in the presence of PdCl2 both the disk and ring currents of the Pd5.0μM/GC remained almost unchanged over 17 hours. At this potential, a diffusion-limited condition was reached and the current density of the disk was 1.98 mA cm-2. After the current decreased over 24 hours, the Pd5.0μM/GC electrode was withdrawn from the solution and placed into a new O2-saturated 5.0 μM PdCl2/0.1 M HClO4 solution, and again a potentiostatic (0.45 V vs. RHE) stability test was executed for another 24 hours. As shown in Figure S19b, both the disk and ring current densities recovered in the new 5.0 μM PdCl2/0.1 M HClO4 solution. These experiments provide strong evidence that when Pd is not mechanically stable on a conductive support, a dynamic equilibrium exists, whereby Pd nanoparticles desorb and new Pd is deposited in the presence of the precursor.

2.5.2

Kinetics of ORR at in situ-deposited Pd. Additional voltammetry experiments and data

analysis were performed to elucidate mechanistic details of oxygen reduction at the in situdeposited catalysts. The electro-kinetic Tafel equation26-28 and the associated Tafel slopes are often used in microkinetic analyses to determine the rate-determining step of a multi-electron electrochemical process.29 Figure S20 provides a Tafel plot of ORR voltammetry data for in situdeposited Pd1.0µM as well as that for Pt (the prototypical four-electron O2 reduction catalyst) for comparison and to benchmark the methodology. The kinetics of ORR at Pt have been extensively characterized,26 and the measured Tafel slope of 75 mV dec-1 is within the variance reported in prior studies, including for nanoparticulate Pt,30 which report a range of 50-80 mV dec-1 at low overpotentials. In HClO4, this slope indicates that the rate-determining step at low-overpotentials for Pt is the second electron transfer step (Equation 4; resulting in the formation of *O).26 In contrast, at analogous low overpotentials, in situ-deposited Pd1.0µM is associated with a Tafel slope of 114 mV dec-1. Kinetic analyses of the ORR mechanism that consider all elementary steps26 report that a Tafel slope near 120 mV dec-1 indicates that the rate-determining step at low-


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overpotentials is the first electron transfer step (Equation 7; resulting in the formation of *OOH); this rate-determining ORR step is therefore assigned for the in situ-deposited Pd electrocatalyst.

As described above, the ATO support was found to stabilize the in situ-deposited Pd during ORR in pure 0.1 M HClO4 solution. Therefore, the ORR kinetics of the catalysts on ATO supports was further analyzed. Figures 5a,b show ORR RRDE voltammetry data and H2O2 selectivity for in situ-deposited Pd2.8μM/ATO recorded in O2-saturated pure 0.1 M HClO4. The maximum disk current density over the potential range examined for Pd2.8μM/ATO (2.3 mA cm-2) is smaller than that for Pd2.8μM/GC (3.9 mA cm-2). However, the H2O2 selectivities are similar for both samples (60%-80%). Figure 5c provides rotation rate-dependent voltammetry of Pd2.8μM/ATO. The current density increases monotonically with increasingly cathodic potential; in these conditions the ORR does not reach a plateau current density. A conservative assignment therefore is that in the potential range 0.05-0.20 V vs. RHE the reaction is under mixed kinetic and mass transfer control. The rotation rate-dependent polarization curves were used to calculate the corresponding electron transfer number (n) through the K-L equation (Figure 5d). The electron transfer number was calculated to be approximately 3 at four different potentials. Given the H2O2 selectivity of this catalyst (Figure 5b), this electron transfer number is numerically consistent with an average value accounting for the proportion of current associated with approximately 70% two-electron reduction and 30% four-electron reduction.


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Figure 5. Electrochemical analysis of in situ-deposited Pd2.8μM/ATO in O2-saturated pure 0.1M HClO4. (a) ORR RRDE linear sweep voltammogram. 10 mV s-1. 1600 rpm. (b) H2O2 selectivity. (c) Rotation rate-dependent voltammograms. 5 mV s-1. (d) Koutecky-Levich plots for Pd2.8μM/ATO at various potentials, with calculated electron transfer numbers.

