Publications | Raul A. Marquez

2025

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Insights into catalyst degradation during alkaline water electrolysis under variable operation

R.A. Marquez, J.T. Bender , A.M. Aleman , and 11 more authors

Energy & Environmental Science. Check out this video explaining the relevance of this study in variable operation of water electrolyzers , Jun 2025

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Energy conversion technologies that are key to decarbonization efforts face significant durability challenges due to variable operation. Understanding the impact of variable operation on catalytic stability and identifying the key variables that dictate degradation is crucial for developing robust technologies. Here, we present a comprehensive investigation of the effects of variable operation on liquid alkaline water electrolysis. Our findings reveal that variable operation induces severe degradation of Ni, Fe, and Co catalytic films that is not observed during steady-state operation. By systematically interrogating the electrode discharge during simulated shutdown tests using Raman spectroscopy and mass spectrometry techniques, we uncover significant alterations caused by reverse currents in real time. These include changes in crystal structure, composition, film thickness, electronic conductivity, and dissolution rates. Lab-scale electrolyzer experiments further highlight the impact of variable operation on catalyst materials under relevant conditions. Finally, we provide guidelines for leveraging these insights to advance electrocatalysis research. This work underscores the importance of integrating realistic stressors into stability testing and offers practical guidelines for catalyst design, performance evaluation, and industrial implementation. Collectively, insights from this study will drive the development of more resilient energy conversion technologies.
ACS

Tracking Local pH Dynamics during Water Electrolysis via In-Line Continuous Flow Raman Spectroscopy

R.A. Marquez, J.T. Bender , S.C. Cunha , and 7 more authors

ACS Energy Letters. Check out this video explaining the relevance of this study in variable operation of water electrolyzers , Apr 2025

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Here, we introduce in-line continuous flow Raman spectroscopy (CFRS) as a spectroelectrochemical platform for quantifying interfacial pH swings generated during water-splitting. By monitoring phosphate ion speciation and controlling the hydrodynamics with a flow cell, we measure pH swings as a function of current density, flow rate, and distance from the electrode. Comparison with theoretical models reveals the impact of bulk pH, boundary layer thickness, and bubble dynamics at high current densities. Collectively, these findings establish CFRS as a platform for quantitatively investigating pH dynamics, offering critical insights for advancing electrochemical energy conversion technologies.
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Formation of polysulfides as a smart strategy to selectively detect H2S in a Bi(iii)-based MOF material

V.B. Lopez-Cervantes , J.L. Obeso , J.G. Flores , and 11 more authors

Chemical Science, Feb 2025

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SU-101 was demonstrated to be an effective and efficient detector for H2S, due to the facile generation of polysulfides, with a remarkable H2S selectivity. Raman and XPS analyses confirmed the formation of Sn2− and S42− polysulfide species after the H2S adsorption (at 0.05 bar, 0.1 bar and 1 bar), without compromising the structural integrity of SU-101. The detection mechanism involves rigidification of the structure by the formation of the polysulfides and blockage of the ligand–metal charge transfer (LMCT) process, which increased the radiative emission. Additionally, theoretical simulations were carried out in order to demonstrate that the interaction of the polysulfide molecules inside the pores of SU-101 is energetically stable. Remarkably, the limit of H2S detection was calculated to be as low as approximately 22 ppm. Finally, SU-101 is nominated as a promising candidate for implementing toxic waste valorisation (i.e., capture of toxic H2S) toward relevant applications in accurate molecular sensing.

2024

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Transition metal incorporation: electrochemical, structure, and chemical effects on nickel oxyhydroxide oxygen-evolution electrocatalysts

R.A. Marquez, E. Kalokowski , M. Espinosa , and 12 more authors

Energy & Environmental Science. Check out a poster and a video related to this study , Jan 2024

