Oxygen Reduction Reaction Rate Research Articles

Effect of the Electrolyte on the Oxygen Reduction Reaction with PCN-224(Co).

Electrocatalysis in metal-organic frameworks is an interplay between the diffusion of charges, the intrinsic catalytic rate, and the mass-transport of reactants through the pores. Here a systematic study is carried out to investigate the role of the electrolyte nature and concentration on the oxygen reduction reaction (ORR) with the PCN-224(Co) MOF in aqueous electrolyte. It was found that the ORR activity is slightly influenced by the nature of the ions in solution, providing that the ionic strength is high enough to minimize the resistivity during the measurement. The ORR activity was found to be 1.3-1.5 times lower in lithium acetate compared to sodium acetate, while the ORR activity in cesium acetate was 1.3-1.6 times higher compared to the activity in sodium acetate. Moreover, there was no dependency found of the ORR catalysis on the size of the anion, buffer concentration, or oxygen concentration. These findings suggest that ORR catalysis in PCN-224(Co) is limited by the intrinsic ORR rate at the active site rather than charge transport through the porous structure or substrate transport in the pores. Therefore, optimization of ORR catalysis with this MOF might be achieved by the optimization of the electronics at the cobalt active site.

Revealing the Potential-Dependent Rate-Determining Step of Oxygen Reduction Reaction on Single-Atom Catalysts.

Single-atom catalysts (SACs) have attracted widespread attention due to their potential to replace platinum-based catalysts in achieving efficient oxygen reduction reaction (ORR), yet the rational optimization of SACs remains challenging due to their elusive reaction mechanisms. Herein, by employing ab initio molecular dynamics simulations and a thermodynamic integration method, we have constructed the potential-dependent free energetics of ORR on a single iron atom catalyst dispersed on nitrogen-doped graphene (Fe-N4/C) and further integrated these parameters into a microkinetic model. We demonstrate that the rate-determining step (RDS) of the ORR on SACs is potential-dependent rather than invariant within the operative potential range. Specifically, under the charge-neutral condition, the RDS is calculated to be water desorption with the highest barrier, while as the potential increases, it gradually transitions to the protonation of *OH species, O2* species, and O* species, regardless of the protonation of *OH species as the potential-determining step. Moreover, we reveal the critical role of the dynamic adsorption of axially adsorbed water in facilitating the release of the single-atom site, thus enhancing the ORR rate. Our work has resolved the long-standing controversies over the RDS of ORR on SACs and implies that the step with the lowest exothermicity is not always synonymous with the RDS, highlighting the importance of examining the kinetic barriers under realistic potential conditions for understanding the electrocatalytic performance.

Determination Method of Reaction Rate and Mass Transfer Parameters in Cathode Catalyst Layer in PEFC

IntroductionIt is important to optimize the cathode catalyst layer (CCL) because the oxygen reduction reaction (ORR) in the CCL is sluggish. Our previous study showed that the mass transfer resistance in the CCL can be quantified by measuring the ORR rate dependency on the oxygen partial pressure at fixed temperature[1,2]. In this study, polarization curves were measured and analyzed at varied oxygen partial pressure and temperature. The objective of this study is to establish the quantification methods of mass transfer and reaction rate parameters, which explain the behavior of polarization curves at various conditions.ExperimentalA cell whose active area was 20 mm × 20 mm was used in experiments. The membrane electrode assembly (MEA, Eiwa Corp.) consists of proton exchange membrane (PEM, Chemours NafionTM, NR-212) and two catalyst layers of each side of the PEM. Platinum catalyst supported on Ketjen Black (Pt/C weight ratio: 1.0) and a NafionTM ionomer (ionomer/carbon weight ratio: 1.0) consists of the catalyst layer. Oxygen and nitrogen were supplied to the cathode in order to vary the oxygen partial pressure. H2, N2, and O2 were humidified in bubblers to keep the inlet relative humidity (RH) at 0.8. The cell temperature was varied at 50, 60, 70, and 80 °C. The Oxygen mole fraction at the cathode was also varied at 1.5, 3, 6, 10, and 21 % on a dry basis.Results and DiscussionPolarization curves at varied oxygen partial pressure, p Oc and temperature are shown in Figs. 1–4. Since the reaction and mass transfer rate are improved at higher temperature, the current density at fixed cathode electromotive force, E cm increases when temperature increases as far as RH is identical. Arrhenius plot at the E cm of 0.2 V is shown in Fig. 5. Since the reaction rate is proportional to the oxygen partial pressure[3,4], the current divided by the oxygen partial pressure on the vertical axis in Fig. 5 is proportional to the observed reaction rate constant. Fig. 5 shows that the apparent activation energy decreases as temperature increases at the p Oc range of 2.5 to 7.7 kPa. This result indicates that the reaction rate saturates when temperature increases due to the effect of oxygen permeation resistance of the ionomer layer. The observed reaction rate constant decreases at higher p Oc , particularly at 15.9 kPa, due to the effect of proton transfer resistance. Fig 6. shows the apparent activation energies plotted against E cm . Both apparent activation energies of the ORR did not depend on E cm . Tafel slope, b c should be the function of temperature, T as expressed in Eq. (1): b c=RT/4aF, (1)where R, a, and F is the gas constant, transfer coefficient, and Faraday constant, respectively. The ORR rate constant, k vc ° is the exponential function of the inverse of the temperature. Considering Eq. (1), k vc ° should be expressed as follows: k vc° = k vc0exp(-E/RT-E c/b c) = k vc0exp(-(E+4aFE c)/RT), (2)where E is the activation energy. Although the apparent activation energy of the ORR should increase when E cm increases, the measurement results were independent of E cm , especially below 0.65 V. It indicates that the Tafel slope does not depend on temperature. The apparent activation energies of the ORR and of the oxygen permeance through the ionomer layer were obtained as 52 kJ/mol and 15 kJ/mol, respectively.Reaction rate constant, effective oxygen diffusion coefficient, effective proton conductivity, and oxygen permeance through the ionomer layer can be determined by applying the dimensionless isothermal 1D cathode model[1, 2, 4] to the obtained polarization curves.ConclusionsThe polarization curves at varied oxygen partial pressure and temperature were measured and analyzed. As the activation energy of the reaction is higher than that of diffusion and permeation, the observed activation energy in the Arrhenius plot at varied cathode electromotive force decreased when temperature increases. The observed activation energy of the ORR did not depend on the cathode electromotive force.AcknowledgementsThis work was supported by the FC-Platform Program: Development of design-for-purpose numerical simulators for attaining long life and high performance project (FY 2020–2023) conducted by the New Energy and Industrial Technology Development Organization (NEDO), Japan. This work was supported by Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (KAKENHI) Grant Number 22KJ1899.

