Electrochemical Conversion Research Articles

Manganese Oxide Gas Diffusion Electrode Conditioning and Electrochemical Doping for Reversible Oxygen Reduction/Evolution Reactions

Developing durable and active bifunctional gas diffusion electrodes for reversible oxygen reduction/evolution (ORR/OER) reaction catalysis is crucial yet challenging in the development of numerous electrochemical energy conversion systems, including regenerative fuel cells and rechargeable metal-air batteries. This research is focused on developing manganese oxide-based gas diffusion electrodes for reversible oxygen electrodes.Core-porous shell Mn/Mn3O4 (abbreviated as MnOx) nanoparticles (QSI) demonstrated increased ORR/OER activities compared to γ-MnO2 (EMD) and b-MnO2.[1] The unique behaviour of core-shell MnOx is attributed to the charge transfer between the low-valent Mn core and the high-valent Mn3O4 shell, thereby regenerating Mn(III) active sites through comproportionation.[1]The stability of transition-metal-based reversible oxygen electrodes remains a challenge. As shown in Figure 1, the deterioration of Mn/Mn3O4 during ORR and OER cycling can be attributed to three aspects: Mn active sites oxidation (Figure 1a), less active MnOx phase formation (Figure 1b), and material loss (carbon corrosion and dissolved MnO4 - formation) (Figure 1c).[2] As a result, the QSI MnOx electrode transformed from pristine to inactive status within 15 galvanostatic ORR/OER cycles. To improve the stability of the electrode, a set of in-situ electrochemical conditioning and electric field-driven metallic cation incorporation into MnOx GDE was investigated. Among the explored metal cations, the Ni2+ incorporation showed the most significant improvement in terms of activity and cyclic stability. The Ni-MnOx GDE demonstrated excellent stability for over 120 galvanostatic cycles at ±10 mA cm-2 in O2-saturated KOH without losing OER activity, surpassing the Pt/C-IrO2 benchmark.[3]These findings position the MnOx electrodes as promising and sustainable alternatives to precious metal-based electrodes for ORR/OER catalysis and provide valuable insights into designing reversible oxygen electrodes for energy conversion and storage in alkaline systems, contributing to a green future.[1] Pei, Y., Wilkinson, D. P., & Gyenge, E. L. Insights into the electrochemical behavior of manganese oxides as catalysts for the oxygen reduction and evolution reactions: monometallic core‐shell Mn/Mn3O4. Small, 2023, 19, 2204585.[2] Pei, Y., Wilkinson, D. P., & Gyenge, E. L. Gas Diffusion Electrode Design and Conditioning with Manganese (III/IV) Oxide Catalyst for Reversible Oxygen Reduction/Evolution Reactions. ACS Energy & Fuels (2023), 37, 23, 19278–19291.[3] Pei, Y., Wu, W. Y., Wilkinson, D. P., & Gyenge, E. L. High-Performance Reversible Oxygen Reduction/Evolution Gas Diffusion Electrodes with Multivalent Cation Doped Core-Shell Mn/Mn3O4 Catalysts. ChemElectroChem, in press. Figure 1

Atomic Layer Deposition of Cu Catalysts on Gas Diffusion Electrodes for Electrochemical CO2 Reduction

Electrochemical CO2 conversion is gaining attention as an important process for carbon fixation through the use of renewable electricity to address the climate change crisis. Copper is a unique single-element catalyst that can induce C-C coupling reactions to generate high-value-added compounds. The triple phase boundary has been successfully controlled at the catalyst interface using gas diffusion electrodes (GDEs) over the past few years, resulting in high C-C coupling performance. Therefore, methods to engineer the catalyst interface on the 3D structure of GDE are of significant importance. Generally, the catalyst on the GDE is introduced through physical vaper deposition (PVD) using sputtering or E-beam evaporation, or through solution methods such as spray coating and drop-casting of nano-particles. However, the porous and tortuous 3D structure of the GDE often result in limited conformality and difficulty in obtaining a uniform and controlled infiltration of the catalyst into the support structure.In this study, we synthesized Cu catalysts directly onto GDE surfaces using plasma-enhanced atomic layer deposition (PE-ALD), which allowed for precise tuning of the catalyst size and loading at the nanoscale [1]. The synthesized catalysts were confirmed to be polycrystalline Cu nanoparticles using grazing incidence X-ray diffraction. To demonstrate the advantages of the conformal and tunable control of PE-ALD deposition, we deposited the same areal mass loading of Cu catalyst onto the GDE substrates using PVD, and compared the CO2 reduction activity in an H-cell environment. The PE-ALD Cu catalyst showed more than 3 times higher current density than the PVD catalyst at the same electrode potential, a low (~10%) Faradaic efficiency (FE) for the hydrogen evolution reaction (HER), and greater than 75% FE for C-C coupling to form C2+ products including ethylene and ethanol, which is comparable to the highest performances reported to date for metallic Cu catalysts in a H-cell environment. Furthermore, stable operation was observed for a duration of 15 hours, with minimal changes in activity and selectivity. In conclusion, the introduction of catalysts using PE-ALD on GDE electrodes represents a powerful new synthesis method that can increase both C-C coupling performance and selectivity compared to PVD deposition, due to its adjustable Cu nanoparticle size and high conformality. Reference 1) J. D. Lenef, S.Y. Lee, K. M. Fuelling, K. E. Rivera Cruz, A. Prajapati, D. O. D. Cornejo, T. H. Cho, K. Sun, E. Alvarado, T. S. Arthur, C. A. Roberts, C. Hahn, C. C. L. McCrory, N. P. Dasgupta, Nano Lett. 23, 10779-10787 (2023)

(Invited) Dissolution of Organic Overlayers Modified Platinum Single Crystal Interfaces in Acidic Medium

