Oxygen Reduction Reaction Rate Research Articles

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.

Photocatalytic H2 O2 Generation Reaction with a Benchmark Rate at Air-Liquid-Solid Joint Interfaces.

The rapid charge recombination, low selectivity for two-electron oxygen reduction reaction (ORR), and limited O2 diffusion rate hinder the practical applications of photocatalytic H2 O2 generation. Herein, a triphase photocatalytic system in which the H2 O2 generation occurs at the air-liquid-solid joint interfaces is developed, using polymeric carbon nitride (PCN). The introduction of pyrrole units and cyano group into PCN can promote the activation of oxygen molecules and facilitate the spatial separation of HOMO and LUMO orbits, hence improving the charge carrier separation efficiency and enhancing the formation of H2 O2 . Importantly, the gas-liquid-solid triphase interface system allows for the rapid transport of oxygen from the air to the reaction interface, overcoming the low solubility and slow diffusion of oxygen in the water in conventional liquid reaction systems. The triphase system shows a benchmark H2 O2 generation rate over PCN-based materials in pure water (2063.21µmol g-1 h-1 ), which is an approximate tenfold enhancement as compared to powder photocatalyst (215.44µmol g-1 h-1 ). Simulation and electrochemical tests reveal that the rapid oxygen diffusion rate of triphase interface can promote charge separation and provide more O2 to generate H2 O2 . This work provides a promising strategy for constructing an efficient and sustainable H2 O2 production system.

Development of an electrochemical system to measure high-temperature oxygen reduction for the electrochemical evaluation of Pt/C and non-precious metal catalysts

In polymer electrolyte fuel cells, the oxygen reduction reaction (ORR) rate increases at high temperatures. Therefore, ORR catalysts must be designed for use at high temperatures and require evaluation. To address the problem of decreased dissolved oxygen concentrations in conventional channel flow double electrode (CFDE) systems, this study develops a CFDE system capable of electrochemical measurement at 120 °C that can evaluate the ORR activity of Pt-supported carbon (Pt/C) and carbon-based non-precious metal catalysts (NPMC). The ORR activity increased with an increasing reaction temperature, regardless of the catalyst, and the current density ratios at 120 °C and room temperature were estimated to be 2.4 and 5.4 for Pt/C and NPMC, respectively. This indicates that the NPMC ORR activity increased significantly with the increasing reaction temperature. This increased activation energy, which is approximately 2.3 times greater than that of Pt/C at the overpotential of 0.38 V, contributes to a significant increase in the ORR activity at higher temperatures.

Modification of the Fe,Co–N/C catalysts for oxygen reduction reaction by a chemical post-treatment with oxidizing agents

The transition-metals carbon catalysts belong to intensively studied alternatives to the Pt-based catalysts promoting the oxygen reduction reaction (ORR) in electrochemical fuel cells (FCs). Commonly studied Fe,Co–N–C composites are usually obtained through pyrolysis and used as such or after a chemical post-treatment with an oxidizing agent. This treatment is applied to remove inactive metal species, thus, promoting the electrochemical activity. The impact of an oxidizing agent is poorly addressed in the literature, while its nature may negatively affect the catalyst's performance. Herein we report the first comparative study on the effect of post-treatment with the most common oxidizing agents such as HCl, HNO3, H2SO4 and H2O2, on the structural and electrochemical properties of the Fe,Co–N–C catalyst. The catalyst is made by pyrolysis of the Co,Zn-ZIF metal-organic framework enriched with iron and nitrogen. Its structure was observed withstanding the action of mineral acids but suffers from hydrogen peroxide. The treatment with either nitric or chloric acid may improve the electrochemical performance up to 4%, while other agents decrease that by 6% and slow down the ORR rate. These findings are useful for the careful design of post-treatment procedures for carbon catalysts.

