Unitized Regenerative Fuel Cell Research Articles

Advancements in bifunctional catalysts for unitized regenerative fuel cells: exploring polaronic conduction and heterostructure designs

This study investigates the development and performance evaluation of a bifunctional catalyst tailored for unitized regenerative fuel cells (URFCs), capable of facilitating both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in fuel cell and electrolyzer modes. The primary challenge addressed is the creation of electrocatalysts exhibiting high activity and corrosion resistance for oxygen reduction and water oxidation at the oxygen electrode. A novel catalyst structure based on La0.5Sr0.5Fe0.5Ti0.5O3(LSFT), i.e. LSFT/ZnO/LSFT with a thickness of ∼2 μ m, is explored within an environmentally friendly medium. This catalyst demonstrates superior performance characteristics, including reduced overpotential in HER and enhanced stability during oxygen/hydrogen evolution processes in neutral medium. This study identifies the formation of interfacial polarons and polaronic charge modulation resulting from the incorporation of ZnO in LSFT, leading to multifunctional OER/HER behavior. Notably, the proposed interfacial small polaron mechanism offers valuable insights into complex interfacial phenomena and holds promise for applications in diverse heterostructures involving layered 2D materials and transition metal oxides. Moreover, the robust LSFT/ZnO/LSFT catalyst exhibits exceptional stability, maintaining for 168 h of oxygen evolution at a constant potential of approximately 1.66 V for a current density of 1 A cm−2 in a neutral medium. These findings mark a significant advancement in URFC technology and present promising avenues for clean energy storage solutions.

Interaction of Catalysts for Unitized Regenerative Fuel Cells

There is a need for flexible and efficient storage facilities so that the integration of renewable energies and thus the energy transition can succeed. Intermediate storage of energy in the form of hydrogen by electrolysers and effective reconversion to electricity by fuel cells is a possibility. The unitized regenerative fuel cell (URFC) is a single-unit device that combines an electrolyser and a fuel cell, which makes the system costs much cheaper compared to two separate devices. Currently, the focus of URFC research, similar to proton exchange membrane (PEM) electrolysers, is on the development of new catalysts and electrodes with the aim of reducing costs through manufacturing and design optimization [1,2]. In this context, equally important is the increase of electrochemical activity on the oxygen side since oxygen evolution (OER) and oxygen reduction (ORR) are the speed-determining steps comparatively to their counter reactions on the hydrogen side. Due to high potentials and the corrosive oxygen atmosphere, there are additional requirements for electrode stability on the oxygen side. According to the current state of the art, the materials currently used as OER and ORR catalysts in single cell systems are resorted to and combined for the bifunctional oxygen electrode [3,4]. An effective working catalyst layer is crucial for high power densities in both operating directions and thus a research focus for URFCs.To further improve the stability and activity of the catalysts, support materials are used [5]. Important for support materials are good electrical conductivities and a high surface area, to allow a fine dispersion. This maximizes the relative electrochemical catalyst surface area (ECSA), thus leading to a reduction in costs and catalyst layer thickness, which in turn can lead to a reduction in mass transport resistances. A thin catalyst layer would also have advantages for alternating operation between electrolysis and fuel cell mode, since improved removal of liquid water from the catalyst layer can be ensured here.In this study we prepared supported and unsupported Ir and Pt catalysts. We investigated the distribution of catalyst particles on the support, measured by transmission electron microscope (TEM), and the crystallite sizes and structures by X-ray diffraction (XRD). Bifunctional electrodes consisting of supported and unsupported catalysts were investigated by rotating disk electrode (RDE) and single cell measurements. In this talk, we will take a closer look at interactions between Pt and Ir and their influence on OER and ORR.

