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Polyoxometalates (POMs) with transition metals (Co, Cu, Fe, Mn, Ni) of Keggin structure and lamellar-stacked multi-layer morphology were synthesized. They were subsequently explored as bifunctional electrocatalysts for oxygen electrodes, i.e., oxygen reduction (ORR) and evolution (OER) reaction, for aqueous rechargeable metal-air batteries in alkaline media. The lowest Tafel slope (85 mV dec−1) value and the highest OER current density of 93.8 mA cm−2 were obtained for the Fe-POM electrocatalyst. Similar OER electrochemical catalytic activity was noticed for the Co-POM electrocatalyst. This behavior was confirmed by electrochemical impedance spectroscopy, where Fe-POM gave the lowest charge transfer resistance of 3.35 Ω, followed by Co-POM with Rct of 15.04 Ω, during the OER. Additionally, Tafel slope values of 85 and 109 mV dec−1 were calculated for Fe-POM and Co-POM, respectively, during the ORR. The ORR at Fe-POM proceeded by mixed two- and four-electron pathways, while ORR at Co-POM proceeded exclusively by the four-electron pathway. Finally, capacitance studies were conducted on the synthesized POMs. [ABSTRACT FROM AUTHOR]

Climate change is showing its impacts now more than ever. The intense use of fossil fuels and the resulting CO2 emissions are mainly to blame, accentuating the need to develop further the available energy conversion and storage technologies, which are regarded as effective solutions to maximize the use of intermittent renewable energy sources and reduce global CO2 emissions. This work comprehensively overviews the most recent progress and trends in the use of transition metal-based electrocatalysts for three crucial reactions in electrochemical energy conversion and storage, namely, the oxygen evolution (OER), oxygen reduction (ORR), and hydrogen evolution (HER) reactions. By analyzing the state-of-the-art polyoxometalates (POMs) and metal-organic frameworks (MOFs), the performance of these two promising types of materials for OER, ORR, and HER is compared to that of more traditional transition metal oxides and alloy-based electrocatalysts. Both catalytic activity and stability are highly influenced by the adsorption energies of the intermediate species formed in each reaction, which are very sensitive to changes in the microstructure and chemical microenvironment. POMs and MOFs allow these aspects to be easily modified to fine-tune the catalytic performances. Therefore, their chemical tunability and versatility make it possible to tailor such properties to obtain higher electrocatalytic activities, or even to obtain derived materials with more compelling properties towards these reactions. [ABSTRACT FROM AUTHOR]

Developing bifunctional oxygen electrode materials with superior activity for oxygen reduction (ORR) and oxygen evolution (OER) reactions is essential for advancing regenerative fuel cell and rechargeable metal–air battery technologies. This present work deals with the synthesis and characterization of electrocatalysts containing Pt and Ni nanoparticles supported on nitrogen-doped mixed metal oxides (Mn2O3-NiO) and the systematic evaluation of their bifunctional ORR/OER performance in an alkaline medium. These electrocatalysts have been successfully synthesized by a simple and fast microwave method. PtNi/Mn2O3-NiO-N with a binary metal oxide-to-N ratio of 1:2 demonstrated the best performance among the studied materials regarding bifunctional electrocatalytic activity (∆E = 0.96 V) and robust stability. [ABSTRACT FROM AUTHOR]

The new 3D coordination polymer (CP) [Mn(L)(HCOO)]n (Mn-CP) [L = 4-(pyridin-4-ylcarbamoyl)benzoate] was synthesised via a hydrothermal reaction using the pyridyl amide functionalized benzoic acid HL. It was characterized by elemental, FT-IR spectroscopy, single-crystal and powder X-ray diffraction (PXRD) analyses. Its structural features were disclosed by single-crystal X-ray diffraction analysis, which revealed a 3D structure with the monoclinic space group P21/c. Its performance as an electrocatalyst for oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions was tested in both acidic (0.5 M H2SO4) and alkaline (0.1 M KOH) media. A distinct reduction peak was observed at 0.53 V vs. RHE in 0.1 M KOH, which corresponds to the oxygen reduction, thus clearly demonstrating the material's activity for the ORR. Tafel analysis revealed a Tafel slope of 101 mV dec−1 with mixed kinetics of 2e− and 4e− pathways indicated by the Koutecky–Levich analysis. Conversely, the ORR peak was not present in 0.5 M H2SO4 indicating no activity of Mn-CP for this reaction in acidic media. In addition, Mn-CP demonstrated a noteworthy activity toward OER and HER in acidic media, in contrast to what was observed in 0.1 M KOH. [ABSTRACT FROM AUTHOR]

