Non-precious Metal-based Electrocatalysts Research Articles

One-dimensional screw-like MoS2 with oxygen partially replacing sulfur as an electrocatalyst for the N2 reduction reaction

• One-dimensional screw-like MoS 2 with oxygen partially replacing sulfur nanosheets are active for NRR. • 1D-MoS 2-x O y nanosheets achieve a large NH 3 yield of 5.56 × 10 -8 mol s −1 cm −2 at −0.35 V vs. RHE. • 1D-MoS 2-x O y nanosheets attain a relatively high Faradaic efficiency of 9.86% at −0.3 V vs. RHE. • 1D-MoS 2-x O y nanosheets show high electrochemical stability. Electrocatalytic nitrogen reduction reaction (NRR) is considered to be an energy-saving method for the production of NH 3 . However, it is still very challenging to achieve a high NH 3 yield and efficiency under ambient conditions. Herein, we for the first time synthesize one-dimensional screw-like MoS 2 with oxygen partially replacing sulfur (1D-MoS 2-x O y ) nanosheets by a one-step solvothermal method as an efficient non-noble metal electrocatalyst for NRR. In a 0.1 M HCl solution, 1D-MoS 2-x O y exhibits excellent NRR performance, with a remarkable average ammonia yield rate of 5.56 × 10 -8 mol s −1 cm −2 at −0.35 V, and a high Faradaic efficiency of 9.86% at −0.3 V. The isotope labeling experiment proves that the generation of NH 3 is completely derived from the feed-in N 2 . Density functional theory calculations show that MoS 2 with oxygen partially replacing sulfur can reduce the Gibbs free energy of the potential determination step (PDS). Besides, the unique screw-like structure can provide more N 2 adsorption sites for NRR. This work proposes a promising strategy for the reasonable design of non-precious metal-based NRR electrocatalysts.

Molybdenum-induced tuning 3d-orbital electron filling degree of CoSe2 for alkaline hydrogen and oxygen evolution reactions

The development of high-performance non-precious metal-based robust bifunctional electrocatalyst for both hydrogen evolution reaction (HER) and oxygen evolution reactions (OER) in alkaline media is essential for the electrochemical overall water splitting technologies. Herein, we demonstrate that the HER/OER performance of CoSe2 can be significantly enhanced by tuning the 3d-orbital electron filling degree through Mo doping. Both density functional theory (DFT) calculations and experimental results imply that the doping of Mo with higher proportion of the unoccupied d-orbital (Pun) could not only serve as the active center for water adsorption to enhance the water molecule activation, but also modulate the electronic structures of Co metal center leading to the optimized adsorption strength of *H. As expected, the obtained Mo-CoSe2 exhibits a remarkable bifunctional performance with overpotential of only 85 mV for HER and 245 mV for OER to achieve the current density of 10 mA/cm2 in alkaline media. This work will provide a valuable insight to design highly efficient bifunctional electrocatalyst towards HER and OER.

MOF-driven ultrafine Co9S8 nanocrystals embedded in N, S-Codoped Multilayer-Assembled carbon nanoplates for efficient bifunctional oxygen electrocatalysis

• Co 9 S 8 /NSCP is made by polymer assembly, metal-organic coordination, and pyrolysis. • Co 9 S 8 nanocrystals are only 6 nm embedded in multilayer-assembled carbon nanoplates. • Co 9 S 8 /NSCP shows excellent catalytic performance towards both ORR and OER. • The reversible oxygen potential of Co 9 S 8 /NSCP is only 0.7 V, one of the best. • This work opens the door towards rational design of 3D carbon-based nanocrystals. To achieve broad commercialization of regenerative fuel cells and rechargeable metal-air batteries, the development of non-precious metal-based bifunctional oxygen electrocatalysts is critical. Herein, we report the controlled synthesis of MOF-derived ultrafine Co 9 S 8 nanocrystals (∼6 nm) embedded in nitrogen- and sulfur- co-doped multilayer-assembled carbon nanoplates (Co 9 S 8 /NSCP) for highly efficient oxygen electrocatalysis through polymer self-assembly, metal–organic coordination, and subsequent space-confined pyrolysis. The resultant Co 9 S 8 /NSCP shows excellent catalytic activity towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with high half-wave potential (0.9 V) and low overpotential (370 mV), separately. The extremely low reversible oxygen overpotential of 0.7 V is achieved in 0.1 M KOH electrolyte, much better than those of single-component counterparts (Co 9 S 8 or carbon), comparable or even superior to those of noble metal catalysts and recently reported advanced catalysts. The superior performance of Co 9 S 8 /NSCP can be attributed to the synergistic effect of highly exposed Co 9 S 8 nanocrystals and heteroatom-enriched 3D carbon superstructure, enabling large surface area, protective graphitic layers, and abundant active sites towards fast catalytic kinetics and excellent durability. This work will open the door towards the rational construction of 3D carbon-based nanocrystals for a wide range of applications beyond catalysis.

