bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of Surface States and Mobility on Charge Transport in BiVO4 Investigated By Surface Photovoltage Spectroscopy
Authors: Fengler Steffen , Emmler Thomas , Wolpert Charline , Schieda Mauricio , Villa Vidaller María , Klassen Thomas , Dittrich Thomas
Published: May 2020

In order to enable mass production of photocatalytically active electrodes for direct conversion of solar energy into hydrogen, abundant materials and inexpensive preparation methods are needed. BiVO4 photoelectrodes prepared by aerosol deposition satisfy these requirements, in addition offering reasonable photocurrent densities, and can potentially contribute to the establishment of a prospective economy based on hydrogen as energy source.

Nevertheless, in order to allow the economically feasible implementation of BiVO4 electrodes for direct water splitting, there is need for further improvement of the electrodes efficiency.

In particular, an increased understanding of factors limiting the photocurrent is needed. In this work, BiVO4 photoelectrodes, prepared by aerosol deposition of BiVO4 particles on FTO-substrates, were investigated by surface photovoltage (SPV) as well as photocurrent and Mott-Schottky measurements.

Surface photovoltage spectroscopy under front and back illumination gives evidence of limitation of charge transport by a significant lower mobility of electrons compared to holes, leading to a nearly tenfold higher photocurrent, when illuminated from the back, compared to illumination from the front. Furthermore, the presence of traps for holes as well as for electrons on the BiVO4 surface has been confirmed by transient surface photovoltage spectroscopy and their influence on the photocurrent is discussed.

Figure 1
(Invited) In-Depth Understanding of Photoelectrochemical Water Splitting Systems By Operando Electrochemical Techniques
Authors: Tilley David
Published: May 2020

In this talk, I will show how operando electrochemical techniques can be used to gain a deeper understanding of the underlying photophysical charge carrier processes taking place in photoelectrodes under practical water splitting conditions. High performance semiconductor-based electrodes are often multilayer systems (for maximizing the photovoltage, stability, etc.), and the relatively high number of interfaces in these devices makes characterization challenging. The characterization of both photocathodes and photoanodes will be discussed utilizing the dual working electrode (DWE) technique as well as photoelectrochemical impedance spectroscopy (PEIS). Using a certain architecture of DWE that preserves the catalytic interface, the performance of water splitting photoelectrodes can be de-convoluted to observe changes in the photovoltaic output separately from changes in the surface catalyst, rapidly identifying mechanisms of degradation in these systems.[1] This technique can also be used to detect the band position of the surface layer under operation in photoanode systems, which provided evidence for a certain mechanism of charge transfer.[2] Finally, I will show how PEIS can be used to characterize complex systems such as Au/Cu2O/Ga2O3/TiO2/RuOx, identifying the problematic interfaces in this promising system.[3]

[1] Cui, W.; Niu, W.; Wick-Joliat, R.; Moehl, T.; Tilley, S. D.* Operando Deconvolution of Photovoltaic and Electrocatalytic Performance in ALD TiO2 Protected Water Splitting Photocathodes Chem. Sci.,
2018,
9,6062-6067. doi: 10.1039/C8SC01453A

[2] Cui, W.; Moehl, T.; Siol, S.; Tilley, S. D.*
Operando electrochemical study of charge carrier processes in water splitting photoanodes protected by atomic layer deposited TiO2
Sustainable Energy Fuels
2019,
3, 3085 - 3092. doi: 10.1039/C9SE00399A

[3] Moehl, T.; Cui, W.; Wick-Joliat, R.; Tilley, S. D.* Resistance-based analysis of limiting interfaces in multilayer water splitting photocathodes by impedance spectroscopy Sustainable Energy Fuels,
2019, 3, 2067–2075. doi: 10.1039/C9SE00248K
Revealing the Surface Corrosion Chemistry and Kinetic Stabilization at Photoelectrochemical Interfaces for Solar-Driven Water-Splitting
Authors: Yu Weilai , Lewis Nathan S
Published: May 2020

Stability is a crucial consideration for practical applications of integrated solar-fuel devices. For solar water-splitting reactions, the requirement of strong acidic/alkaline electrolytes poses serious concerns over the materials stability of semiconductors, electrocatalysts and membranes. Especially, the stability of semiconductor materials can be even more complicated due to its dependence on applied potentials, ambient environment and light/dark conditions etc. However, systematic understandings based on experimental results are lacking for many materials. Herein, we would like to address the material stability for a series of 3-5 semiconductors (InP, GaP and GaAs etc.) used as photocathodes for the hydrogen-evolution reaction (HER) at pH=0/14. To minimize any impact of ambient oxygen, the long-term corrosion evaluation experiments and post-electrochemistry XPS surface characterizations were carefully performed under air-free conditions. The electrode-to-electrolyte leaching of ions over time was quantitatively analyzed by ICP-MS. Combined with considerations of the Pourbaix diagram, both the thermodynamics and kinetics of existing corrosion reactions can be experimentally determined. Significant differences in material stability and surface conditions were revealed between bare semiconductor surfaces and semiconductor/catalyst interfaces, where a surface kinetic stabilization effect has been emphasized. Based on the current understanding, we demonstrate significant promises of these semiconductors to be used as robust light-absorbers for solar water-splitting.
Stable Photoelectrochemical Hydrogen Evolution for 1000 Hours at 14% Efficiency in a Monolithic Device
Authors: Kistler Tobias A , Um Min Young , Danilovic Nemanja , Agbo Peter
Published: May 2020