Nickel-doped ceria (Ce1−xNixO2−δ) nanopowders (7 to 5 nm in size) synthesized by the cation complexation method with 5, 10, 15, and 20 Ni at.% are studied with respect to their electrochemical activity for the oxygen reduction (ORR) and oxygen evolution (OER) reactions in alkaline medium. One finds good bifunctional electrocatalytic activity of the four Ce1−xNixO2−δ electrocatalysts. The Tafel analysis of the ORR in the 0.57–0.78 V vs. RHE potential window leads to slopes in the 70–108 mV dec−1 range. The number of electrons exchanged during ORR is between 2 and 2.7. The OER Tafel slopes are determined to be in the range 192 –281 mV dec−1. OER activation energies are found to range between 28 and 43 kJ mol−1. The specific capacitance of Ce1−xNixO2−δ electrocatalysts measured at a scan rate of 100 mV s−1 varies between 0.7 and 1.4 Fg−1. The results demonstrate that Ce1−xNixO2−δ nanopowders can act as bifunctional electrocatalysts for ORR/OER for potential application in the oxygen electrode of devices such as rechargeable metal–air batteries. [ABSTRACT FROM AUTHOR]

Copper-silver and cobalt-silver alloy nanoparticles deposited on reduced graphene oxide (CuAg/rGO and CoAg/rGO) were synthesized and examined as electrocatalysts for oxygen reduction reaction (ORR) and hydrogen peroxide reduction reaction (HPRR) in alkaline media. Characterization of the prepared samples was done by transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction analysis (XRD), and scanning electron microscopy with integrated energy-dispersive X-ray spectroscopy (SEM-EDS). CuAg/rGO and CoAg/rGO nanoparticles diameter ranged from 0.4 to 9.2 nm. The Ag loading was ca. 40 wt.% for both electrocatalysts, with that for Cu and Co being 35 and 17 wt.%, respectively. CoAg/rGO electrocatalyst showed a Tafel slope of 109 mV dec−1, significantly lower than that for CuAg/rGO (184 mV dec−1), suggesting faster ORR kinetics. Additionally, a higher diffusion current density was obtained for CoAg/rGO (−2.63 mA cm−2) than for CuAg/rGO (−1.74 mA cm−2). The average value of the number of electrons transferred during ORR was 2.8 for CuAg/rGO and 3.3 for CoAg/rGO electrocatalyst, further confirming the higher ORR activity of the latter. On the other hand, CuAg/rGO showed higher peak current densities (−3.96 mA cm−2) for HPRR compared to those recorded for CoAg/rGO electrocatalyst (−1.96 mA cm−2). [ABSTRACT FROM AUTHOR]

Platinum–dysprosium (Pt–Dy) alloys prepared by the arc melting technique are assessed as potential electrodes for the oxygen reduction reaction (ORR) using voltammetry and chronoamperometry in alkaline media. A relatively small change (10 at.%) in the alloy composition brought a notable difference in the alloys' performance for the ORR. Pt40Dy60 electrode, i.e., the electrode with a lower amount of Pt, was identified to have a higher activity towards ORR as evidenced by lower overpotential and higher current densities under identical experimental conditions. Furthermore, DFT calculations point out the unique single-atom-like coordination and electronic structure of Pt atoms in the Pt40Dy60 surface as responsible for enhanced ORR activity compared to the alloy with a higher Pt content. Additionally, Pt–Dy alloys showed activity in the oxygen evolution reaction (OER), with the OER current density lower than that of pure Pt. [ABSTRACT FROM AUTHOR]

