Degradation Of Cell Performance Research Articles

Oxidation of nickel in solid oxide cells during electrochemical operation: Experimental evidence, theoretical analysis, and an alternative hypothesis on the nickel migration

Solid oxide cells with nickel/yttrium-stabilized zirconia (Ni/YSZ) cermet electrodes exhibit Ni migration which can cause severe cell performance degradation. The experimentally reported migration behavior of Ni is complicated, and the mechanisms remain under debate. This work discusses the possible mechanism of Ni migration related to the oxidation of Ni at the Ni-electrolyte interfaces under polarization via combined experimental study and theoretical analysis. In the experiments, NiO is found at the Ni-YSZ interfaces in the active layer in both tested fuel cells and electrolyzer cells, despite that the nominal oxygen partial pressure at the hydrogen electrode is well below the thermodynamic threshold for Ni oxidation. Due to the volume expansion during Ni oxidation and the outward diffusion nature of NiO growth, Ni oxidation and reduction of NiO back to Ni can cause Ni relocation. Thermodynamic analysis shows that the oxygen partial pressure near the Ni-electrolyte interface can be significantly higher than the hydrogen electrode under polarization, which can cause Ni oxidation and concentration increase of the gaseous Ni(OH)2, and the latter accelerates the transport of Ni. As such, a new hypothesis for Ni migration in solid oxide cells is proposed in which the interfacial oxidation of Ni plays an essential role.

Numerical analysis of the electrochemical dissolution of iridium catalyst and evaluation of its effect on the performance of polymer electrolyte membrane water electrolyzers

A physicochemical model of a water electrolyzer with a polymer electrolyte membrane (PEM) was developed, taking into account the electrochemical dissolution of an anodic iridium catalyst. The dependencies of the rates of iridium loss and electrolysis voltage increase upon the current density were calculated in order to analyze the effect of the iridium dissolution on degradation of the electrolysis cell (EC) performance. As an estimated characteristic of the techno-economic costs of the electrolysis process, the amount of iridium loss from the anode catalyst layer (as a result of electrochemical dissolution) in the course of the generation of 1 kg of hydrogen was calculated. Data were analyzed and a number of regularities of the iridium dissolution and its influence on the rate of degradation of the EC performance were found. In particular, the most efficient ECs in terms of electrolysis voltage (energy consumption for gas production) are, simultaneously, the most unstable (prone to performance degradation) in relation to the iridium dissolution process. An aim of current requirements for water electrolyzers includes reducing the specific consumption of iridium required for hydrogen generation.

Suppressed Voltage Deficit and Degradation of Perovskite Solar Cells by Regulating the Mineralization of Lead Iodide.

Both the uncoordinated Pb2+ and excess PbI2 in perovskite film will create defects and perturb carrier collection, thus leading to the open-circuit voltage (VOC ) loss and inducing rapid performance degradation of perovskite solar cells (PSCs). Herein, an additive of 3-aminothiophene-2-carboxamide (3-AzTca) that contains amide and amino and features a large molecular size is introduced to improve the quality of perovskite film. The interplay of size effect and adequate bonding strength between 3-AzTca and uncoordinated Pb2+ regulates the mineralization of PbI2 and generates low-dimensional PbI2 phase, thereby boosting the crystallization of perovskite. The decreased defect states result in suppressed nonradiative recombination and reduced VOC loss. The power conversion efficiency (PCE) of modified PSC is improved to 22.79% with a high VOC of 1.22V. Moreover, the decomposition of PbI2 and perovskite films is also retarded, yielding enhanced device stability. This study provides an effective method to minimize the concentration of uncoordinated Pb2+ and improve the PCE and stability of PSCs.

