Electronic modulation induced by decorating single-atomic Fe-Co pairs with Fe-Co alloy clusters toward enhanced ORR/OER activity

The effective utilization of clean energy and finding alternatives to fossil resources are highly important to ensure the sustainability of human society and are always among the major goals of both chemistry and material science research. Advanced electrochemical devices, such as fuel cells, water electrolysers and metal-air batteries, represent the most promising strategies for clean-energy utilization. In an electrochemical device, the redox reactions are spatially separated by a membrane, allowing direct extraction/transfer of electrons at an electrode-electrolyte interface, which leads to higher intrinsic energy conversion efficiencies, milder process conditions, easy product separation and excellent design features for coupling to renewable energy infrastructure. The performance of such electrochemical processes is fundamentally determined by the physicochemical properties of the electrochemical interfaces, encompassing both the electrocatalyst and the structure of the adjacent electrochemical double layer. Specifically, electrocatalysts play key roles in electrochemical reactions and often limit the performance of entire systems due to their insufficient activity, low durability or high cost. Ideally, the rate, efficiency, and selectivity of the above electrochemical reactions can be substantially improved by developing high-performance electrocatalyst. One of the central tasks for chemists and material scientists is to design and fabricate the high-efficient efficiency but low-cost electrocatalysts systems. The current promising electrochemical reactions mainly focus on the realization of the reversible conversion between chemical and electricity energy, e.g., the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and hydrogen evolution reaction (HER). Coupling of the above electrochemical reactions provide a solid foundation for various essential electrochemical devices, such as direct hydrogen fuel cells (HOR + ORR); electrolysers (OER + HER); rechargeable zinc (Zn)-air battery (ORR + OER). Therefore, this thesis aims to design and synthesize high-performance electrocatalysts for HER, ORR and OER based on earth-abundant materials with proper hierarchical 2D or 3D nanostructures. Combined with the advanced characterization techniques and density functional theory (DFT) calculations, the relationship between the electrochemical activity and active sites of these earth-abundant electrocatalysts were detailedly explored and confirmed. Furthermore, to emphasize the hierarchical 2D or 3D nanostructures, the actual performance of these electrocatalysts was all evaluated in practical devices including Zn-air battery and proton exchange membrane fuel cell (PEMFC), specifically as follows: (1) The vast majority of the reported HER electrocatalysts performs poorly under alkaline conditions due to the sluggish water dissociation kinetics. In the first work, a hybridization catalyst construction concept is presented to dramatically enhance the alkaline HER activities of catalysts based on 2D transition metal dichalcogenides (TMDs) (MoS2 and WS2). A series of ultrathin 2D-hybrids are synthesized via facile controllable growth of 3d metal (Ni, Co, Fe, Mn) hydroxides on the monolayer 2D-TMD nanosheets. The resultant Ni(OH)2 and Co(OH)2 hybridized ultrathin MoS2 and WS2 nanosheet catalysts exhibit significantly enhanced alkaline HER activity and stability compared to their bare counterparts. The combined theoretical and experimental studies confirm that the formation of the heterostructured boundaries by suitable hybridization of the TMD and 3d metal hydroxides is responsible for the improved alkaline HER activities because of the enhanced water dissociation step and lowers the corresponding kinetic energy barrier by the hybridized 3d metal hydroxides. (2) Nitrogen-coordinated iron atoms on carbon matrix (Fe-N-C) materials are the most active Pt-group-metal-free ORR catalysts but still suffering their low stability and relatively lower activity compared to platinum-based materials. In the second work, Fe and Ni dual sites atomically dispersed in hierarchically ordered macroporous carbon support (Fe-Ni/N-HOMC) was designed and successfully prepared. Isolated atomic Fe- N4 and Ni-N4 active sites were confirmed via various characterizations. The ORR activity and stability of Fe-Ni/N-HOMC in both acid and alkaline electrolyte were much higher than commercial Pt/C and the mono-Fe doping counterpart, which was among the state-of-the-art ORR electrocatalysts. In addition, this 3D ordered interconnected macroporous structure with abundant mesopores and micropores could greatly increase the accessible ORR active site and also enhance the mass transport during the ORR process. When employed as cathodes for PEMFC, we found the excellent ORR activity of Fe-Ni/N-HOMC was completely translated to the cathode in the fuel cell. (3) High-performance bifunctional electrocatalysts with ORR and OER activity is the key to developing efficient rechargeable Zn-air batteries. In the third work, a high-performance bifunctional electrocatalysts for both OER and ORR were synthesized via further hybridizing as-prepared Fe-Ni/N-HOMC with NiFe layer double hydroxides (LDHs). Layered double hydroxides (LDHs) have been reported to be promising OER electrocatalysts with ultrahigh OER performances. The as-synthesized new composites exhibited almost the same ORR activity as Fe-Ni/N-HOMC, revealing that hybridization of NiFe-LDHs would not deteriorate the initial ORR activity. Moreover, the remarkable enhancement of OER activity was observed after the hybridization, which was attributed to the strong coupling of uniformly dispersed small NiFe-LDH nanoparticles with the carbon substrate. The prototype Zn-air battery was assembled using these new composites, which displayed the ultralow voltage gap and long-term stability. (4) Compared with Fe-N-C or Co-N-C based ORR electrocatalysts, the Cu-nitrogen-carbon composites were attracted little attention. However, the natural multicopper oxidases (MCOs) enzymes, such as laccase, can serve as efficient ORR catalyst with almost no overpotential. Inspired by their tris-copper centers in MCO, one novel Cu-nitrogen-carbon composite (Cu SAs/N-CS) with atomic Cu coordination sites were synthesized via the pyrolysis of the Cu-involved metal-organic-framework. The copper contents in Cu SAs/N-CS reaches as high as 3.17 wt.%, and the average distances of adjacent copper sites was around only 3.1 Å. Due to the synergetic effect of abundant single atomic copper active sites with closer distance and ultrathin carbon nanosheet structure, Cu SAs/N-CS exhibited superior ORR activity exceeding commercial Pt/C catalyst, methanol tolerance, and long-term stability in both alkaline and neutral electrolyte. In summary, four kinds of new composites were successfully designed and prepared as high-performance electrocatalysts for HER, ORR and OER. Multi-dimensional heterostructures, atomic metal coordination sites and 3D hierarchically porous structure were designed and observed, which contributed greatly to improve activities of these composites. This thesis suggests several new viewpoints in the design of electrocatalysts based on earth-abundant materials: (i) offering new strategies for the preparation of novel 2D and 3D heterostructures as electrocatalysts; (ii) expanding methods for the synthesis of atomic metal coordination sites and evaluating their activities for ORR; (iii) evaluating the practical performances of achieved electrocatalysts in proton exchange membrane fuel cell and Zn-air battery; (iv) attempting to explain reaction mechanisms of some electrocatalysts by DFT calculation.

