PdâAg Alloy Electrocatalysts for CO2 Reduction: Composition Tuning

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Pd-Ag Alloy Electrocatalysts for CO2 Reduction: Composition Tuning to Break Scaling Relationship Jiachang Zeng, Wenbiao Zhang, Yang Yang, Dan Li, Xiang Yu, and Qingsheng Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11729 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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ACS Applied Materials & Interfaces

Pd-Ag Alloy Electrocatalysts for CO2 Reduction: Composition Tuning to Break Scaling Relationship Jiachang Zeng,† Wenbiao Zhang,† Yang Yang,† Dan Li,† Xiang Yu,†,‡ and Qingsheng Gao*,†

†Department

of Chemistry, College of Chemistry and Materials Science, Jinan

University, Guangzhou 510632, P. R. China

‡Analytic

and Testing Centre, Jinan University, Guangzhou 510632, P.R. China

KEYWORDS: electrocatalysts, alloy, breaking scaling relationship, CO2 reduction reaction, electronic engineering

ABSTRACT: Constructing solid-solution-alloy electrocatalysts with tunable surface electronic configuration is the key to optimize intermediate bindings and thereby to promote the activity and selectivity of CO2 reduction reaction (CO2RR). Herein, Pd1-xAgx alloy electrocatalysts are investigated as a platform to uncover the electronic effects on the CO2RR. The optimal Pd0.75Ag0.25/C affords a superior CO Faradaic efficiency of 95.3% at -0.6 V (vs. RHE) in 0.5 M

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KHCO3, performing at the high-level of recently reported electrocatalysts. Experimental and theoretical analysis further evidence that varying the composition of Pd1-xAgx alloys can effectively alter the electronic configurations, and consequently break the inherent scaling relationship of binding energy of different intermediates (*COOH and *CO). Among Pd1-xAgx, the Pd0.75Ag0.25 gains the obviously weakened *CO and *H bindings, but the well-retained binding with *COOH, contributing to the facilitated kinetics toward CO product. Elucidating a feasible way to break the scaling relationship and further uncover the underlying mechanism, this work will inspire new design strategies toward active and selective electrocatalysts. INTRODUCTION The increasing consumption of fossil fuels has raised the atmospheric CO2 level, posing great challenges to global climate and environment.1 In the meantime, CO2 is also a potential carbon resource. Thereby its effective utilization is vital to balance energy supplement and environmental issues.2-4 Electrochemical CO2 reduction reaction (CO2RR) powered by renewable electricity sources is a promising route, thanks to the distinct strengths like mild operation conditions, no need for H2 feeding and near-neutral pH environment.5-8 Typically, electrocatalysts are required to address the huge kinetic barriers and rationalize the reaction pathways toward value-added chemicals, e.g., CO, formate, hydrocarbon, oxygenated compounds, etc. Metallic catalysts have been extensively investigated. For instance, Sn, Pb and Bi can effectively reduce CO2 to formic acid,9-13 and noble metals (e.g., Pd, Ag and Au) produce CO,14-16 associated with their different electronic configurations and intermediate bindings. And Cu electrodes, which have a moderate binding with *CO (* denotes a surface active-site), are efficient for the reduction to hydrocarbon.17,18 It should be noticed that, among various reduction


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in aqueous electrolytes, the competitive hydrogen evolution reaction (HER) should be inhibited to achieve a satisfactory Faradaic efficiency (FE) for CO2 conversion.19-21 In general, the catalytic activity and selectivity of CO2RR depend on the binding energy and chemisorption configuration of reaction intermediates.22 An inherent challenge is that the intermediate bindings follow conventional scaling relationships due to the similar surfaceadsorbate bonds.23,24 For example, the bindings of C-bound intermediates (e.g., *COOH, *CO, and *COH) usually follow the similar trend when the d-band center of catalysts shifts,22 resulting in uncoordinated bindings and poor kinetics. In the case of Pd, the strong Pd-C bond can effectively reduce the positive Gibbs free energy for generating *COOH (G*COOH), however its strong binding with *CO leads to a thermodynamic limitation toward CO product.6 As a result, pure Pd electrodes usually suffer from a low CO FE of 30 ~ 60%.