Gold-silver (AuAg) nanoparticles were deposited on reduced graphene oxide (rGO), with Au wt% varying from 34.6 to 46.2 and Ag wt% ranging from 31.7 to 35.3. Three AuAg/rGO samples were characterized by inductively coupled plasma optical emission spectroscopy, transmission and scanning electron microscopy with energy-dispersive X-ray spectroscopy, and X-ray diffraction analysis. Subsequently, AuAg/rGO materials were studied using the rotating disk electrode as electrocatalysts for oxygen reduction reaction (ORR) for potential application in alkaline fuel cells. AuAg/rGO-3 (with ca. 34.6 wt% Au and 35.3 wt% Ag) showed the most promising performance, with high ORR activity and good stability, as evidenced by the highest current density (-4.45 mA cm-2 at 0.5 V), the lowest Tafel slope (103 mV dec-1), and the highest number of electrons exchanged (4.1). [ABSTRACT FROM AUTHOR]

Polarization curves and complex kinetics of oxygen reduction (ORR) and borohydride oxidation (BOR) reaction were evaluated at thin Au layer on Ti and TiO2 electrodes. TiO2 electrodes prepared included amorphous TiO2 (AM−TiO2) and anatase (A−TiO2). The electrodes structure and nanotube arrays morphology were observed by XRD and SEM, respectively. All electrodes show activity for both ORR and BOR. The use of Au layer over A−TiO2 produces the strongest synergistic effect for ORR with exchange of 3 electrons. On the other hand, the strongest effect for BOR was observed in case of Au/Ti. [ABSTRACT FROM AUTHOR]

Three zeolites (A, X and Y) with Ag incorporated in their cages are tested as bifunctional electrocatalysts for direct borohydride fuel cells (DBFCs). Ag zeolites are characterised by XRD, FTIR, nitrogen sorption and SEM, as well as by electrochemical methods. Ag zeolite shows good activity for the anodic reaction, i.e., borohydride oxidation reaction (BOR) in alkaline media, presenting low onset potential and high current densities. BOR kinetic and faradaic parameters at AgY are calculated, with the number of exchanged electrons being 4.3. The BOR at AgY was found to be of 0.6 order with respect to BH4− concentration with an activation energy of 13 kJ mol−1. Regarding ORR, AgX shows the best performance with the highest current densities and good stability over time. A two-electron pathway is observed for ORR at AgX and specific capacitance was calculated to be 5.61 F g−1. The DBFC performance is investigated based on half-cell measurements of the AgY anode and Pt/C cathode. [ABSTRACT FROM AUTHOR]

Four TiO2 electrodes have been investigated for the oxygen reduction reaction (ORR) in 2 M NaOH. Anodic oxidation of Ti-foil was used as the final or the first step in the preparation of four electrodes based on TiO2: amorphous (Ti/AM-TiO2), two anatase forms (Ti/A400-TiO2 and Ti/A550-TiO2), and, finally, lithium-anatase form (Ti/A-Li0.25TiO2), all of them obtained as TiO2 nanotube arrays. The electrodes' structure was examined by Raman spectroscopy. Cyclic voltammetry and chronoamperometry were employed for the study of kinetics of ORR. Ti/A-Li0.25TiO2 and Ti/A400-TiO2 electrodes showed the highest catalytic activity for ORR with the highest cathodic peak current density and the lowest Tafel slope values. Number of electrons exchanged during ORR at TiO2 electrodes was in the range from 2.3 to 2.8. Additionally, TiO2 electrodes showed good stability as well as acceptable selectivity in the presence of borohydride, methanol or ethanol as typical fuels for fuel cells. Thus this study introduces a simple, low-cost preparation route of TiO2-based electrodes with high efficiency for ORR for energy conversion and storage applications. [ABSTRACT FROM AUTHOR]