The material design of highly active catalysts for energy storage applications (e.g., metal-air batteries and fuel cells) is crucial for solving global energy problems. The oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is used for charging and discharging rechargeable metal-air batteries, respectively. Although precious metal-based catalysts facilitate these reactions, the kinetics of the OER and ORR are rather sluggish due to their multistep electron transfer. It is therefore important to explore the mechanism of the OER and ORR to discover the key factors for highly active catalysts. As demonstrated by Suntivich et al. (2011)[1],[2], when the number of electrons in the eg orbital is close to unity for transition metals, perovskite oxides exhibit maximum OER and ORR activities. In other words, Mn3+ (t2g 3 eg 1 for both surface and bulk), Co3+ (t2g 5 eg 1 for surface), and Ni3+ (t2g 6 eg 1 for both surface and bulk) become OER and ORR active sites for Mn3+, Co3+ and Ni3+ based compounds . However, LaMnO3 exhibits a significantly lower specific OER activity[3] compared with LaCoO3 and LaNiO3 (~6% of LaNiO3 at 1.8 V vs. RHE). Also, Mn3O4 exhibits a low specific OER activity[4] (40% of Mn2O3 at 1.8 V vs. RHE) similar to LaMnO3. It is therefore important to explore what causes degradation of the OER activity of Mn3+-based (t2g 3 eg 1) compounds to improve their catalytic activity. We attempt to improve the performance of Mn3+-based oxides by controlling the degradation factors of their OER activities. To directly compare the OER activities of Mn3+-based oxides containing more than two cation sites, Mn3+ concentrations at the octahedral site and their initial crystal structures were maintained. Mn3-x Co x O4 (0 ≤ x < 1), a series of tetragonally distorted spinel compounds, satisfies this condition, as their octahedral sites remain occupied by only Mn3+ ions . For Mn3+-based oxides with a single cation site, Mn3+ was directly substituted by Co3+.We systematically studied the OER and ORR performances of Mn3+-based oxides by Co3+-substitution. Nanoparticles were prepared for the evaluation of catalytic activity to minimize the influence of geometric factors. Electrochemical measurements were conducted using a rotating ring disk electrode rotator (RRDE-3A, BAS Inc., Japan) at 1600 rpm, in combination with a bipotentiostat (ALS Co., Ltd, Japan). In addition, a Pt wire counter electrode, and an Hg/HgO reference electrode (International Chemistry Co., Ltd., Japan) filled with 0.10 M KOH (Nacalai Tesque, Inc., Japan) were used. Electrochemical measurements were conducted with O2 saturation (30 min bubbling O2 gas through the solution) at ~25 °C, where the equilibrium potential of the O2/H2O redox couple was fixed at 0.304 V vs. Hg/HgO (or 1.23 V vs. RHE). During OER current density measurements for each sample, the potential of the sample-modified GC was controlled from 0.3-0.9 V vs. Hg/HgO (1.226-1.826 V vs. RHE) at 10 mV/s. For all measurements, the OER current density was iR-corrected (R = ~43 Ω) using the measured solution resistance, and capacitance-corrected by averaging the anodic and cathodic scans to remove the influence of the current related to the formation of an electrical double layer. With an increase in Co content, signicant improvements in the OER and ORR activities were observed for Mn3+-based oxides. For example, Mn2.1Co0.9O4 exhibited a high specific OER activity (1700 % of Mn3O4 at 1.76 V vs. RHE) and a long-term stability over 100 cycles [5]. The OER activities of Mn3+-based oxides increased linearly with the suppression of the Jahn–Teller distortion. We examined whether the electronic state of Mn3+ changes in favor of enhancing the OER and ORR activities. When the Jahn-Teller distortion of Mn3+O6 octahedra is suppressed due to the increase in Co content, the splitting of the Mn3+ eg orbitals becomes smaller and the electron occupying the Mn3+ eg orbital shifts to a higher energy level. Thus, the overlap of the antibonding Mn3+ eg orbitals with the O 2p orbitals of the oxygen adsorbate becomes stronger. The OER and ORR activities of Mn3+-based oxides should therefore be enhanced due to the stronger binding of OER intermediates to the catalytic surface. We therefore conclude that the suppression of Jahn–Teller distortion enhances the OER and ORR activities of Mn3+-based oxides. Our result suggests a future application of Mn3+-based oxides as bifunctional catalysts for metal-air batteries. [1] J. Suntivich et al., Science, 2011, 334, 1383. [2] J. Suntivich et al., Nature Chemistry, 2011, 3, 546. [3] B. Han et al., Phys. Chem. Chem. Phys., 2015, 17, 22576. [4] A. Ramirez et al., J. Phys. Chem. C, 2014, 118, 14073. [5] S. Hirai et al., RSC Adv., 2016, 6, 2019.

