Research Article Cite This: ACS Catal. 2019, 9, 6837−6845
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Stability Limits of Ni-Based Hydrogen Oxidation Electrocatalysts for Anion Exchange Membrane Fuel Cells Elena S. Davydova,*,†,⊥ Florian D. Speck,‡,§,⊥ Michael T.Y. Paul,‡ Dario R. Dekel,*,†,∥ and Serhiy Cherevko*,‡ †
The Wolfson Department of Chemical Engineering, Technion−Israel Institute of Technology, 3200003 Haifa, Israel Helmholtz-Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Forschungszentrum Jülich GmbH, 91058 Erlangen, Germany § Department of Chemical and Biological Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany ∥ The Nancy and Stephen Grand Technion Energy Program (GTEP), Technion−Israel Institute of Technology, 3200003 Haifa, Israel
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‡
S Supporting Information *
ABSTRACT: Among the non-noble-metal electrocatalysts for the hydrogen oxidation reaction (HOR) in anion exchange membrane fuel cells (AEMFCs), Ni-based nanoparticles have shown the highest reported activities. In this work, we investigated the chemical and electrochemical stability of representative Ni-based electrocatalysts. For this, carbonsupported monometallic Ni and bimetallic Ni3M (M = Co, Fe, Cu, Mo) nanoparticles were synthesized and tested using a set of complementary techniques. It was found that Mo suffers from intense dissolution due to thermodynamic instability. Cu was stable below 0.4 VRHE, though it undergoes noticeable electrochemical transient dissolution if the potential range is extended to 0.5 VRHE and higher. However, Ni, Co, and Fe showed negligible dissolution up to 0.7 VRHE. Despite the absence of dissolution, all catalysts lose their HOR activity if they are cycled to these high potentials. Physicochemical characterization of the aged catalysts revealed full oxidation of the metal nanoparticles, which could be responsible for the performance deterioration. Although our results demonstrate that, besides Ni3Mo, all studied materials show high stability under operating potentials of AEMFCs, if fuel starvation in AEMFCs results in high anodic potentials, cell activation and operation strategies are needed to prevent the passivation of the catalysts. These results present critical insights toward the design and development of affordable Ni-based electrocatalysts for AEMFCs as well as provide a better understanding of the operation strategies for the stability of AEMFCs. KEYWORDS: stability, anion exchange membrane fuel cell (AEMFC), nickel, electrocatalyst, hydrogen oxidation reaction (HOR)
1. INTRODUCTION The latest advances in anion exchange membrane fuel cells (AEMFC) have clearly revealed that this new technology may provide a real alternative to the more mature proton exchange membrane fuel cells (PEMFC).1−18 Currently, such high performance is, however, only achievable when noble-metal catalysts are utilized. While the performances of many platinum-group-metal (PGM)-free cathode electrocatalysts are already competitive against noble metals,19−29 we still rely on PGM materials to catalyze the alkaline hydrogen oxidation reaction (HOR) in AEMFCs.13,30−33 Indeed, a recent comprehensive review study of electrocatalysis in alkaline medium reveals that the best PGM-free anode materials developed so far still have 2 orders of magnitude lower exchange current density in comparison to PGM electrocatalysts.34 To be truly competitive against PEMFCs, © XXXX American Chemical Society
the development of more active PGM-free HOR catalysts is imperative. Currently, the most promising PGM-free AEMFC electrocatalysts for HOR are Ni-based alloys or mixtures such as Ni-Mo,17 Ni-Cu,35−37 Ni-Mo-Co,38 Ni-Ag,39 and N-doped Ni40 among others. Recent publications have shown promising performance of some of these materials. For example, Serov et al. have shown that, using Ni-Mo17 and Ni-Cu35 anode electrocatalysts, AEMFC performances as high as 120 mW cm−2geom (ca. 50 mA cm−2geom at 0.8 V) and 350 mW cm−2geom (ca. 100 mA cm−2geom at 0.8 V) were achieved, respectively. A high mass activity of 22 A gNi−1 by electrodeposited monometallic Ni/C was also reported by Oschepkov et al.41 Received: April 17, 2019 Revised: June 6, 2019 Published: June 19, 2019 6837
DOI: 10.1021/acscatal.9b01582 ACS Catal. 2019, 9, 6837−6845
Research Article