The development of a fully-integrated, photoelectrochemical (PEC) device coupling water oxidation to hydrogen evolution using a III-V triple-junction photovoltaic (PV) embedded in a Nafion membrane is reported. This architecture is genuinely monolithic, with wireless catalyst integration being achieved via compression of metal sputter-coated, carbon electrodes against the front and back PV contacts. The resulting MEA-type, sandwich structure minimizes the path length for proton conduction through the membrane ionomer, while simultaneously preventing PV light attenuation by the catalyst layer, a common issue for monolithic PEC structures. While the wireless nature of monolithic PEC devices typically prevents the measurement of current flow and faradaic efficiencies, we circumvent this complication through the placement of an electrical shunt between the PV and the cathode catalyst layer, rerouting charge generated at the PV through a potentiostat prior to cathodic proton reduction. Using this device architecture, we demonstrate significant enhancements in device stability and longevity, by transitioning from a liquid-water to water-vapor anode. Our use of a gas-fed anode enables 1000 hours of cumulative device operation at a peak solar-to-hydrogen efficiency of 14%, during simulated, solar illumination at 1 sun and outdoor, diurnal cycling. Vapor-fed water oxidation is shown to reduce drops in device performance by mitigating the corrosion effects that are commonly associated with full-aqueous immersion of the electrochemical and photovoltaic elements in PEC devices. Causes for observed fluctuations in device performance are determined through the collection of real-time, current-voltage data, paired with an analytical method permitting the deconvolution of PV-driven and catalyst-driven performance losses.

Figure 1
Optimization of Hematite to Improve PEC Performance in Water Splitting
Authors: Diale Mmantsae
Published: May 2020

Thus far, semiconductor materials explored to achieve theoretical solar to hydrogen efficiency (STH), included cadmium selenide (CdSe), zinc oxide (ZnO), copper(I)oxide (Cu2O), tungsten trioxide (WO3) and hematite (α-Fe2O3). Of these materials, hematite has received much attention for photoelectrochemical water splitting attributed to its stability in aqueous solution, a small band gap of 1.90 eV-2.20 eV, non-toxicity and abundance. However, it has associated limitations such as high electron-hole recombination rate, short hole diffusion length (2-4 nm) accompanied by short excited lifetime of 10 ps, and poor minority charge carrier mobility of 0.1 cm2 V-1s-1, leading to low photocurrent during water splitting. Furthermore, hematite promises a maximum theoretical STH efficiency of ~16.8 %, with photocurrent densities above 12 mAcm-2, but the reported findings are far below 10 mAcm-2. Using interfacial layers , annealing and doping, we have prolonged lifetimes and increased the photocurrent. In this work, hematite thin films and nanostructures were synthesized using dip coating and spray pyrolysis.
(Invited) Electro-Chemo-Mechanical Equilibrium of Point Defects at Interfaces
Authors: Chueh William C.
Published: May 2020

Formation and migration of point defects underlie nearly all materials for energy transformation, such as electrodes and solid electrolytes for lithium-ion batteries and solid oxide fuel cells. The distribution of point defects such as lithium and oxygen vacancies reflect electro-chemo-mechanical equilibria. In this talk, I will present a general theory framework for understanding how mechanical contribution plays a key role in determining the space-charge effect at interfaces.
(Invited) Strain Effects on Li Ion Conductivity – Migration, Defect Correlations, and Structural Changes
Authors: Zguns Pjotrs , Yildiz Bilge
Published: May 2020

Elastic or plastic strain at cathode/electrolyte interfaces can affect the charge transfer kinetics in all-solid-state Li-ion batteries. Quantifying this effect both for Li ion conduction within the solid electrolyte material and for the Li transfer ability at the interface is important for understanding the kinetics of solid state batteries. Depending on the strain tensor, the impact could be beneficial or, alternatively, detrimental. While it is generally known that isotropic tensile strain increases the ionic conductivity and compressive strain decreases it [1], how different strain fields act upon Li ion conduction is not yet assessed. Existing studies show a non-trivial decrease/increase of Li-ion migration energies depending on relative alignment of migration direction and strain field [2], making quantitative estimates of conductivity changes difficult to predict. Here, we study the effect of strain on Li ion conductivity of β-Li3PS4 by means of Ab Initio Molecular Dynamics (AIMD) simulations. Ionic conductivity is directly calculated from AIMD trajectories using Nernst–Einstein equation, as it is linearly proportional to the diffusivity of the center-of-mass of all Li-ions [3]. Our calculations indicate that biaxial strain could substantially promote correlated motions of Li-ions, which in turn could increase ionic conductivity of β-Li3PS4. The effect depends on direction of applied strain, and the underlying structural changes increasing ionic conductivity are discussed.