25-27

Ideal catalysts are

expected to break the scaling relationship, showing a strong binding with *COOH, but weak with *CO.22,28-30 Introducing a secondary metal to form uniform alloys is feasible, because it can provide fine-tuned electronic/surface configurations.27,31-35 Regarding the relatively weaker CO binding due to insufficient d vacancies, Au and Ag are the candidates to engineer Pd-based alloys toward well-coordinated *COOH and *CO.23 Meanwhile, solid solution alloys of Pd-Au and Pd-Ag can be varied within a large composition range, enabling the easy electronic/surface modulation.36 As been recently highlighted, the electron re-distribution in Pd-Au alloy weakened the Pd-C bond and balance *COOH and *CO bindings, accomplishing the high efficiency and mass activity toward CO.37 In comparison with high-cost Au, Ag is more cost-efficient to regulate the electrocatalysis on Pd-based alloys in an economic manner. Although Pd-Ag has been extensively studied in alcohol electroxidation,38-40 it has been rarely investigated for CO2RR. Recently, AgPd nanodendrite-modified Au nanoprisms were used to electrocatalyze



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CO2 to formate in aqueous electrolytes (FE < 50%) and to CO in organic electrolytes (FE: 85 ~ 87%),41 however such ternary system is complex to understand the working mechanism. It is still unknown that how the tailored electronic configuration of Pd-Ag influences the intermediate bindings and the overall reaction. Herein, a series of Pd1-xAgx electrocatalysts are investigated as a platform to uncover the electronic effects on electrocatalytic CO2RR. The optimal Pd0.75Ag0.25/C affords a high CO FE of 95.3% at -0.6 V (vs. RHE) in 0.5 M KHCO3, performing at the high-level of recent reports. The well-retained performance in long-term tests ensures its satisfactory stability. Experimental and theoretical investigations further evidence that varying the composition of Pd1-xAgx can effectively alter the electronic configurations, and thereby break the scaling relationship of binding energy of *COOH and *CO intermediates. Owing to the rich d electrons of Ag and its strong interactions with Pd, the Pd1-xAgx with a low Ag/Pd ratio (e.g., Pd0.75Ag0.25) can obviously weaken *CO binding, but still retain the low G*COOH, enabling the facilitated kinetics toward CO product. As Ag is excessive in alloys, the *COOH binding is obviously weakened, and its formation turns to a thermodynamic limitation. Besides, the weakened *H binding after introducing Ag disfavors the competitive HER. The above factors, as a result of breaking scaling relationship in Pd0.75Ag0.25, is responsible for the efficient CO2RR. RESULTS AND DISCUSSION A series of Pd1-xAgx alloy nanoparticles supported on activated carbon (Pd1-xAgx/C) was obtained via reducing Pd(NO3)2 and AgNO3 by NaBH4, and varying Pd/Ag feeding ratio results in different composition that is determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Table S1 in Supporting Information). They are accordingly denoted as Pd0.75Ag0.25/C, Pd0.50Ag0.50/C and Pd0.25Ag0.75/C. The total metal (Pd and Ag) loading in the


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above samples are kept at a similar level (40.0 ~ 48.0 wt.%). Figures 1a and 1b display the X-ray diffraction (XRD) patterns of the Pd1-xAgx/C, which have been already aligned with Si (JCPDS No. 27-1402) standard to evaluate the possible peak shifts. The Pd/C and Ag/C clearly identify the characteristic patterns of a fcc structure (Pd: JCPDS No. 46-1043, Ag: JCPDS No. 04-0783), in which the peaks of (111), (200), (220), (311) and (222) are observed (Figure 1a). In comparison, the Pd1-xAgx/C presents the diffraction peaks locating between those of Pd and Ag, indicating the formation of Pd1-xAgx alloys.42 Figure 1b further shows that the patterns of Pd1xAgx/C

gradually shift to low 2 along with increasing Ag, which could be attributed to the

enlarged lattice spacing after replacing Pd with larger Ag atoms.39,43,44

Figure 1. (a) XRD patterns and (b) partially magnified ones of Pd1-xAgx/C with different compositions. The chemical states of Pd/C, Ag/C, and Pd1-xAgx/C were studied by X-ray photoelectron spectroscopy (XPS). As shown in Figure 2a, the Pd 3d profiles can be deconvolved to two couples that are ascribed to metallic Pd0 and Pd2+ species, respectively. The former presents the electronic properties in alloys, while the latter is associated with the surface oxidation of metal as exposed to air. Thereby, the peaks of Pd2+ 3d3/2 (343.3 eV) and 3d5/2 (338.2 eV) are fixed in



