Cathodic Conditions Research Articles

Insights into surface alkaline-earth segregation on the surface of La0.3Me0.7Fe0.7Cr0.3O3−δ (Me = Sr, Ca) porous electrodes for oxygen reduction reaction

Surface alkaline-earth segregation is a common problem encountered for the cathodes of solid oxide fuels cells based on perovskites (ABO3) containing alkaline-earth acceptor dopants. In this work, the porous electrodes of La0.3Me0.7Fe0.7Cr0.3O3−δ (Me = Sr, Ca) were employed as the model specimens to look into the surface segregation issue. The two electrodes were cathodically polarized by continuously testing their cathodic current polarization curves. The electrocatalytic activity of the two electrodes was examined in relation with their cathodic polarization history. Moreover, the crystal and surface structures of the cathodically polarized electrodes were analyzed. An appreciable segregation of strontium onto the surface was detected for cathodically polarized La0.3Sr0.7Fe0.7Cr0.3O3−δ electrode, leading to an obvious deceleration of disassociative adsorption of molecular oxygen on the surface and thus degradation of the electrode activity. In contrast, La0.3Ca0.7Fe0.7Cr0.3O3−δ electrode displayed nearly negligible surface calcium segregation and degradation of the electrocatalytic activity after experiencing an identical cathodic polarization history. The disparity of the two electrodes was related to their A-site compositions. This work exemplifies the importance of minimizing A-site cation size mismatch in stabilizing the crystal structure of the perovskite electrodes under cathodic polarization conditions and therefore alleviating the segregation of A-site alkaline-earth cations onto the electrode surface.

Hydroxide melt induced corrosion of Ni at elevated temperatures under steam electrolysis conditions

Molten alkali electrolyte based high temperature water electrolysis (HTWE) process offers an efficient pathway to integrate renewable energy resources for large scale and economic hydrogen production. Long term and stable operation of these systems require an in-depth understanding of materials stability under anodic and cathodic exposure conditions of the cell and cell stacks. In the present study, we report our findings on the corrosion of Ni in the presence and absence of (Li–Na) hydroxide melt at 600 °C under oxidizing and reducing conditions representative of HTWE. While the Ni electrode was found to remain thermodynamically stable in metallic form under cathodic (reducing) exposure conditions, the corrosion rate in molten hydroxide under oxidizing conditions was found to be non-parabolic in nature. A cyclic voltammetry study provides the breakdown of the passive metal-oxide surface layer at the anodic overpotential region between 0.45 V and 2 V in molten hydroxide under oxidizing conditions. A thermochemical process for accelerated corrosion based on the oxide scale fluxing in hydroxide melt has been developed.

Electrochemical performance and distribution of relaxation times analysis of tungsten stabilized La0·5Sr0·5Fe0·9W0·1O3-δ electrode for symmetric solid oxide fuel cells

W-doped La0·5Sr0·5Fe0·9W0·1O3-δ (LSFW) was prepared and evaluated as a symmetric electrode for solid oxide fuel cells (SSOFCs). Phase and structural stability of LSFW under both reducing and oxidizing atmospheres was studied. The oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) mechanisms were investigated by using electrochemical impedance spectra (EIS) and distribution of relaxation times (DRT). Electrode polarization resistance (Rp) of LSFW are 0.08 and 0.16 Ω cm2 in air and wet hydrogen at 800 °C, respectively. DRT results indicate that the rate-limiting step of LSFW at 800 °C in cathodic conditions and anodic conditions are related to oxygen diffusion and hydrogen adsorption/diffusion, respectively. A La0·8Sr0.2Ga0.8Mg0·2O3-δ (LSGM) electrolyte-supported single cell using LSFW electrodes shows a maximum power density of 617.3 mW cm−2 at 800 °C with considerable stability and reversibility, which enables LSFW a promising SOFCs symmetric electrode material.