Electrocatalysts design has been an important researching discipline in the filed of electrochemical energy conversion and storage, among which, surface (organic) modification has becoming an efficient, selective, and flexible approach in electrocatalysis study and applications. In the focus of direct modulation on electrocatalysis interfaces, the functionalization of metallic catalysts with organic ligands (i. e. functional organic molecule, ionomers, ionic liquids) has demonstrated its capacity on improving catalytic activity and selectivity in electrochemical reactions such as ORR, CO2RR, as well as oxidation processes. However the electrocatalysts stability behavior and surface degradation dynamics of such modified systems are still not sufficiently studied.To fully decipher the potentials of surface/interface modification in electrocatalysis, we focus our studies on the dissolution behavior of functional organics (i. e. melamine, immidazolium ionic liquids) modified model Pt (hkl) single crystal facets, in acidic medium. In regard of the excellent system sensitivity of our inductively coupled plasma mass spectrometer (ICP-MS, detection limit down to 0.1-1 ng/L), we employed a customized scanning flow cell (SFC) coupled on-line monitoring system to probe the potential-resolved surface metal dissolution dynamics. With designed electrochemical system configuration, we acquired results demonstrating the delicate stability behavior changes over transient and accumulative dissolution study on (modified) Pt facets. In combination of other state-of-art operando spectroscopy methods (electrochemical infrared reflection absorption spectroscopy, electrochemical scanning tunneling microscopy, Raman spectroscopy, etc.), we further comprehended the surface oxidation process, surface blockage/modification mechanisms on electrocatalysis interface. Our research conferred a model stability study approach on surface modification of electrodes and electrocatalysts, providing important insights into the dynamic interfacial interaction between functional modifier and electrocatalysts.

Embedded Micro-Interdigitated Flow Fields for Efficient Intercalation-Based Desalination at Seawater Scale

During the past decade, intensifying water crises around the globe have motivated research in electrochemical desalination. Among such technologies, Faradaic deionization (FDI) has shown great promise for seawater desalination with the use of intercalation electrode materials that possess at least ten-fold higher charge storage concentration (~5 M) than seawater salinity (~0.5 M). Despite many efforts focused either on developing electrode materials or modifying flow-cell architectures, the promise of intercalative FDI was not previously realized experimentally. This work demonstrates for the first time salt removal approaching seawater-level salinity by using a flow cell containing symmetric cation intercalation electrodes separated by an anion-exchange membrane [Do et al., Energy Environ. Sci., 16, 3025–3039 (2023)]. We show removal of 96% of salt from ~500 mM NaCl feeds with ~50% water recovery at a thermodynamic energy efficiency (TEE) of 7%. In addition, the flow cell readily produces fresh water (< 0.017 M NaCl) from brackish water with 40% TEE and substantially lowered hypersaline brine concentration from 0.78 M down to 0.23 M at 11.7% TEE. We note that (1) scaling up the charge capacity of intercalation electrodes is crucial to improve salt removal in FDI and (2) rational design of flow-through electrodes is equally important to make FDI more energy-efficient by mitigating pumping energy consumption. Upscaling was achieved by using large-area electrodes with high areal loading of nickel hexacyanoferrate (NiHCF), up to 21 mg cm-2. Pumping energy was drastically reduced by embedding an interdigitated array of 70 µm wide channels into the NiHCF electrodes using laser micromaching to form novel embedded, micro-interdigitated flow fields (eµ-IDFFs). The dimensions of the eµ-IDFFs were tailored for our low-permeability electrodes based on analytical and numerical modeling, resulting in two orders of magnitude improvement in the apparent hydraulic permeability of these electrodes while minimizing active-material loss. Evidence of water transport between diluted and concentrated streams, as well as feed-concentration-dependent charge efficiency were reported, which potentially contributed to energy efficiency and water recovery losses, motivating their investigation to further improve FDI performance. In addition to their application toward desalination, the present eµ-IDFFs show promise for use in electrochemical energy storage and conversion processes that employ nanomaterials

Sustainable and Tunable Electrochemical CO2 Conversion to Chemicals and Fuels at Industrial Scale Using Metal-Oxide-Based Materials

The most challenging deal we face these years is the need to lower greenhouse gas (GHG) emissions and tackle climate change; though calls to reduce it are growing louder yearly, emissions remain high. CO2 is the key contributor. For this reason, the synthesis of high-added-value products from CO2 conversion is a promising approach to mitigate the problem.1 Among the different alternatives, exploiting CO2 via electrochemical reduction under mild conditions (ambient pressure and temperature) represents an opportunity to support a low-carbon economy.2 The electrocatalytic (EC) CO2 reduction (CO2R) driven by renewable energy can be exploited for the future energy transition, for the carbon storage into valuable products like syngas (H2/CO mixtures), organic acids (formic acid) and chemicals/fuels (C1+ alcohols).3 A big challenge for the industrialisation of this technology is to find low-cost electrocatalysts, efficient reactors and process conditions. In the efforts to develop efficient, selective and stable materials, we have exploited the current knowledge of thermocatalytic CO2 hydrogenation to develop noble-metal-free CO2R electrocatalysts.4 For instance, Cu/Zn/Al synthesised catalyst producing methanol and CO from the CO2 thermocatalytic (TC) hydrogenation (at H2 pressure (P) of 30 bar and temperature (T) > 200 oC) promotes the formation of methanol (⁓32% of FE) during the EC CO2R in a gas-diffusion-electrode system; while operating in the liquid phase, the same catalyst produces syngas with a tunable composition (95% of FE at the most positive applied potential) and other liquid C2+ products (in both cases at ambient T, P).4 On the other hand, Cu/ZnO electrocatalyst has also been tested at industrially relevant current densities in liquid phase configuration.5 We demonstrated through ex-situ characterisations that the presence of ZnO nanoparticles in the mixed Cu/ZnO catalyst plays an important role in forming and stabilising mixed oxidation states of copper and Cu1+/Cu0 interfaces in the electrocatalyst (in bulk and surface). These interfaces seem to promote CO dimerisation to ethanol. Indeed, ethanol was produced with the Cu/ZnO catalyst, reaching ethanol productivity of about 5.3 mmol∙gcat -1∙h-1 in a liquid-phase configuration at ambient conditions. The Cu/Zn/Al and Cu/ZnO electrocatalysts also have been tested in a catholyte-free configuration with an increased selectivity to ethylene, reaching approx. 60% and 70% of FE, respectively. Here, Cu catalyst structure transformed, on average, completely to metallic with a very thin layer of Cu1+ during testing, which seems to promote the selectivity towards C2H4, demonstrating that the reaction pathways for EC CO2R are largely determined by transport limitations rather than only by the intrinsic properties of the electrocatalysts. Our results open a promising path for the prospective implementation of metal-oxide nanostructures for CO2 conversion to the chemicals and fuels of the future. Acknowledgements This work has received financial support by the EU H2020 SunCOChem Project, grant agreement: 862192. References Guzmán, H., Russo, N. & Hernández, S. CO2 valorisation towards alcohols by Cu-based electrocatalysts: challenges and perspectives. Green Chem. 23, 1896–1920 (2021).Romero Cuellar, N. S., Wiesner-Fleischer, K., Fleischer, M., Rucki, A. & Hinrichsen, O. Advantages of CO over CO2 as reactant for electrochemical reduction to ethylene, ethanol and n-propanol on gas diffusion electrodes at high current densities. Electrochim. Acta 307, 164–175 (2019).Guzmán, H., Farkhondehfal, M. A., Rodulfo Tolod, K., Russo, N. & Hernández, S. Photo/electrocatalytic hydrogen exploitation for CO2 reduction toward solar fuels production. in Solar Hydrogen Production Processes, Systems and Technologies 560 (Elsevier Inc., 2019). doi:10.1016/C2017-0-02289-9.Guzmán, H. et al. How to make sustainable CO2 conversion to Methanol: Thermocatalytic versus electrocatalytic technology. Chem. Eng. J. 417, 127973 (2021).Guzmán, H. et al. CO2 Conversion to Alcohols over Cu/ZnO Catalysts: Prospective Synergies between Electrocatalytic and Thermocatalytic Routes. ACS Appl. Mater. Interfaces 14, 517−530 (2022).