[1] Y. Wang, W.D. Richards, S.P. Ong, L.J. Miara, J.C. Kim, Y. Mo and G. Ceder, Design principles for solid-state lithium superionic conductors, Nat. Mater.
14
, 1026–1031 (2015)

[2] C. O’Rourke and B. Morgan, Interfacial strain effects on lithium diffusion pathways in the spinel solid electrolyte Li-doped MgAl2O4, Phys. Rev. M
2, 045403 (2018)

[3] A. Marcolongo and N. Marzari, Ionic correlations and failure of Nernst-Einstein relation in solid-state electrolytes, Phys. Rev. M
1, 025402 (2017)
Improving Chemo-Mechanical Stability of Proton-Conducting Perovskites
Authors: Chen Ting , Jing Yuhang , Leonard Kwati , Anderson Lawrence O , Matsumoto Hiroshige , Aluru Narayana R. , Perry Nicola H.
Published: May 2020

The superior ionic conductivity of selected proton-conducting perovskites enables development of intermediate temperature electrochemical devices, such as solid oxide fuel/electrolysis cells, gas separation membranes, and electrochemical reactors. The lower operating temperature, compared to those of most oxide ion-conductors, can benefit the durability of the devices, as most degradation mechanisms are thermally activated. On the other hand, the chemical strains associated with hydration/dehydration during processing or operation can be considerably larger than those seen in their oxide ion-conducting counterparts during oxygen loss/gain. There is therefore a significant need to develop proton conductors with lower coefficients of chemical expansion (CCE), which is the chemical strain normalized to the change in defect (proton) concentration. Since design principles for engineering CCEs are not well developed, we have previously been investigating the role of various structural and chemical features, such as size of the oxygen vacancy and charge localization.

In this presentation, I will focus on our more recent work investigating the role of octahedral distortions and deviations away from the perfect cubic perovskite structure. By substituting differently sized isovalent cations on the A- and B-site of perovskites, it is possible to engineer the tolerance factor to vary the symmetry and distortions. We have therefore synthesized a series of (Ba,Sr)(Zr,Ce)0.9Y0.1O3-x perovskites with gradually varying tolerance factor. To measure hydration strains, isothermal in situ X-ray diffraction and dilatometry were performed at temperature points in the range ~360-680 °C, in varying humidities. To evaluate the corresponding changes in proton concentration, isothermal thermogravimetric analysis was applied under identical conditions; an intermediate, constant oxygen partial pressure was chosen to avoid the possibility of any simultaneous redox reactions contributing significantly to mass changes and strains. The combined results show that by diminishing the perovskite tolerance factor in this family of zirconate/cerate proton conductors, the coefficient of chemical expansion can be lowered by 500%. Further in situ X-ray diffraction analysis of powder vs. bulk samples, and density functional theory simulations of single crystals, have revealed that there are both crystal chemical and microstructural contributions to the trend seen. The results show that inducing octahedral rotations and lowering symmetry, by altering the cation radius ratio, is an effective route to increase chemo-mechanical stability by lowering CCEs.
(Invited) Electrodeposition at Solid-Solid Interfaces
Authors: Mukherjee Partha P. , Verma Ankit , Mistry Aashutosh
Published: May 2020

The development of next generation batteries is predicated on utilization of lithium metal anodes with solid electrolytes. Solid electrolytes offer attractive features of improved safety and reduced predilection for dendritic lithium growth through mechanical rigidity. However, the inherent solid-solid interfaces in these systems are not always stable necessitating fundamental understanding of the electrodeposition characteristics at solid-solid interfaces. In particular, coupled transport-kinetics-mechanics complexations arise with dependence on solid electrolyte structure, and interfacial stochasticity dependent on chemical and mechanical wettabilities. Furthermore, the formation of interphase layer complicates the phase-interface pairs with varying kinetic, transport and mechanical properties. In this presentation, the mechanistic underpinnings of the electrodeposition stability at the solid-solid interface for solid electrolyte and lithium metal systems will be examined.
Heterogeneous Damage in Li-Ion Batteries: Experimental Analysis and Theoretical Modeling
Authors: Zhao Kejie
Published: May 2020

We assess the heterogeneous electrochemistry and mechanics in a composite electrode of commercial batteries using synchrotron X-ray tomography analysis and microstructure-resolved computational modeling. We visualize the morphological defects at multi-scales ranging from the macroscopic composite, particle ensembles, to individual single particles. Particle fracture and interfacial debonding are identified in a large set of tomographic data of active particles. Mechanical failure in the regime near the separator is more severe than toward the current collector. The active particles close to the separator experience deeper charge and discharge over cycles and thus are more mechanically loaded. The difference in the Li activity originates from the polarization of the electrolyte potential and the non-uniform distribution of the activation energy for the charge transfer reaction. We model the kinetics of intergranular fracture and interfacial degradation to confirm that the various Li activities are the major cause of the heterogeneous damage. The interfacial failure may reconstruct the conductive network and redistribute the electrochemical activities that render a dynamic nature of electrochemistry and mechanics evolving over time in the composite electrodes. We further quantify the influence of the mechanical damage on the metrics of battery performance. We simulate the electrochemical impedance profile to build a relationship between the interfacial debonding and the impedance of electron transport and surface charge transfer. The mechanical failure disrupts the conduction path of electrons and results in significant polarization and capacity loss in batteries.
Mechano-Electro-Chemical Insights into the Danger of Using Precious Metal Electrodes for Oxygen Surface Exchange Measurements
Authors: Ma Yuxi , Nicholas Jason D.
Published: May 2020

This talk will explore the effect intentionally added Pt impurities have on the oxygen surface exchange coefficient of praseodymium doped ceria, as determined from wafer curvature measurements.
Engineering of B-Metal Exsolution for High Performance Anode Materials in Solid Oxide Fuel Cells
Authors: Han Jeong Woo , Kim Kyeounghak
Published: May 2020

B-metal exsolution has great potential to synthesize well-distributed metal nanoparticles (NPs) on the surface of oxide support materials. In previous works, we found the different exsolution tendencies of different kinds of transition metals on double-layered PrBaMn2O5+δ
(L-PBMO). In particular, the exsolved NPs showed higher catalytic activity and stability for fuel oxidation, such as dry reforming of methane. To further engineer that concept, we have explored the possibility and detailed mechanism for various kinds of B-metal exsolution on L-PBMO. Based on our density functional theory (DFT) results, we will introduce (1) a possible driving force for B-metal NPs exsolution, (2) thermodynamically favorable mechanism for alloy NPs exsolution, and (3) how to boost B-metal exsolution to further improve cell performance SOFCs with the experimental verifications.

