Advanced Nickel-Based Catalysts for Urea Oxidation Reaction: Challenges and Developments

Highly selective urea electrooxidation coupled with efficient hydrogen evolution

Electrochemical urea oxidation offers a sustainable avenue for H2 production and wastewater denitrification within the water-energy nexus; however, its wide application is limited by detrimental cyanate or nitrite production instead of innocuous N2. Herein we demonstrate that atomically isolated asymmetric Ni-O-Ti sites on Ti foam anode achieve a N2 selectivity of 99%, surpassing the connected symmetric Ni-O-Ni counterparts in documented Ni-based electrocatalysts with N2 selectivity below 55%, and also deliver a H2 evolution rate of 22.0 mL h-1 when coupled to a Pt counter cathode under 213 mA cm-2 at 1.40 VRHE. These asymmetric sites, featuring oxygenophilic Ti adjacent to Ni, favor interaction with the carbonyl over amino groups in urea, thus preventing premature resonant C⎓N bond breakage before intramolecular N-N coupling towards N2 evolution. A prototype device powered by a commercial Si photovoltaic cell is further developed for solar-powered on-site urine processing and decentralized H2 production.

Unraveling the Oxidation Kinetics Through Electronic Structure Regulation of MnCo2O4.5@Ni3S2 p-n Junction for Urea-Assisted Electrocatalytic Activity

A promising strategy to boost electrocatalytic performance is via assembly of hetero-nanostructured electrocatalysts that delivers the essential specific surface area and also active sites by lowering the reaction barrier. However, the challenges associated with the intricate designs and mechanisms remain underexplored. Therefore, the present study constructs a p-n junction in a free-standing MnCo2O4.5@Ni3S2 on Ni-Foam. The space-charge region's electrical characteristics is dramatically altered by the formed p-n junction, which enhances the electron transfer process for urea-assisted electrocatalytic water splitting (UOR). The optimal MnCo2O4.5@Ni3S2 electrocatalyst results in greater oxygen evolution reactivity (OER) than pure systems, delivering an overpotential of only 240 mV. Remarkably, upon employing as UOR electrode the required potential decreases to 30 mV. The impressive performance of the designed catalyst is attributed to the enhanced electrical conductivity, greater number of electrochemical active sites, and improved redox activity due to the junction interface formed between p-MnCo2O4.5 and n-Ni3S2. There are strong indications that the in situ formed extreme-surface NiOOH, starting from Ni3S2, boosts the electrocatalytic activity, i.e., the electrochemical  surface reconstruction generates the active species. In conclusion, this work presents a high-performance p-n junction design for broad use, together with a viable and affordable UOR electrocatalyst.

Barium-induced lattice expansion of Ni(OH)2: enhancing catalytic urea oxidation activity for energy-saving H2 production

Constructing an environmentally friendly and efficient electrocatalyst holds important and profound significance for energy-efficient hydrogen production. Replacing the oxygen evolution reaction with a lower potential urea oxidation reaction (UOR) may save energy in water electrolysis to produce hydrogen. The UOR is characterized by its high energy barrier, which results in slow reaction kinetics. In this study, we introduced Ba(OH)2 into Ni(OH)2 to form uniform nanosheets. Due to the introduction of Ba2+, the lattice expansion of Ni(OH)2 was triggered, leading to significant improvement in UOR activity. The catalyst achieved a current density of 100 mA cm-2 at only 1.316 V and exhibited remarkable stability over time. Density functional theory (DFT) calculations demonstrate that the Ba-Ni(OH)2 site significantly reduces the energy barrier for urea adsorption, intermediate steps, and desorption. This work provides a novel and environmentally friendly strategy for constructing energy-efficient and highly efficient catalysts through the doping of alkaline earth metals.

