bluuebunny/crossref_metadata_2025 huggingface dataset:
(Invited) Resolving a Catalytic Mechanism on an Electrode Surface: Using Time Resolution to Identify Theoretical Descriptors
Authors: Cuk Tanja
Published: November 2022

Catalytic mechanisms at electrode surfaces guide the development of electrochemically controlled energy storing reactions and chemical synthesis. The intermediate steps of these mechanisms are challenging to identify experimentally but are critical to understanding the speed, stability, and selectivity of product evolution for a given energy input. One of the most studied reactions is the water-splitting reaction which breaks apart H2O into H2 and O2. In the laboratory group, we employ time-resolved optical and vibrational spectroscopy to deconstruct the energy intensive part, the oxygen evolution reaction (OER), into its component parts. The first step of this oxidative reaction, which is to release a proton and take an electron from water, has often been thought of as a theoretical “descriptor” that differentiates the OER activity of materials: if this is hard to do, then the material is less energy efficient and if it is easy to do, then a later, bond-forming step limits the energy efficiency. Yet, the formation free energy of this step had yet to be measured experimentally. Here, I will begin by summarizing the work done previously by the group: the structure and kinetics of forming these first reaction intermediates (Ti-OH*) by their optical and vibrational signatures upon driving OER with an ultrafast photo-trigger of the electrode surface (SrTiO3). I will then discuss the most recent work, which shows that by separating the reaction steps in time and optically detecting these intermediates created upon hole-transfer from the valence band, we can determine their formation free energy1. In addition to the optical spectroscopy, we employ reaction isotherm methodology borrowed from surface studies of reactions. By tuning the pH, we obtain a Langmuir isotherm on the driven surface (Figure 1) regulated by the proton transfer that increases surface hydroxylation for higher pH prior to photo-triggering OER. From the midpoint of this isotherm, one can obtain the formation free energy of Ti-OH* from valence band holes. Such an application of isotherms is new in that the product is not the final one, for which one generally knows the reaction free energy, but rather an intermediate of a reaction. Overall, the innovation of the work is to utilize kinetic measurements to obtain the calculated free energy of an intermediate reaction step at a material surface.

1I. Vinogradov, S. Singh, H. Lyle, M. Paolino, A. Mandal*, J. Rossmeisl, T. Cuk
*,
Nature Materials, 2022, 21, 88.

Figure 1: The time-resolved optical data as a function of pH show how the population of intermediates generates a reaction isotherm (black trace). The emission (in blue) counts the intermediate Ti-OH* population.

Figure 1
(Invited) Small Polarons and Surface Defects in Metal Oxide Photocatalysts Studied Using Xuv Reflection-Absorption Spectroscopy
Authors: Baker Lawrence
Published: November 2022

In photocatalytic transition metal oxides, the surface electronic structure and carrier kinetics are complicated by charge trapping at defects as well as by carrier self-trapping as small polarons. These processes are unique to the material interface, where surface carrier dynamics deviate from bulk behavior due to the presence of surface defect states, undercoordination, or interface bonding. This talk will highlight the ability to directly visualize these processes with surface sensitivity, chemical state resolution, and ultrafast time resolution using extreme ultraviolet (XUV) reflection-absorption spectroscopy. This method measures element-specific, core-to-valence spectra analogous to x-ray absorption, but with the added benefits of surface sensitivity and ultrafast time resolution. Using this method, we investigate surface trapping and recombination in α-Fe2O3 and TiO2 as two prototypical metal oxide photocatalysts. In α-Fe2O3, we observe ultrafast polaron formation with a rate that is three times slower than observed in bulk. Kinetic analysis reveals that this is a result of increased lattice relaxation energy for polaron formation at a surface, and we show that these rates can be rationally tuned by surface molecular functionalization. In contrast, in photoexcited TiO2 we observe that hot holes trap as small polarons at oxygen vacancy defects in approximately 50 fs, and hot electrons couple to polar optical phonons leading to vibrational coherence and large polaron formation in approximately 1 ps. These findings provide examples of the unique opportunities enabled by XUV spectroscopy for understanding surface electron dynamics in photocatalytic systems.
(Invited) Small Polarons in Metal-Oxide Based Solar Absorber Materials
Authors: Huda Muhammad N.
Published: November 2022

The key materials for photo-electrochemical (PEC) processes are the photoelectrocatalysts that are required to absorb sunlight, form electron-hole pairs, and transport the charge carriers efficiently towards the electrolyte-semiconductor interface. Metal-oxides are thought to be the most stable materials under intense interfacial reactive conditions. However, the overall efficiencies of these metal-oxide photoelectrocatalysts are not as high as expected, even for those with near-suitable bandgap energies required for efficient solar energy absorptions. One of the main reasons is attributed to the poor transport properties in metal-oxides with strongly correlated electrons. Typically, strongly correlated systems exhibit polaron conduction via electron-phonon interactions. Polarons act like a bottleneck for photoconductions in such metal-oxides. Recently, there have been significant efforts to characterize polaronic states and their effects on transport properties in photoactive materials. First-principles theoretical and computational studies can reveal the electronic origin of the polaron formation and, therefore, can suggest how to improve overall transport properties in such photoactive materials. In this presentation, representative metal-oxides photo-catalysts will be considered to demonstrate the small polaron formation mechanism. The polaronic states in metal-oxides will be depicted from detailed electronic structure calculations by density functional theory (DFT) based methods. We will show how these polaronic states affect the PEC cell's transport properties and photo-voltages. We will also discuss how these computational results compare with the recent experimental outcome. Some part of the work was funded by National Science Foundation, USA. Computations were performed at the Texas Advanced Computing Center (TACC).
(Invited) Semipermeable Oxide Coatings for Selective Z-Scheme Photocatalytic Water Splitting
Authors: Esposito Daniel V. , Stinson Robert , Stinson William D. H. , Wang Yinxian , Aydin Fikret , Pham Tuan Anh , Ogitsu Tadashi , Hurst Katherine , Barrera Luisa , Bala Chandran Rohini , Chen Zejie , Ardo Shane
Published: November 2022

