Water electrolysis is considered as the most promising technology for hydrogen production. Much research has been devoted to developing efficient electrocatalysts for hydrogen production via the hydrogen evolution reaction (HER) and oxygen production via the oxygen evolution reaction (OER). The optimum electrocatalysts can drive down the energy costs needed for water splitting via lowering the overpotential. A number of cobalt (Co)-based materials have been developed over past years as non-noble-metal heterogeneous electrocatalysts for HER and OER. Recent progress in this field is summarized here, especially highlighting several important bifunctional catalysts. Various approaches to improve or optimize the electrocatalysts are introduced. Finally, the current existing challenges and the future working directions for enhancing the performance of Co-implicated electrocatalysts are proposed.

The use of electrocatalysts with low metal content (metal-deficient) or metal free for the hydrogen evolution reaction (HER) can prevent or decrease metal ion release, which reduces environmental impact; development of such catalysts with high activity and durability over the whole pH range is thus highly desired but still remains a huge challenge. Herein, we describe the direct growth of a film consisting of interconnected Co-entrapped, N-doped carbon nanotubes on carbon cloth using chemical vapor deposition from dicyanodiamine using a Co₃O₄ nanowire array as catalyst. This integrated architecture is used as a flexible 3D electrode for the electrolytic hydrogen evolution with outstanding catalytic activity and durability in acidic media. Moreover, this electrode is also highly efficient under neutral and basic conditions. It offers us an attractive carbon-based metal-deficient HER catalyst outperforming most transition-metal and all metal-free/deficient catalysts.

Active and stable electrocatalysts made from earth-abundant elements are key to water splitting for hydrogen production through electrolysis. The growth of NiSe nanowire film on nickel foam (NiSe/NF) in situ by hydrothermal treatment of NF using NaHSe as Se source is presented. When used as a 3D oxygen evolution electrode, the NiSe/NF exhibits high activity with an overpotential of 270 mV required to achieve 20 mA cm⁻² and strong durability in 1.0 M KOH, and the NiOOH species formed at the NiSe surface serves as the actual catalytic site. The system is also highly efficient for catalyzing the hydrogen evolution reaction in basic media. This bifunctional electrode enables a high-performance alkaline water electrolyzer with 10 mA cm⁻² at a cell voltage of 1.63 V.

Active and stable electrocatalysts made from earth-abundant elements are key to water splitting for hydrogen production through electrolysis. The growth of NiSe nanowire film on nickel foam (NiSe/NF) in situ by hydrothermal treatment of NF using NaHSe as Se source is presented. When used as a 3D oxygen evolution electrode, the NiSe/NF exhibits high activity with an overpotential of 270 mV required to achieve 20 mA cm⁻² and strong durability in 1.0 M KOH, and the NiOOH species formed at the NiSe surface serves as the actual catalytic site. The system is also highly efficient for catalyzing the hydrogen evolution reaction in basic media. This bifunctional electrode enables a high-performance alkaline water electrolyzer with 10 mA cm⁻² at a cell voltage of 1.63 V.

It is highly desired but still challenging to develop active nonprecious metal hydrogen evolution reaction (HER) electrocatalysts operating under all pH conditions. Herein, the development of three-dimensional N-doped carbon-coated tungsten oxynitride nanowire arrays on carbon cloth as a highly efficient and durable HER cathode was explored. The material delivers current densities of 10 and 100 mA cm⁻² at overpotentials of 106 and 172 mV, respectively, in acidic medium, and it also performs well in neutral and basic electrolytes.

The detection of specific DNA sequences plays an important role in the identification of disease-causing pathogens and genetic diseases, and photochemical water splitting offers a promising avenue to sustainable, environmentally friendly hydrogen production. Cobalt–phosphorus nanowires (CoP NWs) show a high fluorescence quenching ability and different affinity toward single- versus double-stranded DNA. Based on this result, the utilization of CoP NWs as fluorescent DNA nanosensors with a detection limit of 100 pM and a selectivity down to single-base mismatch was demonstrated. The use of a thrombin-specific DNA aptamer also enabled the selective detection of thrombin. The photoinduced electron transfer from the excited dye that labels the oligonucleotide probe to the CoP semiconductor led to efficient fluorescence quenching, and largely enhanced the photocatalytic evolution of hydrogen from water under visible light.

The detection of specific DNA sequences plays an important role in the identification of disease-causing pathogens and genetic diseases, and photochemical water splitting offers a promising avenue to sustainable, environmentally friendly hydrogen production. Cobalt–phosphorus nanowires (CoP NWs) show a high fluorescence quenching ability and different affinity toward single- versus double-stranded DNA. Based on this result, the utilization of CoP NWs as fluorescent DNA nanosensors with a detection limit of 100 pM and a selectivity down to single-base mismatch was demonstrated. The use of a thrombin-specific DNA aptamer also enabled the selective detection of thrombin. The photoinduced electron transfer from the excited dye that labels the oligonucleotide probe to the CoP semiconductor led to efficient fluorescence quenching, and largely enhanced the photocatalytic evolution of hydrogen from water under visible light.