Oxygen Concentration and Reaction Rate Profiles Analysis of Polymer Electrolyte Fuel Cell

Introduction A one-dimensional (1D) model of polymer electrolyte fuel cells (PEFCs) is especially suitable for performance diagnostics, design optimization, and material evaluation, due to its high computational efficiency. Uniform distributions of reactants are commonly assumed in 1D models, i.e., a single representative reactant concentration inside the gas channel is required. However, this concentration is strongly dependent on the flow field. Although plenty of 1D models with sufficient accuracy as 3D models have been reported, the effects of the flow field have not been sufficiently discussed in the literature. In this work, efforts have been put into developing a generalized theory suitable for various flow fields to quantitatively analyze the in-plane reaction rate profile, so that the flow-field-sensitive representative reactant concentration can be employed to further fill the gaps in the 1D models. Analytical model A series of a plug flow reactor (PFR) and a continuous stirred tank reactor (CSTR) was proposed to explain the unideal flow and the macromixing behaviors inside a gas channel coupled with a gas diffusion layer (GDL), as shown in Fig. 1(a).[1] The fraction of PFR’s space time to total space time was defined as the characterization factor γ, which should be the function of flow field shape and flow rate. Furthermore, under-rib oxygen concentration and current density profiles were taken into account by introducing the dimensionless moduli.[2] Especially, the under-rib convection can be evaluated by Péclet modulus Pe, by increasing which will improve the under-rib mass transfer and the effectiveness factor f. The in-plane profile of the ohmic resistance of the membrane was assumed uniform. The oxygen reduction reaction (ORR) was regarded as a surface reaction occurring on the interface of cathode catalyst layer (CCL) and GDL. The ORR rate was assumed to be proportional to oxygen partial pressure and independent of relative humidity (RH). Results and Discussion The characterization factors γ of different flow fields were determined according to the residence time distribution simulated by the computational fluid dynamics (CFD) model. The ORR rate constant at each electrical potential was obtained from the measured polarization curve of a parallel gas channel (PN0) with 0.35 mm channel depth, by employing the analytical model mentioned above. Fig. 1(c) shows the polarization curves of parallel channels with staggered 1-mm-long partially narrowed structures (PSN1) and PN0 with different channel depths. Compared to the PN0, the under-rib convection improves the oxygen mass transport and the cell performance, where f increases 17 % at 0.35 V in the case of the PSN1. On the other hand, increasing the channel depth restrains the oxygen diffusion and f decreases by 18 % at 0.35 V. The analytical model results show good agreement with experiment data, which indicates that the effects of the flow field can be summarized by the parameters γ and f.The analytical model provides the in-plane oxygen mole fraction and current density profiles. Fig. 2 shows the oxygen partial pressure distribution in a single gas channel calculated by the CFD model and analytical model. The mean oxygen partial pressure on the under-channel CCL-GDL interface calculated by the analytical model approximately equals the CFD result. By employing the ohmic resistance estimated according to the inlet conditions, although the current density is underestimated where RH is high, the integral mean of the in-plane current density offered by the analytical model has less than 5 % error compared to the CFD results, as shown in Fig. 3.The analytical model also gives an effective way to estimate the integral mean in-plane oxygen partial pressure. Fig. 4 demonstrates the relationships among integral mean, inlet, and outlet oxygen partial pressures. When the reaction rate constant and effectiveness factor are fixed, Fig. 4 also reveals the relationships between total and local current densities. If the parameters γ and f are determined, the integral mean of the oxygen partial pressure can be obtained when the outlet oxygen partial pressure or oxygen conversion is known. Conclusions The analytical model quantitatively demonstrates the effects of the flow field shape and provides a quick and effective way to estimate the reaction rate profile, based on the fixed in-plane ohmic resistance assumption. This model was verified by experiment and CFD model in case of the oxygen conversion less than 0.33 and can be applied to ORR kinetics determination and rapid performance prediction. Reference [1] Y. Ma et al., J. Electrochem. Soc., 170, 084506 (2023).[2] Y. Ma et al., ECS Meet. Abstr., MA2023-02, 1867 (2023). Figure 1

Developing Cobalt-Free, Strontium Titanium Ferrite Based Air Electrodes for Solid Oxide Cells

