Lithium-ion battery performance degrades over time due to complex aging mechanisms, including lithium inventory loss, structural changes in electrode materials, and increases in contact resistance. Understanding these processes is essential for improving battery performance and extending cycle life. This work presents a combined electrochemical impedance spectroscopy and distribution of relaxation times approach using a three-electrode configuration to investigate aging and state-of-charge (SOC)-dependent internal resistances in NMC811 & boxv;graphite Li-ion cells. By simultaneously probing the cathode, anode, and full-cell responses and deconvoluting overlapping impedance features using DRT, we identify distinct contributions from particle-particle resistance, SEI resistance, intercalation (charge-transfer) kinetics, and solid-state diffusion. Our results show that cathode processes dominate impedance growth at high SOC due to structural degradation and surface reconstruction, whereas anode contributions arise mainly at low SOC through increasing intercalation-kinetic limitations. SEI-related resistance stabilizes after the initial cycles, while contact resistance and diffusion impedances grow steadily with aging. These trends correlate closely with SEM, XRD, and Raman evidence of particle cracking, lattice strain, and structural disorder, establishing strong links between electrochemical signatures and physical degradation. Overall, this methodology provides a quantitative, component-resolved framework for diagnosing electrode-specific failure modes and guiding materials or interface-engineering strategies to enhance the lifetime of lithium-ion cells.

The transference number is a critical parameter in solid-state batteries, as it determines how efficiently Li + ions move through the electrolyte to reach the electrode interface. We measured the transference number using a symmetric coin cell.

Metal oxide is being researched as a potential replacement for the graphite anode in lithium-ion batteries. This material offers several advantages as an alternative anode for advanced rechargeable batteries, including higher theoretical capacity, superior discharge potential, affordability, natural availability, and safety characteristics [1]. Furthermore, when formed as nanoparticles, increased surface area enhances electrode activity and facilitates faster ion transport, which improves lithium mobility in lithium-ion batteries. However, the stability of the nanoparticles is considered a significant challenge. In this work, we present the synthesis conditions and characterization of stable spinel manganese oxide nanoparticles with various sizes and morphologies and high crystallinity [2]. We demonstrate that it is possible to control the size and shape of these nanocrystals under specific conditions by using the polyol process [3]. These nanomaterials were tested in half-cell configurations using lithium metal as the counter and reference electrodes. Figure 1 (a) presents the TEM images of Mn₃O₄ nanocrystals, which exhibit uniform particle sizes in the range of approximately 7–10 nm. A nanocomposite anode was fabricated by combining these Mn₃O₄ nanoparticles with graphite. Figure 1 (b) displays the first formation cycle of the composite anode, demonstrating enhanced specific capacity relative to conventional graphite which has specific capacity of 350 mAh/g, indicative of improved electrochemical performance. In this study, we are developing composite anode materials with varying weight percentages of metal oxides and different sizes and morphologies to enhance capacity, rate capability, and stability. References: He, S. Wu, N. Zhao, C. Shi, E. Liu, and J. Li, Carbon-encapsulated Fe 3 O 4 nanoparticles as a high-rate lithium ion battery anode material, ACS Nano, 2013, 7, 4459–4469. Rhadfi, J.-Y. Piquemal, L. Sicard, F. Herbst, E. Briot, M. Benedetti, and A. Atlamsani, Polyol-made Mn 3 O 4 nanocrystals as efficient Fenton-like catalysts, Appl. Catal. A, 2010, 386, 132–139. Rhadfi, L. Sicard, F. Testard, O. Taché, A. Atlamsani, E. Anxolabéhère-Mallart, Le Y. Du, L. Binet, and J.-Y. Piquemal, A comprehensive study of the mechanism of formation of polyol-made hausmannite nanoparticles: from molecular species to solid precipitation J. Phys. Chem. C, 2012, 116, 5516. Figure 1