Similar to natural photosynthesis, Z-scheme photocatalytic water splitting relies on two different light absorbing components that are coupled by a redox active mediator that shuttles charge between them. Such a two-absorber system possesses several advantages over single absorber photocatalytic system, including higher theoretical solar-to-hydrogen conversion efficiency, relaxed band alignment requirements, and the potential for inherently safe operation whereby H2 and O2 evolution occur in separated compartments. However, a major disadvantage and challenge for Z-scheme photocatalysis is that the presence of a redox mediator introduces two undesirable back-reactions on top of parasitic H2 oxidation and O2 reduction reactions that can occur in a single absorber photocatalytic system. Previous research efforts have identified the use of semi-permeable oxide coatings as an attractive approach to suppress these thermodynamically favored redox reactions while still permitting the desired water splitting and mediator redox reactions to occur. Here, we present a combined experimental and computational approach based on model thin films that is used to (i) probe the performance limits of oxide-encapsulated photocatalysts, (ii) quantify the effects of coating defects on performance, and (iii) guide the rational design of coatings aimed at maximizing the solar-to-hydrogen conversion efficiency of a target photocatalytic system. This work specifically focusses on the development of silicon and titanium oxide coatings for Z-scheme water splitting based on a Fe(II)/Fe(III) mediator, showing that the best coatings can achieve selectivities > 90 % towards the H2 and O2 evolution reactions over undesired Fe(II)/Fe(III) back reactions. Another key finding from this work is that coating defects can have a significant influence on the performance of encapsulated electrodes, as revealed by scanning electrochemical microscopy (SECM) measurements that were used to locally quantify the parasitic back reaction rates around individual defects to determine their impact on the global selectivity of an encapsulated electrode.
(Invited) Operando Unraveling Photothermal-Promoted Dynamic Active Sites Generation in Spinel Oxide for Markedly Enhanced Oxygen Evolution
Authors: Lin Zhiqun
Published: November 2022

As the electrochemical reconstruction into highly active species is anticipated to arise under the oxidative conditions, activating and tailoring the surface reconstruction via a rational and effective strategy is pivotal to improve the oxygen evolution reaction (OER) activity. In this work, we advance a robust process, that is, the photothermal effect originated from the spinel oxide catalyst, to render its dynamic transition into active metal oxyhydroxides species that triggers highly efficient oxygen evolution. Significantly, the reaction kinetics of OER are improved due to the lowered activation energy, as modulated by the photothermal energy-promoted surface reconstruction. More importantly, the operando Raman spectroelectrochemistry study and first-principle calculation collectively unveil the mechanism of photothermal-assisted enhancement in OER reaction.
The Effect of Photoanode Interface and Surface Treatment in Dye-Sensitized Photo-Electrochemical Systems for Effective Lignin Decomposition
Authors: Kang Hyeong Cheol , Kim Saerona , Yoo Kicheon , Leem Gyu , Lee Jae-Joon
Published: November 2022

Traditionally, Dye-sensitized photo electrochemical cells (DSPECs) have been targeted for solar-driven water splitting as production of fuels in aqueous media. Recently, hydrogen atom transfer incorporated dye-sensitized photo electrochemical cell (HAT-DSPEC) which used photocatalyst and homogeneous catalysts incorporating system was suggested for the decomposition process of lignin. This HAT-DSPEC system representing conversion efficiencies over 90% resulted in the formation of the oxidized ketone product from model compounds under simulated solar illumination with an applied bias 0.4 V vs Ag/Ag+
for 24 hrs. However, they have several issues such as long reaction time, low stability, and high bias potential.

To overcome these problems, in this study, dye-sensitized photovoltaics’ advanced technic is applied. In HAT-DSPEC system, photo-anode (TiO2@Dye) has important role to produce oxoammonium species by oxidizing nitroxyl derivatives, such as 4-acetomido TEMPO (ACT). And these oxoammonium species act as a catalytic reaction with lignin model compounds (LMCs) to oxidizing of 1o or 2o alcohol and cleavage Caryl-Cα bond cleavage. However, in photo-anode, lots of charge recombination and electron back transfer occurs on the FTO and TiO2 surface and it induce low current and reactivity. For effectively producing oxoammonium species from photoelectrode, using blocking layer to passivate FTO surface from electrolytes and applying TiCl4 post-treatment for fast charge transport in TiO2 by enhancing interconnection of TiO2 nanoparticles. The photocurrent response of the m-TiO2@Ru470 photoelectrode in a nonaqueous electrolyte was examined with increasing mediator concentration during light on/off cycles under AM 1.5G illumination (100 mW/cm2), with an applied bias 0.1 V vs Ag/Ag+. Three kinds of electrode condition such as m-TiO2, BL/m-TiO2, BL/m-TiO2/TiCl4 post treatment was examined under same condition and treated photoelectrode (BL/m-TiO2/TiCl4) showed a significant increase in maximum photocurrent up to approximately 367.69 μA/cm2 with 3 mM ACT mediator, while BL/m-TiO2 exhibits 26.75 μA/cm2 and m-TiO2 1.26 μA/cm2. This result indicates that the blocking layer and TiCl4 post treatment enhanced electron lifetime and collection efficiency in the photoelectrode, as well as enhancing the capability of producing oxoammonium species.
Synthesized Pectin from Food Waste and Its Application in Electrolyte Gelation for CO2 Reduction
Authors: Wilson Nathan W. , Botte Gerardine G.
Published: November 2022

