Synergistic etching of nickel foam by Fe3+ and Cl- ions to synthesize nickel-iron-layered double hydroxide nanolayers with abundant oxygen vacancies for superior urea oxidation.

Urea electrolysis is an appealing topic for hydrogen production due to its ability to extract hydrogen at a lower potential. However, it is plagued by sluggish kinetics and noble-metal catalyst requirements. Herein, we developed nickel-iron-layered double hydroxide (NiFe-LDH) nanolayers with abundant oxygen vacancies (OV) via synergistically etching nickel foam with Fe3+ and Cl- ions, enabling the efficient conversion of urea into H2 and N2. The synthesized OV-NiFe-LDH exhibits a lower potential (1.30 vs. reversible hydrogen electrode, RHE) for achieving 10 mA cm-2 in the urea oxidation reaction (UOR), surpassing most recently reported Ni-based electrodes. OV provides favorable conductivity and a large surface area, which results in a 4.1-fold in electron transport and a 5.1-fold increase in catalyst reactive sites. Density Functional Theory (DFT) calculations indicate that OV can lower the adsorption energy of urea, and enhance the bonding strength of *CONHNH, giving rise to improved UOR. This study provides a viable path toward economical and efficient production of high-purity hydrogen.

Ni doped amorphous FeOOH layer as ultrafast hole transfer channel for enhanced PEC performance of BiVO4.

Bismuth vanadate (BiVO4), as the potential and prospective photocatalyst, has been limited by the issue of poor separation and transfer of charge carrier for photoelectrocatalytic (PEC) water oxidation. Here, a significant increase of surface injection efficiency for BiVO4 is realized by the rationally designed Ni doped FeOOH (Ni:FeOOH) layer growing on BiVO4 photoanode (Ni:FeOOH/BiVO4), in which doped Ni2+ can induce partial-charge of FeOOH to serve as ultrafast transfer channel for hole transfer and transportation at the semiconductor/electrolyte interface. In addition, the Ni:FeOOH/BiVO4 shows the ηsurface value of 81.6 %, which is 3.28-fold and 1.47-fold of BiVO4 and FeOOH/BiVO4, respectively. The photocurrent density of Ni:FeOOH/BiVO4 is 4.21 mA cm-2 at 1.23 V vs. RHE, with the onset potential cathodically shifting 237 mV over BiVO4 and a long-term stability for suppressing surface charge recombination. The UPS and UV-Vis spectra have confirmed the type-II band alignment between Ni:FeOOH and BiVO4 for promoting carrier transfer. This facile and effective spin-coating method could deposit oxygen evolution catalysts (OECs) availably onto photoanodes with enhanced PEC water splitting.

Nitrogen-containing wastewater fuel cells for total nitrogen removal and energy recovery based on Cl•/ClO• oxidation of ammonia nitrogen.

The excess nitrogen discharge into water bodies has resulted in extensive water pollution and human health risks, which has become a critical global issue. Moreover, nitrogenous wastewater contains considerable chemical energy contributed by organic pollutants and nitrogenous compounds. Therefore, the treatment of various kinds of nitrogen-containing wastewater for nitrogen removal and energy recovery is of significance. Biological methode and advanced oxidation processes (AOPs) are the main methods for nitrogen removal. However, biological treatment is easily inhibited by high-salinity, high ammonia nitrogen (NH3-N/NH4+-N), nitrite and toxic organics in wastewater, which limits its application. AOPs mainly induce in situ generation of highly reactive species, such as hydroxyl radical (HO•), sulfate radical (SO4•-) and chlorine radicals (Cl•, ClO•, Cl2•-), for nitrogen removal. Nevertheless, HO• shows low reactivity and N2 selectivity towards NH3-N/NH4+-N oxidation, and SO4•- also demonstrates unsatisfactory NH3-N/NH4+-N removal. It has been shown that Cl•/ClO• can efficiently remove NH3-N/NH4+-N with high N2 selectivity. The generation of Cl•/ClO• can be triggered by various techniques, among which the PEC technique shows great potential due to its higher efficiency for Cl•/ClO• generation and eco-friendly approach for pollutants degradation and energy recovery by utilizing solar energy. Cl•/ClO• oxidation of NH3-N/NH4+-N and nitrate nitrogen (NO3--N) reduction can be strengthened through the design of photoanode and cathode materials, respectively. Coupling with this two pathways, an exhaustive total nitrogen (TN) removal system is designed for complete TN removal. When introducing the mechanism into photocatalytic fuel cells (PFCs), the concept of nitrogen-containing wastewater fuel cells (NFCs) is proposed to treat several typical types of nitrogen-containing wastewater, achieving high-efficiency TN removal, organics degradation, toxic chlorate control, and energy recovery simultaneously. Recent research progress in this field is reviewed, summarized and discussed, and in-depth perspectives are proposed, providing new ideas for the resource treatment of nitrogen-containing wastewater.