2.5.3 Extension to other systems toward generalization of findings. This study has thus far demonstrated that Pd ORR electrocatalysts with high selectivity toward H2O2 result from in situ electrodeposition in O2-saturated 0.1 M HClO4 solution with low PdCl2 precursor concentrations. In this section, alternative relevant precursors, electrolytes, and metals are considered, as a first attempt to generalize findings. To assess whether the chloride and perchlorate ions were crucial


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aspects of this chemistry, in situ deposition was executed with PdSO4 as a precursor and H2SO4 as the acidic supporting electrolyte. Figures 6a,b show ORR RRDE voltammetry results in O2saturated pure 0.1 M H2SO4 for Pd1.0μM deposited in O2-saturated 1.0 μM PdSO4/0.1 M H2SO4, contrasted with analogous results from use of PdCl2 and HClO4 as described in previous sections. The results indicate that in situ Pd deposition in the presence of only the sulfate anion also yields highly H2O2-selective ORR electrocatalysts over the entire potential range examined, beginning with current onset. It is evident from the data that the sulfate anion increases the cathodic overpotential required for Pd deposition and for associated O2 reduction (which requires prior or concurrent Pd deposition). The cathodic shift in onset potential in the presence of the sulfate anion is readily explained by the well-known phenomenon of sulfate adsorption on electrode surfaces.31

Given the observed efficacy of in situ deposition for synthesizing H2O2-selective Pd electrocatalysts, further experiments were conducted to examine whether Pt, the prototypical fourelectron ORR electrocatalyst, could similarly be engineered to have properties promoting the twoelectron reduction of O2. To this end, the CV-based in situ deposition procedure was applied for synthesis of Pt in O2-saturated 0.1 M HClO4 containing 0.3 or 2.8 μM K2PtCl6. The RRDE voltammetry and H2O2 selectivity results in Figures 6c,d indicate that Pt0.3μM is indeed associated with a much higher H2O2 selectivity than bulk Pt, as well as for Pt deposited in situ from the higher concentration (2.8 μM) precursor solution. Pt0.3μM is associated with H2O2 selectivity ranging from 45% to 90% over the potential range examined. Expectedly and as is usually observed for twoelectron O2 reduction catalysts, the total current density is lower for Pt0.3μM than for Pt2.8μM. The selectivity and total current densities for these two samples are also consistent with observations made above regarding the influence of the deposited quantity of metal (correlated with particle size) on H2O2 production rates. Although preliminary, these results verify that in situ deposition


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of platinum group metals, known for their fast kinetics and high ORR selectivity for H2O, can be engineered to possess tunable selectivity.

Figure 6. (a) ORR RRDE linear sweep voltammograms in O2-saturated pure 0.1M HClO4 for in situ-deposited Pd1.0μM/GC deposited from PdCl2 and PdSO4 precursors, respectively. 10 mV s-1; 1600 rpm. (b) H2O2 selectivity associated with data in (a). (c) ORR RRDE linear sweep voltammograms in O2-saturated pure 0.1M HClO4 for in situ-deposited Pt0.3μM and Pt2.8μM. 10 mV s-1; 1600 rpm. (d) H2O2 selectivity associated with data in (c).


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2.5.4 Summary of mechanistic insights and material property correlates of high ORR H2O2 selectivity at in situ-deposited metal electrocatalysts. The origins of H2O2 selectivity associated with in situ-deposited Pd can be assessed in the context of the physical and electrochemical properties revealed in this study. Voltammetry and chronoamperometry data indicate that total O2 reduction current correlates positively with precursor concentration in the deposition solution (and quantity of metal deposited), and that H2O2 selectivity correlates negatively with precursor concentration. The maximum H2O2 partial current density, reflective of the rate of H2O2 production, is dependent on the desired (device-specific) operating potential of the cathode. Microscopy analysis of representative electrocatalysts with very dissimilar ORR performance indicates that high H2O2 selectivity correlates with the co-existence of two physical properties: small metal particle size (less than ca. 5 nm diameter) and lack of crystalline order. It is found that the existence of either of these features alone is insufficient to yield highly H2O2-selective electrocatalysts. It has been reported that Pd electrodeposition from solutions with higher Pd precursor concentrations result in crystalline Pd particles,21 which is consistent with results here and the observed deleterious effect of crystallinity on H2O2 selectivity. The electrodeposition of Pd onto supports proceeds via the cathodic reduction of Pd2+ ions (in chloride concentrations on the order of one micromolar, no complexation occurs).32 Important information about the in situ electrodeposition process itself is revealed by features of the deposition RRDE voltammograms. As shown in Figure 3a and Figure S16, during the first cathodic scan of a clean GC electrode in the presence of Pd2+ and O2, the onset potential of first cycle is more cathodic than those of all subsequent cycles. Because ring H2O2 oxidation current commences simultaneously with cathodic current onset, and because H2O2 is the only species that can be oxidized in these conditions, all results are consistent with the fact that the cathodic