(Invited) Development of Multifunctional Nanoscale Reactors for Electrocatalytic Oxidation of Dimethyl Ether Fuel

There has been growing interest in utilizing small (simple) organic molecules, as alternative fuels to hydrogen, for electrochemical energy conversion in fuel cells. Dimethyl ether (DME) is one of the possible choices in this respect. But electrooxidation of DME is rather slow at ambient conditions. Obviously, there is a need to develop novel electrocatalytic materials.Bimetallic PtSn catalysts were found to be almost as active as PtRu for methanol oxidation, and PtSn was even more efficient toward the electrooxidation of ethanol where breaking of C-C bond is required. While PtRu forms a single-phase homogeneous (intermetallic) alloy in which Ru is largely metallic, tin does not maintain its metallic state in PtSn (heterogenous alloy) and is transformed readily to such oxygenated species as Sn oxides or hydroxides. During operation of PtSn, the Pt component retains its mostly metallic character and act independently as Pt0 catalyst. By analogy to Ru, the Sn “bifunctional agent” tends to activate the water dissociative adsorption and increases population of chemisorbed hydroxyl groups to diminish the Pt-poisoning effect through enhanced oxidation of CO-type intermediates. Nevertheless, the DME oxidation on both PtRu or PtSn is still kinetically less favorable, relative to the systems’ performance toward methanol. While Sn, and particularly Ru alloyed atoms activate readily the water dissociative adsorption on Pt and permit the oxidative stripping of the poisoning CO-type adsorbates, the feasibility of the enhancement of the of the DME adsorption and activation through alteration of the electronic structure of Pt catalytic sites has not been fully explained. By analogy to the ethanol oxidation, Pt sites in the heterogeneous PtSn alloy are active toward dissociation of the DME molecule but the Sn additive is less efficient than Ru (from intermetallic PtRu) in the oxidative removal of poisoning adsorbates. But PtRu is probably the most active during oxidation of methanol, and this small organic molecule is also the DME-oxidation-reaction intermediate. Nevertheless, in comparison to bare Pt, electrooxidation of DME starts at PtRu and PtSn at less positive potentials.We pursue a concept of utilization of multicomponent hybrid electrocatalytic systems or nanoreactors utilizing zirconia oxide matrices for supporting and activation of primarily PtSn nanoparticles supported onto Vulcan XC-72 carbon black carriers. Electrocatalytic activity of Vulcan-supported PtSn (PtSn/V) nanostructured alloys supported onto ZrO2 matrices has been significantly enhanced toward electrooxidation of DME in acid medium (0.5 mol dm-3 H2SO4) by decorating PtSn/V with bimetallic PtRu nanoparticles. The enhancement effect concerns both shifting the onset potential for the DME-oxidation toward less positive values and increase of the DME electrocatalytic current densities recorded under both cyclic voltammetric and chronoamperometric conditions. The activating capabilities of ruthenium nanostructures seem to originate from the existence (even below 0.45 V vs. RHE) of reactive ruthenium oxo/ hydroxo groups on their surfaces capable of inducing the oxidative removal of poisoning (CO-type) adsorbates from the neighboring platinum catalytic sites. In this respect, the Ru-oxo species seem to support activity of Sn forming with Pt the PtSn heterogenous alloy. The PtSn/V admixed with PtRu decorated ZrO2 exhibit very high activity toward the oxidation of methanol which is also an important DME-oxidation intermediate. On the whole, the hybrid materials composed of Vulcan-supported PtSn decorated with Ru or PtRu nanoparticles decorated zirconia oxide seem to act as multifunctional nanoreactors inducing not only stripping of poisoning adsorbates but also catalyzing oxidation of the DME-reaction intermediates (methanol). Acknowledgement Authors acknowledge financial support of the National Science Center (Poland) under Opus Project 2018/29/B/ST5/02627.

Dissolution Stability of Graphene-Modified Pt(111)