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Understanding how electrode materials evolve in energy conversion and storage devices is critical to optimizing their performance. We report a comprehensive investigation into the impact of in situ metal incorporation on nickel oxyhydroxide oxygen evolution reaction (OER) electrocatalysts, encompassing four multivalent cations: Fe, Co, Mn, and Cu. We found that adding trace amounts of these cations to alkaline electrolytes alters the electrocatalytic and energy storage properties of NiOxHy films after electrochemical conditioning. As opposed to the well-known increase in OER activity induced by Fe, in situ incorporation of trace Co and Mn cations increases the total capacitance, while Cu incorporation does not proceed. We show that increasing Fe and Co concentrations leads to a maximum electrochemical performance attributed to a saturation threshold in the metal uptake. Depth profiling measurements reveal that metal incorporation is confined to the surface of the film, resulting in an interstratified structure that partially retains the more active, disordered phase at the surface. Building upon solid-state chemistry principles, we provide an in-depth discussion of four critical factors determining the occurrence of in situ metal incorporation, underscoring its nature as a cation exchange process. To further support this concept, we manipulate cation exchange by shifting the solubility equilibrium and via ion complexation. By providing a better understanding of in situ metal incorporation, our results underscore its potential as a strategy for manipulating the surface chemical composition, thus advancing the development of electrochemical energy materials.
ACS

A Guide to Electrocatalyst Stability Using Lab-Scale Alkaline Water Electrolyzers

R.A. Marquez, M. Espinosa , E. Kalokowski , and 5 more authors

ACS Energy Letters. Check out a supplementary video tutorial and CAD files of the water electrolyzer , Jan 2024

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This work, inspired by similar advances in CO2 electrolysis, represents the next step in AWE electrocatalysis research. As illustrated in Figure 1, lab-scale electrochemical flow cells harmonize stability evaluation and mark an advancement toward meeting industrial requirements. This initial effort aims to establish a straightforward protocol and comparison platform for evaluating and validating electrocatalysts in AWE. We hope this guide facilitates the transition to more realistic testing and promotes rigorous examination of electrocatalytic stability.
ACS

Redefining the Stability of Water Oxidation Electrocatalysts: Insights from Materials Databases and Machine Learning

R.A. Marquez, E.E. Oefelein , T.V. Le , and 3 more authors

ACS Materials Letters.Check out this GitHub Link for more details on the machine learning models. , Jun 2024

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Research on electrochemical water splitting has experienced significant growth in interest in transition metal borides, carbides, pnictides, and chalcogenides, owing to their notable catalytic performance. These materials, collectively called X-ides, are often considered promising electrocatalysts for the oxygen evolution reaction (OER). However, under the strongly oxidizing conditions of the OER, transition metal X-ides often act as precatalysts, undergoing in situ reconstruction to a different, catalytically active phase. Discrepancies exist in the literature, with some studies claiming the absence of such transformations. Building upon previous efforts to elucidate catalytic performance trends in the community, this Perspective discusses a more nuanced approach to X-ide research, emphasizing the need to reassess our understanding of their chemical stability and the significance of the in situ reconstruction process. By discussing the role of experimental and computational databases, we present strategies for predicting X-ide stability and stress the importance of thorough experimental validation. Moreover, we highlight the use of machine learning to extract meaningful insights from these data and urge the community to adopt a standardized, systematic reporting of X-ide performance. Finally, we provide strategic guidelines and directions to advance transition metal X-ide research, ultimately enhancing their future application for a sustainable hydrogen economy.
ACS

A Perspective on Protective Carbon Shells for Improved Stability of Alkaline Water Oxidation Electrocatalysts

L.A. Smith , K. Kawashima , R.A. Marquez, and 1 more author

ACS Materials Letters, Jun 2024

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In recent years, carbon shells have garnered attention as possible protective layers that can be applied to oxygen evolution reaction (OER) electrocatalysts to prevent corrosion under alkaline conditions. However, thermodynamic considerations and experimental results indicate that these carbon shells are subject to corrosion themselves, limiting their applicability as protective coatings against oxidation. Herein, the thermodynamics of carbon corrosion are presented with spectrometric, spectroscopic, crystallographic, microscopy, and electrochemical measurements, emphasizing the inevitable degradation of carbon under alkaline OER conditions. Recent work focused on suppressing this carbon corrosion is then discussed alongside future directions for carbon shells in water oxidation research.
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From pollution to energy storage: leveraging hydrogen sulfide with SU-101 cathodes in lithium–sulfur batteries

