Biomass derivative-based fibrous perovskite electrocatalysts with a hierarchical porous structure for oxygen reduction in alkaline media

The CaMnO₃ perovskites have been proposed as promising electrocatalysts for the oxygen reduction reaction (ORR) as substitutes for the noble metal. However, their ORR catalytic activities still need to be further improved. Herein, fibrous CaMnO₃ catalysts with hierarchical mesoporous/macroporous structures for the ORR were prepared through a simple ion exchange process with biomass derivative calcium alginate as a precursor. Optimization of the synthesis conditions, in particular, the La doping content, is conducted. The as-prepared CaMnO₃ shows superior ORR performance in alkaline media with a limited diffusion density of 6.13 mA cm⁻². Furthermore, the ORR performance was further promoted by La-doping. The optimal La-doped CaMnO₃ sample exhibits the most positive onset potential of −0.01 V, the most positive half-wave potential of −0.28 V (VS. Ag/AgCl), and a greatly enhanced limited diffusion density of 6.76 mA cm⁻². The improvement of catalytic activity for ORR can be attributed to the porous structure, the rich surface chemisorbed oxygen and the redox couple Mn³⁺/Mn⁴⁺. The as-prepared perovskite fibers have potential as low cost and efficient ORR catalysts.     

Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction

Spinel-type nickel cobaltite with numerous oxygen vacancies is successfully synthesized by hydrothermal and thermal reduction using hydrogen. The effects of oxygen vacancies on the electrochemical activity and stability for the oxygen reduction reaction are investigated. The prepared catalyst displays significantly enhanced oxygen reduction reaction (ORR) catalytic performance under alkaline conditions, which is comparable to that of commercial Pt/C. The oxygen-deficient NiCo₂O₄ exhibits a very high limiting current density of −5.44 mA cm⁻² with onset and half-wave potentials of 0.93 and 0.78 V versus the reversible hydrogen electrode (RHE), respectively. Additionally, it shows excellent durability and resistance to methanol. The enhanced ORR activity and stability of the catalyst can be ascribed to the synergistic effects of the relatively large electrochemical surface area, more exposed active sites, and good electrical conductivity derived from abundant oxygen vacancies.     

Spinel oxides wrapped on electrospun carbon nanofibers: Superior electrocatalysts boosted by enhanced conductivity and rich oxygen vacancies

Spinel oxides have been considered as promising precious metal-free catalysts for oxygen reduction reaction (ORR). However, the poor intrinsic conductivity and moderate electrocatalytic performance hinder their practical applications. Hence, various strategies have been explored and reported in addressing the issues. Herein, an elaborate approach for enhancing the ORR performance of spinel NiCo₂O₄ is proposed, by combining the decoration of NiCo₂O₄ nanoparticles on electrospun carbon nanofibers and defect engineering with rich oxygen vacancies on NiCo₂O₄ nanoparticles through a facilely controlling on calcination circumstance, which could not only increase more active sites and improve the intrinsic catalytic activity, but also render an excellent stability for long-term operation. Thus, the as-prepared hybrid exhibits significantly improved ORR electrocatalytic performance, including a high limited current density of −5.8 mA cm⁻², a positively shifting of the onset potential at 0.88 V and half-wave potential at 0.76 V (vs. RHE). The performance of rechargeable Zn-air battery based on the as-prepared catalyst surpasses the one based on Pt/C catalyst significantly. This work can be also applied to other metal oxides based electrocatalysts, and then provided an avenue for the realization of metal-air batteries and fuel cells with high efficient and cost-effective.     

Reduced CoFe2O4/graphene composite with rich oxygen vacancies as a high efficient electrocatalyst for oxygen evolution reaction

