Pt-Rh Alloy Catalysts for Hydrogen Generation developed by Direct Current/Pulse Method

Highly active Pt-Rh alloy catalyst coatings were developed by direct current (DC) and pulse current (PC) electrodeposition method for ecient hydrogen evolution reaction (HER) catalysts. The coatings were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM) linear sweep voltammetry (LSV) studies. Pt-Rh alloy catalyst coatings showed almost similar behavior as pure platinum metal.The Pt-Rh coating were obtained by PC method showed low over potentialfor HER. Very low slope of 41.2 mV/dec is obtained represents the Volmer-Tafel mechanistic path way for HER process. Chronopotentiometry was conducted and amount of hydrogen collected during these experiments was 24 ml for PC method 75% duty cycle sample.


Introduction
Techniques for exploiting promising clean, renewable energy sources need to be established due to the depletion of traditional fossil fuels and the gradual degradation of the environment. Due to its excellent theoretical mass energy density (120 mj kg − 1 ) and zero greenhouse emissions, hydrogen is a highly e cient and environmental friendly energy medium [1][2][3]. Hydrogen can be generated by water electrolysis e ciently electrochemically using a catalysts with long-term stability. It is necessary to enable large-scale practical applications that are affordable. Platinumis the best catalyst for HER because of high activity with zero over potential [4]. But, platinum is costly ($988 per ounce) and low abundant makes it di cult to use in large scale HER process [5][6][7] Recently, large amount of research work has been carried out to develop the alternatives to platinum catalyst. In this direction, catalyst based on molybdenum, nickel, cobalt, Molybdenum oxides, sul des, selenides compounds have been explored [8][9][10][11][12]. These catalysts show quite good HER activity in acidic medium. However, these catalysts do not compete with Pt in terms of over potential and current densities. In addition, the preparation of these compounds is highly complex and environmentally unfriendly.
In spite of numerous efforts, only 4% of H 2 was produced by water electrolysis due to high cost of the process [13]. These all are due to the high energy losses in electrolysis, lack of state-of-the-art electrodes, the key barriers to the wider use of water electrolysis.
Platinum is the best catalyst for hydrogen generation because it is having zero over potential. The only hindrance of using noble metals and its alloys is low abundance and costly. Using of bulk noble metal is not favorable to use in hydrogen generation. Catalytic activities mainly depend on the surface properties and by using this principle, thin layer of noble metal and its alloy coating on base metal usage as catalyst is good alternative and cost-effective method in hydrogen generation. Pt-Rh alloy coating is more durable and superior than pure platinum coatings [14]. Pt-Rh coating is best option on boiler water reactors because of neutron activation considerations. Alloying of Pt with Rh is another method to achieve lower over potential with higher e ciency in hydrogen generation. In this work Pt-Rh coating was developed on SS 304 with very small amount of 0.5 µg/cm 2 noble metal. The Pt-Rh coatings were performed by both DC and PC methods at different conditions. These developed coatings are characterized and studied by electrochemical methods to know the e ciency in hydrogen generation.  3 purchased from Arora Matthey Limited, Kolkata was used as the main content in the electrolyte for Electrodeposition process.1.5 ml Platinum solution and 0.075 ml of Rhodium solution from the 5Q plating solution was taken to prepare the electrolyte solution. This solution is marked to 50 ml by adding 5 ml ConcH 2 SO 4 and distilled water. A Pt-Rh alloy coating was developed on SS 304 with an exposed surface area of 1.7 cm 2 from a bath solution containing 50 ml. The anode and cathode were held parallel to each other during Electrodeposition. To achieve a smooth nish, stainless steel panels are manually polished, cleaned ultrasonically then electrochemically cleaned before electroplating.
The SS 304 circular form cathode with a diameter of 15 mm and a thickness of 1 mm was used during electrodeposition for coating with a total surface area of 1.7 cm 2 , while the remaining portion of the back side was masked during the deposition process. Platinized titanium was used as an anode.
Electrodeposition was carried out by using a current source N6705B Key sight of direct current (DC) and pulse current (PC) for 18 minutes. After each deposition, the coatings obtained were rinsed carefully with distilled water and then air dried. Amount of coating was estimated by weighing the cathode before and after coating.