Depth-dependent understanding of the degradation mechanisms for typical LiNi0.5Co0.2Mn0.3O2/artificial graphite lithium-ion batteries upon high-temperature storage

With the expanding application fields of lithium-ion batteries (LIBs), some LIBs may experience periods of higher-temperature storage, e.g., in electric vehicles (EVs) parked under sunlight in hot summers. However, reports on cell behaviors and performance degradation mechanisms during high-temperature storage have rarely been released. Thus, there are still issues requiring in-depth investigation, including the (i) effects of operating conditions on cell performance, (ii) reasons for cell swelling, (iii) substances of generated solid byproducts and gases, and (iv) causes for reversible and irreversible capacity losses. To answer these questions, the performances of ∼2 Ah LiNi0.5Co0.2Mn0.3O2/artificial graphite pouch cells stored at various temperatures and voltages are studied. Interestingly, the cell is more sensitive to temperature than voltage. After storage at 4.4 V and 60 °C for 20 days, the cell exhibits a severe voltage drop and swelling, low capacity retention and recovery, and high impedance. The mechanisms are understood through advanced characterizations. The results indicate that early cell degradation is caused by a series of chain reactions involved in electrolyte and electrode/electrolyte interphase film decomposition and structural evolution in the outermost regions of the electrodes. We believe that these findings provide a theoretical basis for developing high-temperature resistant materials and LIBs.

Investigation of an electrode reversal method and degradation recovery mechanisms of PEM fuel cell

Considering the high initial cost of proton exchange membrane fuel cells, improving their durability is significant for their commercialization. This work proposes a performance recovery strategy by reversing the cathode and anode (“cathode/anode reversal”) and then operating under a high current density load. It is systematic to investigate the mechanisms of performance recovery due to recovery strategy and the mechanisms of irreversible degradation of cell performance under a dynamic load cycling condition. The experimental results show that the cell performance is significantly improved after implementing the recovery strategy, with recovery rate reaching 54%@800 mA cm−2. The characterization test results show that the mechanisms of performance recovery are mainly the reduction of platinum oxide, the desorption/removal of sulfate, and the removal of accumulated water. The main mechanisms of irreversible performance attenuation are the coarsening of Pt particles, the formation of Pt band, carbon support corrosion, and CL mechanical degradation. The recovery strategy, recovery effect, recovery mechanisms, and irreversible degradation mechanisms that are discussed in this study provide a reference for designing recovery strategies and improving electrode materials and structures.

A novel domino-like snow removal system for roof PV arrays: Feasibility, performance, and economic benefits

Snow accumulation on PV modules hinders normal power generation which is a challenge for the wide-scope application of photovoltaics in snowy regions. In this paper, a domino-like snow removal system (DSRS) based on photovoltaics self-heating (PVSH) was designed and investigated to overcome this application challenge. The domino-like snow removal strategy is first proposed, whose core idea is to use the energy from uncovered PV modules to accomplish snow removal in PV systems string by string. The process is almost free of utility power consumption and is suitable for both off-grid and grid-connected PV systems. The principle of PVSH is explained and its long-term effect on PV modules was first verified by comprehensive experiments. The entire process of snow removal is physically modeled. A novel circuit topology and control flow were designed to implement the domino-like snow removal strategy. The 1st generation prototype of the DSRS was manufactured and two-year outdoor experiments were conducted to validate the snow removal performance. Furthermore, the economic benefits of DSRS were analyzed through detailed installation cost calculations and conservative snow effect projections. The experimental results demonstrate that long-term PVSH does not contribute to the performance degradation of PV cells. The DSRS shows high snow removal efficiency with extremely low power consumption compared to similar methods, which has been tested to have outstanding adaptability in handling different types of snow under complex climatic conditions. In addition to a convenient and low-cost installation, the payback period of the DSRS varies between 2.99 and 4.3 years according to the scale of PV systems. So the 25-year return on investment (ROI) of DSRS can reach up to 857.2 %. This paper presents a systematic work around the feasibility, performance, and economic benefits of the domino-like snow removal system and confirms it is an excellent solution to removing snow on PV modules and has great potential to promote PV deployment where the snow covers for a few months in winter.

