Anodic Operating Conditions Research Articles

Engineering Iridium-Based Oxygen Evolution Reaction Electrocatalysts for Proton Exchange Membrane Water Electrolyzers

The electrocatalytic water splitting technology, especially proton exchange membrane water electrolyzers (PEMWEs), is one of the core hydrogen production technologies to achieve carbon-free energy cycling. However, high acid corrosion and anodic potential operating conditions pose serious challenges for the development of PEMWEs. It is urgent to explore highly active, durable, and compatible anodic catalysts for the complex oxygen evolution reaction (OER). Hitherto, iridium (Ir)-based electrocatalysts (IBCs) with trade-off catalytic activity and stability are the primary candidates for the OER. Yet, the continued huge consumption of expensive and scarce Ir species severely hinders the widespread deployment of PEMWEs. It is necessary to systematically understand the latest research progress in IBCs to guide the controllable construction of catalysts with low-Ir loading to meet industrial demands. In this review, the conventional OER catalytic mechanism and Ir species-related failure modes are introduced. Subsequently, four common classes of IBCs, including Ir-based metals, oxides, perovskites, and pyrochlores, as well as attempts to correlate structural features of catalysts with their performance are reviewed. Within this scenario, the actual performances of bright representative IBCs applied in practical PEMWEs are also discussed. At the end of this review, unresolved issues and challenges in the field are proposed with a view to formulating effective strategies to break the bottleneck of commercial deployment of PEMWEs.

Understanding the effects of operating conditions on the water management in anion exchange membrane fuel cells

While water management in Anion Exchange Membrane Fuel Cells (AEMFCs) is seen as crucial for performance and durability, measurements of water management within operating cells remain few. In this work, we measured the performance of an H2/O2 AEMFC based on a low-density polyethylene membrane AEM, PtRu/C anode and Fe–N–C cathode for various combinations of inlet relative humidities, gas pressures and flow rates. Simultaneously, the anode and cathode outlet relative humidities were measured using humidity sensors, from which the amount of water removed from the cell could be quantified for each side. The data shows that back-diffusion of water from anode to cathode results in increasing outlet relative humidity with increased current density, both at anode and cathode. Water produced in the AEMFC was thus removed from both sides. The maximum current density was found to be strongly connected to anode flooding, which is also the main cause for hysteresis during potentiodynamic scans. However, under atypical operating conditions, the cell performance may also be limited by low humidity at the cathode, with associated low ionomer conductivity. Overall, it is concluded that water management of AEMFC with thin AEMs can be achieved by playing with both the anode and cathode operating conditions.

Towards outstanding performance of direct urea fuel cells through optimization of anode catalyst layer and operating conditions

Highlights • The cell performances and characteristics of DUFCs were investigated. • Optimization was performed by varying operating conditions and anode parameters. • Cause for the difference in performance was analyzed in relation to MEA properties. • DUFCs with the optimum conditions had outstanding performance. Direct urea fuel cells (DUFCs) are expected to be ideal devices to protect the ecology and generate energy because they convert urea from urea-rich wastewater and/or urine directly into electrical energy and turn “waste” into “power.” This study investigated the operating conditions of direct urea fuel cells (DUFCs) with commercial catalysts to achieve maximum performance and to accumulate knowledge regarding the electrochemical environment of DUFC full-cell membrane electrode assemblies (MEAs). The cell performance was optimized by experimentally varying the operating conditions (activation process, cell temperature, and reactant concentration) and anode parameters (ionomer ratio and catalyst loading mount). The results were evaluated by comparing the open-circuit voltage (OCV) and maximum power density obtained from the polarization curve. Additionally, the features of MEAs in DUFCs were explored from the viewpoint of performance improvement for the anion exchange membrane (AEM), supply of the liquid anode, and thickness of the bulky catalyst layer, which all have a comprehensive effect on cell performance. The best performance was realized when the single-cell was operated under 0.3 M urea in 1.0 M KOH at 50 ℃ using a KOH pretreatment membrane, and the cell exhibited the highest maximum power density of 57.18 mW cm −2 , To the best of our knowledge, the MEA with the optimum conditions introduced showed the best performance among the DUFCs. This suggests that scientific understanding and analysis of the optimal operating conditions have considerable potential for the development of new catalysts and MEA materials.