In this study, tin (Sn) nanoparticles are demonstrated to effectively catalyze the reduction of CO2 to formate in an alkaline medium. Catalytically active Sn-based nanoparticles, supported on carbon black (Sn/C) and highly conductive graphene nanosheets (Sn/GN), present a promising approach to mitigating atmospheric CO2 emissions when integrated with capture technologies. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were employed to evaluate the prepared catalysts in CO2-saturated 0.5 M KHCO3 using a three-electrode rotating disk electrode (RDE) configuration. The results revealed a significantly lower charge-transfer resistance for graphene-supported tin compared to carbon black-supported tin. The CO2 reduction to formate was further demonstrated in a full electrochemical cell setup resembling the architecture of a low-temperature polymer electrolyte fuel cell (PEFC) operating in an alkaline medium with an anion exchange membrane (AEM). Performance tests were conducted with both triple-serpentine and parallel flow field architectures, showing flow rate-dependent behavior. Additionally, an ex-situ RDE technique was utilized to detect and quantify formate production during CO2 reduction in the full-cell configuration. This work highlights the importance of catalyst support materials and flow field design in optimizing CO2 electroreduction systems.

The demand for electric vehicles (EV) and electric vertical take-off and landing (eVTOL) aircraft is growing exponentially. The lithium-ion battery (LIB) is commonly selected for such applications due to its promising relatively high energy and power density. However, range limitations, capacity fade, and durability of Li-ion systems limit EVs. These questions become even more vital when discussing aerospace applications of eVTOL, which have more stringent requirements for safety, power, and durability. Additionally, cell-to-cell-variations (CtCv) play a major role in pack performance. To address the current needs, this work employs a characterization method combining the techniques of electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT), allowing for better characterization of cell performance and degradation at relevant high C-rates. In this work, cells are cycled at high-rate discharge (10C and 25C) at various temperatures to mimic eVTOL applications, with characterization tests being performed at beginning of life and every 50 cycles for a total of 500 cycles. By applying DRT to the EIS data, the electrochemical processes inside the cell can be deconvoluted and examined [1-3]. Gaussian peaks are fit to the DRT distribution function, and the resistance of each process can be predicted, thereby monitoring changes in HFR, interfacial particle-particle interactions, SEI/CEI interphase layers, and charge transfer/intercalation kinetics at various states of charge (SOC) and cycle number. Furthermore, diffusion related processes can also be identified and estimated from DC current-interrupt/application methods. Degradation tests were performed to simulate various thermal boundary conditions faced by eVTOLs ranging from 20°C to -20°C. The results reveal that, at high discharge rates operating at room temperature, cells reached up to 90°C and signatures of degradation in the SEI layer are seen in the DRT data. Other cells cycled at 0°C show evidence of lithium plating due to the cold environment. Active cooling at 20°C showed the least degradation as identified from the various internal processes. This method of testing provides an approach to comprehensively analyze individual internal degradation modes of a Li-ion battery. References: [1] Wan, Ting Hei et al. “Influence of the Discretization Methods on the Distribution of Relaxation Times Deconvolution: Implementing Radial Basis Functions with DRTtools” Electrochimica Acta, Vol. 1842015, http://dx.doi.org/10.1016/j.electacta.2015.09.097 [2] Iurilli, Pietro et al. “Detection of Lithium-Ion Cells’ Degradation through Deconvolution of Electrochemical Impedance Spectroscopy with Distribution of Relaxation Time” Energy Technology, Vol. 10, 2022, https://doi.org/10.1002/ente.202200547 [3] Chen, Xiang et al. “Detection of lithium plating in lithium-ion batteries by distribution of relaxation times” Journal of Power Sources, Vol. 496. 2021. https://doi.org/10.1016/j.jpowsour.2021.229867 Figure 1