R.A. Marquez, J.L. Obeso , R. Reddy Vaidyula , and 6 more authors

Journal of Materials Chemistry A, Jun 2024

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Despite growing interest in developing metal–organic frameworks to capture toxic emissions, the potential for revalorizing these emissions has largely been overlooked. Captivated by the unique ability of SU-101 to transform H2S into polysulfides spontaneously, here we demonstrate how this remarkable capability can be leveraged to power lithium–sulfur batteries. Our proof-of-concept demonstrates how hydrogen sulfide emissions, efficiently captured by the SU-101 metal–organic framework, can be directly converted and utilized in a lithium–sulfur battery. Despite demonstrating a modest initial capacity of about 85 mA h g−1 and capacity retention of approximately 54%, analogous to other MOF-based Li–S batteries reported in the literature, the SU-101-Sat cathode delivered a durable and stable performance across 1000 cycles and maintained 99.87% coulombic efficiency. The stable response is credited to the controlled release of polysulfides generated from captured hydrogen sulfide. Such resilience is advantageous for developing compact batteries for extended use in special applications, including wearable technology, satellite components, and weather monitoring instruments. These findings open new pathways for waste valorization by employing metal–organic frameworks designed to spontaneously convert toxic gas emissions into valuable feedstocks, serving as potential candidates for more sustainable electrochemical energy conversion and storage devices.
ACS

Teaching Electrochemical Energy Conversion and Storage through Active Learning: Insights from Science Workshops

R.A. Marquez, E. Kalokowski , M. Espinosa , and 4 more authors

Journal of Chemical Education. Check out the supplementary slides for mini-lectures and the student and instructor handouts for experiments , Aug 2024

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Electrochemical energy conversion and storage devices are pivotal in transforming our society and advancing sustainability. Therefore, educating students in electrochemistry, the fundamental backbone of these technologies, is essential for preparing a new generation of professionals and raising public awareness of the role of these technologies in mitigating environmental challenges. However, a critical challenge lies in teaching electrochemistry through captivating and interactive approaches, particularly for younger learners. Herein, we outline a week-long workshop designed to immerse high school and undergraduate students in the world of electrochemical energy conversion and storage. The workshop was meticulously crafted to ensure a comprehensive exploration of electrochemistry fundamentals, operational principles of energy devices, real-world applications, and their societal impacts. Through mini-lectures, demonstrations, class discussions, educational games, and collaborative projects based on active learning, this workshop aims to improve the students’ understanding of electrochemistry and promote an appreciation for its critical role in society. Course evaluations indicate that our approach cultivates a stimulating learning environment. This initiative serves as a model for future educational programs in electrochemistry, aiming to equip students with the knowledge and inspiration needed to contribute to a sustainable future.

2023

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Electrochemically Engineered Domain: Nickel–Hydroxide/Nickel Nitride Composite for Alkaline HER Electrocatalysis

C. Chukwuneke , K. Kawashima , H. Li , and 5 more authors

Journal of Materials Chemistry A, Dec 2023

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Boosting hydrogen evolution reaction (HER) performance in alkaline media has been a topic of interest in a world that continuously strives for cleaner energy conversion systems. The paucity of protons provides a challenge to performing the HER in an alkaline environment and is a major impediment to decreasing the energy requirement needed to produce hydrogen. Here, we improve the HER activity of nickel-nitride by electrochemically anodizing the surface which aids water dissociation in alkaline HER. We experimentally show that by oxidizing the nickel nitride on a nickel foam (Ni3N/NF) surface using cyclic voltammetry (CV) cycles in Fe-unpurified KOH media, Ni3N can be converted to a Ni(OH)2/Ni3N composite. Activation of the Ni3N/NF with 50 CV cycles resulted in a low HER overpotential of 172 and 207 mV at the current densities of 50 and 100 mA cm−2, respectively. The 50 CV cycle Ni3N/NF also had the minimum charge transfer resistance at 6 Ω compared to all other prepared samples, and the Tafel slope is 100.87 mV dec−1.
ACS

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

K. Kawashima , R.A. Marquez, L.A. Smith , and 6 more authors

Chemical Reviews. Kawashima, K. and Marquez, R.A. contributed equally to this work , Nov 2023

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Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of “catalytically active sites/species/phases” in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.
ACS

