High Current Density Region Research Articles

Self‐supported thin‐film electrode consisting of transition metal borides for highly efficient hydrogen evolution

AbstractTransition metal borides (TMBs) are a new class of promising electrocatalysts for hydrogen generation by water splitting. However, the synthesis of robust all‐in‐one electrodes is challenging for practical applications. Herein, a facile solid‐state boronization strategy is reported to synthesize a series of self‐supported TMBs thin films (TMB‐TFs) with large area and high catalytic activity. Among them, MoB thin film (MoB‐TF) exhibits the highest activity toward electrocatalytic hydrogen evolution reaction (HER), displaying a low overpotential (η10 = 191 and 219 mV at 10 mA cm−2) and a small Tafel slope (60.25 and 61.91 mV dec−1) in 0.5 M H2SO4 and 1.0 M KOH, respectively. Moreover, it outperforms the commercial Pt/C at the high current density region, demonstrating potential applications in industrially electrochemical water splitting. Theoretical study reveals that both surfaces terminated by TM and B atoms can serve as the active sites and the H* binding strength of TMBs is correlated with the p band center of B atoms. This work provides a new pathway for the potential application of TMBs in large‐scale hydrogen production.

Evaluation of hydrogen evolution activity by bubbles growth rates as descriptor

Evaluation of the electrocatalytic behavior in the high current density region is critical toward the applications in the industrial water electrolysis system. Here, we utilized the bubble growth rate as descriptor for evaluation of hydrogen evolution reaction. Ag electrode was selected as a model electrode. Pressure-controlled cell was used for the evaluation of the bubble behavior under the wider range in the overpotential. Hydrogen bubble evolution behavior was evaluated by video observation under electrochemical potential control. Even in the presence of the complex processes such as nucleation, growth and detachment of the bubbles, the hydrogen evolution behavior is evaluated with respect to the classical Tafel analyses even in high overpotential region. These analyses provide the methods for the rapid screening toward the material discovery of the green hydrogen production in high current density region.

Driving a deficient cathodic environment using anode to control selectivity for CO2 electroreduction

The anode has not been extensively studied in electrochemical CO2 reduction reaction (CO2RR) systems because it is believed to have no significant effect on the catalytic selectivity and activity of the cathode. Herein, we report that the anode can indeed influence the catalytic activity of the CO2RR by regulating the transfer of water and ions in a membrane electrode assembly. Despite employing identical cathode conditions, we observed optimal ethylene production at different current densities for distinct anode substrates: carbon paper (CP) and Ti meshes (TM). TM shows optimal ethylene Faradaic efficiency (31.1 %) in the high current density region, while CP demonstrates optimal ethylene Faradaic efficiency (20 %) in the low current density region. Based on the results of in situ/operando analysis and transparent cell tests, CP creates an environment deficient in water and cations, thereby promoting local pH effects. Consequently, this leads to a higher valence state of Cu during the CO2RR, resulting in increased catalytic activity for ethylene production. However, in the high-current-density region, the catalytic activity of CP degrades due to the uneven distribution of the reaction. Our study contributes to a deeper understanding of the reaction environment for CO2RR and offers insights for optimizing the catalytic activity of CO2RR systems.

Nanocrystalline transition metal tetraborides as efficient electrocatalysts for hydrogen evolution reaction at the large current density

While great efforts have been made to improve the electrocatalytic activity of existing materials toward hydrogen evolution reaction (HER), it is also importance for searching new type of nonprecious HER catalysts to realize the practical hydrogen evolution. Herein, we firstly report nanocrystalline transition metal tetraborides (TMB4, TM=W and Mo) as an efficient HER electrocatalyst has been synthesized by a single-step solid-state reaction. The optimized nanocrystalline WB4 exhibits an overpotential as low as 172 mV at 10 mA/cm2 and small Tafel slope of 63 mV/dec in 0.5 M H2SO4. Moreover, the nanocrystalline WB4 outperforms the commercial Pt/C at high current density region, confirming potential applications in industrially electrochemical water splitting. Theoretical study reveals that high intrinsic HER activity of WB4 is originated from its large work function that contributes to the weak hydrogen-adsorption energy. Therefore, this work provides new insights for development of robust nanocrystalline electrocatalysts for efficient HER.