Synthesis and electrocatalytic performance of N-doped graphene embedded with Co/CoO nanoparticles towards oxygen evolution and reduction reactions

N-doped graphene decorated with cobalt-based nanoparticles (Co/CoO/NG-T) were obtained via a 1,10-phenanthroline assisted pyrolysis method. The optimized Co/CoO/NG-800 showed excellent performance with E1/2, ORR of 0.82 V vs. RHE and ηOER, 10 mA cm-2 of 407 mV. A good stability was obtained with only 20 mV negative drift in the half-wave potential after 2000 cycles, and a retention rate of 94% after 5 h. The prominent performance is attributed to the electronic interaction between Co-based species and N-doped graphene, and the presence of active pyridinic-N moieties. This work provides a revealing paradigm on the design of efficient non-precious metal-based electrocatalysts.

Co/N-codoped carbon nanotube hollow polyhedron hybrid derived from salt-encapsulated core-shell ZIF-8@ZIF-67 for efficient oxygen reduction reaction

The rational design and construction of non-precious metal-based electrocatalysts with high activity and stability toward oxygen reduction reactions (ORR) are essential but challenging. In this work, a novel hybrid nanoporous structure of Co nanoparticles uniformly embedded in hollow porous carbon doped with nitrogen (CoNHPC) was constructed by a melt-evaporation-pyrolysis method, in which NaCl is utilized as an auxiliary template. The introduction to NaCl in this process served as the confining template to prevent nitrogen loss and promote graphitization, and the intercalating agent to form a mesoporous product. Due to the composite hierarchical porous structure, higher graphitization degree, and desirable nitrogen bonding type, the as-prepared CoNHPC-920 presents outstanding ORR electrocatalytic activity in terms of a half-wave potential of 0.87 V (vs. RHE) and onset potential of 0.97 V (vs. RHE). The composite material also shows higher selectivity and outstanding durability with most transition metal porous carbon materials reported. The strategy proposed in this study may provide ideas about metal nitrogen-carbon type catalysts enriched with carbon nanotubes and bare leaky hollow structures of the construction of advanced ORR electrocatalysts, opening up a new pathway.

Aminouracil-assisted synthesis of CoFe decorated bougainvillea-like N-doped carbon nanoflowers for boosting Zn–air battery and water electrolysis

Highly efficient, low-cost, and long-term stable catalysts are of great significance as promising alternatives to noble metal-based catalysts for oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in energy-related devices. Herein, CoFe decorated bougainvillea-like N-doped carbon nanoflowers (CoFe-NCNFs) are synthesized through a one-step aminouracil-assisted pyrolysis strategy. Benefit from the abundant active sites from the flower-like structures, along with synergistic effect between multi-compositions, the catalyst displays distinct improvement on the ORR with the more positive onset potential ( E onset = 1.06 V) and half-wave potential ( E 1/2 = 0.85 V), coupled with the small overpotential ( η ) of 323 mV and 212 mV at 10 mA cm −2 for OER and HER, respectively. In view of the trifunctional properties, a rechargeable Zn–air battery is constructed by self-assembly, possessing large power density (116.1 mW cm −2 ), small voltage gap (0.865 V at 5 mA cm −2 ), and superior durability for 110 h. Also, the catalyst demonstrated excellent catalytic activity and stability in water electrolysis ( η = 1.65 V at 10 mA cm −2 for 20 h). This research offers a breakthrough in developing advanced multifunctional non-precious metal-based electrocatalysts in metal–air batteries, water electrolysis, and other energy-correlated devices. • CoFe-NCNFs were fabricated by a facile pyrolysis method. • The unique structure offers enlarged specific active area and improved stability. • Multi-component synergetic effect brings out trifunctional catalytic activities. • The catalyst performs well in Zn–air battery and water electrolysis.

Facile Synthesis of 1T-Phase MoS2 Nanosheets on N-Doped Carbon Nanotubes towards Highly Efficient Hydrogen Evolution.

