Manuel Edgardo Gomez Winkler

The nitrate reduction reaction (NO₃RR) offers a sustainable route to ammonia under ambient conditions, yet its practical implementation is hindered by sluggish kinetics and poor understanding of catalyst evolution under bias. Here, we used Co₃O₄ nanoplates as model electrocatalysts for NO₃RR and NO₂RR, and systematically evaluated their morphological and structural evolution after an accelerated electrochemical aging protocol (1000 CV cycles). By comparing pristine and aged electrodes, we probe changes in charge-transfer dynamics, interfacial resistance, surface defect density, nanoparticle aggregation, and the Co²⁺/Co³⁺ ratio, using in situ and ex situ electrochemical and spectroscopic techniques. Density functional theory calculations provide insights into the evolution of surface energetics and the adsorption behavior of key intermediates. The aged catalyst exhibits faster charge-transfer kinetics and lower resistance, associated with surface reconstruction and modified adsorption properties. Specifically, weakened *NO₂ adsorption and a more thermoneutral *H binding promote hydrogen evolution (HER), shifting product selectivity. Although the Faradaic efficiency toward NH₃ of the aged surface decreases, higher yield rates are achieved due to enhanced kinetics. These findings underscore that dynamic changes in Co₃O₄ morphology and surface chemistry govern the stabilization of intermediates and performance over time. Understanding such dynamic behavior provides a foundation for the rational design of catalysts and efficient NO₃^– usage.

Cobalt- and copper-based oxides have emerged as cost-effective electrocatalysts for the electrochemical nitrate reduction reaction (NO3RR) to ammonia. However, the cathodic potentials required for NO3RR induce irreversible structural transformations that often compromise catalyst stability and selectivity, depending on the applied electrochemical protocol. To understand the resulting dynamic structure-performance relationships and improve nitrate-to-ammonia conversion, a tandem Co3O4-CuxO electrocatalyst was prepared by electrodeposition followed by thermal treatment, and two surface activation strategies were employed: by cycles of cyclic voltammetry (CV) or by holding at constant potential by chronoamperometry (CA). The CA-reconstructed Co/Cu mixed oxide-derived electrocatalyst exhibited higher faradaic efficiency (FE) toward ammonia across the entire potential window studied (0.00 VRHE to 0.40 VRHE). The reconstruction effects induced by both electrochemical protocols were systematically investigated, revealing morphology, structural, and composition changes that underpin the improved nitrate-to-ammonia conversion. Furthermore, in situ and online electrochemical techniques were employed to identify intermediates and active sites, providing new mechanistic insights into the electrochemical nitrate-to-ammonia conversion pathway. These findings contribute to understanding dynamic reconstruction phenomena and offer design guidelines for more stable and selective mixed oxide electrocatalysts for sustainable ammonia production.

Single-Atom Catalysts (SAC) have emerged as a promising class of materials for various catalytic applications, including the electrochemical nitrate reduction reaction (eNO3RR) and consequently ammonia production. While the efficiency and selectivity of these materials have been extensively highlighted for the eNO3RR, the in situ evolution to their structure and composition during electrocatalysis are largely unexplored and lacks catalyst design principles. To solve this, we investigated a series of high utilization metal-nitrogen-carbon (MNC) SACs (M = Cr, Fe, Co, Ni, and Cu) for eNO3RR. Except for CuNC, which selectively produced nitrite, all catalysts exhibited Faradaic efficiencies (FE) for ammonia exceeding 50%. NiNC demonstrated the highest performance (FE of 78.0 ± 2.9% at −0.4 V versus reversible hydrogen electrode (RHE) at pH 13 and maximum ammonia production rate of 615.7 ± 176.5 µmol·h−1· cm_geo^-2), corresponding to an energy efficiency (EE) of 15.1 ± 1.4% at −0.6 VRHE), followed by CoNC. In situ Synchrotron X-ray fluorescence (SXRF) mapping at various cathodic potentials (from open circuit potential to 0.0 VRHE and then −0.6 VRHE at 100 mV steps) revealed significant mobility of Ni within the carbon matrix, leading to the formation of metallic clusters from 0.0 VRHE. Similar in situ metal clustering was observed for CoNC. Structure-activity plots were generated from both MNC literature and results obtained here, finding a clear trend between OH binding energy and turnover frequency, with the high activity of NiNC and CoNC in this work explained by their stronger OH binding in the metallic structure compared to their SAC coordination. This work therefore reveals the structure-activity-stability of MNCs for eNO3RR and provides a simple descriptor for identifying highly active eNO3RR catalysts and their in situ structural evolution.