Coatings characterization
The morphology analysis of coated samples was conducted by scanning electron microscopy (FESEM) (ZEISS sigma) after electroplating. SEM assessment explored the surface morphology and porosity of DC and PC coating on SS304. Platinum-coated samples were mounted on a sample holder and SEM Electrochemical studies The electrochemical cell was designed for quantitative measurement of hydrogen, where the electrodeposited Pt-Rh electrode was subjected to cathodic polarization, respectively. Electrodeposited alloy coatings obtained under different deposition conditions were used as the test electrode and platinum electrode with high surface area was used as the counter electrode. Saturated calomel electrode (SCE) was used as the reference electrode. All potentials reported in the present study are with reference to SCE and converted to standard hydrogen electrode (SHE) Electrochemical behavior of the coatings, in terms of HER, was evaluated by subjecting it to cyclic voltammetry, linear sweep voltammetry and chronopotentiometry studies in 0.5M H 2 SO 4 medium, using computer controlled using electrochemical workstation (compactostath 10800 from Ivium Technologies, Netherlands). The cell was tted with a graduated gas collector where the liberated hydrogen replaces corresponding amount of solution. This facility allows relating the amount of gas liberated at given time for electrode materials deposited at a given current density. The schematic diagram of the setup is given in Figure 1.

Results And Discussion
In order to get a good desirable coating, bath parameters like pH, temperature, Current density, and composition plays a very important role in the coating. Optimized bath conditions are given in table 1.  Figure 2. The 75% duty cycle coating exhibited the smaller and uniform grains result in the smoother coating surface compared to remaining coatings. In75% PC coating we have observed more number of pores with uniform size. In other coatings, pores size was uneven. The degree of uniformity decreased as duty cycle percentagedecreases the value from 75% to 25% while porewith larger and uneven size has been observed in DC coating samples. As observed in gures, duty cycle has a signi cant effect on the surface morphology. All these coatings were highly adhesion and bright. The 75% duty cycle sample showed more uniformity than other coatings.
Atomic Force Microscopy AFM is an important instrument for studying the surface morphology at nano-to-microscale and has become a modern approach for studying the effect of the different electrodeposition methods like DC and PC at various duty cycle. AFM research includes details on average roughness (S a ) on the surface of coated SS304 sample, which helps to assess the effectiveness of the coated sample by electrodeposition method as shown in the Table 2. The 3D images of Pt-Rh alloy coating for DC, 25%, 50%and 75%duty cycle are shown in Figures (3a) to (3d). Sample obtained by 75% duty cycle by PC method showed lesser roughness than other coatings which indicates Pt-Rh is deposited quite uniformly. The decrease in roughness value (Table 2. and Figure 3 (a))is due to the reduction of the grain size on metal (SS 304) surface. It can be concluded that higher the crystal grain size, the greater the roughness of the coatings.
The instability of the duty cycle has a signi cant effect on the surface morphology of their crystalline measurements. In our studies, good coating was achieved in 75% duty cycle and it gives smaller grain size with lower roughness. In DC coating, su cient time was not available to achieve the small grain size with uniform coating.

Energy Dispersive X-Ray Spectroscopy
The EDX (using 20 keV beam energy Model: Oxford Instrument 250) analysis was performed to measure the amount of metal ions on the substrate in the coating and offers valuable information on the percentage of speci c metal ions in the coating. Details are given in Table3 and Figure 4. In the course of our studies EDX were observed for both DC and PC techniques. It also contains trace amount of iron, chromium, nickel and manganese. It shows more amount of iron along with Platinum and Rhodium it may due to porosity in the coating as shown in Figure 2.

