Leveraging Commercial Silver Inks as Oxidation Reduction Reaction

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Leveraging Commercial Silver Inks as Oxidation Reduction Reaction Catalysts in Alkaline Medium Shlomi Polani, Naftali Kanovsky, and David Zitoun ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00714 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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ACS Applied Nano Materials

Leveraging Commercial Silver Inks as Oxidation Reduction Reaction Catalysts in Alkaline Medium Shlomi Polani,† Naftali Kanovsky† and David Zitoun,*, † †

Bar Ilan University, Department of Chemistry and Bar Ilan Institute of Nanotechnology and Advanced Materials (BINA), Ramat Gan 5290002, Israel

ABSTRACT Alkaline exchange membrane fuel cells (AEMFCs) development is impeded by the lack of low-cost processes for fabricating catalyst layers. One bottleneck lies in using large amounts of precious group metals during the layer deposition process by spray-painting. One possible solution would be to leverage inkjet printing technologies known for their full recovery of the printed material. Recent advances in printed electronics has led to the commercialization of silver (Ag) inks. We demonstrate the electrocatalytic properties (oxygen reduction reaction (ORR)) of Ag ink. Moreover, we show that a simple galvanic replacement reaction (GRR) on the Ag ink yields Ag0.9M0.1 (M = Pt or Pd) hollow nanoparticles. The resultant electrocatalyst demonstrated high activity for the ORR in alkaline medium. The Ag colloidal dispersion (the ink) reacted with a minute amount of platinum group metals precursors (PGMs). X-ray diffraction (XRD) and electron microscopy confirmed the hollow morphology and the formation of Pt or Pd-rich surfaces. The onset voltage for alkaline ORR activities follows the trend Ag0.9Pt0.1 > Ag0.9Pd0.1 > Ag. These experiments are a first step toward inkjet printing usage for fabricating catalytic layers. KEYWORDS: Oxygen reduction reaction; electrocatalysis; alkaline; ink-jet; silver nanoparticles Fuel cells are currently top candidates for power sources for electric buses and passenger vehicles. However, their drawback is their high cost of metal catalysts.1 Alkaline fuel cells allow the use of nonplatinum group metal catalysts (non PGM) in a less aggressive medium (hydroxide, basic pH).2 The alkaline exchange membrane fuel cells (AEMFCs), based on a hydroxide-conducting membrane, operate at moderate temperatures (60°C - 80°C) with no added liquid electrolyte, maintaining the advantages of the mainstream proton exchange membrane fuel cell (PEMFC) technology for transportation.3,4,5,6 Currently, the industry utilizes spraying methods to deposit the catalyst ink for fabricating the

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membrane electrode assembly (MEA). This method has two major disadvantages. First, a considerable amount of catalyst is lost during the loading of the catalyst layer to the MEA. Second, it is difficult to control the electrode morphology which is crucial for addressing the issue of mass transport and cannot be neglected in the electrode design process.7 Inkjet technology is used for various applications, such as organic thin films, transistors, light-emitting diodes, solar cells, conductive structures, memory devices and sensors. Inkjet printing allows for additive manufacturing processes with adjustable thickness and designs with high resolution deposition, reduces material wastage, lowers cost, and is scalable for large area manufacturing.8 Commercial silver (Ag) ink technology enables low sintering temperature and the low resistivity of the printed patterns (< 2.5 x ρ Ag bulk) is compatible with the membrane and the operation conditions of PEMFC. Moreover, the higher metal loading (40–60 %) is suitable for mass production. Currently, Ag is much more affordable than any PGM (for instance, it is 1/60 the price of Pt) (Figure S10). While Pt and Pt-based are the most efficient electrocatalysts for ORR,9 silver is a promising candidate to replace platinum due to the similar mechanisms and kinetics for alkaline ORR catalysis10–11 and the higher Ag overpotential could be reduced by alloying Ag with other selected metals.12,13 Ag-PGM structures have been widely studied for alkaline ORR. One strategy to produce Ag-PGM alloys is through a galvanic replacement reaction (GRR) with PGMs, which is feasible due to the high difference in their standard reduction potential.14–15 Silver based cathodes are carbon-free, which improves their durability since the lifetime of cathodes is mainly limited by carbon corrosion and structural degradation. The carbon is slowly oxidized due to attack by HO2− radicals formed as an intermediate during oxygen reduction.11 Conversely, the oxidation of Ag yields Ag2O, one of the rare oxides that is catalytically active and electronically conductive. Here, we use GRR directly on the industrial ink which is advantageous for the following reasons: the performance of the electrocatalyst will improve due to the formation of hollow AgM nanoshells with a thin and controllable shell layer. This will ensure larger surface areas to better utilize the precious group metals and should decrease the losses inherent to the current state of technology. Additionally, the use of a colloidal dispersion (the ink) will optimize the formulation of the cathode layer material and minimize the losses of material compared to the standard spray-painting method. Lastly, the modified catalyst is formulated as an ink for printing the catalyst layer on the MEA. This feature allows us to design and fabricate the catalyst layer on the MEA according to different flow regimes.