Navigating iR Compensation: Practical Considerations for Accurate Study of Oxygen Evolution Catalytic Electrodes

Y.J. Son , R.A. Marquez, K. Kawashima , and 4 more authors

ACS Energy Letters, Sep 2023

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In this Viewpoint, we study the iR compensation for the alkaline OER system as a case study. We aim to identify the experimental pitfalls that can arise when applying iR compensation to the study of catalytic electrodes. First, we discuss several cases in which the same catalytic electrode exhibits different OER activity depending on the methods used for Ru measurement and iR compensation. To understand the underlying causes behind this issue, we investigate the roles of different Ru components in iR compensation depending on the electrode properties and electrochemical testing conditions. Furthermore, we demonstrate that the results obtained from potentiodynamic analysis of the electrode’s redox reaction can vary depending on the method and degree of iR compensation. These variations are attributed to a change in the actual scan rate near the redox peak during potentiodynamic analysis and the occurrence of electron -rapping phenomena within the electrode. Lastly, we provide practical guidelines on how to correctly measure and interpret the Ru value and how to select the proper iR compensation method for studying the redox reaction and electrocatalytic activity of the electrode.
ACS

Getting the Basics Right: Preparing Alkaline Electrolytes for Electrochemical Applications

R.A. Marquez, K. Kawashima , Y.J. Son , and 5 more authors

ACS Energy Letters. Check out the supplementary spreadsheet for volumetric and pH titrations of KOH/NaOH electrolytes. , Jan 2023

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In this Viewpoint, we describe six practices to prepare, characterize, and validate the quality of alkaline electrolytes used in electrochemical energy systems. By adapting previous procedures and performing specific measurements, we provide a unified protocol to prepare and purify alkaline electrolytes, standardize their concentrations, determine their elemental compositions, provide statistical metrics, and examine their electrochemical properties.
Elsevier

Key concepts for understanding alkaline oxygen evolution reaction at the atomic/molecular scale

Y.J. Son , K. Kawashima , R.A. Marquez, and 3 more authors

Current Opinion in Electrochemistry, Jun 2023

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Understanding and improving the sluggish oxygen evolution reaction (OER) is critical for efficient water electrolysis. However, given that the OER is a multi-step, inner-sphere electrochemical reaction, understanding the OER at the atomic/molecular scale is difficult due to its complex redox chemistry and convoluted reaction mechanism. Moreover, recent discoveries of new types of active site, active species, reaction mechanism, and the driving force of the OER further complicate our understanding of the OER. Herein, a review of the key concepts of the OER at the atomic/molecular scale is given, focusing on providing their clear definitions, principles, categories, and commonalities and differences. Also, the role of each concept in determining the OER activity is discussed.
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Unveiling the reaction mechanism of capacity reactivation in silver vanadate cathodes for aqueous zinc-ion batteries

Z. Wang , J. Diao , K. Kawashima , and 6 more authors

Journal of Materials Chemistry A, Aug 2023

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Aqueous zinc-ion batteries are regarded as promising candidates for future energy storage devices because of their high safety. Due to the dissolution in the aqueous electrolytes, most vanadate-based zinc-ion batteries suffer from continuous capacity fading. In some cases, a capacity reactivation process can be observed in vanadate-based cathodes after capacity decay. Herein, we employed electrochemical methods and characterization techniques to study the reaction mechanism of capacity reactivation in Ag0.33V2O5 cathodes. Our preliminary results suggested that the reactivation is due to an in situ crystalline structure evolution in the cathode materials. Under the electrochemical condition, a new phase Zn3(OH)2V2O7·2H2O irreversibly formed on the initial cathodes, which prevented the dissolution of vanadate and further resulted in the capacity increase. Moreover, we confirmed the unique intercalation pseudocapacitive behavior in the reconstructed Zn3(OH)2V2O7·2H2O, which provided fast ionic diffusion to facilitate electrochemical performance. Accordingly, our study offers a new understanding of the capacity change of the vanadate-based cathode materials and provides a more general explanation of the capacity reactivation in aqueous zinc-ion batteries.
ACS

Accurate Potentials of Hg/HgO Electrodes: Practical Parameters for Reporting Alkaline Water Electrolysis Overpotentials

