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•The charge process of Zn-air batteries is investigated by in-situ characterization.•The gas evolution behavior during charging can be divided into three stages.•The ideal ...“self-clean” state is found at the low current and high catalyst loading.
Although tremendous efforts are made in developing stable Zn electrodes and bifunctional catalysts in rechargeable Zn–air batteries, the charging process is few reported. Herein, an in-depth investigation into the gas evolution on the air electrode is conducted by in-situ characterization. It is found that the charging process can be divided into three stages: no obvious bubbles, small bubbles owing to the oxygen evolution, and large bubbles owing to oxygen evolution and carbon corrosion. The post analyses illustrate that the severe bubble formation can deteriorate the electrochemical performance of the air electrode. With the catalyst loading increases, the bubbles on the surface become smaller and sparser. At low current densities, the air electrode can keep “self-clean” from bubbles, which is the ideal state for the charge. Whereas with an increase of the current density, the large-bubble stage is enlarged, and more bubbles are attached to the surface, leading to extra charge impedance. As the extent of discharge/charge reduces, the three stages disappear, and only small bubbles can be found on the surface, which is favourable for the stable operation. This work provides a profound understanding of the charging behaviors on the air electrode, facilitating the development of high-performance Zn-air and other metal-air batteries.
Proton exchange membrane fuel cell (PEMFC) has been used in supplying power for Unmanned Underwater Vehicle which operate in a closed environment and dead-ended anode and cathode (DEAC) mode is ...deemed as an effective way to enhance the fuel utilization rate. Catalyst is an important factor that influences the performance and durability of PEMFC, especially in DEAC mode. In this paper, the degradation characteristics of PEMFCs with Pt black and Pt/C catalyst after 100 h operation have been investigated by electrochemical techniques and morphological characterization methods. It's shown that the degradation of Pt black catalyst layer (CL) was more severe than that of Pt/C CL. The difference of performance degradation is due to the dominant decay mechanism of these two catalysts is different. According to SEM, TEM and XPS results, the decay of Pt black catalyst is mainly caused by Pt agglomeration and oxidation, causing a higher ohmic resistance, higher mass transfer resistance and severer degradation of performance. The degradation of Pt/C catalyst is mainly due to the reduction of electrochemical surface area and carbon corrosion because the larger carbon corrosion makes micropores and the thicker supporting structure, resulting in the performance degradation.
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•Pt/C catalyst shows better durability than that of Pt black under dead-ended anode and cathode mode.•Severe catalyst degradation occurs at the outlet of cathode.•The decay of Pt/C catalyst was mainly resulted from carbon corrosion.•The decay of Pt black catalyst was mainly caused by the migration and dissolution of Pt particles.
This paper presents an overview of the causes and consequences of the carbon corrosion mechanism and summarizes the corresponding mitigation strategies for various operating conditions. The high ...potential at the cathode caused by the anode hydrogen-oxygen interface leads to the occurrence of carbon corrosion reactions during the startup and shutdown process. System strategies, including gas purge and auxiliary load, have been developed to alleviate the performance decay during startup and shutdown processes. Improper water management will cause local flooding to accelerate the carbon corrosion. Many novel flow fields have been proposed to solve water management problems, such as an optimized 3D flow field, baffle flow field and porous media flow field. The carbon corrosion under normal operation is not as serious as that under start-up and flooding, but it is serious enough to affect the later performance of the PEMFC after long-term operation. Varieties of new supports, such as mesoporous carbon, graphitized carbon, carbon nanotubes (CNTs), carbon nanofibers (CNF), and metal oxides, have been developed to improve the durability of the catalyst support. This review aims to provide a clear understanding of the carbon corrosion mechanisms, thereby helping researchers to prolong the lifetime of PEMFCs.
Carbon corrosion in three operating conditions. Display omitted
•An overview of carbon corrosion mechanism under three conditions was addressed.•Corresponding mitigation strategies for carbon corrosion were presented.•Future goals of mitigation strategies about carbon corrosion were suggested.