Unconventional Substitution for BiVO4 to Enhance Photoelectrocatalytic Performance by Accelerating Polaron Hopping.

Bismuth vanadate (BiVO4) as a fascinating semiconductor for photoelectrocatalytic (PEC) water oxidation with suitable band gap (Eg) has been limited by the issue of poor separation and transportation of charge carriers. Herein, we propose an unconventional substitution of V5+ sites by Ti4+ in BiVO4 (Ti:BiVO4) for the similar ionic radii and accelerated polaron hopping. Ti:BiVO4 increased the photocurrent density 1.90 times up to 2.51 mA cm-2 at 1.23 V vs RHE and increased the charge carrier density 1.81 times to 5.86 × 1018 cm-3. Compared with bare BiVO4, Ti:BiVO4 improves the bulk separation efficiency to 88.3% at 1.23 V vs RHE. The DFT calculations have illustrated that Ti-doping modification could decrease the polaron hopping energy barrier, narrow the Eg, and decrease the overpotential of the oxygen evolution reaction (OER) concurrently. With further spin-coated FeOOH cocatalyst, the photoanode has a photocurrent density of 3.99 mA cm-2 at 1.23 V vs RHE. The excellent PEC performance of FeOOH/Ti:BiVO4 is attributed to the synergistic effect of the FeOOH layer and Ti doping, which could promote charge carrier separation and transfer by expediting polaron migration.

Facile, Controllable, and Ultrathin NiFe-LDH In Situ Grown on a Ni Foam by Ultrasonic Self-Etching for Highly Efficient Urine Conversion.

As the primary source of nitrogen pollutants in domestic sewage, urine is also an alternative for H2 production via electrochemical processes. However, it suffers from sluggish kinetics and noble-metal catalyst requirement. Here, we report a non-precious ultrathin NiFe-layered double hydroxide catalyst for the remarkable conversion of urea into N2 and H2, which is in situ grown on a Ni foam via ultrasonic self-etching in Fe3+/ethylene glycol (EG). EG regulates the etching rate of Fe3+, resulting in an ultrathin nanosheet structure with the aid of ultrasonication. This structure dramatically promotes the dehydrogenation process via decreasing the nanolayer thickness from 120 to 3.4 nm and leads to a 4.8-fold increase in the generation of active sites. It exhibits record urea oxidation kinetics (390.8 mA·cm-2 at 1.5 V vs RHE) with excellent stability (120 h), which is 11.8 times better than that of commercial Pt/C catalyst (33.1 mA·cm-2). Tests with real urine at 20 mA cm-2 achieve 74% total nitrogen removal and 2853 µmol·h-1 of H2 production. This study provides an attractive landscape for producing H2 by consuming urine biowastes.

Type-II Heterojunction CdIn2S4/BiVO4 Coupling with CQDs to Improve PEC Water Splitting Performance Synergistically.