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The nanoshell electrocatalysts were synthesized by first diluting a commercial silver ink consisting of Ag NPs with a well-defined particle size distribution (PSD), a mean size of 64 ± 20 nm in diameter and a polyhedral morphology as shown on the SEM image (Figure 1A) and previously reported by Polani et al.16 The Ag NPs were used as a chemical sacrificial template in the synthesis of AgM (M = Pt or Pd) nanocrystals which exhibited hollow interiors as well as controlled surface composition. The synthesis was based on the GRR between metallic Ag and MCl4-2(aq). 2Ag0 + MII = 2AgI + M0

(M = Pt or Pd)

Figure 1B-C shows HRSEM images for AgM nanoshells. It can be observed that all the AgM nanostructures display well-defined shapes and a relatively narrow PSD. The hollow interiors in the nanoshells can be clearly visualized from the TEM images due to differences in mass thickness contrast. From the analysis of the SEM images (Figures 1B–C and SI Figures S2 and S3), the size distribution of the AgPt and AgPd nanoshells are respectively 80 ± 20 and 90 ± 30 nm in diameter (see Figures S5 and S6 in the SI). The atomic composition in both AgM nanostructures was determined by statistical EDS analyses and the stoichiometry was set as Ag0.9M0.1. Therefore, the AgM NPs have similar morphologies, which differ only in the shell composition.



Figure 1. SEM of the pristine Ag ink (A); SEM of the Pt and Pd modified Ag ink (B-C) and HRTEM with corresponding FFT for Ag0.9Pt0.1 (D) and Ag0.9Pd0.1 (E) nanoshells.

Figures 1–3 depict HRTEM, HAADF-STEM and EDS images for AgM nanoshells. HRTEM and the selected


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area FFT (insets) results for the AgM nanoshells indicate that in addition to their hollow interiors both materials displayed a polycrystalline surface (Figure 1 D and E). This can be due to the polyhedral nature of the Ag NPs used as sacrificial templates for the synthesis. The lattice fringe spacing is 2.35 Å corresponding to Ag {111} planes. HAADF-STEM images and EDX line scans for both AgM nanostructures (Figures 2-3) support the formation of hollow interiors and reveal some structural details such as porous walls. EDX elemental maps for Ag (green), Pt and Pd (red) (Figures 2 and 3b–d) show that both Ag and M are uniformly distributed in both AgM nanostructures.

Figure 2. HAADF-STEM images of hollow Ag0.9Pt0.1 NPs with an EDS line scan (A); EDS mapping of elemental Pd (B) and Ag (C); and the corresponding overlay (D)



Figure 3. HAADF-STEM images of hollow Ag0.9Pd0.1 NPs with an EDS line scan (A); EDS mapping of elemental Pd (B) and Ag (C); and the corresponding overlay (D)

The formation of hollow structures from galvanic replacement was observed on faceted Ag nanocrystals. The driving force for the reaction is nucleation of Pt or Pd on several areas of the nanocrystals, usually the vertices, as observed in single crystalline nano-octahedra,17 which act as the anode of the galvanic cell. On the facets of the nanocrystals, the oxidation of silver leaches out cations, which further precipitate with chloride. Overall, the reaction yields incomplete shells, which appear as empty bowls. The structures of the Ag polyhedral and the AgM nanoshells, and the resulting products, were further characterized using XRD (Figure S7). The Ag nanocrystals are face-entered cubic (FCC) with a lattice parameter of 4.086 Å. In all diffractions, the strong peak near 2θ = 38.115° and the weaker peaks at