Study on gas accumulation in polymer electrolyte membrane water electrolyzer considering two-phase flow

In this study, we analyze gas accumulation in a polymer electrolyte membrane water electrolyzer by considering two-phase flows in the porous transport layer on the anode and cathode sides. First, effect of mass flow rate on gas accumulation is analyzed. The transport of liquid water occurred differently on the anode and cathode sides. While the transport of liquid water on the anode side decreased in the high current density region, that on the cathode side increased with current density. The limiting current density increased by 1.2 A/cm2 when the mass flow rate was increased. Next, we study the effect of contact angle of the porous transport layer. When the contact angle was decreased from 75° to 30°, gas saturation decreased by 0.04. The use of a hydrophilic porous layer on the cathode side, however, hindered the transport of liquid water into the channel because the porous transport layer tended to retain liquid water. Finally, we study the effect of porosity of the porous transport layer. When the porosity was 80%, the mass flow rate of liquid water into the porous layer was 0.1 g/s, and gas saturation was approximately 0.25 at 0.8 A/cm2.

Structural and Transport Properties of Hydrophilic and Hydrophobic Modified Ionomers in Proton Exchange Membrane Fuel Cells.

The sluggish commercial application of proton exchange membrane fuel cells (PEMFCs) with low Pt loading is chiefly hindered by concentration polarization loss, particularly at high current density regions. Addressing this, our study concentrates on the ionomer membranes in the cathode catalyst layer (CCL) and explores the potential of incorporating additional hydrophilic or hydrophobic components to modify these ionomers. Therefore, an all-atom model was constructed and for the ionomer and hydrophilic and hydrophobic modifications were implemented via incorporating SiO2 and PTFE, respectively. The investigation was conducted via molecular dynamics (MD) simulations to predict the morphology and structure of the ionomer and analyze the kinetic properties of oxygen molecules and protons. The simulation results elaborate that the hydrophilic and hydrophobic modifications favor the phase separation and the self-diffusion coefficients of oxygen molecules and protons are enhanced. Considering the hydration level of the ionomer films, hydrophilic modification facilitates mass transfer under low-hydration-level conditions, while hydrophobic modification is more effective in optimizing mass transfer as the hydration level increases. The optimal contents of SiO2 and PTFE for each hydration level in this work are 9.6% and 45%, respectively. This work proposes a reliable model and presents a detailed analysis of hydrophilic and hydrophobic modifications, which provides theoretical guidance for quantitative preparations of various composite membranes.

Single Cell Performance and Durability of Pt and Pt-Based Catalysts Supported on Mesoporous Carbon

Introduction In Japan, an extremely high cell performance, i.e., 0.84 V at 0.2 A/cm2 and 0.70 V at 3.0 A/cm2, is required for polymer electrolyte fuel cells (PEFCs) used in fuel cell electric vehicles after 2030 [1]. In addition to high oxygen reduction reaction (ORR) activity of the Pt and the Pt-based cathode catalysts for increasing cell voltage in the low current density (LCD) region, high oxygen diffusivity is crucial for achieving the cell voltage in the high current density (HCD) region [2]. Therefore, it is considered that carbon supports for the catalyst NPs would play an important role for realizing the high cell performance. In this study, a mesoporous carbon (MPC) was selected as the support for Pt and Pd@Pt core-shell structured catalyst NPs and cell performance of the catalysts were compared with that of the catalyst supported on a conventional Ketjen Black (KB). Furthermore, durability of the MPC support against a high potential range was investigated. Experimental KB-600JD (SBET: 1,335 m2/g, LION) and MPC (CNovel MH-18, SBET 1,334 m2/g, TOYO TANSO [3]) were used as carbon supports. The Pt/MPC catalyst, Pt diameter of 2.5 nm and metal loading of 50 wt.%, was synthesized through impregnation of Pt(NO2)2(NH3)2 followed by thermal reduction in N2 at 400oC. Pt/Pd/KB and Pt/Pd/MPC core-shell catalysts were synthesized by a direct displacement reaction (DDR) method [4]. Diameter of Pd@Pt NPs was 5.8 nm and Pt and Pd metal loading were 20 wt.% and 30 wt.%, respectively. Cyclic voltammogram was recorded in Ar-saturated 0.1 M HClO4 at 25oC with a scan rate of 50 mV/s and ORR activity was evaluated in O2-saturated 0.1 M HClO4 at 25oC with a positive potential scan rate of 10 mV/s at 1,600 rpm. Fuel cell performance was evaluated by using a single cell with an active area of 1 cm2. Nafion DE2020 was used as an ionomer for the catalyst ink preparation (I/C ratio was set to 0.83-1.2) and Nafion NRE 211 with 25 µm in thickness was used as a membrane. Pt loading in the cathode catalyst layer was set to 0.1 mg/cm2. H2 and air were supplied to the anode and the cathode by 418 NmL/min. and 988 NmL/min., respectively, and pressurized to 150 kPa at gas outlets. The cell temperature was set to 80oC and humidified from 35% to 95% relative humidity (RH). H2 utilization was 5% at a current density of 3.0 A/cm2. Durability of the carbon supports was evaluated by using the single cell. H2 and N2 were fed to anode and cathode, respectively, and cathode potential was swept from 1.0 to 1.5 V vs. RHE with a scan rate of 500 mV/s for 1,000 cycles. Results and Discussion Pore size distribution of the carbon supports is shown in Figure 1. Both KB-600JD and MPC (CNovel MH-18) carbon supports have mesopores, and the MPC exhibits a narrower pore size distribution centered at 4 nm. Figure 2 demonstrates I-V performance of Pd@Pt core-shell catalysts, indicating that Pt/Pd/MPC catalyst shows a higher cell voltage in HCD region (> 2 A/cm2) than that of Pt/Pd/KB-600JD catalyst. Interconnectivity of the mesopores analyzed by 3D-TEM observation of the carbon supports is depicted in Figure 3. It is clear that density of interconnected mesopores is much higher in the MPC support, which is considered to contribute to the higher cell voltage in HCD region of the Pt/Pd/MPC catalyst. Figure 4 displays cross-sectional SEM image of the Pt/Pd/MPC catalyst. In addition to the well-interconnected mesopores (Figure 3), macropores coexist in the MPC support, which is also considered to contribute to the higher cell voltage in HCD region.A high potential durability test was performed by using potential cycling of 1.0-1.5 V vs. RHE for 1,000 cycles at 80oC. Thickness of Pt/MPC cathode catalyst layer was decreased by 36% after the durability test, while the decrease was mitigated to 14% with a heat treatment of the MPC support at 2,200oC in Ar. (Figures 5). As is well known, it was demonstrated that the high temperature heat treatment has a positive effect for durability improvement of the carbon support against the high potential range.This study was partly supported by NEDO, Japan.