Bidirectional electroactive microbial biofilms and the role of biogenic sulfur in charge storage and release

SummaryThe formation of combined electrogenic/electrotrophic biofilms from marine sediments for the development of microbial energy storage systems was studied. Sediment samples from the German coasts of the Baltic and the North Sea were used as inocula for biofilm formation. Anodic biofilm cultivation was applied for a fast and reproducible biofilm formation. North-Sea- and Baltic-Sea-derived biofilms yielded comparable anodic current densities of about 7.2 A m−2. The anodic cultivation was followed by a potential reversal regime, transitioning the electrode potential from 0.2 V to −0.8 V every 2 h to switch between anodic and cathodic conditions. The charge-discharge behavior was studied, revealing an electrochemical conversion of biogenic elemental sulfur as major charge-discharge mechanism. The microbial sequencing revealed strong differences between North- and Baltic-Sea-derived biofilms; however with a large number of known sulfur-converting and electrochemically active bacteria in both biofilms.

Effect of cathode conditions on performance of direct borohydride–hydrogen peroxide fuel cell system for space exploration

Fuel cell tests are conducted under various cathode conditions to investigate the effect of cathode conditions on the performance of a direct borohydride–hydrogen peroxide fuel cell (DBHPFC) system. The efficiency and mass of the virtual DBHPFC system are estimated based on experimental data obtained from the fuel cell tests. The type of cathode electrocatalyst considerably affects the performance and mass of the fuel cell system. Ni is the most suitable for the cathode owing to its high fuel utilization efficiency and low cost. The H2O2 concentration has a minimal effect on the performance of the fuel cell system but considerably affects the mass of the fuel cell system. The mass of the fuel cell system negatively correlates with the H2O2 concentration. The H3PO4 concentration and operating temperature have a negligible effect on the performance and mass of the fuel cell system; 5 wt% H3PO4 is sufficient to suppress the decomposition reaction and improve the fuel cell performance. Operation at room temperature is recommended for high fuel utilization efficiency. The decomposition reaction rate affects the efficiency and mass of the DBHPFC system. Consequently, the decomposition and electrochemical reaction rates should be considered when determining suitable cathode conditions for the DBHPFC system.

Electrochemically-active carbon nanotube coatings for biofouling mitigation: Cleaning kinetics and energy consumption for cathodic and anodic regimes

Biofouling is a major obstacle in engineered systems exposed to aqueous conditions. Many attempts have been made to engineer the surface properties of materials to render them resistant to biofouling. These modifications typically rely on passive antimicrobial or anti-adhesive surface coatings that prevent the deposition of bacteria or inactivate them once they reach the surface. However, no surface modification strategy completely prevents biofilm formation, and, over time, surfaces will be fouled and require cleaning. In this work, we demonstrate the capacity of electrochemical carbon nanotube coatings in dispersing biofilms formed on the surface. A systematic analysis of the biofilm removal kinetics in function of applied current density is made to identify the optimal current conditions needed for efficient surface cleaning. Operating the electrochemically active surface as a cathode produces superior results compared to when it is operated as an anode. Specifically, the 5.00 A m−2 and 2.50 A m−2 cathodic conditions produced rapid cleaning, with complete biofilm dispersal after 2 min of operation. Surface cleaning is attributed to the generation of microbubbles on the surface that scours the surface to remove the adhered biofilm. Energy consumption analyses indicate that the 2.50 A m−2 cathodic condition offers the best combination of cleaning kinetics and energy consumption achieving 99% biofilm removal at an energy cost of ~$ 0.0318 m−2. This approach can be competitive compared to the current chemical cleaning strategies, while offering an opportunity for a more sustainable and integrated approach for biofouling management in engineered systems.

Magnetic Filling of Microporous Silicon: An Interlink of Optical and Magnetic Behavior