K. Kawashima , R.A. Marquez, Y.J. Son , and 5 more authors

ACS Catalysis, Jan 2023

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Nowadays, as molarity is most frequently used in the field of electrocatalytic water splitting, we urgently need the calculated potential values of the Hg/HgO electrodes (x M MOH, M = Na or K) that can be utilized for the Hg/HgO-to-RHE potential conversion and can also act as benchmarks for judging the reliability of each user Hg/HgO electrode. To address these concerns, we herein discuss three points: (i) calculating the potentials of the Hg/HgO electrodes containing NaOH/KOH internal aqueous solution with different molar concentrations (M), (ii) how to calibrate/check the Hg/HgO electrode potentials, and (iii) additional instructions/precautions that should be considered when using Hg/HgO reference electrodes.

2022

ACS

Tailoring 3D-Printed Electrodes for Enhanced Water Splitting

R.A. Marquez, K. Kawashima , Y.J. Son , and 6 more authors

ACS Applied Materials & Interfaces. Checkout the bubble evolution recordings under stagnant conditions, controlled flow rate, and porous electrodes. , Sep 2022

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Alkaline water electrolysis, a promising technology for clean energy storage, is constrained by extrinsic factors in addition to intrinsic electrocatalytic activity. To begin to compare between catalytic materials for electrolysis applications, these extrinsic factors must first be understood and controlled. Here, we modify extrinsic electrode properties and study the effects of bubble release to examine how the electrode and surface design impact the performance of water electrolysis. We fabricate robust and cost-effective electrodes through a sequential three-dimensional (3D) printing and metal deposition procedure. Through a systematic assessment of the deposition procedure, we confirm the close relationship between extrinsic electrode properties (i.e., wettability, surface roughness, and electrochemically active surface area) and electrochemical performance. Modifying the electrode geometry, size, and electrolyte flow rate results in an overpotential decrease and different bubble diameters and lifetimes for the hydrogen (HER) and oxygen evolution reactions (OER). Hence, we demonstrate the essential role of the electrode architecture and forced electrolyte convection on bubble release. Additionally, we confirm the suitability of ordered, Ni-coated 3D porous structures by evaluating the HER/OER performance, bubble dissipation, and long-term stability. Finally, we utilize the 3D porous electrode as a support for studying a benchmark NiFe electrocatalyst, confirming the robustness and effectiveness of 3D-printed electrodes for testing electrocatalytic materials while extrinsic properties are precisely controlled. Overall, we demonstrate that tailoring electrode architectures and surface properties result in precise tuning of extrinsic electrode properties, providing more reproducible and comparable conditions for testing the efficiency of electrode materials for water electrolysis.
ACS

Effects of Electrochemical Conditioning on Nickel-Based Oxygen Evolution Electrocatalysts

Y.J. Son , S. Kim , V. Leung , and 10 more authors

ACS Catalysis, Aug 2022

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Electrochemical conditioning via chronopotentiometry (CP) and cyclic voltammetry (CV) is essential for the activation of oxygen evolution reaction (OER) electrocatalysts. While many reports have activated OER electrocatalysts using either CP or CV, the inherent differences between these two electrochemical conditioning methods for the activation of OER electrocatalytic materials have yet to be explored. Here, we investigate the effects of CP and CV electrochemical conditioning on a Ni-based OER precatalyst and substrate in Fe-purified and Fe-unpurified KOH electrolytes by employing (i) Ni foil, (ii) NiSe precatalyst films with different thicknesses on the fluorine-doped tin oxide glass substrate, and (iii) NiSe precatalyst films on Ni foil substrates. It was found that CV electrochemical conditioning can result in a higher degree of in situ oxidation and Fe incorporation for Ni-based precatalysts and substrates compared to CP electrochemical conditioning. In turn, this brought about different material properties (e.g., in situ oxidized layer thickness, composition, crystallinity, and morphology) and electrochemical characteristics (e.g., active surface area, electron transport limitation, and intrinsic activity) of Ni-based electrocatalysts, thereby not only affecting their OER activity but also complicating the interpretation of the origin of OER activity. This study identifies the distinct effects of CP and CV electrochemical conditioning on Ni-based OER electrocatalysts and provides insight into the choice of the electrochemical conditioning method to better investigate OER electrocatalysts.