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reduction of Pd2+ and cathodic reduction of O2 proceed simultaneously. The two reactions compete: cathodic reduction of O2 consumes electrons and is associated with chemical occupation of surface sites. This reduces the overall rate of cathodic reduction of Pd2+, which prevents the formation and/or aggregation of Pd nanoparticles. It is logical to infer that the involvement of exposed surface Pd in O2 reduction additionally prevents Pd migration and restricts the formation of crystalline order. In order to verify that the existence of competing reduction reactions is an integral feature of the in situ deposition process, control experiments were performed wherein Pd ORR electrocatalysts were deposited by Pd ion reduction in the absence of O2. Figure S21 shows that Pd2.8µM deposited in N2-saturated 2.8 µM PdCl2/0.1 M HClO4 solution yielded electrocatalysts associated with significantly decreased H2O2 selectivity. These results confirm the importance of concurrent O2 reduction during Pd deposition. The binding of the reaction intermediates directly influences the catalytic activity and selectivity. The *OOH or *OH binding energy (ΔG*OOH = ΔG*OH + 3.2, where * indicates a surface-bound species) is a key determining factor for catalyst activity and selectivity. In the simplest interpretation, in a Sabatier volcano plot for this reaction the most active catalyst lies at the peak and is associated with an optimum species binding energy. Catalysts, whose *OOH binding energies are lower, are expected to yield increased H2O2 selectivity, but lower activity.19 Crystalline Pd surfaces bind *OH very strongly, which is the origin of this material’s strong propensity to fully reduce O2 to H2O. It has been reported that *OH binding can be weakened through disordered surface structures due to the existence of a distribution of local coordination environments at surface active sites.33 Based on the observations made in this report and on


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analyses of the related literature, it is possible that the *OOH binding energy for in situ-deposited Pd is lower than that of crystalline Pd and closer to an optimum value. In addition to electronic effects, geometric effects can also play a role in controlling H2O2 selectivity. Isolated active sites favoring H2O2 production can be realized through alloying active atoms with inert atoms,19,34 covering active atoms with amorphous carbon,35 and anchoring isolated atoms on supports.25,36 Unlike the closely-packed active sites in crystalline metals, more comfortable packing of ‘soft’ atoms exists in amorphous metals due to natural consequence of strain relaxation,37 which perhaps results in a decreased probability for the existence of two adjacent hard Pd atoms. Additionally, disordered Pd most likely contains hollow sites that interact weakly with oxygen, and therefore active sites can be isolated by oxygens. Based on this reasoning, O2 dissociation is would be expected to be suppressed at in situ-deposited Pd (sub-5 nm) surfaces, due to the reduction of the quantity of adjacent hard Pd atoms. Mechanistic investigations of the observed high H2O2 selectivity associated with nanoscale amorphous Pd particles is ongoing, and definitive explanations are expected to require a firstprinciples approach and computational efforts, which are beyond the scope of this first report. 3. CONCLUSION This study has demonstrated that Pd, which, like other platinum group metals, is generally understood to strictly catalyze the four-electron reduction of O2 to H2O, can be made highly selective toward two-electron reduction to H2O2 when it is deposited in situ during O2 reduction. When deposited from solutions with the lowest precursor concentration examined, selectivities above 95% were observed in the relevant potential range. In situ-deposited electrodes were measured to generate in standard acidic conditions the second-highest partial kinetic current density for H2O2 production reported in the known literature. The magnitude of the potential-