Long side-chain N-heterocyclic cation based ionic liquid grafted poly(terphenyl piperidinium) membranes for anion exchange membrane fuel cell applications

Anion exchange membrane fuel cells (AEMFCs) bring out unique advantages including the use of cheap and plentiful catalytic materials, fast oxygen reduction reaction rate at cathode and low cost of membranes. However, the development of performing and durable anion exchange membranes (AEMs) is still the major challenge for the ultimate industrial adoption of AEMFC. Herein, the ether-free poly(p-terphenyl-co-N-methyl-piperidine) (PTP) is synthesized and employed as the matrix for AEM. Four ionic liquids of long side-chain N-heterocyclic cations bromide are synthesized and chosen as the quaternization reagents for PTP. Thus, four flexible and long side-chain cationic groups grafted PTP based AEMs are fabricated. The effect of end-group cations on the properties of long side-chain AEMs has been investigated systematically. The presence of long side-chain cations endows AEMs with high conductivity and low swellings simultaneously. Among all the membranes, N-methylpiperidinium grafted AEM (PTP-C5-MPi) exhibits the highest alkaline stability owing to ether-free polymer chain, long side-chain linkage and piperidinium cation, which retains 87 % of its original ion conductivity after storage in 1.0 M KOH solution for 2118 h. Furthermore, the H2–O2 fuel cell based on PTP-C5-MPi is performed to determine the technical feasibility.

Electrochemical activity of copper doped Sr2.7Pr0.3Fe2O7−δ cathode catalysts for solid oxide fuel cells

Due to their great efficiency, low cost and pollution, and flexible fuel choice, solid oxide fuel cells (SOFCs) have been applied to energy conversion filed. However, one challenge for developing SOFCs is sluggish oxygen reduction reaction (ORR) rate. Herein, a series of Ruddlesden-Popper-type Sr2.7Pr0.3Fe2–xCuxO7–δ (SPFCx) oxides are successfully synthesized and thoroughly assessed as potential SOFCs cathodes. Among all compositions, Sr2.7Pr0.3Fe1·9Cu0·1O7–δ (SPFC10) possesses the best electrocatalysis performance and improved oxygen reduction kinetics, with the lowest polarization resistance (Rp, 0.13 Ω cm2) at 700 °C. Cu-doping will contribute to improving oxygen transport kinetics, increasing oxygen vacancy concentration, and O 2p band centers close to the Fermi level, hence, enhancing the electrocatalysis performance. At 700 °C, the maximum output of the single cell with SPFC10 cathode is 704 mW cm−2. These results highlight a highly active SPFC10 cathode with Ruddlesden-Popper structure for SOFCs.

Machine learning and Shapley Additive Explanation-based interpretable prediction of the electrocatalytic performance of N-doped carbon materials

Enhancing the kinetic rate of cathodic oxygen reduction reaction (ORR) by catalysts is the key to improve the performance of microbial fuel cells (MFCs). Metal-free ORR catalysts represented by nitrogen-doped carbon materials have been extensively investigated and have shown excellent catalytic effects for oxygen reduction reaction. However, it is difficult to clarify the coupling effect physicochemical properties of nitrogen-doped carbon materials on their catalytic effect (i.e. electricity production performance of MFCs) by traditional experimental methods. Therefore, in this study, six machine learning models were combined with SHAP to develop prediction models for the power density ratio of MFCs for reflecting the catalytic performance of nitrogen-doped carbon materials by using physicochemical properties, such as elemental composition, functional group structure, and pore structure, as input features. The gradient boosting regression (GBR) model was found to have the highest prediction accuracy on the test set, with R2 and RMSE of 0.86 and 0.09, respectively. The SHAP method was used to interpret the output of the GBR model and reveal the mechanism of interaction between different characteristic variables. It was found that pyridine nitrogen is the most important input characteristic of the nitrogen elements, as its corresponding SHAP value reaches above 0.1. Surprisingly, an increase in the content of oxygen significantly attenuates the extent to which changes in nitrogen content affect the system, although its effect on power density prediction is inconsiderable. In addition, the optimal range of important physicochemical properties of nitrogen-doped carbon materials was obtained by the SHAP method. The carbon materials have relatively high catalytic performance under the conditions of C(at%) between 85% and 90%, N(at%) > 2.5%, Pyridine-N(at%) > 30%, Vtotal between 0.3 and 0.6 cm3g−1, and SBET > 1000 m2g−1. This study can provide theoretical guidance for subsequent experimental design of carbon-based nitrogen-doped electrocatalysts.