Achieving Outstanding Performance of Direct Urea Fuel Cells Through Optimization of Anode Catalyst Layer and Operating Conditions

Achieving Outstanding Performance of Direct Urea Fuel Cells Through Optimization of Anode Catalyst Layer and Operating Conditions

An ab initio study of the oxygen defect formation and oxide ion migration in (Sr1-xPrx)2FeO4±δ

The Sr n+1 Fe n O 3n+1 Ruddlesden-Popper (RP) perovskite family displays promising oxygen permeability and serves as a host stoichiometry for the design of solid oxide electrode materials. A strategy to tune electronic and ionic properties is the introduction of substitutional dopants like Pr 3+ to the A-site. In this study, we investigate the bulk structural, electronic, and oxygen migration properties for the n = 1 RP perovskite (Sr 1-x Pr x ) 2 FeO 4± δ (x = 0, 0.125, 0.25, 0.375, and 0.5). The oxygen partial pressure is adjusted to elucidate how anodic and cathodic operating conditions influence the formation of oxygen defects. Under anodic conditions, the oxygen vacancy is the dominant oxide defect for all dopant-configurations. Under cathodic conditions, oxygen vacancy defects dominate for configurations from x = 0 to x = 0.375 while the oxygen peroxide interstitial defect becomes the primary defect for x = 0.5. Next, we examine the relationship between Pr 3+ concentration, iron oxidation state, and charge compensation with defect formation to explain the trends in vacancy and peroxide interstitial formation energies. Results clarify the role of lanthanide A-site substitutional dopants on the electronic conductivity and oxide defect formation and migration in Sr-based RP perovskites. These atomic-scale insights suggest design directions for RP perovskites in SOFCs. • Doping strategy of Sr 2 FeO 4 with Pr 3+ for targeted electronic and ionic properties. • Mechanism of vacancy defect formation outlined in terms of charge delocalization. • Mechanism of interstitial defect formation outlined in terms of A-site identity. • Studied all non-symmetrical vacancy-mediated migration paths for given Pr 3+ value.

In situ growth of LaSr(Fe,Mo)O4 ceramic anodes with exsolved Fe–Ni nanoparticles for SOFCs: Electrochemical performance and stability in H2, CO, and syngas

Fe–Ni nanoparticle–decorated LaSr(Fe,Mo)O4 Ruddlesden–Popper (R–P) perovskite anodes, named R–LSFMNx, were prepared in situ by reducing perovskites La0.5Sr0.5Fe0.9Mo0.1–xNixO3–δ (LSFMNx; x = 0.03–0.07) under SOFC anode operating conditions. Electrolyte–supported single cells with a configuration of R–LSFMNx|La0.9Sr0.1Ga0.8Mg0.2O3–δ (LSGM)|Ba0.5Sr0.5Co0.9Nb0.1O3–δ were used to evaluate the electrochemical performances and redox/long–term stability of the R–LSFMNx anodes fuelled by H2, CO, and simulated syngases (x% H2/CO; x = 50–10). EIS analyses indicated that the increased Ni level in the exsolved Fe–Ni nanocatalysts significantly promotes fuel diffusion/adsorption/dissociation, which plays a rate–limiting role in the anode fuel oxidation. Furthermore, the incremental Ni in Fe–Ni alloy also enhances the anode redox/long–term stability and carbon resistance/tolerance, and the R–LSFMN0.07 anode, i.e., Ni level in Fe–Ni alloy attaining ∼14 mol.%, displays the optimal stability and carbon resistance/tolerance. Finally, the potential of the R–LSFMN0.07 anode for direct utilization of syngas was demonstrated by the characterization of the electrochemical performance and stability based on the R–LSFMN0.07 anode cell.