Pt plays a leading role in electrocatalysis, being the best pure metal catalyst for the oxygen reduction reaction (ORR) taking place at the cathode of the commercial polymer electrolyte membrane fuel cells (PEMFCs). In addition, it is usually used in the cathode to catalyze the hydrogen evolution reaction (HER) in water electrolyzers for hydrogen production. Apart from activity, stability is another critical parameter to consider when evaluating the performance of electrochemical energy conversion devices. Even pure Pt catalysts degrade under real-life conditions due to electro-oxidation and dissolution processes. Fundamental studies with well-defined Pt surfaces are mandatory for understanding these phenomena and developing new electrocatalysts with improved activity and stability.1 Different types of protection on the surface of electrocatalysts, such as carbon layers or ceria surrounding metal nanoparticles,2-4 have been investigated to enhance their stability while maintaining or improving their activity towards a reaction of interest. Here, we propose the modification of Pt(111) single crystal electrode with an adlayer of graphene as a model system to evaluate the protection mechanism of carbon-capped catalysts.In this work, the stability of graphene-modified Pt(111) in 0.1 M HClO4 is studied by on-line inductively coupled plasma mass spectrometry (ICP-MS) measurements using an electrochemical scanning flow cell (SFC). The Pt(111) surface was modified by a fast and easy transfer method of a commercially available chemical vapor deposited (CVD) graphene single-layer. The cyclic voltammetry results are comparable to those previously reported for surfaces prepared by direct CVD on the Pt(111) electrode.5 To test the quality of the graphene adlayer we checked the electrocatalytic activities towards different reactions of interest, such as ORR, HER and hydrogen oxidation reaction (HOR), and compared the results with the previous works.6 The graphene-modified Pt(111) surface presents enhanced stability since no dissolution is detected for a cyclic voltammetry experiment at 10 mV s-1 with an upper potential limit (UPL) of 1.3 V vs. RHE. In contrast, evident Pt dissolution is observed for the unmodified surface. Different accelerated stress test (AST) protocols consisting in groups of 50 cyclic voltammetries at 200 mV s-1 were performed up to three different UPL, namely 1.3 V, 1.5 V and 1.7 V vs. RHE, showing almost no dissolution over the protocol when UPL = 1.3 V, while the integrity of the graphene adlayer was quickly compromised for UPL = 1.7 V as pointed out by the quick increase in Pt dissolution. The degradation of the graphene layer in the different conditions was additionally characterized by Raman spectroscopy and atomic force microscopy (AFM). The results from this work provide useful information about the behavior of carbon capping agents on Pt surfaces, which can help to design new electrocatalytic materials with improved stability. References (1) Fuchs, T.; Briega-Martos, V.; Drnec, J.; Stubb, N.; Martens, I.; Calle-Vallejo, F.; Harrington, D. A.; Cherevko, S.; Magnussen, O. M. Anodic and Cathodic Platinum Dissolution Processes Involve Different Oxide Species. Angew. Chem. Int. Edit. 2023, 62, e202304293.(2) Sgarbi, R.; Doan, H.; Martin, V.; Chatenet, M. Tailoring the Durability of Carbon-Coated Pd Catalysts Towards Hydrogen Oxidation Reaction (HOR) in Alkaline Media. Electrocatalysis 2023, 14 (2), 267-278.(3) Kim, H.; Kwon, G. H.; Han, S. O.; Robertson, A. Platinum Encapsulated within a Bacterial Nanocellulosic-Graphene Nanosandwich as a Durable Thin-Film Fuel Cell Catalyst. ACS Appl. Energy Mater. 2021, 4 (2), 1286-1293.(4) Miller, H. A.; Vizza, F.; Marelli, M.; Zadick, A.; Dubau, L.; Chatenet, M.; Geiger, S.; Cherevko, S.; Doan, H.; Pavlicek, R. K.; et al. Highly active nanostructured palladium-ceria electrocatalysts for the hydrogen oxidation reaction in alkaline medium. Nano Energy 2017, 33, 293-305.(5) Fu, Y. C.; Rudnev, A. V.; Wiberg, G. K. H.; Arenz, M. Single Graphene Layer on Pt(111) Creates Confined Electrochemical Environment via Selective Ion Transport. Angew. Chem. Int. Edit. 2017, 56 (42), 12883-12887.(6) Shih, A. J.; Arulmozhi, N.; Koper, M. T. M. Electrocatalysis under Cover: Enhanced Hydrogen Evolution via Defective Graphene-Covered Pt(111). ACS Catal. 2021, 11 (17), 10892-10901.

Technical Strategies on Building Loosened Polymertic-Chain Structure at Platinum-Perfluorinated Sufonicacid Ionomer Interface

For a complete electrification of transportation sector, the hydrogen fuel cells and lithium-ion batteries are considered as promising candidates for electro-chemical energy storage and conversion devices that may utilize the renewable energy. Between them, the PEFCs offer the decisive advantages, such as faster fueling and longer driving range that is especially advantageous for the heavy-duty applications. However, the spread of this technology is blocked by scarcity of platinum group metals (PGMs). Although the state-of-the-art fuel cell electric vehicles (FCEVs) have succeeded in reducing the PGM usage around 20 g per a passenger vehicle until now, over four-folds amount of PGMs must be further reduced to achieve sustainability. Hence, increasing Pt utilization rate in such system is key to everything; however, the performance of PEFC electrode has been severely aggravated under the presence of perfluorinated sulfonicacid (PFSA) ionomer. In this work, we have shown the key comparative features of technical approaches that have been made within literatures in the recent decade including our previous work. More specifically, the electrode ionomer homogeneity of hydrophilic-hydrophobic domains as well as polymeric chain densification at Pt-ionomer interface has been discussed. Finally, some technical pathways to simultaneously resolve the dry accessibility as well as specific adsorption of ionomer are drawn with detailed information. This paper provides a key summary that may be helpful in defining the notorious "RDE versus MEA" issue and also categorizing the possible solutions that are either previously done or will be covered in the near future.

(Invited) Overview of Ion-Conducting Polymer Membranes and Membrane Separators for Water Electrolysis

Traditional liquid alkaline electrolyzer uses a porous diaphragm to allow diffusion of hydroxide ion from a feed of concentrated KOH solution while separating anode and cathode and suppressing mixing of produced H2 and O2 gases. Although liquid alkaline electrolyzer is considered as a matured technology, recent renaissance of new electrode separators suggests there is significant potential for improvement in efficiency and cost reduction by lowering area specific resistance and increasing operation current density.Ion-conducting polymers (e.g., proton or hydroxide) are used as a key component of polymer electrolyte membranes, such as acidic proton exchange membrane (PEM) and alkaline anion exchange membrane (AEM), in electrochemical energy conversion and storage technologies including water electrolyzers. For example, the state-of-the-art PEM electrolyzers have used Nafion for a PEM and ionomer catalyst binder for decades, though it is not ideal proton-conducting membrane material for those applications. Recent pressure on perfluoroalkyl substrates (PFASs) from government and environmental activist groups due to their environmental and health hazard has attracted the development of alternative polymer electrolyte membranes based on hydrocarbon polymers within electrochemical society. Compared to perfluorosulfonated Nafion, hydrocarbon ion-conducting polymers (both PEMs and AEMs) can offer advantages of flexible synthetic tunability, lower gas permeability, and importantly lower production cost.In this tutorial, an overview of recent progress in the development of advanced porous membrane separators, PEMs and AEMs, their key membrane properties, and the state-of-the-art performance in applications of water electrolyzers will be discussed. Figure 1