In the frame of this work microporous silicon (mPSi) is filled with magnetic metals (Ni, Co) resulting in a composite system with specific magnetic properties. The results are compared with mesoporous silicon (PSi) loaded with Ni nanoparticles (NPs) of comparable size. The mPSi offers luminescence in the visible, whereat this behavior is due to the small structure size in the range of a few nanometers (2 – 5 nm) [1]. The morphology of mesoporous silicon shows straight pores with a diameter up to 50 nm.The mPSi samples are fabricated by anodization of a moderately doped p-type silicon wafer in a 10 wt% hydrofluoric acid solution. The challenging metal filling of these pores is carried out under cathodic conditions by pulsed electrodeposition. As electrolytes aqueous NiSO4 and CoSO4 solutions are employed. The microporous silicon offers a branched morphology and thus also the deposited nanosized metal structures are interconnected. The structure size of the porous samples is estimated by photoluminescence measurements to 5 nm which also determines the size of the deposits. Mesoporous silicon is produced by anodization of a highly n-doped silicon wafer resulting in straight pores of about 50 nm in diameter [2]. The loading of these samples with Ni NPs is also performed by electrodeposition.Magnetic filling of meso- and microporous silicon, respectively means a change in the morphology as well as in the structure size of the magnetic precipitations and both influence the magnetic behavior of the composite system drastically. In the case of mPSi also the luminescence is influenced in its intensity and wavelength by the magnetic filling. The magnetic properties are investigated with a Vibrating Sample Magnetometer (VSM) and a SQUID, respectively. A luminescence spectrometer is used to measure the intensity and the peak position of the photoluminescence spectrum. The structure is investigated by SEM and TEM.Field dependent magnetization measurements show a distinct difference between Ni filled mPSi and PSi loaded with Ni NPs. Considering mPSi filled with Ni and Co a strong magnetic anisotropy between the two magnetization directions applying a magnetic field perpendicular and parallel to the surface, respectively is observed, whereas in the case of Co this behavior is even more pronounced. Figure 1 depicts the hysteresis of mPSi and mesoporous silicon filled with Ni. The coercivities do not vary in a broad range between Ni and Co samples but the saturation field differs drastically between the two materials. In contrast mesoporous silicon loaded with Ni NPs offers only a small magnetic anisotropy caused by weak magnetic interactions between the Ni deposits.Since the microporous silicon offers photoluminescence the huge magnetic anisotropy is of interest for magneto-optical applications.Figure 1: Comparison of field dependent magnetization of mPSi and mesoporous silicon filled with Ni.[1] Handbook of Porous Silicon, Ed. L. Canham, Springer Int. Publishing, 2018.[2] P. Granitzer, K. Rumpf, Semiconductor Science and Technology 31, 4004, 2016. Figure 1

Electrochemical Nitrate Reduction to Ammonia in a PEM Cell: Maximizing Faradaic Efficiency and Single Pass Conversion

Polymer electrolyte membrane (PEM) electrolysis is considered a promising technology for the renewable hydrogen production from renewable energy, due to the high energy efficiency and robust design of the PEM cells.1 Nevertheless, despite their attractiveness, PEM cells did not find wide implementation for other electrochemical transformations than water splitting. The main reason is the low faradaic efficiencies of reduction reactions at the cathode of PEM cells, due to the competing hydrogen evolution reaction (HER), highly favorable in the acidic electrolyte environment of the PEM cell. We investigated the electrochemical nitrate reduction reaction at the cathode side of a PEM cell, aiming at reaching high faradaic efficiency and selectivity towards a single reaction product, and at the same time, high single pass nitrate conversion.Nitrates represent one of the most prominent ground water pollutants, and significant efforts are targeted for their removal. Reductive abatement treatment typically yields nitrogen, which is environmentally benign but does not represent an economically attractive reaction product.2 Nitrates can be converted electrochemically to nitrogen (1), but also ammonia (2), which is an important building block in the chemical industry and the main pillar of fertilizer production.3-5 By enabling this transformation in the PEM cell, added-value can be added to an otherwise waste chemical, by making use of renewable electricity that can power PEM cells, which is highly relevant for a future circular economy.2NO3 − + 12H+ + 10e− → N2 + 6H2O E° = 1.25 V (vs NHE) (1)NO3 − + 10H+ + 8e− → NH4 + + 3H2O E° = 0.875 V (vs NHE) (2)We investigated four different catalysts for their activity, selectivity and faradaic efficiency towards ammonia formation: copper, palladium, rhodium and ruthenium. Of the four, we identified ruthenium as the most active and selective catalyst. We were able to limit the extent of the competing HER, by precisely tuning the supply of reactant and catalyst amount at the anode of the PEM cell. By this, we show the importance of matching the cathode and anode reaction conditions. This approach allowed us to reach 94% faradaic efficiency towards ammonia, one of the highest values reported to date. At the same time, we aimed at maximizing the single pass nitrate conversion in the cell. We found that mass transport limitations play an important role in achieving this goal. Nevertheless, by recirculating the reactant feed through the cell, we obtained 93% single pass nitrate conversion after 8 h of electrolysis at 10 mA/cm2 current density.References Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., A comprehensive review on PEM water electrolysis. Int. J. Hydrogen Energy 2013, 38 (12), 4901-4934.Barrabés, N.; Sá, J., Catalytic nitrate removal from water, past, present and future perspectives. Appl. Catal., B 2011, 104 (1), 1-5.Duca, M.; Koper, M. T., Powering denitrification: the perspectives of electrocatalytic nitrate reduction. Energy Environ. Sci. 2012, 5 (12), 9726-9742.Rosca, V.; Duca, M.; de Groot, M. T.; Koper, M. T. M., Nitrogen cycle electrocatalysis. Chem. Rev. 2009, 109 (6), 2209-2244.McEnaney, J. M.; Blair, S. J.; Nielander, A. C.; Schwalbe, J. A.; Koshy, D. M.; Cargnello, M.; Jaramillo, T. F., Electrolyte engineering for efficient electrochemical nitrate reduction to ammonia on a titanium electrode. ACS Sustainable Chem. Eng. 2020, 8 (7), 2672-2681. Figure 1