Corrosion inhibition of methanol towards stainless steel bipolar plate for direct formic acid fuel cell

The corrosion properties of AISI316L stainless steel (316 L SS) as bipolar plates are investigated under aqueous acid methanol solutions (0.05 M H2SO4 + 2 ppm HF + 10 M HCOOH + x M CH3OH (x = 0, 3, 6 and 9) solutions at 70 °C) to simulate the varied anodic operating conditions of direct formic acid fuel cells (DFAFCs). When the methanol content is higher, the potentiodynamic, potentiostatic polarisation and EIS tests of the 316 L SS bipolar plates all show excellent corrosion resistance. The surface morphology and the glow discharge mass spectrometer (GDMS) illustrate that the surface corrosion on 316 L SS bipolar plates is slowed down when the methanol concentration is increased. These results indicate the methanol plays the role in retarding the corrosion rate of the 316 L SS in simulated DFAFCs anodic operating conditions by restricting the proton conductivity in the test solutions. The sample tested in higher content methanol solution has smoother corroded surface and thinner passivation film, which contributes to a lower interfacial contact resistances (ICR) value.

Impact of the anode operating conditions on the liquid water distribution in the cathode gas diffusion layer

Previous experimental results indicate that the humidification conditions at the anode have an impact on the liquid water distribution in the cathode gas diffusion layer. Numerical simulations are developed to reproduce and analyze this effect. Results consistent with the experimental results are first obtained by playing with the partition coefficients of an advanced pore network model computing the liquid water formation and transfer in the cathode gas diffusion layer (GDL) for a large range of operating conditions. Then, a model for the full anode – cathode assembly is developed by combining the pore network model of the cathode GDL and a 1D model describing the heat and water transfer in the various components of the anode-cathode assembly. This enables one to generalize the dry – wet regime diagram introduced in a previous work by incorporating the effect of the humidity condition at the anode.

Corrosion behavior of tantalum coatings on AISI 316L stainless steel substrate for bipolar plates of PEM fuel cells

Corrosion resistance of tantalum coatings 30 μm thick deposited by chemical vapor deposition on SS316L coupons has been evaluated by electrochemical impedance spectroscopy (EIS). To this end, anodic and cathodic operating conditions of proton exchange membrane fuel cells (PEMFC) have been simulated in a three-electrode heated corrosion cell. Interfacial contact resistance (ICR), contact angle and durability tests have been performed in long-term tests (>100 h) polarizing the electrode to 1.193 V vs. Ag/AgCl. Results obtained by different experimental techniques show a dense coating structure with a high polarization resistance, mainly formed by surface crystals of α-Ta (bcc), Ta2O5 and carbon. An atomic ratio (in %) of oxide to metallic species (Taox/Tamet) of 4.8 was verified from XPS spectra, which is slightly increased to 6.23 after the anodizing treatment. The modified surface composition yielded a coating capacity higher than the amorphous oxide, favoring the in-plane electrical conduction. After the treatment, no noticeable changes were observed neither in surface morphology nor in contact angle (>90°). ICR values in the range of 22.3–32.6 mΩ cm2 were obtained for a clamping pressure of 140 N cm−2. No morphological changes or loss of coating adherence were observed during the long-term tests.

Experimental Investigation of Ion Beam Extraction in Filament-Driven Negative-Carbon-Ion Sources With Various Geometrical Configurations for Accelerator Mass Spectrometry

A negative carbon-ion source was designed for accelerator mass spectrometry for application in the medical field to minimize cross-contamination and to increase the measurement efficiency. The ion source was designed by adjusting the distance between the plasma and the extraction electrodes and by changing the anode structure. Various prototype ion source configurations were fabricated to construct an ion source test stand to confirm the carbon beam extraction. The transport efficiency of the negative carbon beam was experimentally investigated using a filament-driven ion source to determine the geometrical effect of the anode configuration. The carbon-ion source was operated under carbon dioxide gas flowing at 3.0 sccm at a high voltage of −30.0 kV and an applied arc current of up to 0.5 A and various magnetic fields. The optimized anode structure and operating conditions of the ion source with the beam characteristics must be verified to measure the beam current with various ion source parameters. The effects of various operating and structural conditions on the beam current are evaluated in this article.