MOF-Derived Trifunctional High Entropy Sulfides as an Electrocatalyst for Alkaline Unitized Regenerative Fuel Cell Application

Electrochemical devices such as electrolyzers (EL), fuel cells (FC), metal-air batteries (MAB), and unitized regenerative fuel cells (URFC) are key components in our drive toward a renewable energy-based power infrastructure. The potential scale of these devices requires the usage of catalytic materials that exhibit not only excellent catalytic activity but also commendable durability. Conventional precious metal catalysts such as Pt, Ir, Rh, Ru, and Pd are utilized for variable deployments during electrocatalysis. 1 Recent trends in alloying, tuning the band structure, nanosizing of particles, and application of earth-abundant transition metals, have immensely increased the economic feasibility and practical applications of a new genre of electrochemical devices. 2 Here, strategically designed materials, such as metal-organic frameworks (MOF), offers a new route for synthesizing the alloys and their derivatives with high porosities and excellent catalytic activity. 3 High entropy alloys (HEA) containing multiple (four or more) components of metallic oxides, sulfides, and phosphides have emerged as a new class of materials. Effects such as lattice distortion, entropy stabilization, cocktail effect, and formation of multiple oxidation states in such HEA enable access to multiple catalytic centers for catalyzing more than one small molecule activation reaction. 4 The current work focuses on the synthesis of FeCuCoMnNi-based high entropy sulfide (HES) via a unique MOF route. This HES material successfully demonstrates three distinct catalytic reactions: oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The surface morphology and phase of the HES can be critically curated via sulfurizing the multi-metal MOF precursor, which was monitored with a battery of surface characterization techniques. This HES displayed an impressive bifunctionality index of 0.80 V and 1.87 V for application in alkaline URFC (ORR/OER) and EL (HER/OER), where moderate to low overpotentials were observed for OER (η OER = 380 mV), ORR (η ORR = 420 mV), and HER (η HER = 260 mV). The superior electrochemical performance of this HES is attributed to the surface reconstruction mechanism upon the inclusion of transition metals such as Fe, Co, and Mn to Ni leading to the formation of the key Ni-OOH layer. Along with excellent performance, the HES exhibited good durability at +1.5 V vs. RHE up to 12 hours while retaining ~75% of the initial current density, indicating the presence of a high configurational entropy in the material. References Huynh, M., Ozel, T., Liu, C., Lau, E. C., & Nocera, D. G. 2017 Chemical science, 8(7), 4779-4794.Xu, J., Li, J., Xiong, D., Zhang, B., Liu, Y., Wu, K. H., ... & Liu, L. 2018 Chemical science, 9(14), 3470-3476.Karmakar, A., Kumar, N., Samanta, P., Desai, A. V., & Ghosh, S. K. 2016 Chemistry–A European Journal, 22(3), 864-868.Miracle, D. B., Miller, J. D., Senkov, O. N., Woodward, C., Uchic, M. D., & Tiley, J. 2014 Entropy, 16(1), 494-525 Figure 1

Bifunctional TiCo electrocatalysts based on N-doped graphene cryogels for the oxygen evolution and reduction reactions

Three-dimensional N-doped graphene cryogels (NGC) modified with Ti and Co nanoparticles are investigated as bifunctional oxygen electrocatalysts for potential application in unitized regenerative fuel cells (URFCs). These composites have been successfully produced via a combined solvothermal, freeze-drying, and thermal treatment procedure. Crystallographic structure, surface properties, and morphology are analyzed to study the influence of each metal and the bimetallic combination in forming active sites for oxygen reduction (ORR) and oxygen evolution (OER) reactions. The intrinsic catalytic activity of the composites was determined by rotating disk electrode and further assessed in a gas diffusion electrode to test them in more realistic conditions for URFCs in 6 M KOH. Results revealed the remarkable bifunctional activity of Co/NGC and bimetallic TiCo/NGC composites. The synergistic effect of the metallic cobalt and surface atomic composition of Co3O4 distributed on the N-doped graphene cryogel in Co/NGC is responsible for the high bifunctional performance. Moreover, the partial substitution of cobalt for a less critical raw material, such as titanium, allows the electrocatalyst to maintain great bifunctional activity and stability. The nitrogen doping of the graphene cryogel structure can influence the anatase/rutile titania ratio that forms during pyrolysis. As a result, defects, oxygen vacancies, and Ti3+ species are formed, which, when combined with Co nanoparticles, contribute to the outstanding performance and stability of the TiCo/NGC composite. The excellent performance of N-doped graphene cryogel-based electrocatalysts in GDE conditions indicates their potential use in electrochemical processes, which involve gas-phase reactions.