Catalyst Layer Structure Properties and Electrode Performance with the Addition of Polyethylene Glycol

Reducing the platinum loading in Proton Exchange Membrane Fuel Cells to mitigation costs is of topical interest. Lowering the Pt content beyond critical thresholds has a direct impact on the kinetic conversion efficiency of O2, and result in local oxygen transport resistance in the catalyst layer at high current densities. Improvements in the Pt use efficiency can be achieved by affecting the catalyst layer structure properties with improved Pt dispersion and increased ionomer interface1-3.In this study, we are investigating how the structure properties of the catalyst layer are affected by the addition of polyethylene glycol (PEG) during the ink step. PEG has a high boiling point (>200°C) and is known to have pore forming properties4. It has also been reported to extend the reach of the ionomer and its resulting protonic connectivity. Increasing the porosity in the catalyst layer can increase the triple phase boundaries and reduce interfacial resistances 5.Membrane electrode assemblies (MEAs) have been made with inks containing increasing wt.% PEG. The resulting inks were coated onto membranes to form catalyst layers containing 0.2 mgPt/cm2. The addition of PEG affected the binding of the ionomer and the catalyst, which will likely affect the packing of the agglomerates in the resulting catalyst layer structure. The catalyst layer thickness, surface area and porosity were determined. The coated substrates were assembled into single cells for electrochemical evaluation. The resulting MEAs showed that the inclusion of PEG reduced the kinetic losses at low current density due to improved electronic connectivity between the catalyst and the ionomer. Overall, the performance increased by ≈30mV at 1A/cm2.However, PEG content higher than 3% resulted in a significant decrease in performance in the high current density region. This is due to a higher packing density and therefore higher mass transport resistance with higher PEG content.The inclusion of PEG into the catalyst ink reduced the amount of flammable organic content and improved the MEA performance especially at low current density. Catalyst layer deposition was smoother with reduced cracking. Ultimately, the ambitions for low loading MEAs can be achieved by optimising the structure performance properties in the catalyst layer. PEG is considered a viable candidate for performance enhancement for low loading MEAs, yet a number of practical obstacles were experienced and require some optimization.

Improvement of Current Efficiency of PEM Electrolyzer for Organic Chemical Hydride Energy Carrier Production with Water Electrolysis by Humidified Air Supply to Anode