The utilization of in-situ resources such as food waste or plants grown on Mars combined with the available Martian regolith and atmosphere presents a path towards battery separators with increased sustainability, energy density, and durability. Separators that focus on CO2 reduction take advantage of both a highly relevant source of materials on Mars and a solution to an increasingly relevant problem of the increase of atmospheric CO2 on Earth.

In this paper we evaluate the electrochemical and thermal properties of gel electrolytes synthesized from food waste and their performance in CO2 reduction. Our work focuses on the extraction of pectins from soybean hull and tomato pomace as sources of inedible biomass. Cross-linking and ionic activity of the pectin-based gel electrolytes are determined by the integration of monovalent and divalent cations. A biomass-based polymer electrolyte provides a novel method of using in-situ resources for chemical conversion and energy storage that is critical for long-term space travel.
(Invited) Understanding CO2 and Water Supply in Zero-Gap Membrane Electrode Assembly Based Electrochemical CO2 Reduction Reaction
Authors: Hwang Yun Jeong
Published: November 2022

Due to the global agenda to achieve net zero carbon emission, the needs to develop carbon capture and utilization technology has been sharply emerging. Although electrochemical CO2 reduction (CO2R) to useful carbon chemicals is challenging to meet the industrial requirements, recent studies have made a significant improvement in the performances both of the product selectivity and current density. Gas-diffusion electrode configurations allow direct CO2 gas-feeding to the catalyst layer and several hundred mA/cm2 of current density have been demonstrated with a zero-gap type of membrane electrolyzer assembly (MEA) electrolyzer. However, many underlying phenomena are still not clearly understood.

Among various components of MEA, here, I will focus on the cathode catalyst, and balanced CO2 / water supply at the catalyst/membrane boundary to understand the CO2R activity. We demonstrate the importance of the water supply from the anolyte for the selective CO2R in the zero-gap MEA electrolyzer. We confirm that hydrogen is mostly originated from the aqueous anolyte for the production of the ethylene from CO2R with controlled labelling experiments. In terms of the Cu catalyst morphology, the alkaline environment near the catalyst layer leads a significant morphology changes during the pre-treatment steps for the ionomer activation, which contributes to increase in the CO2R activity. The microenvironment behavior is simulated and compares with the experimental CO2R MEA performance to understand the mass flow and CO2R activity, which also support the catalyst thickness dependence. For the practical application consideration, we also study the CO2R activity for CO production with the low concentration (~ 10%) of CO2 feeding and amine-captured direct CO2 reduction condition. Weak HER activity is important especially with the low concentration of CO2 because the slower CO2 reduction rate lead to more hydrogen compared to 100 % of CO2 gas feeding. These studies will give insight to the intrinsic and extrinsic factors determining the activities of CO2R MEA.
Intrinsic Catalytic Activity and Active Phase for Oxygen Evolution in Layered Double Hydroxide Electrocatalysts
Authors: Dionigi Fabio , Zeng Zhenhua , Zhu Jing , Merzdorf Thomas , Klingenhof Malte , Li Wei-Xue , Greeley Jeffrey , Strasser Peter
Published: November 2022

The NiFe layered double hydroxides (LDHs) are among the most active electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes.1 The incorporation of Fe dramatically increases the catalytic activity of Ni hydroxides.2 However, even the most active NiFe LDH catalyst still shows a considerable overpotential for the OER. Understanding the nature of the catalytic active sites and the catalytic mechanism are key challenges to develop better OER electrocatalysts.

In this contribution, atomic-scale details of the catalytic active phase will be presented, showing that NiFe LDHs are oxidized under applied anodic potentials from as-prepared α-phases to activated γ-phases.3 The interlayer and in-plane lattice parameters of the OER-active γ-phase were obtained by performing wide angle X-ray scattering (WAXS) measurements on NiFe LDH nanoplatelets during operating electrochemical conditions and were characterized by a contraction of about 8%. Operando WAXS was then performed for other selected catalysts belonging to the transition metal LDH family of materials. Structural similarities of the catalytically active phases will be highlighted.

Finally, in order to derive activity-structure relationships, an approach is presented to calculate intrinsic catalytic activities, which are challenging to evaluate for this class of materials. The presented method is based on electrochemical active surface area normalization obtained by impedance spectroscopy measurements under OER.4

References

F. Dionigi and P. Strasser, Advanced Energy Materials, 2016, 6 1600621.

L. Trotochaud, S. L. Young, J. K. Ranney and S. W. Boettcher, Journal of the American Chemical Society, 2014, 136: 6744-6753.

F. Dionigi, Z. Zeng, I. Sinev, T. Merzdorf, S. Deshpande, M. B. Lopez, S. Kunze, I. Zegkinoglou, H. Sarodnik, D. Fan, A. Bergmann, J. Drnec, J. Ferreira de Araujo, M. Gliech, D. Teschner, J. Zhu, W.-X. Li, J. Greeley, B. Roldan Cuenya and P. Strasser, Nat Commun, 2020, 11 2522.

