Content Number: 17
Author Name: Syed Jawad Rasheed
Author I’d: SBPWNC – A17
Educational Institution: University of Sialkot, Pakistan
Content Title: Electrocatalytic Water Splitting as a Source of Renewable Energy
1. Introduction
World is facing global problem of huge population, to meet their needs, energy resources were declining as well as environment issues were arising due to emission of greenhouse gases. Which leads into severe health conditions and global warming as well as energy crisis in future [1]. Unfortunately, we are using only 5% of energy from renewable resources, remaining 95% still obtained from fossil fuels and others that causing environment pollution [2]. Fossil fuel burning emit CO2 was main reason behind global warming as it increasing world’s temperature by 1.5℃ in just last 20 years. It’s necessary to shift on eco-friendly energy resources as soon as possible. While hydrogen emerged as suitable, eco-friendly and cost-effective energy source. Hydrogen has energy density of 140 MJ kj-1 while petrol and coal have 44 MJ kg-1 and 24 MJ kg-1 respectively, which means hydrogen had much higher energy as compared to our non-renewable energy resources [3]. Moreover, hydrogen is clean source of energy as compared to hydrocarbons [7].
Use of fossil fuel as energy source causing serious threats for life regarding their health issues as well as energy shortage. Now a days CO2 volume in environment become twice as compared to before industrial revolution [4]. To meet that problem, researchers are trying to get renewable energy sources such as hydrogen and solar power plants. Hydrogen emerges as an efficient energy source due to its availability, inexpensiveness, cleanliness, ability be stored and great energy density [5]. As hydrogen is present in most of compounds but rarely present in its molecular form [3, 6]. Hydrogen fuel produced by industry was obtained from reforming of natural fuel, and this process was energy consuming as well as unwanted greenhouse gases were emitted which results in global warming. It is necessary to obtain hydrogen from material other than natural fuel like fossils and must be obtained by eco-friendly system and inexpensive [3].
2. Water Splitting
Fujishima and Honda were the first one who devised the method of PEC water splitting into O2 and H2 in 1972 using TiO2 anode [5]. Water splitting is very useful technique for generation of hydrogen and oxygen on large scale with cost effectiveness and great efficiency. Water splitting involves hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [7]. As we know hydrogen is efficient and clean source of energy but mass production of hydrogen is difficult. To overcome this problem electrocatalysts were synthesized which were able to reduce its overpotential and catalyze water splitting problem [8].
3. Electrocatalysts for Water Splitting
3.1. Metal Nanoparticles
In the process of water splitting, its highly difficult to control the HER and OER, to control those processes NPs of rare metals were required, that increased production cost [9]. Platinum as an electrocatalyst, used in ORR and HER as cathode and in OER as anode in the presence of water as electrolyte. After platinum, iridium and rhodium were also used as electrocatalyst, but problem was there that they can be corroded and they were highly expensive [10]. In water splitting system, an electrode may have several attributes such as high surface area, lower overpotential, stability, inexpensive, conductivity, selectivity and specificity. Theoretical cell voltage, that required for the electrocatalytic water splitting was 1.23 V [7].
Transition metals such as Mn, Co, Ni and Fe and their metal oxides were used for electrocatalytic splitting of water due to their abundance in earth’s crust, stability at wide range of pH, lower health effects, environment friendly and good electrical properties [1]. Several semiconductors including include TiO2, ZnO, CuO and Fe2O3 were synthesized for hydrogen production from water splitting [11]. Iron was used as electrocatalyst for HER due to its inexpensiveness and high efficiency. Iron oxides were mostly used for this purpose [7]. Iron doping in electrocatalysts increases its interface in numbers that resulted in increased efficiency in HER [8]. Nickel doping enhanced the process of HER and OER by reducing resistance in catalyst [9]. Ni can utilize light energy to catalyze reaction, usually act as photoactive electrocatalyst [12].Unfortunately these oxides had disadvantage of their large band gap and reunion of electron and hole pair on their surface, which resulted in less number of active sites available, and lower efficiency [2].
3.2. Metal-organic Frameworks and Spinel Compounds
Metal-organic frameworks (MOFs) were also used by researchers for electrocatalytic splitting of water. But there was a problem they exhibit lower catalytic efficacy and sustainability, to overcome these problems metal NPs are decorated on MOFs that resulted in enhanced electrocatalytic activity. In the MOF structure metal NPs and oxygen atoms are present in organized way. After calcination of MOF, metal and metal oxides are embed in its surface. V and GO doping of MOF highly enhanced the electrocatalytic activity of catalyst by dropping its overpotential and increasing its corrosion resistance [8].
Spinel compounds having general formula AB2X4 where A is divalent transition metal and B is trivalent transition metal and X representing chalcogens such as oxygen were emerged as efficient electro catalyst for water splitting. They also shoed resistance towards corrosion on metals, that’s why they were used as electrocatalyst for HER widely [1]. Moreover, binary spinel compounds were stable and had great efficiency as electrocatalyst in water splitting [10].
3.3. Graphene based catalysts
Other than metallic semiconductors, some non-metallic oxides were also present such as graphene oxide. Graphene oxide was 2D carbon sheet with no bad effects on health, cheep, had large surface area and very effective material. Large surface area means more absorbance of water molecules on the surface of electrocatalyst, a greater number of water molecules absorbed results in the more production of hydrogen. Graphene oxide was also present in its reduced form known as reduced graphene oxide rGO, that obtained by removal of oxygen from GO and achieving aromaticity by carbon-to-carbon double bond [11]. rGO has increased catalytic efficiency than GO due to oxygen in GO can reverse the reaction that make itself less efficient than rGO [2]. GO and rGO as electrocatalyst were more stable and efficient as they can act as catalyst in wide range of pH [4]. They were used in several fuel cells, their blending with metal NPs enhance their activity, conductivity and durability [13].
Silver nanoparticles dispersed on rGO was synthesized, that were capable of producing four times more anodic current as compared to carbon anode [13]. A bifunctional electrocatalyst synthesized by decorating rGO with Ni NPs and NiO for hydrogen fuel production. Ni/NiO-rGO had enhanced electrocatalyst and conductive properties as compared to Ni/NiO as well as rGO. Moreover, they were highly stable catalysts as they withstand up to 1200 s with maximum potential in HER and OER [6]. Fe/MgO-rGO was synthesized and found efficient in reducing band gap as well as to avoid reunion of electron hole pair. It generates hydrogen gas from water splitting in deionized water as well [2]. ZnO decorated in surface of rGO had given a porous morphology. ZnO/rGO had 12 folds more conductivity of electrons, ease in synthesis and les electron pair recombination phenomenon as compared to ZnO. In which rGO played an important role that it enhanced the interfacial area of electrocatalyst as well as stopped electrons to rejoining hole pairs in ZnO surface resulting in stability of active sites, [14] as well as ZnO NPs helps in the movement of electrons in between sheets of GO [15].
4. Conclusion
Hydrogen fuel can be used as an alternate to other non-renewable energy resources as it can be recycled as well as less harmful to nature. To obtain hydrogen fuel, electrocatalytic splitting of water molecules emerge as efficient, cost effective and eco friendly approach. The byproduct of water splitting is oxygen gas, which is fruitful for nature as oxygen is necessary for respiration. In future water splitting for energy needs can be used globally as an alternate of non-renewable energy resource.
References
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