Tribocorrosion Behavior of Typical Austenitic, Martensitic, and Ferritic Stainless Steels in 3.5% NaCl Solution

Tribocorrosion behaviors of typical austenitic (ASTM 316), martensitic (ASTM S41500), and ferritic (ASTM S44660) stainless steels were investigated in a 3.5% NaCl solution under cathodic protection conditions, and variations in the microstructures and chemical compositions of worn surfaces were further analyzed. The results showed that the friction coefficient and wear rate of austenitic steels were lower than those of ferritic and martensitic steels in 3.5% NaCl solution with cathodic protection. Additionally, the tribocorrosion properties of ferritic steels were obviously unaffected by load. Wear mechanisms of the three typical stainless steels mainly included abrasive wear at lower loads and plastic deformation at a higher loads. Furthermore, in austenitic steels, variations in microstructure and chemical composition confirmed that a higher load was beneficial to the transformation of austenite to martensite; thus, the hardness and wear resistance were improved. Compared with austenitic steels, the phase compositions of martensitic and ferritic steel were more stable during sliding, and the surface hardness of martensitic steel increased due to the surface phase and microstructure transformations induced by sliding. In addition, the total damage to steel was mainly caused by mechanical wear and the interaction between corrosion and wear. Therefore, ASTM S44660 and 316 were suitable for high-load corrosion-wear environments and low-load corrosion-wear environments, respectively, providing guidance for material selection under practical conditions.

Influence of additive manufacturing by laser powder bed fusion on the susceptibility of Alloy 718 to hydrogen embrittlement

ABSTRACT The susceptibility of additively manufactured (AM) and heat-treated Alloy 718 to hydrogen embrittlement was compared to that of wrought Alloy 718. Specimens were prepared from laser powder bed fusion (LPBF) coupons built with the long axis in the x-direction (horizontal printing) and with the long axis in the z-direction (vertical printing). Despite the heat treatment, coupons printed horizontally demonstrated retained elongated grains parallel to the build, z-direction, while the coupons printed vertically had a relatively uniform and small grain structure perpendicular to the build direction, but a highly elongated grain structure parallel to the build direction. Slow strain rate testing under standardised cathodic charging conditions in 0.1 M H2SO4 indicated a greater susceptibility to hydrogen embrittlement of the test specimens manufactured from coupons printed horizontally, attributed to cracks more readily propagating along the elongated direction of the microstructure.

Electrodeposition of conductive PAMT/PPY bilayer composite coatings on 316L stainless steel plate for PEMFC application