Composition-Dependent Dynamics of Nickel-Iron-Cobalt Metal Catalysts for Oxygen Evolution Reaction

Water electrolyzers, such as proton exchange membrane water electrolyzers (PEM) and anion exchange membrane water electrolyzers (AEM) are important technologies for sustainable electrochemical energy conversion. The oxygen evolution reaction (OER) at the anode is a pivotal half-reaction in these systems. Despite substantial progress over the years, OER kinetics remain a major source of inefficiency in these devices, necessitating the development of stable catalysts. Currently, precious metal-based catalysts dominate the landscape of OER catalysts for these devices. As non-precious alternatives, nickel-iron (Ni/Fe) catalysts have emerged as particularly promising in alkaline conditions. However, they exhibit dynamic behavior during electrocatalysis—for instance, oxidizing to an active oxyhydroxide phase during the OER. This dynamic nature makes it challenging to establish clear structure-activity-stability relationships. Incorporating cobalt (Co) into the catalyst composition has been found to influence the activity and material conversion of mixed Ni-Fe-Co oxides and (oxy)hydroxides. Methods for rationally activating and stabilizing Ni-Fe-based catalysts via electrochemical and chemical (e.g. composition) routes are necessary for enhancing scalability and accessibility in hydrogen production devices.Herein, our goal is to understand and exploit activation protocols to enhance the OER performance of as-synthesized nickel-iron-cobalt metal thin films, addressing three primary objectives: synthesizing and characterizing homogeneous metal catalysts, investigating the impact of different potential ranges and scan rates on catalyst oxidation and surface conversion from the metal, and assessing how the addition of cobalt influences conversion rates and current density of the catalyst. We use cyclic voltammetry to facilitate and disentangle conversion (integrated reductive peak for Ni and/or Co2+ → 3+ transition) and OER performance (current density (mA cm-2) at η = 350 mV). For all compositions tested of (Ni80Fe20)100-xCox, x = 0 - 5 % we observe that the most effective conversion requires both redox and oxygen evolution. To further investigate the relationship between conversion and performance catalysts were tested in varying electrolytes such as 1M KOH, 0.1M KOH, and an ionomer overlayer to understand how these impacted conversion.Subsequently, post-analysis techniques such as X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma mass spectrometry optical emission spectroscopy (ICP-OES) were conducted to gain insights into the surface composition and elemental distribution of the catalysts following potential cycling. Additionally, electrochemical mass spectrometry (EC-MS) was employed to quantify the faradaic efficiency of oxygen, providing an understanding of at what point the catalyst begins converting and how much of the OER current is directed towards OER versus metal conversion.This project advances the fundamental understanding of NiFeCo catalyst conversion, shedding light on the critical role of catalyst composition and investigating the conditions that foster optimal conversion efficiency. The use of physical vapor deposition for thin film catalysts offers distinct advantages, making deposition and scaling processes facile. This capability, coupled with the gained insights into conversion dynamics, sets the stage for in situ optimization of the surface to achieve the active oxyhydroxide phase with the highest performance for AEM water electrolysis.

(Invited) Two-Dimensional Layered Materials and Their Mixed-Dimensional Hybrids for Electrochemical Applications

Compared with their 3D counterparts, two-dimensional (2D) van der Waals (vdW) materials exhibit quantum confinement where charge carriers are spatially confined at the physical boundaries. Particularly, when mixing 2D materials with other low-dimensional (LD) materials, they exhibit enormous potential in electrochemical energy applications due to the unique properties arising from reduced dimensionality and, more importantly, material integration synergy.In this work, 2D transition metal dichalcogenides (MoS2) and their mixed low-dimensional hybrids (MLDHs) are introduced with an emphasis on the hybrid structure construction using the one-step solvothermal synthesis technique and their electrochemical applications. Fundamental insight into the synergistic effect of the MLDHs integration in terms of active site exposure, surface area enlargement, electrical conductivity improvement, and structural phase engineering for advancing the development of Li-ion batteries (LIBs) and electrocatalytic hydrogen evolution reaction (HER) will also be discussed. Specifically, in the case of LIBs, 2D-based binary hybrids (i.e., MoS2/MXene) deliver a Li-ion storage capacity of 540 mA h g−1 at 100 mA g−1 and a stable cycle performance up to 300 cycles. In sharp contrast, the 2D-based ternary MLDHs (i.e., MoS2/MXene/CNT) exhibit a higher Li-ion storage capacity (1500 mAh g−1 at 100 mA g−1), outstanding cycle stability (up to 300 cycles) and excellent rate capability (500 mAh g−1 at 4000 mA g−1) as an anode of LIBs. In the case of HER, the MLDH heterostructures show an overpotential of 169 mV with a low Tafel slope of 51 mV/dec and can be cycled to 1000 cycles. Leveraging the unique microreactor platform based on the 2D vdW platform, a mechanistic understanding of charge transport dynamics at the electrified electrode and current collector interface will be highlighted.The knowledge gained on how mixed-dimensional physics and chemistry will shed light on the design principle of the electrode materials for electrochemical energy storage and conversion and deepen the understanding of the process-structural-property-performance (PSPP) relationship of the vdW-based hybrid structures.