Performance Evaluation of Aquaponics-Waste-Based Biochar as a Cathode Catalyst in Sediment Microbial Fuel Cells for Integrated Multitrophic Aquaculture Systems

The sustainable development of aquaculture faces a significant challenge due to the need for the frequent treatment of aquacultural waste. This research presents a pioneering solution by concurrently utilizing aquacultural waste to produce biochar and enhancing a sediment microbial fuel cell (SMFC)’s treatment efficacy for waste generated from the integrated multitrophic aquaculture (IMTA) system. The water quality parameters of the aquacultural pond water were analyzed, and synthetic wastewater was prepared to validate the system’s efficiency. Over a period of more than 50 days, the SMFC was operated and monitored, yielding an average chemical oxygen demand (COD) removal efficiency of 86.31 ± 2.18%. The maximum operating voltage of the SMFC reached 0.422 V on the 21st day of operation when connected to an external resistance of 975 Ω. A novel-activated aquacultural biochar catalyst was synthesized from aquaponics waste and used as a cathode catalyst, substantially improving the SMFC’s performance. Characterization studies demonstrated that the aquacultural biochar catalyst was an active electrocatalyst, accelerating the oxygen reduction reaction rate and leading to increased power output and overall efficiency of the SMFC. The SMFC utilizing the aquacultural-waste-based biochar cathode catalyst showcased the highest maximum power density, with a range of 101.63 mW/m2 (1693.83 mW/m3), and the lowest internal resistance, indicating superior performance. These results validate the reliability of implementing SMFCs in actual aquaculture systems. A novel modular design for SMFC reactor-assisted small-scale integrated poultry–fish culture systems is proposed for further practical application in real-life aquaculture settings. This research contributes to finding sustainable and effective methods for waste treatment for aquaculture, promoting the development of environmentally friendly practices in the industry.

Neodymium-Doped Zinc Oxide Nanoparticles Catalytic Cathode for Enhanced Efficiency of Microbial Desalination Cells

The Microbial Desalination Cell is a novel method for desalinating water that also generates energy via substrate oxidation. The MDC comprises three chambers: the anode chamber, the desalination chamber, and the cathode chamber. The fundamental problem with the technology is that it generates very little power during the oxygen reduction reaction (ORR). One solution to this issue is to use a highly active cathode catalyst, which effectively increases the ORR rate. Neodymium-doped ZnO nanoparticles were produced and employed as a cathode catalyst in the three-chambered MDC1 to improve performance. Zn1−xNdxO nanocrystalline samples containing x = 0.0, 0.03, 0.6, and 0.10 were synthesized efficiently through the cost-efficient sol-gel method. Transmission electron microscopy (TEM) and X-ray diffraction techniques revealed the nanocrystalline nature and the phase purity of the Zn1−xNdxO samples. The structural properties of ZnO nanostructured materials were elucidated by Rietveld refinement of the XRD patterns, which showed displacement of Zn and O ions and revealed changes in the electron density around the Zn-O bond with Nd substitution. The local features of light emission from Zn1−xNdxO samples have been studied with photoluminescence. The UV and green-yellow emissions originate from the exciton transition and the transition between the Nd3+ deep level, oxygen vacancy and interstitial oxygen. The results were compared to MDC-2, which did not have a catalyst on the cathode. Both MDCs were tested using a saline water solution containing 15 g/L of NaCl to measure their desalination performance. The better reduction kinetics was confirmed by cyclic voltammetry of the MDC-1 cathode. MDC-1 had a higher desalination efficiency (77.02% ± 2.0%) due to the presence of an Nd-doped ZnO catalyst than MDC-2 (59.3% ± 8.3%). MDC-1’s maximum power density of 3.65 W/m3 was 2.78 times greater than MDC-2’s (0.78 W/m3). Furthermore, the coulombic efficiency of MDC-1 was found to be (8.8 ± 0.3%), which was much higher than that of MDC-2 (4.56 ± 0.2%). As a result, the Nd-doped ZnO-based catalyst developed in this study can potentially improve ORR in MDC cathodes, enabling them to generate more power.

