One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction ...(ORR) occurs, due to the current need of precious metals to catalyze this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-nitrogen-carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for a sufficiently long time in an operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in a liquid electrolyte and a PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.
The electrical performance of a proton exchange membrane fuel cell is limited by the slow oxygen reduction reaction (ORR) kinetics. Catalytic improvements for the ORR have been obtained on alloyed ...PtM/C or M-rich-core@Pt-rich-shell/C catalysts (where M is an early or late transition metal) in comparison to pure Pt/C, due to a combination of strain and ligand effects. However, the effect of the fine nanostructure of the nanomaterials on the ORR kinetics remains underinvestigated. Here, nanometer-sized PtNi/C electrocatalysts with low Ni content (∼15 atom %) but different nanostructures and different densities of grain boundary were synthesized: solid, hollow, or “sea sponge” PtNi/C nanoalloys, and solid Ni-core@Pt-shell/C nanoparticles. These nanostructures were characterized by transmission and scanning transmission electron microscopy, X-ray energy dispersive spectroscopy, synchrotron wide-angle X-ray scattering (WAXS), atomic absorption spectroscopy, and electrochemical techniques. Their electrocatalytic activities for the ORR were determined and structure–activity relationships established. The results showed the following: (i) The compression of the Pt lattice by ca. 15 atom % Ni provides mild ORR activity enhancement in comparison to pure Pt/C. (ii) Highly defective PtNi/C nanostructures feature up to 9.3-fold enhancement of the ORR specific activity over a commercial Pt/C material with similar crystallite size. (iii) The enhancement of the ORR kinetics can be ascribed to the presence of structural defects, as shown by two independent parameters: the microstrain determined from WAXS and the average COads electrooxidation potential (μ1 CO) determined from COads stripping measurements. This work indicates that, at fixed Ni content, ORR activity can be tuned by nanostructuring and suggests that targeting structural disorder is a promising approach to improve the electrocatalytic properties of mono- or bimetallic nanocatalysts.
Fe‐N‐C catalysts containing atomic FeNx sites are promising candidates as precious‐metal‐free catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. The durability of ...Fe‐N‐C catalysts in fuel cells has been extensively studied using accelerated stress tests (AST). Herein we reveal stronger degradation of the Fe‐N‐C structure and four‐times higher ORR activity loss when performing load cycling AST in O2‐ vs. Ar‐saturated pH 1 electrolyte. Raman spectroscopy results show carbon corrosion after AST in O2, even when cycling at low potentials, while no corrosion occurred after any load cycling AST in Ar. The load‐cycling AST in O2 leads to loss of a significant fraction of FeNx sites, as shown by energy dispersive X‐ray spectroscopy analyses, and to the formation of Fe oxides. The results support that the unexpected carbon corrosion occurring at such low potential in the presence of O2 is due to reactive oxygen species produced between H2O2 and Fe sites via Fenton reactions.
This corrosion: The stability of Fe‐N‐C electrocatalysts in fuel cells is often studied with accelerated stress tests (ASTs). A new AST performed in O2 not Ar shows reactive oxygen species (ROS) are produced from H2O2 (from the oxygen reduction reaction) and FeNx sites. They cause carbon corrosion and the loss of catalytic FeNx sites, which are transformed into Fe oxides.
Alkaline fuel cells and electrolyzers have attracted increasing attention from the electrochemical community, and one of their supposed advantages is their greater electrode material stability in ...comparison to their proton-exchange membrane analogues. However, the stability of the core materials of fuel cells and electrolyzers in an alkaline environment cannot be taken for granted and remains understudied so far. Herein, using in situ Fourier transform infrared spectroscopy (FTIR), identical-location transmission electron microscopy (IL-TEM), X-ray photoelectron spectroscopy (XPS), and COads stripping techniques, we provide physical and chemical evidence that Pt-based nanocatalysts catalyze the electrochemical corrosion of the carbon support (Vulcan XC72). This is due to more facile oxidation of oxygen-containing surface groups of the carbon support upon adsorption of hydroxyl groups on the Pt-based surface. The degradation mechanism is, to some extent, similar for other carbon-supported Pt-group-metal (PGM) electrocatalysts. We propose that the extent of degradation of PGM/C nanoparticles in alkaline electrolytes scales with the electrocatalyst’s activity to electrooxidize CO, thereby providing a marker of the material’s propensity for degradation in an alkaline environment.