FeNi supported on carbon sponge for efficient urea oxidation in direct urea fuel cell

The direct urea fuel cell (DUFC) is a power generation equipment with urea-rich wastewater or urine as fuel source. It has the unique ability to purify sewage while simultaneously generating electricity, making it a highly efficient and environmentally friendly option. In this paper, pomegranate seed-like Ni nano-blocks and Fe nanosheets were synthesized by electrodeposition and chemical reduction and attached to the carbonized melamine sponge matrix. The N-doped carbon sponge (NCS) provided a large number of polyhedral holes, which allowed for efficient gas escape through channels. The combination of Fe reduces the initial urea oxidation potential, reaction activation energy and reaction resistance. The synthesized FeNi supported on N-doped carbon sponge composite (FeNi@NCS) has a catalytic current density of 625 mA cm-2 and a Tafel slope of 42.51 mV dec-1 for urea electrooxidation reaction (UOR). Assembling the direct urine-hydrogen peroxide fuel cell (DUrHPFC) resulted in the highest performance output. The open circuit voltage (OCV) was 0.98 V, and the peak power density reached 9.61 mW cm-2. The results show that the prepared catalyst provides an opportunity to solve the problems that hinder the development of urea green cycle at present.

Spherical Ni/NiO nanoparticles decorated on nanoporous carbon (NNC) as an active electrode material for urea and water oxidation reactions

Herein, we report a chemical method for scalable synthesis of spherical Ni/NiO nanoparticle-decorated nanoporous carbon (NNC) based electrocatalytic system using a simple and easy chemical method with ultra-high activity towards urea electrooxidation. Morphological analysis by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) confirms the formation of Ni/NiO NPs on highly nanoporous carbon with an average size of ∼50 nm. X-ray diffraction (XRD) confirms NNC with a face-centred cubic (FCC) crystal structure. Ni/NiO NPs intercalated with nanoporous carbon exhibited the best electrocatalytic performance towards urea oxidation with an ultra-low onset potential of ∼0.33 V vs. SCE, and faster electrokinetic mechanism confirmed from Tafel slope (∼45 mV dec-1), EIS Rct (∼6.98 Ω), and long term durability for 7 h at 10 mA cm-2 with high CO poisoning tolerance. This work affords noble metal-free electrocatalysts for novel appliances and remarkable potential for urea determination, hydrogen generation, real-time water remediation, and energy conversion.

Divalent Oxidation State Ni as an Active Intermediate in Prussian Blue Analogues for Electrocatalytic Urea Oxidation

Urea degradation is one of the most crucial links in the natural nitrogen cycle. Exploring the real active species in the urea electro-oxidation process is of great significance for understanding the urea electro-oxidation mechanism and designing catalysts. A highly active and stable Prussian blue analogue catalyst (PBA@NiFe/NF) loaded on nickel foam was synthesized for electro-oxidation of urea. In situ Raman spectra revealed that Ni in PBA@NiFe/NF was able to maintain a stable divalent nickel (Ni(II)) state for up to 3.5 h during the initial urea oxidation process, which is rarely reported in previous research studies. In addition, with the participation of iron, the Ni-Fe bimetallic center significantly improves the electro-oxidation of urea. Our work provides a new idea for prolonging the Ni(II) activity in electrocatalytic oxidation of urea.

Synthesis of Anion Exchange Membranes Containing PVDF/PES and Either PEI or Fumion®

In this work, the preparation of dense blended membranes, from blends of poly(vinylidene fluoride) (PVDF), poly(ether sulfone) (PES) and polyethyleneimine (PEI) or Fumion^®, with possible applications in alkaline fuel cell (AEMFC) is reported. The blended PEI/Fumion^® membranes were prepared under a controlled air atmosphere by a solvent evaporation method, and were characterized regarding water uptake, swelling ratio, thermogravimetric analysis (TGA), infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), ion exchange capacity (IEC), OH^- conductivity and novel hydroxide ion exchange rate (HIER), which is related to the mass transport capacity of the OH^- ions through the membrane. The effect of the chemical composition on its morphological and anion exchange properties was evaluated. It was expected that the usage of a commercial ionomer Fumion^®, in the blended membranes would result in better features in the electrical/ionic conductivity behaviour. However, two of the membranes containing PEI exhibited a higher HIER and OH^- conductivity than Fumion^® membranes, and were excellent option for potential applications in AEMFC, considering their performance and the cost of Fumion^®-based membranes.