The development of effective and inexpensive hydrogen evolution reaction (HER) electrocatalysts for future renewable energy systems is highly desired. The strongly acidic conditions in proton exchange membranes create a need for acid-stable HER catalysts. A nanohybrid that consists of carbon nanotubes decorated with CoP nanocrystals (CoP/CNT) was prepared by the low-temperature phosphidation of a Co₃O₄/CNT precursor. As a novel non-noble-metal HER catalyst operating in acidic electrolytes, the nanohybrid exhibits an onset overpotential of as low as 40 mV, a Tafel slope of 54 mV dec⁻¹, an exchange current density of 0.13 mA cm⁻², and a Faradaic efficiency of nearly 100 %. This catalyst maintains its catalytic activity for at least 18 hours and only requires overpotentials of 70 and 122 mV to attain current densities of 2 and 10 mA cm⁻², respectively.

The development of effective and inexpensive hydrogen evolution reaction (HER) electrocatalysts for future renewable energy systems is highly desired. The strongly acidic conditions in proton exchange membranes create a need for acid-stable HER catalysts. A nanohybrid that consists of carbon nanotubes decorated with CoP nanocrystals (CoP/CNT) was prepared by the low-temperature phosphidation of a Co₃O₄/CNT precursor. As a novel non-noble-metal HER catalyst operating in acidic electrolytes, the nanohybrid exhibits an onset overpotential of as low as 40 mV, a Tafel slope of 54 mV dec⁻¹, an exchange current density of 0.13 mA cm⁻², and a Faradaic efficiency of nearly 100 %. This catalyst maintains its catalytic activity for at least 18 hours and only requires overpotentials of 70 and 122 mV to attain current densities of 2 and 10 mA cm⁻², respectively.

Iron is the cheapest and one of the most abundant transition metals. Natural [FeFe]-hydrogenases exhibit remarkably high activity in hydrogen evolution, but they suffer from high oxygen sensitivity and difficulty in scale-up. Herein, an FeP nanowire array was developed on Ti plate (FeP NA/Ti) from its β-FeOOH NA/Ti precursor through a low-temperature phosphidation reaction. When applied as self-supported 3D hydrogen evolution cathode, the FeP NA/Ti electrode shows exceptionally high catalytic activity and good durability, and it only requires overpotentials of 55 and 127 mV to afford current densities of 10 and 100 mA cm², respectively. The excellent electrocatalytic performance is promising for applications as non-noble-metal HER catalyst with a high performance–price ratio in electrochemical water splitting for large-scale hydrogen fuel production.

Iron is the cheapest and one of the most abundant transition metals. Natural [FeFe]-hydrogenases exhibit remarkably high activity in hydrogen evolution, but they suffer from high oxygen sensitivity and difficulty in scale-up. Herein, an FeP nanowire array was developed on Ti plate (FeP NA/Ti) from its β-FeOOH NA/Ti precursor through a low-temperature phosphidation reaction. When applied as self-supported 3D hydrogen evolution cathode, the FeP NA/Ti electrode shows exceptionally high catalytic activity and good durability, and it only requires overpotentials of 55 and 127 mV to afford current densities of 10 and 100 mA cm², respectively. The excellent electrocatalytic performance is promising for applications as non-noble-metal HER catalyst with a high performance–price ratio in electrochemical water splitting for large-scale hydrogen fuel production.

Graphitic C₃N₄ (g-C₃N₄) is used as a low-cost organic oxygen evolution reaction (OER) electrocatalyst. The integration of ultrathin g-C₃N₄ nanosheets with graphene leads to g-C₃N₄/graphene composites with high OER activity and good durability. X-ray photoelectron spectroscopy (XPS) studies suggest that the OER activity results from pyridinic-N-related active sites. This catalyst provides an alternative to OER catalysts based on transition metals.

Searching for inexpensive hydrogen evolution reaction (HER) electrocatalysts with high activity has attracted considerable research interest in the past years. Reported herein is the topotactic fabrication of self-supported Cu₃P nanowire arrays on commercial porous copper foam (Cu₃P NW/CF) from its Cu(OH)₂ NW/CF precursor by a low-temperature phosphidation reaction. Remarkably, as an integrated three-dimensional hydrogen-evolving cathode operating in acidic electrolytes, Cu₃P NW/CF maintains its activity for at least 25 hours and exhibits an onset overpotential of 62 mV, a Tafel slope of 67 mV dec⁻¹, and a Faradaic efficiency close to 100 %. Catalytic current density can approach 10 mA cm⁻² at an overpotential of 143 mV.

Searching for inexpensive hydrogen evolution reaction (HER) electrocatalysts with high activity has attracted considerable research interest in the past years. Reported herein is the topotactic fabrication of self-supported Cu₃P nanowire arrays on commercial porous copper foam (Cu₃P NW/CF) from its Cu(OH)₂ NW/CF precursor by a low-temperature phosphidation reaction. Remarkably, as an integrated three-dimensional hydrogen-evolving cathode operating in acidic electrolytes, Cu₃P NW/CF maintains its activity for at least 25 hours and exhibits an onset overpotential of 62 mV, a Tafel slope of 67 mV dec⁻¹, and a Faradaic efficiency close to 100 %. Catalytic current density can approach 10 mA cm⁻² at an overpotential of 143 mV.