1T-phase molybdenum disulfide is supposed to be one of the non-precious metal-based electrocatalysts for the hydrogen evolution reaction with the highest potential. Herein, 1T-MoS2 nanosheets were anchored on N-doped carbon nanotubes by a simple hydrothermal process with the assistance of urea promotion transition of the 1T phase. Based on the 1T-MoS2 nanosheets anchored on the N-doped carbon nanotubes structures, 1T-MoS2 nanosheets can be said to have highly exposed active sites from edges and the basal plane, and the dopant N in carbon nanotubes can promote electron transfer between N-doped carbon nanotubes and 1T-MoS2 nanosheets. With the synergistic effects of this structure, the excellent 1T-MoS2/ N-doped carbon nanotubes catalyst has a small overpotential of 150 mV at 10 mA cm−2, a relatively low Tafel slope of 63 mV dec−1, and superior stability. This work proposes a new strategy to design high-performance hydrogen evolution reaction catalysts.

Iron and chromium co-doped cobalt phosphide porous nanosheets as robust bifunctional electrocatalyst for efficient water splitting

It is still a huge challenge to develop highly efficient and low-cost non-precious metal-based electrocatalysts for overall water splitting in alkaline electrolytes. Herein, Cr and Fe co-doped CoP porous mesh nanosheets (Mesh-CrFe-CoP NSs) were synthesized through hydrolysis reaction, ion exchange etching and subsequent low-temperature phosphating process. The Mesh-CrFe-CoP NSs provides overpotentials at a current density of 10 mA cm−2 under alkaline electrolyte of 103.7 mV and 256.4 mV for HER and OER, respectively. Furthermore, when using Mesh-CrFe-CoP NSs as anode and cathode, the water splitting system could afford a current density of 10 mA cm–2 at 1.55 V, which is better than an electrolytic cell composed of 20% Pt/C and RuO2. The excellent electrocatalytic performance of Mesh-CrFe-CoP NSs is attributed to the co-doping and porous nanostructure. Specifically, the Cr and Fe co-doped porous CoP nanosheets electrocatalyst not only provided abundant exposure active sites, accelerated the entry of liquid and the diffusion of gas, but also regulated the electronic environment of active sites, and thus enhanced the electrochemical performance. This work proposes a strategy for the rational design of highly efficient and stable non-precious metal co-doped phosphide electrocatalysts in the of electrochemical water splitting.

An active ZnxNi1-xS@Mo2C/carbon cloth electrode as efficient catalyst for water electrolysis

Electrolysis of water is a promising method for production of the green-energy hydrogen, which is commonly assisted by high powerful catalyst. At present, developing low-cost and highly efficient electrocatalysts has attracted increasing attention. Herein, a novel composited catalyst of ZnxNi1-xS@Mo2C loaded on conductive carbon cloth (ZnxNi1-xS@Mo2C/CC) is successfully prepared by the hydrothermal method. Due to the abundant active sites of defect structures induced by the surface engineering, the prepared ZnxNi1-xS@Mo2C/CC shows outstanding electrocatalytic performances in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) of water splitting, with low overpotential of 230 mV and 150 mV at 10 mA cm−2, respectively. The ZnxNi1-xS@Mo2C/CC as an OER/HER bifunctional catalyst for overall water splitting affords a stable voltage of 1.56 V @current density of 10 mA cm−2. Moreover, the catalyst shows an outstanding stability after a series of electrochemical tests. This work expands the multi-field applications of carbon cloth-supported materials as efficient non-precious metal-based electrocatalysts, with great importance in sustainable energy-relative technologies.

Single-Atom Co Doped in Ultrathin WO3 Arrays for the Enhanced Hydrogen Evolution Reaction in a Wide pH Range.

Owing to the scarcity of Pt, low-cost, stable, and efficient nonprecious metal-based electrocatalysts that can be applied in a wide pH range for the hydrogen evolution reaction (HER) are urgently required. Herein, a highly efficient and robust HER catalyst that is applicable at all pH values is fabricated, containing isolated Co-single atomic sites anchored in the self-supported WO3 arrays grown on Cu foam. At a current density of 10 mA cm-2, the HER overpotentials are 117, 105, and 149 mV at pH values of 0, 7, and 14, respectively, which are significantly lower than those of the undoped WO3, suggesting superior electrocatalytic H2-evolution activity at all pH values. The catalyst also exhibits long-term stability over a wide pH range, particularly in an acidic medium over 24 h, owing to the excellent anticorrosion properties of WO3. Density functional theory calculations prove that the enhanced HER activity is attributed to the isolated Co sites because these optimize the adsorption energy of H* species on WO3. Moreover, the high electrical conductivity of Co-doped WO3 and the three-dimensional array structure supported on the porous metal support afford a catalyst with suitable HER kinetics to enhance the catalytic performance.