The state-of-the-art Li-ion battery has energy density plateauing at ~300 Wh/kg [1]. Replacing the graphite-based anode with Li metal is one promising pathway to increase energy density. However, a lithium metal anode is prone to non-uniform plating/striping that leads to capacity decay and dendrite formation. Solid-state batteries (SSBs) hold great promise for next-generation energy storage systems due to their potential to address safety concerns, provide energy density, and enable the use of high-energy electrode materials such as lithium (Li) metal. Additionally, SSBs exhibit greater mechanical stability and can limit dendritic growth [2]. Furthermore, solid electrolytes show much higher thermal stability, are non-toxic, and have high energy density. Among the various types of solid-state electrolytes, composite polymer electrolytes (CPEs) have emerged as a promising option owing to their excellent ionic conductivity, mechanical flexibility, and compatibility with Li metal electrodes. Polyethylene oxide (PEO) polymer electrolyte has very good interfacial contact between electrode and very flexible in nature [3]. But PEO by itself is unstable. A PEO composite with solid ceramic offers the potential for better stability with high conductivity as well as improved physical properties. Our research focuses on elucidating key parameters that affect the electrochemical behavior and stability of these systems, aiming to enhance their overall performance and longevity. One of the primary factors under investigation is the composition and morphology of the composite polymer electrolyte. We explored PEO polymer and LLZTO ceramic fillers to optimize the electrolyte's mechanical strength, Li-ion conductivity, and interfacial stability with Li metal electrodes [2]. Through systematic characterization techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and impedance spectroscopy, we gain insights into the microstructure and ion transport properties of the CPE. The combination of PEO and Polyvinylidene fluoride (PVdF) polymer creates a remarkably robust and flexible polymer electrolyte, renowned for its exceptional mechanical and electrochemical stability. Figure 1 (a) shows the cycling profile of PEO/PVdF electrolyte at different C rates with LFP cathode and Li metal as anode. Compared to PEO membrane, the composite exhibits higher ohmic polarization. However, this composite polymer electrolyte demonstrates better stability and high-rate capability than a pure PEO membrane. At 1-C rate charging and discharging condition cell shows 99% columbic efficiency and good capacity retention. In Figure 1(b) and (c) show the SEM image of PVdF and PEO composite membrane respectively. Which shows PEO completely covers the PVdF network and helps better contact with solid electrode. Reference: Bapi Bera, Anirban Roy, Douglas Aaron, and Matthew M Mench, “Understanding the Transport Phenomena in Solid State Battery (SSB)”, Electrochemical Society Meeting Abstracts-241, 2022, 1, 45-45. Yanda Fu et al., “Surface Defects Reinforced Polymer-Ceramic Interfacial Anchoring for High-Rate Flexible Solid-State Batteries”, Adv. Funct. Mater. 2023, 33, 2210845. Sahore, Z. Du, X. C. Chen, W. B. Hawley, A. S. Westover, and N. J. Dudney, Practical considerations for testing polymer electrolytes for high-energy solid-state batteries, ACS Energy Lett. 2021, 6, 2240-2247. Figure 1

Aluminum-air batteries (AAB) are regarded as one of the most promising beyond-lithium high-energy-density storage candidates. This paper introduces a three-dimensional (3D) Al 7075 anode enabled by femtosecond laser and friction-stir process which, along with a special double-face anode architecture provides world-class performance. Electrochemical characterizations prove that the corrosion resistance of the modified 3D Al 7075 FSP anode was enhanced, and electrochemically active surface area (ECSA) was increased compared with that of normal Al 7075 anode. Friction-stir processing reduced the mean grain size from 30 mu m to 3 mu m. The discharge performance of 3D Al 7075 FSP anode is shown to be quite stable, and the average values of energy density are significantly increased from 2256 mWh g-1 to 2941 mWh g-1 at 100 mA cm-2. In a double-face flowing Al-air battery system, the 3D Al 7075 FSP anode exhibited significantly better electrocatalytic performance (discharge voltage of 0.76 V at 400 mA cm-2, and power density of 337.8 mW cm-2) than that of a commercial Al 7075 anode.

A high temperature ammonia treatment was applied to carbon felt electrodes to enhance vanadium redox flow battery (VRFB) performance. Samples were heated to 900 degrees C in the presence of ammonia gas for up to 4 h. While all heating times resulted in an overall improvement in current density at 80% voltage efficiency, samples treated for 4 h showed the greatest increase in current density (325%) compared to untreated carbon felt. Raman spectroscopy showed a 74% increase in edge sites as a result of the 4 h treatment. Electrochemical surface area increased by 142% and scanning electron microscopy showed the appearance of pores on felt fiber surfaces, indicating that the performance improvement may be due to enhanced surface area in addition to functionalization. Impedance spectroscopy showed decreased charge transfer resistance and increased durability (during cycling) compared to other published electrode treatments. These results indicate that heated ammonia can be used to increase the performance of electrodes for vanadium flow battery applications, with excellent durability.