Tailoring the Mass Transport to Achieve High-Performing Unitized Regenerative Fuel Cells Considering Practical Operating Conditions

Unitized regenerative fuel cells (URFCs) convert electrical energy to chemical bonds in hydrogen during charge and convert chemical energy to output electricity during discharge, offering a promising solution to long-term energy storage. Recent studies indicate that the round-trip-voltaic efficiency (RTE) and longevity of URFCs are limited by complex mass transport during charging and discharging. Here, we first investigate how different porous transport layer (PTL) structures can impact URFC performance. The preferred PTL has a low tortuosity and high porosity, leading to a high RTE above 50% at 1 A cm−2 using Nafion 212. Moreover, thicker membranes, such as Solvay 90, are required to ensure mechanical stability and minimize H2 crossover when operating under high differential pressure. Although this assembly inevitably leads to a higher ohmic loss, the RTE can be improved by further tailoring the electrode structures to facilitate mass transport by using supported catalyst, which still achieves over 50% RTEs at 1 A cm−2. Optimization of porous structure to mitigate mass transport resistance with appropriate materials down selection considering practical application requirements can be a key design principle for achieving high-performing URFCs.

Two-phase mass transporting under various operating conditions during mode switching of unitized regenerative fuel cell with non-uniform flow channels

ABSTRACT The electrolyzing and fuel cell (FC) modes appear in a unitized regenerative FC. Understanding the operating condition impacts on the dynamic behaviors during mode-switching procedure facilitates better handling of cell working states and increasing the performance behaviors. In the present paper, a two-dimensional transient numerical model is employed to discuss the influence of operating conditions on two-phase species distributions when switching the cell mode into FC. According to the numerical results, it is concluded that when the cell switches from electrolytic cell to the mode of FC, reducing the operating cell voltage can significantly improve the cell performance. At the same time, concentration polarization and temperature heterogeneity are serious. Increasing the gas inlet velocity can improve the cell performance. Providing the reaction gas with the same concentration as the FC mode in advance can improve the temperature uniformity and cell current density values when starting switching the mode but hardly impacts the parameter value and currents after stabilization.

Simulation Research of the Energy Storage System based on Unitized Regenerative Fuel Cell

The Unitized Regenerative Fuel Cell (URFC) is an advanced energy storage system that integrates a fuel cell and an electrolysis cell. In this paper, MATLAB/Simulink is used to construct mathematical models of URFC. Through the steady-state simulation, it is found that with the increase in temperature, the output voltage of fuel cell mode increases, and the electrolysis mode voltage decreases. The highest efficiency reaches 41.8% at 80°C. As the pressure increases, the performance of URFC improves but requires good packaging and gas sealing. 1 MPa is the optimal pressure value considering URFC operating efficiency.

(Invited) Synthesis and Characterization of New Catalyst Materials for Electrocatalysis