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dependent partial (H2O2) current density was found to be controlled by the precursor concentration and deposition conditions, which suggests that the reported in situ deposition technique is flexible and can potentially be used to tune production rates based on the aqueous H2O2 concentration required for the end-use application. Comprehensive physical characterization indicated that high H2O2 selectivity is obtained only when two physical properties co-exist: small metal particle size (less than ca. 5 nm diameter) and lack of crystalline order. Results show that greater particle crystallinity results in higher selectivity toward the four-electron reduction of H2O2 to H2O (the pathway traditionally observed for platinum group metals). This study has also reported efforts taken to generalize the in situ deposition technique. The H2O2 yield enhancement was observed for all inert, conductive supports studied – glassy carbon, commercial activated carbon, graphene, and antimony-doped tin oxide. It was also demonstrated that even Pt, the prototypical four-electron O2 reduction catalyst in fuel cell cathodes, can be engineered to be highly selective to H2O2 when synthesized in situ in relevant conditions. Taken together, all electrochemical and physical characterization reported suggests that in situ synthesis is a promising approach to engineer O2 reduction electrocatalysts with tunable product selectivity and activity.

ASSOCIATED CONTENT Supporting Information. Experimental section and additional results as noted in the text (PDF). AUTHOR INFORMATION Corresponding Author *E-mail for C.X.K.: [email protected]. Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT C.X.K. acknowledges support from the University of California, Davis, CA. Y.L.W. acknowledges support from the China Scholarship Council. The authors acknowledge Simon Bare and Adam Hoffman of the Stanford Synchrotron Light Source and Co-ACCESS for providing Xray absorption spectroscopy measurements and discussion. The authors acknowledge J. Anibal Boscoboinik of the Center for Functional Nanomaterials at Brookhaven National Laboratory for additional support and discussion. The authors acknowledge Austin Cole and Peter Green of the UC Davis Interdisciplinary Center for providing Plasma Mass Spectrometry (ICP-MS) measurements. REFERENCES 1. U.S. Energy Information Administration. Annual Energy Outlook 2019 with Projections to 2050: Washington, DC, 2019; www.eia.gov, accessed April 1, 2019). 2. Exxon Mobil Cooperation. The Outlook for Energy: A View to 2040: Irving, TX, 2018; (exxonmobil.com/energyoutlook, accessed April 1, 2019). 3. Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science 2009, 323, 760-763. 4. Yang, S.; Verdaguer-Casadevall, A.; Arnarson, L.; Silvioli, L.; Čolić, V.; Frydendal, R.; Rossmeisl, J.; Chorkendorff, I.; Stephens, I. E. L. Toward the Decentralized Electrochemical Production of H2O2: A Focus on the Catalysis. ACS Catal. 2018, 8, 4064-4081. 5. Jiang, Y.; Ni, P.; Chen, C.; Lu, Y.; Yang, P.; Kong, B.; Fisher, A.; Wang, X. Selective Electrochemical H2O2 Production through Two-Electron Oxygen Electrochemistry. Adv. Energy Mater. 2018, 8, 1801909. 6. Ciriminna, R.; Albanese, L.; Meneguzzo, F.; Pagliaro, M. Hydrogen Peroxide: A Key Chemical for Today’s Sustainable Development. ChemSusChem 2016, 9, 3374-3381. 7. Chen, Z.; Chen, S.; Siahrostami, S.; Chakthranont, P.; Hahn, C.; Nordlund, D.; Dimosthenis,