Introduciton To improve hydrogen energy carrier synthesis, we have studied toluene electrohydrogenation and water decomposition using proton exchange membrane (PEM) electrolysis. The cathode reaction is electrohydrogenation of toluene to synthesis methylcyclohexane instead of hydrogen evolution reaction of water electrolysis. Only side reaction of the cathode is hydrogen evolution reaction, which was affected by electroosmotic water from the anode to cathode. Sulfuric acid solution feeds as anolyte that is not only reactant reduces water transportation and increases current efficiency [1, 2]. As practical system, water transportation from anode to cathode should be suppressed without sulfuric acid utilizationIn this study, to improve current efficiency by water management, we have investigated humidified air feed to reduce water transfer to cathode compare with liquid water feed. Experimental A single cell electrolyzer made of titanium with 16 cm2 of projected electrode area was used. Figure 1 shows the schematic drawing of toluene direct hydrogenation electrolyzers with humidified air feed. Membrane-electrodes assembly was consisted of Nafion® 117 (DuPont), 0.6 mg cm-2 of PtRu/C (TEC61E54, TKK, I/C = 0.8) cathode catalyst layer and 2.0 mg cm-2 of IrOx (TEC77100, TKK, I/IrOx = 0.28). Cathode backing was a carbon paper (10BC, SGL) of 0.02 mg cm-2 of Pt loading, which works flow filed of toluene-methylcyclohexane, too. Anode baking as a Pt plated sintered Ti web (Bekaert), and anode flow filed was serpentine type. 10 mL min-1 of toluene or 10% toluene – methylcyclohexane was fed to cathode. 5.0 L min-1 (313 mL min-1 cm-2) of fully humidified air or liquid water was fed to cathode. Operation temperature was 80oC. Internal resistance was determined by high frequency intercept of real axis of the electrochemical impedance spectroscopy in the current density region between 0.1 to 0.6 A cm-2. The water transportation through the membrane was determined by separated water in outlet liquid from cathode during constant current electrolysis, and the Faraday efficiency was determined by generated gas volume from the cathode, simultaneously. Results and discussion Figure 2 shows the cell voltage and current efficiency as a function of the current density for 10% toluene-methylcyclohexane or 100% toluene to cathode and 5.0 L min-1 of fully humidified air or liquid water to anode feed. Cell voltages were almost same for all experiments in less than 0.3 Acm-2 of current density region, while cell voltages of humidified air feed were higher than that of liquid water feed in higher current density region. Here, internal resistance of humidified air and liquid water feed were 0.29 and 0.21 Ω cm2. Therefore, membrane condition of the humidified air feed was drier than that of the liquid water feed. Water transportation through the membrane around 0.2 mg min-1 cm-2, which was less than 1/10 of the liquid water or 1 M H2SO4 feed electrolysis. The current efficiency for 100% toluene feed was more than 95% at 1 A cm-2, and slightly decreased with current density. In this case, the current efficiency of the liquid water feed was a little higher than that of the humidified air feed. The current efficiency of the humidified air feed was more than 95% up to 0.5 Acm-2, while the current efficiency of the liquid water feed decreased 95% around 0.2 Acm-2. In addition, the current efficiency of 10% toluene-methylcyclohexane significantly decrease with current density.The current efficiency would be affected by the cathode potential and toluene transportation in the catalyst layer. For the 100% toluene feed, toluene transportation would less affect and cathode potential would much affect to the current efficiency. Larger internal resistance of the humidified air feed would correspond to dry condition of cathode ionomer, and it would decrease the cathode potential. Therefore, the current efficiency of humidified air was slightly lower than liquid water feed. On the other hand, the current efficiency of 10% toluene feed would be controlled by toluene transportation in the cathode catalyst layer and is significantly affected by the water transportation through the membrane. Therefore, the current efficiency of the humidified air feed significantly higher than that of the liquid water feed. Acknowledgement This study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project (P14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References Nagasawa, K. Tanimoto, J. Koike, K. Ikegami, S. Mitsushima, J Power Sources, 439, 227070 (2019).Oi, K. Nagasawa, T. Takamura, Y. Misu, K. Matsuoka, S. Mitsushima, ECS Meeting s, MA2021-02, 1737 (2021). Figure 1

Towards Low Iridium Loading in Proton-Exchange Membrane Water Electrolysis: Full-Cell Performance of IrOx@TiO2 Core-Shell Particles in Anode Catalyst Layers

