Oxide‐based ORR catalysts

The exploration of noble metal-free catalysts with efficient electrochemical performance toward oxygen reduction reaction in the acid electrolyte is very important for the development of fuel cells technology. Novel pyrolyzed heteroatom-doped Fe/N/C catalysts have been regarded as the most efficient electrocatalytic materials for ORR due to their tunable electronic structure, and distinctive chemical and physical properties. Herein, nitrogen- and sulfur-doped (Fe/N/C and Fe/N/C-S) electrocatalysts were synthesized using ferric chloride hexahydrate as the Fe precursor, N-rich polymer as N precursor, and Ketjen Black EC-600 (KJ600) as the carbon supports. Among these electrocatalysts, the as prepared S and N-doped Fe/N/C-S reveals the paramount ORR activity with a positive half-wave potential value (E1/2) 0.82 at 0.80 V vs. RHE in 0.1 mol/L H2SO4 solution, which is comparable to the commercial Pt/C (Pt 20 wt%) electrocatalyst. The mass activity of the Fe/N/C-S catalyst can reach 45% (12.7 A g−1 at 0.8 V) and 70% (5.3 A g−1 at 0.95 V) of the Pt/C electrocatalyst in acidic and alkaline solutions. As result, ORR activity of PGM-free electrocatalysts measured by the rotating-ring disk electrode method increases in the following order: Fe/N/C<Fe/N/C-S, in both basic and acidic medium. This scientific work offers a facile approach to design and synthesizes efficient heteroatom-doped catalytic materials for electrochemical reactions in energy devices.

Solid oxide fuel cell (SOFC) technology requires further maturations to become commercialized broadly. Many existing problems are expected to be solved by intermediate temperature SOFC (IT-SOFC). On the other hand, electrochemical resistances of cathode materials increase rapidly at lower temperature. So it is necessary to understand the underlying mechanisms of oxygen reduction reaction and develop new materials. Although many experimental and theoretical researches for this purpose are in progress, traditional trial-and-error materials search must be inefficient and take too long time. Considering the above situation, we explore critical factors that can explain the oxygen exchange rate of perovskite oxide cathode in SOFCs. It has been said that the bulk ionic conductivity is strongly related to the oxygen exchange rate, while the electronic conductivity is not as far as it is high enough. However, we found that the combination of ionic and electronic conductivities shows much stronger correlation than ionic conductivity alone while analyzing reported experimental data on 18 materials in figure 1. Materials which have high oxygen exchange rate can be explained by the multiplication of bulk ionic conductivity and electronic conductivity while others explained by the ionic conductivity. Fig. 1 Relationship between oxygen exchange rate and bulk electronic conductivity, ionic conductivity, and the model function with both conductivities. Figure 1

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Transition metal selenides have attracted intensive interest as cost-effective electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) because of the continuous thrust in sustainable energy conversion. In this article a Mn-based bifunctional electrocatalyst, MnSe has been identified which shows efficient OER and ORR activity in alkaline medium. The catalytic activity could be further enhanced by using multiwalled carbon nanotubes (MWCNT) asadditives which increases the charge transfer and electronic conductivity of the catalyst composite. This MnSe@MWCNT catalyst composite exhibits a very low overpotential of 210 mV at 50 mA cm-2 when deposited on Ni foam, which outperforms state-of-the-art RuO2 as well as other oxide and Mn-based electrocatalysts. Furthermore, the composite’s facile OER kinetics was evidenced by its small Tafel slope of 54.76 mV dec–1 and low charge transfer resistance, indicating quick transport of the reactant species. The MnSe@MWCNT also exhibited efficient electrocatalytic activity for ORR with a Eonset of 0.94 V, which is among the best reported till date for chalcogenide based ORR electrocatalysts. More importantly, this MnSe-based ORR electrocatalyst exhibits high degree of methanol tolerance, showing no degradation of catalyst performance in presence of copious quantities of methanol, thereby out-performing state-of-the-art Pt electrocatalyst. The catalyst compositie also exhibited exceptional functional and compositional stability for OER and ORR after prolonged period of continuous operation in alkaline medium. The surface Raman analysis after OER revealed the retention of manganese selenide surface with evidence of Mn-oxo coordination confirming the formation of mixed anionic (oxy)selenide as the active surface for OER. Such efficient bifunctional OER and ORR activity makes this MnSe based catalyst attractive for overall electrolysis in regenerative as well as direct methanol fuel cells.