The state-of-the-art Li-ion battery has energy density plateauing at ~300 Wh/kg. Replacing the graphite-based anode with Li metal is the easiest way to increase energy density. However, a lithium metal anode is prone to non-uniform plating/striping that leads to capacity decay and dendrite formation. Dendrites trigger short-circuiting and possible explosions as the liquid electrolytes that are used in Li-ion batteries are flammable. Solid-state batteries (SSBs) have the potential to enable Li-metal anodes as they are typically less reactive and nonflammable. Additionally, SSBs exhibit greater mechanical stability and can prevent dendritic growth [1,2]. Furthermore, solid electrolytes show much higher thermal stability, are non-toxic, and have high energy density, making the solid-state battery one of the best choices for the next generation of energy storage devices. Solid polymer electrolytes are an important class of materials for making solid-state batteries commercially viable. These have the potential to increase energy density and decrease contact resistance between anode and separator by formation of a suitable solid-electrolyte-interphase (SEI) [1]. However, this technology still has major hurdles to overcome, like lower Li-ion conductivity when compared to state-of-art ceramic separators. In recent years, garnet-type lithium oxide perovskites have gained attractiveness as state-of-art ceramic separators for SSBs. LLZTO is one such ceramic electrolyte that is being thoroughly investigated by researchers as it shows very high Li-ion conductivity at room temperature [2]. However, these materials suffer from poor interfacial contact. Recently, Yang, et al., [3] combined the best of both worlds with a new type of solid polymer separator which has better physical contact between separator and lithium and good li-ion conductivity at room temperature. In this work, we investigate the transport of Li-ions across both a solid polymer electrolyte and LLZTO solid electrolyte using a symmetric Li-cell configuration. Fig. 1 (a) and (c) show the Li plating/stripping cycling performance in a symmetric cell. The cell voltage measured during plating and stripping is to be very high for LLZTO compared to polymer electrolyte. A possible explanation may be due to high interfacial resistance arising between solid ceramic and lithium metal. Impedance spectroscopy was performed on both LLZTO and polymer separators after each current density step (24 h) and shown in Fig. 1 (b) and (d) respectively. The impedance increased with cycling for the LLZTO separator but decreased with cycling for polymer electrolyte. This may indicate that better interfacial contact between Li and polymer exists and that these connections may become more established while cycling. Furthermore, the transport of Li-ions across the separators will be analyzed using the transference number calculated using the Bruce-Vincent method. The influence of temperature and separator thickness on the transference number will also be used to characterize the nature of ion transport across such solid electrolyte separators. Such deep understanding of the transport mechanism is needed to minimize the different losses in SSBs and make it commercially viable. Figure 1: Li plating/stripping cycling performance of the (a) LLZTO electrolyte, and (c) PEO polymer electrolyte at different current density, with 12 minutes for each plating/stripping half cycle, for a total of 72 h at 70 ℃ temperature and their corresponding impedance are shown in (b) and (d) respectively. References R. Sahore, Z. Du, X. C. Chen, W. B. Hawley, A. S. Westover, and N. J. Dudney, Practical considerations for testing polymer electrolytes for high-energy solid-state batteries, ACS Energy Lett. 2021, 6, 2240-2247. A. Parejiya, R. Amin, M. B. Dixit, R. Essehli, C. J. Jafta, D. L. Wood, III, and I. Belharouak, Improving contact impedance via electrochemical pulses applied to lithium−solid electrolyte interface in solid-state batteries, ACS Energy Lett. 2021, 6, 3669−3675. Yang at al, Copper-coordinated cellulose ion conductors for solid-state batteries, Nature, 2021, 598, 590−596. Figure 1

This work quantifies in-plane spatial heterogeneity (polymer electrolyte fuel cell cathode inlet vs outlet) in Pt particle size growth and distribution as a function of nitrogen (N2) flow rate during a square-wave accelerated stress test (AST). The average Pt particle sizes for membrane electrode assemblies (MEAs) subjected to N2 flow rates ranging from 4–16 sccm cm−2 are in the range 9–10.5 nm at the end-of-life (EOL) with similar electrochemically active surface area (ECSA) loss (∼65%). However, Pt particle size at EOL exhibits spatial heterogeneity: greater Pt particle size growth occurs near the flow field outlet than the inlet. The spatial heterogeneity for a fully-humidified N2 flow is believed to originate from non-uniform humidification (outlet is more humidified than the inlet) across the cell for a co-flow arrangement. A first-order rate model for ECSA loss predicts linear increase of the rate constant with N2 flow rate. The polarization losses of the aged MEAs over a wide range of operating conditions increase with N2 flow rate. From the results of this work, for holistically assessing durability of Pt catalysts in fuel cells at high humidity conditions, it is recommended to include purge gas flow rate as a stressor during an AST.