Hydrogen as a promising energy carrier to bridge the gap between supply and demand of renewable energies can be produced by proton-exchange membrane water electrolysis (PEMWE). However, the scarcity of iridium limits the widespread and large-scale implementation of PEMWE. Thus, the state-of-the-art iridium loading for the oxygen evolution reaction (OER) of ~1-2 mgIr·cm-2 must be distinctly reduced.1, 2 A decrease in loading involves a reduction of the catalyst layer (CL) thickness. The reduction is, however, only possible down to a certain threshold (≤ 0.5 mgIr·cm-2), below which the CL shows cracks leading to incomplete connection of the catalyst particles.1, 3 To overcome this issue, we developed a novel catalyst structure with reduced iridium content (< 50 wt% Ir). The synthesized catalyst exhibits a core-shell structure consisting of an insulating TiO2 core (d ≈ 1 µm) coated with iridium oxide (IrOx@TiO2). This optimized structure enables a reduction of the loading while maintaining the thickness and, thus, the electrical connection within the CL.In two related contributions, we demonstrate the suitability of the catalyst by a comprehensive analysis: The first contribution mainly focuses on structural optimization, activity, and stability investigations of the catalyst powder. In this contribution, we optimize the catalyst layer aiming towards high catalyst utilization and in-plane conductivity at iridium loadings below 0.5 mgIr·cm-2. PEMWE full-cell performance is investigated in catalyst-coated membrane (CCM) configuration and the structure and thickness of the CL are characterized by SEM cross-sections. The results of the full-cell tests with our catalyst in comparison to a state-of-the-art IrO2/TiO2 catalyst are shown in Fig. 1. The polarization curve of our catalyst exhibits high OER activity and performance at low loadings (Graph A: 1.76 V @ 1.5 A·cm-2). In comparison to the state-of-the-art catalyst (Graph B), our catalyst outperforms the commercial catalyst in terms of activity. This becomes evident when comparing the HFR-free polarization curves in the low current density region. It could be assigned to a higher catalyst utilization due to the maintained CL thickness or, more likely, to a higher activity of the catalyst itself.1 The performance difference between our and the state-of-the-art catalyst in higher current density regions mainly stems from ohmic losses in the electrode as can be seen from the high frequency resistance (HFR) in Fig. 1. These ohmic losses, however, can supposedly be assigned to the material itself and not to a decreased in-plane conductivity due to a disconnection of parts of the CL.In conclusion, we successfully synthesized an IrOx@TiO2 catalyst with a core-shell structure. This catalyst enables the fabrication of catalyst layers with decreased loadings below 0.5 mgIr·cm-2 while maintaining the catalyst utilization demonstrated in full-cell testing. Therefore, our newly developed catalyst contributes to a scale-up of the PEMWE technology by drastically reducing the demand for the scarce element iridium.Figure 1: Polarization curves, HFR-free polarization curves and HFR values at 80°C (ambient pressure, 100 mlH2O·min-1 on anode) of CCMs fabricated with different anode catalyst types and loading. A: in-house core-shell catalyst with 0.2 mgIr·cm-2 and B: commercial catalyst with 0.3 mgIr·cm-2 (IrO2/TiO2 with 75 wt% Ir, Umicore).References M. Möckl, M. F. Ernst, M. Kornherr, F. Allebrod, M. Bernt, J. Byrknes, C. Eickes, C. Gebauer, A. Moskovtseva and H. A. Gasteiger, J. Electrochem. Soc., 169(6), 64505 (2022).C. Minke, M. Suermann, B. Bensmann and R. Hanke-Rauschenbach, Int. J. of Hydrog. Energy, 46(46), 23581–23590 (2021).M. Bernt, A. Siebel and H. A. Gasteiger, J. Electrochem. Soc., 165(5), F305-F314 (2018). Figure 1

Investigating Oxygen Reduction Performance of Fe-N-C Catalysts for Proton Exchange Membrane Fuel Cells

Non-precious Fe-N-C catalysts have reached high activity levels for acidic oxygen reduction reaction (ORR) in rotating ring disk electrode (RRDE) setup approaching to that of the platinum group metals (PGM) when compared at high catalyst loadings of 0.6-1.0 mgFe-N-C cm-2. The improvements in the ORR activity of Fe-N-Cs can be attributed to new synthetic routes developed in recent years that have led to increased Fe-N4 active site density and utilization and/or better intrinsic activity/turnover frequency (TOF) of those sites.1-3 Despite impressive activities in half-cells, Fe-N-Cs still require quite high loadings in the range of 4.0-5.0 mg cm-2 in proton exchange membrane fuel cells (PEMFCs), which makes the Fe-N-C cathodes 4-5x thicker than Pt-based cathodes (~100 µm for Fe-N-C layer vs ~10-20 µm for Pt layer). Thick Fe-N-C cathodes suffer from quick performance drop in medium to high current density region due to mass-transport related losses. Reducing the Fe-N-C loading levels in RRDE and in PEMFC without comprising the ORR activity would need further improvements in active site density, site utilization and TOF. In this talk, I will present our work on how morphological tailoring of carbon framework in Fe-N-Cs affects the utilization of active sites and water adsorption-desorption behaviour with the aim of improving the ORR performance and mass-transport properties. Active-site imprinted carbon frameworks are used to prepare Fe-N-Cs that consist of purely Fe-N4 sites without any other iron species.1,4,5 Besides activity, performance stability against extensive voltage cycling in RRDE and long-term durability in PEMFC single-cells is another important criterion that Fe-N-Cs need to fulfil for becoming a viable substitute to PGM catalysts. Very recently, encouraging results on the improvement of Fe-N-C durability in PEMFC tests have been reported using radical scavengers and by coating a thin carbon layer on the surface of Fe-N-C.6,7 However, significantly more efforts are required in that direction for reaching the durability targets of thousands of hours. Second part of the talk will discuss the performance stability of Fe-N-Cs.