Stainless steel fulfills most of the requirements as bipolar plates in Proton Exchange Membrane Fuel Cell. However, it undergoes severe corrosion in fuel cell operating condition. This can be resolved by coating the stainless steel with corrosion resistive conducting polymers. In this study, homogeneous and adherent conductive Poly(2-amino-5-mercapto-1,3,4- thiadiazole)/Polypyrrole (PAMT/PPY) mono and bilayer polymer composite coatings are electrosynthesized on 316L SS in 0.5 M H2SO4 by cyclic voltammetry and chronopotentiometry. The hydrophobicity and surface morphology of the coatings are analyzed by contact angle and scanning electron microscopy respectively. The polymer coatings are evaluated in 0.5 M H2SO4 medium by potentiodynamic polarization and impedance techniques at 25 °C. The polarization results reveal that PAMT on PPY composite coating shifts the Ecorr of the 316L SS towards noble direction. The EIS study reveals that the Rf value of PAMT on PPY coating is significantly higher by three orders (x103 Ωcm2) of magnitude than uncoated 316L SS. The corrosion performance of the coatings in simulated PEMFC environment is investigated by potentiodynamic and potentiostatic studies. Results show that the PAMT on PPY and PPY on PAMT bilayer coatings are stable and increased the corrosion potential by about 410–470 mV and 275–310 mV (SCE) in simulated cathodic and anodic conditions respectively. This investigation reports that the PAMT on PPY bilayer coating is serving as a good physical barrier and protecting the 316L SS against corrosion in PEMFC environment.

Photoelectrochemical Stability under Anodic and Cathodic Conditions of Meso-Tetra-(4-Sulfonatophenyl)-Porphyrinato Cobalt (II) Immobilized in Polypyrrole Thin Films.

Cobalt porphyrins have emerged as promising catalysts for electrochemical and photoelectrochemical applications because of their good performance, low cost and the abundance of cobalt in the earth. Herein, a negatively charged porphyrin meso-tetra-(4-sulfonatophenyl)-porphin (TPPS) was immobilized in polypyrrole (PPy) during the electro-polymerization, and then it was metallized with cobalt to obtain meso-tetra-(4-sulfonatophenyl)-porphyrinato cobalt (II) (CoTPPS) as a dopant in PPy. The coatings were evaluated as photoelectrodes towards thiosulfate oxidation and oxygen reduction. For comparison purposes, the photoelectrochemical behavior of ClO4−-doped polypyrrole films was also evaluated. Characterizations by chronoamperometry, UV-Vis spectroscopy and Raman spectroscopy showed that polypyrrole is stable under anodic and cathodic conditions, but CoTPPS and TPPS immobilized in PPy are degraded during the anodic process. Thus, decreases in photocurrent of up to 87% and 97% for CoTPPS-doped PPy and TPPS-doped PPy were observed after a 30-min chronoamperometry test. On the other hand, good stability of CoTPPS and TPPS immobilized in PPy was observed during photoelectrochemical oxygen reduction, which was reflected in almost constant photocurrents obtained by chronoamperometry. These findings are relevant to understanding the role of CoTPPS as a catalyst or pre-catalyst in photoelectrochemical applications such as water splitting. In addition, these results could pave the way for further research to include CoTPPS-doped PPy in the design of novel photocathodes.

Selective and High Current CO2 Electro-Reduction to Multicarbon Products in Near-Neutral KCl Electrolytes.

Reducing CO2 to value-added multicarbon (C2+) fuels and chemicals using renewable energy is a viable way to circumvent CO2 buildup in the atmosphere and facilitate closing the carbon cycle. To date it remains a challenge to achieve high product selectivity and long-term stability of electrocatalytic carbon dioxide reduction reaction (CO2RR) especially at practically relevant high current levels >100 mA cm-2. Here, we report a simple electrodeposited Cu electrocatalyst on a hydrophobic porous gas-diffusion layer (GDL) electrode affording stable and selective CO2RR to C2+ products in near-neutral KCl electrolytes. By directing the CO2 stream to fully submerged hydrophobic GDLs in a H-cell, high C2+ partial current densities near 100 mA cm-2 were achieved. In a flow-cell setup, the Cu/GDL cathode in 2 M KCl afforded stable CO2RR superior to that in widely used KOH electrolytes. We found that Cu etching/corrosion associated with trace oxygen played a role in the catalyst instability in alkaline media under cathodic CO2RR conditions, a problem largely suppressed in near-neutral electrolyte. A two-electrode CO2 electrolyzer was constructed with a Cu/GDL cathode in KCl catholyte and an anode comprised of nickel-iron hydroxide on nickel foam (NiFe/NF) in a KOH anolyte separated by Nafion membrane. By periodically adding HCl to the KCl catholyte to compensate the increasing pH and remove accumulated (bi)carbonates, we observed little decay over ∼30 h in flow-cell CO2RR activity and selectivity at 150 mA cm-2 with a high Faradaic efficiency (FE) of ∼75% and energy efficiency of 40% for C2+ products.