Bismuth vanadate (BiVO4) has been considered as a promising photoelectrocatalytic (PEC) semiconductor, but suffers from severe hole recombination, attributed to the short hole-diffusion length and the low carrier mobility. Herein, a type-II heterojunction CdIn2S4/BiVO4 is designed to improve the photocurrent density from 1.22 (pristine BiVO4) to 2.68 mA cm-2 at 1.23 V vs the reversible hydrogen electrode (RHE), accelerating the bulk separation of photogenerated carriers by the built-in field from the matched energy band. With the introduction of CQDs, CQDs/CdIn2S4/BiVO4 increases the photocurrent density to 4.84 mA cm-2, enhancing the light absorption and cathodically shifting its onset potential, due to the synergetic effect of the heterojunction and CQDs. Compared with BiVO4, CQDs/CdIn2S4/BiVO4 promotes the bulk separation efficiency to 94.6% and the surface injection efficiency to 72.2%. Additionally, spin-coating of FeOOH on CQDs/CdIn2S4/BiVO4 could further improve the PEC performance and keep a long stability for water splitting. The density function theory (DFT) calculations illustrated that the type-II heterojunction CdIn2S4/BiVO4 could decrease the oxygen evolution reaction (OER) overpotential and accelerate bulk charge separation for the built-in field of the aligned band structure.

Highly efficient removal of total nitrogen and dissolved organic compound in waste reverse osmosis concentrate mediated by chlorine radical on 3D Co3O4 nanowires anode.

Reverse osmosis concentrate (ROC) from wastewater reclamation has posed significant disposal challenges due to its highly concentrated NH3-N, chloride ion and bio-refractory organics, and developing technologies for their removal are essential. Herein, we developed an efficient electrochemical system to remove total nitrogen and dissolved organic compound (DOC) simultaneously mediated by chlorine radical (Cl•), which is generated by activation of chloride ion existing in ROC on an inexpensive, three-dimensional Co3O4 nanowires. Results showed that the total nitrogen and total organic carbon removal were 98.2% and 56.9% in 60 min for synthetic ROC with 56 mg/L of NH3-N and 20 mg/L of DOC. The utilization of Co3O4 nanowires enhanced NH3-N degradation by 2.58 times compared with Co3O4 nanoplates, which were 1.69 and 17.5 times these of RuO2 and Pt. We found that structural Co3+/Co2+ acts as cyclic catalysis to produce Cl• via single-electron transfer, which convert NH3-N to N2 and lead to faster DOC degradation. This architecture provides abundant catalytic sites and sufficient accessibility of reactants. Small amount of nitrate generated by oxidation of NH3-N was further reduced to N2 on Pd-Cu/NF cathode. These findings provide new insights for utilization of waste Cl- and development of novel electrochemical system for ROC disposal.

The design of high performance photoanode of CQDs/TiO2/WO3 based on DFT alignment of lattice parameter and energy band, and charge distribution.

Photoanode is the key issue for photoelectrocatalytic (PEC) water splitting and organics degradation. However, it always faces several restrictions including severe photocorrosion, low charge separation and transfer efficiencies, poor visible light harvesting, and sluggish interfacial reaction kinetics, which often required a variety of modifications with only low improvements achieved. Herein, a high performance CQDs/TiO2/WO3 photoanode was designed on the basis of density function theory (DFT) alignment of lattice parameters and energy band, and charge distribution. The TiO2/WO3 heterojunction can abate photocorrosion through the hetero-epitaxial growth of TiO2 (001) on WO3 (002) for the lattice mismatch <3% eliminating dangling bonds, with high corrosion resistance and photostability of TiO2. As the built-in field constructed by a staggered band alignment structure with the valence band offset (VBO) of 0.51 eV, the photogenerated carriers transfer and separation are promoted dramatically. Through the DFT calculations, the sunlight absorption wavelength can be extended, and the interfacial reaction kinetics can be expedited with the modification of carbon quantum dots (CQDs) on TiO2/WO3, due to the narrower bandgap (Eg) and the accumulation of electrons at TiO2 side. The DFT designed CQDs/TiO2/WO3 photoanode significantly increase photocurrent density from 0.90 to 2.03 mA cm-2 at 1.23 V, charge separation efficiency from 56.3 to 79.2% and charge injection efficiency from 51.2 to 70.4%, and extend light absorption edge from 455 to 463 nm over pristine WO3, with better photostability and lower holes-to-water resistance.