Comparison of Different Carbon Supports By Simulation of Agglomeration in Polymer Electrolyte Fuel Cell Catalyst Ink

Polymer electrolyte fuel cells (PEFCs) are one of the most possible power sources for heavy-duty vehicles like trucks due to their high conversion efficiency and low environmental impact. Since platinum which is often used as a catalyst is expensive, it is essential to reduce the amount of platinum by increasing the power density. Regarding materials, it was reported that the activity is increased by supporting platinum on porous carbon [1]. However, there is a voltage drop in the high current density region comparing non-porous carbon. This is caused by oxygen transport properties, but the relationship with the carbon structure is unknown. Oxygen moves through the voids in the catalyst layer, so it is necessary to control the porous structure that forms the voids. This porous structure depends on the conditions of the catalyst layer fabrication process. In particular, the dispersion process is important to determine the size of the carbon agglomerate, or the backbone of the catalyst layer. In this study, an agglomeration model in catalyst ink was developed, and the dispersion process was verified by simulation under various conditions to elucidate the factors that affect agglomeration.The discrete element method (DEM) was used for the simulation. In this study, aggregates were approximated into spheres, and the forces between the particles were the drag force from the fluid, Brownian motion, and particle-particle interactions. Based on DLVO theory, the particle-particle interactions were given as an attractive force due to the van der Waals interaction and a repulsive force due to the overlapping of the electric double layer. In addition, the effect of ionomer was considered. The ionomer effect was introduced as a repulsive force and modeled with the same equation as the repulsion due to the electric double layer. The dielectric dependence equation of the ionomer shape that Magnus et al. modeled [2] was incorporated. As the amount of ionomer added increases, a binder effect is observed between the particles. This effect was also introduced using the adsorption data of ionomers on carbon. The carbon was assumed to be non-porous (Vulcan) and porous (Ketjen). Considering the difference in the platinum ratio on the surface depending on the carbon porosity, the Hamaker constant, which affects the van der Waals force, was determined. Also, the zeta potential, which affects the electric double layer force, was used at different values for carbons. In this study, the calculations were started with the particles uniformly dispersed in the ink. Periodic boundary conditions were used as boundary conditions. Using the above numerical model, simulations were conducted with different water-alcohol ratios and ionomer carbon ratios (I/C).As a result, it was confirmed that differences in the agglomerate size for two carbon supports. This result implies that the composition of the ink and the type of carbon affect the catalyst layer structure and cell performance.AcknowledgmentThis study was carried out under the project of Development of Advanced PEFC Utilization Technologies supported by the NEDO.Reference[1] Y. C. Park et al., J. Pow. Sou r., 315, 179 (2016).[2] M. So et al., Int. J. Hydrog. Energy, 44, 28984 (2019).

Investigation of nonlinear accelerated degradation mechanism in fuel cell stack under dynamic driving cycles from polarization processes

The nonlinear accelerated degradation severely limits the lifetime of fuel cells. This paper presents a comprehensive analysis of polarization processes during accelerated performance degradation. The double trap model is applied to reveal kinetic degradation, and an equivalent circuit model based on the distribution of relaxation times is proposed to identify and quantify the polarization loss and low-frequency inductive loop. Combined with monitoring dynamic voltage, the results show that the accelerated performance decline is dominated by the growth of platinum oxides rather than kinetic degradation and structural damage. The platinum oxides reduce the cathode mixed potential gradually, leading to the accelerated decay occurring earlier at low current density. Meanwhile, the high oxide coverage hinders oxygen diffusion and water removal, resulting in sudden and drastic concentration polarization in the high current density region. Furthermore, the formation and reduction of platinum oxide affect voltage stability and steady performance of loading and unloading processes due to their potential dependency. The present study helps in modeling and predicting the nonlinear accelerated ageing to improve the lifetime of fuel cells.

(Digital Presentation) Quantifying the Effect of Potential Cycling Conditions on the Resulting Performance of Stainless Steel as an Anode for Alkaline Water Electrolysis

