On the electrocatalytical oxygen reduction reaction activity and stability of quaternary RhMo-doped PtNi/C octahedral nanocrystals

Recently proposed bimetallic octahedral Pt–Ni electrocatalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC) cathodes suffer from particle instabilities in the form of Ni corrosion and shape degradation. Advanced trimetallic Pt-based electrocatalysts have contributed to their catalytic performance and stability. In this work, we propose and analyse a novel quaternary octahedral (oh-)Pt nanoalloy concept with two distinct metals serving as stabilizing surface dopants. An efficient solvothermal one-pot strategy was developed for the preparation of shape-controlled oh-PtNi catalysts doped with Rh and Mo in its surface. The as-prepared quaternary octahedral PtNi(RhMo) catalysts showed exceptionally high ORR performance accompanied by improved activity and shape integrity after stability tests compared to previously reported bi- and tri-metallic systems. Synthesis, performance characteristics and degradation behaviour are investigated targeting deeper understanding for catalyst system improvement strategies. A number of different operando and on-line analysis techniques were employed to monitor the structural and elemental evolution, including identical location scanning transmission electron microscopy and energy dispersive X-ray analysis (IL-STEM-EDX), operando wide angle X-ray spectroscopy (WAXS), and on-line scanning flow cell inductively coupled plasma mass spectrometry (SFC-ICP-MS). Our studies show that doping PtNi octahedral catalysts with small amounts of Rh and Mo suppresses detrimental Pt diffusion and thus offers an attractive new family of shaped Pt alloy catalysts for deployment in PEMFC cathode layers.


Synthesis of PtNi/C octahedral NPs.
Synthesis of PtNi/C Ni-rich with 6 nm edge length, PtNi: 0.64 mg Pt(acac)2, 200 mg Ni(acac)2, 0.64 g PVP (10k) and 0.4 g benzoic acid in 40 mL benzyl alcohol (BA). The 100 mL pressure glass flask was then stoppered and stirred vigorously for 1 h at RT. Then the flask was heated to 150 ℃ with a ramp of 5 ℃ min -1 . After 12 h of reaction, the reaction solution was allowed to cool to RT, then 100 mg of carbon (XC 72R) in 10 ml of BA was added to the reaction solution after stirring overnight. After stirring overnight, the solution was purified with a mixture of ethanol and acetone using a centrifuge.

Synthesis of PtNi(RhMo)/C octahedral NPs.
In a typical synthesis of PtNi(RhMo)/C Ni-rich with 9 nm edge length: 0.64 mg Pt(acac)2, 200 mg Ni(acac)2, 20 mg Mo(CO)6, 20 mg Rh(acac)3, 0.64 g PVP (10k) and 0.4 g benzoic acid in 40 mL BA. The 100 mL pressure glass flask was then stoppered and placed in a heating mantle for 1 h with vigorous stirring at 60 ℃. Then the flask was heated to 150 ℃ with a ramp of 5 ℃ min -1 . After 12 h of reaction, the reaction solution was allowed to cool to RT and then 100 mg of carbon (XC 72R) in 10 ml of BA was added to the reaction solution after stirring overnight. After stirring overnight, the solution was purified with a mixture of ethanol and acetone using a centrifuge.

Synthesis of PtNi(Rh)/C NPs.
In a typical synthesis of PtNi(Rh)/C Ni-rich: 0.64 mg Pt(acac)2, 200 mg Ni(acac)2, 20 mg Rh(acac)3, 0.64 g PVP (10k) and 0.4 g benzoic acid in 40 mL BA. The 100 mL pressure glass flask was then stoppered and placed in a heating mantle for 1 h with vigorous stirring at 60 ℃. Then the flask was heated to 150 ℃ with a ramp of 5 ℃ min -1 . After 12 h of reaction, the reaction solution was allowed to cool to RT, then the solution was purified with a mixture of ethanol and acetone using a centrifuge. We also tried to increase Rh(acac)3 amount to 40 mg and use different Rh precursor like Rh(acac)(CO)2 13 mg.