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further analysis. The blue-shift of Pd0 binding energy becomes obvious with increasing Ag in the Pd1-xAgx/C. Accordingly, the Ag0 couple in the Pd1-xAgx/C presents a gradual red-shift to lower binding energy with increasing Ag (Figure 2b). As previously evidenced,7,27 the d-band overlapping of metal atoms in alloys enables electronic interactions and thereby electron redistribution in Pd1-xAgx, which should be accounting for the variation in XPS.

Figure 2. XPS spectra of (a) Pd 3d and (b) Ag 3d in Pd1-xAgx/C. The nanostructures of electrocatalysts were investigated by transition electronic microscopy (TEM). The overall observation shows that the metal or alloy nanoparticles are homogeneously dispersed on activated carbon supports (Figure S1 in Supporting Information). And the corresponding size-distribution histograms identify the average diameters of 4.0 ± 0.5 nm in Pd/C, Pd0.75Ag0.25/C, Pd0.50Ag0.50/C and Pd0.25Ag0.75/C. A slightly larger size (5.5 ± 0.5 nm) is observed in Ag/C, which agrees with the relatively shaper diffraction peaks of Ag in the XRD pattern (Figure 1a). As a modeling sample, the Pd0.75Ag0.25/C is taken for further analysis. A typical SEM observation shows dispersive particles in the Pd0.75Ag0.25/C, which should be ascribed to carbon supports (Figure S2 in Supporting Information). Furthermore, the Pd0.75Ag0.25 nanoparticles with uniform dispersion are observed on carbon supports, as displayed in TEM images (Figures 3a and 3b). The visible lattice fringe of 2.27 Å is between the (111) lattice


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spacing of pure Pd (2.25 Å) and Ag (2.36 Å), corresponding to the one of alloy phase.27,45 In addition, the energy dispersive spectroscopy (EDS) confirms the presence of Pd, Ag and C, and the elemental mapping shows their uniform distribution on the samples (Figures 3c and 3d).

Figure 3. (a) TEM, (b) HR-TEM, (c) EDS profile and (d) the corresponding elements mapping of the Pd0.75Ag0.25/C. The CO2RR activities were investigated in a CO2-saturated KHCO3 aqueous solution. Figure S3 in Supporting Information shows the linear sweep voltammetry (LSV) results of the Pd0.75Ag0.25/C in 0.5 M KHCO3 saturated with N2 or CO2. Visibly, the Pd0.75Ag0.25/C with CO2 saturation exhibits a higher current density than the case saturated with N2, indicating the effective CO2 reduction in test. Figure 4a displays the LSV curves for the Pd1-xAgx/C, Pd/C and Ag/C. The applied potential is between 0 and -1.1 V (vs. RHE), and the sweep rate is 10 mV s-1. Obviously, the Pd0.75Ag0.25/C exhibits the highest current density among Pd1-xAgx/C, Pd/C and Ag/C, indicating its outstanding activity. Determined by gas chromatography (GC), CO and H2 are the main gaseous products. And no obvious liquid product is detected by 1H NMR (Figure S4



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in Supporting Information). Figure 4b summarizes the FEs of CO on various catalysts from -0.2 to -1.1V (vs. RHE), which shows a clear dependence on alloy composition. The Pd0.75Ag0.25/C affords an outstanding CO FE of 95.3% at -0.6 V, higher than those of Pd/C, Ag/C, and other Pd1-xAgx/C. This performance is at the high level of recently reported Pd-based electrocatalysts (Table S2 in Supporting Information). The FEs of CO and H2 at -0.6 V associated with alloy compositions are depicted in Figure 4c. The former presents a volcano trend, while the latter is invert-volcano, resulting in the maximum for CO2RR on the Pd0.75Ag0.25/C.