Oxygen evolution reaction (OER) is regarded as a limit-efficiency process in electrochemical water splitting generally, which needs to develop the effective and low-cost non-noble metal electrocatalysts. Oxygen vacancies have been verified to be beneficial to enhance the electrocatalytic performance of catalysts. Herein, we report the facile synthesis of reduced CoFe₂O₄/graphene (r-CFO/rGO) composite with rich oxygen vacancies by a citric acid assisted sol-gel method, heat treatment process and the sodium borohydride (NaBH₄) reduction. The introduction of graphene and freezing dry technique prevents the restacking of GO and the aggregation of CFO nanoparticles (NPs) and increases the electronic conductivity of the catalyst. Fast heating rate and low anneal temperature favors to obtain low crystallinity and lattice defects for CFO. NaBH₄ reduction treatment further creates the rich oxygen vacancies and electrocatalytic active sites. The obtained r-CFO/rGO with high specific surface area (108 m² g⁻¹), low crystallinity and rich oxygen vacancies demonstrates a superior electrocatalytic activity with the smaller Tafel slope (68 mV dec⁻¹), lower overpotential (300 mV) at the current density of 10 mA cm⁻², and higher durability compared with the commercial RuO₂ catalyst. This green, low-cost method can be extended to fabricate similar composites with rich defects for wide applications.     

Ni/Ce co-doping δ-MnO2 nanosheets with oxygen vacancy for enhanced electrocatalytic oxygen evolution reaction

The design and manufacture of strongly engaged, low-cost, and resilient oxygen evolution reaction (OER) electrocatalysts is the most challenging task in electrochemical hydrolysis. Herein, Ce and Ni co-doped MnO₂ (NiCe/MnO₂) nanosheets (NSs) with oxygen vacancy (V_O) and abundant active sites have been prepared in one step employing a defect strategy. The co-doping of Ce/Ni on the one hand reduced the catalyst particle size and increased the specific surface area, which promoted the exposure of more active sites. On the other hand, heteroatom doping altered the species the crystalline surface, stimulating the formation of Vo and thus activating the catalyst performance simultaneously. The OER performance of NiCe/MnO₂ NSs was significantly enhanced over the pure δ-MnO₂, with an overpotential of 170 mV (10 mA cm^?2), which was verified by density functional theory. This work shows a straightforward and practical method for making non-precious metal electrocatalysts with high electrochemical hydrolysis performance.     

Template-assisted polymerization-pyrolysis derived mesoporous carbon anchored with Fe/Fe3C and Fe−NX species as efficient oxygen reduction catalysts for Zn-air battery

Constructing highly efficient and durable non-noble metal modified carbon catalysts for oxygen reduction reaction (ORR) in the whole pH range is essential for energy conversion devices but still remains a challenge. Herein, the Fe/Fe₃C nanoparticles and Fe-N_X species anchored on the interconnected mesoporous carbon materials are fabricated through an economical and facile template-assisted polymerization-pyrolysis strategy. The catalyst exhibits unique features with the electronic interaction between Fe/Fe₃C and Fe−N_X, large specific surface area and high mesoporous structure as well as nitrogen doping in porous carbon skeletons, which can effectively catalyze ORR over the full pH range. In an alkaline electrolyte, the optimized catalyst displays favorable ORR performance with positive onset potential (E_onset = 0.91 V), half-wave potential (E_1/2 = 0.83 V), long-term cycles stability and methanol tolerance, exceeding those for the commercial Pt/C. Furthermore, the prepared catalyst could be directly assembled into the alkaline Zn−air battery that exhibits the open-circuit voltage of 1.44 V, high power density of 96.0 mW cm⁻² and long-term durability. Therefore, the template-assisted polymerization-pyrolysis strategy provides a promising route for designing high-performance non-noble metal decorated ORR electrocatalysts.     

Zeolitic-imidazolate-framework-derived Fe-NC catalysts towards efficient oxygen reduction reaction

The development of non-precious metal catalysts to replace scarce and expensive Pt-based catalysts is critical for oxygen reduction reactions (ORR), where zeolitic-imidazolate-framework-derived (ZIF-derived) iron-based electrocatalysts hold a promising prospect. Herein, Fe₃O₄ was used as Fe source, and ZIF-8 was used as C and N source to prepare Fe-NC catalysts. Specifically, the half-wave potential (E_1/2) of the Fe-NC reached 0.90 V, which was higher than commercial Pt/C catalysts (0.87 V), and the overpotential of OER reached 327 mV. In addition, the power density tested in Zn-air batteries upped to 129.59 mW cm⁻², surpassing that of the Pt/C (108.93 mW cm⁻²). The superior performance was attributed to the effective introduction of Fe, the large specific surface area (851.6 m² g⁻¹), relatively regular porous structure and the high degree of graphitization.     