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44.299° and 64.443° correspond to the {111}, {200} and {220} planes of Ag respectively. After the replacement reaction, diffraction peaks of Ag ({111}, {200} and {220}) are still present and predominant (Figure S7A). The nanoshells keep the FCC phase of the Ag sacrificial template and a slight shift in the diffraction peaks towards the M pure phase peaks which can be observed (high angles). This small shift can be explained due to the low amount of PGM and the increased width of the peaks associated with nanostructures. After the metal etching, the AgCl phase is clearly seen due to the low Ksp of Ag+(aq) and Cl-(aq) (Ksp = 1.8*10-10) and the precipitation of AgCl(s). Due to the ink-like nature of the electrocatalyst, we drop-casted it on a glass slide and used grazing incidence XRD. In the following, the electrocatalysis is measured on the crude ink. However, the AgCl(s) by-product can be dissolved by using a saturated NaCl solution according to the equilibrium AgCl(s) + Cl-(aq)= AgCl2-(aq) and the diffraction pattern of the catalyst after washing shows the absence of any AgCl(s) secondary phase (Figure. S7B). Figure 4 shows the cyclic voltammetry of the pristine Ag and AgM nanoshell inks after the replacement reaction with Pt or Pd. During the anodic scan, a peak was observed between 1.1 and 1.35 V, which correlates to the formation of Ag2O on the Ag and AgM catalysts surface. The Ag2O was then reduced during the cathodic scan with a sharp peak at 1.08 V. For the AgM electrocatalyst, the silver oxide reduction peak is still present around 1V with a small shift to low potentials and a second reduction peak from Pt or Pd occurs as a broad peak at 0.5 and 0.65 V respectively. The ratio between the cathodic peaks of Ag/Pt and Ag/Pd could be calculated from the ratio between the charges calculated for the two peaks. The silver surface ratio was determined to be 85 and 91 % for the on the AgPt and AgPd respectively. The electrochemical stability of the catalysts was assessed by cycling the catalyst for 250 cycles from 0.05 until 1.35 V vs. RHE. The catalyst layer has been modified with Nafion® to provide a mechanical strength to the catalyst layer. The electrochemical window is very wide compared to the cathodic potential in an actual fuel cell. We chose this protocol in order to follow the ratio between the cathodic peaks of Ag and Pt (or Pd) during the cycling. Figure S8 displays the cyclic voltammograms of cycles #50, #150 and #250. Both the amplitude and the ratio between the two cathodic peaks remain constant, signaling that the ECSA of the catalyst and its surface composition remain identical to the pristine ones during the cycling. The catalysts activities were determined by ORR measurements carried out in O2 saturated 0.1 M KOH at 900 RPM (Figure 4). The activity follows the order Ag0.9Pt0.1> Ag0.9Pd0.1>Ag. The AgPt catalyst displayed a current density of 0.037 mA/cm2 (at 900 mV, iR free vs RHE) and the AgPd had a current density of 0.03



mA/cm2, an improvement from 0.017 and 0.01 mA/cm2 for the pristine Ag ink.

A

B

Figure 4. Cyclic voltammetry in 0.1 M KOH saturated with Ar at 50 mV/s scan rate (inset) mass activities (A); ORR comparison of the different catalysts at 900 rpm in 0.1 M KOH saturated with O2 with a 10 mV/s scan rate (B)