Figure 4. CO2RR performance on a series of Pd1-xAgx/C in CO2-saturated 0.5 M KHCO3. (a) Polarization curves, (b) FEs of CO at various applied potentials, (c) FEs of CO and H2 at -0.6 V vs. RHE), and (d) Tafel plots for CO production. To gain kinetic insights for CO2RR, the Tafel analysis involving current density for CO generation (jCO) at various potentials was conducted. Typically, the Tafel slope of 118 mV dec-1


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suggests that the rate-determining step (RDS) is the generation of *CO2•- or *COOH intermediate by the initial one-electron transfer step, while a Tafel slope of 59 mV dec-1 indicates a fast equilibrium process for CO2 discharging and a subsequent slower chemical reaction as the RDS.3,46,47 As shown in Figure 4d, the slope for the Pd0.75Ag0.25/C is 60.7 mV dec-1, close to that of the Pd/C (65.6 mV dec-1) and Pd0.50Ag0.50/C (67.1 mV dec-1), but obviously lower than that of the Ag/C (109.8 mV dec-1) and Pd0.25Ag0.75/C (90.1 mV dec-1). This implies the step following CO2 protonation is rate-determining on the Pd/C, Pd0.75Ag0.25/C and Pd0.50Ag0.50/C, while the RDS is the formation of *COOH on the Ag/C and Pd0.25Ag0.75/C. Furthermore, the reaction order for CO2RR was further analyzed on the Pd0.75Ag0.25/C with respect to CO2 and HCO3-. As depicted in Figures S5a and S5b in Supporting Information, the slopes of linear fitting for logjCO vs. logPCO2 and logjCO vs. log[HCO3-] are close to zero, indicating an independence on the CO2 partial pressure and HCO3- concentration in the range considered here.48-50 This is consistent with the Tafel analysis. A RDS following CO2 protonation suggested by a low Tafel slop of 60.7 mV dec-1 is usually independent of reactant CO2. According to previous reports,26,51,52 the reaction rate over Pd and Pd-rich Pd1-xAgx alloys is limited by the *CO desorption from metal surface.

Figure 5. Stability test over the Pd0.75Ag0.25/C (-0.6 V vs. RHE). The insets display TEM images before and after electrolysis. (Scale bar: 20 nm)



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The durability experiment over the optimal Pd0.75Ag0.25/C was investigated at -0.6 V (vs. RHE) with long time electrolysis (Figure 5). A total current density around -3.0 mA cm-2 and the CO FE over 90% present negligible decrease over 20 hours. Meanwhile, the Pd0.75Ag0.25/C before and after electrocatalytic test was further investigated by TEM. As shown in the inset of Figure 5, the alloys retain the consistent size distribution, in accordance with the excellent stability of CO2RR performance. Density functional theory (DFT) calculations were performed to investigate the energetics of the key reaction intermediates (e.g., *COOH, *CO and *H) involved in the pathways of CO2RR and HER. The commonly observed (111) facet in TEM investigation was adopted to simulate the binding energy change. The details for modeling are described in the experimental section, and the relevant models are displayed in Figure S6 of Supporting Information. The projected density of states (PDOS) of different Pd1-xAgx is shown in Figure 6a. Visibly, the Pd1xAgx

alloys present a down-shift of the d-band center as the content of Ag increases. According

to d-band theory, a negative shift of d-band center away from the Fermi level (EF) normally leads to weaker binding between intermediates and catalyst surface.53,54 Our calculation for the *COOH, *CO and *H on the Pd1-xAgx (111) identifies the consistent evolution. It’s clear that Pd has strong adsorption of *COOH and *CO. Introducing Ag into Pd brings a drastic attenuation of *CO binding, but quite slight for *COOH (Figure 6b), in the cases with a low Ag content (e.g., Pd0.75Ag0.25). When the Ag further increases, the *COOH obviously turns weak on Pd0.25Ag0.75 and Ag, while the *CO suffer negligible variation. Such difference change in *CO and *COOH bindings along with the d-band center energies of Pd1-xAgx could be ascribed to the coupling via 5σC=O and πCOOH, respectively, which differ in orbital energy and symmetry. In the conventional scaling relationship, the adsorption energy of *CO cannot be optimally tuned without affecting