Alkaline water electrolysis through the use of the anion exchange membrane (AEM) has great potential. The current industry standard for water electrolysis utilise diaphragm-based traditional alkaline water electrolysers (AWEs), however using AEM water electrolysers (AEMWEs) introduces benefits including greater current densities, superior hydrogen purity, simpler cell/stack designs and less corrosive electrolytes. These advantages are additions to the pre-existing benefits to AWE, namely the possibility of utilising inexpensive catalyst materials for both hydrogen and oxygen evolution.Great attention has been payed to stainless steel (SS) as an anode for AEMWE due to its fair activity, low cost and good stability. Potential cycling (PC) is one method of electrochemically modifying the surface of various SS structures to increase electrochemical activity, however the PC conditions thus far reported in literature are rife with variation. As such, the full extent of PC conditions on SS remain unreported. Herein, we seek to fill this gap in literature by potential cycling a series of SS felt (SSF) electrodes under varied scan rates and ranges. Special attention is payed to surface conditions due to the intricate nuances affecting the electrocatalytic activity of the surface oxide layer.Two redox couples are clearly visible in cyclic voltammetry (CV) in the range 0-1.90 VRHE, termed whole-range, namely the Fe+2/Fe+3 at 0-0.65 VRHE and Ni+2/Ni+3 at 1.50-1.75 VRHE. The SSF electrodes were PC over one of these ranges at either slow, intermediate or fast scan rates, producing six different SSF electrodes.Initiating measurements were carried out to ascertain the baseline performance, including whole-range CV, electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV). The pristine SSF surface was exceedingly sensitive to the whole-range CV sweeps during initial measurements. Thus, this work is divided between SSF electrodes with and without CV in the initial measurements, termed with and without pre-cycling.Without pre-cycling, the decline in charge transfer resistance (Rct) before and after PC was more than tenfold for all scan speeds. Scan speed was relevant, where slow and fast scan speeds resulted in a mean Rct reduction of 84.15 and 90.59% for fast and slow scan speeds respectively. Post-experimental surface analysis with X-ray photoelectron spectroscopy revealed a decline in the average oxidation state of the principle elements in the SSF electrodes (Fe, Ni and Cr) and a surface depletion of iron. The relative increase in surface concentration of nickel and chromium was highly correlated with the reduction of Rct. Evaluation of affiliated Tafel slopes and LSV curves reveals similar trends, where the latter indicates a fair performance increment between 8-21% for fast and slow scan speeds.With pre-cycling the decline in Rct was lower, in the range of 18-30% and 3-7% for the SSF electrodes cycled around the Fe+2/Fe+3 and Ni+2/Ni+3 redox couple respectively. The influence of scan rate was the same for these SSF electrodes as with those unexposed to pre-cycling. Tafel analysis reveals two dominant slopes, where PC elicits small changes in the low current density slope and greater changes in the high current density region for the Fe+2/Fe+3 cycled SSF electrodes. These trends are also seen in LSV curves, where the greatest improvement is clearly seen for the SSF electrode cycled with a slow scan rate.Tafel analysis shows that the SSF electrodes cycled around the Ni+2/Ni+3 redox couple display a small decline in kinetics following PC, which corresponds to the rather meagre improvement in Rct affiliated with the PC conditions. The same trends are also seen by comparing the before and after LSV curves.Additional measurements documenting the full cell performance of these anodes is necessary and will be featured in the unabridged version of this paper, in addition to a more thorough characterisation of the surface conditions.

Defect-Engineered ZIF-Derived Non-Pt Cathode Catalyst at 1.5mgcm-2 Loading for Proton Exchange Membrane Fuel Cells.

Due to the sluggish kinetics of the oxygen reduction reaction (ORR) by non-Pt based catalyst, high loading of catalyst is required to achieve satisfactory fuel cell performance, which inevitably leads to the increase of the catalyst layer thickness with serious mass transport resistance. Herein, a defective zeolitic imidazolate framework (ZIF) derived Co/Fe-N-C catalyst with small mesopores (2-4nm) and high density of CoFe atomic active sites are prepared by regulating the Fe dosage and pyrolysis temperature. Molecular dynamics simulation and electrochemical tests indicate that >2nm mesopores show insignificant influence on the diffusion process of O2 and H2 O molecules, leading to the high utilization of active sites and low mass transport resistance. The proton exchange membrane fuel cell (PEMFC) shows a high-power density of 755mWcm-2 with only 1.5mgcm-2 of non-Pt catalyst in the cathode. No apparent performance loss caused by concentration difference can be observed, in particular in the high current density region (1Acm-2 ). This work emphasizes the importance of small mesopore design in the Co/Fe-N-C catalyst, which is anticipated to provide essential guidance for the application of non-Pt catalysts.

The relative humidity sensitivity of fuel cell catalyst layer with solid or porous carbon support structure

The effective transport of proton and reactant within the electrode is heavily influenced by the pore structure of the carbon support hosting both Pt particles and ionomers. To this end, the impact of carbon structure on the Pt position, ionomer distribution, and voltage losses under various relative humidity (RH) conditions has been systematically investigated. The findings reveal that low-loading solid carbon-based catalysts display impressive performance at low RH, while high-loading porous carbon-based catalysts stand out at high RH. Furthermore, porous carbon-based catalysts exhibit high mass activity at low current density due to their reduced susceptibility to the poisonous effects of sulfonate groups. Meanwhile, solid carbon catalysts facilitate a more uniform ionomer thin-film and create a more active three-phase interface area, resulting in a satisfactory performance at the high current density region and low local-O2 transport resistance.