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To extract the kinetics of the ORR, Koutecky–Levich plots for the following AgM nanoshell and Ag catalysts were used (Figure S9). Linear sweep voltammograms were taken at 225, 400, 625, 900, and 1600 RPM and fitted to the Koutecky–Levich equation to determine the average number of electrons transferred during the ORR. The slight drift observed may be due to absence of any ionomer in the ink, as the aim of this communication is to show the direct use of an ink as ORR electrocatalyst. The Koutecky–Levich plots for all the catalysts show linear dependence at all four potentials. This linear plot indicates that these reactions have first order kinetics with respect to oxygen. From the Koutecky-Levich plots, the average number of electrons transferred for the AgM and Ag was ∼4, which corresponds to the full reduction of oxygen (Figure S9). This result is in good agreement with previous studies on bulk and large nanoparticles (as in the present study) concluded that the mechanism is a pseudo 4-electron pathway.10 A similar mechanistic study on the alloy might be more speculative. However, a mechanistic study of ORR on silver alloys is beyond the scope of a communication on the leverage of Ag inks as electrocatalysts. Onset potentials for silver-based ORR electrocatalysts were recently reviewed, and a wide discrepancy was found, with values spanning from 0.83 V to 1.02 V vs. RHE.2 The pristine and modified catalysts reported herein display onset potentials above 0.9 V vs. RHE. We calculated the mass activity at 0.9 V vs. RHE (iR free) and reported the values in the inset of Figure 4A. Pristine Ag reaches a mass activity of 57 mA/mg, while AgPd and AgPt attain 90 and 106 mA/mg, respectively. These values are consistent with a previous report on carbon-supported silver-based bimetallic catalysts18 and are similar to the mass activity of Pt/C (~ 100 mA/mg at 0.9 V vs. RHE iR free).2 The optimal loading of PGM on Ag results from a balance between the electrocatalytic activity and the cost. The galvanic replacement reaction used during the synthesis yields the exact planned stoichiometry, the composition of the bimetallic catalysts Ag0.9M0.1 correspond to the molar ratio between the pristine Ag ink and the PGM complex. The GRR leads to formation of a PGM-rich surface, which behaves similarly to the PGM. In the case of our study, even a 10% PGM loading already provides excellent ORR activity. We believe a further decrease of the PGM amount could optimize the mass activity. The demonstration of ORR activity is a first step on the pathway towards printing membrane electrode assembly. We emphasize that the ORR activity was obtained on a commercial ink with further washing of the surfactants. One of the key factor for the electrochemical activity is the high metallic weight fraction in the ethylene glycol solution (50 %wt Ag) and the very low fraction of stabilizing polymer (0.5



%wt polyvinylpyrrolidone). We did not observe any inhibition of the electrocatalysis due to the stabilizing polymer. After the demonstration of ORR activity for the pristine and modified inks, there is a long way ahead before matching the stable and well-optimized process of membrane electrode assembly. The design of a catalyst layer depends not only on the activity of the catalyst (which is reported in this study), but also on the formulation of a catalyst ink layer. This communication provides a proof of concept that a catalyst layer can be produced by inkjet printing from a commercial or modified silver ink. At this stage, printing a membrane electrode assembly would require considerable effort and time and such an endeavor is beyond the scope of a short communication. Looking forward, we expect to successfully deposit an electrode with this method in the following months, considering that the ink can be easily processed by inkjet printing as we have recently shown.16 In conclusion, we measured the ORR of commercial Ag ink in an alkaline medium. To the best of our knowledge, such measurement was not reported previously. A simple galvanic replacement reaction (GRR) on the Ag ink yielded Ag0.9M0.1 (M = Pt or Pd) hollow nanoparticles. The hollow particles consist of a polycrystalline Ag shell covered with Pt or Pd. The resultant electro-catalyst demonstrated even higher ORR activity in an alkaline medium. The Ag colloidal dispersion (the ink) reacted with a minute amount of platinum group metals precursors (PGMs). X-ray diffraction (XRD) and electron microscopy confirmed the hollow morphology and the formation of Pt or Pd-rich surfaces. The onset voltage for alkaline ORR activities follows the trend Ag0.9Pt0.1 > Ag0.9Pd0.1 > Ag. We regard these experiments as a first step toward the use of inkjet printing for the fabrication of the catalytic layer.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Funding Sources This work is supported by the Planning & Budgeting Committee of the Council of High Education and the Prime Minister Office of Israel, in the framework of the INREP project. Conflict of Interest The authors declare that they have no conflict of interest.

ACKNOWLEDGMENTS The authors thank PV Nano Cell for supplying the commercial ink used in this study. The authors thank Victor Shokhen for his scientific understanding and his ability to convey it through graphical design in the best way possible. The authors thank Dr. Mechael Kanovsky for proofreading the article. The authors thank Dr. Fernando De La Vega and Semyon Melamed for providing the silver ink and sharing their insights with us in the course of this research.

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Leveraging Commercial Silver Inks as Oxidation Reduction Reaction

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