Oxygen-reducing bidirectional microbial electrodes: A mini-review

Aerobic bidirectional microbial electrodes (ABME) oxidize matter in anodic conditions (substrate supply, no oxygen) and reduce oxygen in cathodic conditions (oxygen supply, no substrate). The different experimental protocols used to design them, their electrochemical performance and the microbial populations involved are reviewed here. In spite of a limited number of studies, ABMEs have already shown promising capabilities to solve the issues of oxygen crossover and pH control in microbial fuel cells. They may also broaden the field of efficient electroactive bacteria to phyla other than Proteobacteria.

Layered oxygen-deficient double perovskite GdBaFe2O5+δ as electrode material for symmetrical solid-oxide fuel cells

A highly efficient and stable catalyst is critically required for simultaneous oxygen reduction and fuel oxidation in symmetrical solid-oxide fuel cells (SSOFCs). Herein, a novel material named as layered oxygen-deficient double perovskite GdBaFe2O5+δ (GBFO; δ = 0.25 and 0.5) for SSOFC electrodes was synthesized in oxidizing and reducing atmospheres and exhibited excellent structural stability under cathode and anode conditions. La0.9Sr0.1Ga0.8Mg0.2O3–δ (LSGM) and Ce0.8Sm0.2O1.9 (SDC) were used as electrolyte and buffer layer, respectively. The polarization resistance values of the GBFO electrodes on LSGM electrolytes were as low as 0.054 and 0.974 Ω cm2 at 850°C in air and H2, respectively. The maximum power densities obtained from a cell with the configuration of GBFO/SDC/LSGM (200 μm)/SDC/GBFO were 1053, 868, and 197 mW cm−2 at 850°C with H2, wet syngas, and wet CH4 as fuel, respectively. The formation mechanism of oxygen vacancies and the structural stability of GBFO were also analyzed and discussed. GBFO showed good electrochemical stability and resistance to carbon deposition based on a 120-hour test with wet CH4 as fuel. These findings indicate the potential use of this material in SSOFCs running on hydrocarbon fuels.

Hydrogen embrittlement processes in microalloyed steel notched tensile samples

In this work, two microalloyed steels, one used for oil&gas pipelines and the other one for structural components in hydrotreating reactors in petrochemical industries, have been studied under hydrogen embrittlement (HE) conditions. They have been cathodically charged with hydrogen under different aggressive conditions, its hydrogen content being determined afterwards. In order to know the effect of triaxiality on the HE behaviour of this steel, two different types of axisymmetric round notched tensile specimens have been tested under continuous hydrogen charging processes, whilst varying cathodic charging conditions and loading rate. The effects on the global mechanical characterization have been analysed, as well as the correlation between the observed fracture micromechanisms with both local environmental (as hydrogen concentration) and mechanical conditions (as hydrostatic stress and plastic strain determined with a finite elements analysis). Also, subcritical cracking due to embrittlement has been analysed and compared with those obtained by fracture mechanics standardised specimens.

Tailoring the Electrocatalytic Activity and Selectivity of Pt(111) through Cathodic Corrosion

Developing active and selective electrocatalysts is of prime importance in shifting society toward utilizing renewable energy sources. Such developments often require catalysts with particular morphologies because many chemical reactions are sensitive to the catalyst structure. However, it can be challenging to synthesize and stabilize shape-controlled catalysts. It would therefore be desirable to tailor the morphology of an existing electrocatalyst to exhibit favorable catalytic properties. In this work, such morphological alteration is explored using cathodic corrosion: an electrochemical etching process that can generate geometrical shapes on metallic surfaces. Specifically, cathodic corrosion is utilized to tailor a well-defined Pt(111) electrode for oxygen reduction and glycerol oxidation catalysis. Both the oxygen reduction activity and the glycerol oxidation selectivity can be tailored by controlling the cathodic corrosion conditions. These results demonstrate the specific potential of cathodic corrosion for tailoring catalyst activity and selectivity.