Interface engineering assisted Fe-Ni3S2/Ni2P heterostructure as a high-performance bifunctional electrocatalyst for OER and HER

Rational development and construction of electrochemical water splitting catalysts with excellent activity and super-durability utilizing earth-abundant elements are extremely desirable and imperative for energy crisis. Herein, a self-supported hierarchical Fe-doped nickel sulfide/nickel phosphide (nanoflower-linked nanosheet arrays grown vertically on nickel foam) heterostructure electrocatalyst was obtained via a successive structural transformation to realize high-efficiency alkaline OER and HER performance. Benefiting from the novel nanoflower-sheet crosslinked structure on NF, electronic coupling reconstruction and abundant edge sites, as-synthesized Fe-Ni3S2/Ni2P electrocatalyst exhibits remarkable electrocatalytic OER and HER activity in both low and high current density regions. Specifically, the Fe-Ni3S2/Ni2P electrode achieved low overpotentials of 172, 277 and 357 mV at current densities of 10, 50 and 100 mA·cm−2 for OER, respectively. A series of experimental results confirmed that the surface reconfiguration behavior of Fe-Ni3S2/Ni2P heterostructure led to the formation of amorphous M-OH bond system, which was answerable to the prominent OER catalyst properties. This work confirms the potential of designing robust bifunctional electrocatalysts for alkaline water electrocatalysis by carefully combining doping engineering as well as interfacial effects.

A novel cathode flow field for PEMFC and its performance analysis

A good flow field design is important to the proton exchange membrane fuel cell (PEMFC) performance, especially under a high current density region, which is dominated by concentration polarization. Motivated by the variable cross-section channel idea, in this study, a novel flow field containing a converging-diverging (C-D) pattern is proposed. A three-dimensional multiphase model is established to study its performance. The numerical results show that it outperforms the conventional straight channel and only depth-variant channel. In the novel flow field the enhanced under land cross flow and higher effective mass transfer coefficient both improve the reactant transport. The effects of operating conditions, like stoichiometric ratio and operating pressure, on cell output performance are studied. It is found that a higher promotion rate can be obtained by increasing the stoichiometric ratio, but increasing the operating pressure has little effect. The droplet dynamic behavior in the C-D channel and straight channel are studied, and the results prove the better drainage capability of the novel flow field.

Self-supported electrodes to enhance mass transfer for high-performance anion exchange membrane water electrolyzer

The rational design of electrodes is essential to achieve efficient hydrogen production via water electrolysis. This study confirms the crucial role of physical properties of electrodes in enhancing the mass transfer augmenting performance of the electrolyzer, particularly in the high-current–density region. The electrodeposition method is used to fabricate self-supported NiMo and NiFe electrodes with high wettability, porosity, and gas permeability. Then, various combinations of membrane electrode assembly with as-prepared and commercial electrodes are used to provide insight into the electrode design for mass-transfer behavior in anion exchange membrane water electrolyzer (AEMWE). The overpotentials of AEMWE with self-supported electrode pairs significantly decrease, particularly in the high-current–density region. It demonstrates a current density of 7.5 A/cm2 at a lower heating value efficiency of 50 % and excellent stability at a current density of 1.00 A/cm2 for 100 h. These results emphasize the crucial role of the rational electrode design in achieving high-performance AEMWE.

Construction of catalyst layer network structure for proton exchange membrane fuel cell derived from polymeric dispersion

A rational design of the structure of catalyst layer (CL) is required for proton exchange membrane fuel cells to attain outstanding performance and excellent stability. It is crucial to have a profound comprehension of the correlations existing between the properties (catalyst ink), network structures of CL and proton exchange membrane fuel cells’ performance for the rational design of the structure of CL. This study deeply investigates the effects of a series of alcohol solvents on the properties and network structure of CL. The results demonstrate that the CL aggregates in higher ε solution show smaller particle sizes, and the sulfonic acid groups (∼SO3H) tend to extend more outward due to the strong dissociation. A more continuous and homogeneous ionomer distribution around Pt/C aggregates is observed in the CL, which improves the electrochemically active surface area (ECSA) and performance of the electrode. But, the electrode has a poor performance at high current density regions due to the mass transfer resistance. Based on this, a two-step solvent control strategy is proposed to maintain uniform ionomer and aggerates distribution and optimize the mass transfer for CL. The performance of the cell improves from 0.555 V to 0.615 V at 2000 mA·cm−2.