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*COOH.30 When the *CO is weakened to promote desorption, the *COOH turns so weak that its formation becomes a thermodynamic limit (e.g., the case of Ag). Such incoordination leads to poor kinetics of CO2RR. In sharp contrast, the composition fine-tune of the Pd1-xAgx can retain the strong binding of *COOH, and at the same time earn the obviously weakened *CO binding at a low Ag/Pd ratio (Pd0.75Ag0.25, marked in Figure 6b), as a result of breaking scaling relationship. This coordination between *CO and *COOH is of great importance to boost the kinetics toward CO product. Besides, the gradually weakened *H binding is observed after introducing Ag (Figure 6c), which disfavors the HER kinetics.

Figure 6. (a) Projected density of states (PDOS) of different Pd1-xAgx catalysts. White bar indicates its d band center and Gray dashed line indicates EF. Binding energies of (b) *COOH, *CO and (c) H* on Pd1-xAgx surfaces. PDOS plots of (d) C atom in *COOH, (e) C atom in *CO and (f) H atom in H* with Pd atom in pure Pd, Pd1-xAgx alloys and Ag atom in pure Ag surfaces. ACS Paragon Plus Environment 11


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The varied bindings of intermediates can be further interpreted in view of orbital overlapping that gives insight into the interactions between absorbed intermediates and catalytic sites.55-57 Figures 6d and 6e depict the PDOS of the C and metal atoms in chemisorbed *COOH and *CO on Pd1-xAgx (111), and Figure 6f presents that of H and metal in *H configurations. There are two main orbital overlaps (a, b) between C 2p and metal d orbitals in Figures 6d and 6e. In the case of *COOH, the large overlap between C2p and Pdd in b area suggests the strong interactions between the C and Pd atoms.56,58-60 Such overlap keeps similar at low Ag/Pd ratio (Pd0.75Ag0.25), but it obviously declines with the further increasing Ag (Figure 6d), indicating the weaker binding of *COOH. This is consistent with the trends in *COOH binding energy diagram. By contrast, the overlap between C2p and Pdd in *CO configuration is drastically reduced once Ag is introduced (Figure 6e), agreeing with the attenuation of *CO binding. Such different variations illustrate the broken scaling relationship in Pd1-xAgx. In addition, the continuously shrinking overlap can be observed between H1s and Pdd over Pd1-xAgx surface (Figure 6f). The calculated Gibbs free energy (G) diagrams are depicted in Figure 7a, representing CO2

reduction

to

CO

through

following

elementary

reaction

steps:3,51

CO2(aq.) + H + (aq) + e - + * → * COOH(aq) * COOH(aq) + H + (aq) + e - → * CO(aq) + H2O(l) * CO(aq) → CO(g) + * Where the asterisk (*) represents a vacant catalytically active site. The configurations of CO2, *COOH, *CO and CO over metal surface are shown in the top panel of Figure 7a, taking Pd0.75Ag0.25 as a model. According to the calculated G diagram, the activation of CO2 through the formation of a *COOH intermediate (G*COOH) is the potential-determining step. Although


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the Pd has a negative G*COOH (-0.08 eV), it suffers from a quite large uphill energy barrier to desorb *CO. This agrees with the experimental kinetics analysis that suggests a RDS of *CO desorption. The G*COOH increases to positive values as Ag increases in Pd1-xAgx. In comparison with Pd, the Pd0.75Ag0.25 and Pd0.50Ag0.50 present slightly higher value of G*COOH, but obviously lower energy barriers from *CO to gaseous CO. As a result, the RDS to desorb *CO is successfully overcome, while the fast CO2 protonation is well-retained. In particular, the Pd0.75Ag0.25 deserves the lowest barrier for *CO desorption, consistent with the best CO2RR performance on the Pd0.75Ag0.25/C. When Ag further increases in Pd0.25Ag0.75 and Ag, the G*COOH turns to much positive, suggesting a RDS of initial one-electron transfer step. This is consistent with the variation of Tafel slope. Meanwhile, the G diagrams for the HER were also calculated via a computational hydrogen electrode (CHE) model (Figure 7b). The formation energies of *H (G*H) are distinctly ascending to positive values with the increase of Ag content, indicating the weakened H-binding on alloy surface and the prohibited initial step of HER (Volmer reaction).61,62 Comparing the free energy diagrams of the desired CO2RR and unwanted H2 evolution reaction over Pd1-xAgx surfaces, the G*COOH (0.07 eV) is lower than G*H (0.11 eV) over Pd0.75Ag0.25 surface, which indicates the more favorable CO2RR.63 This calculation is in good accordance with the experimental results.