In this talk I will discuss some of our recent explorations of new catalyst materials for electrocatalysis for renewable fuels and energy, including the synthesis and characterization of these materials. First, I will briefly describe our report of an efficient acid-stable N2-plasma treated hafnium oxyhydroxide electrocatalyst for hydrogen evolution and oxidation reactions (HER and HOR). Lack of a highly active, stable, earth-abundant electrocatalyst for carrying out HER and HOR in strongly acidic conditions is a major technical challenge for developing economical polymer electrolyte membrane (PEM)-based electrolyzers and fuel cells. We found that processing Hf oxide with an atmospheric N2 plasma forms an acid-insoluble hafnium oxynitride material, and we propose that under electrochemical environments this material is transformed into an active oxynitride hydroxide that demonstrates unprecedented high catalytic activity and stability for both HER and HOR in strong acidic media for earth-abundant materials. The zero onset potentials and high current densities demonstrate that this material is a promising alternative to Pt group metal (PGM) catalysts, opening new opportunities to develop technologically and economically viable unitized regenerative fuel cell (URFC) systems and cost-effective PEM electrolyzers. Furthermore, these results have broad implications for using nitrogen incorporation (e.g. by N2 plasma treatment) to activate other non-conductive compounds and films to form new active electrocatalysts.I will discuss our recent observations on the reaction-driven restructuring of defective PtSe2 into an ultrastable electrocatalyst for the oxygen reduction reaction (ORR), which is of interest for accelerating the commercialization of PEM fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). We have found that the restructured defective PtSe2 electrocatalyst had 1.3 times the specific activity and 2.6 times the mass activity of a commercial Pt/C catalyst, and maintained this superior ORR activity even after 126,000 cycles in accelerated durability tests.I will describe our report of a highly active hafnium-modified iridium oxide (IrHfxOy) electrocatalyst for the oxygen evolution reaction (OER), a reaction that limits development of energy efficient and economically viable a wide range of energy conversion devices. Iridium oxide (IrOx) is the current benchmark electrocatalyst for OER due to its superior performance and excellent stability, but large scale applications using IrOx are impractical due to its low abundance and high cost. We have found an IrHfxOy electrocatalyst that demonstrated ten times higher activity in alkaline conditions (pH = 11) and four times higher activity in acid conditions (pH = 1) than an IrOx electrocatalyst. The highest intrinsic mass activity of the IrHfxOy catalyst in acid conditions was calculated as 6950 A gIrOx -1 at an overpotential (η) of 0.3 V. I will also discuss our research on phosphorus-doped Hf-Ru oxide (denoted here as HfxRuyPO) electrocatalysts for OER in which we demonstrated a strategy for efficiently promoting the durability of RuO2. This research was motivated by the need to address the rapid corrosion and precipitation of highly efficient precious metal (Ir, Ru, and Rh) nanoparticle catalysts in the harsh environment of OER in PEM electrolyzers for hydrogen fuel generation. We found that when the HfxRuyPO catalysts possessed a Hf:Ru atomic ratio of 1:2, they exhibited a highly efficient and stable OER activity over 40 hours. By XPS and Raman analyses, the HfPO component was found to play a significant role in improving the durability of RuO2 through the formation of a HfOOH phase, which also possessed a higher OER activity and lower impedance.Most recently we are investigating metal-organic frameworks (MOFs) and MOF-derived (MOF-d) materials as electrocatalysts for renewable fuels and energy applications.

Low loading inkjet printed bifunctional electrodes for proton exchange membrane unitized regenerative fuel cells

Inkjet printing is evaluated as a feasible technique for unitized regenerative fuel cells (URFCs) electrode fabrication and to study the optimal ionomer and catalyst loading. A physical mixture of Pt, IrOx, and Nafion ionomer was inkjet printed directly on a Nafion membrane to fabricate varying loading bifunctional oxygen electrodes for a URFC and characterized via scanning electrode microscopy and cyclic voltammetry. The results show that the two catalyst do not interfere with one another and that an optimal electrode requires low ionomer loading, i.e., 10 wt%, and significantly lower amount of the IrOx catalyst compared to platinum, i.e., 3:1 Pt-IrOx ratio. A catalyst loading study shows that the highest round-trip efficiency is obtained at around 0.67 mgPt+IrOx/cm2 and further increasing the amount of catalyst does not result in a significant cell performance improvement either in water electrolysis or fuel cell mode. Overall, inkjet printing technique was used to fabricate thin bifunctional electrodes reaching round-trip efficiencies of up to 50% at 500 mA/cm2 using a bifunctional oxygen electrode with only 0.67 mgPt+IrOx/cm2 PGM catalyst loading, resulting in the highest efficiency by amount of catalyst reported to date.

Role of a Unitized Regenerative Fuel Cell in Remote Area Power Supply: A Review

This manuscript presents a thorough review of unitized regenerative fuel cells (URFCs) and their importance in Remote Area Power Supply (RAPS). In RAPS systems that utilize solar and hydrogen power, which typically include photovoltaic modules, a proton exchange membrane (PEM) electrolyzer, hydrogen gas storage, and PEM fuel cells, the cost of these systems is currently higher compared to conventional RAPS systems that employ diesel generators or batteries. URFCs offer a potential solution to reduce the expenses of solar hydrogen renewable energy systems in RAPS by combining the functionalities of the electrolyzer and fuel cell into a single unit, thereby eliminating the need to purchase separate and costly electrolyzer and fuel cell units. URFCs are particularly well-suited for RAPS applications because the electrolyzer and fuel cell do not need to operate simultaneously. In electrolyzer mode, URFCs function similarly to stand-alone electrolyzers. However, in fuel cell mode, the performance of URFCs is inferior to that of stand-alone fuel cells. The presented review summarizes the past, present, and future of URFCs with details on the operating modes of URFCs, limitations and technical challenges, and applications. Solar hydrogen renewable energy applications in RAPS and challenges facing solar hydrogen renewable energy in the RAPS is discussed in detail.