Impact of OER Catalyst Activity and Stability on PEMFC Fuel Starvation Caused Cell Reversal Tolerance

Polymer electrolyte membrane fuel cells (PEMFCs) are considered as one of the most promising alternatives to internal combustion engines (ICEs). In a fuel cell electric vehicle (FCEV), the technology provides high energy conversion efficiency and power density at relatively low operating temperatures, with zero or low greenhouse gases emissions. Materials and systems research and development over the last decade have significantly improved the output power and lifetime of the PEMFC stacks. For wide adoption of PEMFCs to replace ICEs, continuing improvements in durability and cost are required. Cell voltage polarity reversal can result in irreversible damage of materials in the PEMFC membrane electrode assembly (MEA), gas diffusion layer (GDL), and even the bipolar plates [1], especially in fuel starvation caused cell reversal events. A number of circumstances can lead to fuel starvation, such as ice formation and severe water flooding which block the anode reactant channels, a sudden change in reactant demand due to load change, and improper reactant control during fuel cell start-up [2]. In fuel starvation caused cell reversal events, carbon oxidation (MEA material degradation) or oxygen evolution reaction (OER, i.e., water electrolysis) will take place to sustain the current generated from unaffected neighbor cells in the fuel cell stack. To avoid the detrimental oxidation of MEA materials, the water electrolysis process is preferred to be promoted by incorporating OER catalysts into the anode catalyst layer. Ruthenium and iridium based oxides are state-of-art PEM water electrolyzer (PEMWE) catalysts. In spite of their different anode layer design and operating conditions, the OER activity and stability of these catalysts have been investigated as the reversal tolerant catalyst (RTC) in PEMFCs [3]. Ru based catalysts provide high OER catalytic activity and increase the fuel cell reversal tolerance. However, they are prone to degradation due to insufficient stability in the PEMFC environment and operating condition. In PEMWE, the catalyst is expected to be stable at potentials higher than 1.4V, on the other hand, in PEMFC, the OER catalyst in the anode is required to be robust to potential swings between 0V to 1V (vs. SHE), because of the fuel cell start-up and shutdown processes. Considering the structure of the anode compartment, the OER catalyst is in direct contact with the electrode substrate in PEMWE, whereas in PEMFC, carbon supported platinum (Pt/C) catalyst of the hydrogen oxidation reaction (HOR) is also incorporated in the anode layer. The presence of Pt/C catalyst adds further complication toward the development of reversal tolerant anode since the carbon corrosion resistance is also required to be considered. Figure 1.a reveals an example in which using the same OER catalyst with similar loading, the cell reversal tolerance for two anode HOR catalyst designs can be significantly different. In this study, the impact of OER catalyst activity and stability on fuel cell reversal tolerance and durability is explored. Also, the strategic pathways to improve both the reversal tolerance and durability of OER catalysts in PEMFC are discussed. It has been demonstrated that these approaches can provide remarkable reversal tolerance without a significant trade-off in the performance and durability of the fuel cell. Figure 1.b shows that by catalyst treatment, both cell reversal tolerance and durability of the anode layer have been improved.

Improvement and Stable High Bioelectricity Generation Using Alkaliphilic Oceanobacillus iheyensis BS1(2) in Microbial Fuel Cells and Effect of Different Anodic Operating Conditions

Improvement and Stable High Bioelectricity Generation Using Alkaliphilic Oceanobacillus iheyensis BS1(2) in Microbial Fuel Cells and Effect of Different Anodic Operating Conditions

The Performance of a Direct Borohydride/Peroxide Fuel Cell Using Graphite Felts as Electrodes

A direct borohydride/peroxide fuel cell (DBPFC) generates electrical power by recirculating liquid anolyte and catholyte between the stack and reservoirs, which is similar to the operation of flow batteries. To enhance the accessibility of the catalyst layer to the liquid anolyte/catholyte, graphite felts are employed as the porous diffusion layer of a single-cell DBPFC instead of carbon paper/cloth. The effects of the type of anode alkaline solution and operating conditions, including flow rate and temperature of the anolyte/catholyte, on DBPFC performance are investigated and discussed. The durability of the DBPFC is also evaluated by galvanostatic discharge at 0.1 A∙cm−2 for over 50 h. The results of this preliminary study show that a DBPFC with porous graphite electrodes can provide a maximum power density of 0.24 W∙cm−2 at 0.8 V. The performance of the DBPFC drops slightly after 50 h of operation; however, the discharge capacity shows no significant decrease.

