Ag-Cd-B-P ternary alloy with efficient electrocatalytic activity towards hydrogen evolution reaction (HER)

Recently, a stable and effective Pt-free electrocatalyst is hugely required for hydrogen generation through water splitting. In this article, we present an easy synthesis of very small particle sized electrocatalysts Ag-Cd-B-P and Cd-Ag-B-P with good catalytic activity and stability for electrochemical hydrogen evolution reaction in acidic medium. Ag-Cd-B-P catalyst exihibits better performance than Cd-Ag-B-P with lower onset potential of - 0.25 V, higher electrochemical surface area of 0.50 cm2, excellent current density of 1.18 A/cm2 at – 1.29 V, low tafel slop of 118 mV/dec and small overpotential of 50 mV to achieve 10 mA/cm2 current density. The reported Ag-Cd-B-P electrocatalyst shows good storage stability of 2 months in normal condition with only ∼2 % change in onset potential and current density.


Introduction
The continuous rising of environmental pollution, economic development and energy shortage, stimulates a search for clean and sustainable energy. As a green and sustainable energy source, hydrogen has become one of the most assuring alternatives of fossil fuels [1]. Electrocatalytic water splitting is a green and sustainable process to generate hydrogen without harmful by-products [2]. Hydrogen evolution reaction (HER) is the cathodic part of water splitting reaction and requires a highly effective electrocatalyst to obtain lower overpotential [3]. The noble metals including Pt-group metals show ideal electrocatalytic activity towards HER, but the high price and insufficiency make them limit their wide industrial application [4]. Transition metal based materials including oxides, borides, phosphides, nitrides, sulphides, selenides, tellurides, carbides [3,[5][6][7] and also metal free materials [8] give satisfactory results as electrocatalyst to replace the Pt based materials. Among them transition metal borides and phosphides are widely studied as efficient electrocatalyst. Patel et al. first synthesized Co-P-B powder catalyst with B/P molar ratio 2.5 and applied in hydrogen production [9]. In comparison with Co-P and Co-B, enhanced catalytic efficiency was shown by Co-P-B due to presence of synergic effect of P and B atoms and higher active surface area. Ternary boride phosphide was also studied as catalyst for hydrogen generation which also more efficient than only boride or phosphide [10,11]. Recently, Ag and Cd based nanomaterials which are comparatively inexpensive electrocatalysts were widely studied and an excellent activity was shown in the field of HER [12,13].
To the best of our information, this nanomaterial is the first ternary boride phosphide which is used as electrocatalyst for hydrogen evolution reaction and gives a good performance comparing with other elctrocatalysts available. In this work, we have synthesized very small particle sized Ag-Cd-B-P and Cd-Ag-B-P nanomaterials by an easy and facile chemical reduction process at room temperature. The elctrocatalytic activity of the nanomaterials towards HER was studied in acidic condition. Prepared Ag-Cd-B-P powder catalyst shows lower onset potential of -0.25 V and small overpotential of 50 mV to achieve 10 mA/cm 2 current density. A very impressive storage stability of this elctrocatalyst was observed with only ~2% change in current density and onset potential after two months of storage at normal condition.

Chemical reagents and apparatus
All the chemical reagents were of analytical grade and used without any additional purification. Silver nitrate (AgNO3) and cadmium chloride (CdCl2.2.5H2O) were purchased from SRL and Laboratory Rasayana Chemicals. Sodium borohydride (NaBH4) and di-sodium hydrogen orthophosphate (Na2HPO4) were purchased from TCI chemicals (India) and CDH chemicals. Solvents including ethanol and dimethylsulfoxide (DMSO) were acquired from TCI chemicals (India) and Merck (India). The whole analysis was carried out in room temperature and stock solutions were prepared using distilled water. The powder X-ray diffraction (XRD) spectra of the nanomaterials were conducted using a Rigaku Smartlab X-Ray diffractometer. Zeiss-SUPRA 55 was used for field emission scanning electron microscopy (FE-SEM) study. The electrochemical analysis was carried out using CH instrument (USA, model number 440D).

Synthesis of Ag-Cd-B-P and Cd-Ag-B-P
Ag-Cd-B-P electrocatalyst was synthesized according to the previously reported procedure with some modifications [9]. Briefly, 1 mmol AgNO3 (0.169 g) and 0.1 mmol CdCl2 (0.0228 g) were taken in a round bottom flask with 20 ml deionised water. The mixture solution was stirred to get a homogeneous solution. After 30 minutes, 0.4 mmol Na2HPO4 (0.056 g) and 1 mmol NaBH4 (0.037 g) were added to the previous solution and stirred for 10 h. The solution was cooled down to room temperature naturally. The black precipitation of final product was centrifuged at 10,000 rpm by washing several times with water and ethanol. The obtained product was dried in vacuum at 65⁰C for 12 h. The synthesized product was further calcinated at 300⁰C for 3 h. Cd-Ag-B-P nanoparticles were prepared by similar method as described above only by changing the amount of silver and cadmium salt. In this case, 1 mmol CdCl2 (0.228 g) and 0.1 mmol AgNO3 (0.0169 g) were taken with previously described materials.