Variation of Two-Phase Species Distribution inside a Unitized Regenerative Fuel Cell during an Electrolytic Cell Mode

Variation of Two-Phase Species Distribution inside a Unitized Regenerative Fuel Cell during an Electrolytic Cell Mode

Experimental study on a unitized regenerative fuel cell operated in constant electrode mode: Effect of cell design and operating conditions

Constant electrode (CE) operation of a unitized regenerative fuel cell (URFC) facilitates the optimization of each electrode. In this work, bipolar plates (BPPs) with different flow fields (FFs) are assessed, and a graphite BPP is compared with a titanium BPP in the cathode. Platinum coated titanium felt and carbon cloth and paper gas diffusion layers (GDLs) are tested in the cathode. The carbon BPP and GDLs operated with higher ohmic resistance than the titanium alternatives. However, the titanium GDL caused flooding in FC mode. Carbon GDLs can be applied in the cathode, where carbon paper operated with higher performance than carbon cloth. Titanium GDL is still required in the anode to resist the corrosive environment. A double-serpentine FF in the cathode and a parallel design in the anode were applied. Optimization of the URFC operating conditions under FC mode provided the best results at 80 °C, 2 barabs, with saturated air and dry hydrogen and stoichiometry ratios of 2 and 8. This resulted in a round-trip efficiency of 56% at 0.1 A cm−2, 48% at 0.3 A cm−2 and 22% at 1 A cm−2. In comparison, 56%, 52% and 37% were obtained with a discrete regenerative fuel cell (DRFC).

Characterization of bipolar plates manufactured with various Pb/C ratios for unitized regenerative fuel cell system.

The weight reduction of the bipolar plate (BP) is essential for commercializing unitized regenerative fuel cells (URFCs). In order to lighten the weight of the bipolar plate, we have used Pb/C composite powder as a cost-effective primary material, which is a mixture of low-density graphite and lead. Further, varied lead-carbon weight ratios (1: 8, 1:4, 1:1, 4:1, and 8:1) were investigated for fabricating the bipolar plate by hot-pressing process adding styrene-butadiene rubber (SBR) as a binder. The specific surface area, porosity, and microstructure characteristics corresponding to the varied lead-graphite ratio of the prepared bipolar plates were studied. The relative difference in conductivity upon the compressibility of the plates is also examined. Finally, the wettability and electrochemical properties of the prepared bipolar plates were evaluated through water contact angle and cyclic voltammetry analysis.

Optimization research on round-trip efficiency of a CHP system based on 10 kW-grade unitized regenerative fuel cell

The unitized regenerative fuel cell (URFC) integrates fuel cell and water electrolyzer into the same device, and the round-trip efficiency (RTE) is considered as an important criterion for evaluating its comprehensive performance. A model of URFC-based combined heat and power (CHP) system is developed, which takes into account the energy consumption of the components, and conducts orthogonal matrix analysis in three dimensions, including operating current density, temperature and pressure, to quantitatively evaluate the RTE of the CHP system based on URFC. The results show that the RTE of the system exceeds 75 % when the operating current density in FC mode lower than 0.6 A/cm2, and the operating current density in WE mode is between 1.4 A/cm2 and 3 A/cm2. The RTE of the system increases at lower FC temperatures, although the electrical output efficiency is lower. Compared to the working pressure of 1 atm in FC mode, the RTE of the system increases by 0.6 % at the working pressure of 3 atm. In addition, an adaptive optimization strategy is proposed, by adjusting the working current density in the two operating modes and controlling the temperature in the FC mode, which enables a proportional delivery of thermal and electrical energy according to demand and ensures high system efficiency.

KW-grade unitized regenerative fuel cell stack design for high round-trip efficiencies

