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Electrocatalysts for hydrogen production through water electrolysis

Copper Foam
Copper Foam

The utilization of global energy has significantly increased due to population growth and industrial intervention. Energy is essential for the global economy and social modernization, with non-renewable sources fulfilling approximately 65% of the worldwide energy requirement. However, these sources emit harmful greenhouse gases, leading to global warming, environmental pollution, and damage to components and systems, which have severe impacts on society and industry. The decreasing supply of fossil fuels and the increase in CO2 emissions have resulted in a demand for renewable energy production, with a target of 10 TW production by 2050. Renewable energy sources include solar, wind, hydroelectricity, nuclear, biomass, and ocean thermal. Electrochemical methods are recognized as the most efficient process for the storage and conversion of renewable energy. Hydrogen, as a renewable energy source, is significant as it only emits water as a by-product without any carbon emissions. It can be used as an alternative to carbon-based fuels due to its high energy efficiency, environmental friendliness, and lack of contamination. Hydrogen has a high energy density of 140 MJ/kg, which is twice that of regular solid fuels. Currently, global hydrogen production is around 500 billion cubic meters (b.m3) per year. It is primarily used in various industrial applications, particularly in the oil and gas sector, fertilizer industry, fuel cell technology, petrochemical industry, and petroleum and metal refining industry.

Currently, hydrogen gas is produced from a variety of renewable and nonrenewable sources, including fossil fuels, biomass, water electrolysis, wind, solar, hydro, and nuclear energy. There is a growing focus on developing eco-friendly energy policies that utilize renewable resources for hydrogen production. Electrochemical water splitting has emerged as a promising method for producing hydrogen energy, converting electrical energy from solar, marine, and wind sources into chemical energy. Among the various hydrogen production techniques, electrolysis offers the potential for high-density and eco-friendly hydrogen production. However, the efficiency of water electrolysis is limited by high power consumption and low hydrogen release rates. Currently, only 4% of hydrogen gas is produced globally through electrochemical water electrolysis, primarily due to the high cost and scarcity of noble electrocatalysts such as platinum and ruthenium. These factors hinder their widespread commercial application. Therefore, there is a need to explore cost-effective and highly efficient non-precious metal electrocatalysts for hydrogen evolution and oxygen evolution reactions. Metal organic frameworks (MOFs) composites and transition-metal based compounds, including metal oxides, hydroxides, sulfides, phosphides, nitrides, and selenides, have shown promising electrocatalytic properties in HER and OER processes.

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The search for an economical, affordable, and highly efficient non-precious electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is a compelling task. In order to enhance efficiency and reduce hydrogen power consumption, it is crucial to dedicate significant efforts towards the development of inexpensive electrocatalysts that can improve hydrogen production efficiency and decrease energy consumption. This paper provides a comprehensive overview of various hydrogen production methods, highlighting recent advancements in economic and efficiency-based electrocatalysts for OER and HER. The analysis of recently developed electrocatalysts, including their nanoparticle structures and electrochemical properties, is of great importance in improving energy efficiency, catalytic activity, and durability to meet future energy demands. However, it should be noted that the morphological structure of the electrocatalyst significantly affects the electrocatalytic properties of HER and OER catalysts. This review extensively explores different types of non-precious metal-based electrocatalysts for HER and OER in hydrogen production. The fundamentals, catalytic reaction mechanism, design, characterization, and performance of these electrocatalysts are emphasized. Finally, a comprehensive discussion on the major challenges, possibilities, and future research directions, along with critical recommendations, is provided to address the necessary steps before the full commercialization of water electrolysis systems.

Electrochemical water splitting is considered to be a highly environmentally friendly method for hydrogen production, as it does not result in any carbon emissions. This process involves the occurrence of half-cell reactions, namely the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode.

During electrolysis, a direct current power supply is employed, which is sourced from sustainable energy sources such as wind, solar, and biomass energy. However, the water electrolysis method has only achieved a hydrogen yield of 4% thus far, primarily due to economic challenges and the high cost of the electrocatalysts used in the overall process. Furthermore, it is anticipated that these expenses will increase as a result of greater reliance on renewable resources. The decomposition reaction of water necessitates a specific energy input, which is influenced by factors such as temperature and pressure.