The development of stable air electrodes for Solid Oxide Cells operating at reduced temperatures (≤700°C) creates unique material design challenges. These electrodes must not only show excellent oxygen evolution and oxygen reduction reaction rates, but also be able to maintain these fast rates over the 40,000hr lifetimes envisioned for solid oxide cells. One material of significant interest for this is Sr(Ti0.3Fe0.7)O3-δ (STF), which is a mixed ionic-electronic conducting (MIEC) perovskite that has shown performance similar to that of LaxSr1-xCoyFe1-yO3-δ (LSCF) (1). Performance and stability substantially better than that of LSCF can be achieved by substitution of as little as 7% Cobalt onto the B-site of STF. (2) Reducing or eliminating the Cobalt while maintaining performance is desirable to avoid issues with its toxicity, availability, and cost. In this work, we explore Cobalt-free STF-based air electrodes; strategies explored include composites of STF and Gd0.1Ce0.9O2-δ (GDC), PrOX infiltration, and alternative B-site substitutions of STF with Ni and Cu.One of the aims of GDC-STF composite electrodes is to improve the thermal expansion coefficient (TCE) match and adhesion with the GDC reaction barrier layer, presumably making them more robust in thermal cycling. The 50% STF – 50 % GDC composites yielded polarization resistance values similar to those of pure STF electrodes, ~ 0.2 Wcm2 at 700C in air. Over the temperature range of 200°C-850°C, the coefficient of thermal expansion (CTE) of STF-GDC closely matches the CTE of GDC (~12×10-6 K-1), representing a significant improvement compared to STF. As previously discussed by Lu et al (3), the performance of pure STF electrodes can be significantly improved by infiltration with PrOx. Here we show that infiltration of STF-GDC composites with 1, 2, or 3 PrOx infiltration steps significantly decreases initial polarization resistance, e.g., from 0.5 to < 0.2 Wcm2 at 650C, with two infiltration steps yield the lowest polarization resistance. Furthermore, long term stability tests over >2 kh show that STF-GDC electrode degradation rates can be decreased by a factor of 4 through infiltration.Additional results for STF substituted with alternatives to Co, including Cu, Ni, and Cu-Ni, will be presented, along with the effects of exsolution induced by pre-reduction. S.-L. Zhang, D. Cox, H. Yang, B.-K. Park, C.-X. Li, C.-J. Li and S. A. Barnett, Journal of Materials Chemistry A, 7, 21447 (2019).S.-L. Zhang, H. Wang, M. Y. Lu, A.-P. Zhang, L. V. Mogni, Q. Liu, C.-X. Li, C.-J. Li and S. A. Barnett, Energy & Environmental Science, 11, 1870 (2018).M. Y. Lu, R. Scipioni, B.-K. Park, T. Yang, Y. A. Chart and S. A. Barnett, Materials Today Energy, 14, 100362 (2019).

Analysis of Entropy Generation Due to Heat and Mass Transfer in Porous Electrode of PEFC

During operation of polymer electrolyte fuel cells (PEFCs), the complicated multi-physics phenomena including heat and mass transfer, ionic and electronic transport and electrochemical reaction simultaneously occur in porous electrodes. It has been well known that the kinetic-theory based oxygen reduction reaction (ORR) and two-phase transport within cathode electrodes cause the severe overpotentials and performance degradation. In the previous studies, the energy loss in fuel cells and electrochemical devices has been experimentally investigated by using electrochemical impedance spectroscopy (EIS) and classified into the activation, ohmic and concentration polarization. EIS technique is a powerful method to analyze the sources of their performance loss, however, it cannot locally identify the key factors and transport processes causing the loss. To further enhance the power density and efficiency of PEFCs, it’s necessary to discriminate the critical phenomena influencing on the performance degradation using an alternative approach and design the better electrode/channel structure for reducing the energy loss. Several researchers attempted to apply the analysis of entropy generation to the elucidation of performance loss mechanisms in fuel cells and electrochemical devices [1-3]. Entropy is a physical property representing the thermodynamic irreversibility. The entropy generation analysis enables to identify the dominant phenomena inducing the overpotential in such devices and contributes to design the optimum structure minimizing the energy dissipation. Kjelstrup and Røsjorde [1] numerically estimated the local entropy production in various layers of a one-dimensional PEFC and found that the dissipation of energy due to charge transport in the electrolyte membrane is more important than the that of diffusion in the electrode. The author introduced the entropy generation analysis into the two-phase flow simulation for the cathode diffusion media of a PEFC and succeeded to locally estimate the entropy production rate due to oxygen diffusion within the porous electrode in the past work [3]. This study further analyzed the distributions of entropy production rate due to the heat transfer, diffusion and electrochemical reaction in the cathode porous electrode in more detail using the conventional two-phase flow PEFC model and extracted the significant process affecting the cathode overpotential. The author also investigated the effect of land/channel geometry on the local entropy generations of these phenomena in the electrode and discussed how to design its structure reducing the entropy generation.Fig. 1 shows the numerical results of the two-phase flow simulation and the entropy generation analysis in the cathode electrode. The two-phase flow model used in this study was developed by Natarajan and Nguyen [4]. (a), (b) and (c) denote the through-plane distribution of temperature across the cell and the profiles of oxygen concentration and water saturation in the cathode gas diffusion layer (GDL), respectively. (d), (e) and (f) are the distributions of entropy production rate due to heat flux, oxygen diffusion and ORR in the electrode. The fuel cell was operated for 50 s at the temperature of 80 oC and the current density of 1.0 A/cm2. It was noted that the entropy generation due to the electrochemical reaction gives a larger effect on the cathode overpotential than that of diffusion and heat transfer. That is very well known. The entropy production rate of ORR at the catalyst layer under the channel is larger than that under the land because the sufficient oxygen supply progresses the reaction more actively. Furthermore, the land/channel geometry has an influence on the entropy generation of diffusion and heat transfer in the cathode electrode. The entropy production rates due to oxygen diffusion and heat flux become remarkably high under the boundary between the channel and land owing to the large gradients of its concentration and temperature. To reduce the entropy generation in the porous electrode with the land/channel configuration, it’s necessary to design the cell structure uniformizing the reaction distribution and alleviating the gradients of current, concentration and temperature.