2021

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Electrochemical behavior of a Ni3N OER precatalyst in Fe-purified alkaline media: the impact of self-oxidation and Fe incorporation

K. Kawashima , R.A. Marquez-Montes, H. Li , and 4 more authors

Materials Advances, Feb 2021

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Nickel nitride (Ni3N) is known as one of the promising precatalysts for the electrochemical oxygen evolution reaction (OER) under alkaline conditions. Due to its relatively low oxidation resistance, Ni3N is electrochemically self-oxidized into nickel oxides/oxyhydroxides (electroactive sites) during the OER. However, we lack a full understanding of the effects of Ni3N self-oxidation and Fe impurity incorporation into Ni3N from electrolyte towards OER activity. Here, we report on our examination of the compositional and structural transformation of Ni3N precatalyst layers on Ni foams (Ni3N/Ni foam) during extended periods of OER testing in Fe-purified and unpurified KOH media using both a standard three-electrode cell and a flow cell, and discuss their electrocatalytic properties. After the OER tests in both KOH media, the Ni3N surfaces were converted into amorphous, nano-porous nickel oxide/(oxy)hydroxide surfaces. In the Fe-purified electrolyte, a decrease in OER activity was confirmed after the OER test because of the formation of pure NiOOH with low OER activity and electrical conductivity. Conversely, in the unpurified electrolyte, a continuous increase in OER activity was observed over the OER testing, which may have resulted from the Fe incorporation into the self-oxidation-formed NiOOH. Our experimental findings revealed that Fe impurities play an essential role in obtaining notable OER activity using the Ni3N precatalyst. Additionally, our Ni3N/Ni foam electrode exhibited a low OER overpotential of 262 mV to reach a geometric current density of 10 mA cmgeo−2 in a flow cell with unpurified electrolyte.
RSC

Mass transport-enhanced electrodeposition of Ni–S–P–O films on nickel foam for electrochemical water splitting

R.A. Marquez-Montes, K. Kawashima , Y.J. Son , and 4 more authors

Journal of Materials Chemistry A, Feb 2021

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Electrochemical water splitting is one of the most promising approaches for sustainable energy conversion and storage toward a future hydrogen society. This demands durable and affordable electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). In this study, we report the preparation of uniform Ni–P–O, Ni–S–O, and Ni–S–P–O electrocatalytic films on nickel foam (NF) substrates via flow cell-assisted electrodeposition. Remarkably, electrodeposition onto 12 cm2 substrates was optimized by strategically varying critical parameters. The high quality and reproducibility of the materials is attributed to the use of a 3D-printed flow cell with a tailored design. Then, the as-fabricated electrodes were tested for overall water splitting in the same flow cell under alkaline conditions. The best-performing sample, NiSP/NF, required relatively low overpotentials of 93 mV for the HER and 259 mV for the OER to produce a current density of 10 mA cm−2. Importantly, the electrodeposited films underwent oxidation into amorphous nickel (oxy)hydroxides and oxidized S and P species, improving both HER and OER performance. The superior electrocatalytic performance of the Ni–S–P–O films originates from the unique reconstruction process during the HER/OER. Furthermore, the overall water splitting test using the NiSP/NF couple required a low cell voltage of only 1.85 V to deliver a current density of 100 mA cm−2. Overall, we demonstrate that high-quality electrocatalysts can be obtained using a simple and reproducible electrodeposition method in a robust 3D-printed flow cell.
Elsevier

Seasonal evidences of microplastics in environmental matrices of a tourist dominated urban estuary in Gulf of Mexico, Mexico

L.J. Sánchez-Hernández , P. Ramírez-Romero , F. Rodríguez-González , and 4 more authors