Abstract: Carbon materials, prepared by using different methods with ionic liquids, are compared as electrocatalysts for the oxygen reduction reaction (ORR). Materials were synthesized through the hydrothermal carbonization of glucose and by using the same method in the presence of 1‐butyl‐3‐methylimidazolium methanesulfonate [bmim][MeSO3] as an additive. Another two carbon‐based materials were prepared by using ionic‐liquid‐based methods: ionothermal carbonization of glucose using [bmim][MeSO3] as a recyclable medium for the carbonization reaction and by direct carbonization of the ionic liquid in a one‐step method using [bmim][MeSO3] as the precursor for N‐ and S‐doped porous carbon (Carb‐IL). Characterization results showed the possibility of morphology and porosity control by using [bmim][MeSO3]. All materials were subsequently tested for the ORR in alkaline media. Carb‐IL showed enhanced and stable electrocatalytic ORR activity, even in the presence of methanol, ethanol, and borohydride, opening the possibility for its application in fuel cells. [ABSTRACT FROM AUTHOR]

Palladium-nickel nanoparticles supported on different tin oxide-carbon composites, namely PdNi/(SnO 2 -KB600), PdNi/(SnO 2 -KB300) and PdNi/(SnO 2 -graphene), as well as on Vulcan XC-72 (PdNi/Vulcan), are prepared and characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). The electrocatalytic activity of each material for the oxygen reduction reaction (ORR) and borohydride oxidation reaction (BOR) is studied in alkaline media by voltammetric techniques using rotating disk electrode (RDE) and rotating ring disk electrode (RRDE). ORR and BOR parameters, such as number of exchanged electrons, kinetic current density, Tafel slope and activation energy, are calculated. ORR n values at 0.2 V are close to 4 for PdNi/(SnO 2 -KB600) and PdNi/Vulcan, and close to 2 for PdNi/(SnO 2 -KB300) and PdNi/(SnO 2 -graphene). BOR n values range from 1.9 for PdNi/(SnO 2 -graphene) to 3.4 for PdNi/(SnO 2 -KB300). The materials stability is examined by chronoamperometry. The obtained results show that PdNi nanoparticles anchored on SnO 2 -KB600 support are good electrocatalyst candidates for both ORR and BOR in alkaline media. [ABSTRACT FROM AUTHOR]

Molybdenum carbide (Mo2C)-based electrocatalysts were prepared using two different carbon supports, commercial carbon nanotubes (CNTs) and synthesised carbon xerogel (CXG), to be studied from the point of view of both capacitive and electrocatalytic properties. Cation type (K+ or Na+) in the alkaline electrolyte solution did not affect the rate of formation of the electrical double layer at a low scan rate of 10 mV s−1. Conversely, the different mobility of these cations through the electrolyte was found to be crucial for the rate of double-layer formation at higher scan rates. Molybdenum carbide supported on carbon xerogel (Mo2C/CXG) showed ca. 3 times higher double-layer capacity amounting to 75 mF cm−2 compared to molybdenum carbide supported on carbon nanotubes (Mo2C/CNT) with a value of 23 mF cm−2 due to having more than double the surface area size. The electrocatalytic properties of carbon-supported molybdenum carbides for the oxygen reduction reaction in alkaline media were evaluated using linear scan voltammetry with a rotating disk electrode. The studied materials demonstrated good electrocatalytic performance with Mo2C/CXG delivering higher current densities at more positive onset and half-wave potential. The number of electrons exchanged during oxygen reduction reaction (ORR) was calculated to be 3, suggesting a combination of four- and two-electron mechanism. [ABSTRACT FROM AUTHOR]