Electrocatalytic Upgrading of CO2 to Light Olefins in Protonic Ceramic Electrochemical Cells

Light olefins (ethylene, propylene, butylene) are primary building blocks for chemical manufacturing of large volume products including organic chemicals, plastics, and even sustainable aviation fuels. Currently, the dominant commercial process for olefin production is steam cracking of naphtha or ethane, which is highly energy intensive and accounts for a large fraction of global CO2 emissions. To decarbonize olefin production processes, electrochemical CO2-to-olefin conversion coupled with renewable or low-emission energy sources including electricity and heat offers an attractive alternative. Previously, extensive research efforts have been devoted to low temperature electrochemical CO2 reduction in aqueous media, i.e., occurring below 100 °C. This approach, however, still faces enormous scientific and technological challenges in improving CO2 utilization, energy efficiency, and product selectivity. In this work, we have developed a new electrochemical process for direct conversion of CO2 to ethylene at intermediate temperatures (350-500°C) in an electrochemical membrane reactor. This process is based on high-performance protonic ceramic electrochemical cells (PCECs) that integrate CO2 hydrogenation reaction to produce olefins in the cathode and concurrent hydrogen generation reaction in the anode to provide hydrogen for CO2 conversion. Highly active and selective catalysts for CO2-to-olefin conversion have been identified and integrated into the PCECs. Both the catalytic and electrochemical performances of the integrated reactor system are evaluated and optimized. In contrast to the use of molecular H2 in conventional thermal catalytic pathways, the active hydrogen in PCECs could be sourced from different feedstocks including H2O, light alkanes, and ammonia, etc. This work also provides valuable insights into the selection and design of efficient catalysts for electrochemical CO2 conversion at elevated temperatures.

Assessing Nonlinear Polarization in Electrochemical Cells Using AC Impedance Spectroscopy

Electrochemical energy storage and conversion devices are used to power a range of electrical loads including portable electronics, electric vehicles, and utility grids. The energy efficiency of these systems decreases with increasing current load due to energy losses (in the form of heat) associated with different processes. Electrochemical impedance spectroscopy (EIS) is a nondestructive technique commonly used to assess rate-limiting steps in electrochemical devices. Most EIS measurements for energy storage devices are only performed around open-circuit and do not probe system behavior under non-zero biases where these devices operate. Despite its widespread use, this approach does not provide information about the relative overpotentials associated with coupled nonlinear processes (e.g., diffusion limited redox reactions) commonly encountered in these systems. Furthermore, ambiguous terms such as equivalent series resistance and interfacial resistance can lead to confusion and data misinterpretation. For example, Ohm’s law cannot be applied to calculate nonlinear overpotentials using a resistance that was measured from a locally linear response.To determine overpotentials in nonlinear systems using EIS, one must integrate the individual resistive elements of the equivalent circuit as a function of steady state current. Recent experimental work has used this method to decouple ohmic, kinetic, and transport overpotentials in redox flow batteries. In these prior studies, impedance elements were derived, but analytical expressions for the corresponding overpotentials were not presented.While integrated resistance measurements are a powerful experimental tool, a more rigorous mathematical framework describing the approach is needed. The present work bridges this gap by: (i) deriving a general impedance expression for coupled charge transfer and diffusion processes and (ii) integrating the individual resistive elements as a function of steady-state current to obtain global polarization relationships. Analytical expressions for diffusion overpotentials are obtained for special cases, and numerical methods are applied when closed-form expressions do not exist. The work considers three model electrode geometries (planar, cylindrical, and spherical) and two diffusion boundary types (reflective and transmission). Finally, this treatment is extended to macroscopically homogenous porous electrodes which are relevant for describing practical devices.This presentation will highlight how measuring impedance at different steady-state biases provides critical insights on the rate-limiting processes in energy storage devices. While this work focuses on a simple Faradaic charge transfer process, the method can be used to evaluate other nonlinear phenomena in electrochemical power supplies. For example, the resistance of passive films (e.g., solid electrolyte interphase layer) in Li-ion batteries is typically only probed near open-circuit. Similarly, literature on solid-state batteries commonly uses ambiguous terms such as interfacial resistance to describe rate-limiting processes. For these more complex systems, integrating the resistive elements can provide insights on how these phenomena influence device efficiency. Overall, combining these experimental observations with physical models derived using the approach illustrated herein represents a powerful tool to guide component/system design and improve device performance. Acknowledgements This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE). This work is sponsored by the U.S. Department of Energy in the Office of Electricity through the Energy Storage Research Program and the Office of Energy Efficiency and Renewable Energy (EERE) in the Vehicle Technologies Office (VTO) through the Advanced Battery Materials Research (BMR) Program.

Advancements in Electrocatalysts for Low-Temperature CO2 Electrolysis: Progress and Insights from the ECO2Fuel Project

The Electrochemical CO2 Reduction Reaction (CO2RR) represents a cutting-edge approach for converting CO2 into valuable chemicals and fuels, marking a significant stride in energy storage through chemical bonds. This innovative method ingeniously merges the well-established water splitting reaction with Fischer-Tropsch synthesis, traditionally dependent on high temperatures and pressures. However, uniquely, CO2RR facilitates this process under ambient conditions, combining hydrogenation with water splitting at room temperature and normal atmospheric pressure. In this context, the ECO2Fuel project, an initiative under the European Green Deal, aims to develop, implement, and validate the first-ever 1MW low-temperature direct electrochemical CO2 conversion system, poised to produce economically viable and sustainable e-fuels and chemicals.Despite its transformative potential, the ECO2Fuel project acknowledges and addresses several pivotal challenges, including enhancing catalyst performance and selectivity, electrode longevity, mass transport optimization, averting electrolyte flooding, and mitigating the hydrogen evolution reaction.This presentation will predominantly concentrate on refining the gas diffusion electrode at the cathode, for large-scale fuel and alcohol production via CO2RR. This electrode is composed of a gas diffusion layer, ensuring efficient CO2 transport; a catalyst layer where the reduction transpires; and a top adhesion layer to prevent catalyst delamination. These components collectively influence the electrode’s selectivity, durability, and stability.Our discourse will delve into the intricate aspects of the catalyst layer and porous transport layer design, along with electrode manufacturing techniques, demonstrating their critical roles in achieving high faradaic efficiencies for carbonaceous fuel production. Furthermore, we will explore the nuanced operational modalities of the CO2 electrolyzer cell, focusing on strategies to curtail the parasitic side reaction of hydrogen evolution, thereby enhancing the system's overall efficiency and productivity.This project has received funding from the European Union’s Horizon 2020 research and Innovation program under grant agreement no. 202037389.