Chemosphere, Aug 2021

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The study evaluated the spatial and seasonal variations of microplastic abundances in water, sediments, and commercial fishes of a semi-urban tourist impacted estuary in the Gulf of Mexico, Mexico. The prevalence of microplastics (MPs) elucidated diffuse sources namely long-range transport, domestic, agricultural, fishing, industrial and recreational activities and the local climatic conditions. Seasonally, the mean abundances of MPs in both water and sediments were high during Nortes (strong winds) followed by the dry and rainy seasons. Overall, black and blue colored MPs dominated the region and all the recovered plastics were fibers. The commercial fishes (n = 187) contained 881 MPs in their gastrointestinal tracts, suggesting that the food web of the estuary is highly prone to microplastic contamination. SEM images of extracted plastic fibers presented surface morphologies that are impacted by physical strains. Further, the elemental characterization of fibers using EDX displayed significant peaks of Al, As, Cl, Cr, Cu, Pb, and Zn that were used as additives during the production of plastics. The main types of polymers included low-density polyethylene, polyester, polypropylene, polycarbonate, rayon, polyvinyl chloride, polyacrylonitrile, polyamide, nylon and polyethylene terephthalate. MP abundances demonstrated in this study elucidate that estuaries are a major conduit for land-derived plastics to the ocean and the results will aid in implementing remedial/clean up actions of the estuary for better conservation of the ecosystem.

2020

ACS

Simultaneous sulfite electrolysis and hydrogen production using Ni foam-based three-dimensional electrodes

R.A. Marquez-Montes, K. Kawashima , K.M. Vo , and 4 more authors

Environmental Science & Technology, Sep 2020

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The electrochemical oxidation of sulfite ions offers encouraging advantages for large-scale hydrogen production, while sulfur dioxide emissions can be effectively used to obtain value-added byproducts. Herein, the performance and stability during sulfite electrolysis under alkaline conditions are evaluated. Nickel foam (NF) substrates were functionalized as the anode and cathode through electrochemical deposition of palladium and chemical oxidation to carry out the sulfite electro-oxidation and hydrogen evolution reactions, respectively. A combined analytical approach in which a robust electrochemical flow cell was coupled to different in situ and ex situ measurements was successfully implemented to monitor the activity and stability during electrolysis. Overall, satisfactory sulfite conversion and hydrogen production efficiencies (>90%) at 10 mA·cm–2 were mainly attributed to the use of NF in three-dimensional electrodes with a large surface area and enhanced mass transfer. Furthermore, stabilization processes associated with electrochemical dissolution and sulfur crossover through the membrane induced specific changes in the chemical and physical properties of the electrodes after electrolysis. This study demonstrates that NF-based electrocatalysts can be incorporated in an efficient electrochemical flow cell system for sulfite electrolysis and hydrogen production, with potential applications at a large scale.
ACS

Evaluation of a V8C7 Anode for Oxygen Evolution in Alkaline Media: Unusual Morphological Behavior

K. Kawashima , C.L. Cao , H. Li , and 4 more authors

ACS Sustainable Chemistry & Engineering, Aug 2020

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Metallic vanadium carbide (V8C7) with cubic symmetry is examined as an oxygen evolution reaction (OER) precatalyst in alkaline media. Herein, we used quasi in situ scanning electron microscopy and energy-dispersive X-ray spectrometry to investigate the structural transformation of the precatalyst V8C7 microparticles during extended cyclic voltammetric (CV) OER testing. Interestingly, an anisotropic morphological transformation (from a distorted sphere to a cuboid) of V8C7 was observed. Our theoretical and experimental results strongly suggest that this morphological change happens due to the selective self-oxidation and dissolution of the V8C7(110) and (111) surfaces and the subsequent exposure of the relatively stable (100), (010), and (001) surfaces. These results also suggest that these stable facets are preferable for the OER, and a current density of 10 mA·cm–2 was delivered at an overpotential of 503 mV after the extended CV OER testing.
Elsevier

Implementing a sustainable photochemical step to produce value-added products in flue gas desulfurization

R.E. Orozco-Mena , R.A. Marquez, K.I. Mora-Domínguez , and 4 more authors

Chemical Engineering Journal, Feb 2020

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Coal consumption has led to a worrying surge in flue gas emissions, aggravating health and environmental issues worldwide. Therefore, novel approaches supported by sustainable technologies are needed to exploit flue gas emissions effectively. Here, we evaluate an ammonia-based flue gas desulfurization process based on the photochemical capture of CO2 and SO2 for producing hydrogen and ammonium-based fertilizers. The kinetics and mechanistic pathways for sulfite photooxidation are examined, while process feasibility and environmental relevance are evaluated by process simulation and life cycle assessment. Results show that increasing the initial sulfite concentration results in faster reaction rates, following first-order kinetics with an activation energy of 32.9 kJ/mol. Moreover, kinetics is ascribed to two competing photoinduced radical-mediated reaction pathways. Finally, powering the process by renewable energy leads to positive human health, ecosystems, and resources scores in life cycle assessment, supporting the sustainability of the process. Overall, this approach offers essential insights for large-scale hydrogen production through the photooxidation of captured flue gas emissions.
Elsevier