In addition to L-PBMO, we also introduce the bond length of B-metal (Co) with lattice oxygen in perovskite structure (Co-doped SrTiO3, STC) as a key descriptor to represent the degree of Co exsolution on STC. Interestingly, we found that the Co exsolution can be facilitated under biaxial tensile strain (larger bond length of Co-O). By examining the bond length of Co-O under different strain states, we demonstrated that the weaker bond strength of Co-O is originated from the larger bond length of Co-O under tensile strain. This results in easier bond breaking between Co and O, thereby facilitating Co exsolution. To confirm our hypothesis (controlling Co-O bond length), we doped the larger ionic radius of A-site cation (Ba) into Sr in STC to induce tensile strain around Co. As we expected, the exsolution of Co is more preferred than than of Ba on STO, which is a more practical way to induce tensile strain than the mechanically applied strategy. As an application of exsolved Co NPs, we showed enhanced catalytic activity for CO oxidation.
(Invited) Ammonia As an Energy Vector
Authors: Soloveichik Grigorii L.
Published: May 2020

The Advanced Research Projects Agency (ARPA-E) funds high risk, high reward transformational research to reduce energy related emissions, reduce imports of energy from foreign sources, improve energy efficiency across all economic sectors, and ensure US technological lead in advanced energy technologies, including electrochemical technologies for energy transformation and storage for distributed, grid scale and automotive applications.

Ammonia is produced at a scale of about 180 million tons per year and is easily stored and transported. Currently it is mostly used as a fertilizer and a feedstock for production of various chemicals. However, its physical properties, high energy density, the absence of carbon emissions, availability of feedstock (air and water), and well developed synthesis and infrastructure make ammonia almost ideal energy vector. Produced using renewable energy, it can be used for long duration energy storage and as a zero-emission transportation fuel as well as a hydrogen carrier. To address these technological challenges, ARPA-E launched the Renewable Electricity to Fuels through Utilization of Energy-dense Liquids (REFUEL) program. This program and, in part, OPEN program fund the development of transformational technologies for the conversion of intermittent renewable energy, water and air to energy-dense carbon neutral liquid fuels including ammonia and their conversion to hydrogen or back to electricity using fuel cells. All these directions will be covered in the presentation with emphasis on ammonia fuel cells.

Fuel cells provide the most efficient conversion of ammonia to electricity. Indirect fueling includes the intermediate ammonia cracking to hydrogen (Industrial process runs at 850oC but temperature could be substantially lower with advanced catalysts), which can be purified to PEM fuel cell grade. Direct fueling requires less capital expenses, and can be used with different types of fuel cells (SOFC, PCFC, AEMFC and molten carbonate fuel cells). Performance of direct ammonia fuel cells (DAFC) was demonstrated to be very close to the performance of the same cells fueled by hydrogen (with correction for the dilution). Issues of crossover (for polymer membranes), electrocatalyst stability (nitridation), cell architecture, and operation parameters will be discussed.

Recent advances in the development of novel ammonia synthesis, its use for hydrogen generation, and DAFCs that is being funded by ARPA-E via REFUEL and OPEN 2018 programs will be presented. Comparison of DAFCs with combustion engines fueled by ammonia for energy applications will be given.
(Invited) Electrochemical Nitrogen Reduction to Ammonia on Transition Metal Nitrides
Authors: Xu Bingjun
Published: May 2020

Renewable production of ammonia, a building block for most fertilizers, via the electrochemical nitrogen reduction reaction (ENRR) is desirable; however, selective electrocatalysts are lacking. Here we show that transition metal nitrides are a family of active, selective, and stable ENRR catalysts. In particular, vanadium nitride nanoparticles have an ENRR rate and a Faradaic efficiency of 3.3 × 10−10 mol s−1 cm−2 and 6.0% at −0.1 V within 1 h,

respectively. ENRR with 15N2 as the feed produces both 14NH3 and 15NH3, which indicates that the reaction follows a Mars−van Krevelen mechanism. Ex situ X-ray photoelectron spectroscopy characterization of fresh and spent VN catalysts reveals that multiple vanadium oxide, oxynitride, and nitride species are present on the surface and identified vanadium oxynitride as the active phase in the ENRR, which is supported by operando X-ray absorption spectroscopic results. Quantitative isotopic label studies are conducted to determine the density of the initial and steady-state active surface N sites.
(Invited) Plasma-Activated Synthesis of Ammonia Using Nitrogen Gas and Water As Feedstocks
Authors: Toth Joseph , Lacks Daniel J. , Renner Julie N. , Sankaran R. Mohan
Published: May 2020

The reduction of nitrogen to ammonia by the Haber-Bosch (H-B) process entails large energy and environmental costs that motivate research on alternative strategies. A major issue with the H-B process is the need for hydrogen gas that currently comes from fossil fuels (steam methane reforming). Plasma activation is a potential alternative as it was historically the first industrial method employed to fix nitrogen, but by oxidation. More recently, plasmas have been combined with solid catalyst materials to reduce nitrogen to ammonia at lower pressures and temperatures than the H-B process, but also use hydrogen gas as a feedstock.