Amount of Platinum -Rhodium Alloy in the coating
Pt-Rh coating was done by PC and DC method. Weight of the cathode coupons were taken before and after the experiments. Amount of alloy in the coating was given in the below Table 4. The electrocatalytic activity of Pt-Rh was rst evaluated for H 2 evolution in anacidic medium asshown in Figure 5.All the samples show good hydrogen evolution reaction. Pt -Rh coated by PC method at 75% duty cycle shows very low over potential for hydrogen evolution. Pt-Rh catalyst deposited by 75% duty cycle sample shows over potential similar to that of pure Pt for HER. These results con rm the better performance of the sample (75% duty cycle). 25% and 50% sample shows more over potential and also less current observed during hydrogen evolution reaction. It was anticipated that Pt-Rh is very active for HER. Indeed, the Pt-Rh catalyst exhibited a catalytic onset at nearly zero over potential and catalytic current rapidly rose for the sample obtained at 75% duty cycle. Vibrant H 2 bubble growth and release from the surface were observed upon further cathodic sweeping. The catalytic activity of Pt-Rh electrocatalysts were evaluated by Tafel plots as shown in the Figure 6.
The Tafel slopes provides insight mechanism of HER process. The kinetics parameter of electrocatalytic HER is expressed in terms of either Volmer-Heyrovsky or Volmer-Tafel mechanistic pathways. The Tafel slopes of 41.2, 63.6 and 83.1 mV/dec for Pt-Rh catalysts coated by PC method using 75%, 50% and 25% duty cycles, respectively from Table 4. Very low Tafel slope of 41.2 mV/dec of 75% duty cycle deposited Pt-Rh catalysts implies that better electrocatalytic performance which is also higher than the DC coated sample (69.9 mV/dec). The low Tafel slope of 41.2 mV/dec implies that the Volmer-Tafel reaction mechanism and the rate determining step is the desorption of hydrogen atoms [20].

Chronopotentiometry
The simplest way to estimate the electrocatalytic activity of the electrodes to monitor the electrode potential at constant current density applied over a su cient period of time [19].The chronopotentiometry study for the evolution of hydrogen on Pt-Rh coatings obtained by different methods, were made at a constant current of -0.3 mA cm -2 for a duration of 21,600 seconds. The nature of chrono-potentiograms for the alloy coatings obtained by different methods is given in Figure 7. The electrocatalytic performance of these coatings were evaluated by measuring the amount of H 2 liberated in rst 150 seconds as shown in Figure 8. The electrodeposited catalysts show constant and stabilized potential for HER. This phenomenon is ascribed to the formation of hydrogen bubble on the surface of the electrode.
Coating with uniform grain size and porosity gives more hydrogen evolution reaction. In electrolysis, formed hydrogen gas detached from the cathode only after it became a bigger in size. During this time contact between cathode and electrolyte was seized and reduces the hydrogen evolution. If the sample has more uniform pores, these gas bubbles avoid the ohomic resistance and enhance the catalytic activity of the coating [21].It may be seen that the coating obtained by75% duty cycle gives more volume of hydrogen evolution compare to other samples. This sample has more porosity with uniform coating this enhance the hydrogen generation activity of the catalyst.

Conclusions
Pt-Rh Alloy Coating was successfully done by the PC and DC method. Bath parameters were optimized to get the desired coating. SEM infers the smaller grain size of the coating obtained by PC method at 75% duty cycle. AFM analysis showcases the surface roughness and this is con rmed the SEM results. Linear sweep voltammetry gives the evidence of zero over potential of developed alloy coating and also it gave more current than pure platinum coating. Small Tafel slopes con rms the e cient catalysts and follows the Volmer-Tafel reaction mechanism for the HER. Chronopotentiometry values again con rm overall results by giving more volume of hydrogen collection during electrolysis. This work has the potential to commercialize for industrial applications.