Performance and biotoxicity of electro-Fenton treatment of bisphenol A: Evaluation of copper recovered from microbial fuel cell cathodes for subsequent catalytic applications

A three-tank microbial fuel cell (MFC) is successfully constructed to achieve electromigration and is used in the electro-Fenton process to treat bisphenol A (BPA). The MFC system exhibits excellent mobility and efficiently aggregates Cu+ to the cathode. Under optimal operating conditions, the MFC achieves a maximum power density of 31.4 mW/m2, which is a 3.4-fold increase over that at 0.5 kΩ. Increasing the external resistance increased the MFC power output (ca 1.5 times) and copper ion migration (ca 4.6 times) while reducing the internal resistance of the system (ca 25.3 %). Surface analysis of the cathode carbon cloth shows a 5.5-fold increase in copper content at 1 kΩ over that at 0.5 kΩ. This also increases the oxygen reduction reaction rate, thereby increasing the H2O2 yield by 1.5 times. The recovered copper cathode at 1 kΩ exhibits the best catalytic degradation of BPA, removing 99.7 % of BPA in 180 min, increasing 1.4 and 1.8 times that obtained at 1.5 and 0.5 kΩ, respectively. The optimal operating conditions significantly increased the abundance of electrochemically active bacteria (Azospirillaceae) (3.2 %−41.7 %), indicating that the optimized MFC is favorable for the rapid acclimatization of electrochemically active bacteria.

ZIF-67 기반의 산소 환원 반응用 촉매 합성을 통한 Zinc-Air Primary Battery의 효율 개선에 관한 연구

Zinc-air batteries are attracting attention nowadays due to their environment-friendliness, stability, and high efficiency. However new catalyst development is necessary for commercialization with a slow reaction rate of ORR (Oxygen Reduction Reaction) occurring in the cathode and low efficiency compared to the price of Pt/C catalysts. This study produced a catalyst based on the ZIF-67 (Zeolic Imidazolate Framework-67) to compensate for this, and the effects of PVP addition, carbonization, and sulfidation on the catalyst efficiency were confirmed in the ZIF-67 synthesis process. As a result of the analysis, when 0.52 mmol of PVP was added and carbonization and sulfidation processes were performed, high efficiency, high driving force, stability, and life comparable to Pt/C catalyst were shown. Therefore, the ZIF-67-based oxygen reduction reaction catalyst is expected to contribute to the commercialization of Zinc-air batteries by replacing the existing Pt/C catalyst.

Designing highly efficient electrocatalyst for ORR and OER of transition metals doped g-C9N7: A density functional theory study

The application of fuel cells and metal-air batteries is limited by the rate of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Thus, the aim of this study is to find an efficient electrocatalyst to accelerate the reaction rate by DFT method. The structures of VIIB, VIII, IB, and IIB groups transition metals doped g-C9N7 as superb bifunctional electrocatalysts for ORR and OER are probed. The results show that Fe-, Pd-, and Au–C9N7 possess the superb ORR performance with the ORR overpotentials values of 0.35, 0.40, and 0.39 V, which gives them better performance than Pt(111). Moreover, Rh-, Pd-, and Pt–C9N7 have better OER performance with the OER overpotentials values of 0.30, 0.57, and 0.43 V, which makes them comparable to RuO2(110) and IrO2(110). Finally, the calculated bifunctional index (BI) results show that Pd–C9N7 can be used as an excellent bifunctional electrocatalyst because it has a minimum BI value of 0.97 V. This research has the potential to provide valuable theoretical direction into the use of M − C9N7 as electrocatalysts for both the ORR and OER.

CuO vs. AgO: A comparative study of cathode catalysts for boosting oxygen reduction in microbial desalination cells

Microbial Desalination Cell (MDC) is an innovative approach for water desalination that concurrently produces energy through the process of substrate oxidation. The primary issue of the technique is its limited power on capabilities during the oxygen reduction reaction (ORR). While the depletion of oxygen is a significant process in electrochemical energy conversion, its slow kinetics hinder its utilization in MDCs. One potential resolution to this problem is the utilization of a cathode catalyst with a high level of activity, which effectively enhances the rate of ORR. Different concentrations of copper oxide (CuO) nanoparticles (NPs) were used to investigate the behavior of ORR and its impact on the performance of the MDC. The optimal concentration of silver oxide (AgO) NPs was utilized to conduct a comparative analysis of both types of NPs. The size of CuO NPs and AgO NPs was found to be 65 nm and 55 nm respectively by material characterization studies. The power density achieved in the absence of the catalyst was 1.1 W/m3, and this value increased to 4.9 W/m3 with the incorporation of 3 mg/cm2 AgO. The use of a catalyst loading of 3 mg/cm2 has been determined to be efficacious in enhancing the desalination rate in MDC. The electrochemical investigations conducted also showed an improvement in the electrocatalytic behavior of cathodic kinetics while operating under these specific afterward. Therefore, the utilization of AgO catalyst exhibits potential as a viable option for the expansion of MDC processes, thereby contributing to the achievement of sustainable development objectives.

Optimized microbial fuel cell-powered electro-Fenton processes to enhance electricity and bisphenol A removal by varying external resistance and electrolyte concentrations

This study is the first to investigate the effects of external resistance and electrolyte concentration on the performance of a bioelectro-Fenton (BEF) system, involving measurements of power density, H2O2 generation, and bisphenol A (BPA) removal efficiency. With optimized operating conditions (external resistance of 1.12 kΩ and cathodic NaCl concentration of 1,657 mg/L), the BEF system achieved a maximum power density of 38.59 mW/m2, which is about 3.5 times higher than with 1 kΩ external resistance and no NaCl. This system featured a 71.7 % reduction in total internal resistance. The optimized BEF also accelerated the oxygen reduction reaction rate, increasing H2O2 generation by 4.4 times compared to the unoptimized system. Moreover, it exhibited superior BPA degradation performance, removing over 99 % of BPA within 14 hs, representing a 1.1 to 3.3-fold improvement over the unoptimized BEF. By the fifth cycle (70 h), the optimized BEF still removed 70 % of BPA. Optimizing the operating conditions significantly increased the abundance of electrochemically active bacteria (Pseudomonadaceae) from 2.2 % to 20 %, facilitating rapid acclimation. The study demonstrates the strong potential of an optimized BEF system for removing persistent pollutants.