The Pt-based expensive catalysts and sluggish kinetics at cathode in oxygen reduction reaction (ORR) hinder the rapid commercialization of fuel cells. The quest for cheap, non-noble metal catalysts to replace Pt-based catalysts has thus become a critical issue in the field of fuel cells. The carbon black (CB) and CB supported catalyst have been explored with the ultimate goal of finding a substitute for Pt-based catalysts in fuel cells. In the present work, we synthesized Zn-based MOF (1), 1 selectivity gives H2O2 followed by two-electron pathways. However, sample 1 modification might be needed to enhance its selectivity for the generation of H2O. Two composites of MOFs with carbon black and 1 were prepared to increase the H2O yield, called 1⊃CB and 1⊃SCB. The electrochemical generation of H2O2 was analyzed by the rotating ring disk electrode (RRDE) using catalyst 1. Following the addition of CB, H2O2 yields decreased from above 93% (1) to 59% and 75% for 1⊃CB and 1⊃SCB, respectively. CB modified catalysts moved towards four-electron pathways due to the conductive nature of CB. Electrochemical Impedance Spectroscopy (EIS) has also been performed to study in detail the conductivity effect of CB and kinetic behavior of ORR in alkaline electrolyte. This research opens up a new path for ORR to advance non-precious metal catalysts based on MOFs. Synopsis: This paper describes how we synthesized carbon Black (CB) supported MOF: Zn-based MOF (1), 1 selectivity gives H2O2 (two-electron pathways). Two composites of MOFs (1⊃CB and 1⊃SCB) were prepared to increase the H2O yield. After addition of CB, H2O2 yields decreased from above 93% (1) to 75% (1⊃SCB) and 59% (1⊃CB).

With the U.S. Department of Energy (DOE) 2020 durability target for transportation applications of 5000 hours in mind [1], several studies have been performed to understand the factors influencing load cycle durability of the cathode catalyst layer subjected to various operating conditions. In general, high upper potential limit (> 1 V vs RHE), high temperature (> 90 °C) and high relative humidity (RH) during potential cycling have been shown to exacerbate degradation [2]. It is known that Electrochemical surface area (ECSA) loss occurs by Pt dissolution attributed to particle size growth via modified Ostwald ripening, crystal migration, detachment from carbon support due to carbon corrosion, and precipitation in the membrane by chemical reduction due to hydrogen crossover [3]. Accelerated stress tests (ASTs) are often used to mimic material component degradation similar to real automotive driving conditions. Different cycle profiles have been shown to cause varying rates of degradation depending on the operating conditions. The influence of flow rate of reactant gases on degradation, however, has received little attention. Stariha et al. [4], showed that a relatively high flow rate AST had the highest degradation acceleration factor and resulted in non-uniform decay rates when compared to low flow rate conditions of nitrogen gas (N2) at the cathode. However, this phenomenon has not been further studied in detail. The current study is aimed at elucidating the effect of flow rate on cathode catalyst layer degradation subjected to a standard DOE square-wave AST protocol. Single cell studies with 5-cm2 active area were performed using Nafion® XL membrane (Ion Power Inc.) and SGL-22 BB gas diffusion electrodes (GDEs) as MEA materials and 1-serpentine flow field. All the MEA samples used in the present study had nominally identical Pt loading and were subjected to a consistent conditioning procedure prior to AST. A DOE square-wave cycling protocol [4] was executed: H2/N2 flows at anode/cathode, voltage cycling between 0.6 V and 0.95 V, and 3 seconds hold at each voltage. 30,000 cycles (total test time of 50 hours) were performed at 100% RH, 80°C, and atmospheric pressure. Three different cathode N2 flow rates of 20 sccm, 40 sccm and 80 sccm were used during the AST with fixed H2 flow of 40 sccm at the anode. ECSA loss was tracked using cyclic voltammograms at several intervals during the AST. Complete electrochemical characterization at beginning and the end of life (BOL and EOL), along with ex-situ diagnostic techniques such as TEM and GIXRD, were used to obtain particle size distribution and spatial degradation profiles. The results indicate higher performance loss (at EOL) for 80 sccm flow of N2 at the cathode compared to 20 sccm and 40 sccm flow rates, most likely due to inhomogeneous Pt loss from inlet to the outlet due to variations in local flow conditions by fully saturated N2 flow at different flow rates. References: [1] Fuel Cells 2016 Multi-Year Research, Development, and Demonstration Plan. [2] Matthew M. Mench, Emin Caglan Kumbur and T. Nejat Veziroglu, 2012, Polymer Electrolyte Fuel Cell Degradation, Academic Press - Elsevier, USA. [3] Y. Shao-Horn et al., “Instability of Supported Platinum Nanoparticles in Low-Temperature Fuel Cells”, Topics in Catalysis, 46, 285-305, 2007. [4] S. Stariha et al., “Recent Advances in Catalyst Accelerated Stress Tests for Polymer Electrolyte Membrane Fuel Cells”, Journal of the Electrochemical Society, 165, F492, 2018.