F. Dionigi, J. Zhu, Z. Zeng, T. Merzdorf, H. Sarodnik, M. Gliech, L. Pan, W.-X. , Li, J. Greeley, and P. Strasser, Angew Chem Int Edit, 60, 14446 – 14457 (2021).
(Invited) Electrocatalyst Development for Efficient CO2 Electrolysis
Authors: Kauffman Douglas
Published: November 2022

This talk will discuss recent efforts at the National Energy Technology Laboratory (NETL) to develop efficient electrocatalysts that convert CO2 and water into C1 products, such as carbon monoxide (CO) and formic acid (HCOOH). The first example combines advanced surface science techniques, computational modeling, and electrochemical evaluation to track the size-dependent CO2 reduction vs. HER selectivity of silver nanocatalysts with sub-nanometer resolution. Understanding size-dependent catalytic properties is important to maximize the utilization of costly, state-of-the-art catalyst materials. Experimental efforts synthesized and evaluated Ag nanocatalysts with controlled diameters between 2-6 nm, and we identified ~4 nm Ag particles as the optimum balance of activity, selectivity, and catalyst utilization. Computational modeling techniques, including DFT, transition state theory, and microkinetic modeling identified key catalytically active surface sites that were responsible for HER vs. CO2 reduction, determined the respective active sites’ population as a function of particle size, and then predicted HER vs. CO2 reduction rates and selectivity for Ag nanocatalyst with diameters between 1-10 nm. Our computational results predicted ~4 nm particle provided the highest combination of product selectivity, catalyst utilization, and catalytic activity based on a total metal basis, which was in near perfect agreement with experiential results. These experimental and computational results should guide future catalyst design efforts by placing a lower bound on the size of practical Ag catalysts. A second example will highlight efforts to develop 3D structured and chemically-doped SnO2 catalysts for the conversion of CO2 into formate and formic acid. We coupled novel synthetic techniques with advanced synchrotron characterization, computational modeling, and evaluation MEA-style electrolyzer devices. In the case of 3D structured SnO2 catalysts we found CO2 conversion activity was substantially improved by increasing the electrocatalytically active surface area, while doping SnO2 catalysts with heteroatoms created new active sites that lowered the CO2 conversion overpotential and boosted performance.
(Invited) Perspectives of Advanced Electrolysis for Green Chemicals Manufacturing
Authors: Xu Hui , Macauley Natalia , Zhang Tianyu , Zhong Sichen , Pupucevski Max , Lattimer Judith
Published: November 2022

An electrolyzer can utilize “off peak” electricity from solar or wind farms to produce hydrogen or other fuels. These chemicals can subsequently be operated in a fuel cell mode to generate electricity or used for other industrial applications. The power to green chemicals alone is poised to become a multi-billion-dollar market for on-site water electrolysis systems over the next decade. Some examples include green H2, NH3 or hydrocarbon fuels (e.g., methanol, ethanol, and ethylene)

However, current green chemical production from electrolysis has not achieved viable commercialization due to expensive materials and low efficiency even if “free” electricity from renewable energy can be secured. Giner has been a world leader in researching, developing and manufacturing electrolyzers for green chemical synthesis. We have been committed to addressing the challenges of materials (catalyst, membrane, and bipolar plates) and processing conditions to improve electrolyzer efficiency, extend lifetime, and lower capital cost. These efforts include: 1) developing low-cost durable ion exchange membranes; 2) lowering platinum-group metal (PGM) catalyst loading; 2) discovering non-precious metal catalysts; 4) increasing corrosion resistance of separators and other hardware; 5) optimizing operating conditions.

These efforts have therefore enhanced the conversion rate and process efficiency. For example, the electrochemical production of ammonia from nitrogen and water using rationally designed catalysts can lead to tremendous energy savings compared to conventional Haber-Bosch process. The electrochemical conversion of CO2 and water to ethylene can be significantly boosted by novel membrane design and optimizing optimal operating conditions. Due to high efficiency and lowered cost, the electrolysis technologies for green chemical production may become strongly competitive and complementary to conventional chemical manufactuaring industry.
(Invited) Design of Ceramic Fuel Electrode for Solid Oxide Cells for Direct Oxidation of Hydrocarbon Fuels and for Direct CO2 Electrolysis
Authors: Chen Fanglin (Frank) (Frank)
Published: November 2022

Large scale consumption of fossil fuels has led to the rapid increase in the atmospheric CO2 concentration, resulting in very adverse environmental concerns and frequent natural disasters. There is a grand challenge to improve conversion efficiency of fossil fuels and mitigate CO2 emmissions for terrestial applications. To enable long-term manned space exploration, there are critical needs in recycling CO2 to oxygen for life support and in situ generation of propellants using CO2 feedstock for ascent vehicles. Solid oxide cells (SOCs) have a great promise for energy conversion and storage for both terrestrial applications and space explorations. In the fuel cell mode, SOCs can convert chemical energy to electrical energy with high efficiency and fuel flexibility. In the electrolysis mode, SOCs can efficiently utilize CO2 as feedstock for chemical synthesis. The conventional Ni-based fuel electrode is vulnerable to deactivation by carbon build-up (coking) from direct oxidation of hydrocarbon fuels or for direct electrolysis of CO2, and suffers volume instability on redox cycling. This talk will report novel heterogeneous functional materials as SOC fuel electrodes that possess a combined property of good coking resistance and redox cyclability, enabling SOCs to be operated on hydrocarbon fuels for electricity generation in the fuel cell mode and for direct CO2 electrolysis for fuel synthesis in the electrolysis mode.