Electrochemical study
All the electrochemical studies were carried out using a nanomaterial-modified pencil graphite electrode (PGE). The pencil graphite leads with diameter 0.5 mm were purchased from Hi Par Camlin Ltd. (India) and micropipette tips were acquired from Tarsons Products Pvt. Ltd. The PGE which was the working electrode was fashioned in our laboratory using the pencil graphite lead. Pre-treatment of pencil graphite lead was carried out before the electrode fabrication using 6.0 M nitric acid and subsequently 2 times washing with water. After that, the surface of the lead was cleaned with cotton and dried at room temperature. The dried lead was then accommodated in a micropipette tip keeping 4.0 mm of pencil lead outside through the narrow end of micropipette tip to modify in future. A metallic wire was tied with the lead at the wider end of the micropipette tip to generate electrical contact. To modify the pencil lead, 2.0 mg nanomaterial was dispersed in 0.5 ml DMSO by ultrasonication for 1 h to get a homogeneous ink. The prepared ink (6 L) was drop-casted onto the clean surface of PGE and dried. This method was replicated three times to get a good coating. Typically, in a three electrode electrochemical cell, a Pt wire and Ag/AgCl electrode were used as IOP Conf. Series: Materials Science and Engineering 577 (2019) 012122 IOP Publishing doi:10.1088/1757-899X/577/1/012122 3 counter and reference electrode, respectively and the prepared modified PGE was used as working electrode for the electrochemical analysis. To analyse the electrochemical activities of four nanomaterials towards HER, 1.0 M H2SO4 (pH = 0) solution was taken as supporting electrolyte with scan rate of 50 mV/s throughout the measurement. Prior to the electrochemical analysis nitrogen gas was purged into the electrolyte solution for 30 min to remove the dissolved oxygen completely and sealed the electrochemical cell. After performing 10 cycles, stable polarisation curves were recorded. All the potentials were calculated keeping reversible hydrogen electrode (RHE) as reference using the equation: is the standard Ag/AgCl electrode potential at 25 ⁰C and EAg/AgCl is the potential measured during the experiment against Ag/AgCl electrode. The geometrical surface area of PGE was remained constant at 0.089 cm 2 . For HER, 1.0 M (pH = 0) H2SO4 was used, therefore, ERHE = EAg/AgCl + 0.210. The equation used to calculate overpotential () which was ERHE -1.23. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were used to investigate the electrochemical activity of prepared electrocatalysts.

Tafel plots
The electrocatalytic activity of the nanomaterials towards HER was calculated using Tafel plots applying the equation [14]: This equation correlates with overpotential (and current density (j) with the slope 2.3 . Tafel slope gives the idea about HER reaction pathway. There are three types of reaction pathway including Tafel (slope 30 mV/dec), Heyrovsky (slope 40 mV/dec) and Volmer (slope 120 mV/dec) reaction. Lower tafel slop with rising current density, indicates a slower increment of overpotential which means a better reaction pathway for HER.

Characterization of nanomaterials
The XRD spectra of the prepared electrocatalysts are shown in Fig. 1  Morphological characterization of prepared nanomaterials was carried out by FE-SEM analysis. We can see in Fig. 2 both the powder electrocatalysts show particle like morphology and particles are arranged homogeneously. Ag-Cd-B-P contains (Fig. 2 (A and B)) smaller particles having size range between 20 to 30 nm. In Fig. 2 (C and D) Cd-Ag-B-P exhibits homogeneously distributed particles with size range 30 to 40 nm. The small particle size of the fabricated nanomaterials can be due to the use of strong reducing agent NaBH4 during the synthesis process. A fast reduction of metal ions is caused because of NaBH4 and the particles are not allowed to enlarge more than little nanometers. This type of morphological structure is advantageous in increasing the electroactive surface area.