Ammonia gas treatments of varying temperature were performed on carbon felt electrodes in this study. Their physical and electrochemical properties were investigated. Carbon felt electrodes are often used in vanadium redox flow batteries (VRFBs). Without processing, carbon felt has poor wettability which frustrates electrochemical activity. This material must be modified to improve performance, which has been historically accomplished with thermal treatments ostensibly to promote oxygen functional groups, although this has been the subject of debate as morphological impacts have also been observed. [1-3] Many attempts have been made to improve performance of carbon felt in VRFBs, but some of the more successful modifications have resulted in an increase in the specific surface area. [4-6] Previous research at the University of Tennessee Knoxville suggests edge sites play a role in improving performance. Preliminary experiments on carbon felt performed by this lab with ammonia at high temperatures have shown promising results. Additional experiments were performed at 900°C by varying length of treatment time, finding four hours to be optimal. [3] New experiments have been done to investigate the kinetic effects of temperature. For these experiments, commercially available carbon felts (SIGRACELL® GFD3 by SGL Carbon, Meitingen, Germany) were modified by exposure to flowing ammonia gas through a furnace at (500°C, 700°C, 900°C, and 1100°C) varying temperatures for four hours. The samples were tested in a single-cell flow battery using cyclic voltammetry (CV), polarization curves, and electrochemical impedance spectroscopy (EIS). This treatment results in a significant increase in electrochemical surface area and performance. The physical properties were characterized using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and Raman spectroscopy. This was done to study the mechanism and kinetic effects of ammonia on the carbon felt material as temperature increases during the treatment itself. Samples treated at the highest temperatures exhibited a notable loss of mass, which is observed in the SEM images in Figure 1. This resulted in a significant increase in edge sites and active surface area, along with a substantial increase in cell performance, which was achieved with the ammonia-modified carbon felts described in this work, with over three times the current density at 80% voltage efficiency compared to untreated felt. Sun, B. and M. Skyllas-Kazacos, Modification of graphite electrode materials for vanadium redox flow battery application—I. Thermal treatment.Electrochimica Acta, 1992. 37(7): p. 1253-1260. Pezeshki, A.M., "Impedance-Resolved Performance and Durability in Redox Flow Batteries. " PhD diss., University of Tennessee, 2016. Gass, K., High Performance Vanadium Redox Flow Battery Electrodes. [manuscript under submission], 2021. Lu, W., et al., High-performance porous uncharged membranes for vanadium flow battery applications created by tuning cohesive and swelling forces.Energy & Environmental Science, 2016. 9(7): p. 2319-2325. Zhou, X.L., et al., A high-performance dual-scale porous electrode for vanadium redox flow batteries.Journal of Power Sources, 2016. 325: p. 329-336. Wei, L., et al., Highly catalytic hollow Ti3C2Tx MXene spheres decorated graphite felt electrode for vanadium redox flow batteries.Energy Storage Materials, 2020. 25: p. 885-892. Figure 1. SEM images at 30,000X magnification of carbon felt electrodes: a) untreated, b) 4 hours NH3 treated at 500°C, c) 4 hours NH3 treated at 700°C, d) 4 hours NH3 treated at 1100°C. Scale was identical for all micrographs Figure 1