(Invited) 3D Printing of 2D Materials for Optimized Electrochemical Performance

Electrochemical energy storage (EES) and conversion devices (e.g. batteries, supercapacitors, and reactors) are emerging as primary methods for global efforts to shift energy dependence from limited fossil fuels towards sustainable and renewable resources. These devices, while showing great potential for meeting some key metrics set by conventional technologies, still face significant limitations. For example, an EES device tends to exhibit large energy density (e.g. lithium-ion battery) or power density (e.g. supercapacitor), but not both. This inability of a single device to simultaneously achieve both metrics represents a major obstacle to widespread adoption of EES devices. Similarly, many catalytic processes depend on expensive platinum group metals (PGM) because non-PGM materials have poor performance or durability. Though the integration of 2D materials (e.g. graphene, dichalcogenides, MXene, etc.) into electrochemical devices has yielded some exciting results towards tackling these issues, significant improvements are still needed. One approach to optimizing the performance of these devices is to focus on one of the fundamental processes that occur in these systems: mass (or charge) transport. The efficient transport of ions within EES and conversion devices is critical to realizing better performance and durability. The pore structure of the electrode is a key factor in determining this transport phenomena, but in many cases, engineering the pore structure in a highly deterministic fashion can be challenging. In this work, we explore a number of additive manufacturing methods (e.g. direct ink write, projection microstereolithography, etc.) to engineer the pore structure of device electrodes. We also determine effective electrode geometries using both simple theory and topology optimization techniques. The topology optimization couples the solution of the forward electrochemical problem over the full electrode domain with gradient-based optimization. The output of our code is a three-dimensional CAD representation which optimizes over specific performance metrics and which can be used to print functional electrodes. This work provides a systematic path toward automated design and fabrication of engineered electrodes with precise control over the fluid and species distribution.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Understanding Reaction Kinetics of Iron Oxide (α- Fe2O3) Reduction with Single Entity Measurements

Emitting minimal amount of CO2 during the production of steel is one of the major challenges that can be overcome by producing iron using electrowinning method. The reaction of interest is at the cathode where the direct reduction of Iron oxide (α-Fe2O3) to iron takes place. In this study, we are investigating the electrochemical conversion of hematite (α-Fe2O3) Nanoparticles to iron in an extremely alkaline medium (50 wt% NaOH) using Single Entity Electrochemistry (SEE).Single Entity Electrochemistry is the study of single atom, molecule or particle colliding with the surface of the electrode usually by diffusion or migration. For these experiments, we are developing alkaline stable carbon-fiber (C)/Nickel (Ni) ultra-microelectrode (UME), prepared using multiple pathways such as plasma-focused ion beam.A spike in current transient during chronoamperometry (CA) experiments is observed when a hematite particle collides stochastically with the UME and gets reduced in the process. Each spike is the reduction event of one hematite nanoparticle to iron. Individual reduction events, that last 0.2-0.3 s and resulting in 0.1-0.2 nC of charge being passed were observed at Ni and C microelectrodes, this shows good agreement with the average particle radius of 100 nm. The shape of the current transient was distinct from previously reported transients for metal deposition at cathode1-3. However, A. Allanore, et.al. model was used to support a mechanism for the direct oxide reduction process at nanoscale particles4.The results presented here illustrate the reaction kinetics and the mode of reduction propagation in a single nano-sized hematite particle in alkaline conditions. The kinetics of hematite reduction in alkaline medium is considerably slow as observed from the slope of current spikes in SEE experiments. Furthermore, considering Allanore’s model, the reduction propagation occurs from the outer surface to the inner core.(1) Luo, L.; White, H. S. Electrogeneration of single nanobubbles at sub-50-nm-radius platinum nanodisk electrodes. Langmuir 2013, 29 (35), 11169-11175.(2) Xiao, X.; Fan, F.-R. F.; Zhou, J.; Bard, A. J. Current transients in single nanoparticle collision events. Journal of the American Chemical Society 2008, 130 (49), 16669-16677.(3) Bard, A. J.; Zhou, H.; Kwon, S. J. Electrochemistry of single nanoparticles via electrocatalytic amplification. Israel Journal of Chemistry 2010, 50 (3), 267-276.(4) Allanore, A.; Lavelaine, H.; Valentin, G.; Birat, J.; Delcroix, P.; Lapicque, F. Observation and modeling of the reduction of hematite particles to metal in alkaline solution by electrolysis. Electrochimica Acta 2010, 55 (12), 4007-4013.

Electrochemical Conversion of Low-Cost Glycerol to Valuable Products

Over the last years, biodiesel production, a renewable fuel derived from biomass, has significantly increased, reaching an output of 40 million tons annually. This surge in production has resulted in excessive glycerol production, as a byproduct, accounting for 10 wt.% of biodiesel production. By 2025, it is predicted that glycerol production will amount to 6.3 million tons.1 However, the low demand for glycerol not only causes high disposal costs for this waste byproduct but also poses a threat to the ecosystem. Consequently, the valorization of glycerol is crucial for enhancing the biodiesel industry's economic viability and environmental sustainability.Among all the techniques for the valorization of glycerol, such as catalytic oxidation, dehydration, and hydrogenolysis, the electrochemical oxidation of glycerol stands out for its cost-effectiveness, simplicity, and eco-friendliness. The glycerol electrooxidation reaction (GOR) can yield a wide range of valuable chemicals, such as C3 products (glyceraldehyde, lactic acid, glyceric acid, tartronic acid), C2 products (glycolic acid, oxalic acid, acetic acid), and C1 products (formic acid). Among all the products, C3 products have notably higher economic value compared to others. In particular, glyceric acid stood out by having a price of over 200 folds of glycerol and having the highest worldwide demand among all the GOR products.2 However, a significant technical challenge lies in developing a highly selective catalyst for C3 products showing high activity, and stability. Thus far, most of the catalyst developed for GOR includes noble metals or their alloys, which limits their usage in realistic systems due to their high cost and scarcity.3 Therefore, developing a cost-effective catalyst with high selectivity, activity, and stability is necessary to make the valorization of glycerol economically viable.In this work, we demonstrate a bimetallic catalyst that, despite its low noble metal content, achieves high selectivity towards glyceric acid, along with notable catalytic activity and stability. Our analysis focuses on the impact of noble metal amount on activity and stability, and parametric analysis to improve the selectivity of C3 products. Furthermore, we outline the reasons for the discrepancies in the quantification of GOR products, especially in applying H-NMR analysis, and discuss the methods to improve the precision in GOR product analysis.