Lithium-air batteries have become attractive due to their extremely high theoretical energy density. However, conventional lithium-air cells operating with nonaqueous electrolytes suffer from poor cycle life due to the clogging of the porous air cathodes by insoluble discharge products, contamination of the organic electrolyte by moist air, and decomposition of the electrolyte during cycling. Attempts to overcome these problems by developing compatible electrolyte/catalyst combinations have invariably been disappointing. These difficulties could be overcome by adopting a new cell configuration consisting of a lithium metal anode in the conventional organic electrolyte separated by a lithium-ion conducting solid electrolyte from the air electrode in an aqueous catholyte. Accordingly, this presentation will focus on the development of catholytes that can avoid corrosion of the solid electrolyte as well as efficient, inexpensive electroctalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).A buffer catholyte solution with a moderate pH, consisting of phosphoric acid and a supporting salt, has been developed to keep the solid electrolyte stable and reduce the internal resistance and overpotential. With a high operating voltage and the utilization of all three protons of phosphoric acid, the buffer catholyte enables a rechargeable lithium-air cell with high energy density. Further increase in power density has been realized by increasing the solid-electrolyte conductivity and the operating temperature to 40 oC.The biggest challenge with lithium-air cells is the large overpotentials associated with the ORR and OER. Nanocrystalline iridium oxide has been the best OER catalyst, but it is expensive. In this regard, the low-temperature form of lithium cobalt oxide (designated as LT-lithium cobalt oxide) that adopts a lithiated spinel structure has been developed as an inexpensive, efficient electrocatalyst for OER. Its OER activity is much higher than that of the high-temperature form of lithium cobalt oxide (designated as HT-lithium cobalt oxide), which has a layered structure and is used as a cathode in lithium-ion batteries, and even slightly higher than that of iridium oxide. Although the ORR activity of LT-lithium cobalt oxide is low, a combination of high ORR and OER activity could be realized by chemically extracting lithium from the LT-lithium cobalt oxide, demonstrating the lithium-deficient, spinel-type LT-lithium cobalt oxide as a bifunctional catalyst for rechargeable metal-air batteries. The high OER and ORR activities of spinel-type LT-lithium cobalt oxides are attributed to cubane-like subunits and a pinning of the cobalt-3d energy with the top of the oxygen-2p band. Furthermore, a novel 3-D O- and N-doped carbon nanoweb (ON-CNW) has been developed as an inexpensive, metal-free catalyst for ORR. The 3-D nanoweb structure provides an ideal backbone support for catalytically active sites due to the fast electron and mass transport properties compared to 1-D or 2-D structures. Also, the synergistic effect between the O and N groups creates highly active pyridone groups all over the nanoweb surface that significantly improves the ORR activity. With a hybrid lithium-air cell, the ON-CNW exhibits performance close to that of commercial Pt/C. In addition, a novel hybrid lithium-air cell configuration with decoupled ORR and OER electrodes has been developed. With the conventional bifunctional air cathode that supports ORR during discharge and OER during charge, the ORR catalyst and carbon support suffer from degradation under the highly oxidizing conditions during the charge process, resulting in rapid increase in overpotential upon cycling and limiting the cycle life. In contrast, the hybrid lithium-air cell with decoupled ORR and OER electrodes eliminates the degradation problems and leads to high efficiency with good cycle life.