ACS Catalysis In an earlier work,42 the same authors identified that the kinetics of HOR at the Ni electrocatalyst depends on the presence of oxygenated species at its surface. More recently, Davydova et al.43,44 have provided a systematic study on the role of transition-metal elements on the electronic and oxophilic properties of the Ni-based catalysts, which was revealed to be correlated to their catalytic activities. A correlation between HOR kinetics and the H- as well as OH-binding energy (denoted as HBE and OHBE, respectively) was presented, suggesting a competitive adsorption of H and OH species. However, none of the studies have focused on the stability of these catalysts, which is critical for the success in the development of PGM-free HOR electrocatalysts. The available data on the stability of Ni and Ni-based alloys are scarce, and the operating conditions relevant for AEMFCs have rarely been addressed. These conditions include high pH medium, relatively elevated temperatures, hydrogen (usually humid) atmosphere, and applied electrochemical potentials.45−48 Since performance deterioration can be attributed to numerous degradation mechanisms,49−51 conclusive results on the intrinsic stability of different HOR catalysts under relevant AEMFC conditions may be difficult to interpret. Herein, we systematically studied the stability of Ni3M catalysts in alkaline media by applying a set of techniques for the quantification of materials oxidation and dissolution. An electrochemical scanning flow cell (SFC) connected online to an inductively coupled plasma mass spectrometer (ICP-MS) was used to measure the dependence of metal dissolution at anodic potentials.52 As an initial orientation point in the analysis of the obtained dissolution data, the thermodynamic data provided by Pourbaix was used.53 Thereby, we can directly correlate our experimental data with the thermodynamic redox transitions for the discussion of electrochemical catalyst dissolution at AEMFC relevant electrode potentials. Accelerated stress tests (ASTs) of a thin-layer rotating-disk electrode (TL-RDE) were performed to examine the HOR electrocatalysis durability during potential cycling. To mimic the condition of fuel starvation in the AEMFC anode, and to characterize the long-term changes in Ni-based electrocatalysts, a chemical aging procedure in an aerated alkaline medium was used in combination with ex situ bulk electrolyte ICP-MS analysis. Finally, physiochemical analyses were performed using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), energy-dispersive X-ray spectra (EDS), high-resolution scanning electron microscopy (HR-SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and H2-temperatureprogrammed reduction (H2-TPR).
for electrochemical investigations of the electrocatalysts, while the electrolyte that flowed across the working electrode was analyzed in real time using a PerkinElmer NexION 350x ICPMS. The SFC was connected to a Gamry Reference 600 Potentiostat (Gamry, USA), a graphite counter electrode, and a Metrohm (Metrohm, Switzerland) Ag/AgCl reference electrode. The reference electrode was regularly calibrated against a reversible hydrogen electrode (RHE), and all potentials in this work are reported versus RHE. The Ni-M electrocatalyst inks were drop-casted onto a glassy-carbon (GC) plate, resulting in spots of approximately 1.4 mm in diameter and loadings of 115 μg cm−2. The SFC setup utilized 0.05 M KOH (Fe, Ni, Cu ≤0.0005%, Merck Emsure) with a flow rate of 190 μL min−1. Each ink spot on the GC was used as the working electrode by connecting the spots with the SFC. The spots were contacted by the SFC at open circuit potential (OCP), and a potentiodynamic control protocol was started shortly after. This electrochemical protocol consisted of pairs of two CVs starting at −0.05 VRHE going to three different upper potential limits (0.3, 0.5, and 0.7 VRHE) at 5 mV s−1. By scanning from −0.05 VRHE we ensured that the oxidation transitions of Fe, Co, and Ni are within the cycling range; only Mo with a thermodynamic redox potential E° < −0.2 VRHE could not be scanned due to interference by the evolution of hydrogen. The CV pairs were separated by a potential hold at 0.0 VRHE to make sure that the transient dissolution retreats to that of the baseline level. The ICP-MS was calibrated daily using a four-point calibration slope created from standard solutions (Merck Centripur) containing intentional amounts of the investigated metal salts in a 0.05 M KOH solution. Note that 56Fe analysis was done in the dynamic reaction chamber mode of the instrument to avoid interference