The general philosophy to design novel SOC fuel electrodes is to develop ceramic materials that are redox-stable, possess mixed ionic and electronic mixed conductivity, and have catalytic activities for fuel oxidation and CO2 splitting reaction. Different types of ceramic fuel electrode materials have been explored and characterized as SOC fuel electrodes. The phase formation, redox-stability, electrical conductivity, electrochemical performance, and tolerance to coking and redox-cycling have been systematically evaluated. By judicious design of ceramic-based heterogeneous functional materials, high performance redox-flexible ceramic fuel electrodes can be achieved for direct oxidation of hydrocarbon fuels and for direct CO2 electrolysis.

Acknowledgements

Financial support from the U.S. Department of Energy (DE-EE0009427), National Science Foundation (DMR–1832809) and NASA EPSCoR (Grant # 80NSSC20M0233) is greatly appreciated.
Confined Oxygen Promotes Radical Generation for Methane Oxidation Toward Liquid Oxygenates
Authors: Oh Choeulwoo , Oh Hyung-Suk
Published: November 2022

The activation of methane (CH4) gas to produce value-added liquid hydrocarbons has been a challenging issue in catalytic research. Electrochemical oxidation is one of the alternative methods to oxidize methane and can be usually performed at ambient temperature, which can solve the problems of methane steam reforming caused by high temperature. However, electrochemical direct methane oxidation still have limitations that it should competes with oxygen evolution reaction, resulting in low efficiency. Also, oxygenates generated from methane oxidation can be easily overoxidized to carbon dioxide which is not appropriate product for the purpose of electrochemical system to make liquid hydrocarbons , suggesting solution to transport problem in the industry.

Therefore, we suggest new system to convert methane into liquid products via oxygen reduction reaction using cathode. Recently in some papers, oxygen is used as an oxidant in photocatalytic system to oxidize methane into liquid hydrocarbons. Herein, hydrogen peroxide (H2O2) generated from oxygen reduction reaction can produce hydroxyl radical, superoxide radical, and hydroperoxide radical with strong activity to break first C-H bond from CH4. By combining methyl radical activated from methane and radicals generated from H2O2, we could make formic acid with high selectivity. Through the reduction reaction on cathode and oxidation reaction by radicals, intermediate products, such as methanol and methyl hydroperoxide, were finally converted into formic acid. Furthermore, overoxidation, one of important limitation in direct methane oxidation, can be prevented by using cathode where reduction reaction occurs on electrode.

In this work, we synthesized metal organic framework (MOF)-derived cobalt single atom using ZIF-67. The cobalt single atom has high faradaic efficiency (FE) in acidic media with low onset potential, which is over 80% at 0.5 V (vs RHE). To efficiently utilize H2O2 produced by oxygen reduction reaction, additional hydrophobic layer was introduced with gas diffusion electrode (GDE) by coating poly tetra fluoro ethylene (PTFE) on catalyst layer. Additional PTFE layer on GDE confined H2O2 and CH4 together, offering higher possibilities to be reacted. In addition, liquid oxygenates generated from the reaction have hydrophilic properties that prevents further reactions by reducing the diffusion of liquid products toward cathode due to hydrophobic layer. By applying new system into flow cell, the low solubility of methane in the electrolyte can be alleviated to achieve high production rate. At last, This new system using oxygen reduction reaction to oxidize methane can make a new pathway for many chemical reaction area in the point that the system can be applied in any industrial reactions where H2O2 included.

Figure 1
Sp-D Orbital Hybridization Driven Metal-Graphene Catalysts for Electrochemical CO2 Reduction to Formic Acid
Authors: Cho Jinwon , Choi Ji Il , Drexler Matthew N , Alamgir Faisal M. , Jang Seung Soon
Published: November 2022

The efficient transformation of readily-available CO2 into renewable energy sources is a crucial for the decarbonization. Formic acid (FA), the simplest combination of hydrogen and CO2, is one of the most promising hydrogen storage compounds for its high hydrogen density, non-toxicity, and great stability under ambient temperature and pressure. However, thermochemical conversion of CO2 to FA on the noble metal and metal alloy catalysts has been a challenge for a decade since the reaction is thermodynamically unfavorable and kinetically difficult due to the nature of relatively weak adsorption of CO2 on the metal surfaces in comparison to HCOO, and thus requires very high pressure/temperature. Electrochemical conversion of CO2 on the conventional carbon black supported noble metal and metal alloy catalysts, in contrast, operates at ambient conditions, but suffer low stability through dissolution and agglomeration, and requires high overpotentials to overcome a competitive hydrogen evolution reaction (HER). In this context, we investigate a highly active and stable electrocatalyst obtained by depositing transition metal monolayer on graphene (M/Gr) using DFT method, with several experimental whenever tractable. It is found that the hybridization of sp and d orbitals between graphene and metal significantly reduces the local density of states of d-band of metals and increases s-and p-orbital of graphene near the fermi level at the same time, which leads to a strong covalent bonding between metal monolayer and graphene. The covalent bonding manifests in an epitaxial-like metal growth on Gr, which has been experimentally observed for Pt/Gr, Au/Gr and Pd/Gr systems. The calculation of the orbital-resolved density of states for d band of M specifically reveals that sharp dz
2, dyz, and dxz orbital peaks below the fermi level play a decisive role in the formation of covalent bonding between M and Gr. In addition, it is found that the charge polarization on graphene in Pt/Gr and Pd/Gr not only enables a deposition of another metallic monolayer on the graphene, thus forming Pt/Gr/M or Pd/Gr/M sandwich structures, but also induces a transition from physisorption to chemisorption for M = Ag, Au, and Cu cases. Finally, by evaluating the overpotentials required for CO2 reduction to FA, it is discovered that, among M/Gr, Pt/Gr/M, and Pd/Gr/M electrocatalysts, Ni/Gr, Pd/Gr, Pt/Gr/Ag, and Pt/Gr have excellent activity and selectivity toward electrosynthesis of FA.
(Invited) Practical Assessment of the Electrochemical Conversion of Methane to Methanol at Scale
Authors: Soucie Hanna , Elam Matthew , Mustain William Earl
Published: November 2022