Electrocatalytic activity and electrochemical surface area
Electrocatalytic activity and electrochemical surface area of the synthesized nanomaterials were studied with CV, taking potential range of -0.8 V to 1.2 V. The analysis was carried out at 100 mV/s scan rate using 0.2 M potassium ferrocyanide (K4[Fe(CN)6]) and 1.0 M KCl as electroactive probe molecule and supporting electrolyte respectively. The electrocatalytic activity of two nanomaterials and blank PGE is shown in the Fig. 3. Higher current was obtained from Ag-Cd-B-P (742 A) modified PGE in comparison with Cd-Ag-B-P (546 A) modified PGE. In Fig. 3, the extra peaks appeared in the CV plots of nanomaterial modified PGE, except the potassium ferrocyanide peaks are due to the presence of silver. Electrochemical surface area was calculated following Randles-Sevcik equation [15]: (3) Where Ip (A), n, D, v, C 0 , A are peak current, number of electron transfer during the reaction, diffusion coefficient, scan rate, concentration of potassium ferrocyanide and electrochemical surface area respectively. Normally, in this redox reaction Fe 2+ converts to Fe 3+ that means one electron transfer (n = 1) occurs. Diffusion coefficient (D) has the definite value of 0.76×10 5 cm 2 /s. The calculated electrochemical surface area, peak current and roughness factor are given in Table 1. We can see that modification of PGE with Ag-Cd-B-P has higher elctroactive surface area (0.50 cm 2 ) than that of Cd-Ag-B-P (0.37 cm 2 ) due to increment of charge transfer. The roughness factor is higher in the case of Ag-Cd-B-P modified PGE (5.62) than Cd-Ag-B-P modified PGE (4.14).

Electrochemical behaviour towards HER
The optimization processes of some important parameters including scan rate, supporting electrolyte concentration and mass loading amount of the catalysts were performed before studying HER activity of nanomaterials. We know that smaller onset potential and larger current density indicates a better electrocatalytic activity. The optimization study is shown in Fig. 4 with LSV polarization curve. At first scan rate was optimized (Fig. 4A) by recording LSV with different scan rates (100 mV/s, 50 mV/s, 10 mV/s, 5 mV/s and 2 mV/s). Electrolytes were optimized (Fig. 4B) by using H2SO4 with different concentrations (0.1 N, 0.5 N, 1 N, 2 N and 3 N). Mass of the electrocatalyst loaded onto the bare PGE was optimized (Fig. 4C) by coating different amount of nanomaterial (1 mg, 2 mg, 5 mg and 8 mg) on PGE. It was noticed that lower onset potential as well as higher current density is acquired at 50 mV/s scan rate with 1.0 M supporting electrolyte (H2SO4) concentration. For the case of loading amount of nanomaterials it was seen that with 2 mg catalyst coating the better result was obtained. After optimisation, onset potentials (highest cathodic potential at which the product of a reaction is obtained) of two nanomaterials were compared with optimized parameters using CV and LSV (shown in Fig. 5). In the LSV plot (Fig. 5 A) performed within the potential range 0 to -1.3 V, it was observed that Ag-Cd-B-P showed best electrochemical activity with lowest onset potential (-0.25 V) as well as high current density (1.18 A/cm 2 at -1.29 V). The other nanomaterial Cd-Ag-B-P showed onset potential of -0.34 V and current density 0.76 A/cm 2 at -1.29 V. Lower the overpotential value better the electrocatalytic activity of the nanomaterials. In the case of Ag-Cd-B-P, the calculated overpotential value was 50 mV which was much smaller than that of Cd-Ag-B-P (108 mV). From the Fig. 5 (B) we can observe that CV curve was studied under same reaction condition with LSV and Ag-Cd-B-P modified electrode shows higher current density than Cd-Ag-B-P electrode. Tafel slops were obtained by plotting the linear portion of LSV curve with optimized parameters in non-stirred solution. For Ag-Cd-B-P the calculated tafel slop was 118 mV/dec and that for Cd-Ag-B-P was 192 mV/dec shown in Fig. 6. So it can be concluded that the two nanomaterial modified electrodes adopt Volmer reaction pathway and Ag-Cd-B-P modified electrode shows much lower tafel slop than Cd-Ag-B-P modified electrode.

Stability Study
Stability is very important for an efficient electrocatalyst used in water splitting. To prepare a cost effective and good catalyst for HER, stability also plays a vital role. At first the cyclic stability of Ag-Cd-B-P was studied with CV and LSV within the potential range of 0 -1.3 V. It was observed in Fig.  7(A, B) that after 200 and 500 cycles no such significant change was occurred in onset potential and current density. CV and LSV runs were also compared before and after the whole electrochemical performance within the same potential range shown in Fig. 7(C, D). The recorded current densities in the LSV curve (Fig. 7 C) were 1.18 A/cm 2 and 1.13 A/cm 2 at -1.29 V for initial and after the whole electrochemical study respectively. There was almost no change in onset potential before and after the electrochemical performance. The storage stability of Ag-Cd-B-P was also examined with LSV analysis after 1 month and 2 months of storage at normal conditions in room temperature. In Fig. 9 we can observe that a slight change (~2 %) in onset potential and current density was occurred after 2 months of storage period.

Conclusion
We have synthesized Ag-Cd-B-P and Cd-Ag-B-P by a facile chemical reduction process by changing the amount of silver and cadmium. Different characterization methods confirm the successful synthesis of the nanomaterials. Between the two, Ag-Cd-B-P showed better electrocatalytic property towards HER than Cd-Ag-B-P with small onset potential and higher current density. Tafel plot also