from the 40Ar16O dimers. More details on the experimental setup can be found in our previous publications.52,54,55 2.2.2. Long-Term Cycling Stability. Cyclic voltammetry (CV) with TL-RDE using a conventional three-electrode setup was used to characterize the long-term electrochemical cycling stability of the catalysts. The experiment was performed by a WaveDriver 20 Bipotentiostat/Galvanostat (Pine Research). For the TL-RDE tests, 10 mg of the catalyst was dispersed in 2 mL of an isopropyl alcohol/water mixture (3/1 v/v). Nafion (10 wt % in H2O, ρ = 1.05 g mL−1, Sigma-Aldrich) was added to the catalyst to get a weight ratio of Nafion to catalyst of 0.40/1. This suspension was drop-cast onto a glassy-carbon disk (5.0 mm, PTFE shroud, Pine Research, USA) to form a catalyst layer with a loading of 400 μgcat cm−2. The electrode was air-dried for about 1 h and mounted on the rotating shaft. The experiments were conducted at 25 °C in 0.05 M KOH aqueous solution in a water-jacketed electrochemical cell. A Pt wire (Pine Research, USA) isolated in a fritted-glass tube was used as the counter electrode. All potential values were registered vs Hg/HgO (4.2 M KOH, RE-61AP, ALS Co., Japan) and afterward presented as versus RHE. The following cycling stability protocol was applied using RDE and CV methods. Similar protocols were also used in previously reported literature.56−58 After the working electrode was immersed into the electrolyte, it was purged with H2 (electrolyzer SPH-500, H2 purity 99.999%, H2 flow 0−500 mL min−1, Jinan Mao An Instrument Co., Ltd., China) until the open circuit potential stabilized (step 1). The potential was then scanned up to five cycles with a sweep rate of 1 mV s−1 at a rotation speed of 1600 rpm to register the HOR polarization curves (step 2). The exchange current density (i0) was
2. EXPERIMENTAL SECTION 2.1. Catalyst Synthesis. A series of electrocatalysts composed of carbon (VXCMAX22, Cabot, USA)-supported bimetallic Ni3M nanoparticles, where M includes Co, Fe, Cu, and Mo, were prepared by reducing salt precursors in sodium borohydride-based solutions. Carbon-supported monometallic Ni nanoparticles were also prepared for use as reference. The complete synthetic procedures and analyses of the catalysts are described in Figure S1 and Table S1 in the Supporting Information. Further details can be also found in our previous publication.43 2.2. Stability Tests. 2.2.1. Online ICP-MS Stability. The potential-dependent online dissolution data were gathered using a custom setup. A LabVIEW-controlled SFC was used 6838
DOI: 10.1021/acscatal.9b01582 ACS Catal. 2019, 9, 6837−6845
Research Article
ACS Catalysis
Survey spectra were registered in a wide energy range of 0− 1400 eV. Utility multiplex spectra were taken for different peaks in the low energy range window at an intermediate resolution. The calculation accuracies were ±2%, ±5%, ±10%, and ±20% for atomic concentrations of around 50%, 20%, 5%, and 1%, respectively. The H2-TPR spectra for the materials were obtained using an AutoChem 2920 instrument (Micromeritics GmbH, Germany) operated in flowing mode. A thermal conductivity detector (TCD) was used to determine the H2 concentration. The samples (0.06−0.1 g) were placed in a quartz reactor. Adsorbed water was removed by heating the samples at 200 °C for 60 min with an Ar flow of 50 mL min−1. Afterward, the gas was switched to a mixture of 10 vol % (H2/Ar) with a flow rate of 50 mL min−1 until the baseline of the TCD signal was obtained. The H2-TPR was carried out between room temperature and 800 °C, with a temperature ramp of 10 °C min−1 using a mixture of 10 vol % (H2/Ar) at a flow rate of 50 mL min−1. After the desired temperatures were reached, the samples were cooled under a flow of Ar. Quantitative analysis of the catalyst samples using ICP-MS was done by fully oxidizing the carbon support of 5 mg (total weight) of catalyst in a tube furnace at 500 °C in air for 5 h. Subsequently, the crucible containing the residual metal component was boiled in 40 mL of HCl (30 vol %, Suprapur) for 2−3 h until roughly 5 mL of acid remained. The remaining acid was diluted with Milli-Q water to 50 mL. This solution was further diluted with 0.1 M HClO4 solution to contain approximately 1−10 μg L−1 of the metal of interest and analyzed by the ICP-MS. The calibration was done similarly to the online-SFC calibration described previously, only the matrix of the standard solutions was adapted to that of the analyte.