Nano-flower like CoFe-layered double hydroxide@reduced graphene oxide with efficient oxygen reduction reaction for high-power air-cathode microbial fuel cells

Microbial fuel cells (MFCs) are “self-sufficient” renewable energy sources that utilize the redox processes inherent in microbial metabolism to generate an electrical potential. However, the cathodic region of these fuel cells typically utilizes an oxygen reduction reaction (ORR) operating at high overpotential and slow reaction kinetics significantly, restricting the power generation capacity of MFCs. Herein, to improve the cathodic reduction efficiency, we synthesized CoFe-layered double hydroxides (LDH) on partially-reduced graphene oxide (p-rGO) via electrostatic interaction. In contrast to pure CoFe-LDH, CoFe-LDH@p-rGO composite possessed a three-dimensional uniform porous nanoflower-like morphology, giving the catalyst a high surface area for efficient catalysis. The proportions of CoFe-LDH to p-rGO were optimized to minimize layer stacking through strong π-π bond and van der Waals force, maximizing the overal electron transfer numbers n (3.66), with the lowest charge transfer resistance Rct (38.5 Ω). As a result, this novel catalyst combined the rich Co/Fe metal center catalytic active sites and interlayer structure of CoFe-LDH, with the high electrical conductivity of p-rGO to accelerate the electron transfer rate of ORR. Air-cathode microbial fuel cell (ACMFC) utilizing CoFe-LDH@p-rGO catalyst showed comparable output voltage of 423 mV to Pt/C catalyst, and a comparably high-power density of 204 mW m−2. This work provides a new path towards development of nanostructured non-noble metal catalysts for the optimization of oxygen reduction reaction in high-power ACMFCs.

Highly efficient heterogeneous electro-Fenton degradation of organic pollutants using a FeNi-OH/NF cathode

The electro-Fenton (EF) technology provided a promising way to degrade organic pollutants by in-situ producing H2O2 and decomposing it into hydroxyl radicals (OH). However, the degradation rate is restricted by the two-electron oxygen reduction reaction (ORR) rate of H2O2 generation and the production efficiency of OH, and the efficient EF cathode is important. Herein, we designed an efficient heterogeneous EF system with a Fe-doped β-Ni(OH)2 nanosheets/nickel foam (FeNi-OH/NF) cathode for phenol removal in water. FeNi-OH enhanced the electrochemically active area for H2O2 production and produced OH for pollutant degradation. The FeNi-OH/NF cathode demonstrates excellent heterogeneous EF catalytic activity with a degradation efficiency of phenol (99.4%) and total organic carbon (67.4%) in 2 h. The experiments and density functional theory (DFT) calculations further confirmed that the Fe-doped β-Ni(OH)2 on the nickel foam enhanced H2O2 adsorption and accelerated Fe3+/Fe2+ circulation, promoting the reaction of H2O2 with Fe2+ to produce OH for phenol degradation. This study provides a novel and efficient heterogeneous EF cathode for accelerating pollutant degradation securely and quickly.

Plasma-engineered cobalt nanoparticle encapsulated N-doped graphene nanoplatelets as high-performance oxygen reduction reaction electrocatalysts for aluminum–air batteries

The increasing demand for continuous and eco-friendly alternative energy has prompted interest in aluminum–air (Al–air) batteries owing to their high energy density and safety. On the other hand, the sluggish oxygen reduction reaction (ORR) rate in alkaline electrolytes, which adversely affects the efficiency of the Al–air battery, is still a challenge. A transition metal electrocatalyst consisting of cobalt nanoparticles encapsulated with N-doped graphene nanoplatelets (Co@N/GNP) was synthesized by plasma engineering. In this design, the active Co nanoparticle acted as the ORR active site and the core-shell structure of dominant pyridinic-N graphene led to outstanding catalytic performance (E1/2 = 0.87 V, Eonset = 0.98 V) in an alkaline electrolyte. Furthermore, when applied as a cathode catalyst in practical Al–air batteries, Co@N/GNP showed excellent performance with a peak power density of 143 mW cm-2.

GDE vs RDE: Impact of operation conditions on intrinsic catalytic parameters of FeNC catalyst for the oxygen reduction reaction

We present a comprehensive comparison of the catalytic evaluation of a FeNC catalyst using two different approaches, i.e., employing the rotating ring disk electrode (RRDE) technique and a gas diffusion electrode (GDE) setup. With both techniques, the FeNC catalyst is evaluated in acidic as well as in alkaline environment. The oxygen reduction reaction (ORR) is studied and parameters such as ORR kinetic activity, Tafel slope, and the apparent activation energy are determined at different applied temperatures/overpotentials. The obtained results offer clear evidence that in alkaline medium, the GDE offers reaction conditions with improved reactant mass transport enabling an evaluation of the FeNC catalyst closer to membrane electrode assembly (MEA) testing. As compared to the RRDE evaluation, the ORR rate is higher in the GDE setup and the potential range for extracting kinetics and activation energies is substantially improved. However, in acidic environment only minor differences between the two reaction environments (RRDE and GDE) are observed. The difference between alkaline and acidic conditions is rationalized based on the different reaction pathway in the two environments.