Interrogation of Oxidative Pulsed Methods for the Stabilization of Copper Electrodes for CO2 Electrolysis.

Using copper (Cu) as an electrocatalyst uniquely produces multicarbon products (C2+-products) during the CO2 reduction reaction (CO2RR). However, the CO2RR stability of Cu is presently 3 orders of magnitude shorter than required for commercial operation. One means of substantially increasing Cu catalyst lifetimes is through periodic oxidative processes, such as cathodic-anodic pulsing. Despite 100-fold improvements, these oxidative methods only delay, but do not circumvent, degradation. Here, we provide an interrogation of chemical and electrochemical Cu oxidative processes to identify the mechanistic processes leading to stable CO2RR through electrochemical and in situ Raman spectroscopy measurements. We first examine chemical oxidation using an open-circuit potential (OCP), identifying that copper oxidation is regulated by the transient behavior of the OCP curve and limited by the rate of the oxygen reduction reaction (ORR). Increasing O2 flux to the cathode subsequently increased ORR rates, both extending lifetimes and reducing "off" times by 3-fold. In a separate approach, the formation of Cu2O is achieved through electrochemical oxidation. Here, we establish the minimum electrode potentials required for fast Cu oxidation (-0.28 V vs Ag/AgCl, 1 M KHCO3) by accounting for transient local pH changes and tracking oxidation charge transfer. Lastly, we performed a stability test resulting in a 20-fold increase in stable ethylene production versus the continuous case, finding that spatial copper migration is slowed but not mitigated by oxidative pulsing approaches alone.

Advanced electrocatalysts for fuel cells: Evolution of active sites and synergistic properties of catalysts and carrier materials

AbstractProton exchange‐membrane fuel cell (PEMFC) is a clean and efficient type of energy storage device. However, the sluggish reaction rate of the cathode oxygen reduction reaction (ORR) has been a significant problem in its development. This review reports the recent progress of advanced electrocatalysts focusing on the interface/surface electronic structure and exploring the synergistic relationship of precious‐based and non‐precious metal‐based catalysts and support materials. The support materials contain non‐metal (C/N/Si, etc.) and metal‐based structures, which have demonstrated a crucial role in the synergistic enhancement of electrocatalytic properties, especially for high‐temperature fuel cell systems. To improve the strong interaction, some exciting synergistic strategies by doping and coating heterogeneous elements or connecting polymeric ligands containing carbon and nitrogen were also shown herein. Besides the typical role of the crystal surface, phase structure, lattice strain, etc., the evolution of structure‐performance relations was also highlighted in real‐time tests. The advanced in situ characterization techniques were also reviewed to emphasize the accurate structure‐performance relations. Finally, the challenge and prospect for developing the ORR electrocatalysts were concluded for commercial applications in low‐ and high‐temperature fuel cell systems.

Systematic exploration of N, O coordination number on the catalytic performance for oxygen reduction and oxygen evolution

The reaction rate of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is an important factor restricting its application. In this work, we have systematic explorated of N, O coordination number on the catalytic performance for ORR and OER. Firstly, a variety of stability analysis methods show that most of the catalysts have excellent stability. Secondly, a series of excellent bifunctional catalysts are screened by volcanic maps, ORR overpotential (ηORR), OER overpotential (ηOER), and bifunctional index (BI), such as, RhN3O1-1 (ηORR = 0.28 V, ηOER = 0.34 V, BI = 0.62 V) and CoN3O1-1 (ηORR = 0.33 V, ηOER = 0.37 V, BI = 0.70 V). In particular, RhN3O1-1 has the smallest BI value, indicating that it has the best bifunctional catalytic activity. This study offers insights into how the coordination environment affects the activity of OER/ORR.

Mitigating hydrogen evolution reaction and corrosion of zinc in electrically rechargeable zinc-air batteries using nanofluid electrolytes

Among various metal-air batteries, zinc-air batteries (ZABs) have received tremendous attention owing to their abundant resources, cost-free fuel from the atmosphere and ease of fabrication. The performance of ZABs depends on the bifunctional electrocatalyst deployed at the air-cathode and the hydrogen evolution reaction (HER) occurring at the anode. While the rate of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) improve the performance of ZAB, HER causes zinc corrosion and thereby affects the performance of ZAB. The corrosion of zinc is attributed to the fact that HER is thermodynamically favoured since the standard reduction potential of zinc is more negative than hydrogen. Herein, the onset and overpotential of HER are increased by the uniform dispersion of metal oxide nanoparticles in the native electrolyte (6 M KOH), thereby reducing zinc corrosion. Dispersion of 0.1 wt% of SiOx or ZnO nanoparticles increases the HER overpotential to 547 and 679 mV, respectively. The microscopic investigations confirm the inhibition of corrosion and dendritic growth of zinc. Besides these, nanofluid electrolytes enhance the kinetics of ORR and OER, thereby improving the performance of electrically rechargeable ZABs. Also, nanofluid electrolytes are stable over three months.