Abstract PtNi nanoparticles (NPs) are synthesised by microwave irradiation technique and anchored onto three binary metal oxide (BMO) supports, namely Mn 2 O 3 -ZrO 2 , Mn 2 O 3 -V 2 O 5 and Mn 2 O 3 -WO 3 , prepared by solid-state dispersion method. The BMO supports are characterised using SEM, FTIR, N 2 -sorption and electrical conductivity measurements. XRD, XPS and TEM analysis confirm the formation of PtNi NPs on the BMO supports. Pt and Ni content over the support materials is set to 10 wt.% for each element. These electrocatalysts activity for oxygen reduction (ORR) and borohydride oxidation (BOR) reaction in alkaline media is assessed for the first time using voltammetric and chronoamperometric techniques. All three PtNi electrocatalysts revealed activity for ORR and BOR, with PtNi/(Mn 2 O 3 - ZrO 2) exhibiting the highest current densities. The ORR onset potentials were observed to range from 0.84 to 0.97 V vs. RHE, with Tafel slopes ranging from 0.101 to 0.230 V dec−1. BOR activation energies were found to range from 27 to 30 kJ mol−1. Obtained results point out PtNi/(Mn 2 O 3 - ZrO 2) as suitable electrocatalyst for fuel cell applications, particularly for BOR, with lower catalyst price due to partial replacement of the noble metal by a transition metal and improved stability achieved by introducing a binary metal oxide support. Graphical abstract Image 1 Highlights • Mn 2 O 3 -MO (ZrO 2 , V 2 O 5 , WO 3) supports are prepared by solid-state dispersion method. • MO choice affects materials' morphology, texture and electrical conductivity. • Supports are decorated with PtNi nanoparticles by microwave irradiation technique. • Electrocatalytic activity of PtNi/(Mn 2 O 3 -MO) is tested for ORR and BOR. • PtNi/(Mn 2 O 3 - ZrO 2) exhibits the best electrocatalytic performance towards BOR. [ABSTRACT FROM AUTHOR]

Porous cobalt (III) oxide (Co 3 O 4) and mixed cobalt (III) oxide - tin oxide (Co 3 O 4 /SnO 2) were prepared by a novel template-based hydrothermal method resulting in their spherical morphology as confirmed by thorough physico-chemical characterisation. Two oxides were systematically examined as bifunctional electrocatalysts for oxygen reduction (ORR) and evolution (OER) reaction in alkaline media by voltammetry with rotating disk electrode, electrochemical impedance spectroscopy, and chronoamperometry. Low-cost Co 3 O 4 and Co 3 O 4 /SnO 2 electrocatalysts showed excellent ORR performance with low onset and half-wave potential, low Tafel slope, and the number of exchange electrons near 4, comparable to the commercial Pt/C electrocatalyst. Low OER onset potential of 1.52 and 1.57 V was observed for Co 3 O 4 and Co 3 O 4 /SnO 2 , respectively, with low charge transfer resistance under anodic polarization conditions. Finally, to test bifunctional activity and durability of the two electrocatalyst, switch OER/ORR test was carried out. [Display omitted] • Spherical Co 3 O 4 and Co 3 O 4 /SnO 2 were synthesized by the hydrothermal method. • Both electrocatalysts showed excellent ORR performance comparable with Pt/C. • OER at Co 3 O 4 electrocatalyst starts ca. 50 mV earlier than at Co 3 O 4 /SnO 2. • Co 3 O 4 /SnO 2 gave comparable OER current densities to Co 3 O 4. • Co 3 O 4 and Co 3 O 4 /SnO 2 could be promising candidates for metal-air batteries. [ABSTRACT FROM AUTHOR]

• Platinum (Pt) nanoparticles (NPs) are prepared by microwave irradiation. • Pt NPs are anchored on MnO and binary metal oxides, MnO-NiO and MnO-TiO 2. • Electrocatalysts are assessed for oxygen reduction and borohydride oxidation. • Pt/MnO-NiO is a promising DBFC cathode electrocatalyst, with n of 4 for ORR. Integrating transition metal oxides with precious metals is a strategic approach to designing cost-effective electrocatalysts with enhanced stability. Herein, platinum (Pt) nanoparticles (NPs) were prepared by microwave irradiation and anchored onto MnO and two binary metal oxides, MnO-NiO and MnO-TiO 2 , obtained by solid-state dispersion. Voltammetric and electrochemical impedance spectroscopy techniques evaluated their performance for oxygen reduction reaction (ORR) and borohydride oxidation reaction (BOR) in alkaline media. Tafel slope and the number of exchanged electrons, n, were determined to compare the three electrocatalysts' performance for fuel cell applications. Pt/MnO-NiO revealed a Tafel slope of 177 mV dec–1 for ORR and an n value of ca. 4 and 3 e- for ORR and BOR, respectively. These findings demonstrate that Pt NPs supported on binary metal oxide supports, particularly Pt/MnO-NiO, are promising electrocatalysts for ORR and BOR in alkaline media, thus recommending their use in direct borohydride fuel cells. [ABSTRACT FROM AUTHOR]