Ultra-Low Platinum Nano-Alloy Encapsulated in Carbon Nanotube Matrix: A Practical PEM Fuel Cell Electrocatalyst

Hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) are the crucial PEM fuel cell reactions. However, the sluggish kinetics of these reactions particularly ORR require efficient catalysts to reduce the larger voltage losses and boost the electrochemical activity. Hence developing efficient and durable platinum (Pt)-based electrocatalysts is of enormous significance for practical PEM fuel cell. The state-of the-art carbon black supported Pt nanoparticles (Pt/C) catalyst has sever activity and stability issues. The tiny Pt nanoparticles often undergo the dissolution and redeposition under electrochemical operation, which would destroy their initial state of highly uniform dispersion and high activity. Meanwhile, the amorphous carbon support suffers from the corrosion and oxidation at the cathodic working conditions, which leads to the initial interaction disruption of Pt and carbon support. It causes Pt nanoparticles detachment and agglomeration with a consequent significant activity degradation. Herein we report an integrated ultra-low Pt nanoalloy encapsulated in nitrogen-doped carbon nanotubes matrix (PtCoNi@NCNTs) for efficient oxygen reduction in practical fuel cell. This hybrid Pt-based catalyst achieves a higher mass activity of 3.46 A mgPt -1 at 0.9 V vs. RHE with a negligible stability decay after 10,000 cycles. More importantly, this excellent half-cell activity can be fully transformed into practical full cell. The fuel cell based on this hybrid electrocatalyst delivers a power density of 11.2 W mgPt -1 cathode, which is 8 times of commercial Pt/C 20 % based cells (1.4 W mgPt -1 cathode). Furthermore, PtCoNi@NCNTs also demonstrates an excellent fuel-cell durability by retaining its initial current after 10 h operation at the constant voltage of 0.7 V without any loss. This excellent performance is ascribed to the synergetic response by the integration of encapsulated Pt nano-alloy and hierarchical nitrogen-doped carbon nanotube matrix. It is well acknowledged that the fine tuning of transition metals in the PtM alloy offers extra Pt structural strain that controls the weakening of surface bonds and results in a hike in the electrocatalytic activity. Moreover, the incorporation of metal-nitrogen-carbon (M-N-C) sites also additionally boosts catalytic activity, where the hierarchical architecture is not only the robust substrate to anchor/embed ternary alloy nanoparticles but also ensures sufficient contact of the reactant on the catalysts, which is always ignored in the previous catalysts for fuel cells. Respectable hydrophilic NCNTs matrix restricts comprehensive contact between electrolyte and active sites thereby regulating the oxygen diffusion to the active sites and avoid flooding, hence the enhanced mass transport boosts fuel cell performance. Decent hydrophilic nature of the PtCoNi@NCNTs catalyst facilitates the three-phase interface at the cathode that offers efficient mass transport and excellent conductivity for efficient oxygen reduction reaction. Higher graphitic properties offer advanced durability to the NCNTs support against corrosion and oxidation at cathodic working conditions, while few layers carbon coatings shield the Pt nanoalloy against dissolution in acidic conditions thereby offering pronounced durability for long term fuel cell operation. Therefore, such novel graphitic NCNTs support material endows superior durability to the electrocatalyst in real working conditions of PEM fuel cell. In this work the existing gaps between the RDE activity and fuel cell performance have been investigated and optimized magnificently, which endorses the complete transformation of RDE activity to the MEAs level. This work may provide significant insights in designing the low-Pt integrated highly durable electrocatalysts for practical PEM fuel cells. Figure 1

Magnéli Phase Ti9O17- Catalyst Support Materials for ORR

The durability of catalysts in polymer electrolyte membrane fuel cells (PEMFCs) is an important issue that must be addressed before their commercialization since catalyst durability directly reflects the life and cost of fuel cell power-generation systems [1]. Although many factors could play a role in the mechanism of catalyst degradation, the major contributors are believed to be i) dissolution of Pt and re-deposition (Ostwald ripening), ii) coalescence via crystal migration, and iii) detachment of Pt particles from the carbon support [2]. Among these, Pt detachment is strongly affected by the corrosion of the carbon support materials. Conventional carbon blacks, such as Vulcan XC72, are superior catalyst-support materials because of their cost and physicochemical properties. However, carbon is not stable based on thermodynamic considerations under the cathode conditions in a PEMFC: C + 2H2O → CO2 + 4H+ + 4e− E0 = 0.207 V vs. RHE Although this reaction proceeds slowly at potentials <0.8 V in a PEMFC [3], corrosion reactions are significantly accelerated under high potential conditions. Therefore, alternative catalyst-support materials that are highly oxidation-resistant are necessary. In order to improve the stability of the catalyst support, considerable work has been done over the past few decades on non-carbon support materials [4].In this presentation, we have focused on oxygen-deficient, sub-stoichiometric titanium oxide (TinO2n−1, known as Magnéli phase) as a potential oxidation-resistant cathode catalyst support in PEFCs. This leads to improving specific activity for the oxygen reduction reaction (ORR) due to the formation of Pt–Ti alloy particles on the TiOx support, as well as the improve dispersion of Pt due to the deposition of smaller catalyst particles. The Magnéli phase Ti9O17 is synthesized and characterized (Fig. 1) by XRD. The Pt/Ti9O17 will be synthesized, and the ORR activity and stability will be investigated. A comparative study will be performed to those of a conventional Pt/C catalyst.