Morphological and Structural and Elemental Characterization.
Transmission electron microscopy (TEM). Micrographs were acquired using a FEI Tecnai G2 20 S-TWIN with a LaB6 cathode at an accelerating voltage of 200 kV and a resolution limit of 0.24 nm. Samples were dispersed in ethanol and dropped onto a Cu grid. High resolution (HR)TEM and scanning TEM-energy dispersive spectroscopy (STEM-EDS). The high-resolution EDS elemental maps were acquired on a Thermo-Fisher probe corrected Titan 80-20 electron microscope fitted with a four quadrant Super-X EDX detector. The experiments were carried out at an operating at a voltage of 80 kV, to minimize particle damage and maximize X-ray counts, with a probe convergence angle of 25mrad. EDX quantification was carried out using the partial ionization cross section approach. Identical location (IL) -STEM measurements. 10µl of an aqueous catalyst suspension (0.05gcat l -1 ) was drop-casted on a gold TEM finder grid (S147A9, Plano GmbH) and dried. After acquiring several high angle annular dark field STEM images of different regions and the corresponding elemental maps, the catalyst-coated TEM grid was used as a working electrode for the AST consisting of 10,800 cycles between 0.6 and 0.95 VRHE at 1V s -1 in Ar-saturated 0.1 M HClO4 at RT. A Gaskatel HydroFlex RHE was used as the reference electrode and a graphite rod as the counter electrode. The potential was controlled with a Bio-Logic Science Instruments SP-50 potentiostat. The ohmic potential drop (iR drop) was measured and corrected using the current interruption method. After AST, the grid was rinsed with ultrapure water to remove electrolyte residues and dried. Subsequently, the grid was analyzed again at identical locations. Inductively coupled plasma -optical emission spectrometry (ICP-OES) was used to determine the elemental composition of the various catalysts. Samples were prepared by dissolving the catalyst powders in a mixture of H2SO4, HNO3 and HCl (1:1:3). The solutions were heated from room temperature to 180 °C in 10 min using a Microwave Discover SP -D (CEM Corporation) and maintained at this temperature for 20 min. Finally, the solutions were diluted with Milli-Q® water, filtered and brought to a known volume. To calculate the concentration of the different solutions, 5 standards were prepared for Pt, Ni and Mo with concentrations of 0, 1, 4, 7 and 12 mg/L of each element. On-line ICP-MS measurements Stability of the oh-PtNi and the Rh-, and Mo-doped PtNi electrocatalyst samples was studied with on-line ICP-MS. The setup consists of a custom-designed and manufactured propylene carbonate scanning flow-cell (SFC). The outlet of the cell was coupled to the inlet of the ICP-MS (Perkin Elmer NexION 350X). A GC rod was used as a counter electrode that was connected to the SFC via a Tconnector on the inlet side. An Ag/AgCl/ 3 M KCl reference electrode (Metrohm) was connected to the cell via a capillary channel on the outlet side. All potentials in the manuscript are reported against the reversible hydrogen electrode (RHE). Electrocatalyst spots were drop-casted on a GC plate (SIGRADUR, A = 25 cm 2 ), which was used as the working electrode. Inks were prepared according to the recipe described below in the section "Electrochemical measurements." The only difference is that only 0.2 µl (instead of 10 µl) of catalyst suspension was drop-casted onto the polished glassy carbon plate resulting in spot diameters between 1200-1500 µm. The working electrode was sitting on an XYZ translation stage (Physik Instrumente M-403), allowing rapid navigation in between samples. Electrochemical protocols were performed using a Gamry Reference 600 potentiostat. An in-house developed LabVIEW software controlled all instruments, including the stages, gas control box, and the potentiostat. ICP-MS was calibrated daily by a four-point calibration slope prepared from standard solutions (Pt, Ni, Mo, Rh, Re, Co, In -Merck Centripur). 187 Re, 58 Co, 115 In were used as internal standards. The sample and internal standard streams were merged via a Y-connector right before the nebulizer of the ICP-MS. The Ar-purged electrolyte flow was controlled by the peristaltic pump of the ICP-MS (Elemental Scientific, M2 pump) with an average flow-rate of 3.48 µl s −1 . Electrochemical measurements were performed in 0.1 M HClO4 electrolyte solution saturated with Ar. Contact with the SFC was established at OCP. The system was held at this potential for 5 min, followed by 10,800 cycles between 0.6-0.95 VRHE applying 1 V s -1 scan rate. The protocol was finished with performing one CV in the potential window of 0.6-1.5 VRHE applying 10 mV s -1 scan rate and a potentiostatic hold at 0.6 VRHE for 5 min (allowing the ICP-MS signal to return to its baseline). The composition of the electrocatalysts after the AST protocol was calculated using the composition and Pt mass loading determined by ICP-OES measurements. Only mass loss before the final CV was considered.

Synchrotron Wide-Angle X-Ray Scattering (WAXS) Measurements.
WAXS measurements were performed at ID31 beamline of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The samples were measured in grazing-incident geometry in a customized three-electrode-operando-cell connected to a BioLogic SP-300 potentiostat. Catalyst films were prepared via drop-casting of ink on a mirror-polished glassy carbon cylinder that was connected as working electrode from the cylinders bottom to the cell. A Pt wire acted as counter electrode and a leak less miniature Ag/AgCl electrode as reference electrode (ET072, eDAQ). The electrolyte (0.1 M HClO4, Suprapur  , Merck) was continuously pumped through the cell at a flow rate of 20 mL•min -1 using a peristaltic pump. The high-energy X-ray beam (78 keV) was focused on the sample and the scattered signal collected using a Dectris Pilatus CdTe 2M detector positioned 850 mm behind the sample. The energy, detector distance and tilts were calibrated using a standard CeO2 powder and the 2D diffraction patterns were reduced to the presented 1D curves using the pyFAI software package

Rietveld Refinements.
Rietveld refinement of the WAXS patterns was performed to extract the phase structure, crystallite size and lattice parameter using the Fm3m structure of Pt metal (with possibly Ni oxide) and the Fullprof software. The instrumental resolution function was determined by the refinement of a CeO2 standard sample. Thomson-Cox-Hastings profile function was adopted. The background of patterns was described by an interpolated set of points with refinable intensities. When the contribution of the glassy carbon electrode substrate was visible on the diffraction patterns, the Rietveld analysis was restricted to the preserved Pt [220] Bragg reflection.