Comprehensive Analysis of Solid Oxide Fuel Cell Performance Degradation Mechanism, Prediction, and Optimization Studies

Solid oxide fuel cell (SOFC) performance degradation analysis and optimization studies are important prerequisites for its commercialization. Reviewing and summarizing SOFC performance degradation studies can help researchers identify research gaps and increase investment in weak areas. In this study, to help researchers purposely improve system performance, degradation mechanism analysis, degradation performance prediction, and degradation performance optimization studies are sorted out. In the review, it is found that the degradation mechanism analysis studies can help to improve the system structure. Degradation mechanism analysis studies can be performed at the stack level and system level, respectively. Degradation performance prediction can help to take measures to mitigate degradation in advance. The main tools of prediction study can be divided into model-based, data-based, electrochemical impedance spectroscopy-based, and image-based approaches. Degradation performance optimization can improve the system performance based on degradation mechanism analysis and performance prediction results. The optimization study focuses on two aspects of constitutive improvement and health controller design. However, the existing research is not yet complete. In-depth studies on performance degradation are still needed to achieve further SOFC commercialization. This paper summarizes mainstream research methods, as well as deficiencies that can provide partial theoretical guidance for SOFC performance enhancement.

Degradation Analysis by X-ray Absorption Spectroscopy for LiNbO3 Coating of Sulfide-Based All-Solid-State Battery Cathode.

The surface coating of cathode active material in all-solid-state batteries using sulfide-based solid electrolytes is well-known to be a fundamental technology, and LiNbO3 is one of the most representative materials. The half cells using the cathode mixture of Li6PS5Cl/LiNbO3-coated LiNi0.5Co0.2Mn0.3O2 were exposed in harsh conditions at 60 °C and 4.25-4.55 V vs Li/Li+ and analyzed by transmission electron microscope/energy dispersive X-ray spectroscopy (TEM/EDS) and X-ray absorption spectroscopy (XAS). TEM/EDS observation shows that Nb element derived from LiNbO3 coating had remained at the interface, which means that Nb element had not migrated to the solid electrolyte and active material. On the other hand, the XAS spectra of Nb L3-edge changed corresponding to cell performance degradation. From the comparison with the spectra of the reference materials of the Li-Nb-O system, the XAS spectral changes were assigned to the decomposition reaction which released Li and O from the LiNbO3 coating. The side reaction is presumed to cause to the oxidization deterioration of sulfide electrolyte at the interface of Li6PS5Cl/LiNbO3-coated LiNi0.5Co0.2Mn0.3O2.

Investigation of the influence of Pt/C percentage and humidity on the voltage decay rate of proton exchange membrane fuel cell

AbstractVoltage degradation is the main parameter that determines the effective usable life of a fuel cell, here the influence of Pt/C% and relative humidity (RH%) on the voltage decay rate of a fuel cell is experimentally evaluated and reported. This study implements a stress test with frequent interrupts of purging for determining the durability of the fuel cells. In the course of the 1456 h stress test for each membrane electrode assembly (MEA), the polarization curve and electrochemical impedance spectroscopy (EIS) were measured. The experimental results make it evident that the lowest voltage degradation was 6 µV/h for MEA with 40% Pt/C tested under 70% RH while the highest was 183 µV/h recorded for MEA with 20% Pt/C tested under 90% RH. From EIS results, the ohmic resistance increased for all the tested MEAs, which is also reflected in the performance degradation. Field emission scanning electron microscopy images also indicate the delamination of MEA layers which in turn increases the electron transfer resistance. So, the decisive factors for the fuel cell performance degradation were flooding, catalyst surface contamination, and delamination of MEA layers due to repeated mechanical and chemical stresses.

A novel water content sensor design and its application for in-situ water content measurement inside the flow channel of PEMFC

In-situ water content detection in proton exchange fuel cell (PEMFC) is of great importance to its water management. In this study, flexible thin film water content microsensor (WCMS) were fabricated and deployed for real-time measurement of water distribution inside the channel of proton exchange membrane fuel cells. The effect of inserting WCMS on the cell performance was confirmed to be negligible. Besides, three WCMS were used to monitor the variation of water distribution in the upstream, midstream, and downstream of the flow channel, respectively. The experimental results show that the WCMS can efficiently detect the change of water content inside the fuel cell in real time. The co-occurrence phenomenon of water column clogging in the flow channel and degradation of cell performance were observed. This work provides a novel design of WCMS and its application on the water content distribution inside flow channel of fuel cells.