Fe2O3 nanoparticles immobilized on N and S codoped C as an efficient multifunctional catalyst for oxygen reduction reaction and overall water electrolysis

Currently, multifunctional electrocatalysts with superior performance are very vital for developing various clean and regenerated energy systems. Herein, an effective multifunctional electrocatalyst comprising Fe₂O₃ nanoparticles immobilized on N and S codoped C has been synthesized via heat-treatment of Fe(II) complex at 800 °C (denoted as Fe₂O₃/NS-C-800). Favorable features including the introduction of maghemite nanoparticles, N/S-codoping effect, and close contact between the Fe₂O₃ nanoparticles and NS-C ender the Fe₂O₃/NS-C-800 with high multifunctional catalytic performance. The onset potential (0.97 V) and half-wave potential (0.81 V) of the Fe₂O₃/NS-C-800 towards oxygen reduction reaction (ORR) are comparable to Pt/C (0.99 and 0.82 V). The Fe₂O₃/NS-C-800 also exhibits high oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activity with low OER and HER overpotentials of 0.37 and −0.27 V at 10 mA cm⁻², respectively. In addition, higher ORR, OER and HER stabilities than Pt/C are observed for the Fe₂O₃/NS-C-800. More importantly, the assembled water electrolyzer using the Fe₂O₃/NS-C-800 as the anode and cathode exhibits a high stability at a water electrolysis current density of 10 mA cm⁻². The present study offers a new promising non-noble multifunctional catalyst for future application in renewable energy technologies.     

NiOx nanoparticles supported on polyethylenimine functionalized CNTs as efficient electrocatalysts for supercapacitor and oxygen evolution reaction

Ni oxide based nanoparticles (NPs) have been widely used as electrocatalysts in the electrochemical energy storage and conversion applications. In this paper, NiO_x NPs are successfully synthesized by the self-assembly of Ni precursor onto polyethylenimine functionalized carbon nanotubes (PEI-CNTs) assisted with microwave radiation. NiO_x NPs with size around 2–3 nm are homogenously dispersed on the PEI-CNTs supports with no aggregation. The electrochemical activity of NiO_x NPs on PEI-CNTs, NiO_x/PEI-CNTs, as effective electrocatalysts is studied for supercapacitor and oxygen evolution reaction in alkaline solutions. NiO_x/PEI-CNTs show a capacitance of 1728 and 1576 F g⁻¹ based on active material, and 221 and 394 F g⁻¹ based on total catalyst loading on 12.5% and 25% NiO_x/PEI-CNTs, respectively, which is substantially higher than 152 F g⁻¹ of unsupported NiO. The NiO_x/PEI-CNTs electrodes exhibit reversible and stale capacitance of ∼1200 F g⁻¹ based on active materials after 2000 cycles at a high current density of 10 A g⁻¹. NiO_x/PEI-CNTs also exhibit significantly higher activities for oxygen evolution reaction (OER) of water electrolysis, achieving a current density of 100 A g⁻¹ at an overpotential of 0.35 V for 25% NiO_x/PEI-CNTs. It is believed that the uniformly dispersed nano-sized NiO_x NPs and synergistic effect between the NiO_x NPs and PEI-CNTs is attributed to the high electrocatalytic performance of NiO_x/PEI-CNTs electrocatalysts. The results demonstrate that NiO_x NPs supported on PEI-CNTs are highly effective electrocatalysts for electrochemical energy storage and conversion applications.     

Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transition-metal doping

Developing active and durable electrocatalysts for oxygen reduction reaction (ORR) is of great significance in proton exchange membrane fuel cells (PEMFCs). Herein, we develop a facile strategy to synthesize PtCu nanoparticles with enhanced ORR performance through morphology tuning and transition-metal doping. Two distinct PtCu nanoparticles, namely nanooctahedrons (NOs) and nanospheres (NSs), are selectively synthesized in presence or absence of W(CO)₆ via a facile one-pot method. Furthermore, by introducing a small amount of third transition metal, M-doped (M = Sc, Y, La, Gd, Fe) PtCu NOs are obtained. Electrocatalytic results suggest that the ORR performance of PtCu NOs is better than that of PtCu NSs due to the morphology advantages. And the ORR performance of PtCuM NOs is further promoted since the doping effect of transition metals compared to that of PtCu NOs. Particularly, PtCuSc NOs exhibit remarkable mass activity (1.652 mA μg⁻¹_Pt) and specific activity (2.093 mA cm⁻²), which are 9.9 and 7.2 times higher than that of commercial Pt/C catalysts at 0.8 V (vs. RHE). Moreover, after accelerated stability tests, the loss of mass activity for PtCuSc NOs is only 9.2%, which is much lower than that of PtCu NOs (16.5%) and commercial Pt/C (44.3%). This work provides a feasible idea to boost the ORR performances of Pt-based nanoparticles.     