Here, we present our efforts to develop plasma-activated processes for the reductive synthesis of ammonia using nitrogen and water as feedstocks. One configuration is a hybrid electrochemical system in which a microplasma jet impinging on an aqueous electrolyte surface replaces a metal electrode and reactions are carried out at the interface of the plasma and liquid bath. A second configuration is a hybrid aerosol system in which water droplets are introduced into a larger-volume plasma and reactions are carried out at the interface of the plasma and droplet surface. In both cases, the ammonia is collected in the liquid phase and measured by chemical assay methods. We will discuss process optimization, product selectivity, and energy consumption for both systems. Insights into the reaction mechanisms will also be addressed and in particular, the possible role of water vapor.
(Invited) Direct Ammonia Polymer Electrolyte Fuel Cell
Authors: Wang Teng , Zhao Yun , Achrai Ben , Tamir Gal , Abbasi Reza , Setzler Brian P. , Gottesfeld Shimshon , Yan Yushan
Published: May 2020

Ammonia is a liquid fuel of high energy density and low storage pressure. These properties facilitate storage, distribution and fueling, as required for the introduction and acceptance of an alternative fuel. Distributed as liquid, ammonia cost is projected to be 30% lower than that of hydrogen distributed as compressed gas.1 The structure of direct ammonia fuel cells (DAFCs) is illustrated in Figure 1a.

The hydroxide exchange membrane employed in our DAFCs work was the poly(aryl piperidinium) (PAP) membrane developed at the University of Delaware2 and at the spin-off company, W7energy LLC. Following optimization of the composition of the catalyst layers and the operation conditions, the performance of an ammonia oxygen cell at Tcell= 95 oC reached the peak power level of 580 mW cm-2 (shown in Figure 1b).

A remaining significant challenge has been the DAFCs performance stability, with testing at constant current showing unstable voltage followed by the fall of the voltage to zero after only a few hours. Recognizing the origin of the instability as liquid management in the DAFCs anode, recent optimization of the anode structure improved substantially the voltage stability with the earliest test showing this improvement, which is depicted in Figure 1c.

1. Y. Zhao, B. P. Setzler, J. Wang, J. Nash, T. Wang, B. Xu, and Y. Yan, Joule, 3, 2472-2484 (2019)

2. J. Wang, Y. Zhao, B. P. Setzler, S. Rojas-Carbonell, C. B. Yehuda, A. Amel, M. Page, L. Wang, K. Hu, L. Shi, S. Gottesfeld, B. Xu, and Y. Yan, Nature Energy, 4, 392–398 (2019)

Figure 1
(Invited) NH3 Generation from 2D MoS2 Catalysts
Authors: Zhang Hanyu , Metzroth Lucy , Miller Elisa M.
Published: May 2020

We use metallic MoS2 as a catalyst for N2 fixation and NH3 generation. Based on the current literature with regard to MoS2 catalytic performance for NH3 generation, it is tracking with catalytic performance for hydrogen evolution. For example, if you increase edge sites with 2D MoS2 structuring or incorporate more vacancies/defect sites, the hydrogen evolution reaction improves. The same is true of NH3 formation. If we take this one step further, hydrogen generation further increases if MoS2 is converted to its thermodynamically unstable metallic state. Therefore, we extend this correlation to NH3 generation. We expect that the N2 fixation and NH3 generation will improve if we use metallic MoS2 nanosheets. This hypothesis is tested and reported. We will also comment on the selectivity for the H2 versus NH3 generation from this catalyst.
(Invited) Critical Assessment of Aqueous Electrochemical Synthesis of NH3: N2 Reduction Versus NOx Reduction
Authors: Choi Jaecheol , Suryanto Bryan H R , Du Hoang-Long , Chatti Manjunath , Simonov Alexandr N , MacFarlane Douglas R.
Published: May 2020

As an ideal alternative to the Haber-Bosch process, the renewable energy-powered electrochemical N2 reduction to NH3 is a promising approach since the electrochemical reduction can occur under mild conditions; only water and N2 are consumed during overall nitrogen reduction reaction.[1] Recently, there have been many efforts devoted to developing electrocatalysts for energy efficient NH3 synthesis from N2; however, it remains very challenging due to the thermodynamic inertness of the dinitrogen triple bond. The electrochemical reduction of N2 to NH3 (the nitrogen reduction reaction “NRR”) requires the consecutive six-electron/proton transfer reactions to proceed and this leads to the sluggish kinetics. In addition, the competitive hydrogen evolution reaction (HER, Eº = 0 V vs. RHE) can concomitantly occur at similar potentials to the NRR (Eº = 0.092 V vs. RHE), resulting in both low faradaic efficiency (< 10 %) and low yield rates (< 10-10 mole cm-2 s-1) for NH3 synthesis. Such poor conversion efficiency and yield rates also make it more difficult to confirm the origins of the NH3 production, ie whether it genuinely comes from electrocatalytic NRR, as opposed to some other readily reducible N-containing contaminants (NO, NO2, N2O and doped N atoms in the materials) under reducing potentials.