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S.; Nørskov, J. K.; Bao, Z.; Jaramillo, T. F. Development of A Reactor with Carbon Catalysts for Modular-Scale, Low-Cost Electrochemical Generation of H2O2. React. Chem. Eng. 2017, 2, 239-245. 8. Freakley, S. J.; He, Q.; Harrhy, J. H.; Lu, L.; Crole, D. A.; Morgan, D. J.; Ntainjua, E. N.; Edwards, J. K.; Carley, A. F.; Borisevich, A. Y.; Kiely, C. J.; Hutchings, G. J. Palladium-Tin Catalysts for the Direct Synthesis of H2O2 with High Selectivity. Science 2016, 351, 965-968. 9. Samanta, C. Direct Synthesis of Hydrogen Peroxide from Hydrogen and Oxygen: An Overview of Recent Developments in the Process. Appl. Catal. A: Gen. 2008, 350, 133-149. 10. Campos-Martin, J. M.; Blanco-Brieva, G.; Fierro, J. L. G. Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone Process. Angew. Chem. Int. Ed. 2006, 45, 6962-6984. 11. Viswanathan, V.; Hansen, H. A.; Rossmeisl, J.; Nørskov, J. K. Unifying the 2e– and 4e– Reduction of Oxygen on Metal Surfaces. J. Phys. Chem. Lett. 2012, 3, 2948-2951. 12. Kulkarni, A.; Siahrostami, S.; Patel, A.; Nørskov, J. K. Understanding Catalytic Activity Trends in the Oxygen Reduction Reaction. Chem. Rev. 2018, 118, 2302-2312. 13. Rankin, R. B.; Greeley, J. Trends in Selective Hydrogen Peroxide Production on Transition Metal Surfaces from First Principles. ACS Catal. 2012, 2, 2664-2672. 14. Pizzutilo, E.; Kasian, O.; Choi, C. H.; Cherevko, S.; Hutchings, G. J.; Mayrhofer, K. J. J.; Freakley, S. J. Electrocatalytic Synthesis of Hydrogen Peroxide on Au-Pd Nanoparticles: From Fundamentals to Continuous Production. Chem. Phys. Lett. 2017, 683, 436-442. 15. Pizzutilo, E.; Freakley, S. J.; Cherevko, S.; Venkatesan, S.; Hutchings, G. J.; Liebscher, C. H.; Dehm, G.; Mayrhofer, K. J. J. Gold-Palladium Bimetallic Catalyst Stability: Consequences for Hydrogen Peroxide Selectivity. ACS Catal. 2017, 7, 5699-5705. 16. Rahul, R.; Singh, R. K.; Bera, B.; Devivaraprasad, R.; Neergat, M. The Role of Surface Oxygenated-Species and Adsorbed Hydrogen in the Oxygen Reduction Reaction (ORR) Mechanism and Product Selectivity on Pd-Based Catalysts in Acid Media. Phys. Chem. Chem. Phys. 2015, 17, 15146-15155. 17. Okumura, K.; Yoshimoto, R.; Uruga, T.; Tanida, H.; Kato, K.; Yokota, S.; Niwa, M. EnergyDispersive XAFS Studies on the Spontaneous Dispersion of PdO and the Formation of Stable Pd Clusters in Zeolites. J. Phys. Chem. B 2004, 108, 6250-6255. 18. Kou, Y.; Zhang, B.; Niu, J.; Li, S.; Wang, H.; Tanaka, T.; Yoshida, S. Amorphous Features of Working Catalysts: XAFS and XPS Characterization of Mn/Na2WO4/SiO2 as Used for the