Adaptive optimization strategy of air supply for automotive polymer electrolyte membrane fuel cell in life cycle

• Adaptive optimization matching method of the PEMFC system is presented. • The adaptive model of degradation stack can be universally applied in life cycle. • Validation is completed on experimental data of commercialized fuel cell engine. • Effects of air compressor parameters on the system efficiency are investigated. • System efficiency could be improved above 5% by adaptive optimization method. In this study, an adaptive optimization matching method of the air supply is developed to maintain the high-efficiency operation of the automotive polymer electrolyte membrane fuel cell (PEMFC) system in the life cycle. A 1-D non-isothermal model of the PEMFC stack with 150 kW designed power and a centrifugal air compressor model are developed, considering the fuel cell performance degradation. The genetic algorithm (GA) is used to optimize the overall system efficiency under various output powers to achieve adaptive matching. The 1-D stack model is validated with the experimental test results at two states (before and after 800 h degradation), considering the effect of degradation on the matching strategies. Through the optimization method, the centrifugal air compressor is adaptively matched with the stack of the proposed two states to develop the compressor matching strategies under various stack conditions individually. It is found that the efficiency of the system with this optimized method is 3.8% higher than that of the system without an optimized method under the full system power range. In addition, the new matching strategy between the air compressor and the stack after degradation is exploited by the adaptive optimization method. With the help of this method, the efficiencies of the system and the stack are 5.7% and 2.9% higher than that of the matching strategy without adaptive updating. It is shown that this adaptive optimization method not only improves the output efficiency of the stack but also reduces the additional parasitic power consumed by the compressor.

CFD simulation of hot spot in PEM fuel cell with diverging and converging flow channels

Hot spot formation is one of the most critical factors contributing to the membrane layer deterioration and, hence, fuel cell performance degradation. When local hot spot patches develop, the long-term endurance of a PEM fuel cell catalyst and membrane layer is dramatically reduced. The reactive flow gas distribution inside a PEM fuel cell with a 3-D gas diffusion layer has been investigated using divergent and convergent gas flow channels. COMSOL Multiphysics 4.4a was used for simulation studies. A three-dimensional gas flow channel for a single-phase, a non-isothermal simulation model of cathode side PEM fuel cells was created to investigate the impact of the local hot spot patch on the cathode side PEM fuel cells. The effect of local hot spot patches on the catalyst and membrane layer is investigated in this work, using a transient-state modeling approach. The conclusions have important implications for evaluating the internal reaction uniformity in the PEM fuel cell using divergent and convergent flow channels and developing a theoretical basis for evaluating local hot spot patches on the catalyst and membrane layer surface. Maximum surface temperature of the divergent flow channel was found to be 4% lower than that of the convergent flow channel. The studies aim to examine the possibility of altered variation of local hot spots on the catalyst and membrane layer due to changes in depth-dependent flow fields.