Highly stable and methanol tolerant oxygen reduction reaction electrocatalyst Co/CoO/SnO@N-C nanocubes by one-step introduction of functional components

To develop high-performance non-precious metal-based electrocatalysts for oxygen reduction reaction (ORR) is an urgent demand. We herein report an ingenious strategy to develop highly selective and competitive precious metal-free ORR electrocatalyst Co/CoO/SnO encapsulated in N-rich mesoporous carbon (CCS@NPC) nanocubes, via a simple one-step introduction of all functional components. Atomically mixed composites with significantly enhanced catalytic activity were obtained by adopting amorphous mesoporous ternary metal oxides CoSnO3 nanocubes as precursor and template. The synergistic effect of the triphase nanohybrids (Co, CoO & SnO) anchored in nitrogen-doped mesoporous carbon nanocubes promoted the catalytic reactions, owning to the anisotropic morphology, great heterojunction interfaces, and structural stability. The large-scale prepared CCS@NPC exhibited excellent electrocatalytic activity toward ORR with an admirable onset potential (1.01 V) and diffusion-limited current density (5.88 mA cm−2). The CCS@NPC showed stronger stability (20 h) and higher methanol tolerance with the concentration up to 8.0 M, compared to that of commercial Pt/C.

Ni(OH)2 microspheres in situ self-grown on ultra-thin layered g-C3N4 as a heterojunction electrocatalyst for oxygen evolution reaction

Oxygen evolution reaction (OER), as a part reaction of the overall water splitting, is deemed as a prospective technology for large-scale energy storage. However, the sluggish kinetics (large overpotential) and the expensive cost of noble metal-based electrocatalysts (RuO2 and IrO2) restrain the widespread usage of OER. Hence, the discovery of non-precious metal-based electrocatalysts with tiny overpotential, satisfactory current density and outstanding stability has become urgent. In this work, we have prepared ultra-thin layered g-C3N4 by a two-step thermal peeling method and synthesized Ni(OH)2/g-C3N4 composite by a simple one-step solvothermal method. Benefiting from the layered g-C3N4 as substrate, the overpotential of Ni(OH)2/g-C3N4 composite was just 240 mV at 10 mA cm−2, which gives an unexpected improvement in the electrochemical performance of the composite samples compared to pure Ni(OH)2 (η = 460 mV). Additionally, the formation of heterojunction effectively reduces the resistance to electron transport in OER, the resistance of Ni(OH)2/g-C3N4 (Rct = 38.8 Ω) is far smaller than those of bare Ni(OH)2 (Rct = 41.6 Ω) and layered g-C3N4 (Rct = 43.6 Ω) at the open circuit potential. Ni(OH)2/g-C3N4 composite samples display outstanding electrochemical stability, maintaining 85% of initial current density for 12 h. Moreover, density functional theory (DFT) demonstrates that ΔG3 of Ni(OH)2/g-C3N4 composite (η = 0.46 V) is much lower than that of bare Ni(OH)2 (η = 0.64 V). These undeniable results demonstrate that Ni(OH)2/g-C3N4 composite materials is promising alternative materials to replace precious metal-based OER electrocatalysts in the field of water splitting.

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 E1/2 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 Ej = 10 of 1.67 V and a potential gap of 0.833 V between Ej = 10 and E1/2 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.

Highly exfoliated NiPS3 nanosheets as efficient electrocatalyst for high yield ammonia production

The development of electrocatalytic nitrogen reduction reaction (NRR) at ambient conditions as an alternative to traditional high-temperature ammonia synthesis is a vibrant research topic due to the potential to significantly reduce the energy consumption required for the production of ammonia. Several noble metal-based materials have already been identified as highly active electrocatalysts for this purpose. However, the development of non-precious metal-based electrocatalysts is necessary for realizing cost-effective ammonia synthesis at large scales. This work has explored the potential of the less exploited transition metal phosphorus trichalcogenide NiPS3 as an NRR electrocatalyst. Excellent NRR activities have been achieved by exfoliating bulk NiPS3 into bi- or tri-layered nanosheets. The NH3 yield attained using exfoliated NiPS3 is 118 μg h−1 mgcat-1 with a Faraday efficiency of > 17 % at an applied potential of −0.4 V vs RHE. The NRR performance of exfoliated NiPS3 in terms of NH3 yield surpasses many non-noble metal-based catalysts reported in the literature. Further, it also exhibited a stable NRR activity of > 90% even after several repetitive cycles. Plane-wave DFT calculations at the GGA + U level have been used to investigate the reaction pathways. It could be shown that the NNR follows an associative mechanism, with the very first hydrogenation step being the potential determining step.