calculated in the micropolarization potential range according to eq 1:59 i0(αc + αa) =
RT i F η
(1)
The variables αa and αc are the anodic and cathodic transfer coefficients, respectively, i is the measured current density averaged for the range of overpotential values (η) between 5 and 50 mV (in A cm−2Ni), T is the temperature of the electrolyte (in K), R is the gas constant, and F is the Faraday constant. After the HOR polarizations, the gas flow was changed to Ar until the OCP was stabilized (step 3). The potential was then swept in the range between 0.0 and 0.4 VRHE at a rate of 1 mV s−1 (step 4) to record the CVs. The anodic peak was integrated and used as an in situ method to determine the electrochemically active surface area (ECSA) of Ni with the specific charge density of 514 μC cm−2Ni.40 After (1000 cycles) the potential cycles either between 0−0.3 VRHE or 0−0.7 VRHE at a sweep rate of 20 mV s−1 was performed (step 5), steps 1−4 were then again repeated. The 0.05 M KOH electrolyte and the catalyst layer after the cyclic stability tests were also collected for the bulk electrolyte elemental analysis using ICP-MS. 2.2.3. Chemical Stability. Catalyst samples (40−50 mg of the catalysts and 50 mg of VXCMAX22 carbon black as the reference) were each placed in a polypropylene vial containing 50 mL of 1 M KOH. The mixtures were equilibrated at ambient atmosphere, capped, heated to 60 °C in a silicon oil bath, and maintained for 30 days. The solid phase of the mixtures was then separated from the KOH solution via centrifugation at 21000 rpm for 15 min, followed by the removal of the supernatant and the addition and resuspension of the solid in fresh solvent. This process was repeated five times with Milli-Q water, and the washed samples were placed overnight in a vacuum oven at 80 °C. The ICP-MS analysis was performed with remnant KOH solution as well as the chemically aged catalyst solids that were collected. 2.3. Physical and Chemical Characterization. The TEM images were obtained with an FEI Tecnai T20 LaB6 microscope operated at 200 kV. The catalysts were dispersed in isopropyl alcohol (2 mgcat per 10 mL) by an ultrasound bath for 2 h, spray-casted on to a 300 mesh Cu grid coated with holey carbon (Agar Scientific, USA), and left to dry on the TEM grids under ambient conditions. The EDS spectra, STEM images, and elemental maps were collected by a Zeiss Ultra-Plus HR-SEM instrument. The EDS spectra were registered at accelerating voltages in the range of 5−15 kV with a data collection time of 50−150 s. STEM element mapping was done at 30 kV on holey carbon 200 mesh Cu TEM grids with the applied catalysts. XRD patterns were collected using a Rigaku Smartlab (Rigaku Co., Japan) diffractometer with a Cu X-ray source (λ = 1.5406 Å). The patterns were recorded in mediumresolution parallel beam geometry with a tube current of 150 mA and a tube voltage of 45 kV in θ/2θ scan mode. A scan rate of 1° min−1 and a step resolution of 0.01° was used. The diffraction patterns were analyzed using the Inorganic Crystal Structure Database (ICSD). The XPS measurements were performed under ultrahigh vacuum (2.5 × 10−10 Torr base pressure) using a 5600 Multi-Technique System (PHI, USA). The samples were irradiated with an Al Kα monochromated source (1486.6 eV), and photoelectrons were analyzed by a Spherical Capacitor Analyzer using a slit aperture of 0.8 mm.