Optimization of the electrooxidation of aqueous ammonium sulfite for hydrogen production at near-neutral pH using response surface methodology

R.A. Marquez-Montes, R.E. Orozco-Mena , A.A. Camacho-Dávila , and 3 more authors

International Journal of Hydrogen Energy, May 2020

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Sulfur-based thermochemical cycles, such as the hybrid sulfur-ammonia (HySA) cycle, offer a valuable approach in which hydrogen is produced by exploiting sulfur dioxide (potentially pollutant emissions) through the electrochemical oxidation of aqueous sulfite. In this study, the effect of pH on electrooxidation rate was assessed by comparing different reaction scenarios. Then, a Central Composite Design (CCD) combined with a Response Surface Methodology (RSM) was used to optimize batch electrooxidation of ammonium sulfite at near-neutral pH. Results show that the use of an anion exchange membrane (AEM) greatly improves sulfite electrooxidation rate while pH is effectively stabilized. Furthermore, a second-order model that relates applied potential and sulfite concentration with the normalized half-life of the reaction was obtained and verified experimentally at long-term batch electrooxidations. A good agreement between the model and experimental tests, adequate hydrogen recoveries and low sulfur crossover through the membrane demonstrate practical robustness of this approach.
ACS

Electrochemical engineering assessment of a novel 3D-printed filter-press electrochemical reactor for multipurpose laboratory applications

R.A. Marquez-Montes, V.H. Collins-Martínez , I. Pérez-Reyes , and 3 more authors

ACS Sustainable Chemistry & Engineering. Check out recordings of ink injection tests. , Feb 2020

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Flow devices fabricated by means of 3D-printing offer an economic and effective approach for testing different electrochemical systems at the laboratory scale. Here, the fabrication and optimization of a novel filter-press electrochemical reactor is described. 3D-printing is used to obtain critical components of the device as a sustainable and efficient manufacturing approach. Hydrodynamics and mass transfer of different flow distributors, turbulence promoters, and nickel foam, as a three-dimensional (3D) electrode, were evaluated by a convenient set of well-known techniques for filter-press reactor characterization. Furthermore, the chemical stability of 3D-printed materials was assessed in several electrolytes used for common electrochemical applications. Designed configurations and geometries exhibited enhanced turbulence and large mass transfer coefficients, which make them adequate for processes such as electrosynthesis, electrodeposition, and electrochemical water splitting. Ultimately, superior performance was validated for nickel foam, demonstrating robustness of the reactor for realistic evaluation of electrocatalytic materials. Therefore, the proposed electrochemical reactor provides a low-cost and versatile alternative for testing electrochemical systems in a wide range of applications.

2019

Elsevier

Sulfur dioxide exploitation by electrochemical oxidation of sulfite in near-neutral pH electrolytes: A kinetics and mechanistic study

R.A. Marquez-Montes, R.E. Orozco-Mena , D. Lardizábal-Gutiérrez , and 3 more authors

Electrochemistry Communications, Jul 2019

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Electrochemical oxidation of sulfite ions offers an efficient and profitable approach to the conversion of sulfur dioxide, a harmful air pollutant, into valuable by-products via flue gas desulfurization. Here, the electrochemical oxidation of sulfite in near-neutral pH electrolytes is studied in order to determine kinetic parameters and a reaction mechanism. Sulfite electrooxidation on palladium is demonstrated at pH 7.5 and 8.5, the latter being comparable to platinum. Anodic charge transfer coefficients and non-linear reaction orders are linked to a mechanism which involves sulfite adsorption at low potentials. This study proves that sulfite electrooxidation on palladium at near-neutral pH is a useful approach for sulfur dioxide exploitation.