As one of the most promising power sources in converting and storing chemical energy, aqueous zinc-air batteries (ZABs) stand out because of their high theoretical energy density, low cost, high safety of aqueous electrolyte, and environmental friendliness. Three main types of ZABs including primary ZABs, mechanically rechargeable ZABs, and electrically rechargeable ZABs have been developed up to now.[1] Primary ZABs for single use are usually discarded after the Zn anode is consumed or passivated although other parts of battery remain intact, leading to enormous waste. To address it, the depleted anode and failure electrolyte can be physically replaced to recover the capacity in mechanically rechargeable ZABs. But the cumbersome disassembly/reassembly process and the performance deterioration after every recharge process limit its practical application. In addition, it is not feasible to replace zinc in some integrated systems, such as the large-scale ZAB stack. Electrically rechargeable ZABs with bifunctional oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) catalysts could be returned to the original state by electrochemical charging process, but also face some formidable challenges, such as dendritic zinc deposition, electrode deformation, and hydrogen evolution, limiting the long-term cycling performance. Besides, sluggish kinetics of ORR/OER and Zn passivation result in low energy efficiency, limited depth of discharge and power density. Especially in the resource-constrained and off-grid region, there isn’t external electricity source to charge the electrically rechargeable ZABs which limits their applications. Moreover, the reactions inherently take place on the surface of Zn anode and air cathode, in which the storage and conversion capability of the cell would be essentially constrained by the surface area of electrode and the volume of electrode compartment.The concept of redox targeting (RT) of battery materials offers a feasible solution to address these issues by employing redox mediators (RMs). To tackle the above challenges, here we demonstrate a redox-mediated Zn-air fuel cell (RM-ZAFC) based on the RT reactions of cobalt triisopropanolamine complex (CoTiPA) with O2 and 7,8-dihydroxy-2-phenazinesulfonic acid (DHPS) with Zn metal in the catholyte and anolyte, respectively. Both the 4-electron (4 e-) ORR and Zn oxidation reactions are liberated from the electrode surface and occur inside separate reactor tanks resorting to the RT processes. Upon operation, Co(III)TiPA in the catholyte is electrochemically reduced to Co(II)TiPA on the cathode and chemically oxidized back through ORR in the gas diffusion tank (GDT) into which O2 is fed. In conventional ZABs, ORR takes place on the triple-phase reaction interface of the heterogeneous electrocatalyst, and the catalytic performance is closely related to the density of exposed active site, electrical conductivity and reaction energy barrier. As compared, the redox-mediated ORR in GDT occurs homogeneously in the catholyte in which the dissolved RM could provide a large quantity of rection sites with tunable reaction energy barrier. For the negative side, DHPS-2H is firstly oxidized to DHPS on the anode and circulated into anodic tank, where it is regenerated by oxidizing Zn to ZnO. The discharge product ZnO is formed and deposited on the Zn granules loaded in the tank. After fully discharged, the cell can be feasibly refueled by replacing the Zn granules in the tank instead of disassembling and reassembling the whole cell. In this RM-ZAFC, the energy storage unit is decoupled from the power generation unit, thus offering greater operation flexibility and system scalability with modular design than the conventional ZABs. Compared with the commercial hydrogen tank used in hydrogen fuel cells, Zn granules loaded in the anodic tank present much higher volumetric capacity, greater safety and ease for storage and transportation. Therefore, as an energy conversion and storage device, RM-ZAFC would have great promise for automotive, backup, and stationary power applications.[2] In addition, we also propose an efficient and zinc regeneration method by RT reaction between RM and the product ZnO in RM-ZAFC to avoid the waste of ZnO and reduce the cost.