Bimetallic ZnCo zeolitic imidazolate framework/polypyrrole-polyaniline derived Co/N-doped carbon for oxygen reduction reaction

Non-noble-metal based materials with high activity for oxygen reduction reaction (ORR) are urgently required to substitute Pt-based materials. Herein, metallic Co/N-doped carbon electrocatalysts were synthesized via a facile pyrolysis method of bimetal ZIF(ZnCo)/polypyrrole-polyaniline (ppy-pani) precursors. The evaporation of Zn and the introduced ppy-pani can lead to the porous structure and effectively hinder the aggregation of Co species, which results in the small nanoparticles uniformly distributed on carbon matrices. A high ECSA and a high content of Co species are obtained after the introduction of ppy-pani, thus resulting in the abundant Co²⁺ species to enhance ORR. Therefore, the optimized ZIF/ppy-pani-750 exhibits a high E_1/2 (~0.86 V) and a low Tafel slope (~43.6 mV dec⁻¹).     

Aligned Co3O4–CoOOH heterostructure nanosheet arrays grown on carbon paper with oxygen vacancies for enhanced and robust oxygen evolution

Developing efficient and stable non-noble metal oxygen evolution reaction (OER) electrocatalysts for sustainable overall water-splitting is extremely desirable but still a great challenge. Herein, we developed a facile strategy to fabricate Co₃O₄–CoOOH heterostructure nanosheet arrays with oxygen vacancies grown on carbon paper (Co₃O₄–CoOOH/CP). Benefiting from the unique 3D architecture, large surface area, synergistic effects between Co₃O₄, CoOOH and oxygen vacancies, the obtained self-supporting Co₃O₄–CoOOH/CP presents excellent electrocatalytic OER activity (low overpotentials of 245 and 390 mV at 10 and 100 mA cm⁻²) and robust long-term stability in alkaline condition. The present strategy provides the opportunities for the future rational design and discovery of high-performance non-noble metal based electrocatalysts for advanced water oxidation and beyond.     

Electrocatalytic performances of oxygen-deficient titanium dioxide nanosheet coupled palladium nanoparticles for oxygen reduction and hydrogen evolution reactions

The development of multifunctional electrocatalysts is crucial for enhancing the efficiency of electrochemical conversion in energy devices. Here we have synthesized TiO_2-x nanosheets (NSs) supported metallic Pd nanoparticles (Pd/TiO_2-x NSs) as an electrocatalyst using a simple impregnation process. High electrochemical surface areas (ECSAs) and strong metal support interactions (SMSI) of the electrocatalyst showed improved ORR performance throughout a wide pH range under ambient conditions. The outstanding durability of the catalyst was proven by the square-wave potential cycling experiment at 60 °C. Additionally, it was shown that Pd/TiO_2-x NSs showed improved HER activity and stability in 0.5 M H₂SO₄. The catalyst had an overpotential of 19.5 mV for the 10 mA cm⁻² and a low Tafel slope of 41 mV dec⁻¹. The catalyst also showed higher stability for about 30 h in HER performance. This work will help in rationally building nanostructured electrocatalysts loaded on carbon-free support for efficient electrochemical energy storage devices.     