Using effluents from two-phase anaerobic digestion to feed a methane-producing microbial electrolysis

The integration of a methane-producing microbial electrolysis cell (MEC) into two-phase anaerobic digestion (TP-AD) was investigated, by using effluents from a pilot-scale TP-AD treating the organic fraction of municipal solid waste. The MEC was aimed at exploiting residual COD of TP-AD effluents at the MEC anode in order to support CO2 removal and methane generation at the MEC cathode (fed by a CO2-rich gas phase, simulating a biogas). Feeding by 2nd phase digestate caused a loss of MEC performance, due to poor biodegradability of digestate COD under chosen anodic operating conditions (+0.2V vs SHE, HRT 13h, organic load 2.3gCOD/Ld). On the other hand, by using the 1st phase fermentate (rich in volatile fatty acids, VFA), good MEC performance was recovered with a current of 60±4mA and a methane production rate of 33±3meq/Ld. However, periodic baskwashing was also necessary to recover fouling effects. Moreover, partial nitrogen removal (228mgN/Ld) from the fermentate was obtained because ammonium was transported across the separation membrane and then recovered through the cathodic concentrated spill (at 3177±97mgN/L). Cation transport also generated net alkalinity which strongly contributed to CO2 removal (besides methane generation).

Performance of buffered ferric chloride as terminal electron acceptor in dual chamber microbial fuel cell

Electrochemical behaviour of buffered ferric chloride (FC) was studied as terminal electron acceptor (TEA) in a dual chamber microbial fuel cell (DCMFC). The DCMFC was operated in batch mode using lactate as substrate and enriched consortium of sulphate reducing and sulphur oxidising bacteria as anodic biocatalysts. To characterise the TEA in terms of open circuit voltage and power density, four DCMFCs were operated individually under similar anodic operating conditions with phosphate buffer (PB), potassium hexacyanoferrate (PF), potassium dichromate (PD) and FC. Results showed that voltage and power generation with FC were highest among the four TEAs. Performance of the four tested buffered catholytes was found to be in the following order: ferric chloride (857mV; 308.7mW/m2) > potassium hexacyanoferrate (571mV; 222mW/m2) > potassium dichromate (517mV; 168.6mW/m2) > phosphate buffer solution (250mV; 16.87mW/m2). With the addition of potassium dichromate, potassium hexacyanoferrate and ferric chloride to phosphate buffer, charge transfer resistance (Rct) declined by 67%, 52.2% and 51.8%, respectively. Ohmic resistance decreased by 36%, 41.6% and 37.6%, respectively. Electrical conductivity of the leachate after coagulation with ferric chloride increased from an initial of 2.54–7.96mS thus further supporting the decreased resistance. Further, with no optimization, 50mM/L of ferric chloride could remove 68.3% of COD having initial value of 9600mg/L in the cathode chamber.

Structural analysis of PrBaMn2O5+δ under SOFC anode conditions by in-situ neutron powder diffraction

Abstract The crystal structure and oxygen stoichiometry of the proposed double perovskite solid oxide fuel cell (SOFC) anode material PrBaMn2O5+δ were determined under SOFC anode conditions via in-situ neutron diffraction. Measurements were performed in reducing atmospheres between 692 K and 984 K. The structure was fit to a tetragonal (space group P4/mmm) layered double perovskite structure with alternating Pr and Ba A-site cation layers. Under all conditions examined, the oxygen sites in the Ba and Mn layers were fully occupied, while the sites in the Pr layer were close to completely vacant. The results of the neutron diffraction experiments are compared to previous thermogravimetric analysis experiments to verify the accuracy of both experiments. PrBaMn2O5+δ was shown to be stable over a wide range of reducing atmospheres similar to anode operating conditions in solid oxide fuel cells without significant structural changes.

