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Complete decarbonization of hard-to-abate industrial sectors is critical to reach the carbon neutrality goal set for 2050. The production of nitrogen-containing fertilizers (N-fertilizers) is responsible for 2.1% of the overall global carbon dioxide emissions. Urea is the most common N-fertilizer, and it is currently produced through the Bosch-Meiser process starting from ammonia (NH3) and carbon dioxide (CO2). Electrochemical production of urea can reduce drastically the emission of greenhouse gases and the energy required for the process. Promising results were recently reported using nitrate (NO3−) and CO2 as reagents with increasing production rate and faradaic efficiency. In this mini-review, we summarize the most recent studies, including reaction mechanisms, electrocatalysts, and detection methods, highlighting the challenges in the field. A roadmap for future developments is envisioned with the scope of reaching industrial requirements.

Within the framework of the circular economy, the waste litchi's skins were upgraded and transformed into electrocatalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). The waste litchi's skins were pyrolyzed, activated, and then used as carbon support for fabricating metal–nitrogen–carbons (M–N–Cs) which belong to a promising class of platinum group metal-free electrocatalysts. The activated char was functionalized with transition metal (Fe, Ni, and Co)- phthalocyanine (Pc) in monometallic and bimetallic fashion by subjecting it to a thermal treatment at 600 and 900°C. The samples functionalized at 900°C showed higher performance for HER due to the formation of metal nanoparticles, whereas the samples functionalized at 600°C showed higher performance for ORR. Particularly, sample Ni–Co 900 had an overpotential of −0.38 V for HER, while the sample Fe 600 was the most active electrocatalyst for ORR by demonstrating the onset potential of ∼0.9 V (a half-wave potential of ∼0.81 V) with the least production of unwanted peroxide anion.

Following the core theme of a circular economy, a novel strategy to upcycle cigarette butt waste into platinum group metal (PGM)-free metal nitrogen carbon (M-N-C) electrocatalysts for oxygen reduction reaction (ORR) is presented. The experimental route was composed of (i) the transformation of the powdered cigarette butts into carbonaceous char via pyrolysis at 450 °C, 600 °C, 750 °C and 900 °C, (ii) the porosity activation with KOH and (iii) the functionalization of the activated chars with iron (II) phthalocyanine (FePc). The electrochemical outcomes obtained by the rotating disk electrode (RRDE) technique revealed that the sample pyrolyzed at 450 °C (i.e., cig_450) outperformed the other counterparts with its highest onset (Eon) and half-wave potentials (E1/2) and demonstrated nearly tetra-electronic ORR in acidic, neutral and alkaline electrolytes, all resulting from the optimal surface chemistry and textural properties.

Metal-nitrogen-carbons (M-N-Cs) as a reliable substitution for platinum-group-metals (PGMs) for oxygen reduction reaction (ORR) are emerging candidates to rationalize the technology of fuel cells. The development of M-N-Cs can further be economized by consuming waste biomass as an inexpensive carbon source for the electrocatalyst support. Herein, we report the simple fabrication and in-depth characterization of electrocatalysts using lignin-derived activated char. The activated char (LAC) was functionalized with metal phthalocyanine (FePc and MnPc) via atmosphere-controlled pyrolysis to produce monometallic M-N-Cs (L_Mn and L_Fe) and bimetallic M1-M2-N-Cs (L_FeMn) electrocatalysts. Raman spectroscopy and transmission electron microscopy (TEM) revealed a defect-rich architecture. XPS confirmed the coexistence of various nitrogen-containing active moieties. L_Fe and L_FeMn demonstrated appreciable ORR in both acidic and alkaline conditions whereas L_FeMn helped in restricting the peroxide yield, particularly in alkaline media. L_Fe and L_FeMn demonstrated remarkable onset potential (Eonset) of ∼0.942 V (vs RHE) with an E1/2 of 0.874 V (vs RHE) in 0.1 M KOH. In acid, L_FeMn had an Eonset of 0.817 V (vs RHE) and an E1/2 of ∼0.76 V (vs RHE). Finally, the L_FeMn as a cathode electrocatalyst was integrated and tested in PEMFC and AEMFC. AEMFC demonstrated optimistic performance with a peak power density of 261 mW cm−2 at the current density of ∼577 mA cm−2.

Mass generation of scrap tires presents a major challenge for environmental safety, however, their upcycling into carbon-based nanomaterials by the virtue of pyrolysis treatments can open up new windows for energy conversion and storage technologies in the context of the circular economy. Herein, we report the synthesis of Fe-N-C oxygen reduction reaction (ORR) electrocatalyst for fuel cell (FC) applications using carbonaceous char derived from scrap tires through microwave-assisted pyrolysis (MAP). The char obtained from MAP was activated with potassium hydroxide and then pyrolyzed at a high temperature to fabricate Fe-N-C after mixing with iron and nitrogen precursors. Finally, the developed Fe-N-C was ball-milled and acid-etched for homogenization and leaching of iron oxide nanoparticles. In this study, structural evaluation during each synthesis step was elucidated and correlated with the ORR activity in all three pHs i.e. acidic, neutral, and alkaline. Moreover, the effect of electrocatalyst loading on ORR kinetics was also analyzed using two different loadings (0.2 and 0.6 mg cm−2) on the rotating ring disk electrode (RRDE). The developed Fe-N-C demonstrated encouraging onset potentials of 0.881, 0.822, and 0.936 V vs RHE in acidic, neutral, and alkaline conditions, respectively. Whereas the ORR activity was slightly reduced after the milling-etching step. Lower peroxide yield together with a tetra-electronic reduction of oxygen was witnessed in acidic and neutral conditions, however, peroxide production was increased in the alkaline medium.

CuS nanoparticles (CuS NPs) are used in a variety of optical, photo-chemical, and photo-electrochemical applications. An unsophisticated but efficient strategy is reported here to enhance the performance of CuS by transforming it into CuS.PbS nanocomposites (NCs). A modified successive ionic layer adsorption and reaction method was employed to combine CuS NPs with PbS quantum-dots in a qualitatively-controlled way. Resultant CuS.PbS NCs were first qualitatively investigated using scanning electron microscopy coupled with an energy-dispersive spectroscope and an X-ray diffractometer. Owing to the improved optical behavior revealed by UV–vis spectroscopy, CuS.PbS NCs were employed first for photo-catalysis and then for electro-catalysis, where optimal CuS.PbS NCs (specifically, CuS.2PbS) ensued an ∼80 % degradation of the toxic Congo red dye. Moreover, the efficacy of the controlled deposition of PbS on CuS NPs was electrochemically investigated and a significant improvement in the imperative parameters, such as the charge transfer resistance at the electrode/electrolyte interface and electron lifetime, was obtained. CuS.2PbS NCs also exhibited air-processable and heat-treatable characteristics which demarcated its suitability for paintable counter electrodes (CEs) in quantum-dot-sensitized solar cells (QDSCs).