Methane is the second-most emitted greenhouse gas (GHG) globally, and it has over 25x more potency toward the GHG effect than carbon dioxide. Methane emissions in the U.S. are equally distributed across the energy, agricultural, and industrial sectors – and occur in relatively small volumes at each site, though the total volume is sizable (e.g., 145 billion cubic meters of methane coextracted with U.S. oil alone per year). To avoid its direct emission, this “waste” methane is typically flared to produce CO2. Methane flaring sites contribute to approximately 2% of global CO2 emissions. These sites represent an opportunity to convert a waste stream into desirable products. Methane (natural gas)-to-chemicals conversion units are not implemented today because of economies of scale. Because each site is relatively small, the construction of a traditional stream-reforming based gas-to-liquid plant is not economically viable. It is also not profitable to build pipelines to these sites to take the natural gas to other locations.

Recently, new concepts have been put forward for methanol production that are quoted as being more modular and possibly more profitable at smaller scales. Many of these concepts revolve around electrochemical and/or biochemical transformations. Proponents of electrochemical methane conversion have claimed that the technology will play a key role in eliminating these flare sites while opening up new business opportunities. Though this sounds very good, all of the research and development in this area remain on the bench scale as they have widely suffered from low current, high voltage losses and low faradaic efficiency. It should also be noted that there are very few studies in the literature that use economic analyses to understand the operational boundaries of a future electrochemical methane-to-methanol plant and to set realistic targets for its operation. Hence, research work is proceeding without clear operating targets in mind.

In this presentation, we will present an economic analysis on an electrochemically driven methane-to-methanol plant. We investigate a wide range of possible production rates (from 2-2000 metric tons per day), operating current densities (10-1000 mA/cm2), operating voltages (Vthermo-2.0 V) and faradaic efficiencies. This analysis also takes into consideration the cost of the electrolyzer stacks, the balance-of-plant, building sizes/layouts, labor force, etc. Over this wide design space, we determine the possible operating space that will allow for profitable operation. While doing so, we test whether electrochemical processes can economically out-compete steam reforming plants at any scale. We also identify the operating modes where electrochemical processes emit fewer greenhouse gases that traditional plants.
Insights into the Role of Electrolyte Ionophore on Electrochemical Reduction of CO2
Authors: Tran Tam , Zhang Laibao , Xu Nengneng , Xia Guanguang , Sahu Sushant , Wang Yudong , Yu Xingwen , Zhou Xiao-Dong
Published: November 2022

Electrochemical reduction of carbon dioxide (CO2) to value-added chemicals and fuels offers a sustainable route to utilize CO2 by exploiting renewable energy source. The electrolyte plays a significant role by taking advantage of the interaction between electrocatalytic surface and electrolyte. In this work, we present a CO2 conversion flow cell with an external compartment for a reference electrode (AgCl/Ag) to investigate the effect of alkaline electrolyte composition on the direct conversion of CO2 to fuels and commodity chemicals under ambient conditions. In this flow cell, a catalytic Ag-based gas diffusion electrode (GDE) was employed for CO2 reduction reaction, while an anode electrode was prepared with a RuO2 catalytic GDE. The gaseous products generated from the cell were analyzed with a Gas Chromatography (GC), while the liquid products were analyzed with Nuclear Magnetic Resonance Spectroscopy (NMR). Electrolytes with various cations (Li+, Na+, K+) and anions (OH-, HCO3
-, CO3
2-) were investigated. With a KOH electrolyte (combination of the cation K+ and anion OH-), high faradaic efficiency (~ 97%) was achieved at -1.115 V vs SHE toward the formation of syngas (CO + H2) and ~ 60% faradaic efficiency was obtained toward the product of ethanol. The overall energy conversion efficiency could be up to ~ 45%. In consideration of faradaic efficiency, energy conversion efficiency, and the production of CO and ethanol, the preferable option of anions followed the order of OH- > HCO3
- > CO3
2-, while the option for cations was in the order of K+ > Na+ > Li+.
CO2 Electroreduction to Fuels Using Solid Oxide Electrodes: Beyond Ni-YSZ
Authors: Kamboj Vipin , Ranjan Chinmoy
Published: November 2022