Towards Uncoated Stainless-Steel Bipolar Plates in Automotive PEM Fuel Cells

Bipolar plates (BPPs) have been identified as a key component in improving the economic viability of proton exchange membrane fuel cells (PEMFCs), as they contribute approximately 80 % of weight and 30 % of total cost in a fuel cell stack [1]. Stainless steel is considered an excellent material for mass-producing BPPs with complex flow field structures in a cost-effective manner. However, the perceived risk of corrosion and subsequent metal ion contamination as well as the inherently high contact resistance of stainless-steel surfaces have compelled manufacturers to add protective coatings to BPPs.In this study, we show new insights into the actual metal ion dissolution mechanisms in operating fuel cell systems and present experimental results which demonstrate that stainless-steel bipolar plates can be used in real life fuel cell applications without protective coatings. The bipolar plates were evaluated, in an operating fuel cell, using a dynamic load cycling protocol based on the New European Drive Cycle with increased humidity levels which simulates realistic conditions known to facilitate metal ion dissolution mechanisms [2]. The post analysis showed no signs of surface dissolution on any of the tested bipolar plates which had been subjected to the endurance/stress hybrid tests. Furthermore, only trace amounts of critical metals were found in the MEAs after testing, i.e., an order of magnitude below the radical scavenger (Mn) concentration. The trace metal contamination was comparable to a reference test without bipolar plates (graphite current collector), rendering metal input from the balance of plant the most likely source of contamination. The observable changes in cell performance and voltage degradation was unrelated to the presence of bipolar plates. A selection of experimental data is shown in Figure 1.As a theoretical explanation for the apparent stability of bare stainless-steel plates, three inter-linked phenomena were identified, akin to a three line of defence model [3]. The first and arguably most important of these phenomena is ionic decoupling between electrode and bipolar plate surface. The gas diffusion layer (GDL) strongly inhibits transport of ions from the electrode to the bipolar plate, which leads to a largely unpolarized bipolar plate and a stable surface potential even during load changes of the fuel cell. These stable potentials at the anode and cathode bipolar plates are only influenced by the immediate surroundings of the plate surfaces and are generally much milder than the encountered potentials at the electrodes. Secondly, metal dissolution from the surface of stainless steel follows a passive-transpassive-passive pattern. This mechanism describes a dissolution process, where a change in conditions at the surface, which destabilizes a previously established passive film, will only lead to limited amounts of (transpassive) metal dissolution, until a new stable passive film of different structure establishes itself. Finally, an operating regime which mitigates start/stop effects and hydrogen starvation during load changes will ensure very stable conditions at the bipolar plates’ surfaces, which strongly mitigates the risk of transpassive dissolution. As a final demonstration of the industrial feasibility of uncoated stainless-steel plates, an experimental alloy was tested under the same conditions. Results showed similar resistance to metal dissolution paired with low contact resistance at the level of C-coated or graphite plates. These results help to legitimize the efforts to industrialize uncoated stainless-steel plates since only the contact resistance challenge remains.[1] B. Pollet, S. Kocha, I. Staffell, ”Current status of automotive fuel cells for sustainable transport,” Current Opinion in Electrochemistry, vol. 16, pp. 90-95, 2019[2] T. Novalin, B. Eriksson, S. Proch, U. Bexell, C. Moffatt, J. Westlinder, C. Lagergren, G. Lindbergh, R. Wreland Lindström, “Trace-metal contamination in proton exchange membrane fuel cells caused by laser-cutting stains on carbon-coated metallic bipolar plates,” Int. J. Hydrogen Energy, Volume 46, Issue 26, 2021[3] T. Novalin, B. Eriksson, S. Proch, U. Bexell, C. Moffatt, J. Westlinder, C. Lagergren, G. Lindbergh, R. Wreland Lindström, “Concepts for preventing metal dissolution from stainless-steel bipolar plates in PEM fuel cells,” Energy Conversion and Management, 2022 Figure 1

Polymer Electrolyte Fuel Cell Degradation Investigations Using X-Ray Computed Tomography

On-going research is aimed at commercializing low temperature fuel cell technology systems as zero emission alternatives for automotive applications in order to reduce greenhouse gas emissions and air pollution. These systems use polymer electrolyte fuel cells (PEFCs) to generate electricity via an electrochemical process using hydrogen and ambient air (oxygen) to produce water. Advantages of PEFCs include quick start-up time, low operating temperature, low weight, high efficiency, and relatively simple design. An important area of current research is the identification of factors which affect fuel cell performance degradation during operation and ultimately, its durability. A technique that has yielded new insights in the investigation and identification of failure modes in fuel cells is lab-based X-ray computed tomography (XCT), which is most advantageous due to its on-demand availability. The Fuel Cell Research Laboratory (FCReL) at Simon Fraser University currently operates Canada’s only facility for multi-length scale XCT, comprising of two state-of-the-art laboratory-based XCT scanners from Carl Zeiss X-ray Microscopy (Zeiss Xradia 520 Versa and 810 Ultra) with complementary resolution and field of view capabilities. This unique combination offers unprecedented access to investigations at multi length scales; with an ability to probe fuel cell components at the micro as well as the nano scale. Figure 1 illustrates the fuel cell holder and its orientation with respect to the X-ray beam as well as an exploded view of the miniature fuel cell design.An overview of recent fuel cell degradation investigations at FCReL using the XCT technique will be shown. The XCT based workflow facilitates determination and quantification of material structure and properties changes resulting from degradation stresses associated with operational parameters such as temperature, relative humidity, and voltage. The non-destructive nature of lab-based XCT visualization coupled with the ability to scan the same fuel cell multiple times without inducing damage [1] has enabled detailed studies of fuel cell degradation evolution in four dimensions (3D space, 1D time) [2 - 4]. The new knowledge gained from this procedure has led to root cause identification with respect to membrane and catalyst layer crack initiation and propagation [5 - 7], sealing issues [8,9], and subsequent mitigation toward enhanced fuel cell durability. Acknowledgement This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Western Economic Diversification Canada, Canada Research Chairs, and Ballard Power Systems.