Porous NiFe alloys synthesized via freeze casting as bifunctional electrocatalysts for oxygen and hydrogen evolution reaction

Binder-free NiFe-based electrocatalyst with aligned pore channels has been prepared by freeze casting and served as a bifunctional catalytic electrode for oxygen and hydrogen evolution reaction (OER and HER). The synergistic effects between Ni and Fe result in the high electrocatalytic performance of porous NiFe electrodes. In 1.0 M KOH, porous Ni7Fe3 attains 100 mA cm−2 at an overpotential of 388 mV with a Tafel slope of 35.8 mV dec−1 for OER, and porous Ni9Fe1 exhibits a low overpotential of 347 mV at 100 mA cm−2 with a Tafel slope of 121.0 mV dec−1 for HER. The Ni9Fe1//Ni9Fe1 requires a low cell voltage of 1.69 V to deliver 10 mA cm−2 current density for overall water splitting. The excellent durability at a high current density of porous NiFe electrodes has been confirmed during OER, HER and overall water splitting. The fine electrocatalytic performances of the porous NiFe-based electrodes owing to the three-dimensionally well-connected scaffolds, aligned pore channels, and bimetallic synergy, offering excellent charge/ion transfer efficiency and sizeable active surface area. Freeze casting can be applied to design and synthesize various three-dimensionally porous non-precious metal-based electrocatalysts with controllable multiphase for energy conversion and storage.

Phosphazene-Based Covalent Organic Polymer Decorated with NiCo2O4 Nanocuboids as a Trifunctional Electrocatalyst: A Unique Replacement for the Conventional Electrocatalysts

Developing nonprecious metal-based electrocatalysts to convert water into green fuels (H2 and O2) is key to address urgent climate and energy challenges. We have prepared an electrocatalyst by the immobilization of NiCo2O4 on a phosphazene-based covalent organic polymer (P-COP) through a facile hydrothermal method. The elemental composition of the P-COP showed the presence of a greater amount of heteroatoms N (6.62%) and P (5.62%) throughout the polymer support. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) were utilized to determine the atomic structure of the nanocuboids, which depicted the formation of an inverse spinel structure. A NiCo2O4-P-COP-based electrode was simultaneously used for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), and it displayed a minimum overpotential of 270 and 130 mV (V vs RHE), respectively, at a current density of 10 mA cm–2. In addition, it acted as an oxygen reduction catalyst with a half-wave potential of 0.83 V (V vs RHE) and a maximum current density of 4.5 mA cm–2. The electrocatalytic activity is comparable with that of the commercially available Pt and RuO2 catalysts. The combined experimental and computational studies confirm that the catalytic centers formed through the interaction between the heteroatoms (N and P) in the phosphazene matrix and metal oxides (Co and Ni) play an important role in its improved durability and electrocatalytic activity.

Metal-organic framework-derived carbon nanotubes with multi-active Fe-N/Fe sites as a bifunctional electrocatalyst for zinc-air battery

Sustainable metal-air batteries demand high-efficiency, environmentally-friendly, and non-precious metal-based electrocatalysts with bifunctionality for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this research, novel functional carbon nanotubes with multi-active sites including well-dispersed single-atom iron throughout the walls and encapsulated ultrafine iron nanoparticles were synthesized as an electrocatalyst (FeNP@Fe-N-C) through one-step pyrolysis of metal-organic frameworks. High-resolution synchrotron powder X-ray diffraction and X-ray absorption spectroscopy were applied to characterize the unique structure of the electrocatalyst. In comparison to the commercial Pt/C and RuO2 electrodes, the newly prepared FeNP@Fe-N-C presented a superb bifunctional performance with its narrow potential difference (Egap) of 0.73 V, which is ascribed to the metallic Fe nanoparticles that boosts the adsorption and activation of oxygen on the active sites with an enhanced O2 adsorption capacity of 7.88 cm3 g−1 and synergistically functionalizes the iron atoms dispersed on the nanotubes. A rechargeable zinc-air battery based on FeNP@Fe-N-C exhibited a superior open-circuit voltage (1.45 V), power density (106.5 mW cm−2), and stable cycling performance. The green technique developed in this work for the fabrication of functional nanotubes raises the prospect of making more efficient electrocatalysts for sustainable energy cells.