Impacts of anions on the electrochemical oxygen reduction reaction activity and stability of Pt/C in alkaline electrolyte

Pt is the benchmark oxygen reduction reaction (ORR) catalyst for the anion exchange membrane fuel cells. In this paper, we have done systematic studies on the influence of anions (F⁻, Cl⁻, Br⁻, I⁻, CO₃²⁻, SO₄²⁻, SO₃²⁻ and S²⁻) to the ORR activity and durability of Pt/C in alkaline condition. It was found that the anions have less impact to Pt/C in alkaline than in acid. I⁻, SO₃²⁻, and S²⁻ have the poisoning effect and the other anions do not influence the ORR activity significantly. In the accelerating durability test, SO₃²⁻ and S²⁻ promote the degradation of the Pt/C catalysts. It was found that the impacts of anions are relative to the standard reduction potential of the anions. The anions with low potential have poisoning impact to the activity and durability of Pt/C. This study provides fundamental understandings of the impacts of anions on Pt/C, which may give insights in design of anion exchange membrane fuel cells.     

Catalytic activity of platinum-cobalt alloy nanoparticles decorated functionalized multiwalled carbon nanotubes for oxygen reduction reaction in PEMFC

The role of functionalized multiwalled carbon nanotubes (MWNTs) decorated with platinum nanoparticles (Pt/f-MWNTs) and platinum-cobalt alloy nanoparticles (Pt₃Co/f-MWNTs) has been investigated for oxygen reduction reaction (ORR) in a proton exchange membrane fuel cell. The electrocatalysts are synthesized by a conventional sodium borohydride reduction method and modified polyol reduction method. The modified polyol reduction method yields better uniform dispersion, higher loading and optimum particle size of Pt and Pt₃Co alloy nanoparticles over the MWNTs compared to the conventional sodium borohydride reduction method. The electrochemical surface area of the electrocatalysts is calculated using cyclic voltammetry. Pt₃Co/f-MWNTs synthesized via modified polyol reduction method yield the highest performance with a maximum power density of 798 mW cm⁻² at 60 °C without any back pressure. The enhanced catalytic activity of Pt₃Co/f-MWNTs toward ORR is attributed to uniform dispersion and optimum particle size of Pt₃Co alloy nanoparticles over the surface of f-MWNTs.     

Hybrid palladium-ceria nanorod electrocatalysts applications in oxygen reduction and ethanol oxidation reactions in alkaline media

Fossil fuel alternatives are being increasingly studied, and alkaline direct ethanol fuel cells (ADEFC) have acquired importance, as to ethanol is a renewable fuel. In this context, the aims of the present study were to synthesize, characterize and evaluate electrocatalytic activity in oxygen reduction reaction (ORR) and ethanol oxidation reaction (EOR) using hybrid electrocatalysts based on Pd nanoparticles and CeO₂ nanorods supported on carbon black for application in ADEFC. The highest OCV, maximum current and power densities obtained using Pd₁₅(CeO₂ NR)₁₀(Vn)₇₅ as the cathode and Pd₁₀(CeO₂ NR)₂₀(Vn)₇₀ as the anode were 1270 mV, 190 mA cm^?2 and 65 mW cm^?2, respectively. These interesting results are justified by the highest I_D/I_G ratio and ECSA, which suggest a high number of oxygenated species, defects and vacancies in these electrocatalysts and by the synergistic effect between CeO₂ NR and Pd nanoparticles. Therefore, these hybrid electrocatalysts are promising for ADEFC applications.     

Highly dispersed MnO nanoparticles supported on N-doped rGO as an efficient oxygen reduction electrocatalyst via high-temperature pyrolysis

Transition metal-based compounds, due to their excellent ORR catalytic performance under alkaline condition, have recently emerged as one of the most promising alternatives to noble metal-based ORR catalysts. It is worth noting that manganese oxide can take an unique advantage for decomposition of intermediate adsorption products H₂O₂ and can effectively reduce O₂ to OH⁻. However, most research has focused on MnO₂, while attention has rarely been paid to MnO catalysts. In addition, under high-temperature pyrolysis condition, MnO is the most stable manganese oxide but MnO nanoparticles easily agglomerate. Hence, it is very difficult to obtain well-dispersed and small-sized MnO nanoparticles. Herein, on the basis of pre-synthesizing uniformly distributed manganese complexes on the reduced graphene oxide (rGO), we innovatively prepare highly dispersed and small-sized MnO nanoparticles (~3.94 nm) via high-temperature pyrolysis, which are uniformly anchored on N-doped reduced graphene oxide (NrGO) as an efficient oxygen reduction electrocatalyst. The as-obtained MnO/NrGO (1050 °C) electrocatalyst achieves satisfactory onset potential (0.942 V) and half-wave potential (0.820 V) under alkaline condition. And the limiting current density is 4.17 mA cm⁻², which is very close to that of Pt/C (20 wt%, JM). Meanwhile, MnO/NrGO (1050 °C) catalyst presents superior longstanding durability and methanol resistance than Pt/C (JM). This work indicates that high-temperature pyrolysis can improve the purity of manganese oxide, simultaneously the defects of NrGO can reduce particle size of MnO nanoparticles, which are greatly beneficial to improve ORR performance. This work provides a new idea for research of MnO catalysts for ORR in the future.     