Electrocatalytic performances of LaNi1−xMgxO3 perovskite oxides as bi-functional catalysts for lithium air batteries

Cathode catalysts with low overpotential for oxygen electrode reaction including oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are required for the batteries that air and an alkaline electrolyte are used. For this purpose, a kind of perovskite metal oxides is promised. Although the electric conductivity is low, LaMnO3shows relatively high activity for both ORR and OER [1, 2]. To support the electric conductivity, some kinds of carbon must be required to practical use. Among the carbon materials, carbon nano fiber, which is partially used in lithium battery, is considered to be favorable because of adequate electric conductivity, high hydrophobicity, low surface area and quite low amount of surface functional groups.In this study, we attempted to prepare a new composite catalyst, that LaMnO3particles are supported on carbon nano fiber, and evaluated catalyst activity for ORR and OER. La(NO3)3 and Mn(NO3)2 aqueous solutions were respectively prepared at desired concentration and well mixed. Then, the solution was poured into (CH3)4NOH aqueous solution. After that, CNF (VGCF, SHOWA DENKO K.K.) was immediately added to the solution and stirred for 1 hour at room temperature. The product was washed with pure water and dried at 120oC for 12 hour. Finally, it was calcined under Ar atmosphere at 600oC for 5, 10 and 20 hour, 800 and 1000oC for 5 hour (600-5, 600-10, 600-20, 800-5 and 1000-5) to obtain LaMnO3 and CNF composites (LaMnO3-CNT).X-ray diffraction (XRD) was used to identify the product. The product was observed by field emission scanning electron microscope (FE-SEM). ORR and OER activity were evaluated in 0.1 mol dm-3 and 6.0 mol dm-3KOH solution by using glassy carbon or Ti substrate. Figure 1 shows FE-SEM image of 800-5. As indicated by allows in this figure, CNF is covered with particles of LaMnO3. In addition, the product that only edge parts were covered with particles and the product that was not covered with particles occasionally.Figure 2 shows XRD patterns of 800-5 and 1000-5. The patterns of 600-5, 600-10, 600-20 and 800-5 show the formation of LaMnO3 and the different crystallite size. In the case of 1000-5, La2O3and MnO are confirmed, suggesting that oxygen in the products should be consumed by CNF oxidation.Figure 3 shows hydrodynamic voltammograms of the samples in 0.1 mol dm-3KOH solution. Reduction current originated from ORR is shown below 0.85 V. From this point of view, 600-10 shows the highest on-set potential (lowest overpotential) among the samples. The detail including OER activity of the samples will be also presented. This work was supported by “Advanced Low Carbon Technology Research and Development Program (ALCA)” of Japan Science and Technology Agency (JST). [1] J Suntivich, H A. Gasteiger, N Yabuuchi, H Nakanishi, J B. Goodenough and Y Shao-Horn, DOI: 10.1038/NCHEM.1069.[2]J O’M. Bockris and Takaaki Otagawa,J Electronchem.Soc. :ELECTROCHEMICAL SCIENCE AND TECHNOLOGY p.290-302 Vol. 131,No. 2.(1984)