Quantification Method of Mass Transfer Resistance in Cathode Catalyst Layer in PEFC

IntroductionTo spread PEFCs, it is important to optimize the cathode catalyst layer (CCL) because the oxygen reduction reaction (ORR) in CCL is sluggish. Our previous study proposed that the mass transfer resistance in the through-plane direction in CCL can be quantified by measuring ORR rate dependency on oxygen partial pressure, effective oxygen diffusion coefficient, ORR rate constant, or CCL thickness[1]. In this study, dependency of the polarization curve on the oxygen partial pressure was measured with commercial supported-platinum catalysts at fixed RH and temperature. The objective of this study is to establish the quantification methods of mass transfer resistances in CCL.TheoryIn our previous study[2, 3], the current density, i is expressed as i = 4F δ (C) k vcm p Oc F e [A/m2], (1)where δ (C) is the cathode catalyst layer (CCL) thickness, k vc is the reaction rate constant per unit volume of CCL [mol/(Pa·m3·s)] which does not include through-plane mass transport resistance. p O is the oxygen partial pressure [Pa]. The subscripts m and c denote PEM–CCL boundary and CCL–gas diffusion layer boundary, respectively. F e is the effectiveness factor, a function of the following 4 dimensionless moduli, M Om , M pm , P Om , and y Oc . F e is defined as the ratio of the observed reaction rate to the reaction rate which does not have the influence of through-plane mass transfer resistance. M Om denotes the ratio of O2 diffusion resistance to the reaction resistance. M pm denotes the ratio of proton transfer resistance to the reaction resistance. P Om is defined as the ratio of O2 diffusion resistance to the convection resistance. y Oc denotes the oxygen mole fraction. Definitions of the dimensionless four moduli are expressed as M Om =δ (C) (k vcm RT/D eO)0.5, (2) M pm = δ (C) (4F k vcm RT/(σ ep b c))0.5, (3) P Om = δ (C) N gm /(C g D eO), (4) y Oc = p Oc /P, (5)where R, T, σ ep, N gm and P denotes the gas constant [J/(mol K)], cell temperature [K], effective proton conductivity [S/m], total gas flux [mol/(m2 s)] and total pressure [Pa], respectively. When the cathode emf at the PEM–CCL boundary, E cm , T, and relative humidity (RH) are fixed, M Om is fixed. F e and M pm are proportional to i/p Oc and p Oc 0.5, respectively. The relationship between F e and M pm shown in Fig. 1 corresponds to that between i/p Oc and p Oc 0.5. Thus, F e and M pm can be determined just by fitting experimental data of i/p Oc plotted against p Oc 0.5 to the theoretical curves.Results and DiscussionPolarization curves shown in Fig. 2 were measured at 80 °C and 0.77 of inlet RH at varied p Oc . As expected from Eq. (1), current densities at a fixed E cm increases when p Oc increases. The current densities at the fixed E cm are plotted against p Oc as shown in Fig. 3. The current density is sublinear against p Oc . This is because M pm expressed in Eq. (3) increased when p Oc is higher. The ORR rate is lower than the value expected from the Tafel equation when mass transfer resistance increases.The dimensionless moduli were determined by fitting the theoretical relationship to the measured i/p Oc vs. p Oc 0.5 graph. From the determined dimensionless moduli, the ORR rate and mass transport parameters, k vc⊕, D eO and σ ep were also obtained and these values are shown in Table 1 with values in literature[4, 5].Effective proton conductivity, σ ep is relatively low compared with values in literature. This resulted from an experimental error in the current density at p Oc of 16 kPa. Lowering the current density at high p Oc leads to decrease in σ ep because M pm increases when p Oc is raised. By extensively measuring the polarization curves dependency on p Oc , the ORR rate parameters and mass transport parameters can be determined more accurately. Recalculated results using the obtained mass transport parameters are shown in Figs. 3 and 4. The recalculated results appropriately reproduced the ORR rate dependency on E cm and p Oc .ConclusionsThe through-plane mass transfer resistance in CCL was quantified by measuring the ORR rate dependency on the oxygen partial pressure. From the determined dimensionless moduli, mass transport parameters were obtained. Recalculated results appropriately reproduced the ORR rate dependency on cathode emf and oxygen partial pressure.AcknowledgementThis work was supported by the FC-Platform Program: Development of design-for-purpose numerical simulators for attaining long life and high performance project (FY 2020–2023) conducted by the New Energy and Industrial Technology Development Organization (NEDO), Japan. Figure 1

Modeling the Environment-Dependent Kinetics of Oxygen Reduction Reaction – a Continuum Model for Electric Double Layer