Herein, we investigate the catalytic nature of nitrogen reduction reaction on three different types of preeminent electrocatalysts from the literature (bismuth, gold and N-doped carbon)[2, 3] using a rigorous experimental protocol developed by our group.[4] It is demonstrated that all of the catalysts are essentially inactive (below LOD) towards dinitrogen reduction to NH3. We also systematically unravel the origins of the reported activity, showing that other N-containing species, particularly ionic/gaseous NOx or doped N atoms in the materials, are strongly active reactants towards NH3 production. Our presentation will conclude with a summary of the critical contaminants leading to false-positive NRR and also provide further protocol recommendations to avoid this outcome.

[1] S.L. Foster, S.I.P. Bakovic, R.D. Duda, S. Maheshwari, R.D. Milton, S.D. Minteer, M.J. Janik, J.N. Renner, L.F. Greenlee, Nature Catalysis, 1 (2018) 490-500.

[2] Y.-C. Hao, Y. Guo, L.-W. Chen, M. Shu, X.-Y. Wang, T.-A. Bu, W.-Y. Gao, N. Zhang, X. Su, X. Feng, Nature Catalysis, 2 (2019) 448.

[3] Y. Liu, Y. Su, X. Quan, X. Fan, S. Chen, H. Yu, H. Zhao, Y. Zhang, J. Zhao, ACS Catalysis, 8 (2018) 1186-1191.

[4] B.H.R. Suryanto, H.-L. Du, D. Wang, J. Chen, A.N. Simonov, D.R. MacFarlane, Nature Catalysis, 2 (2019) 290-296.
Pt-Based Ternary Catalysts for Electrochemical Ammonia Oxidation
Authors: Li Yi , Wu Gang
Published: May 2020

Ammonia (NH3) has proven to be a cost-effective alternative to hydrogen for powering fuel cells via its direct oxidation to nitrogen and water. However, sluggish kinetics of the ammonia oxidation reaction (AOR) have greatly hindered the commercial application of direct ammonia fuel cells (DAFCs). Here we report an efficient AOR catalyst that consists of PtIrNi alloy nanoparticles anchored on a binary support of porous silicon dioxide and carboxyl-functionalized carbon nanotubes (PtIrNi/SiO2-CNT-COOH) through a sonochemical-assisted synthesis strategy. The PtIrNi alloy nanoparticles, with the aid of abundant OHad provided by porous SiO2, and the improved electronic conductivity contributed by CNT, exhibit remarkable catalytic activity for the AOR in alkaline media, with a lower onset potential (~0.40 V vs. RHE) at room temperature, than that of commercial PtIr/C (~0.43 V vs. RHE). Constant-potential density functional theory (DFT) calculations showed that the Pt-Ir ensembles on {100}-terminated surfaces serve as the active site, the introduction of Ni to which raises the center energy of the density of states projected onto the group d-orbitals and thus lowers the theoretical onset potential for *NH2 dehydrogenation to *NH when compared to Pt and Pt3Ir alloy. Varying the NH3 concentration and operation temperature shows a significant influence on the AOR performance of this catalyst. Specifically, AOR activity of the optimal PtIrNi nanoparticle catalyst can be significantly enhanced by elevating the temperature to 80ºC, with a much lower onset potential (~0.32 V vs. RHE), indicating that DAFC can be operated at higher temperature for increased performance.
(Invited) Polymer Electrolyte Membrane Water Electrolyzers – Current Challenges and Perspectives Beyond 2020
Authors: Carmo Marcelo
Published: May 2020

Hydrogen is largely considered the best means by which to store energy coming from renewable and intermittent power sources such as wind and solar. With the growing capacity of out-spread renewable energy sources surpassing the gigawatt range, a storage system of equal magnitude is required, such as the production of electrolytic hydrogen by water electrolysis. Consequently, we envision a rapid market penetration of water electrolyzers in the next years. Nevertheless, the deployment of current, commercially available electrolyzer stacks and systems is conceivable for the current initial period of energy transition and shall go over the next decades. However, electrolyzers still need transformation, rupturing from small, costly and low-efficiency units into game-change electrolyzers much larger than 10 MW, with efficiencies above 80%, power densities greater than 10 W/cm2, and demonstrated durability above 100.000 hours. To achieve these goals, fast and large R&D investment is necessary, strong commitment from government and society; all tied with focused and intensive international collaboration. Over the past 10 years, new companies and projects have appeared, with new leaders being consequently established in this growing “niche”. In this presentation I will show our latest R&D approaches; aiming to increase efficiency, improve durability, and reduce the investment/operating costs of electrolyzers to allow large market penetration of these systems. We believe that only through cutting-edge R&D and significant advancements; we will be able to ultimately establish “green” hydrogen as a commodity in the energy storage market, and contribute to our so much desired environmentally friendly hydrogen economy.
(Invited) Overcoming Technological and Economic Barriers to Generating Renewable Hydrogen with Polymer Electrolyte Membrane Electrolysis
Authors: Mittelsteadt Cortney
Published: May 2020

Water electrolysis, coupled with green energy sources has long held the promise for supplanting steam methane reforming, SMR, for hydrogen generation for ammonia and petroleum refining, together contributing a billion tons of CO2/year. Additionally hydrogen can be used as a store and carrier of energy for mobile and grid applications. Wide adoption has been prevented by unacceptable costs however, both capital and operating, the latter of which is dominated by the cost of the electricity. In those industrial applications where electrolysis has found a niche it has been dominated by liquid alkaline electrolysis, due to its long history of proven durability and low cost materials. Polymer electrolyte membrane electrolysis (PEMEL) has had the potential to operate at slightly higher efficiencies, and much higher current density, while allowing high, and more importantly, differential pressure. PEMEL’s reliance on precious group metal (PGM) catalysts as well as membrane lifetime concerns have generally left them to specialty applications where footprint is a premium, such as sea and space.