Efficient ammonia removal and toxic chlorate control by using BiVO4/WO3 heterojunction photoanode in a self-driven PEC-chlorine system.

The efficient removal of ammonia is a difficult issue in wastewater treatment because ammonia is easily converted to nitrate instead of N2. The oxidation of ammonia by chlorine radical (Cl) is recognized as an effective method. However, the massive generation of toxic byproducts chlorate and nitrate pose great risk for its practical application due to the excessive oxidation capacity of hydroxyl radical. Herein, we propose a novel method to selectively generate Cl for efficient ammonia removal using BiVO4/WO3 photoanode in a self-driven photoelectrocatalytic (PEC) system. Cl was predominantly produced by regulating the valence band edge of WO3 though modifying BiVO4, which tuned the moderate oxidative force of hole to reduce OH generation and thereby inhibited the formation of chlorate and nitrate. The self-driven ammonia degradation was achieved by employing BiVO4/WO3 and Si photovoltaic cells as composite photoanodes to improve light-absorption and electron-hole separation, thus enhancing Cl production. These results showed that 10 mg L-1 of ammonia-N was completely removed (99.3 %) in 120 min with 80.1 % of total nitrogen removal. Toxic byproducts chlorate and nitrate were inhibited by 79.3 % and 31 %, respectively, compared to WO3. This work provides new insights to develop efficient, energy-saving and environment-friendly method for ammonia pollution treatment.

Multistep Surface Trap State Finishing Based on in Situ One-Step MOF Modification over Hematite for Dramatically Enhanced Solar Water Oxidation.

Complex surface dynamics is the key to limit the photoelectrochemical performance of hematite, while its core content is the hole trapping and release by surface traps. Deep traps are accompanied by extremely fast capture rates and extremely slow release rates, which severely suppress the hole transport process. Herein, we proposed a unique method to progressively convert deep traps on the hematite surface for fast hole transfer via in situ one-step metal organic framework modification. This stepwise deep-trap passivation is achieved by hematite corrosion first on the surface and subsequent construction of a porous titanium layer. The gentle trap finishing helps prevent surface losses caused by excessively intense trap passivation. The hematite corrosion can initially passivate 80% of the surface deep traps, while the subsequent porous titanium layer can completely passivate the deep traps. In addition, the accurate optimization of the porous titanium layer can reconstruct the benign shallow traps on the surface, acting as superior oxygen evolution reaction active sites. This sophisticated surface-trap adjustment is accompanied by the rapid reduction of deep traps and the gradual increase of shallow traps, obtaining a superior surface state that is conducive to charge transport and interface catalysis. The obtained treated hematite yields a photocurrent density of 3.08 mA·cm-2 at 1.23 VRHE, increased by 570% compared to the pristine hematite.

Efficient organic pollutants conversion and electricity generation for carbonate-containing wastewater based on carbonate radical reactions initiated by BiVO4-Au/PVC system.

In this paper, we propose an efficient simultaneous refractory organics degradation and electricity generation method for carbonate-containing wastewater based on carbonate radical reactions initiated by a BiVO4-Au/PVC (PVC: photovoltaic cell) system. In the system, nanoporous BiVO4 film and Au modified PVC were used as photoanode and photocathode, respectively. HCO3- was used as the electrolyte. Carbonate radicals, which have lower recombination rates than hydroxyl radicals and strong oxidation abilities, can be generated easily by the capture reaction of hydroxyl radicals with HCO3-, which is one of the most abundant anions in the aquatic environment. The results show that the removal ratios of rhodamine b, methyl orange and methylene blue in the system increased sharply to 77.98 %, 89.15 % and 93.2 % from 18.23 %, 21 % and 23.14 % (BiVO4-Pt/ SO42-), respectively, after 120 min. Meanwhile, the short-circuit current density is up to 2.19-2.41 times larger than the traditional system. Other common ions in natural water minimally affected the properties of the new system. The excellent performance could be ascribed to large amounts of carbonate radicals in the system, which have great potential for efficient carbonate-containing wastewater treatment and energy recovery.