The new copper(II) complexes [Cu(μ-1κ O :2κ ONN′ -HL1)(μ-1κ O :2κ O′ -NO 3)] 2.[Cu(μ-1κ O :2κ ONN′ -HL1)(CH 3 OH)] 2 (NO 3) 2 (1) and [Cu(κ ONN′ -HL2)(μ-1κ OO' :2κ O′ -NO 3)] n (2), derived from the new pro-ligands H 2 L1 = 2-(5,6-dihydroindolo[1,2- c ]quinazolin-6-yl)-5-methylphenol and H 2 L2 = 2-(5,6-dihydroindolo[1,2- c ]quinazolin-6-yl)-4-nitrophenol, were synthesized and characterized by elemental analysis, FT-IR, ESI-MS, and their structural features were unveiled by single-crystal X-ray diffraction analysis. This discloses a dimeric structure for 1 and a polymeric infinite 1D metal-organic chain for 2. The complexes were evaluated as catalysts for the oxidation of toluene, a volatile organic compound (VOC), and for oxygen reduction and water splitting reactions. 1 exhibits a higher activity for the peroxidative conversion of toluene to oxygenated products (total yields up to 38%), whereas 2 demonstrates a superior performance for electrochemical energy conversion applications, i.e., for oxygen reduction (ORR), oxygen evolution (OER) and hydrogen evolution (HER) reactions in an alkaline medium in terms of higher ORR current densities, lower Tafel slope (73 mV dec−1) and higher number of electrons exchanged (3.9), comparable to that of commercial Pt/C. Complex 2 also shows a better performance with lower onset potential and higher current densities for both OER and HER when studied as electrocatalyst for water splitting. Benzimidazole Schiff base copper(II) complexes as catalysts for environmental and energy applications: VOC oxidation, oxygen reduction and water splitting reactions. [Display omitted] Benzimidazole Schiff base copper(II) complex 1 as efficient heterogeneous catalyst for toluene oxidation and complex 2 as effective electrocatalyst for energy conversion reactions. • Two new benzimidazole Schiff base copper(II) complex (1 and 2) was synthesized and characterized. • Complexes were evaluated as catalysts for the oxidation of toluene and as oxygen reduction and water splitting reactions. • For the peroxidative conversion of toluene to oxygenated products, 1 exhibits a higher activity (total yields up to 38%). • Complex 2 exhibits superior performance for electrochemical energy conversion applications. • Complex 2 exhibits better electrocatalysis for both OER and HER with lower onset potential and higher current densities. [ABSTRACT FROM AUTHOR]

Development of cost-effective, bi-functional carbon electrocatalysts via direct carbonization of ionic liquids (bis(cholinium) tetrachlorocobaltate(II) ([Ch] 2 [CoCl 4 ]) and bis(1-butyl-3-methylimidazolium) tetrachlorocobaltate(II) ([Bmim] 2 [CoCl 4 ])) is reported herein for the first time. Carbon electrocatalysts, dual-doped with cobalt and nitrogen, were tested for oxygen reduction (ORR) and oxygen evolution (OER) reactions. Both materials show high bi-functional catalytic activity and excellent stability due to synergistic effects arising from the presence of nitrogen and cobalt. Electrocatalyst prepared by carbonization of [Ch] 2 [CoCl 4 ] show exceptional activity and selectivity toward ORR. Conversely, electrocatalyst prepared from [Bmim] 2 [CoCl 4 ] showed a slightly better OER performance indicating that different catalytic sites are responsible for O 2 reduction and H 2 O oxidation. Parent CoO particles with graphitic nitrogen boost activity for ORR, while elemental Co supported by nitrogen atoms is responsible for OER activity. Finally, energy consumption during electrolytic oxygen production was calculated revealing energy saving when using two materials as anode electrocatalysts. [Display omitted] • Co,N dual-doped carbons are prepared by direct carbonization of ionic liquids. • A next-level of bi-functional electrocatalysts are developed for ORR and OER. • C Ch shows superior electrocatalytic performance for ORR with transfer of 4 e−. • Materials display high OER efficiencies with Tafel slope as low as 115 mV dec−1. • Different highly active catalytic sites are responsible for efficient ORR and OER. [ABSTRACT FROM AUTHOR]

