Incorporation of Bi 2 O 3 Residuals with Metallic Bi as High Performance Electrocatalyst toward Hydrogen Evolution Reaction

: Nanostructured Bismuth-based materials are promising electrodes for highly efﬁcient electrochemical reduction processes such as hydrogen evolution reaction (HER). In this work, a novel sort of nanocomposite made up of partially reduced Bi 2 O 3 into metallic Bi anchored on a 3D network of Ni-foam as a high-performance catalyst for electrochemical hydrogen reduction. The application of the hybrid material for HER is shown. The high catalytic activity of the fabricated electrocatalyst arises from the co-operative effect of Bi/Bi 2 O 3 and Ni-foam which provides a highly effective surface area combined with the highly porous structure of Ni-foam for efﬁcient charge and mass transport. The advantages of the electrode for the electrochemical reduction processes such as high current density, low overpotential, and high stability of the electrode are revealed. An overall comparison of our as-prepared electrocatalyst with recently reported works on related work is done.

In recent decades, Bi-based catalysts [88] have been investigated numerous times because of their low cost, environmental friendliness, high electrochemical active sites and high electrocatalytic performance toward water splitting. In this regard, Bi-based catalysts can be synthesized through a facile electrodeposition route demonstrated influential catalytic performance for HER in neutral medium and was employed as a catalyst toward

Results
The Ni-foam was employed as the platform for the growth of Bi 2 O 3 /Bi by hydrothermal approach. The deposition of Bi 2 O 3 /Bi on Ni-foam leads to a homogenous layer of the substrate by the nanoparticles without blocking the open pores of Ni-foam (Figure 1a,b). The SEM images in Figure 1a,b shows the product before calcination and Figure 1c,d demonstrate product micrographs after calcination. As can be seen, after and before calcination Bi 2 O 3 /Bi nanoparticles have a spherical morphology without further change (Figure 1c,d). The only changes can be ascribed to the particle sizes which the sizes of the particles are in the range of a few nanometers for particle before annealing and a few ten nm after annealing. As can be seen, there are small distances among nanoparticles while some of them are attached to each other [93]. Hydrogen bubbles release during the hydrothermal process would result in dendritic shape morphology or even changes in local pH at Ni-foam surface which leads to a highly porous structure [94]. Moreover, during the hydrothermal process the following reactions would be occurred for the formation of oxidized Bi particles.
CO(NH 2 ) 2 → NH 3 + HCNO (1) HRTEM evaluations have shown that the fabricated nanoparticles are accumulated of fine particles given very high accessible active sites with high surface area (Figure 2a). The nature of the synthesized catalyst was proven through selected area electron diffraction (SAED) (the inset of Figure 2a) and X-ray diffraction (XRD) assessments ( Figure 2b). XRD profile before calcination (Figure 2a black line) demonstrated a general Bi-based material with some weak peaks related to oxidized Bi. However, after calcination, we can see some strong peaks' presence which represents the production of oxidized Bi (orange line in Figure 2b). As it can be seen, a series of XRD peaks on the diffraction pattern of Bi 2 O 3 /Bi composition positioned at demonstrated 2θ at related miller indexes of (012), (107), (116), (214) and (009) at 2θ = 28.5, 59.5, 63.5, 72.5, 73.2, respectively. These miller indexes suggest a well-formed nanostructure of oxidized Bi with a rhombohedral polycrystalline Bi (JCPDS No. 85-1330). HRTEM evaluations have shown that the fabricated nanoparticles are accumulated of fine particles given very high accessible active sites with high surface area ( Figure 2a). The nature of the synthesized catalyst was proven through selected area electron diffraction (SAED) (the inset of Figure 2a) and X-ray diffraction (XRD) assessments ( Figure 2b). XRD profile before calcination (Figure 2a black line) demonstrated a general Bi-based material with some weak peaks related to oxidized Bi. However, after calcination, we can see some strong peaks' presence which represents the production of oxidized Bi (orange line in Figure 2b). As it can be seen, a series of XRD peaks on the diffraction pattern of Bi2O3/Bi composition positioned at demonstrated 2θ at related miller indexes of (012), (107), (116),   Moreover, we used the common method (Debye-Scherrer formula) to estimate particle size by XRD profile 9, 17, 21. The formula is as follows: In the above-mentioned equation, λ represents the X-ray wavelength (1.54 Å), β is the full width half maximum, θ is diffraction angle and D is the particle size 21. The attained nanoparticle size of Bi before and after calcination is ~8 nm and ~26 nm, respectively, Moreover, we used the common method (Debye-Scherrer formula) to estimate particle size by XRD profile 9, 17, 21. The formula is as follows: Catalysts 2021, 11, 1099 4 of 12 In the above-mentioned equation, λ represents the X-ray wavelength (1.54 Å), β is the full width half maximum, θ is diffraction angle and D is the particle size 21. The attained nanoparticle size of Bi before and after calcination is~8 nm and~26 nm, respectively, which is in good agreement with SEM results. We have hypothesized that the fine nanoparticle of Bi 2 O 3 /Bi composition offers a highly accessible site for electrochemical reduction while the present pore structure enables mass transport in the solution during operation. On the other hand, metallic Ni-foam (as substrate) affords fast charge carriage while its porous construction comforts detachment and carriage of gaseous products from the electrode. To demonstrate the feasibility of the concept, we studied the electroactivity of the composite electrode for electroreduction of water. In order to demonstrate Bi oxidization influence, bare Ni-foam and as-deposited Bi electrodes were tested. Electrochemical analysis via linear sweep voltammetry (LSV) displays in Figure 3a toward the HER process. The LSV curves of the Bi 2 O 3 /Bi-NF as-prepared electrode demonstrated excellent HER activity with an overpotential of 180 mV to reach 1 mA·cm −2 . While the bare Ni-foam and Bi/NF electrodes showed a much higher overpotential of 240 mV (for Ni-foam) and 215 mV (for Bi/NF) at a current density of 1 mA·cm −2 . Moreover, the optimized electrode (Bi 2 O 3 /Bi-NF) required a low overpotential of 250 mV to reach a current of 10 mA·cm −2 while higher overpotentials of 300 and 370 mV were required for Bi/NF and Ni-foam electrodes, respectively. This shows the prepared Bi 2 O 3 /Bi-NF electrode had a much excellent HER performance than the other ones which could be due to the presence of partially reduced oxidized Bi at structure and not-reduced Bi 2 O 3 at structure.
To further confirm HER kinetics, the Tafel slope was measured and considered for it, conformed to the Tafel formula ( Figure 3b) were constructed according to the following Equations (6) and (7): In above-mentioned Equations (6) and (7), η demonstrate overpotentials and j also shows current density. In addition, a mechanistic demonstration of electrochemical reactions can be shown by α which is charge transfer coefficient and is a constant. The slope of η-j curve (the cathodic Tafel slope) defines the rate of electrochemical reaction. The value of j at η = 0 is related to exchange current density (j 0 ), which is an important parameter for assessing catalytic activity of a catalyst for water reduction reaction. As can be carried out from the Tafel plot, a lower Tafel slope would result in sharper growth of increasing overpotential 24. As can be seen from Figure 3b, the Bi 2 O 3 /Bi-NF electrode demonstrated a lower Tafel slope of 55 mV/dec which is smaller than that of Bi/NF and Ni-foam electrodes with Tafel slopes of 123 and 115 mV/dec, respectively.