Hierarchical porous N,S-codoped carbon with trapped Mn species for efficient pH-universal electrochemical oxygen reduction in Zn-air battery

Highly efficient and ultrastable nonprecious metal-based electrocatalysts are promising alternatives to platinum (Pt)-based catalysts in electrocatalyzing oxygen reduction reaction (ORR) process. In the present work, nitrogen and sulfur dual-doped hierarchically porous carbon with trapped Mn species (MnNSC-950) was fabricated, which integrates the modulation of the electronic structure for boosted intrinsic activity with the engineering of hierarchical porosity for enriched active sites. The MnNSC-950 shows a positive onset potential of 0.95 V and a large limiting current density of 5.4 mA cm−2, which can rival that of the commercial Pt/C catalyst in 0.1 M KOH, together with a robust long-term stability. Moreover, the MnNSC-950 as ORR catalyst in Zn-air battery achieves a high power density of 107 mW cm−2 and robust cycling performance over 150 h, exceeding that driven by Pt/C cathode. The excellent electrochemical performance is also shown in both neutral and acidic media. This work provides an enabling route to prepare non-noble metal-based catalysts featuring well-engineered geometric and component peculiarities for practical application of energy conversion systems.

Recent development in electrocatalysts for hydrogen production through water electrolysis

Hydrogen is a carbon-free alternative energy source for use in future energy frameworks with the advantages of environment-friendliness and high energy density. Among the numerous hydrogen production techniques, sustainable and high purity of hydrogen can be achieved by water electrolysis. Therefore, developing electrocatalysts for water electrolysis is an emerging field with great importance to the scientific community. On one hand, precious metals are typically used to study the two-half cell reactions, i.e., hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, precious metals (i.e., Pt, Au, Ru, Ag, etc.) as electrocatalysts are expensive and with low availability, which inhibits their practical application. Non-precious metal-based electrocatalysts on the other hand are abundant with low-cost and eco-friendliness and exhibit high electrical conductivity and electrocatalytic performance equivalent to those for noble metals. Thus, these electrocatalysts can replace precious materials in the water electrolysis process. However, considerable research effort must be devoted to the development of these cost-effective and efficient non-precious electrocatalysts. In this review article, we provide key fundamental knowledge of water electrolysis, progress, and challenges of the development of most-studied electrocatalysts in the most desirable electrolytic solutions: alkaline water electrolysis (AWE), solid-oxide electrolysis (SOE), and proton exchange membrane electrolysis (PEME). Lastly, we discuss remaining grand challenges, prospect, and future work with key recommendations that must be done prior to the full commercialization of water electrolysis systems.

Efficient oxygen reduction electrocatalyst derived from facile Fe,N−surface treatment of carbon black

The development of nonprecious metal-based electrocatalysts for oxygen reduction reaction (ORR) is a central task in renewable electrochemical energy conversion and storage technologies. Iron-nitrogen doped carbon-based (Fe−N/C) materials are promising alternatives to Pt-based ORR electrocatalysts. Owing to large specific surface area and outstanding electrical conductivity, carbon black is an inborn support for electrocatalysts. Unfortunately, the direct incorporation of Fe−Nx moieties onto the surface of carbon black has not been realized to date. Herein, Fe−Nx moieties are directly incorporated onto the surface of carbon black through surface modification and the following Fe and N co-doping. The obtained Fe and N co-doped carbon back (Fe−N/CB) catalyst has very large specific surface area and abundant accessible Fe−Nx moieties. As a result, Fe−N/CB electrocatalyst exhibits a more positive half-wave potential (0.86 V) than Pt/C. The Fe−N/CB catalyst also displays better stability and methanol resistance than Pt/C. The Zn-air battery with Fe−N/CB as cathodic catalyst shows a maximum power density of 68 mW cm−2 and a specific capacity of 676 mAh gZn-1. Our finding provides a convenient and low-cost approach to fabricating efficient M−N/C-based catalysts and will be helpful to the development of renewable electrochemical energy conversion and storage technologies.