Alkaline water electrolysis, invented by Troostwijk and Diemann in 1789, is the most advanced and conventional technology for producing hydrogen energy from water. This method has been globally successful in generating megawatts of high-quality hydrogen. The operating temperature range for these systems typically falls between 40 and 90 degrees Celsius, with an efficiency range of 70 to 80 percent. The commonly used electrolytic solutions for alkaline water electrolysis are based on KOH and NaOH, with concentrations varying from 20 to 30 percent. An asbestos membrane is employed to separate the anode loop from the cathode, preventing the mixing of produced gases on the respective electrodes. Nickel (Ni) material is used as both the anodic and cathodic electrodes to prevent this mixing. In this process, water electrolysis occurs at the cathode, causing hydroxyl ions (OH-) to move towards the anode. At the cathode, water molecules (H2O) are reduced to produce one molecule of hydrogen (H2) and two molecules of hydroxyl ions (OH-). At the anode, four molecules of hydroxyl ions (OH-) are oxidized to produce one molecule of oxygen (O2) and two molecules of water (H2O) through the transfer of four electrons. The primary reaction and process for producing hydrogen from alkaline water electrolysis involve the generation of hydrogen at the cathode and oxygen at the anode. These leading product gases are separated by a diaphragm, which allows for the penetration of both OH- and water from this medium. The anode and cathodes are immersed in an electrolyte, typically a liquid NaOH or KOH. Commonly used electrode materials include cobalt, iron, and nickel, with nickel being the most widely used due to its superior electrocatalytic activity and cost-effectiveness.

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Noble metals, including platinum (Pt), palladium (Pd), iridium (Ir), ruthenium (Ru), and rhodium (Rh), are frequently employed in this procedure owing to their ability to offer abundant active sites for hydrogen adsorption. Nevertheless, the utilization of these catalysts in commercial applications is significantly impeded by their exorbitant cost. Consequently, the application of these catalysts is inadequate and largely hindered due to their prohibitively high expenses.

The transitional metals that show the most potential as catalysts for the hydrogen evolution reaction (HER) include iron (Fe), nickel (Ni), copper (Cu), molybdenum (Mo), and tungsten (W). In a study conducted by Miles and Thomas, a voltametric method was employed to investigate the electrocatalytic properties of non-precious metals, with the order of effectiveness being Ni > Mo > Co > Fe > Cu. However, it was observed that nickel exhibited superior electrocatalytic performance for HER in alkaline solutions. Additionally, Co-based electrocatalysts have also been examined for their potential in HER electrolysis.

In the current study of hydrogen evolution reaction (HER), it has been observed that the binding free energy of hydrogen follows a volcano-type connection, which is similar to that of advanced platinum catalysts. This is indicated by the fact that the Gibbs free energy tends to zero. Therefore, it is recommended to use another metal, such as nickel, which has a smaller Gibbs free energy than other non-precious metals, to form nickel alloys such as Ni-Mo, Ni-Co, Ni-Cu, and ternary metals Ni-Mo-Zn. This process is favorable for producing HER catalysts with higher performance. When two transition metals are blended to enhance electrical conductivity, Ni-Co alloys are found to be effective catalysts due to their better inherent electrocatalytic activities and anti-corrosion performance in alkaline solutions.

Numerouschallenges are continued in the hydrogen production for inexpensive non-precious catalysts are following:

(1)Insufficient knowledge of non-precious catalytic mechanism in alkaline solution;

(2)Major problem of corrosion for both OER and HER carbonintensive catalysts in alkaline and acidic solution;

(3)Instability of ceramic catalysts for OER and HER in SOE at high temperature;

(4)PEME electrolysis contained non-active characteristics of transitional metal oxide;

(5)A reduction in potential due to non-precious metals overload at the anode;

(6)The synthesis procedure is complicated for bifunctional catalyst;

(7)Synthesis process can affect the catalysis structure and its performance;

(8)Transition metals low electrical conductivity;

(9)The challenge is to prepare non-precious catalysts that can exceed the precious metal properties;

(10)Determine and resolve the strengths, shortcomings, and hazard of low carbon hydrogen systems.

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There are numerous innovative research opportunities for investigators and scientists to develop modern highly effective catalysts for OER and HER process. The future research recommendations are as follows:

(1)Novel and facile synthetic methods to produce nonprecious metal-based catalysts for OER and HER with high electrocatalytic activity;

(2)Optimize life expectancy, reduce deterioration, and design extrapolation process;

(3)Establish the commercial practicability of hydrogen systems, complete chain of source-to-end;

(4)Plan, design, establish, and experimentally testing of largescale hydrogen system of storing;

(5)Comprehensive understanding and designing of OER and HER electrocatalytic mechanism;

(6)A further investigation in non-precious catalysts for OER and HER in the PEME process;

(7)The research advancement in the non-precious catalyst for OER and HER in the SOE process;

(8)0The future development and application of MOF-based composite catalysts for OER and HER in water electrolysis process;

(9)The catalysts with industrially promising current density (generally > 0.5 A/cm2) are still required. In addition, the development of membrane electrode assembly (MEA) is highly urgent.

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Lu

Our materials research team from Tsinghua University postdoctoral researcher lin and Harbin Institute of Technology researcher Mu, Nanjing University of Technology researcher Wei, they share their expertise in foam metal materials article.

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