In addition to the above-mentioned discussion, the following multistep reactions could be considered for this work, though HER kinetics has not been well-understood up until now [13,20]: MH ads + MH ads → H 2 + 2M (Tafel step; recombination reaction) "M" stands for active sits for H adsorption. The process is begun with the Volmer step (Equation (8)) where the first discharge includes on the electrode surface by adsorbing water molecules. Then, catalytic recombination of adsorbed protons (Equation (9)) and the electro-desorption of adsorbed intermediates (Equation (10)) take place. Based on the Tafel slope, which is in the range of 40-120 mV/dec, we believe that the HER process can be controlled by the Volmer-Heyrovsky mechanism in Equations (9) and (10).
value of j at η = 0 is related to exchange current density (j0), which is an important parameter for assessing catalytic activity of a catalyst for water reduction reaction. As can be carried out from the Tafel plot, a lower Tafel slope would result in sharper growth of increasing overpotential 24. As can be seen from Figure 3b, the Bi2O3/Bi-NF electrode demonstrated a lower Tafel slope of 55 mV/dec which is smaller than that of Bi/NF and Ni-foam electrodes with Tafel slopes of 123 and 115 mV/dec, respectively. In addition to the above-mentioned discussion, the following multistep reactions could be considered for this work, though HER kinetics has not been well-understood up until now [13,20]: MHads + MHads → H2 + 2M (Tafel step; recombination reaction) H2O + MHads + e -→ H2 + M + OH − (Heyrovsky step) (10) We examined the stability of the optimized Bi 2 O 3 /Bi-NF electrocatalyst in the basic electrolyte by CA (the blue line in Figure 3c). The HER stability tests were conducted uninterruptedly from one sample. Changes in the current density in LSV curve are shown in Figure 3c (the red and black line). As seen, the Bi 2 O 3 /Bi-NF electrocatalyst reveals high durability with negligible changes in the current response. This shows good connection between Bi 2 O 3 /Bi particles and Ni-foam platform. Microstructural studies by SEM and XRD have shown some changes in the surface morphology and a little change in the structure of the Bi 2 O 3 /Bi-NF due to the oxidized Bi reduction into metallic Bi of the electrodeposited alloy after CA. As can be seen from Figure 4a, the SEM image shows surface agglomeration which might be due to the reduced Bi 2 O 3 into metallic form and tend to be agglomerate. The structural changes can be carried out from faded XRD peaks or weakened peaks in Figure 4b. As can be seen from Figure 4b, most of the oxidized Bi have been weakened or faded which is as a result of applied reduced potential during HER process. In order to further confirm this result, EDS analysis has been applied to detect the percentage of each element. As can be seen from Figure 4e, more than 80% is metallic Bi, while only a small portion contains Oxygen elements.  Electrochemical impedance spectroscopy (EIS) is conducted to study charge transfer behavior of the catalyst, which is shown by Nyquist plots (Figure 3d). A depressed semicircle can be associated with charge-transfer-controlled effect. Fluctuations in impedance waves could be associated with gaseous product release from the catalyst surface that could generate forced convection and disturb the ionic or electron transfer [94][95][96]. We have found that Bi2O3 growth and deposition on Ni-foam intensely lessening charge transfer resistance (denoted as Rct) of the combined Bi2O3/Bi-NF electrode which results in conductivity enhancement of the electrode. The Bi2O3/Bi-NF electrode showed a small charge transfer resistance (small semi-circle). It is worth noting that the root cause of this is related to the large accessible active sites for doing reduction reaction on the catalyst surface and also good connection between electrode materials for charge transfer. Moreover, it might be due to this reason that the decent electrode material and electrolyte interfacial area could further decrease the resistance for charge transport [97]. Besides, it is seen that co-catalyzing of Bi2O3 with Bi and Ni foam also offers rapid charge transport due to more accessible active sites on electrode surface as reinforced by the double-layer capacitance Electrochemical impedance spectroscopy (EIS) is conducted to study charge transfer behavior of the catalyst, which is shown by Nyquist plots (Figure 3d). A depressed semicircle can be associated with charge-transfer-controlled effect. Fluctuations in impedance waves could be associated with gaseous product release from the catalyst surface that could generate forced convection and disturb the ionic or electron transfer [94][95][96]. We have found that Bi 2 O 3 growth and deposition