Candied Haws-Like Fe–N–C Catalysts with Broadened Carbon Interlayer Spacing for Efficient Zinc–Air Battery

As efficient nonprecious metal catalysts for oxygen reduction reaction (ORR), Fe-N-C materials are one of the most promising alternatives to Pt-based catalysts for fuel cells and metal-air batteries. However, the intrinsically low density of key active sites like FeN4 moieties hampers their commercial applications. Herein, we provide a smart strategy to construct a candied haws-like Fe-N-C catalyst (CH-FeNC) with broadened carbon interplanar spacing (>4 Å), starting with trehalose as a structure-built brick coupled with a zinc-zeolite imidazole framework (ZIF-8) and polyaniline (PANI) and then followed by copyrolysis carbonization of them. The obtained CH-FeNC exhibits half-wave potentials of 0.92 and 0.90 V (vs RHE) before and after 10,000 cycles in 0.1 M KOH, which are superior to the 0.90 and 0.85 V obtained by commercial Pt/C for ORR. The power density of a homemade zinc-air battery equipped with the catalyst is up to 131 mW cm-2, greater than that of Pt/C (124 mW cm-2). The extended X-ray absorption fine structure (EXAFS) results and density functional theory (DFT) theoretical calculations reveal that there exists enriched zigzag or armchair edge-hosted FeN4 active sites, located at the abundant interface between carbon components in this composite. Furthermore, the unique broadened carbon interlayer spacing plays a key role in deciding the ORR rate in alkaline but not in acidic environments because there exists a fifth ligand of active Fe in the form of FeN4 centers coupled with SO42- and ClO4- from acids.

A novel and durable oxygen reduction reaction catalyst with enhanced bio-energy generation in microbial fuel cells based on Ag/Ag2WO4@f-MWCNTs

• Ag/Ag 2 WO 4 @f-MWCNTs was fabricated with a scalable, high efficiency, energy-saving, and time-saving strategy. • Ag/Ag 2 WO 4 @f-MWCNTs catalyst enriches oxygen diffusion and facilitates oxygen reduction reaction in MFCs. • The Ag/Ag 2 WO 4 @f-MWCNTs catalyst indicates high cathodic oxygen reduction reaction rate with 4e – pathway. • The Ag/Ag 2 WO 4 @f-MWCNTs has the potential to be an alternative to Pt/C-based oxygen reduction reaction catalysts. • The usage of this catalyst can substantially improves power production and enables broader applications of MFCs. Microbial fuel cell, as a promising technology, utilizes microorganisms to break down and oxidize the organic matter for harvesting energy. Progress in creating specific compositional functionalities of electrodes that are catalytically active, durable, cost-effective, and energy-recovery efficient is exclusively challenging for microbial fuel cell operation because of its bio-compatibility requirement. The electrodes composition impresses the oxidation-reduction reaction and electron charge-discharge rates. This study explores the use of silver/silver tungstate supported on different kinds of carbon materials, as cathode material for microbial fuel cells. The as-prepared catalysts were synthesized by a simple, environment-friendly, and self-developed surfactant-less method. Among proposed catalysts, the Ag/Ag 2 WO 4 @f-MWCNTs upgraded the oxygen reduction reaction to a four-electron pathway. As well, the optimal catalyst Ag/Ag 2 WO 4 @f-MWCNTs, exhibited excellent oxygen reduction reaction performance activity and stability with onset potential of -0.037 V and kinetic current density of 17.66 mA cm −2 compared with other synthesized catalysts and even the standard Pt/C catalyst. Owing to the more active sites, larger surface, and faster charge transfer, the microbial fuel cell equipped Ag/Ag 2 WO 4 @f-MWCNTs device delivered a high power density and current density of 0.965 W m –2 and 6.43 A m –2 , respectively. Consequently, Ag/Ag 2 WO 4 @f-MWCNTs is an effective catalyst that has the potential to be an alternative to Pt/C-based oxygen reduction reaction catalysts. The usage of this Ag/Ag 2 WO 4 @f-MWCNTs catalyst can therefore substantially improves power production and enable broader applications of microbial fuel cells for renewable electricity generation using waste materials.