Continuous Direct Air Capture and Electrochemical Conversion of CO2 and H2O into Ethylene and Oxygen in Solid Electrolyte Reactor

Chemelectronics LLC has developed economical approach to use solar energy to convert CO2 directly captured from air using carbon dioxide capture conductive sorbent materials coated on carbon foam electrodes into chemical products (ethylene) and oxygen from nick foam electrodes separated with solid polyelectrolytes. We take advantage of commercial off-the-shelf solar panels as electricity to supply the electrochemical reduction of CO2 directly captured from air into chemical products with zero carbon emission.Based on the life cycle analysis, there is no CO2 emission from CO2 capture and conversion. The energy will be produced from solar panels.

A Coherent Workflow for Elevating Transition Metal Oxide Anode Materials in Alkaline Electrolysis to Connect Insights from Micro Powder Analysis with Electrochemical Performance

Increasing environmental concerns have redirected research efforts toward sustainable energy, prominently highlighting hydrogen as a promising solution [1]. Despite considerable advancements in water electrolysis, providing efficient energy conversion and storage, there exists a gap in our comprehensive understanding of the production chain. This includes insights from commercial powder materials to electrode fabrication and their electrocatalytic behavior, representing an underexplored domain that demands closer examination.This study systematically examines transition metal oxide electrocatalysts, aiming to parameterize the process chain within the context of a coherent workflow for advancing transition metal oxide anode materials in alkaline electrolysis. This approach seeks to reveal the interdependencies and comprehend the correlations between material properties, fabrication processes, electrode structure, and the final performance of the cell. Our methodology integrates a range of complementary techniques to ensure a consistent analysis of the physical and chemical properties of the materials. Following the initial characterization of commercial micropowder through transmission electron microscopy and energy-dispersive X-ray spectroscopy, we optimized ink formulations derived from the powder using analytical centrifugation and Hansen parameter calculations [2]. Subsequently, selected inks were transferred to electrodes by coating these inks on Ni plates via ultrasonic spray deposition. We further investigated the impact of post-treatments, including vacuum annealing and surface plasma treatment, on the stability of the electrodes, particularly in terms of delamination prevention.To quantify the underlying micro features of this wide range of electrode surfaces and analyze their characteristics, we developed a framework called multistage data quantification (MSDQ). MSDQ ulitizes microscopical charcterizations such as laser microscopy and atomic force microscopy [3]. The assessed electrodes were finally tested as anodes for alkaline water electrolysis, revealing correlations between electrode properties and electrochemical activity and stability. In line with the recognized structural characteristics, our initial findings indicate that electrodes treated with plasma demonstrate reduced overpotentials and enhanced stability when compared to pristine and plasma treated electrodes, due to increased adhesion and surface properties (e.g., roughness).Our study makes a substantial contribution to the comprehensive evaluation of the alkaline water electrolysis system across its development stages, incorporating valuable insights from earlier steps to continually enhance electrode performance. Our approach is particularly advantageous in enabling the transition from laboratory-scale research to scalable processing, effectively bridging the crucial gap for practical industrial applications [4]. Ehlers, J.C., et al., "Affordable Green Hydrogen from Alkaline Water Electrolysis: Key Research Needs from an Industrial Perspective. " ACS Energy Letters, 2023. 8(3): p. 1502-1509.Bapat, S., et al., "On the state and stability of fuel cell catalyst inks. " Advanced Powder Technology, 2021. 32(10): p. 3845-3859.Jain, A., et al., "Small-Area Observations to Insight: Surface-Feature-Extrapolation of Anodes via Multistage Data Quantification. " Under review, ChemCatChemSegets, D., et al., "Accelerating CO2 electrochemical conversion towards industrial implementation." Nature communications 14.1 (2023): 7950.

Accelerated Development and Optimization of Adiponitrile Production via Kolbe Electrolysis of Biomass-Derived Precursors

Environmental and societal pressures are demanding a transition from fossil-based resources to renewable and sustainable feedstocks for the production of materials. In this context, biomass streams emerge as scalable candidates for creating sustainable chemicals. Electrocatalytic transformation of biomass can facilitate the production of chemicals under mild conditions and can be easily integrated with renewable energy sources. These transformations could lead to the production of sustainable monomers and polymers from biomass, significantly enhancing the sustainability of plastics production. Amongst a large number of plastics manufactured by the chemical industry, Nylon 6,6 is one of the most important fossil-derived polymers for the production of high-performance textiles and structural materials. Central to Nylon 6,6 production is adiponitrile (ADN), predominantly synthesized via energy-intensive thermochemical processes. An alternative route, electrochemical hydrodimerization of acrylonitrile (AN), although successful, still depends on fossil feedstocks. A promising sustainable source for ADN is renewable glutamic acid, derived from hydrolysis of waste proteins.1,2 This process involves transforming glutamic acid to 3-cyanopropanoic acid (CPA) and then to ADN through Kolbe electrolysis, a reaction with a historical background and multiple pathways. Our study utilizes an electrochemical reaction engineering approach to examine the Kolbe electrolysis of CPA to ADN and AN, focusing on a hierarchical research methodology. This approach allowed us to simultaneously accelerate the exploration of numerous reaction parameters while also conducting detailed studies under optimal conditions. This method has enabled us to gain deeper insights into the factors controlling reaction selectivity. We discovered that platinum electrodes favor ADN formation, with selectivity increasing at higher current densities. Optimal CPA concentrations, the presence of larger alkali cations, solvent composition, and controlled pH levels significantly influence ADN production, reducing side reactions such as the oxygen evolution reaction (OER). When using graphite electrodes, an increase in AN production was observed, with the solvent composition playing a crucial role in determining the rate of ADN formation. Our investigation has led to a deeper understanding of the electrochemical conversion of CPA to ADN, revealing key parameters that influence reaction selectivity and rate. The insights obtained from this study are not only pivotal for the specific case of ADN production but also have broader implications for a range of electrochemical decarboxylation reactions. The hierarchical research approach employed here demonstrates its potential for speeding the development of sustainable electrochemical processes. Our insights and methodology used for the electrosynthesis of ADN from biomass-derived feedstocks can be deployed to the deployed to the development of other biomass transformations advancing our ability to produce essential chemicals sustainably.