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Figure 7. Free energy diagrams of (a) CO2RR and (b) HER. CONCLUSION In summary, Pd1-xAgx alloys are investigated as a platform to understand the electronic effects on electrocatalytic CO2RR. As experimentally and therotically evidenced, increasing the Ag/Pd ratio in Pd1-xAgx leads to the obviously altered electronic configurations and accordingly the FE of CO product. The optimal Pd0.75Ag0.25/C affords a high FE of 95.3% at -0.6 V (vs. RHE), and satisfactory long-term durability, performing among the best of recently reported electrocatalysts. The kinetic analysis further identifies the RDS of *CO desorption on the Pd/C, Pd0.75Ag0.25/C and Pd0.50Ag0.50/C, while the CO2 protonation on Ag rich counterparts (Pd0.25Ag0.75/C and Ag/C), which is supported by DFT calcualtion results. Furthermore, the DFT modeling reveals that the scaling relationship of intermediates can be broken at a low Ag content (e.g., Pd0.75Ag0.25), achieving a drastic attenuation of *CO binding, but negligible for *COOH. The resulting low G*COOH and energy barrier for *CO desorption, as well as the positive G*H, are responsible for the efficient CO2RR on the Pd0.75Ag0.25/C. Providing atomic insights to bridge the electronic configurations and the electrocatalytic CO2RR on alloy surfaces, this work is anticipated to shed some light on the rational design of high-performance catalysts. EXPERIMENTAL SECTION Catalyst preparation Pd1-xAgx alloy nanoparticles supported on activated carbon were synthesized via a modified wetting chemistry reduction method, using sodium citrate as the stabilizer and sodium borohydride as the reductant. The composition of Pd1-xAgx was controlled by varying the molar


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ratio of the precursors Pd(NO3)2 and AgNO3, and the particle size was kept as consistent as possible by maintaining the molar ratio of the stabilizer and reductant. Typically, in order to synthesize the Pd0.75Ag0.25/C, 60 mg of activated carbon (Vulcan XC-72R carbon black) was added into 50 mL ultrapure water under a magnetic stirring, and then 0.3 mmol of Pd(NO3)2, 0.1 mmol of AgNO3 and 2.8 mmol of sodium citrate were loaded. After a sonication for 30 min, 10 mL of 2.8 M NaBH4 was slowly added and vigorously stirred for 8 h. The as-prepared mixture was centrifuged at 10,000 rpm for 5 min, and then the product was collected and dried for 15 h in a vacuum oven at 50 °C. For comparison, the Pd0.50Ag0.50, Pd0.25Ag0.75, single-component Pd and Ag catalysts supported on activated carbon were also prepared. Physical characterization TEM and HR-TEM investigations were taken on a JEOL 2100F. EDS attached on TEM and the corresponding elemental mapping were collected on a JEOL 2100F. SEM investigation was undertaken on a Zeiss ULTRA55. XRD analysis was performed on Bruker D8 diffractometer using Cu Kα radiation (λ = 1.54056 A). XPS was processed on a Thermo scientific Escalab 250Xi, using C 1s (284.6 eV) as a reference. 1H NMR experiments were conducted on Bruker DRX300 spectrometer of frequencies of 300 MHz. The Pd and Ag contents in Pd1-xAgx/C were determined by ICP-AES. Electrochemical measurements Catalysts were loaded onto glassy carbon electrodes (GCEs), and tested in a CO2-saturated 0.5 M KHCO3 solution using a typical three-electrode setup. Typically, 4 mg of catalyst and 60.0 μL Nafion solution (5 wt %) were dispersed in 0.5 mL of water-ethanol (volume ratio = 4:1) by sonicating for 30 min to form a homogeneous ink. Then 15 μL of catalyst ink was loaded onto a



