A Comparison of Different Polymeric Materials for Photocatalytic and Electrochemical Water Splitting: A Review

Today 98% hydrogen production is generated from fossil energy sources responsible for an emission of O₂ per year and less than 1% of the world’s hydrogen (green hydrogen) demand is produced from renewable sources. One of major technology for the production of hydrogen (green hydrogen) is Polymer Electrolyte Water Electrolysis (PEWE) It gives us a way for development of PEWE materials and its components for increasing efficiency and lower cost of hydrogen (green hydrogen production) [1,2]. Water electrolyzes or water splitters can be divided into three categories, Alkaline Electrolysis (AE), Solid Oxide Electrolysis (SOE) and Polymer Electrolyte Membrane (PEM) electrolysis. The overall efficiency is also depending upon electrochemical cell’s temperature and pressure [3,4].

Coordination polymers have attracted considerable attention for use in noble-metal-free electrochemical water splitting, for example, by the Hydrogen or Oxygen Evolution Reactions (HER/OER). Electrochemical water splitting is satisfactory method to produce pure hydrogen. To reduce the over potential for catholic Hydrogen Evolution Reaction (HER) is very difficult in hydrogen production. Catalytic hydrolysis of chemical hydrides includes formic acid, sodium borohydride, and ammonia borane are alternative approaches for hydrogen production [5- 15]. Hydrogen production from water electrolysis significantly depends upon the development of catalysts with high efficiencies. Microporous polymer networks have gained widespread attention due to the unique properties of high surface areas, abundant functional groups and chemical and thermal stability. With coordination of ruthenium catalyst in inner space of the HCP-PPh₃ synthesized through cross-linking of PPh₃ and benzene. The assynthesized material would be used for hydrogen production from electrochemical water splitting [16].

Inserting units of free-base porphyrins into PPTA networks can result in two- dimensional (2D) porphyrin-based polymers. Porphyrin moieties can enhance interactions and add charge transport properties used in electrocatalytic applications. The use of metal-free porphyrin-based polymer as bifunctional electrocatalysts directly for both HER and OER. The synthesis of a metal-free porphyrin based crystalline 2D porphyrin-based organic polymer called Porphvlar, get from the condensation of terephthaloyl chloride and tetrakis (4-aminophenyl porphyrin, namely H2TAPP) is an effective bifunctional electrocatalysts for OER and HER in neutral solutions [17-24]. The electrochemical reaction of this material is expose through oxidation and reduction conditions to study its catalytic activity, charge transfer ability and stability. Scanning Electron Microscopy (SEM) images were obtained to evaluate the morphology of the synthesized Porphvlar polymer in different size. Figure 1 & 2 shows that the Porphvlar shows a flake-like morphology. The flakes are stacked in layers of nanometers in thickness. This is different to the rod-like morphology of PPTA threads [25]; (Figure 3).

Figure 1:

Figure 2:Synthesizing HCP-PPh₃.

Figure 3:Preparation of phosphorus containing porous polymer.

The production of H₂ on a large scale Hydrogen Evolution Reaction (HER) and the Oxygen Evolution Reaction (OER) are the two half-cell reactions of electrochemical water splitting. We required highly efficient electrochemical catalysts to proceeds the HER at low overpotentials. We present an efficient approach for preparing porous carbons embedded with transition-metal phosphides (Met P. PCs, where the transition metal center is Met=Mo, Fe) at a large scale through the direct pyrolysis of cationic, phosphorous-based, porous polymer precursors loaded with transition-metal-containing anions under a hydrogen atmosphere. The transition-metal-containing anions and phosphorous-based polymer framework serve as the metal and phosphorous sources for the formation of the Met P. PCs under pyrolytic and reducing conditions. The Met P. PCs process highly specific exhibited electrochemical catalytic activities for the HER in 0.5M H₂SO₄. MoP. PC (0.24mg cm-2 on a glass carbon electrode) exhibited an overpotential of 51mV at 10mA cm^-2 and a Tafel slope of 45mV dec- 1, comparable to those of the commercially available Pt/C catalyst (24mV at 10mA cm^-2, 30mV dec^-1) [25-30]; (Figure 4).