Rechargeable Silicon Redox Batteries

AbstractDespite its high abundance and ease of production, the possibility of using silicon as an active multivalent rechargeable anode has never been explored, until now. As a proof of concept, a novel rechargeable silicon cell, its design and architecture are reported, enabling Si to be reversibly discharged at 1.1 V and charged at 1.5 V. It is proven that during the discharge process silicon is dissolved and upon charging, elemental silicon is deposited. Several discharge–charge cycles are achieved, utilizing heavy doped n‐type Si wafer anodes and hybrid‐based ionic liquid electrolytes, along with dissolved halides, functioning as conversion cathodes. Cell characterizations correlate the discharge and charge processes with the presence of dissolved fluoride and Si species in the electrolyte. The presence of appreciable fluoride species is crucial in both establishing surface activation and battery operation, and their depletion is directly linked to cell performance degradation. Quantum‐chemical calculations based on density functional theory in the framework of a continuum model for solvation in ionic liquids are performed to shed light on Si ion deposition mechanism.

Efficient Perovskite Solar Cells with Enhanced Thermal Stability by Sulfide Treatment

The performance degradation of perovskite solar cells (PSCs) under harsh environment (e.g., heat, moisture, light) is one of the greatest challenges for their commercialization. Herein, a conjugated sulfide 2-mercaptobenzimidazole (2MBI) is applied to significantly improve the photovoltaic properties and thermal stability of PSCs. When treated with heat, 2MBI cross-links with each other on the perovskite surface to facilitate charge transportation, suppress the escape of volatile species, and guide the rearrangement of surface perovskite grains. PSCs with 2MBI modification reach a PCE as high as 21.7% and maintain high-efficiency output during and after thermodestruction at 85 °C, while the unmodified ones suffer severe degradation. Unencapsulated devices after thermodestruction achieve over 98% of initial efficiency after 40-day storage under ambient conditions.

In-situ analysis of anode atmosphere in a flat-tube solid oxide fuel cell operated with dry reforming of methane

In-situ characterization of solid oxide fuel cells (SOFCs) under operating conditions is of great significance to mechanism studies on performance or long-term stability. In this study, real-time visualization of the two-dimensional gas distribution inside a large flat-tube SOFC is realized by built-in gas sampling tubes combined with external gas chromatography, validated by simulating calculation. The gas composition inside anode of the cell fueled with CH4/CO2 is in-situ monitored during a 1000-h discharge test. Decrease in catalytic activity for methane dry reforming during the long-term test differs at different region of the cell, and is related with local carbon deposition and sintering of Ni particles. Based on these experimental results, the mechanism of cell performance degradation of the large flat-tube SOFCs under methane dry reforming conditions is discussed.

The Diffusion Effect of Copper on the Flexible GaInP/GaAs Solar Cells

Electroplating copper is widely used in the fabrication of flexible III-V multijunction solar cell because of its excellent characteristics of lightness, flexibility and conductivity of thin Cu, which can be directly applied as the flexible support and the external electrode. In this letter, we studied the diffusion effect of copper in the flexible GaInP/GaAs solar cells under high temperature annealing, which plays an essential role in achieving a promising ohmic contact between contact layer and metal. However, the degradation in the performance of flexible cell was observed when the annealing procedure was employed at the last step of the device fabricating process. The secondary ion mass spectrometry and electroluminescence results have indicated that the degradation of the solar cell performance after high-temperature annealing is attributed to the excess copper diffusion into active region of the solar cell. High temperature diffusion of copper can be solved by prior-annealing to form ohmic contact.