Water electrolysis is a key technology to convert and store sustainable energy into high-energy-dense hydrogen. However, slow kinetics of oxygen evolution reaction (OER) at the anode hinders overall reaction rate of water splitting and delays realization of hydrogen energy society. OER consists of four electron steps and an ideal catalyst has 1.23 V potential barrier for each step, but a real catalyst has a step which requires higher potential than 1.23 V, i.e., overpotential. Therefore, exploring OER catalyst with minimized OER overpotential is an important challenge in increasing water electrolysis efficiency. On the other side, the direct use of abundant seawater as a splitting reactant has advantage in terms of resources and cost. It can also help commercialization of water electrolysis. Since seawater contains chloride which causes undesirable side reactions and accelerated corrosion of the anode material, appropriate OER catalyst is necessary to avoid these problems.Conventional trial-and-error method in catalyst design usually requires many years of R&D and high cost. Computational screening can preemptively narrow down the candidate group of high-efficient catalyst to reduce time and economic cost. NiFe-based layered double hydroxide (NiFe-LDH) is a promising OER catalyst due to the comparable activity to commercial IrO2catalyst and can provide easily tunable metal composition during synthesis process. Its unique layered structure and reversible oxidation state change in redox condition also have arisen interest of many researchers. In this study, starting from the mechanism study of OER, chloride evolution reaction (ClER), and chloride-induced corrosion on this material, we performed DFT calculations to predict which transition metal dopants can enhance the OER activity of NiFe-LDH without accelerating selectivity and corrosion problems in seawater.NiFe-LDH is known to experience its transition to NiFeOOH phase under OER condition. [001] and [110] facets were set as edge and terrace sides of NiFeOOH, respectively, after their surface energies were investigated. Surface Pourbaix diagram showed that the [001] facet has clean termination without any adsorbate while [110] facet has dissociated-H2O covered termination under alkaline OER condition. OER energetic profile showed that the Fe site on the [110] surface is an OER active site with a theoretical overpotential of 280 mV, and seawater conditions do not significantly affect the OER activity itself. ClER mechanism study revealed that it occurs via *ClOH intermediates on [110] metal sites or *Cl intermediates on deprotonated [001] oxygen sites. Energy calculation of surface chlorination and metal dissolution steps showed that gradual chlorination accelerates metal dissolution and [110] facet is more vulnerable to chloride-induced corrosion than [001] facet.As the third metal candidate of NiFe-LDH, 3d to 5d transition metals excluding heavy metals were screened. Gibbs free energy correction terms such as vibrational entropy, zero-point energy and solvation parameter were calculated only for NiFeOOH case, and applied them to other candidates to simplify the screening process. NiFeOOH [001] surface could not be better than NiFeOOH [110] surface due to the inherent low activity of the [001] facet, but interestingly seven dopants could reduce the overpotential of NiFeOOH [110] surface by affecting the Fe active site or being active sites themselves. Since these dopants increased the oxidation ability of the active site, all of them also lowered chloride oxidation potential. However, fortunately, none of their ClER operating potential was lower than their OER operating condition, which means that there still exist potential windows where 100% OER selectivity can be achieved. In the last screening step, dissolution energies of fully chlorinated metal sites were calculated for the seven candidates, and two cases of them showed negative dissolution energy, which indicates very unstable doping state. As a result, five candidates passed through all three screening criteria: OER activity, OER vs. ClER selectivity, and durability against corrosion. Further, the experimental validation has performed on three promising candidates considering the raw material price, and it revealed that their activity is higher activity than NiFe-LDH. In addition, for the two abandoned cases in the third screening step (about metal dissolution), the experimental data showed that they were not seem to be actually doped into NiFe-LDH, indicating that the computational screening of seawater compatibility really worked. This study supports the utility of theoretical screening in electrochemical catalyst design and presents computational approach method to consider seawater compatibility.

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.

The development of low-cost, stable, and highly efficient electrocatalysts for the bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is crucial for advancing future renewable technologies. In this study, we systematically investigated the OER and ORR performance of subnano clusters across the 3d, 4d, and 5d transition metal (TM) series of varying sizes using first-principles calculations. The fluxional identity of these clusters in the subnanometer regime is reflected in their non-monotonic catalytic activity. We established a size-dependent scaling relationship between OER/ORR intermediates, leading to a reshaping of the activity volcano plot at the subnanometer scale. Our detailed mechanistic investigation revealed a shift in the apex of the activity volcano from the Pt(111) and IrO2 surfaces to the Au11 clusters for both OER and ORR. Late transition metal subnano clusters, specifically Au11, emerged as the best bifunctional electrocatalyst, demonstrating significantly lower overpotential values. Furthermore, we categorized our catalysts into three clusters and employed the Random Forest Regression method to evaluate the impact of non-ab initio electronic features on OER and ORR activities. Interestingly, d-band filling emerged as the primary contributor to the bifunctional activity of the subnano clusters. This work not only provides a comprehensive view of OER and ORR activities but also presents a new pathway for designing and discovering highly efficient bifunctional catalysts.

CoNi nanoalloy-Co-N4 composite active sites embedded in hierarchical porous carbon as bi-functional catalysts for flexible Zn-air battery