Digital Pregnancy Test Powered by an Air-Breathing Paper-Based Microfluidic Fuel Cell Stack Using Human Urine as Fuel

The direct integration of paper-based microfluidic fuel cells (μFC's) toward creating autonomous lateral flow assays has attracted attention. Here, we show that an air-breathing paper-based μFC could be used as a power supply in pregnancy tests by oxidizing the human urine used for the diagnosis. We present an air-breathing paper-based μFC connected to a pregnancy test, and for the first time, as far as we know, it is powered by human urine without needing any external electrolyte. It uses TiO2-Ni as anode and Pt/C as cathode; the performance shows a maximum value of voltage and current and power densities of ∼0.96 V, 1.00 mA cm-2, and 0.23 mW cm-2, respectively. Furthermore, we present a simple design of a paper-based μFC's stack powered with urine that shows a maximum voltage and maximum current and power densities of ∼1.89 V, 2.77 mA cm-2 and 1.38 mW cm-2, respectively, which powers the display of a pregnancy test allowing to see the analysis results.

Recent Development of Nickel-Based Electrocatalysts for Urea Electrolysis in Alkaline Solution

Recently, urea electrolysis has been regarded as an up-and-coming pathway for the sustainability of hydrogen fuel production according to its far lower theoretical and thermodynamic electrolytic cell potential (0.37 V) compared to water electrolysis (1.23 V) and rectification of urea-rich wastewater pollution. The new era of the "hydrogen energy economy" involving urea electrolysis can efficiently promote the development of a low-carbon future. In recent decades, numerous inexpensive and fruitful nickel-based materials (metallic Ni, Ni-alloys, oxides/hydroxides, chalcogenides, nitrides and phosphides) have been explored as potential energy saving monofunctional and bifunctional electrocatalysts for urea electrolysis in alkaline solution. In this review, we start with a discussion about the basics and fundamentals of urea electrolysis, including the urea oxidation reaction (UOR) and the hydrogen evolution reaction (HER), and then discuss the strategies for designing electrocatalysts for the UOR, HER and both reactions (bifunctional). Next, the catalytic performance, mechanisms and factors including morphology, composition and electrode/electrolyte kinetics for the ameliorated and diminished activity of the various aforementioned nickel-based electrocatalysts for urea electrolysis, including monofunctional (UOR or HER) and bifunctional (UOR and HER) types, are summarized. Lastly, the features of persisting challenges, future prospects and expectations of unravelling the bifunctional electrocatalysts for urea-based energy conversion technologies, including urea electrolysis, urea fuel cells and photoelectrochemical urea splitting, are illuminated.

Bimetallic Cu/Fe MOF-Based Nanosheet Film via Binder-Free Drop-Casting Route: A Highly Efficient Urea-Electrolysis Catalyst

Developing efficient electrocatalysts for urea oxidation reaction (UOR) can be a promising alternative strategy to substitute the sluggish oxygen evolution reaction (OER), thereby producing hydrogen at a lower cell-voltage. Herein, we synthesized a binder-free thin film of ultrathin sheets of bimetallic Cu-Fe-based metal–organic frameworks (Cu/Fe-MOFs) on a nickel foam via a drop-casting route. In addition to the scalable route, the drop-casted film-electrode demonstrates the lower UOR potentials of 1.59, 1.58, 1.54, 1.51, 1.43 and 1.37 V vs. RHE to achieve the current densities of 2500, 2000, 1000, 500, 100 and 10 mA cm⁻², respectively. These UOR potentials are relatively lower than that acquired by the pristine Fe-MOF-based film-electrode synthesized via a similar route. For example, at 1.59 V vs. RHE, the Cu/Fe-MOF electrode exhibits a remarkably ultra-high anodic current density of 2500 mA cm⁻², while the pristine Fe-MOF electrode exhibits only 949.10 mA cm⁻². It is worth noting that the Cu/Fe-MOF electrode at this potential exhibits an OER current density of only 725 mA cm⁻², which is far inconsequential as compared to the UOR current densities, implying the profound impact of the bimetallic cores of the MOFs on catalyzing UOR. In addition, the Cu/Fe-MOF electrode also exhibits a long-term electrochemical robustness during UOR.