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 of in situ-Raman measurements are considerably affected by the procedure•In-depth evaluation is achieved by cluster analysis of post mortem-Raman mappings•Carbon corrosion at high potentials ...is amplified by simultaneous laser excitation•Laser-induced degradation also depends on the excitation energy of the laser
In situ-Raman spectro-electrochemistry is frequently used for the investigation of carbon electrode materials, contributing to our fundamental understanding of reaction pathways, surface intermediates or degradation processes. A challenge when using the in situ-Raman technique is to ensure that the measurement results are not influenced by the irradiation with the high-energy laser beam. Therefore, reference experiments with laser irradiation only are commonly carried out to verify that repetitive Raman probing does not affect the electrochemical and structural properties of the investigated electrode material. However, the impact of simultaneous exposure to electrochemical stress and laser excitation is barely considered. In this work, we elucidate these combined effects for in situ-Raman investigation of carbon nanotubes during polarization to high potentials. The results demonstrate that the structural degradation, which is indicated by an increasing D:G-ratio, is significantly intensified by the Raman measurement with a 532 nm laser when the electrode is polarized to potentials higher than 1.3 V vs. Ag|AgCl|KClsat. in strongly acidic environment at the same time. In-depth analysis by subsequent Raman mapping experiments further reveals that the structural degradation caused by high potentials is much more pronounced for the location of Raman investigation of the initial experiment. In addition, the impact of the excitation energy of the laser is examined, and it is indicated that the structural degradation is not amplified when a 785 nm laser excitation is used. These findings highlight the importance of carefully considering concomitant effects when using in situ-techniques.
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•Glassy carbon is not electrochemically inert.•At high anodic potentials oxidation of water and carbon electrodes occurs simultaneously.•Different corrosion behavior of glassy carbon ...in different pH electrolytes was observed.•Carbon oxidation in acidic media forms surface oxides.•Carbon in alkaline media dissolves during the electrochemical oxidation.
Glassy carbon is widely used in electrochemistry due to its properties of high temperature resistance, hardness, low density and low electrical resistance. The present study focuses on the chemical resistance under electrochemical oxidative conditions, which occur under oxygen-involving reactions like oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The electrochemical performance of glassy carbon investigated in alkaline, neutral and acidic media reveal the same chemical processes during the OER but showing different degradation mechanism.
The electrochemical signature of the corrosion in different media could be directly associated with the formation of oxygen functional groups determined by spectroscopic methods like Raman, infrared (IR) and x-ray photoelectron spectroscopy (XPS). The morphology change of the carbon surface caused by carbon oxidation was investigated by microscopy. A rough surface was obtained in the acidic case, whereas dents were seen in alkaline media.
It is assumed that the glassy carbon electrode in acidic media degrades by forming surface oxides by acid catalyzed process leading to ring opening in the graphitic structure and therefore oxidation in the bulk. In alkaline media OH radicals preferentially react with alkyl site chains, leading to oxidation of the edges of carbon layers until they become hydrophilic and dissolve.
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.
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.
•Coupling performance and carbon corrosion model is developed.•Effects of relative humidity and important catalyst layer parameters are explored.•Effects of gradient distributions of catalyst layer ...parameters are investigated.•Optimized structure is proposed with higher performance and stronger durability.
Understanding interaction between carbon corrosion and physicochemical processes inside proton exchange membrane fuel cells (PEMFCs) is important. A one-dimensional carbon corrosion model considering cathode catalyst layer (CCL) structure evolutions is established and coupled with one-dimensional PEMFC performance model. The coupled model is validated and employed to predict CCL structure evolution and PEMFC performance degradation during accelerated stress test. The results find that high relative humidity can enhance performance but accelerate carbon corrosion. Increasing carbon loading and decreasing ionomer weight can improve both performance and durability. Low Pt loading leads to reduced performance and durability. There exists an optimal carbon diameter leading to the best performance. Small carbon diameter can enhance cell durability. Proper gradient CCL design can enhance durability at cost of small initial performance reduction. Finally, an optimized structure is proposed with 21.7 % higher current density at 0.6 V and 5.2 % lower performance loss compared with base case.
Developing durable electrocatalyst for oxygen reduction reaction (ORR) is essential for fuel cell commercialization. Herein, we perform a study of platinum-carbon (Pt/C) degradation mechanisms using ...potential cycling of accelerated durability testing protocols in acidic and alkaline media. Physicochemical results indicate that carbon surface oxides are formed after high-potential cycling in acid causing an increase in the double-layer capacitance and severe ORR activity loss due to Pt poisoning. Whereas, low-potential cycling in acid shows less ORR activity loss, mainly caused by Pt Ostwald ripening, and does not lead to a significant change in double-layer capacitance. In alkaline, the Pt/C catalyst after high-potential cycling shows a decrease of double-layer capacitance over time because of carbon layer dissolution. TEM images reveal larger Pt agglomerates in alkaline, due to high Pt mobility. These findings provide new insights into the role of catalyst and carbon support interface in developing mitigation strategies for stable fuel cell operation.
•Dependence of carbon corrosion and Pt degradation on applied potential cycling.•Pt poisoning due to carbon surface oxides during high-potential cycling in acid.•High Pt mobility in alkaline leading to a formation of larger Pt agglomerates.•Carboxylate group generated in acid affects Pt and carbon interfacial interaction.
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