For proton-exchange-membrane fuel cells (PEMFCs) to achieve broad commercialization, improved energy-conversion efficiency with minimal Pt-based electrocatalyst is required [1]. Because the sluggish rate of oxygen reduction reaction (ORR) limits the efficiency of PEMFCs [2], the efficiency improvement requires a better understanding of ORR kinetics and mechanism to design better catalyst. To understand the ORR mechanism, theoretical and experimental analyses have been conducted. While previous studies reasonably explained the catalyst-dependent activity on single crystal catalysts in 0.1 M perchloric acid solution [3, 4], the explicit effect of electrolyte and related microenvironments [5, 6] is not thoroughly understood. The change in the electrolyte alters the electric-double-layer (EDL) structure and thus the local microenvironment at the electrode/electrolyte interface. Thus, the structure of the EDL should be carefully analyzed to uncover the electrolyte-dependent reaction kinetics. In this talk, we propose a multiscale continuum model to predict the EDL structure and examine the effect of perchloric acid concentration on ORR activity on Pt (111). The model includes Density Potential Functional Theory (DPFT) for electron density and Modified Poisson Boltzmann equation for species’ density and electric potential [7]. Also, the interaction between adsorbents and electric field is taken into account by minimizing the grand potential. After model validation with experimentally measured double-layer capacity data as a function of applied potential and concentration, the effect of the perchloric acid concentration (0.02 M – 0.2 M) on ORR activity is analyzed and discussed. It is shown that the model reproduces the specific activity obtained in the experiments when assuming the oxygen adsorption is limiting the rate, which can be attributed to the large energetic barrier for solvent reorganization [8]. Then, extension of the model to PEMFC ionomer electrolytes will be introduced. Overall, the model framework and findings provide insights into the ORR mechanism and guidance on how to tailor catalyst materials for increased PEMFC performance. Acknowledgements: This work was partially supported by the Hydrogen and Fuel Cell Technologies Office (HFTO), Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE) through the Million Mile Fuel Cell Truck (M2FCT) consortia, technology managers G. Kleen and D. Papageorgopoulos, under contract number DE-AC02- 05CH11231, as well as by a CRADA with Toyota Central R&D Labs., Inc.. Reference [1] D. A. Cullen, K. C. Neyerlin, R. K. Ahluwalia, R. Mukundan, K. L. More, R. L. Borup, A. Z. Weber, D. J. Myers and A. Kusoglu, Nature Energy, 6, 462 (2021).[2] I. E. L. Stephens, A. S. Bondarenko, U. Grønbjerg, J. Rossmeisl and I. Chorkendorff, Energy & Environmental Science, 5 (2012).[3] M. Shibata, M. Inaba, K. Shinozaki, K. Kodama and R. Jinnouchi, Journal of The Electrochemical Society, 169 (2022).[4] H. A. Hansen, V. Viswanathan and J. K. Nørskov, The Journal of Physical Chemistry C, 118, 6706 (2014).[5] K. Kodama, K. Motobayashi, A. Shinohara, N. Hasegawa, K. Kudo, R. Jinnouchi, M. Osawa and Y. Morimoto, ACS Catalysis, 8, 694 (2017).[6] M. Luo and M. T. M. Koper, Nature Catalysis, 5, 615 (2022).[7] J. Huang, J Chem Theory Comput, 19, 1003 (2023).[8] D. T. Limmer, A. P. Willard, P. Madden and D. Chandler, Proceedings of the National Academy of Sciences, 110, 4200 (2013).

Structural Effects on ORR Performances of Carbon-Free Connected Platinum–Cobalt Catalysts with Chemically Ordered Superlattice Structures

Polymer electrolyte fuel cells (PEFCs) have been considered as the promising next-generation energy-conversion technology, owing to their high energy efficiency and clean emissions. However, a sluggish oxygen reduction reaction (ORR) rate and the low durability of conventional catalysts in PEFCs limit the wider commercialization. To address these issues, our group has developed a carbon-free, connected Pt1–Fe1 catalyst with a porous hollow capsule structure. This catalyst comprises the nano-sized network formed by the connection of Pt1–Fe1 nanoparticles and exhibits an enhanced ORR specific activity (9 times higher than a commercial Pt/C) as well as excellent durability against start-stop operations due to the carbon-free structure.[1,2] However, during the load cycle operation, iron is dissolved from the catalyst, leading to the loss of ORR activity and the degradation of polymer electrolytes caused by the Fenton reaction.In this work, carbon-free and iron-free connected Ptx-Co1 catalysts with chemically ordered superlattice structures have been developed for the first time, as shown in Fig. 1(a). In addition, the metal compositions and chemically ordered (superlattice) degrees in the catalysts were controlled to investigate the structural effects on ORR activity and durability of the connected Ptx-Co1 catalysts.Herein, we employed the silica-coating method for the catalyst synthesis, which involves the combination of silica coating and high-temperature annealing. As shown in Fig. 1(b), the surface of the Ptx-Co1 nanoparticles/SiO2 sample was coated with thin silica layers to prevent the detachment of Ptx–Co1 nanoparticles from the SiO2 template as well as the large catalyst agglomeration and coalescence during the annealing process. We controlled the chemically ordered degrees of the connected Pt1–Co1 and Pt3–Co1 catalysts by using different annealing temperatures. The XRD patterns of the catalysts showed the peaks corresponding to L10 type and L12 type chemically ordered structures for the connected Pt1–Co1 and Pt3–Co1 catalysts annealed at 600°C, respectively. These two catalysts possessed ordered degrees (S) of ca. 50–60%. The TEM images of the connected Ptx–Co1 catalysts showed the formation of connected Ptx–Co1 nanonetwork and porous hollow capsule structures.The electrochemical measurements of ORR activity and durability were conducted in a 0.1 M HClO4 electrolyte solution. The connected Pt1–Co1 and Pt3–Co1 catalysts achieved 6- and 10-times higher ORR specific activities than that of Pt/C, respectively. Subsequently, the ORR activity and load cycle durability (0.6 V for 3 s Û 1.0 V for 3 s at 60 °C) of the connected Pt3–Co1 catalyst with different ordered degrees (S = 0%, 28%, and 58%) were evaluated. As the ordered degrees in the connected Pt3–Co1 catalysts were higher, the ORR specific activity was increased. Also, the retention of the specific activity of the ordered catalyst (S = 58%) was higher than that of the disordered catalyst (S = 0%) after 10,000 load cycles. The STEM-EDX line mapping results showed that 83% of the alloyed Co retained in the catalyst with a higher ordered degree even after 10000 load cycles, whereas the disordered and lower ordered catalysts retained lower Co ratios after the test, indicating the suppression effect of an ordered structure on metal dissolution. In addition, the L12 ordered Pt3–Co1 structure remained in the connected catalyst (S = 58%) after load cycle test, confirmed by the TEM diffraction analysis.In summary, we successfully developed the connected Ptx-Co1 catalysts with chemically ordered superlattice structures. In addition, we clarified the structural effects of metal composition and ordered degree in connected Ptx-Co1 catalysts on ORR activity and load cycle durability. These results provide useful guidelines for the design of advanced ORR catalysts towards high-performance PEFCs. Acknowledgement The part of this presentation is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Dimensionless Moduli Governing Under-Rib Transport Phenomenon in Polymer Electrolyte Fuel Cell

