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1. Cooperative Fe sites on transition metal (oxy)hydroxides drive high oxygen evolution activity in base

   https://doi.org/10.1038/s41467-023-43305-z  

   Ou, Yingqing; Twight, Liam P; Samanta, Bipasa; Liu, Lu; Biswas, Santu; Fehrs, Jessica L; Sagui, Nicole A; Villalobos, Javier; Morales-Santelices, Joaquín; Antipin, Denis; et al (December 2023, Nature Communications)

   Fe-containing transition-metal (oxy)hydroxides are highly active oxygen-evolution reaction (OER) electrocatalysts in alkaline media and ubiquitously form across many materials systems. The complexity and dynamics of the Fe sites within the (oxy)hydroxide have slowed understanding of how and where the Fe-based active sites form—information critical for designing catalysts and electrolytes with higher activity and stability. We show that where/how Fe species in the electrolyte incorporate into host Ni or Co (oxy)hydroxides depends on the electrochemical history and structural properties of the host material. Substantially less Fe is incorporated from Fe-spiked electrolyte into Ni (oxy)hydroxide at anodic potentials, past the nominally Ni(2+/3+) redox wave, compared to during potential cycling. The Fe adsorbed under constant anodic potentials leads to impressively high per-Fe OER turn-over frequency (TOF(Fe)) of ~40 s−1 at 350 mV overpotential which we attribute to under-coordinated “surface” Fe. By systematically controlling the concentration of surface Fe, we find TOF(Fe) increases linearly with the Fe concentration. This suggests a changing OER mechanism with increased Fe concentration, consistent with a mechanism involving cooperative Fe sites in FeOx clusters. 

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2. (Invited Talk): Designing smart materials for efficient energy conversion: The story of transition metal chalcogenides

   Nath, Manashi; Masud, Jahangir; Swesi, Abdurazag; De Silva, Umanga; Amin, Bahar; Umapathi, Siddesh; Liyanage, Wipula (March 2018, 255th ACS National Meeting & Exposition, New Orleans, LA, United States, March 18-22, 2018 (2018), ENFL-330)

   Energy harvesting from solar and water has created ripples in materials energy research for the last several decades, complemented by the rise of Hydrogen as a clean fuel. Among these, water electrolysis leading to generation of oxygen and hydrogen, has been one of the most promising routes towards sustainable alternative energy generation and storage, with applications ranging from metal-​air batteries, fuel cells, to solar-​to-​fuel energy conversion systems. In fact, solar water splitting is one of the most promising method to produce Hydrogen without depleting fossil-​fuel based natural resources. However, the efficiency and practical feasibility of water electrolysis is limited by the anodic oxygen evolution reaction (OER)​, which is a kinetically sluggish, electron-​intensive uphill reaction. A slow OER process also slows the other half- cell reaction, i.e. the hydrogen evolution reaction (HER) at the cathode. Hence, designing efficient catalysts for OER process from earth-​abundant resources has been one of the primary concerns for advancing solar water splitting. In the Nath group we have focused on transition metal chalcogenides as efficient OER electrocatalysts. We have proposed the idea that these chalcogenides, specifically, selenides and tellurides will show much better OER catalytic activity due to increasing covalency around the catalytically active transition metal site, compared to the oxides caused by decreasing electronegativity of the anion, which in turn leads to variation of chem. potential around the transition metal center, [e.g. lowering the Ni 2+ -​-​> Ni 3+ oxidn. potential in Ni-​based catalysts where Ni 3+ is the actually catalytically active species]​. Based on such hypothesis, we have synthesized a plethora of transition metal selenides including those based on Ni, Ni-​Fe, Co, and Ni-​Co, which show high catalytic efficiency characterized by low onset potential and overpotential at 10 mA​/cm 2 [Ni 3 Se 2 - 200 - 290 mV; Co 7 Se 8 - 260 mV; FeNi 2 Se 4 -​NrGO - 170 mV (NrGO - N-​doped reduced graphene oxide)​; NiFe 2 Se 4 - 210 mV; CoNi 2 Se 4 - 190 mV; Ni 3 Te 2 - 180 mV]​. 

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3. Dual hydrogen production from electrocatalytic water reduction coupled with formaldehyde oxidation via a copper-silver electrocatalyst

   https://doi.org/10.1038/s41467-023-36142-7  

   Li, Guodong; Han, Guanqun; Wang, Lu; Cui, Xiaoyu; Moehring, Nicole K.; Kidambi, Piran R.; Jiang, De-en; Sun, Yujie (December 2023, Nature Communications)

   Abstract The broad employment of water electrolysis for hydrogen (H 2 ) production is restricted by its large voltage requirement and low energy conversion efficiency because of the sluggish oxygen evolution reaction (OER). Herein, we report a strategy to replace OER with a thermodynamically more favorable reaction, the partial oxidation of formaldehyde to formate under alkaline conditions, using a Cu 3 Ag 7 electrocatalyst. Such a strategy not only produces more valuable anodic product than O 2 but also releases H 2 at the anode with a small voltage input. Density functional theory studies indicate the H 2 C(OH)O intermediate from formaldehyde hydration can be better stabilized on Cu 3 Ag 7 than on Cu or Ag, leading to a lower C-H cleavage barrier. A two-electrode electrolyzer employing an electrocatalyst of Cu 3 Ag 7 (+)||Ni 3 N/Ni(–) can produce H 2 at both anode and cathode simultaneously with an apparent 200% Faradaic efficiency, reaching a current density of 500 mA/cm 2 with a cell voltage of only 0.60 V. 