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Oxidative Coupling of Methane. J. Catal. 1998, 173, 399-408. 19. Siahrostami, S.; Verdaguer-Casadevall, A.; Karamad, M.; Deiana, D.; Malacrida, P.; Wickman, B.; Escudero-Escribano, M.; Paoli, E. A.; Frydendal, R.; Hansen, T. W.; Chorkendorff, I.; Stephens, I. E. L.; Rossmeisl, J. Enabling Direct H2O2 Production through Rational Electrocatalyst Design. Nat. Mater. 2013, 12, 1137-1143. 20. Verdaguer-Casadevall, A.; Deiana, D.; Karamad, M.; Siahrostami, S.; Malacrida, P.; Hansen, T. W.; Rossmeisl, J.; Chorkendorff, I.; Stephens, I. E. L. Trends in the Electrochemical Synthesis of H2O2: Enhancing Activity and Selectivity by Electrocatalytic Site Engineering. Nano Lett. 2014, 14, 1603-1608. 21. Vélez, C. A.; Corchado-García, J.; Rojas-Pérez, A.; Serrano-Alejandro, E. J.; Santos-Homs, C.; Soto-Pérez, J. J.; Cabrera, C. R. Manufacture of Pd/Carbon Vulcan XC-72R Nanoflakes Catalysts for Ethanol Oxidation Reaction in Alkaline Media by RoDSE Method. J. Electrochem. Soc. 2017, 164, D1015-D1021. 22. Wang, J.; Ji, L.; Chen, Z. In Situ Rapid Formation of a Nickel-Iron-Based Electrocatalyst for Water Oxidation. ACS Catal. 2016, 6, 6987-6992. 23. Du, J.; Chen, Z.; Ye, S.; Wiley, B. J.; Meyer, T. J. Copper as a Robust and Transparent Electrocatalyst for Water Oxidation. Angew. Chem. Int. Ed. 2015, 54, 2073-2078. 24. Kanan, M. W.; Nocera, D. G. In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072-1075. 25. Choi, C. H.; Kim, M.; Kwon, H. C.; Cho, S. J.; Yun, S.; Kim, H.-T.; Mayrhofer, K. J. J.; Kim, H.; Choi, M. Tuning Selectivity of Electrochemical Reactions by Atomically Dispersed Platinum Catalyst. Nat. Commun. 2016, 7, 10922. 26. Holewinski, A.; Linic, S. Elementary Mechanisms in Electrocatalysis: Revisiting the ORR Tafel Slope. J. Electrochem. Soc. 2012, 159, H864-H870. 27. Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780786. 28. Wang , S.; Iyyamperumal , E.; Roy, A.; Xue, Y.; Yu, D.; Dai, L. Vertically Aligned BCN Nanotubes as Efficient Metal-Free Electrocatalysts for the Oxygen Reduction Reaction: A Synergetic Effect by Co-Doping with Boron and Nitrogen. Angew. Chem. Int. Ed. 2011, 50, 11756-11760.


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29. Shinagawa, T.; Garcia-Esparza, A. T.; Takanabe, K. Insight on Tafel Slopes from A Microkinetic Analysis of Aqueous Electrocatalysis for Energy Conversion. Sci. Rep. 2015, 5, 13801. 30. Takasu, Y.; Ohashi, N.; Zhang, X. G.; Murakami, Y.; Minagawa, H.; Sato, S.; Yahikozawa, K. Size Effects of Platinum Particles on the Electroreduction of Oxygen. Electrochim. Acta 1996, 41, 2595-2600. 31. Kunimatsu, K.; Samant, M. G.; Seki, H.; Philpott, M. R. A Study of HSO4− and SO42− CoAdsorption on A Platinum Electrode in Sulfuric Acid by In-Situ FT-IR Reflection Absorption Spectroscopy. J. Electroanal. Chem. 1988, 243, 203-208. 32. Colombo, C.; Oates, C. J.; Monhemius, A. J.; Plant, J. A. Complexation of Platinum, Palladium and Rhodium with Inorganic Ligands in the Environment. Geochem.: Explor. Env. A 2008, 8, 91-101. 33. Yan, B.; Krishnamurthy, D.; Hendon, C. H.; Deshpande, S.; Surendranath, Y.; Viswanathan, V. Surface Restructuring of Nickel Sulfide Generates Optimally Coordinated Active Sites for Oxygen Reduction Catalysis, Joule 2017, 1, 600-612. 34. Jirkovský, J. S.; Panas, I.; Ahlberg, E.; Halasa, M.; Romani, S.; Schiffrin, D. J. Single Atom Hot-Spots at Au-Pd Nanoalloys for Electrocatalytic H2O2 Production. J. Am. Chem. Soc. 2011, 133, 19432-19441. 35. Choi, C. H.; Kwon, H. C.; 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. 36. Yang, S.; Kim, J.; Tak, Y. J.; 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. 37. Sheng, H. W.; Luo, W. K.; Alamgir, F. M.; Bai, J. M.; Ma, E. Atomic Packing and Short-toMedium-Range Order in Metallic Glasses. Nature 2006, 439, 419-425.


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In Situ Deposition of Pd during Oxygen Reduction Yields Highly

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