Solid oxide fuel cells (SOFCs) attract attentions as high-efficiency energy-conversion devices. In order to establish high-performance SOFCs, the slow electrode kinetics, which is often a main cause of the energy loss, should be improved. In present SOFCs, mixed ionic-electronic conducting (MIEC) oxides, such as (La,Sr)CoO3 -δ and (La,Sr)(Co,Fe)O3 -δ , are typically used as the cathode materials. In such an MIEC cathode, it is generally believed that electrochemical oxygen reduction reaction takes place mainly at the oxide surfaces (double phase boundary, DPB) and the contribution of the reaction at the electrode/electrolyte/gas interface (triple phase boundary, TPB) is insignificant. Therefore, researches on the kinetics of SOFC cathodic reaction have mostly concerned to investigate the surface reaction on the electrode oxides. In our previous works, the reaction distributions in MIEC SOFC electrodes were experimentally evaluated by using operando micro X-ray absorption spectroscopy (operando μ-XAS) [1, 2]. For more quantitative and precise investigation of the reaction distribution, we proposed a patterned thin film electrode, which is a kind of the columnar electrode simplifying the microstructures of a porous electrode [2]. However, the patterned thin film electrode in our previous work did not have triple phase boundaries, and it was thus impossible to discuss the contribution of TPB reaction to the total reaction. In this work, a patterned thin film electrode having triple phase boundaries was fabricated. (La,Sr)CoO3- d (LSC) was chosen as the model MIEC oxide for SOFC cathodes. The reaction distribution in this new type of the patterned thin film electrode was experimentally determined by using operando μ-XAS, and the results were compared with ones obtained with the old type of the patterned thin film electrode without TPB. It was found the effective reaction area tended to shrink by the introduction of TPB to the patterned thin film electrode. This fact might suggest that the reaction at TPBs in SOFC cathodes is not negligible even when MIEC oxides are used as electrode materials.

The effect of the crystallographic form of MnO2 on the kinetics of oxygen reduction and evolution reaction

Bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are the heart of metal-air batteries, fuel cells, and other energy storage systems. Here, we report a series of a novel class of redox-active viologen-based ionic covalent organic networks (vCONs) which are directly used as metal-free bifunctional electrocatalysts towards ORR and OER applications. These vCONs (named vGC, vGAC, vMEL and vBPDP) were synthesized by the well-known Zincke reaction. The installation of redox-active viologen moieties among the extended covalent organic architectures played a crucial role for exceptional acid/base stability, as well as bifunctional ORR and OER activities, confirmed by the cyclic voltammetry (CV) curves. Among all of them, vBPDP showed high ORR efficiency with a half-wave potential of 0.72 V against a reversible hydrogen electrode (RHE) in 1 M KOH electrolyte. In contrast, vMEL demonstrated high OER activity with an overpotential of 320 mV at a current density of 10 mAcm-2 and a Tafel slope of 109.4 mV dec-1 in 1 M KOH electrolyte solution. This work is exceptional and unique in terms of directly used pristine ionic covalent organic networks that are used as bifunctional (ORR and OER) electrocatalysts without adding any metals or conductive materials.

Solid Oxide Fuel Cell (SOFC) cathode materials rely on the catalytic processes at their surfaces to reduce and incorporate oxygen. For many common cathode materials surface passivation occurs through 'A site' segregation at common operating temperatures [1]. These processes have a negative effect upon the oxygen reduction rates and therefore overall cell performance. Quantifying the true extent these segregation effects have on the exchange properties has not been fully explored but it is clear that the properties of the surface are essential. Many techniques have been employed to study cell properties in-situ, however, surface studies are typically restricted to post mortem or ex-situ analysis due to the necessity of a high vacuum. In this study High Temperature Environmental Scanning Electron Microscopy (HT-ESEM) has been employed to analyse the morphology changes of common SOFC cathode materials in-situ. Samples of La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and La0.6Sr0.4CoO3-x (LSC) were annealed from room temperature to 1000oC at pressure ranges between 3 – 6mbar in atmospheres of Oxygen and Water. Using secondary electron imaging during the thermal annealing cycle morphological and compositional changes were able to be documented at temperature for the first time (Figure 1). This technique has enabled detailed analysis of the surface degradation that occurs during SOFC fuel cell operation. Secondary phase precipitation rate and formation behaviour has been quantified over a range of temperatures and time scales furthering the understanding of SOFC cathode surfaces significantly. [1] M. Kubicek, A. Limbeck, T. Fromling, H. Hutter, and J. Fleig, "Relationship between cation segregation and the electrochemical oxygen reduction kinetics of La0.6Sr0.4Co0.2Fe0.8O3 thin film electrodes" Journal of The Electrochemical Society, vol. 158, no. 6, pp. B727–B734, 2011. Figure 1 - Showing in-situ growth of secondary phase particulates on a polished surface of LSCF Figure 1