Ni-YSZ electrodes form the benchmark in Carbon Dioxide electroreduction to CO. Ni-YSZ due its popular use in Solid Oxide Fuel Cells forms the basis of various process steps. Unfortunately, this material remains active only if a certain amount of hydrogen is supplied along side CO2 to the inlet of the reaction. Consequently, the Ni-YSZ results in CO production via reverse water gas shift (RWGS) reaction when H2 is supplied at the inlet. The currents for the electrode originate from electrolysis of H2O which is formed as a result of reverse water has shift. We have quantified the amount of CO produced from RWGS vs direct CO2 electrolysis in such a system using online mass spectrometry (MS). Electrochemical impedance spectroscopy measurements where analysed using distribution of relaxation times analysis. Under low concentration of H2 the overall impedance is dominated by H2 mass transfer. Effects of concentration of various reactants were measured using online MS. The process of pretreatment of the electrodes and catalyst surface development during reaction were tracked using in situ Raman Spectroscopy and optical spectrocopy. Electrodes were structurally characterised using SEM, TEM, XPS and ICP-OES measurememts. Various new combination of mixed metal oxide electrodes Ni(M)-YSZ were tested and compared with the performance of pure Ni-YSZ. Ni-YSZ electrode when used under pure CO2 results in catastrophic failure of the electrode performance. We have investigated this failure using in situ Raman and Mass spectrometry. The mechanism of CO2 production in pure CO2 stream was found to be drastically different from when a small amount of H2 is used.

Figure 1
Doping Strategies on Improving the Hydration of Doped-BaZrO3 for Hydrogen Production in Proton-Conducting Solid Oxide Electrolysis Cells (p-SOEC)
Authors: Regalado Vera Clarita Yosune , Ding Hanping , Urban-Klaehn Jagoda , Zhou Meng , Luo Hongmei , Ding Dong
Published: November 2022

The proton-conducting electrolyte is a key component for solid oxide electrochemical cells for delivering high-efficient hydrogen production or electricity generation. BaZrO3-based materials have been widely used due to their high proton conductivity and chemical stability. However, some issues such as low performance and electronic leakage can affect their application. In this work, we introduced a type of B-site dopant to induce higher proton concentration in BaZrO3. The proton conduction and hydration behavior as a function of dopant level are studied and compared with the conventional benchmarking electrolyte, 20 mol% yttrium doped BaZrO3 (BZY20). The underlaying mechanism for achieving a higher level of hydration is investigated by various spectroscopy techniques. The electrochemical performances are also evaluated to compare the electrolysis performance and Faradaic efficiency of p-SOEC at different operating conditions to prove the feasibility of using this new electrolyte composition.
(Digital Presentation) A Two-Electron Transition Metal Based Redox Mediator for Dye-Sensitized Solar Cells
Authors: Dalpati Niharika , Kour Ravinder , Delcamp Jared H , Farnum Byron H
Published: November 2022

Since the first report of a Dye Sensitized Solar Cells (DSSC), I3
-/I- has been as staple redox mediator because of its efficient 2e- redox cycle. However, tuning this mediator is challenging given its atomic nature. Here we present the combination of a multielectron redox cycle with a tunable transition metal complex as an alternative redox mediator in DSSCs. This research focuses on using the [NiIV(R2dtc)3]+/NiII(R2dtc)2 redox couple, where dtc- is a bidentate dithiocarbamate ligand with R = ethyl and butyl groups. DSSC device measurements under low light intensity and 1 sun illumination showed the highest efficiency of 8% at 10,000 lux for [NiIV/II(Et2dtc)3/2]+/0, whereas [NiIV/II(Bu2dtc)3/2]+/0 displayed a high of 20% at 4660 lux. To investigate the individual roles of reduced and oxidized mediators, comparative photoelectrochemical studies between reduced mediators, Ni(dtc)2 and I- were performed in a photoelectrochemical cell with an external white light LED along with a photoanode consisting of FTO/TiO2 sensitized with the N3 dye. Ni(Et2dtc)2 and Ni(Bu2dtc)2 showed 75 μA and 100 μA more photocurrent respectively than iodide in the case of the highest irradiance (100 mW/cm2). These studies show the NiII(R2dtc)2 is a superior mediator for dye regeneration. Likewise, the photovoltage was much greater in the case of nickel mediators, NiII(Et2dtc)2 (800 mV) and NiII(Bu2dtc)2 (860 mV) than iodide (560 mV), consistent with ERedox = 0.02 V for the Ni(IV/II) couple, which is 0.32 V greater than ERedox for I3
-/I-. Surprisingly, as [NiIV(R2dtc)3]+ was introduced into the electrolyte the observed photocurrent became dynamic with time and continued to show photoanodic current even at 100% NiIV. These results are consistent with [NiIV(R2dtc)3]+ photodissociation to yield NiII(dtc)2. Preliminary studies on this photodissociation have been done by a UV-Vis kinetic analysis in various solvents. [NiIV(R2dtc)3]+ showed a prolonged dissociation in dichloromethane and chloroform while dissociation was much quicker in acetonitrile. An unknown intermediate was observed in the case of alcohol solvents which suggests a two-step mechanism for dissociation. Taken together, NiIV/II represents alternative redox mediator with outstanding dye regeneration capabilities, but complications due to photodissociation of [NiIV(R2dtc)3]+. Further studies are underway to investigate the impact of photodissociation on the redox cycle and electron-hole recombination in DSSCs.
Pt Island Decorated Au Anode for Formic Acid Oxidation Reaction
Authors: Kim Hyunki , Ahn Sang Hyun
Published: November 2022

For direct formic acid fuel cell (DFAFC), Pt-based catalyst is widely used as an anode. The Pt catalyst shows high activity and stability for formic acid oxidation reaction (FAOR) in acidic condition. For FAOR, it is generally accepted that Pt follows dual pathway including direct and indirect pathway. During the indirect pathway the Pt surface is vulnerable to CO poisoning, which cause the activity degradation. To enhance the CO tolerance, various Pt-based bi/ternary catalysts have been investigated. In recent, Pt–Au alloy catalysts have reported higher current density for direct FAOR pathway compared with pure Pt catalyst. The enhanced FAOR activity is originated from lower CO adsorption energy. However, the alloy structure has low catalyst utilization, which is not applicable to practical device.