Nitrate electroreduction reaction (NO₃RR) to ammonia (NH₃) still faces fundamental and technological challenges. While Cu-based catalysts have been widely explored, their activity and stability relationship are still not fully understood. Here, we systematically monitored the dynamic alterations in the chemical and morphological characteristics of Cu₂O nanocubes (NCs) during NO₃RR in an alkaline electrolyte. In 1 h of electrolysis from -0.10 to -0.60 V vs RHE, the electrocatalyst achieved the maximum NH₃ faradaic efficiency (FE) and yield rate at -0.3 V (94% and 149 μmol h^-1 cm^-2, respectively). Similar efficiency could be found at a lower overpotential (-0.20 V vs RHE) in long-term electrolysis. At -0.20 V vs RHE, the catalyst FE increased from 73% in the first 2 h to ∼90% in 10 h of electrolysis. Electron microscopy revealed the loss of the cubic shape with the formation of sintered domains. In situ Raman, X-ray diffraction (XRD), and in situ Cu K-edge X-ray absorption near-edge spectroscopy (XANES) indicated the reduction of Cu₂O to oxide-derived Cu⁰ (OD-Cu). Nevertheless, a remaining Cu₂O phase was noticed after 1 h of electrolysis at -0.3 V vs RHE. This observation indicates that the activity and selectivity of the initially well-defined Cu₂O NCs are not solely dependent on the initial structure. Instead, it underscores the emergence of an OD-Cu-rich surface, evolving from near-surface to underlying layers over time and playing a crucial role in the reaction pathways. By employing online differential electrochemical mass spectrometry (DEMS) and in situ Fourier transform infrared spectroscopy (FTIR), we experimentally probed the presence of key intermediates (NO and NH₂OH) and byproducts of NO₃RR (N₂ and N₂H _x ) for NH₃ formation. These results show a complex relationship between activity and stability of the nanostructured Cu₂O oxide catalyst for NO₃RR.

Obtaining CO and H₂ from electrochemical CO₂ reduction (CO₂RR) offers a viable alternative to reduce CO₂ emissions and produce chemicals and fuels. Herein, we report a simple strategy for obtaining polycrystalline copper deposited on oxidized graphite felt (Cu-OGF) and its performance on the selective conversion of CO₂ and H₂O to CO and H₂. For the electrode obtaining, graphite felt (GF) was first oxidized (OGF) in order to make the substrate hydrophilic and then copper particles were electrochemically deposited onto OGF. The pH of deposition was investigated, and the CO₂RR activity was assessed for the prepared electrodes at each pH (2.0, 4.0, 6.0, 8.0, and 10.0). It was found that pH 2.0 was the most promising for CO₂RR due to the presence of hexagonal copper microparticles. Fourier transform infrared analysis of the produced gases showed that this is a low-cost catalyst capable of reducing CO₂ and H₂O to CO and H₂, with Faradaic efficiencies between 0.50 and 5.21% for CO and 50.87 to 98.30% for H₂, depending on the experimental conditions. Hence, it is possible for this gas mixture to be used as a fuel gas or to be enriched with CO for use in Fischer-Tropsch processes.

Gelatin (GE), amino-functionalized polyphenolic tannin derivative (TN), and graphene oxide (GO) were associated to yield thermo- and pH-responsive hydrogels for the first time. Durable hydrogel assemblies for drug delivery purposes were developed using the photosensitizer methylene blue (MB) as a drug model. The cooling GE/TN blends provide brittle physical assemblies. To overcome this disadvantage, different GO contents (between 0.31% and 1.02% wt/wt) were added to the GE/TN blend at 89.7/10.3 wt/wt. FTIR and RAMAN spectroscopy analyses characterized the materials, indicating GO presence in the hydrogels. Incorporation studies revealed a total MB (0.50 mg/mL) incorporation into the GE/TN-GO hydrogel matrices. Additionally, the proposed systems present a mechanical behavior similar to gel. The GO presence in the hydrogel matrices increased the elastic modulus from 516 to 1650 Pa. SEM revealed that hydrogels containing MB present higher porosity with interconnected pores. Dissolution and swelling degree studies revealed less stability of the GE/TN-GO-MB hydrogels in SGF medium (pH 1.2) than SIF (pH 6.8). The degradation increased in SIF with the GO content, making the polymeric matrices more hydrophilic. MB release studies revealed a process controlled by Fickian diffusion. Our results point out the pH-responsible behavior of mechanically reinforced GE/TN-GO-MB hydrogels for drug delivery systems purposes.