We explore critical factors that can explain the oxygen exchange rate of perovskite-oxide cathode in solid oxide fuel cell (SOFC). It has been said that the bulk ionic conductivity is strongly related to the oxygen exchange rate, while the electronic conductivity is not as far as it is high enough. However, we found that the combination of ionic and electronic conductivities shows much stronger correlation than ionic conductivity alone while analyzing experimental data reported in the literature. Materials which have high oxygen exchange rate can be explained by the product of bulk ionic and electronic conductivities while others by the ionic conductivity alone.

A dynamic infiltration technique to synthesize nanolayered cathodes for high performance and robust solid oxide fuel cells

Overcoming phase instability of RBaCo2O5+δ (R=Y and Ho) by Sr substitution for application as cathodes in solid oxide fuel cells

The high-temperature solid oxide fuel cells (SOFCs) are the most efficient and green conversion technology for electricity generation from hydrogen-based fuel as compared to conventional thermal power plants. Many efforts have been made to reduce the high operating temperature (>800 °C) to intermediate/low operating temperature (400 °C<T<800 °C) in SOFCs in order to extend their life span, thermal compatibility, cost-effectiveness, and ease of fabrication. However, the major challenges in developing cathode materials for low/intermediate temperature SOFCs include structural stability, catalytic activity for oxygen adsorption and reduction, and tolerance against contaminants such as chromium, boron, and sulfur. This research aims to provide an updated review of the perovskite-based state-of-the-art cathode materials LaSrMnO3 (LSM) and LaSrCOFeO3 (LSCF), as well as the recent trending Ruddlesden-Popper phase (RP) and double perovskite-structured materials SOFCs technology. Our review highlights various strategies such as surface modification, codoping, infiltration/impregnation, and composites with fluorite phases to address the challenges related to LSM/LSCF-based electrode materials and improve their electrocatalytic activity. Moreover, this study also offers insight into the electrochemical performance of the double perovskite oxides and Ruddlesden-Popper phase materials as cathodes for SOFCs.

SrCo0.4Fe0.4Zr0.1Y0.1O3-δ, A new CO2 tolerant cathode for proton-conducting solid oxide fuel cells

Fuel cells have developed of excessive interest as a probable economical, efficient, and clean candidate for alternative and environmental friendly power generation services. Solid Oxide Fuel Cell (SOFC) is an elevated temperature fuel cell, dealing with power generation as well as heat. Up to now many studies have been made to replace platinum, Pt, with a new cathode catalyst for intermediate temperature-solid oxide fuel cells (IT-SOFC) (500 °C&amp;lt;T) range but research has become inadequate. Since Pt sources are limited and very expensive, they could not meet the supply for the commercial fuel cells, the scientists started for searching new materials. There are two important aspects about SOFC cathodes, different cathode materials effect on the electrode electrochemical performance and the oxygen reduction reaction (ORR) kinetics. Understanding in these concepts would lead to improvements of SOFC systems. The production of novel and supreme cathode electrodes used in IT-SOFC is aimed to employ cheaper metals (Fe, Co, Cr, Mn, Gd, and V) by using superior properties of perovskite structure. The reduction of oxygen on metal oxide surface is achieved within the complicated mechanism. The completion of these steps depends on the nature of oxide ion carrier in cathode, atomic formation in crystal structure, and microstructure of cathode materials. The analysis of the impedances required the use of three to four (RQ) circuits in series in the equivalent circuit model. Of the four cathodes synthesized, the vanadium doped cathode on YSZ showed the highest area specific resistance.