Extremely Efficient Decomposition of Ammonia N to N2 Using ClO• from Reactions of HO• and HOCl Generated in Situ on a Novel Bifacial Photoelectroanode.

The conversion of excess ammonia N into harmless N2 is a primary challenge for wastewater treatment. We present here a method to generate ClO• directionally for quick and efficient decomposition of NH4+ N to N2. ClO• was produced and enhanced by a bifacial anode, a front WO3 photoanode and a rear Sb-SnO2 anode, in which HO• generated on WO3 reacts with HClO generated on Sb-SnO2 to form ClO•. Results show that the ammonia decomposition rate of Sb-SnO2/WO3 is 4.4 times than that of WO3 and 3.3 times than that of Sb-SnO2, with achievement of the removal of NH4+ N on Sb-SnO2/WO3 and WO3 being 99.2 and 58.3% in 90 min, respectively. This enhancement is attributed to the high rate constant of ClO• with NH4+ N, which is 2.8 and 34.8 times than those of Cl• and HO•, respectively. The steady-state concentration of ClO• (2.5 × 10-13 M) is 102 times those of HO• and Cl•, and this is further confirmed by kinetic simulations. In combination with the Pd-Cu/NF cathode to form a denitrification exhaustion system, Sb-SnO2/WO3 shows excellent total nitrogen removal (98.4%), which is more effective than WO3 (47.1%) in 90 min. This study provides new insight on the directed ClO• generation and its application on ammonia wastewater treatment.

Efficient purification and chemical energy recovery from urine by using a denitrifying fuel cell.

Urine is a major biomass resource, and its excessive discharge would lead to severe aquatic nitrogen pollution and even eutrophication. In this study, we designed an innovative denitrifying fuel cell (DFC) under illumination to purify urine and convert its chemical energy into electricity. The central ideas include the following: 1) on the anode, chlorine radicals (Cl) and hydroxyl (HO) radicals were induced to react with amine or ammonia in urine into N2, and to mineralize organics into CO2, respectively; 2) on the cathode, NO2- or NO3- generated in the cell was selectively reduced to N2 and tiny NH4+ by Pd/Au/NF; 3) NH4+ was further oxidized to N2 by Cl according to process 1), then the total nitrogen (TN) was ultimately removed by a continuous redox loop between anode and cathode; 4) the separation and migration of charges were strengthened by a self-bias poly-Si/WO3 photoanode. Result indicated that the DFC showed an efficient yield of electricity and almost completely N-removing properties: power density of 2.24 mW cm-2, total nitrogen and total organic carbon (TOC) removal efficiency, respectively 99.02% and 50.76% for artificial urine; and power density of 2.51 mW cm-2, TN and TOC removal efficiency, respectively 98.60% and 54.55% for actual urine. The study proposes a potential and environment-friendly approach by using novel DFC to purify urine and generate electricity.

Increased activity of nitrogen-doped graphene-like carbon sheets modified by iron doping for oxygen reduction.

Rational design and synthesis of Fe-N-codoped carbon materials are promising for replacing commercial Pt/C for oxygen reduction reaction (ORR). Herein, we develop a simple two-step pyrolysis approach to synthesize highly active Fe-N-codoped graphene-like carbon sheets (FeNGC) with active Fe-N-based species for ORR. In this strategy, two-dimensional nitrogen-doped graphene-like carbon sheets (NGC) with a high N-doping level (8.1 at%) and abundant mesoporosity (3.8 nm) are firstly synthesized by co-pyrolysis of biomass carbon source and dicyandiamide, in which dicyandiamide simultaneously serves as a trifunctional role of in situ reaction template, nitrogen source and porogen. Secondly, FeNGCs are prepared by additional iron doping of NGC at high temperatures, in which sheet-like structure is in favor of increased accessibility of N-functional groups to more Fe atoms, thus giving rise to formation of high-density Fe-N-based active sites. The optimized catalyst synthesized at 950 °C (FeNGC-950) demonstrates significantly increased ORR activity with a dominant 4e- reduction process compared to pure NGC in alkaline and acidic solutions, which evidently shows the comparable activity to Pt/C due to the synergy of simultaneously optimized structures and multi-active sites. Moreover, FeNGC-950 has better long-term stability and methanol tolerance than Pt/C both in alkaline and acidic electrolytes. The present strategy paves a new venue to design and prepare various metal-doped carbon materials with great potentials in energy applications.