Unitized regenerative fuel cell (URFC) is a promising electrochemical device, which can function either in fuel cell (FC) mode or water electrolysis (WE) mode. However, few applications of kW-grade URFC stack are available due to their low round-trip efficiency. In this study, a new approach to URFC stack design is provided to break the limitations of round-trip efficiency (RTE) and output power, by introducing the concept of bifunctional flow channels for the first time. Hence, a two-phase flow model based on the volume of fluid (VOF) method for bifunctional flow channels is implemented first. It reveals that URFC suffers from the issues of water adhesion on the channel wall in FC mode and the problem of air film coverage on the GDL surface in WE mode. To address this, water volume fraction in FC mode and gas coverage fraction of GDL in WE mode are proposed as new bifunctional flow channel design index, and the optimized configuration of the bifunctional flow channel is provided to balance the drainage and exhaust capabilities. Eventually, a high power-density URFC stack is demonstrated to achieve a high RTE of 45 %, with an output power of 6.01 kW in FC mode and a power requirement of 18.16 kW in WE mode. Moreover, validation experiments show that the good bifunctional performance is since the little water/gas transport problem of the URFC at high current densities. Further sensitivity experiments of the operating conditions also show that the mass transfer capacity of URFC is well adapted to various operating conditions, which implies that the bifunctional flow channel successfully balances good drainage and exhaust capabilities.

Highly efficient unitized regenerative hydrogen peroxide cycle cell with ultralow overpotential for renewable energy storage

The potential large-scale applications of intermittent renewable energy sources require inexpensive, efficient, and less-resource-demanding energy storage systems to achieve grid balancing. Conventional unitized regenerative fuel cells (URFCs) based on the H2–H2O cycle are promising but suffer from high overpotential and low energy efficiency. Here we demonstrate a highly efficient unitized regenerative hydrogen peroxide cycle cell (UR-HPCC) for the renewable energy storage. The prototype utilizes a carbon-based platinum group metal-free catalyst containing atomically dispersed Co and N dopants (Co–N–C) as the bifunctional oxygen electrode catalyst for the hydrogen peroxide oxidation reaction (HPOR) and two-electron oxygen reduction reaction (2e-ORR) in H2O2 electrolyzer and fuel cell modes, respectively. Further, this prototype exhibits a close-to-zero overpotential with remarkably high round-trip efficiency of over 90%, which is attributed to the ideal catalytic properties of Co–N–C toward HPOR and 2e-ORR. The thermodynamic analysis of these single-intermediate reactions indicates the intrinsic superiority of UR-HPCC in energy efficiency and reversibility over conventional URFCs, thus facilitating the development of future sustainable distributed generation and energy storage systems.

Pyrochlores for Advanced Oxygen Electrocatalysis.

Fuel cells (FCs), water electrolyzers (WEs), unitized regenerative fuel cells (URFCs), and metal-air batteries (MABs) are among the emerging electrochemical technologies for energy storage, fuel (H2), oxidant (O2), and clean energy production. Their commercial applications are hindered by the low oxygen reduction reaction/oxygen evolution reaction (ORR/OER) bifunctional activity (for URFCs and MABs), OER selectivity (brine electrolysis in seawater and Martian environments), and high cost of the benchmark electrocatalysts (OER: RuO2, IrO2 and ORR: Pt/C) which affects the performance and affordability of the devices. Low-cost electrocatalysts with highly symmetric ORR/OER bifunctional activity and high OER selectivity are crucial for large-scale FC, WE, URFC, and MAB application. Recent studies have revealed that tuning the structure of pyrochlore oxides provides a pathway to enhancing OER and ORR activity over a wide range of pH. Pyrochlore oxides commonly contain a cubic A2B2O7-x structure with two types of tetrahedrally coordinated O atoms containing (1) A-O-A and (2) A-O-B types with a cationic radii mismatch of rA/rB > 1.5 and propensity toward oxygen vacancy formation. The variety of pyrochlore oxides and their tunable properties make them attractive for a wide spectrum of applications. Among all the metal oxides, Ru-based pyrochlores (e.g., Pb2Ru2O7-x) exhibit the best bifunctional oxygen electrocatalytic activity, i.e., low bifunctionality index (BI), in alkaline medium. Furthermore, pyrochlores exhibit high OER selectivity in brine electrolytes due to the presence of surface oxygen vacancies, making them suitable for space applications (brine electrolysis on Mars) and coastal hydrogen generation. Their bifunctional activity and selectivity can be further amplified by (1) substituting "A" and "B" sites of pyrochlores (AA'BB'O7-x), (2) tuning metal oxidation states of A and B by varying synthesis conditions, and (3) modulating oxygen vacancy concentration, each of which yield favorable structural and electronic variations. In recent years, research on the synthesis and understanding of pyrochlores has significantly enhanced their viability, offering a new horizon in the quest for economical and active electrocatalysts. However, an account that focuses on critical developments in this field is still lacking.In this Account, we focus on the recent development of a variety of pyrochlore electrocatalysts to understand intrinsic structure-activity-selectivity-stability relationships in these materials. Recent developments and applications of pyrochlore-based electrocatalysts are discussed under the following headings: (1) modulation of crystal and electronic structure of pyrochlores, (2) structure-activity-stability relationships of different pyrochlores for OER and ORR, (3) development of OER-selective pyrochlores for brine electrolysis, and (4) the application of pyrochlores in electrochemical devices. Finally, we highlight some unaddressed issues such as the precise identification of active sites, which can be addressed in the future through advanced in situ and ex situ characterization techniques coupled with the density functional theory-based analyses. This Account provides foundational understanding to guide the comprehensive development of highly active, selective, stable and low-cost structurally engineered pyrochlores for high performance electrochemical devices.