First-Principles Investigation of the Surface Reactivity and Electronic Property of Dopant-Promoted SrTiO3 Catalysts for SOFC Anode Application

The strontium titanium oxide (SrTiO3) has received much attention as an alternative anode material for solid oxide fuel cell (SOFC) owing to its high phase stability and sulfur/carbon resistance in typical SOFC anode operating conditions. However, SrTiO3 has exhibited the low electronic/ionic conductivity and poor electro-catalytic activity toward fuel oxidation, leading to the high overpotential of SOFC. Therefore, it is very imperative to develop highly conductive and active SrTiO3 – based anode catalysts. In this work, we will present some recent density functional theory (DFT)+U calculation results we have on the role of dopants in enhancing the electronic/ionic conductivity and catalytic reactivity of SrTiO3–based catalysts: that is, (1) the origin of improved electronic/ionic conductivity in yttrium(Y)-doped SrTiO3 (2) mechanism of H2 oxidation on the surface of yttrium(Y)-doped SrTiO3 (3) Role of dopant(M) in enhancing H2 oxidation in Sr1-xYxTi1-yMyO3 . This work hints the importance of knowing how to properly tailor the surface reactivity of oxide-based anode materials for achieving wanted electro-catalytic activity for SOFC application.

Why solid oxide cells can be reversibly operated in solid oxide electrolysis cell and fuel cell modes?

High temperature solid oxide cells (SOCs) are attractive for storage and regeneration of renewable energy by operating reversibly in solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC) modes. However, the stability of SOCs, particularly the deterioration of the performance of oxygen electrodes in the SOEC operation mode, is the most critical issue in the development of high performance and durable SOCs. In this study, we investigate in detail the electrochemical activity and stability of La0.8Sr0.2MnO3 (LSM) oxygen electrodes in cyclic SOEC and SOFC modes. The results show that the deterioration of LSM oxygen electrodes caused by anodic polarization can be partially or completely recovered by subsequent cathodic polarization. Using in situ assembled LSM electrodes without pre-sintering, we demonstrate that the deteriorated LSM/YSZ interface can be repaired and regenerated by operating the cells under cathodic polarization conditions. This study for the first time establishes the foundation for the development of truly reversible and stable SOCs for hydrogen fuel production and electricity generation in cyclic SOEC and SOFC operation modes.

Comprehensive understanding of the active thickness in solid oxide fuel cell anodes using experimental, numerical and semi-analytical approach

This paper reports the evaluation of the electrochemically active region in solid oxide fuel cell (SOFC) electrodes through experimental, numerical and semi-analytical approaches. In the experiment, anodes with several different thicknesses were fabricated and their performance was measured to find its dependence on the anode thickness, microstructure and operating conditions. The three-dimensional (3D) microstructure of the anodes was imaged using focused ion beam scanning electron microscopy (FIB-SEM) and the microstructural parameters were quantified. One-dimensional (1D) and 3D numerical simulations based on the actual 3D microstructures were carried out to investigate the active thickness in the anodes. The validity of the numerical models was confirmed by comparing the results with the experiment. The active thickness, i.e., the electrochemically active region within the anode, is discussed using the verified simulation models to find its dependence on various conditions. The active thickness was found to depend on the microstructure and the operating conditions. We then attempted to find a simple expression for the active thickness useful for practical applications with semi-analytical discussion. The developed descriptions expressed the quantitative dependence of the active thickness on the effective ionic conductivity, exchange current density and area-specific current density.

Improvement of power generation using Shewanella putrefaciens mediated bioanode in a single chambered microbial fuel cell: effect of different anodic operating conditions.

Three different approaches were employed to improve single chambered microbial fuel cell (sMFC) performance using Shewanella putrefaciens as biocatalyst. Taguchi design was used to identify the key process parameter (anolyte concentration, CaCl₂ and initial anolyte pH) for maximization of volumetric power. Supplementation of CaCl₂ was found most significant and maximum power density of 4.92 W/m(3) was achieved. In subsequent approaches, effect on power output by riboflavin supplementation to anolyte and anode surface modification using nano-hematite (Fe₂O₃) was observed. Volumetric power density was increased by 44% with addition of 100 nM riboflavin to anolyte while with 0.8 mg/cm(2) nano-Fe₂O₃ impregnated anode power density and columbic efficiency increased by 40% and 33% respectively. Cyclic voltammetry revealed improvement in electrochemical activity of Shewanella with nano-Fe₂O₃ loading and electrochemical impedance depicted inverse relationship between charge transfer resistance and nano-Fe₂O₃ loading. This study suggests anodic improvement strategies for maximization of power output.