The ever-increasing demand for sustainable energy has led to the development of inexpensive energy storage devices. The electrode material is one of the most important parts of an energy storage device; the electrode material has a major impact on the device’s price, sustainability, environmental friendliness, performance and lifetime.Asphaltene, as a high carbon content by-product of the oil sands industry, currently has a higher supply than demand. Furthermore, asphaltene can be used to prepare low-cost carbon fibers (CFs) as the electrode material in energy storage devices. Electrochemical double layer supercapacitors (EDLCs), pseudocapacitors and the air-electrode in Zn-air batteries (ZABs) were prepared in this study by using asphaltene based CF.Activated carbon fibers (ACFs) were prepared by chemically activating the CF derived from asphaltene produced in Alberta, Canada. ACFs were used to prepare stable, high performance and flexible EDLC and birnessite MnO2 type pseudocapacitors. CFs were also used as a conductive base layer for spinel type MnCo2O4, which is an efficient electrocatalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). MnCo2O4 coated CFs were used to prepare stable and high efficiency homemade air electrodes for ZAB.ACFs, that were used as the active material in an aqueous EDLC, had a specific surface area of 2290 m2 g-1 and total porosity (pore volume) of 1.27 cm3 g-1, which includes 0.88 cm3 g-1 of micropores (pore width < 2 nm) and 0.29 cm3 g-1 of mesopores (2 nm < pore width < 50 nm). The maximum specific capacitance (Cs) reached was 311 F g-1 at a specific current (is) of 0.04 A g-1; this was reduced to 248 F g-1 at a specific current of 1 A g-1. Capacitance retention of this EDLC was 91% after 10,000 cycles. This material was later used in an EDLC device with an ionic liquid electrolyte (EMIMBF4, 1-ethyl-3-methylimidazolium tetrafluoroborate); ionic liquid electrolytes provide wider voltage windows. As a result, a specific energy (Es) of 35.7 Wh kg-1 was achieved at a power density (Ps) of 525.4 W kg-1. These values are comparable to energy and power values delivered by some batteries.Birnessite-type MnO2 (δ-MnO2) is a promising material for charge storage devices like pseudocapacitors (slower charge/discharge compared with EDLCs but faster charge/discharge compared with batteries). However, the insulating nature of δ-MnO2 limits its electrochemical performance. Because of the high performance and high conductivity of ACF used in EDLC devices, this material was chosen to fabricate ACF/δ-MnO2 composite electrodes to enhance capacitive performance of δ-MnO2. δ-MnO2 was coated onto ACFs through a hydrothermal process. The crystal structure of δ-MnO2 was subsequently thermally modified to reduce its crystallinity by introducing oxygen deficient defects. These defects acted as active sites to enhance electrolyte ion adsorption/desorption, which improved the capacitive performance. The maximum Cs reached for the composite electrode was 327 F g-1 at a specific current of 0.04 A g-1, which was significantly improved compared to the δ-MnO2 powder (not coated on ACF, 195 F g-1 at 0.04 A g-1). Capacitance retention for the composite electrode was 93% (initial capacitance of 298 F g-1 and final capacitance of 279 F g-1 at 1 A g-1), while the retention for the δ-MnO2 powder was 64% (from 154 F g-1 to 98 F g-1) after 10,000 cycles.CFs were utilized to prepare homemade gas diffusion layers (GDLs) for use in air electrodes in ZABs. Air electrodes were prepared with CF carbonized at three different temperatures, i.e., 500 oC, 800 oC and 1500 oC. the ORR and OER activity of the homemade air-electrodes, as well as commercially purchased electrodes, were tested in 1 M KOH. All homemade electrodes showed much better OER activity than the purchased ones. ORR activity was similar for both commercial electrodes and homemade electrodes prepared with CF at 1500 oC (CF-1500). CF-1500 was coated with spinel type MnCo2O4 via a facile sonication procedure. MnCo2O4 coated CF-1500 had excellent catalytic activity towards both ORR and OER, outperforming the bench mark Pt-RuO2 catalyst. The cycling behavior of CF-100 was very stable with initial and final efficiencies of ~63% and ~58%, respectively, after 200 cycles (100 h) of charge and discharge at 10 mA cm-2.

Perovskite La0.6Sr0.4CoO3-δ (LSC) and Ba0.5Sr0.5Co0.8Fe0.2O3- δ (BSCF) oxides are still low in the catalytic activity of oxygen reduction reaction (ORR) although it exhibits relatively good catalytic activity for oxygen evolution reaction (OER) when using as oxygen electrode catalyst. To improve the ORR activity of the LSC, palladium (Pd) was deposited over the LSC surface with four different weight percent. The physicochemical properties of Pd decorated perovskite La0.6Sr0.4CoO3-δ (Pd/LSC) was studied by using various analytic techniques. It was observed that the palladium is deposited only on the surface of LSC powders in nanostructure forms with a stoichiometric composition, which gives no influence on the crystallinity of LSC. The surface area of the Pd/LSC samples increases with increasing the weight percentage of Pd and, over 10 wt%, dramatically increases due to the formation of Pd porous structure over the perovskite surface. The electrochemical characterization of the Pd/LSC was examined in both aqueous and non-aqueous electrolyte solutions. In aqueous electrolyte, the Pd/LSC materials exhibit higher ORR and OER activities than LSC and KB. The ORR activity rapidly increases with the increase of Pd wt% deposited in order of 5<10<20<30, while the OER activity displays not much different values for all the Pd weight percentages. The Pd/LSC materials were applied as air cathode catalyst of lithium air battery to examine their catalytic performances in non-aqueous electrolyte. The Pd/LSC materials exhibit higher catalytic activities in oxygen electrochemical reactions than LSC and KB. 30wt% Pd/LSC shows the highest discharge capacity and the longest cycle life. It is concluded that the decoration of Pd over perovskite LSC oxide is more effective to increase the ORR activity, resulting in increasing the performance of lithium air battery.