In this study, we report Pt islands decorated porous Au electrode for anode of the DFAFC. The porous Au transport layer was fabricated on carbon fiber by galvanic displacement and subsequent electrochemical etching process. Using Au transport layer as a substrate, Pt self-terminated electrodeposition (SED) was performed. By adjusting the SED condition, the Pt loading and coverage on Au surface cab be easily controlled. The FAOR activity test of Pt–Au electrodes was performed by cyclic voltammetry (CV). It was revealed that the current density for direct FAOR is linearly increased as the Pt coverage on Au surface is lower. By increasing the CV cycles, the current density for direct FAOR is gradually increased owing to enhanced ensemble effect. Furthermore, the cycled electrode was applicated into practical DFAFC single cell. The single cell demonstrates more than two-order higher Pt mass activity compared with previously reported single cell using Pt–Au anode.
Promoting Oxygen Evolution Reaction Induced By Synergetic Geometric and Electronic Effects of Irco Thin-Film Electrocatalysts
Authors: Kim Kyu-Su , Park Shin-Ae , Jung Hyun Dong , Jung Sang-Mun , Woo Hyunje , Ahn Docheon , Park Sarah S. , Back Seoin , Kim Yong-Tae
Published: November 2022

The advancement of water electrolysis technology has seemingly plateaued. Further advances require new strategies to address the key limitations of the oxygen evolution reaction: high overpotential and low stability of electrode materials. Herein, we designed a nanoporous Ir3Co-core@IrO2-shell electrocatalyst with 5 and 3 times higher mass activity and 6 and 2 times higher activity−stability factors than conventional IrO2 nanoparticles and Ir nanoporous electrocatalysts, respectively. The origin of the performance enhancement in the Ir3Co-core@IrO2-shell electrocatalyst was revealed in computational density functional theory calculations. The residual Co after the dealloying process resulted in a down-shift of the d-band center position, thus weakening the −OH* adsorption energy. In addition, the electronic conductivity was enhanced by the three-dimensionally interconnected structure. Under high-current-density operating conditions, the Ohmic losses were reduced by the ultrafast charge transfer pathway provided by the metallic IrCo core.
The Effects of Operating Conditions on the Electrode-Specific Potentials and Overall Performance of a Polymer Exchange Membrane Water Electrolyzer
Authors: Abdelrahman Mohamed Elfadil , Pasmay Fausto N , Komini Babu Siddharth , Mukundan Rangachary , Litster Shawn
Published: November 2022

Proton exchange member water electrolysis (PEMWE) is a crucial technology for the future of clean, sustainable energy systems. However, PEMWE cells use iridium metal as the anode catalyst, which is scarce and prone to high prices and price volatility. To address this problem, researchers have investigated different methods to increase catalyst utilization, thereby reducing catalyst loading and increasing overall MEA durability. One of the governing factors of the durability of both the cathode and anode catalyst is the redox state of the metal catalyst since Pt-dissolution occurs between ~0.9 and 1 V in the acidic conditions of the cell, and IrO2 reduces to Irmetal at potentials below ~1 V which is less stable than the oxide [1]. Past research has found that hydrogen crossover is one of the leading contributors to IrOx reduction since the overall cell potential goes to 0 V when held idle for a while [2]. This study used a custom-made hydrogen reference electrode to deconvolve the electrode-specific potentials under various operating conditions. It was found that anode catalyst loading, feedwater dissolved gas concentration, and cell voltage affect the electrode-specific potentials and the overall cell performance.

[1] S. Song et al. 2008 Int J of Hydrogen Energy 2008;33:4955–61

[2] P. J. Rheinländer and J. Durst 2021 J. Electrochem. Soc. 168 024511
(Invited) Zero-Thermal-Quenching in a Nasicon-Type Phosphor By Na+ Ion Migration
Authors: Noolu Viswanath , Han Joo Hyeong , Im Won Bin
Published: November 2022

Among the many potential Eu2+-activated sodium superionic conductor (NASICON)-based host materials, the Sc3+-based NASICON phosphor (Na3Sc2(PO4)3:Eu2+) is a promising phosphor material for high-power lighting applications owing to its unusual thermal stability at elevated temperatures. It has previously been shown that negative thermal quenching (TQ) can be tailored to zero TQ depending on the Eu2+ concentration. However, the obtained zero-TQ composition has low photoluminescent quantum yields, which hinders its applicability to high-power lighting. Herein, we report a holistic study of the tuning of thermal stability from negative TQ to zero-TQ while preserving the original emission efficiency by introducing Lu3+ ions in Na3Sc2(PO4)3:Eu2+. Furthermore, we fabricated a high-power white light emitting diode (WLED) using optimized Lu3+-doped Na3Sc2(PO4)3:Eu2+ as the blue component, delivering a high color rendering index value of 90 with a high luminous efficacy value of 25 lm/W obtained at a flux current of 1000 mA. Therefore, the findings of this work provide novel scientific insight into the importance of structure-property relationships in designing highly efficient thermally stable phosphors for high-power lighting applications.