[Display omitted] • Unitized fuel cells and their operation mechanisms are introduced. • Oxygen reduction and oxygen evolution pathways in alkaline media are explained. • Transition metal oxides for catalysis of oxygen electrode reactions are reviewed. • Systematic comparison of bifunctional ORR/OER performance of TMOs is given. Among many alternatives to fossil fuel-based energy systems, one of the most promising is based on hydrogen energy and its production and use in unitized regenerative fuel cells as the primary energy conversion devices. However, there are some setbacks and challenges when designing suitable and efficient electrocatalysts for these devices. The most effective and durable catalysts are based on platinum–group metals, with low abundance and unbearably high prices. Many attempts were undertaken to overcome this setback by designing catalysts suitable for massive commercial use. This review paper focuses on recent advances in developing bifunctional catalysts for oxygen reduction and oxygen evolution catalysis in alkaline media, based on abundant transition metal oxides (TMOs): MnO 2 , NiO, and TiO 2. The problem of unifying parameters to compare the effectiveness of different electrocatalysts is emphasized. This review discusses the most promising alternative bifunctional electrocatalysts by comparing the performance of TMOs with some precious metal catalysts used as benchmarks. [ABSTRACT FROM AUTHOR]

Four cerium-zeolites were examined as low-cost bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in an alkaline solution. Cerium-exchanged zeolites, synthetic 13X and natural clinoptilolite were employed as prepared and calcined (cal). Four samples (Ce-13X cal, Ce-13X, Ce-Cli cal, and Ce-Cli) were characterized by X-ray powder diffraction analysis (XRPD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, fluorescence spectroscopy and Brunner-Emmet-Teller (BET) method. All four zeolites exhibited activity for the two studied reactions. Ce-13X cal gave the lowest onset potential accompanied by the highest OER current density. Furthermore, Ce-13X cal showed the most positive ORR onset potential value of 0.84 V and the most positive ORR half-wave potential of 0.75 V (ca. 50 mV more positive than value for the other three cerium-zeolite electrodes) accompanied by the lowest Tafel slope of 80 mV dec−1. Koutecky-Levich analysis indicated that ORR follows 2-electron pathway. Overall, Ce-13X cal showed the best OER/ORR performances in alkaline solution. [Display omitted] • Four cerium-zeolites showed OER/ORR activity in alkaline media. • Ce-13X cal gave the highest OER current density. • The lowest OER onset potential of 1.60 V was observed for Ce-13X cal • Ce-13X cal showed the most positive ORR onset potential value of 0.84 V. [ABSTRACT FROM AUTHOR]

Carbon-supported silver nanoparticles (Ag:NPs/C) were synthesized by gamma irradiation-induced reduction method using the poly(vinyl alcohol) or poly(vinyl alcohol)/chitosan polymer as stabilizer. Prepared samples were characterized using transmission electron microscopy and X-ray diffractometry. Subsequently, Ag:NPs/C were studied using rotating disc and rotating ring disc method as electrocatalysts for ORR (oxygen reduction reaction) and BOR (borohydride oxidation reaction) for potential application in alkaline fuel cells. The synthesis method used herein offers simple and fast approach for catalytic ink preparation, since the ink is prepared in one-step radiation process, simultaneously with Ag + ions reduction. Very high and stable catalytic efficiency toward ORR via 4e − path was evidenced during 4000 square pulse polarization cycles. BOR, accompanied with the simultaneous borohydride ion hydrolysis, was found to proceed at the oxidized Ag surface. [ABSTRACT FROM AUTHOR]