Model of catalyst layers for proton exchange membrane fuel cells: Progress and perspective

<p indent="0mm">The proton exchange membrane fuel cell (PEMFC) is one of the most high-efficient energy conversion devices with zero carbon emission, which is of great significance to achieve the strategic goal of “carbon emission peak and carbon neutrality”. The catalyst directly determines the cell performance and is the core in the large-scale commercial application of PEMFCs. Due to the complex multi-physics and multi-scale process in catalyst layers, it is necessary to establish an accurate numerical model to reveal the internal mechanisms of transport and reactions. The present work reviews the progress of modeling catalyst layers, and the models can be divided into five types: Interface model, homogeneous model, agglomerate model, pore-scale model and particle model. The modeling ideas are introduced, and their typical applications are discussed. The interface model assumes that the catalyst layer is infinitely thin, so it is simple to set up and fast to calculate. It can be used to study the reaction process in fuel cells except the catalyst layer. However, due to the excessively simple assumption, the simulation under high current densities disagrees with the experiment result, which limits the application of the model. The homogeneous model assumes uniform distribution of substances in the catalyst layer. This model completely simulates the reaction and mass transfer process in the cell and has the ability to guide the cell optimization. However, the homogeneity hypothesis does not conform to the image of the catalyst layer observed in the experiment, so the model is not suitable for the study of the catalyst layer with high precision. The agglomerate model assumes that the substances in the catalyst layer agglomerate into a specific geometric shape. There are more parameters about catalyst layers in the agglomerate model, so it can be used to optimize the structure of the catalyst layer. However, some parameters are difficult to be measured experimentally and can only be assumed, which makes the model less scientific. In addition, it also takes the homogeneity hypothesis. The pore-scale model reconstructs the complex structure of the catalyst layer. It has the highest accuracy in theory but lacks perfect reconstruction methods. Regular reconstruction, stochastic reconstruction and process-based reconstruction cannot reflect the real structure characteristics of catalyst layers. Image-based reconstructions and statistical function-based reconstructions are limited by the image resolution and experimental error. Therefore, the pore-scale model is not a perfect model for catalyst layers. The particle model can accurately simulate microcosmic processes, but lacks the ability to combine with macroscopic simulations. So, the catalyst layer model still needs to be further developed. It is suggested to develop advanced experiment methods to ensure the accuracy of catalyst layer model parameters and verify the prediction result of the model. In addition, more complete catalyst layer model with multi-scale and multi-physics should be further developed, incorporating microscopic model results into macroscopic models, thus to fully understand the influence of the microscopic process of catalyst layers on the cell performance. It is also suggested that more research on low platinum, non-platinum catalysts as well as cell performance degradation should be carried out and new models considering their different reaction process and degradation mechanisms should be developed in the future. Moreover, similarity analysis approach is suggested to be adopted for cell models, which can reveal output characteristics and save cell optimization time.

Comprehensive characterization of efficiency limiting defects in the swirl-shaped region of Czochralski silicon

For Czochralski silicon (Cz-Si) solar cells, swirl-shaped regions in silicon wafers could lead to efficiency degradation, usually accompanied by hot spots and thermal breakdown. In this paper, comprehensive characterization methods including electroluminescence (EL), photo-induced current (LBIC), quantum efficiency (QE), cell efficiency, carrier lifetime, scanning electron microscope (SEM), Fourier Transform Infrared Spectroscopy (FTIR), and deep level transient spectroscopy (DLTS) were used to investigate the formation and properties of the Swirl defects. It is found that there exists a strong correlation between the as-grown swirl defects and the degradation in cell performance. The Swirl defects cause an energy level of E v + 0.33 eV in silicon bandgap, with the capture cross section of 2.3 × 10 −15 cm 2 , which is consistent with the feature of silicon/oxide interface states. The swirl defects can further be evolved into stacking faults after annealing at 1050 °C for 16 h, and cause an additional energy level of E v + 0.42 eV with the capture cross section is 1.0 × 10 −14 cm 2 . These results suggest that oxygen precipitates are the vital component or precursor for the formation of swirl-shaped regions in Cz silicon wafers. • A comprehensive study of the nature of Swirl defects in the photovoltaic Czochralski silicon is obtained. • The direct evidence that Swirl defects are related to oxygen precipitates is provided by deep level transient spectroscopy (DLTS) measurements. • The influence of Swirl defects on solar cell performances and material properties is characterized.