Electrochemical reduction of CO2 is a process that can reduce carbon dioxide from the atmosphere and simultaneously produce useful, value-added chemicals, specifically solar fuel [1]. Depending on the catalyst materials, CO2 can be reduced to single carbon or multi carbon products [2]. Sn nanoparticle was used to catalyze CO2 reduction to produce formate in alkaline medium. Sn nanoparticles were synthesized by the sol-gel method with different precursors at room temperature and supported with carbon black and conductive graphene nano-sheets. Graphene nano-sheets were synthesized by the conventional modified Hammer method [3]. X-ray diffraction (XRD) and transmission electron microscope (TEM) enabled insights into phase formation, shape, and size of the Sn nanoparticles. Reference materials included commercial Sn nanoparticles with carbon support (Sn:C_com), commercial Sn nanoparticles with graphene support (Sn:GN_com), Sn nanoparticle synthesised using Sn (II) 2-ethylhexanoate and Sn (II) chloride as tin precursor with carbon support as respectively (Sn:C_Tin-2-hexa) and (Sn:C_SnCl2). Three-electrode chronoamperometry showed that Sn:GN_com enabled high CO2 electrolysis current density (see Figure 1(a)). Electrochemical impedance spectroscopy (EIS) of the prepared catalysts in CO2-saturated 0.1 M KHCO3 was recorded in a three-electrode rotating disk electrode (RDE) configuration (see Figure 1(b)). The charge-transfer resistance was smallest for Sn:GN_com sample, as well. CO2 reduction was performed in a full electrochemical cell using the same architecture as a low temperature polymer electrolyte fuel cell (PEFC) in alkaline medium with anion exchange membrane. Polarization plots show a unique feature (two different slopes); this change in slope is dominated by a distinct change in the area specific resistance (ASR) value (see Figure 1(c)). In this work, we mainly focus on the CO2 electroreduction to produce formate in a full cell with RDE (rotating disc electrode) measurements providing additional insights into catalytic properties. Our primary goal is to understand the basic underlying physical processes during CO2 reduction in the full cell set up and also find inexpensive and efficient catalytic active materials for the reduction process. Figure 1: CO2 electrolysis (a) and impedance spectra (b) in RDE configuration with different catalyst. (c) Polarisation plots for CO2 reduction in full cell configuration with different flow rate References: Wang, C. Niu, and D. Wang, Journal of Colloid and Interface Science 527 (2018) 95–106. Zhao, and J. Wang, Chemical Engineering Journal 293 (2016) 161–170. C.-T. Hsieh, and W.-Y. Chen, Surface & Coatings Technology 205 (2011) 4554–4561. Figure 1

Improvement in catalytic activity of electrochemically treated carbon (relative to the untreated carbon) toward various redox reactions is widely reported in the literature. In this work, the origin of such activity enhancement due to electrochemical treatment in a 1 M H2SO4 electrolyte in a potential range of 1-2.5 V is investigated using physical, electrical, and electrochemical methods. The physical characterizations suggest intercalation of anions (bisulfate) between the graphite layers from the H2SO4 electrolyte. Electrical characterizations (both Hall measurement and Mott-Schottky analysis) show that the samples switch from n-type to p-type behavior upon electrochemical treatment. The improvement in the catalytic activity on electrochemical treatment of carbon is explained on the basis of the change in surface characteristics, carrier concentration (N-D), and active site density. The same is validated with oxygen reduction reaction in alkaline media.

Nitrogen-doped carbon is synthesized by the heat-treatment of carbon in an ammoniacal atmosphere at different temperatures. The active site density and electrochemically active surface area (ESA) of carbon and nitrogen-doped carbon catalysts are estimated from the charge due to oxidation of the adsorbed anthraquinone-2-sulfonate (AQS) probe molecule. In the potential window of interest and over a range of concentrations, there is no unwanted side reaction or polymerization of the probe molecule that interferes with the electrochemical estimation of active site density. Most importantly, the adsorbed AQS can easily be removed from the electrode surface by potential cycling. The ORR activity and active site density of the catalysts derived from AQS-adsorption have similar trends. The active site density and turnover frequency towards ORR estimated using the AQS-adsorption method are in line with those reported in the literature by other methods. On the other hand, the results show that the wetted surface area estimated from the double layer capacitance does not always correlate with catalytic activity.