Electrochemical Measurements.
A typical three-electrode cell was used to perform the electrochemical measurements. A rotating disk electrode (RDE) made of glassy carbon (0.196 cm 2 , Pine Instrument) served as the working electrode. A hydrogen reference electrode (Gaskatel, HydroFlex) and a Pt mesh attached to a Pt wire were used as reference electrode and counter electrode, respectively. A catalyst ink was prepared by mixing 5.5 mg of the as-prepared catalyst powders (PtNi/C and PtNi(RhMo)/C) in water, isopropanol, Nafion (v/v/v= 3.98 ml / 1.00 ml / 0.01 ml), totaling 4.99 ml, and sonicated for 45 min using an ultrasonic horn sonicator in an ice bath. 10 µL of the catalyst ink were dropped onto the working electrode and dried at 60 ℃ for 7 min. The loading amount of the metal Pt for the octahedral PtNi and PtNi(RhMo)/C catalyst was 1.3-1.5 μgpt. For comparison, the commercial Pt/C catalyst JM was used as benchmark catalysts with a Pt loading of 10 μgPt/cm 2 . Cyclic voltammetry (CV) measurements were performed in a 0.1 M HClO4 nitrogen-saturated solution, with first 50 cycles of CVs at a scan rate of 100 mV s -1 for activation and then cycles at a rate of 20 mV s -1 for ECSA acquisition. Oxygen reduction response (ORR) measurements were performed in a 0.1 M HClO4 solution that was purged with oxygen during the measurement. Linear sweep voltammetry (LSV) was performed at a scan rate of 20 mV s -1 and ORR polarization curves were recorded at 1600 rpm. We note that the scan rate has an impact on the mass-normalized current, i.e., higher scan rate usually reports higher performance. In our earlier benchmarking paper, 1 we addressed this issue with reference to the literature. In general, scan rates in the range of 10-20 mV s -1 are recommended because they are not too high, so that capacitive currents are minimized and possible overcorrection is avoided, and also not too low, because in this case the amount of OHads on the surface and adsorption of impurities could be large and reduce activity. For analysis of the ORR activity, LSV curves in oxygen were corrected by LSV curves in nitogen. The ECSAs were determined by integrating the hydrogen adsorption charge on the CV and from CO-stripping charges. For CO-dossing experiments, CO was adsorbed on the electrode by keeping the potential at 0.05 VRHE (dossing potential), for 60 s, after which CO was adsorbed in CO-saturated solution for 25 s, followed by 10 min of N2 flushing. Followed by CVs to CO-stripping and CV after CO oxidation from 0.05 to 1 VRHE and back to 0.05 VRHE. Measurements were performed in a 0.1 M HClO4 N2-saturated solution at a potential scan rate of 50 mV s -1 . Table S1. ICP-OES metallic at%, Pt wt% of octahedral PtNi(L)/C samples, and their corresponding mean edge length distribution in nm for the as-prepared particles.          Figure S10. Rietveld refinement fits (red) of the background subtracted WAXS partial pattern (black) using for one single fcc PtM phase (purple) for a) RhMo-doped PtNi octahedra and one single fcc PtM phase (purple) and one fcc NiO phase (dark red) for b) PtNi octahedra and the difference curve (blue). Figure S11. Dissolution profiles recorded for the oh-PtNi catalyst. Electrochemical measurements were performed in 0.1 M HClO4 electrolyte solution saturated with Ar. Contact with the SFC was established at OCP. The system was held at this potential for 5 min, followed by 10,800 cycles between 0.6-0.95 VRHE applying 1 V s -1 scan rate. The protocol was finished with performing one CV in the potential window of 0.6-1.5 VRHE applying 10 mV s -1 scan rate and a potentiostatic hold at 0.6 VRHE for 5 min (allowing the ICP-MS signal to return to its baseline).  Figure S12. Dissolution profiles recorded for the oh-PtNi(RhMo) catalyst. Electrochemical measurements were performed in 0.1 M HClO4 electrolyte solution saturated with Ar. Contact with the SFC was established at OCP. The system was held at this potential for 5 min, followed by 10,800 cycles between 0.6-0.95 VRHE applying 1 V s -1 scan rate. The protocol was finished with performing one CV in the potential window of 0.6-1.5 VRHE applying 10 mV s -1 scan rate and a potentiostatic hold at 0.6 VRHE for 5 min (allowing the ICP-MS signal to return to its baseline).