3. RESULTS 3.1. Online ICP-MS Dissolution Stability. Electrochemical stability of the Ni3M/C catalysts toward dissolution was studied first by SFC-ICP-MS with results presented in Figure 1 and Table S2. As shown in Figure 1, after some marginal dissolution (Table S2) due to the contact with the catalyst, the Ni, Co, and Fe dissolution signal remained below the detection limit of ICP-MS (ca. 10, 1, and 300 pg cm−2 s−1, respectively) independent of the upper potential limit (UPL) of the CV cycles. Fe and Co showed a small dissolution peak (22 and 3.4 ng cm−2, respectively) upon contact with the electrolyte at OCP. No dissolution of Ni, Fe, and Co was observed upon potential cycling in the range of −0.05 to 0.7 VRHE. In contrast, the dissolution behavior of Cu was completely different. First, the amount of contact Cu dissolution (37.5 ng cm−2) was significantly higher than that measured for Ni, Co, and Fe. Moreover, during potential cycling, a Cu dissolution peak with the onset at ca. 0.43 VRHE can be clearly seen for the CVs going to 0.5 and 0.7 VRHE. The dissolution of Cu increased with respect to the UPL of the CVs, reaching values of 39 and 266 ng cm−2 for UPLs of 0.5 and 0.7 VRHE, respectively. Albeit less pronounced, the dissolution of Mo from the Ni3Mo1/C catalyst during the potential cycles also increased with potential. The overall dissolution of Mo during the cycling (104 ng cm−2) was 1 order of magnitude lower than that during the initial contact of the catalyst with electrolyte at OCP (3211 ng cm−2). Since Mo (in the form of its oxides) was shown to be unstable (Figure 1 and Table S2), in the subsequent tests we focused on the 6839
DOI: 10.1021/acscatal.9b01582 ACS Catal. 2019, 9, 6837−6845
Research Article
ACS Catalysis
Similarly to the SFC-ICP-MS measurements, a potential cycling procedure was used. In this experiment Ni3Fe/C was chosen as a representative catalyst, because all the other catalysts (Ni/C, Ni3Co/C, and Ni3Cu/C) showed properties similar to those of Ni3Fe/C, unless specified otherwise. Complete data on the stabilities of the catalysts are summarized in Table 1. Figure 2 compares the catalytic
Figure 1. Online dissolution data during the potentiodynamic cycling protocol (shown in the upper pane) in 0.05 M KOH within the potential window of −0.05 to 0.7 VRHE for Fe (green), Co (light blue), Ni (dark blue), Cu (orange), and Mo (red). The light gray traces represent the original dissolution data, and the colored components were smoothed for clarity.
dissolution of Ni/C, Ni3Co1/C, Ni3Fe1/C, and Ni3Cu1/C catalysts. 3.2. Long-Term Cycling Stability. The online ICP-MS dissolution measurements have revealed that, in the potential range going up to 0.7 VRHE in 0.05 M KOH at room temperature, Ni/C, Ni3Co/C, and Ni3Fe/C are very stable with regard to metal dissolution. Here we further explore the stability of the same catalysts by investigating the effect of prolonged potential cycling on their HOR activity in the conventional electrochemical cell using the TL-RDE method.
Figure 2. Hydrogen oxidation polarization curves (right panel) for Ni3Fe/C before (blue) and after (red) various degradation protocols schematically shown in the left panel: 1000 cycles in the potential window of 0−0.3 VRHE (A) and 0−0.7 VRHE (B), as well as before and after 30 days of chemical stability test (C). Conditions for electrochemical measurements: 0.05 M KOH, 25 °C, 1 mV s−1, rotation of 1600 rpm.
activity change of Ni3Fe/C before and after 1000 cycles in the potential ranges of 0−0.3 VRHE (Figure 2a) and 0−0.7 VRHE
Table 1. Electrochemical Stability Data for the Ni3M/C Electrocatalysts
a
Section 3.1. bSection 3.2. cSection 3.3. d