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4. Sulfur incorporation into NiFe oxygen evolution electrocatalysts for improved high current density operation

   https://doi.org/10.1039/d2ma00902a  

   Wang, Jiaying; Barforoush, Joseph M.; Leonard, Kevin C. (January 2023, Materials Advances)

   The efficient production of green hydrogen via electrochemical water splitting is important for improving the sustainability and enabling the electrification of the chemical industry. One of the major goals of water electrolysis is to utilize non-precious metal catalysts, which can be accomplished with alkaline electrolyzer technologies. However, there is a continuing need for designing catalysts that can operate in alkaline environments with high efficiencies under high current densities. Here we describe a simple, aqueous-based synthesis method to incorporate sulfur into NiFe-based electrocatalysts for the oxygen evolution reaction (OER). Sulfur incorporation was able to reduce the overpotential for the OER from ca. 350 mV on a NiFe catalyst to ca. 290 mV on the NiFeS catalyst at 100 mA cm −2 on a flat supporting electrode. Electrochemical impedance spectroscopy data showed a small decrease in the charge transfer resistance of the NiFeS catalysts, showing that sulfur incorporation may improve the electronic conductivity. Surface-interrogation scanning electrochemical microscopy (SI-SECM) studies combined with Tafel slope analysis suggested that the NiFeS catalyst was able to have vacant surface sites available under OER conditions and was able to maintain a Tafel slope of 39 mV dec −1 . This is in contrast to the NiFe catalyst, for which the SI-SECM studies showed a saturated surface under OER conditions with the Tafel slope transitioning from 39 mV dec −1 to 118 mV dec −1 . The low Tafel slope enabled the NiFeS catalyst to maintain low overpotentials under high current densities, which we attribute to the ability of the NiFeS catalyst to maintain vacant sites during the OER. 

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5. Structure- and Morphology-Controlled Synthesis of Hexagonal Ni _2–x Zn _x P Nanocrystals and Their Composition-Dependent Electrocatalytic Activity for Hydrogen Evolution Reaction

   https://doi.org/10.1021/acsaem.4c00539  

   Graves, Lisa S; Sarkar, Rajib; Baker, Jordon; Lao, Ka Un; Arachchige, Indika U (July 2024, ACS Applied Energy Materials)

   Nickel phosphides are an emerging class of earth-abundant catalysts for hydrogen generation through water electrolysis. However, the hydrogen evolution reaction (HER) activity of Ni2P is lower than that of benchmark Pt group catalysts. To address this limitation, an integrated theoretical and experimental study was performed to enhance the HER activity and stability of hexagonal Ni2P through doping with synergistic transition metals. Among the nine dopants computationally studied, Zinc emerged as an ideal candidate due to its ability to modulate the hydrogen binding free energy (GH) closer to a thermoneutral value. Consequently, phase pure hexagonal Ni2-xZnxP nanocrystals (NCs) with a solid spherical morphology, variable compositions (x = 0–17.14%), and size in the range of 6.8 ± 1.1 to 9.1 ± 1.1 nm were colloidally synthesized to investigate the HER activity and stability in alkaline electrolytes. As predicted, the HER performance was observed to be composition-dependent with Zn compositions (x) of 0.03, 0.07, and 0.15 demonstrating superior activity with overpotentials (ɳ-10) of 188.67, 170.01, and 135.35 mV, respectively at a current density of -10 mA/cm2, in comparison to Ni2P NCs (216.2 ± 4.4 mV). Conversely, Ni2-xZnxP NCs with x = 0.01, 0.38, 0.44, and 0.50 compositions showed a notable decrease in HER activity, with corresponding ɳ-10 of 225.3 ± 3.2, 269.9 ± 4.3, 276.4 ± 3.7 and 263.9 ± 4.9 mV, respectively. The highest HER active catalyst was determined to be Ni1.85Zn0.15P NCs, featuring a Zn concentration of 5.24%, consistent with composition-dependent GH calculations. The highest performing Ni1.85Zn0.15P NCs displayed a Heyrovsky HER mechanism, enhanced kinetics and electrochemically active surface area (ECSA), and superior corrosion tolerance with a negligible increase of ɳ-10 after 10 h of continuous HER. This study provides critical insights into enhancing the performance of metal phosphides through doping-induced electronic structure variation, paving the way for the design of high-efficiency and durable nanostructures for heterogeneous catalytic studies. 

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