Shape-controlled precious metal nanoparticles have attracted significant research interest in the recent past due to their fundamental and scientific importance. Because of their crystallographic-orientation-dependent properties, these metal nanoparticles have tremendous implications in electrocatalysis. This review aims to discuss the strategies for synthesis of shape-controlled platinum (Pt) and palladium (Pd) nanoparticles and procedures for the surfactant removal, without compromising their surface structural integrity. In particular, the electrocatalysis of oxygen reduction reaction (ORR) on shape-controlled nanoparticles (Pt and Pd) is discussed and the results are analyzed in the context of that reported with single crystal electrodes. Accepted theories on the stability of precious metal nanoparticle surfaces under electrochemical conditions are revisited. Dissolution, reconstruction, and comprehensive views on the factors that contribute to the loss of electrochemically active surface area (ESA) of nanoparticles leading to an inevitable decrease in ORR activity are presented. The contribution of adsorbed electrolyte anions, in-situ generated adsorbates and contaminants toward the ESA reduction are also discussed. Methods for the revival of activity of surfaces contaminated with adsorbed impurities without perturbing the surface structure and its implications to electrocatalysis are reviewed.

Nitrogen-doped carbon (N/C) and graphene (N/G) were synthesized by the established conventional heat-treatment method, and the incorporation of nitrogen into the carbon matrix was confirmed by CHN analysis, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Electrochemical impedance spectroscopy (EIS) of the prepared catalysts in argon -saturated 0.1 M KOH was performed in a three-electrode rotating disk electrode (RDE) configuration. The capacitance derived from the low-pa 200 frequency region of the EIS patterns was used to estimate the effective density of states [D(E-F)] of carbon and its nitrogen-doped counterparts. Moreover, the carrier concentrations (ND) and flat band potentials of the samples were obtained by Mott-Schottky analysis. The metal-free catalyst samples were tested for possible oxygen reduction reaction (ORR) activity in oxygen saturated 0.1 M KOH electrolyte, and the origin of the activity improvement with nitrogen doping of carbon/graphene can be explained on the basis of the effective density of states [D(E-F)], carrier concentration (ND), and flat band potential. The results suggest that N/C-900 has the highest carrier concentration and maximum flat band potential and, therefore, the highest activity for the ORR.

Carbon is heat-treated with a nitrogen-containing precursor (ammonia) to obtain nitrogen-doped carbon and the composition is estimated using CHN and XPS analysis. The active site density of the carbon and nitrogen-doped carbon is quantified using 1,2-dihydroxybenzene (catechol) molecules as an adsorbate in phosphate buffer (pH 7) solution. The features of the voltammograms of the catechol-adsorbed high surface area carbon and nitrogen-doped carbon are similar to that of the polished nitrogen-grafted glassy carbon electrode (GCE) reported in the literature. At the same time, the polished GCE does not show any well-defined catechol adsorption features. It is found that the adsorption charge (obtained by integrating the peak area, after subtracting the background) is in the order of N/C 900 > N/C 1000 > N/C 800 > N/C 700 > C. A similar trend is observed in their oxygen reduction reaction (ORR) activity in 0.1 M KOH. Moreover, the turnover frequency (ToF) of the catalysts is calculated and it is comparable to that reported in the literature using other methods for non-precious catalysts. Therefore, the adsorption charge can be correlated with the active site density of the carbon and nitrogen-doped carbon samples.

Carbon-black is heat-treated with nitrogen-containing precursor (ammonia or melamine) to yield N-doped carbon. The nitrogen content in the sample depends on the type of the precursor and the heat-treatment temperature and it is estimated using CHN analysis. The electrochemical impedance spectroscopy (EIS) patterns of the samples recorded in argon-saturated 0.1 M KOH show a high-frequency equivalent distributed resistance (EDR) and a low-frequency capacitive straight line, tilted slightly away from the y-axis. The EDR and the tilt of the capacitive straight line decrease with increase in nitrogen-content in the N-doped carbon samples. Further, EIS of oxygen reduction reaction (ORR) is recorded with carbon and N-doped carbon in oxygen-saturated 0.1 M KOH. The EIS patterns of ORR suggest that the high-frequency EDR feature is not negligible, unlike that with carbon-supported and unsupported Pt catalysts in both acidic and alkaline media. Therefore, nitrogen content and EDR have important implications in deciding the electrocatalytic activity and the features of the EIS patterns of carbon-based metal-free catalysts. Because of the change in EDR and the capacitance with nitrogen-content, the EIS patterns of ORR with N-doped carbon are complex to analyze.