Investigating Perovskite Oxide Catalysts As Bifunctional Oxygen Electrodes Using Operando XAS

With the increase in renewable energy usage comes the need for energy storage systems due to intermittency issues. Hydrogen storage systems have been identified as one solution. Unitized regenerative fuel cells (URFC) combine electrolyzers and fuel cells in one device, allowing electricity to be stored and used easily. However, the oxygen electrodes are still affected by high overpotentials and slow kinetics. Perovskite oxides have been identified as a class of materials, which are low-cost, tunable, and active for the oxygen reduction (ORR) and evolution (OER) reactions. Here, we investigate perovskites as bifunctional catalysts for ORR and OER in alkaline solution. We examine and compare two strategies for bifunctional catalysts: using one catalyst, which is able to perform OER and ORR vs. a combination of two catalysts, one active for ORR and one active for OER. Frequently, the catalysts’ performances for these two reactions are measured separately.1,2,3 Here, we investigate how these bifunctional catalysts respond to cycling between the OER and ORR regions.Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) is known to be a promising OER catalyst.4,5,6 However, without carbon, it lacks ORR activity.4 La(1-x)SrxMnO3 (LSM) is a promising ORR catalyst.3,7 However, without modification, it has been shown to have limited OER activity.3 Separately, these catalysts lack high performance for both reactions. Here, we combine the two catalysts into a BSCF/LSM/Carbon composite electrode and compare to electrodes prepared from the constituent single material components. In addition, we have synthesized single material perovskites containing both Co and Mn that to the best of our knowledge have never been tested as electrodes for ORR/OER. In order to understand the catalysts’ behaviors under OER and ORR conditions, X-ray adsorption spectroscopy (XAS) was measured continuously while performing cyclic voltammetry. We were able to monitor the continuous changes of the Co, Mn, and Fe oxidation states and local environment during OER and ORR with remarkably high time/applied potential resolution. Our findings illustrate the reversible and irreversible changes that can occur during OER and ORR and provide strategies for future bifunctional catalyst design.References Kirsanova, M. A.; Okatenko, V. D.; Aksyonov, D. A.; Forslund, R. P.; Mefford, J. T.; Stevenson, K. J.; Abakumov, A. M. Bifunctional OER/ORR Catalytic Activity in the Tetrahedral YBaCo 4 O 7.3 Oxide. Mater. Chem. A 2019, 7 (1), 330–341.Elumeeva, K.; Masa, J.; Sierau, J.; Tietz, F.; Muhler, M.; Schuhmann, W. Perovskite-Based Bifunctional Electrocatalysts for Oxygen Evolution and Oxygen Reduction in Alkaline Electrolytes. Acta 2016, 208, 25–32.Xu, W.; Apodaca, N.; Wang, H.; Yan, L.; Chen, G.; Zhou, M.; Ding, D.; Choudhury, P.; Luo, H. A-Site Excessive (La0.8Sr0.2)1+ XMnO3 Perovskite Oxides for Bifunctional Oxygen Catalyst in Alkaline Media. ACS Catal. 2019, 9 (6), 5074–5083.Fabbri, E.; Nachtegaal, M.; Cheng, X.; Schmidt, T. J. Superior Bifunctional Electrocatalytic Activity of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ /Carbon Composite Electrodes: Insight into the Local Electronic Structure. Energy Mater. 2015, 5 (17), 1402033.Fabbri, E.; Nachtegaal, M.; Binninger, T.; Cheng, X.; Kim, B.-J.; Durst, J.; Bozza, F.; Graule, T.; Schäublin, R.; Wiles, L.; Pertoso, M.; Danilovic, N.; Ayers, K. E.; Schmidt, T. J. Dynamic Surface Self-Reconstruction Is the Key of Highly Active Perovskite Nano-Electrocatalysts for Water Splitting. Mater. 2017, 16 (9), 925–931.Kim, B. J.; Fabbri, E.; Abbott, D. F.; Cheng, X.; Clark, A. H.; Nachtegaal, M.; Borlaf, M.; Castelli, I. E.; Graule, T.; Schmidt, T. J. Functional Role of Fe-Doping in Co-Based Perovskite Oxide Catalysts for Oxygen Evolution Reaction. Am. Chem. Soc. 2019, 141 (13), 5231–5240.Tulloch, J.; Donne, S. W. Activity of Perovskite La1−xSrxMnO3 Catalysts towards Oxygen Reduction in Alkaline Electrolytes. Power Sources 2009, 188 (2), 359–366.