The demanding multi-electron transfer process renders the oxygen evolution reaction (OER) a bottleneck for achieving efficient clean hydrogen generation via water electrolysis.[1] Over the past decades, two main categories of catalysts, namely, homogeneous molecular and heterogeneous catalysts, have been implemented for the OER. However, due to the sluggish reaction kinetics and the aggressive reaction media of the OER, the structural integrity of both homogeneous molecular and heterogeneous catalysts faces the dramatic challenges. This calls for a thorough understanding and close monitoring of OER catalysts under their operando reaction conditions.Over the last years, we have been combining a variety of in situ/operando spectroscopy approaches with computational studies toward the comprehensive understanding of our designed catalysts at the atomic level. With this information in hand, we established a full identification of catalytically active species and sites for several systems, some of which are discussed here.First, inspired by nature’s {Mn4CaOx} OER complexes, we recently reported on the design of a tetramer Cu-bipyridyl complex for the OER.[2] Structural characterizations demonstrated a new defect-cubane structure of our designed complex, [Cu4(pyalk)4(OAc)4](ClO4)(HNEt3). We found that this Cu-bipyridyl complex can further undergo structural transformations into two unique complexes under different solution conditions, namely Cu-dimer and Cu-monomers, as revealed by in situ UV-vis and ERP characterizations as well as electrospray ionization mass spectrometry. Specifically, the Cu-monomers can be only formed in presence of carbonate buffer (pH 10.5). Otherwise, the structural transformation into a Cu-dimer complex [Cu2(pyalk)2(OAc)2(H2O)] is dominant under solution conditions. Furthermore, electrochemical characterizations revealed an overpotential of 960 mV to reach a current density of 0.1 mA/cm2 of our designed Cu-dimer catalysts, which is comparable with significant Cu-based OER electrocatalysts. To gain in-depth insights into their conversion processes, postcatalytic characterizations of Cu-based molecular catalysts were carried out based on X-ray photoelectron/absorption (XAS/XPS) spectroscopy appraoches. The results showed that nanosized Cu-oxide-based species were formed in situ in Cu-based molecular catalysts after the OER. Our study highlights the crucial role of the structural integrity of molecular catalysts in solutions for their efficient design.In parallel, we explored the structural transformations of heterogeneous electrocatalysts during the OER. As a typical example, we developed a cost-effective and high-performance NiFe-based coordination polymer (referred to as NiFe-CP) as OER electrocatalyst, which is being investigated as the best-known bimetallic combination for the OER.[3] A central element of our study is the monitoring of true catalytically active species. Results from spectroscopic characterizations revealed a kinetic restructuring of NiFe-CPs into NiFe (oxy)hydroxides during the OER. To further improve the OER activity, we introduced a facile NaBH4-assisted reduction strategy to prepare low-crystalline reduced NiFe-CP (denoted as R-NiFe-CP) OER electrocatalysts with rich structural deficiencies. These catalysts can maintain a very low overpotential of 225 mV at 10 mA/cm2 for over 120 h without any performance decline, outperforming many recent reported bimetallic OER electrocatalysts. As revealed by XAS characterizations and density functional theory (DFT) calculations, engineering of structural deficiencies not only tunes the local electronic structure but also optimizes the rate-determining step towards facilitated OH- adsorption. Noteworthy, the true OER active sites of R-NiFe-CPs originate from the in situ reconstructed Ni-O-Fe motifs. However, fundamental questions, as to (a) the role of engineered structural deficiencies in the generation of active species and (b) facilitating the formation of catalytically active dual oxygen-bridged moieties, need to be answered. Combination of time-resolved operando XAS monitoring and DFT calculations enables the tracking and understanding of the kinetic changes of active species and sites under the operando reaction conditions. We found that the OER of R-NiFe-CPs relies on the in situ formation of crucial high-valent NiIV-O-FeIVO moieties.[4] Furthermore, an anionic engineering strategy through heteroatom sulfur incorporation was carried out to obtain S-R-NiFe-CP showing faster OER kinetics. Importantly, engineered sulfur content promotes the generation of catalytically active S-NiIVO-FeIVO motifs prior to the OER. This offers a lower onset potential to trigger the OER of S-R-NiFe-CPs compared to sulfur-free R-NiFe-CPs. Moreover, our results also suggest a dual-site mechanism pathway of S-R-NiFe-CPs during the OER, in which the O-O bond formed atop the S-NiIVO-FeIVO moieties. Such an anionic modulation strategy for promoting the formation of catalytically active structural moieties and for optimizing the OER kinetics opens an avenue to optimize a wide range of heterogeneous catalysts for the OER.[1] Zhao, Y. et al. Chem. Rev. 2023 , doi.org/10.1021/acs.chemrev.2c00515.[2] Adiyeri Saseendran, D. P. et al. Chem. Comm. 2023, In Revision.[3] Zhao, Y. et al. Adv. Energy Mater. 2020, 10, 2002228.[4] Zhao, Y. et al. ACS Nano 2022, 16, 15318-15327.