A number of concurrent advancements are overcoming these barriers. First, and most importantly, the cost of energy has come down faster than anyone would have predicted. This cannot be over-emphasized, as will be shown, lowering the cost of renewables has done more for green hydrogen than anything that can be achieved by electrochemical scientists and engineers, as energy costs have long dominated the cost of green hydrogen. Further lowering costs will in turn change focus to capital costs and technology opportunities. Second, improvements in membrane stability has greatly increased the durability of PEMEL systems. This has improved the lifetime of the systems while also allowing more aggressive operating conditions of higher pressure, temperature and current density. The challenges and limitations of further increasing these parameters, as well as their impact on cost will be addressed. Finally, PGM catalyst loadings continue to decrease. This reduction, as well as the increase in lifetimes allows a far greater amount of hydrogen to be generated per gram of PGM catalyst.

The last great barrier for PEMEL to achieve parity with SMR, and enable hydrogen for mobility and large scale energy storage is simply scale. PEMEL, and even alkaline electrolysis are at an infancy in terms of scale. As PEM systems grow from traditional 1-100 kW systems to the 1-100 MW systems necessary for mobility, storage and industrial uses the capital costs will come down tremendously. We will look at how low these costs can go as well as consider the scarcity limitation of PGMs as PEMEL increases capacity from current output of < 100 MW/year to 10-100 GW/year necessary to make real impact in these applications.
(Invited) Manufacturing Challenges and Opportunities for PEM Electrolysis
Authors: Capuano Christopher B , Ayers Kathy , Manco Judith , Wiles Luke , Keane Alex
Published: May 2020

Hydrogen generation via electrolysis has gained much attention internationally as not only a sustainable source of fuel for the transportation, but as a carrier of energy to capture stranded renewable due to the carbon-free chemical cycle and response characteristics of the technology. The barrier to entry preventing this technology from being widely adopted is the overall cost associated with both the upfront capital expense as well as the operating cost incurred over the product life. The majority of this cost is driven by 1) low electrical efficiencies due to the high ohmic resistance of thicker membranes required by electrolysis systems to electrochemically compress generated hydrogen for storage and the overpotential associated with the water splitting oxygen evolution reaction (OER) catalysts typically used, 2) high cost of system and cell stack materials, which need to be durable for up to 100,000hrs at a potential of 2V in both reducing and oxidizing environments, and 3) the lack of a robust supply chain for high volume manufacturing to further drive down cost through economies of scale.

With 95% of hydrogen currently being produced through the reformation of cheap natural gas, significant cost reductions are necessary for electrolysis to become a viable alternative to the incumbent. Commercial proton exchange membrane (PEM)-based electrolysis has reached scales of several hundred kg/day, providing a relevant pathway for industrial scale hydrogen generation with tremendous opportunity for continuing cost reduction by leveraging system and manufacturing scaling laws by leveraging advancements in PEM fuel cell materials, manufacturing, and analysis tools. Order of magnitude improvements in some of the highest cost elements are easily achievable and with more research funding being directed towards hydrogen production, the realization of these reductions are occurring at a faster rate. While many improvements in stack design and materials of construction have been identified and implemented, there are still numerous opportunities to explore that would move electrolysis towards a viable, cost-effective alternative to natural gas reformation. This talk will describe some of the areas of success, where research is still needed, and ultimately how each of these improvements translate into cost and the path to reformation parity.
(Organic and Biological Electrochemistry Division Manuel M. Baizer Award Address) Making Electroorganic Synthesis More Practical
Authors: Waldvogel Siegfried R.
Published: May 2020

The direct use of electrochemistry for the generation of reactive intermediates can have major advantages towards conventional synthetic strategies.[1] Less or no reagent waste is generated and new reaction pathways are accessible.[2] In order to exploit the electricity driven conversions for synthetic purposes and to install unique selectivity two modern approaches will be outlined:

Solvent-controlled selective dehydrogenative cross-coupling reactions: A key for this is the use of boron-doped diamond anodes and fluorinated alcohols within the electrolyte.[3] This methodology opened new pathways for innovative and scalable arylation reactions.[4]

New electrode systems for the anodic and cathodic conversion to value-added organic compounds. These systems are capable to go beyond common limits in electro-organic synthesis.

The working horse to identify suitable electrolytic conditions is the electrosynthetic screening approach. This strategy gives also rise to fast optimization and subsequent scale-up.[5] In particular, the second approach will deal with concepts which represent milestones originating from the Baizer process.

References:

[1] E.J. Horn, B.R. Rosen, P.S. Baran, ACS Cent. Sci.
2 (2016) 302-307.

[2] a) S. Möhle, M. Zirbes, E. Rodrigo, T. Gieshoff, A. Wiebe, S. R. Waldvogel, Angew. Chem. Int. Ed. 57 (2018) 6018-6041; b) A. Wiebe, T. Gieshoff, S. Möhle, E. Rodrigo, M. Zirbes, S. R. Waldvogel, Angew. Chem. Int. Ed. 57 (2018) 5594-5619.