Nanostructuring Pt Electrocatalysts Supported on Nanocrystalline SnO2 for Polymer Electrolyte Fuel Cells

Introduction Conventional Pt/C electrocatalysts for PEFCs have a difficulty in durability against start-stop potential cycling, mainly due to degradation of carbon black through electrochemical oxidation especially on the cathode side. SnO2 is known to be stable at a high potential under the PEFC cathode conditions, so that SnO2 is considered as an alternative support material for PEFC cathodes [1-4]. In particular, we have prepared the oxide-core Pt-shell electrocatalyst that can reduce materials cost of PEFCs while developing electrocatalysts with both higher durability and ORR mass activity [3]. In our previous study [5], freeze drying has been successfully applied as an effective procedure for highly dispersing SnO2 nanoparticles on relatively-stable conductive framework, for nanostructuring PEFC electrocatalysts as shown in Fig. 1. Here, the objective of this study is to directly deposit Pt on SnO2 on an atomic level by applying a chemical procedure towards the oxide-core Pt-shell electrocatalyst preparation. Experimental Graphitized carbon black (GCB200, Cabot) was used as the conductive filler of the electrocatalysts acting as the conductive framework. SnO2 was prepared on the conductive filler via the microwave homogeneous precipitation. In order to deposit Pt on the surface of the SnO2, Pt(acac)2 and acetone were added to the Sn-containing solution. Pt/SnO2/GCB was prepared via the freeze drying, and heat treatment was performed to obtain a Pt-based catalyst powder. Nanostructure of the electrocatalysts obtained was observed by FE-SEM and STEM. ICP analysis was also made to quantify Pt loading. Electrochemical characterization of these electrocatalysts was carried out by half-cell measurements. Electrochemical surface area (ESCA) was measured by cyclic voltammetry (CV). Oxygen reduction reaction (ORR) activity was derived from kinetically controlled current density (ik) after the rotating disk electrode (RDE) measurement. Results and discussion We have succeeded to prepare SnO2 nanoparticles with a diameter of about 3 nm on the conductive filler in higher dispersion by applying the microwave homogeneous precipitation and the freeze drying. In addition, it was revealed that Pt nanoparticles were selectively deposited on nanocrystalline SnO2. Furthermore, high-resolution STEM observation revealed that a part of Pt was deposited near an atomic level on the SnO2 surface which could be a Pt-shell of the electrocatalysts, as shown in Fig. 2. Especially in Fig. 2(b), Pt atoms could be distinguished on the nanocrystalline SnO2 particles acting as a core of the electrocatalyst, with a minimized electron-conductive pathway between the Pt catalyst and the GCB conductive framework. Electrochemical properties of such nanostructured electrocatalysts will be reported and discussed. References Y. Nakazato, D. Kawachino, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi and K. Sasaki, J. Electrochem. Soc., 165 (14), F1154 (2018).S. Matsumoto, M. Nagamine, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 165 (14). F1165 (2018).M. Nagamine, Z. Noda, H. Manabe, J. Matsuda, A. Hayashi, and K. Sasaki, ECS Trans., 86 (13), 531 (2018).K. Kakinuma, R. Kobayashi, A. Iiyama, M. Uchida, J. Electrochem. Soc., 165 (15), J3083 (2018).T. Yoshizumi, M. Nagamine, Z. Noda, J. Matsuda, A. Hayashi, and K. Sasaki, ECS Trans., 92 (8), 479 (2019). Figure 1