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GCE of 5 mm in diameter (geometric area: 0.196 cm2). All of the electrochemical measurements were conducted on an electrochemical workstation (CHI 760, CH Instruments, Inc., Shanghai, China), using a saturated calomel electrode (SCE) as the reference electrode, and a graphite electrode as the counter electrode. All of the potentials were calibrated to a reversible hydrogen electrode (RHE), following the equation: E (vs. RHE) = E (vs. SCE) + 0.241 + 0.0591 × pH. The electrocatalytic CO2RR tests were conducted in a gas-tight two-compartment electrochemical cell equipped with a piece of proton exchange membrane (Nafion 117, SigmaAldrich) as the separator, and both of the compartments contained 45 mL electrolyte. Prior to electrolysis, the electrolyte in the cathodic compartment was degassed by CO2 for 30 min. CO2 was delivered into the cathodic compartment at a rate of 35 mL min-1 and was vented directly into the gas-sampling loop of a gas chromatograph (GC). The GC was set to automatically sample every 30 minutes, using high purity nitrogen (N2, 99.999%) as the carrier gas. The column effluent (separated gas mixtures) was quantified by thermal conductivity detector (TCD) and flame ionization detector (FID). It was first passed through the TCD in which hydrogen was quantified; and then, it was passed through a methanizer where CO was converted to methane and subsequently quantified by the FID. Based on the GC peak areas, the partial current densities and FEs of CO and H2 production were calculated, as shown in the following formulas:

jCO =

jH2 =

peak area of CO 2F × flow rate × × (electrode area) -1 α Vm peak area of H2 β

Faraday efficiency =

× flow rate ×

2F × (electrode area) -1 Vm

jCO or jH2 jtotal


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Where α and β are the conversion factors for CO and H2, respectively, based on the calibration of standard samples, Vm = 22.4 L mol-1 and F = 96485 C mol-1. Liquid products were analyzed by 1H NMR spectrometer. After 20 h of electrolysis, 450 μL of the catholyte was mixed with 50 μL D2O and 5 μL of DMSO as internal standard. Computational Methods CASTEP module implemented in Materials Studio was used for the density functional theory (DFT) calculations. Generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) functional was employed for determining the exchange correlation energy. A kinetic energy cutoff of 380 eV was assigned to the plane wave expansions, and reciprocal space was sampled by Monkhorst-Pack scheme with a grid of 5 × 4 × 1. The core electrons were replaced with ultrasoft pseudo-potentials. To implement configuration optimization, the self-consistent field (SCF) tolerance was 1 × 10-5 eV per atom. All Pd1-xAgx alloys crystal structures for calculation adopt a face-centered cubic (fcc) crystal structure, using 2 × 2 supercell. The (111) facets, which were commonly observed crystalline facets of fcc-structure material, were modeled with vacuum widths of 15 Å, using a 2 × 2 five-layer surface slab with top two layers relaxed in all calculations. ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publication at DOI: 10.1021/acsami.XXXXXXX.



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Composition of electrocatalysts determined by ICP, TEM images, SEM image, polarization curves, 1H NMR spectrum, kinetics analysis, calculation models, and comparison of catalytic performance with recently reported Pd-based electrocatalysts. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is financially supported by the National Natural Science Foundation of China (grant nos. 21773093 and 21433002), the Guangdong Natural Science Funds for Distinguished Young Scholars (grant no. 2015A030306014), and the Science and Technology Program of Guangzhou (grant no. 201707010268). REFERENCES (1) Costentin, C.; Robert, M.; Savéant, J.-M., Catalysis of the Electrochemical Reduction of Carbon Dioxide. Chem. Soc. Rev. 2013, 42, 2423-2436. (2) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I. B.; Norskov, J. K.; Jaramillo, T. F., Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, eaad4998.


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PdâAg Alloy Electrocatalysts for CO2 Reduction: Composition Tuning

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PdâAg Alloy Electrocatalysts for CO2 Reduction: Composition Tuning