Highly effective bi‐functional electrochemical activity of Ag 2 O‐PrO 2 / γ‐Al 2 O 3 electrocatalysts towards OER and ORR

The oxygen reduction and evolution reaction (ORR/OER), are the keystones for many energy generating and storage devices, have influence with many electrocatalytic reactions. The sluggish kinetics of these reaction is a challenging situation on the surface of electrodes which imposes large over-potential and affects the energy output of fuel cells. Although the platinum-based electrode materials have excellent activity toward ORR/OER but their high cost limits commercialization of fuel cells. To deal with this problem, platinum-free electrode materials has aroused the considerations of scientists. The present work is proposed to study the oxygen reduction reaction and water oxidation on the surface of xAg2O-yPrO2/γ-Al2O3/Glassy-Carbon. The ORR was studied electrochemically via cyclic voltammetry on the surface of these nano-catalysts while the OER was investigated via cyclic voltammetry and linear sweep voltammetry in alkaline media. The mechanism of ORR on catalysts surface was enquired and the results indicate that ORR followed two electrons pathway in case of 10PrO2/γ-Al2O3/Glassy-Carbon while four electrons reduction pathway is identified on surface of others catalysts which divulged that addition of silver with praseodymium shifted the mechanism from 2e− to 4e− pathway. The electrocatalysts featured the stability during multiple scans and no corrosion of electrode in OER/ORR is observed. Different kinetic parameters for both reactions are determined which affirmed that both ORR and OER on the active surface of catalysts is irreversible and diffusion controlled. 4Ag2O-6PrO2/γ-Al2O3 is indicated to be proficient electro-catalyst among other members of the series, having greater value of mass transport and diffusion coefficients. The bi-functionality of all electrocatalysts are verified by their electrochemical responses in alkaline medium. The catalytic reduction of oxygen and oxidation of water by these electro-catalysts imitates their potential as electrode material in batteries and unitized regenerative fuel cells (URFC) technology for energy production as substitute of platinum electrode.

Performance and Durability of Proton Exchange Membrane Vapor-Fed Unitized Regenerative Fuel Cells

With a growing demand for electricity, clean hydrogen production and usage can be an asset not only to mitigate emissions but for long-term energy storage as well. Hydrogen gas, a high-density energy carrier, can be made through electrolysis in charging mode and generate electricity via a fuel cell in discharging mode in a unitized regenerative fuel cell (URFC). While URFCs reduce cost by combining the charging and discharging modes into a singular device, switching between modes becomes burdensome, and water management is a major challenge. One way to mitigate these issues is to operate the entire system in the vapor phase. Vapor-phase operation simplifies the physics of the system but will introduce losses within the system, primarily ohmic and mass transport during the charging mode. In this study, we explore the performance of a Proton-Exchange-Membrane (PEM)-URFC under vapor-phase conditions and the impact of different PEMs, feed gases, and relative humidity on the performance and durability. By tailoring operating conditions and membrane, the vapor-URFC achieves a roundtrip efficiency of 42% and a lifetime of 50,000 accelerated stress test cycles for fully humidified feeds. Discussion of vapor-URFC for energy storage and extensions to look at various applications shows the promise of this technology.