Recent non-precious-metal catalysts (NPMCs) show promise to replace in the future platinum-based catalysts currently needed for the electroreduction of oxygen (ORR) in proton-exchange membrane fuel ...cells (PEMFCs). Among NPMCs, the most mature subclass of materials is prepared via the pyrolysis of metal (Fe and Co), nitrogen, and carbon precursors (labeled as metal–NC). Such materials often comprise different types of nitrogen groups and metal species, from atomically dispersed metal ions coordinated to nitrogen to metallic or metal–carbide particles, partially or completely embedded in graphene shells. While disentangling the different contributions of these species to the initial ORR activity of metal–NC catalysts with multidunous active sites is complex, following the fate of these different active sites during electrochemical aging is even more difficult. To shed light onto this, herein, six metal–NC catalysts were synthesized and characterized before/after aging with two different accelerated stress tests (AST) simulating PEMFC cathode operating conditions either in steady-state or transient conditions. The samples differed from each other by the nature of the metal (Fe or Co), the metal content, and the heating mode applied during pyrolysis. Catalysts featuring either only atomically dispersed metal-ion sites (metal–N x C y ) or only metal nanoparticles encapsulated in the carbon matrix (metal@N–C) were obtained after pyrolysis of catalyst precursors containing 0.5 or 5.0 wt % of metal, respectively. All six catalysts showed high beginning-of-life ORR mass activity, but the ASTs revealed marked differences in their ORR activity at end-of-life. After the load-cycling AST (10000 cycles), metal–NC catalysts with metal–N x C y sites retained most of their initial activity at 0.8 V (60–100%), while those with metal@N–C particles retained only a small fraction of initial activity (10–20%). Metal–NC catalysts with metal–N x C y sites lost only 25% of their initial ORR activity after 30000 load cycles at 80 °C, thereby reaching the 2020 stability target defined by US Department of Energy. After 10000 start-up/shut-down cycles, no catalyst showed measurable ORR activity at 0.8 V. However, after 1000 start-up/shut-down cycles, most of the metal–NC catalysts initially comprising metal–N x C y sites showed measurable ORR activity at 0.8 V, while those initially comprising metal@N–C particles did not. Energy-dispersive X-ray spectroscopy and Raman spectroscopy measurements of the cycled rotating disk electrodes revealed that demetalation of the catalytic centers and corrosion of the carbon matrix are the main causes of ORR activity decay during load-cycling and start-up/shut-down cycling, respectively. In contrast to what could have been intuitively expected, the metal–N x C y sites are more robust to both demetalation and carbon corrosion than metal@N–C sites.
The impact of the carbon structure, the aging protocol, and the gas atmosphere on the degradation of Pt/C electrocatalysts were studied by electrochemical and spectroscopic methods. Pt ...nanocrystallites loaded onto high-surface area carbon (HSAC), Vulcan XC72, or reinforced-graphite (RG) with identical Pt weight fraction (40 wt %) were submitted to two accelerated stress test (AST) protocols from the Fuel Cell Commercialization Conference of Japan (FCCJ) mimicking load-cycling or start-up/shutdown events in a proton-exchange membrane fuel cell (PEMFC). The load-cycling protocol essentially caused dissolution/redeposition and migration/aggregation/coalescence of the Pt nanocrystallites but led to similar electrochemically active surface area (ECSA) losses for the three Pt/C electrocatalysts. This suggests that the nature of the carbon support plays a minor role in the potential range 0.60 < E < 1.0 V versus RHE. In contrast, the carbon support was strongly corroded under the start-up/shutdown protocol (1.0 < E < 1.5 V versus RHE), resulting in pronounced detachment of the Pt nanocrystallites and massive ECSA losses. Raman spectroscopy and differential electrochemical mass spectrometry were used to shed light on the underlying corrosion mechanisms of structurally ordered and disordered carbon supports in this potential region. Although for Pt/HSAC the start-up/shutdown protocol resulted into preferential oxidation of the more disorganized domains of the carbon support, new structural defects were generated at quasi-graphitic crystallites for Pt/RG. Pt/Vulcan represented an intermediate case. Finally, we show that oxygen affects the surface chemistry of the carbon supports but negligibly influences the ECSA losses for both aging protocols.