on Ni-foam intensely lessening charge transfer resistance (denoted as R ct ) of the combined Bi 2 O 3 /Bi-NF electrode which results in conductivity enhancement of the electrode. The Bi 2 O 3 /Bi-NF electrode showed a small charge transfer resistance (small semi-circle). It is worth noting that the root cause of this is related to the large accessible active sites for doing reduction reaction on the catalyst surface and also good connection between electrode materials for charge transfer. Moreover, it might be due to this reason that the decent electrode material and electrolyte interfacial area could further decrease the resistance for charge transport [97]. Besides, it is seen that co-catalyzing of Bi 2 O 3 with Bi and Ni foam also offers rapid charge transport due to more accessible active sites on electrode surface as reinforced by the double-layer capacitance measurements (Figure 4c,d). There is an ion diffusion limitation linking to the Warburg behavior at low  (Figure 3d) [98]. Further evaluations by EIS at OCP have shown that the ion diffusion at Bi 2 O 3 /Bi-NF surface is controlled by diffusion of OH − ions (with limited diffusion rate) between the electrolyte and the electrode interface. Figure 4 determines that the composition of Bi 2 O 3 in combination with Bi/NF reduces the diffusion resistance of Ni-foam assigning to their co-operative and synergy effect exposed to the alkaline solution that provides a low distance and rapid ion diffusion. In addition, those nanoparticles could reduce the hydrophobicity of overall structure assisting to the diffusion resistance [99].
Capacitance and effective electrocatalytic surface area. To evaluate electrochemical surface area (ECSA), CV technique was employed at various scan rates (10 to 100 mV/s) and double layer capacitance (Cd) was calculated by the following formula [100][101][102][103][104][105][106]: In Equations (11) and (12), i represents non-Nernst current and ν is the scan rate. S stands for the attained surface area by integration CV plot. Equations (11) and (12) are utilized to measure C dl (double layer capacitance) from voltammetry plot where there is not any faradaic charge transfer. It is used to compare electrochemically active surface area of provided electrodes (in this work the electrodes are Bi 2 O 3 /Bi-NF and Ni-foam). As a result of this calculation, we have obtained a C dl of 1.2 mF·cm −2 for Bi 2 O 3 /Bi-NF and a C dl of 0.095 mF·cm −2 for Ni-foam. Which this result shows Bi 2 O 3 /Bi-NF higher ECSA than Ni-foam which is in good agreement with those LSV results. It is worth noting that the capacitance is proportionate to the ECSA for adsorption/desorption of electrolyte ion. To investigate the effect of ECSA of catalysts for HER, it is thus helpful to investigate the ECSA of different catalysts. The acquired C dl of catalysts (Bi 2 O 3 /Bi-NF and Ni-foam) have been assessed as shown in Figure 4c,d. Therefore, the Bi 2 O 3 /Bi-NF electrocatalyst displayed a considerably higher Cdl, which is satisfactory for the essentially promoted HER activity of this catalyst.

Fabrication of Oxidized Bi (Bi 2 O 3 )
Pieces of 2 × 4 cm 2 from Ni foam were prepared through laser cutting and the prepared pieces were washed with acetone and ethanol sonicated for 10 min, respectively. As prepared samples dried under an Ar gas atmosphere and put inside a quartz boat to be dried at 60 • C. Afterward, a facile and reproducible hydrothermal route was employed to synthesize Bi 2 O 3 . In a general process, 1 mmol of Bi(NO 3 )3.5H 2 O was added into 10 mL of 2.5 M HNO 3 aqueous solution, the Bi(NO 3 )3.5H 2 O solution was added gradually. Then, we added the prepared solution into a 30 mL solution containing 5 mmol urea. Therefore, after vigorous blending for 30 min, 10 mL of 3 M sodium hydroxide was added gradually into the prepared solution. After extra stirring for 20 min, the prepared solution along with Ni-foam pieces was transferred into a stainless-steel autoclave with a 50 mL capacity Teflon liner. The autoclave containing all prepared solution was heated at 100 • C for 20 h. The heating ramp was set up at 3 • C/min. Afterward, the autoclave was taken out from the furnace and cooled to ambient temperature. The Ni-foam was taken out from the autoclave and washed several times with ethanol and DI water. The remaining products in the container were also separated through centrifugation and cleaned with DI water and ethanol and methanol three times in each step (in order to get rid of contaminated species), respectively. Then the product was dried under Ar gas flow for 10 h at 50 • C in a tube furnace. Afterward, the prepared product was annealed at 500 • C for 2 h under air condition. The calcination process has been applied to oxidizing metallic Bi.