[3] L. Schulz, S.R. Waldvogel, Synlett 30 (2019) 275-286.

[4] S.R. Waldvogel, S. Lips, M. Selt, B. Riehl, C.J. Kampf, Chem.
Rev., 118 (2018) 6706-6765.

[5] C. Gütz, B. Klöckner, S.R. Waldvogel, Org. Process Res. Dev. 20 (2016) 26-32.
Exploring the Synergistic Relationship between Organic Synthesis and Electrochemistry
Authors: Moeller Kevin D
Published: May 2020

Electrochemistry has long held potential as a tool for constructing organic molecules, and the organic chemistry community is both recognizing and impressively exploiting that potential with increasing regularity. Those efforts have taught us a great deal about how to think about electrochemical reactions, how to use the technique to accomplish new transformations that enrich the synthetic enterprise, and when to employ those methods.

Interestingly, the relationship between organic synthesis and electrochemistry can also be viewed in the opposite direction with the new synthetic chemistry being developed serving as a tool to facilitate and dramatically expand the scope of electrochemical methods. For example, new synthetic chemistry that allows for the site-selective generation of chemical reagents on an array have enabled the construction of complex, addressable molecular surfaces that can in turn be used to probe small molecule interactions with a variety of biological targets. The efforts set the stage for using microelectrode arrays as analytical devices for rapidly screening small molecule libraries in "real-time". The methods allow for the construction of libraries with tunable surfaces, careful control of surface concentrations, and previously unknown levels of quality control over the molecular library. The development of an even greater arsenal of synthetic tools for use on the arrays is underway, an effort that aims to expand the nature and size of the addressable libraries that can be used to probe the world around us.

Not surprisingly, the exploration of these new electrochemical tools is beginning to “give back” to the synthetic arena. A number of preparative scale synthetic applications have now arisen based on the concept of site-selective reagent generation. Chief among these is the use of paired electrochemical reactions for more sustainable reagent generation and the recent discovery that chemical reagents can be confined to specific regions within a preparative reaction; a development that potentially offers a new strategy for controlling the selectivity of a chemical reaction.

In the talk to be given, the interplay between organic synthesis and electrochemistry will be highlighted with an emphasis on how key concepts in physical organic chemistry are used to guide the development of new advances in both areas, and how we think those new developments will shape future efforts to expand the use of electroorganic synthesis.

References:

For a recent review see: Kevin D. Moeller. Chem. Rev.
2018, 118, 4817-4833.
Chemical Hydrogenations Using an Electro-Generated Mediator
Authors: Symes Mark
Published: May 2020

The Electron-Coupled-Proton Buffer (see: Nature Chem.
2013, 5, 403-409; Nature Chem.
2018, 10, 1042-1047; Joule, 2018, 2, 1390-1395) permits the electrochemical oxygen evolution reaction from water to be coupled to the reversible reduction and protonation of a redox mediator (rather than the coupling of the oxygen evolution reaction directly with hydrogen production). Careful selection of this redox mediator, along with judicious choice of the electrode material for the cathode reaction, can allow reduced species to be generated that are thermodynamically unstable with respect to hydrogen generation (Science,
2014, 345, 1326-1330). These reduced mediators can either be used for spontaneous hydrogen generation (decoupled from the oxygen evolution step), or they can be used as soluble sources of hydrogen for performing the hydrogenation of organic molecules (Chem. Commun.
2018, 54, 1093-1096). In this talk, we such discuss current progress and potential future avenues for the use of Electron-Coupled-Proton Buffers as reducing equivalents in organic synthesis.
Phenolic Antioxidants: Redox Properties and Antioxidant Activities
Authors: Martic Sanela
Published: May 2020

Phenolic compounds are known for their antioxidant properties, which is important in the biological setting [1]. The cellular damage induced by reactive oxygen species, ROS, or others leads to diseases. However, foods or drugs which contain phenolic moieties are viable antioxidants which reduce ROS and minimize cell damage. Hence, greater understanding of this class of compounds will lead to an ideal antioxidant with tailored properties for applications in biology, food safety, among others. The relationship between structure-properties-function was evaluated by using voltammetric methods. We have explored redox properties of phenol-based compounds, such as tert-butylhydroxyphenol and its analogues by using Cyclic Voltammetry (CV) as well as Square-Wave Voltammetry (SWV) in order to rank order such compounds based on their redox potentials and structural parameters [2]. We have discovered that under electrochemical conditions, rather than chemical ones, such compounds may undergo cross-coupling reactions leading to photochromic products. The reactivity of such phenolic analogues was also explored by CV in the study with reactive oxygen species, specifically, superoxide anion radical [3]. More recently, we have started exploring compounds with multiple hydroxyl groups, such as flavonoids. The research findings on a variety of hydroxyl-containing aromatics will be presented and described.

1. Ingold, K. U., Pratt, D. A. Advances in radical-trapping antioxidant chemistry in the 21st century: a kinetic and mechanisms perspective. Chem. Rev. 2014, 9022-9046.

2. Zabik, N., Virca, C. N., McCormick, T., Martic-Milne, S. Selective electrochemical versus chemical oxidation of bulky phenols. J. Phys. Chem. B, 2016, 120, 8914–8924.

3. Zabik, N., Anwar, S., Ziu, I., Martic-Milne, S., Electrochemical reactivity of bulky-phenols with superoxide anion radical. Electrochim. Acta, 2019, 296, 174-180