Results from Pt model catalyst surfaces have demonstrated that surface defects, in particular surface concavities, can improve the oxygen reduction reaction (ORR) kinetics. It is, however, a ...challenging task to synthesize nanostructured catalysts with such defective surfaces. Hence, we present a one-step and upscalable top-down approach to produce a Pt/C catalyst (with ∼3 nm Pt nanoparticle diameter). Using high-resolution transmission electron microscopy and tomography, electrochemical techniques, high-energy X-ray measurements, and positron annihilation spectroscopy, we provide evidence of a high density of surface defects (including surface concavities). The ORR activity of the developed catalyst exceeds that of a commercial Pt/C catalyst, at least 2.7 times in terms of specific activity (∼1.62 mA/cm2 Pt at 0.9 V vs the reversible hydrogen electrode) and at least 1.7 times in terms of mass activity (∼712 mA/mgPt), which can be correlated to the enhanced amount of surface defects. In addition, the technique used here reduces the complexity of the synthesis (and therefore production costs) in comparison to state of the art bottom-up techniques.
M-N-C electrocatalysts (where M is Fe or Co) have been investigated for mitigating the dependence on noble metals when catalyzing the oxygen reduction reaction (ORR) for fuel cell technologies in ...acidic or alkaline conditions. Rotating disk and rotating ring-disk electrode measurements for Fe-N-C and Co-N-C catalysts demonstrate promising performances and stability for the ORR, while the activity of main suspected active sites (M-N
x
C
y
and M@N-C) has been discussed on the basis of the known physical-chemical properties of the catalysts in acid and alkaline media. Thereupon, it is observed that atomically dispersed Fe-N
x
C
y
sites reach the highest ORR activity in acid media when amplified by an adequate energy binding between the metallic center and the oxygenated reaction intermediates. In contrast, Fe@N-C core-shell sites reach a maximum ORR mass activity in alkaline media through a synergistic effect involving catalyst particles with metallic iron in the core and nitrogen-doped carbon in the shell.
.
Graphical Abstract
The mechanisms of oxidation of glucose, gluconic acid, and sorbitol have been studied on gold, platinum, and palladium using cyclic voltammetry (CV), differential electrochemical mass spectrometry ...(DEMS), and in situ Fourier transform infrared (FTIR) spectroscopy. The nature of the reactant has a strong impact on the onset of the oxidation reaction. The anomeric function of glucose is oxidized at low potentials on the three surfaces, while gluconic acid and sorbitol poison the surface at low potentials. In addition, the nature of the metal surface leads to different reaction pathways. It is proposed that the oxidation of glucose initiates via the partial dissociative adsorption of glucose into glucose adsorbates and adsorbed H (Had) for the three metal surfaces. These adsorbates are partially combined into H2 on Au and oxidized into water on Pt and Pd. In addition, Au features the best activity, selectivity, and specificity for glucose oxidation into gluconate at low potentials. The study points out a reactant, catalyst, and potential dependent mechanism.
The electrochemical oxidation of carbon is a pivotal problem for low-temperature electrochemical generators, among which are proton-exchange membrane fuel cells (PEMFCs), and (non)aqueous-electrolyte ...Li–air batteries. In this contribution, the structure-sensitivity of the electrochemical corrosion of high-surface area carbon (HSAC) used to support catalytic materials in PEMFC electrodes is investigated in model (liquid electrolyte, 96 h potentiostatic holds at different electrode potentials ranging from 0.40 to 1.40 V at T = 330 K) and real PEMFC operating conditions (solid polymer electrolyte, 12,860 h of operation at constant current). Characterizations from Raman spectroscopy demonstrate that the disordered domains of HSAC supports (amorphous carbon and defective graphite crystallites) are preferentially oxidized at voltages related to the PEMFC cathode (0.40 < E < 1.00 V). Excursions to high electrode potential E > 1.00 V, witnessed during start-up and shut-down of PEMFC systems, accelerate this phenomenon and propagate the electrochemical oxidation to the graphitic domains of the HSAC. Thanks to X-ray photoelectron spectroscopy, a better understanding of the relationships existing between structural changes and carbon surface oxides coverage is also emerging for the first time.
PDF