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.

The word ‘Sustainability’, including carbon neutrality, has dominated the direction of social development over the past decade. In particular, energy conversion reactions through electrochemical methods are one of the efficient methods of obtaining small carbon footprint fuels. The oxygen evolution reaction (OER) is a key step in determining the overall reaction efficiency of fuel-related electrochemical reactions such as CO2 reduction reaction and H2 evolution reaction. However, electron transfer is sluggish for OER due to 4 electrons per one O2 molecule. This promotes multiple studies on the metal oxide electrocatalyst structure. The Alkali-transition metal oxides with the layered structure are one of the attractive OER electrocatalyst series. For example, lithium cobalt oxide (LiCoO2, LCO) presented OER activity through Li+ extraction (delithiation) from the lattice structure. In this work, we investigated the insertion effect of large alkaline cations (A+: Na+, K+, and Cs+) at the delithiated LCO for OER activity and stability. The intercalations of hydrated Na+ and K+ induced significant phase transformation of the delithiated LCO structure. In addition, the relative ratio between Co and alkali metal species determined the average Co oxidation state of LCO. We found that OER activity was improved in the order of Li+ < Na+ < K+, which was associated with the increased Co valence state and the Co-O bond covalency. Consistently, density functional theory (DFT) simulation also predicted the formation of efficient OER active sites by the K+ insertion. In comparison, Cs+ insertion exhibited the highest OER activity and demonstrated different OER processes. Due to the larger Cs+ size, the cation insertion was predominantly achieved at the delithiated LCO surface, resulting in imposing tensile strain to the surface edge. This catalyst showed the significant pH dependency on the OER property, suggesting the lattice-oxygen-based pathway for LCO. However, the bulk structure was preserved with little phase transformation, demonstrating better OER stability than others. In the presentation, I will discuss the catalytic activity responsible for the cation sizes and two different mechanisms in detail. Figure 1

Proton exchange membrane water electrolysis (PEMWE) offers several advantages over other electrolysis technologies including greater energy efficiency, higher production rates, dynamic responses and more compact design. [1] High loading is a major barrier to the path of large-scale production of PEMWE. [2] The state-of-the-art anode catalysts in conventional PEMWEs are mixed iridium and iridium oxides (IrOx) with typical iridium loading from 1 to 3 mg cm-2, which is too high to meet the long term cost target for energy markets [2]. Meanwhile, translation of catalyst development from lab scale to the megawatt scale remains challenging in terms of electrode stability. [3] Herein, a composite catalyst layer of iridium oxide/Nafion (IrOx/Nafion) is developed using reactive spray deposition technology (RSDT) which combines the catalyst synthesis and catalyst-coated membrane (CCM) fabrication into one step. The RSDT-derived IrOx/Nafion showed distinctive features that are beneficial to the oxygen evolution reaction (OER) activity and durability. The iridium particles were about 1-2 nm with very high surface area due to the intrinsic property of resistance to sintering [4]. This resulted in high anodic charge proportional to the oxide surface area and superior activity over planar and electrodeposited IrO2thin films. The OER activity can be further enhanced by increasing the amount of surface oxide of iridium which was achieved by increasing the flame length, flame temperature and oxygen flow rate. In Figure 1, F-1 and F-2 corresponds two different flame conditions with iridium surface oxide percentage of 28% and 100%, respectively. Higher surface oxide percentage (F-2) resulted in superior anodic charge (Figure 1a) and OER activity (Figure 1 b,c). The electrolyzer durability of the RSDT-derived IrOx/Nafion catalyst layer was optimized by increasing the amount of iridium oxide on the iridium particle surface and by decreasing the number of IrOx/Nafion agglomerates on the catalyst layer surface. Ongoing studies of CCMs fully fabricated by the RSDT process showed promising electrolyzer durability of 3300 hours (Figure 1d). Analysis of post-durability test samples will be conducted to investigate the effect of long-term operation on catalyst layer integrity and iridium dissolution and migration in the electrolyte membrane. Figure 1. Cyclic voltammetry (CV) of RSDT-derived IrOx/Nafion catalyst with two flame conditions (a). CV recorded in nitrogen-purged 0.1M HClO4 at 25 oC with 20 mV/s scan rate from 0.0 V to 1.5V (vs. RHE). Quasi-stationary polarization curves for OER recorded from 1.45-1.69 V vs. RHE with 20 mV step and 5 min dwell time (b). (c) Compares the OER mass activity and specific activity of RSDT-derived IrOx/Nafion catalysts and pure IrO2 (adapted from [5]) tested with the same procedure. (d) Shows the electrolyzer MEA durability test of RSDT-derived cathode and anode. Test performed using a circular 86 cm2 cell at 80 oC with 1.8 A cm-2current load and 400 psi hydrogen pressure.