Introduction The performance of polymer electrolyte fuel cell (PEFC) can be considerably improved by promoting the under-rib convection or cross flow between adjacent gas channels, e.g., applying the serpentine and staggered partially narrowed flow fields[1]. Although a lot of efforts have been devoted to analyze the under-rib transport phenomenon, an analytical model which quantitively demonstrates the dominant factors of the under-rib processes has not been established. In this study, a theoretical analysis of the relations between oxygen reduction reaction (ORR) and mass transfer capacity is carried out. The dimensionless moduli which combine physical properties, flow field dimensions and operating conditions are derived, which provides a theory for optimization of flow field and operating condition design for PEFC. Theoretical Analysis The water is assumed to only exist in vapor phase in a steady-state isothermal cell. The gas diffusion layer (GDL) is considered as uniform porous medium, in which Darcy’s law is applicable. The even velocity and oxygen mass fraction profiles in through-plane direction inside GDL is considered. The ORR was experimentally proven as a first order reaction of oxygen partial pressure according to our previous study,[2] and is regarded as a surface reaction, so that the oxygen consumption rate can be expressed as Eq. (1). The ORR rate constant, which is the function of the cathode overpotential or electromotive force, is assumed to be fixed across the rib.The theoretical model was derived from one-dimensioned mass balance of oxygen, which exhibits that the under-rib process is dominated by 3 dimensionless moduli, Péclet number Pe, Thiele number φ and the ratio of oxygen mass fractions θ at two sides of the under-rib GDL, which were defined by Eqs. (2)–(4). Péclet number represents the ratio of oxygen convection rate to oxygen diffusion rate, which is determined by the pressure difference between adjacent channels. Thiele number represents the ratio of oxygen diffusion resistance to reaction resistance. The average oxygen mass fraction can thus be calculated from Eq. (5). Results and Discussion The theoretical model was compared with the computational fluid dynamics (CFD) simulation in the computational domain shown in Fig. 1, where the GDL was placed between two parallel gas channels, instead of being laid under the gas channel, and the active area was only located under the rib. In the CFD simulation, the velocity and pressure distributions were calculated by solving Navier-Stokes equations, and the component distribution dependencies on density, diffusivity and viscosity were also considered. Fig. 2 shows the under-rib average current density at different geometries and operating conditions. The dimensionless moduli calculated from the inlet conditions offer an accurate current density prediction, which demonstrates these moduli dominate the under-rib transport phenomenon.The typical operating condition gives φ = 1.5 and Pe = 4, where the model provides the average current density with high confidence. The model provides lower oxygen mass fraction than the CFD results especially in case of θ = 1, which exists in the staggered partially narrowed flow field. The reason can be attributed to the interface between gas channel and GDL, where the low velocity cannot eliminate the mass transport resistance, so that the model overestimated the oxygen mass fraction at rib boundaries, as shown in Fig. 3.The effects of dimensionless moduli on the current density are exhibited in Fig. 4. In case of θ = 1, when Pe is lower than 5, the under-rib convection cannot obviously boost the under-rib oxygen concentration, which was also reported in our previous study[1]. On the other hand, in case of θ< 1 and Pe > 0, although little increase on Pe remarkably improves the under-rib mass transfer which explains the good performance of serpentine flow field, the benefits from the under-rib convection reduces when Pe is high. Additionally, since oxygen cannot be supplied sufficiently from the gas channel, higher φ gives lower under-rib oxygen concentration. Conclusions The theoretical model for describing the under-rib transport phenomenon was established and verified by CFD calculation, which was found to be dominated by 3 dimensionless moduli. The model demonstrates that the Péclet number is required to be large enough in partially narrowed flow fields to maximize the profits from the under-rib convection. Acknowledgment This work was supported by the FC-Platform Program: Development of design-for-purpose numerical simulators for attaining long life and high performance project (FY 2020–FY 2023) conducted by the New Energy and Industrial Technology Development Organization (NEDO), Japan. Reference [1] Y. Ma et al., ECS Trans., 109 (9), 171–197 (2022).[2] M. Kawase et al., ECS Trans., 75(14), 147–156 (2016). Figure 1

Synthesis of Metal-Free Carbon Aerogel with Nitrogen-Doped from Pyrolysis of Cellulose Aerogels Based on Coir Fibers Using Ammonia-Urea System as Electrocatalyst of Oxygen Reduction Reaction for Cathode in Seawater Batteries

Performance of seawater batteries is still limited by the sluggish nature of oxygen reduction reaction (ORR). One way to optimize the performance of the battery is by utilizing an electrocatalyst with the ability to increase the ORR rate of reaction. Nitrogen-doped carbon aerogel based on coir fiber is one of the alternatives because of its excellent electrocatalytic performance and was made from an abundant material. Carbon aerogel was obtained from pyrolyzing coir fiber-based aerogel cellulose. Nitrogen atom structure in carbon aerogel was affected by the temperature of cellulose aerogel pyrolysis. Pyridinic N content in the Nitrogen-doped carbon aerogel has effective for the activity of the ORR. The electrocatalytic performance of the carbon aerogel synthesized from coir fiber with nitrogen doping at the pyrolysis temperature of 600 °C was investigated in this study. The research process began with the synthesis of cellulose aerogel using urea-ammonia crosslinking precursor. Next, cellulose aerogel was pyrolyzed at 600 °C to create carbon aerogel. FTIR, XRD, CV, LSV, and LP were used to characterize the samples. The carbon aerogel was N-doped with pyridinic N, according to an analysis of the FTIR spectra. The carbon aerogel shows 4-electron ORR with kinetic current density (ik) of 0.74 mA/cm2 and current density (io) of 0.583 μA/cm2, according to electrochemical performance analysis in 3.5% NaCl as electrolyte.