Figure 4: Tafel slopes of MoP.PC.

There are many other organic polymeric materials which are used in electrochemical materials which a react different in different conditions some are used as catalysts included as Ppy\ZnWO₄ nanocomposites [18], triphenylphosphine-hypercrosslinked polymer, tandem polymer cells [14], polypyrrole carbon nanocomposites, Porous organic polymer with carbon nanocomposites [20], PU.PANI.FeS₂ nanocomposites with different electrodes [21] and its mechanism of OER and HER mechanism which are given below Figure 5.

Figure 5:Hydrogen Evolution Reaction (HER) Mechanism.

Figure 6:PEM Electrolyzer Working.

Another way of water splitting through electrochemical way is electrolyzer membrane which is made up of polymeric material as electrode which help them in H2O splitting. The water splitting with electric current the electrolyzers efficiently produce hydrogen [31-40]. The hydrogen production efficiency ηp of Hydrogen H₂ will expresses the ratio of the chemical energy is obtained and how much chemical energy must be supplied to the electrolysis cell. π conjugated polymers [2], ZnO based photoelectrode prepared on the micropatterned Polyurethane Acrylate (PUA) [3], polyaniline polymer [9] gives different RHE and voltages and gives different Tafel slopes value and this material is best for electrolyzers. The mechanism of the Polymer Electrolyte Membrane (PEM) is given as Figure 6.

Different polymeric materials are used for water splitting in different medium for its stability and its efficiency. Following is the comparison table and to compare polymeric material through Tafel slopes (mVdec-1) and Over potential (mV), where on the basis of Tafel slopes as the lowest peak the better performance is achieved. Polyurethane (PU), polyaniline (PANI) with iron sulphide (FeS2) is considered to be the best efficiency for HER in electrochemical water splitting [41-51]. However according to over potential 0. 5MH2SO4.MoP. Polymer compounds gives lowest peaks and Tafel slope at 30mVdec-1 is best compound for water splitting. These experiments uses XRD, SEM,TEM and The Temperature of Tafel slope and efficient polymeric material which one is best given through a graph in which it shows in Figure 7 (Table 1).

Figure 7:

Table 1:

Different Polymer electrolyte water electrolyzer member ionic system is also used at different 9emperature and 50μm thin membranes. It was reveals t that research and development do not need to focus on the development of durable materials for operating at 90 ˚C as an optimal stack temperature can be lower within the relevant current density range, which is limited by efficiency requirements. The polybendezimzole, polythiophenes, polyurethrane acrylte, trimethyl benzyl ammonium group, temperature-based polymers, conjugated polymers, Polyaniline Polymer shows there different RHE vsV value with different temperature at different current densities the water splitting by electrochemical process. Polyaniline polymer and polyurethrane polymers based PEME shows tafel slope at 47 and 21r HER (36Mv RHE at 10Ma cm^-2, 21mVdec^-1), while PANI-Ru is suitable for OER (1.47V RHE at 10mAcm^-2,47mVdec^-1) (Figure 8).

Figure 8:V vs RHE value of polyaniline, polyurethrane.

Over potential low peaks and Tafel slope of 30 and 15mV dec- 1 at 154 and 51mV gives the efficient value of water splitting by electrochemical process use polymeric electrocatalysts and KOH and H2SO4 electrolytes. Another way is Polymeric electrolyzer membrane in which different polymeric material is used best one is polyuretherane material which gives Tafel slope at 47mVdec-1 which is lowest in all other materials. Further improvements have been occurring in water splitting.

The authors gratefully acknowledge the support received for this research work from the Major Project of Fundamental and Application Research of the Department of Chemistry, School of Science, University of Management and Technology, Lahore, Pakistan and Office of Research Innovation and Commercialization UMT Lahore, Pakistan.

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