Creation of Triple Hierarchical Micro-Meso-Macroporous N-doped Carbon Shells with Hollow Cores Toward the Electrocatalytic Oxygen Reduction Reaction.

A series of triple hierarchical micro-meso-macroporous N-doped carbon shells with hollow cores have been successfully prepared via etching N-doped hollow carbon spheres with CO2 at high temperatures. The surface areas, total pore volumes and micropore percentages of the CO2-activated samples evidently increase with increasing activation temperature from 800 to 950 °C, while the N contents show a contrary trend from 7.6 to 3.8 at%. The pyridinic and graphitic nitrogen groups are dominant among various N-containing groups in the samples. The 950 °C-activated sample (CANHCS-950) has the largest surface area (2072 m2 g-1), pore volume (1.96 cm3 g-1), hierarchical micro-mesopore distributions (1.2, 2.6 and 6.2 nm), hollow macropore cores (~91 nm) and highest relative content of pyridinic and graphitic N groups. This triple micro-meso-macropore system could synergistically enhance the activity because macropores could store up the reactant, mesopores could reduce the transport resistance of the reactants to the active sites, and micropores could be in favor of the accumulation of ions. Therefore, the CANHCS-950 with optimized structure shows the optimal and comparable oxygen reduction reaction (ORR) activity but superior methanol tolerance and long-term durability to commercial Pt/C with a 4e--dominant transfer pathway in alkaline media. These excellent properties in combination with good stability and recyclability make CANHCSs among the most promising metal-free ORR electrocatalysts reported so far in practical applications.

In situ formation of nitrogen-doped carbon nanoparticles on hollow carbon spheres as efficient oxygen reduction electrocatalysts.

In situ formation of nitrogen-doped carbon nanoparticles on hollow carbon spheres (NHCSs) were successfully realised via a simple, scalable emulsion polymerization route using melamine as the nitrogen precursor, followed by thermal treatment at 1000 °C in N2. All NHCSs show large BET specific surface areas (648.2-837.7 cm2 g-1) and pore volumes (0.91-1.16 cm3 g-1), evidently superior to N-free hollow carbon spheres (HCSs) (524.3 cm2 g-1 and 0.48 cm3 g-1, respectively). This unique nanocomposite has hierarchical micro-/mesoporosity (1.9 nm and 16.2-19.0 nm). The X-ray photoelectron spectroscopy (XPS) measurements indicate the successful introduction of N atoms into the carbon framework and that the N-doping level can be controlled by changing the amount of melamine. The N-doping by adding melamine during the hydrothermal process not only affects the morphologies and porosities of the final samples, but also improves the electrocatalytic activity compared to N-free HCSs. NHCS-2, prepared with the molar melamine/hexamethylentetramine ratio of 1, showed the best electrocatalytic activity for the oxygen reduction reaction (ORR) in terms of onset potential, half-wave potential and limit current density. The NHCS-2 exhibited not only excellent activity with a mainly four-electron reaction pathway, but also superior long-term durability and methanol tolerance to that of commercial Pt/C in alkaline solution. The excellent electrocatalytic activity of the NHCS-2 is mainly due to its high relative content of pyridinic- and graphitic-N groups as well as unique hierarchical micro-/mesoporosity and a large specific surface area, advantageous for mass transfer and thus improving the electrocatalytic activity.