Hollow NH2-MIL-101@TA derived electrocatalyst for enhanced oxygen reduction reaction and oxygen evolution reaction

Non-precious metal-based electrocatalysts with excellent activity and stability are highly desired for the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, a tannic acid (TA) etching strategy is used to inhibit the metal aggregation and achieve muti-metal doping. The hollow NH₂-MIL-101@TA derived Fe–N–C catalyst exhibits superior ORR catalytic activity with an E_1/2 of 0.872 V and a maximum output power density of 123.4 mW cm⁻² in Zn-air battery. Since TA can easily chelate with metal ions, Fe/Co–N–C and Fe/Ni–N–C are also synthesized. Fe/Ni–N–C manifests exceptional bifunctional activity with an E_j = 10 of 1.67 V and a potential gap of 0.833 V between E_j = 10 and E_1/2 in alkaline electrolyte, which is 45 mV smaller than Pt/C–IrO₂. The improvement of ORR and OER performance of the catalysts via the simple TA etching and chelation method provides a novel strategy for the design and synthesis of efficient electrocatalysts.     

MgO as promoter for electrocatalytic activities of Co3O4–MgO composite via abundant oxygen vacancies and Co2+ ions towards oxygen evolution reaction

Oxygen evolution reaction (OER) is known as bottleneck problem during the water splitting process due to high energy barrier and non-availability of efficient nonprecious electrocatalysts. The cobalt oxide (Co₃O₄) in the spinel phase has limited OER activity and stability in the alkaline media. For this purpose, we have carried out the synthesis of Co₃O₄–MgO (CM) composite by wet chemical method and it offers abundant oxygen vacancies and Co²⁺ concentration for the efficient OER reaction. The effect of different amounts of MgO on the OER activity of Co₃O₄ was also studied. Despite inactivity of MgO towards OER, it creates high density of oxygen vacancies and favored the formation Co²⁺ ions at the surface, thus accelerated the OER kinetics. The physical studies were performed to investigate the morphology, crystalline structure, surface information and chemical composition using several analytical techniques. The optimized CM-0.1 composite produced an overpotential of 274 mV at 10 mAcm⁻² which is lower in value than the pristine Co₃O₄. The significant enhancement in the OER activity was verified by the large value of electrochemical active surface area values 12.8 μFcm⁻² and the low charge transfer resistance of 45.96 Ω for the optimized CM-0.1 composite. The use of abundance materials for the synthesis of CM composite revealed an enhanced OER performance, suggesting the dynamic role of MgO, therefore it could be used for improving the electrochemical properties of extended range of metal oxides for specific application especially energy conversion and storage devices.     

Advanced electrocatalytic activity of praseodymium-deficient copper-based oxygen electrodes for solid oxide fuel cells

Herein praseodymium-deficient Pr_1.90−xCe_0.1CuO₄ oxides are evaluated as potential oxygen electrodes for solid oxide fuel cells (SOFCs). Introducing Pr-deficiency promotes the oxygen vacancy concentration, further improving electrocatalytic activity of the Pr_1.90−xCe_0.1CuO₄ electrodes towards oxygen reduction reaction (ORR). The Pr_1.75Ce_0·.1CuO₄ (P_1·75CC) component exhibits outstanding electrode performance, as supported by a polarization resistance as low as 0.025 Ω cm² and high peak power density of the single cell (1.11 W cm⁻²) at 700 °C. In addition, the rate-limiting steps for ORR kinetics are determined to be the charge transfer reaction and oxygen adsorption/diffusion process on the electrode surface. This work highlights an effective way for developing the cathode candidates with high electrocatalytic activity and superior stability.