Materials Characterizations
Powder X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (Philips, X'pert PRO, Amsterdam, The Netherlands) by using Cu Kα radiation (λ = 1.541874 Å) at a scanning rate of 7 • /min in the 2θ range from 20 • to 80 • . High Resolution Transmission Electron Microscope (HRTEM, JOEL JEM-2010F, Singapore) and a Scanning Electron Microscope (SEM, MIRA III TESCAN, Kohoutovice, Czech Republic) equipped with Energydispersive X-ray spectroscopy (EDX) is a surface analytical technique used to investigate microstructural and morphological properties.

Electrochemical Evaluations
All electrochemical tests for the HER process were conducted by an Autolab PGSTAT 302 N electrochemical workstations (Metrohm Autolab, Amsterdam, The Netherlands) with a standard three-electrode system in 1 M KOH aqueous solution. The working electrode was Bi 2 O 3 /Bi deposited on Ni-foam (Bi 2 O 3 /Bi-NF) electrocatalysts with a size of~one cm 2 . A platinum plate as counter electrode and Ag/AgCl (3 M KCl) as the reference electrode were employed. Linear Sweep Voltammetry (LSV) observations have been conducted with a scan rate of 5 mVs −1 in a potential range of 0 to −1.5 V (vs. Ag/AgCl). CA measurement was applied at −1.35 V (vs. Ag/AgCl) to discover the optimized electrode's durability. Moreover, the charge and ionic transfer and diffusion performance of the prepared electrodes were evaluated through Electrochemical Impedance Spectroscopy (EIS) over a frequency range of 10 −1 Hz to 10 5 Hz and at open circuit potential (OCP) with a 10 mV sinusoidal potential. All the potentials conveyed in this work were referenced to RHE by the addition of (0.197 + 0.059 pH 8). iR-correction was also done [92]. The compensated potential was considered by E compensated = E measured + I × R s , where R s is the series resistance obtained by EIS data [93].

Conclusions
In summary, Bi-based catalysts were synthesized on spongy Ni-foam to prepare active electrochemical material for the HER process. To exhibit the high electrocatalytic performance, the high activity of the Bi 2 O 3 /Bi-NF electrode for the reduction of water in basic electrolytes was revealed. For HER, the general comparative current density 10 mA·cm −2 was reached at a low overpotential of 250 mV as shown. Furthermore, the evaluations through EIS technique analyses demonstrated rapid HER kinetics with a low Tafel slope 55 mV/dec for Bi 2 O 3 /Bi-NF as a result of the co-operative effect of oxidized Bi and metallic Bi in the catalyst structure. These high electrochemical activities proposed highly cooperative effects of oxidized Bi and metallic Bi and Ni-foam substrate on the electrocatalytic performance. Specifically, high electron transport originating from 3D porous structure, that also helped reduce ion diffusion alongside the catalyst surface. Therefore, it can be said that the Bi 2 O 3 /Bi-NF catalyst could offer a good approach for future research in electrochemical redox-reaction, e.g., water splitting and even in electrochemically reduction of CO 2 into value added feedstocks.