Nickel-Based Electrocatalysts for Water Electrolysis

: Current hydrogen production is based on the reforming process leading to the emission 1 of pollutants; therefore, a substitute production method is imminently required. Water electrolysis 2 is an ideal alternative for large-scale hydrogen production, as it does not produce any carbon-based 3 pollutant byproducts. Production of green hydrogen from water electrolysis using intermittent 4 sources (e.g., solar, eolic) would facilitate clean energy storage. However, the electrocatalysts 5 currently required for water electrolysis are noble metals, making this potential option expensive 6 and inaccessible for industrial applications. Therefore, there is a need to develop electrocatalysts 7 based on earth-abundant and low-cost metals. Nickel-based electrocatalysts are a ﬁtting alternative 8 because they are economically accessible. Extensive research has focused on developing nickel- 9 based electrocatalysts for hydrogen and oxygen evolution. Theoretical and experimental work 10 have addressed the elucidation of these electrochemical processes and the role of heteroatoms, 11 structure, and morphology. Even though some works tend to be contradictory, they have lit up 12 the path for efﬁcient nickel-based electrocatalysts. For these reasons, herein, a review of recent 13 progress is presented. 14


Water electrolysis
One of the main motivations to produce molecular hydrogen (H 2 ) is its role as 17 an energy carrier, with an energy density of 140 MJ kg −1 [1], which makes it an ideal 18 alternative for clean energy storage. Water electrolysis, the electrical decomposition of 19 water (H 2 O) into H 2 and molecular oxygen (O 2 ) [2], is considered a clean and efficient 20 H 2 production method since no byproducts are generated, and renewable energies can 21 be used, in contrast to the reforming process, where carbon dioxide (CO 2 ) and carbon 22 monoxide (CO) are generated [3]. Although water electrolysis has been known for more 23 than 200 years and provides high purity H 2 (greater than 99.5%) [4], it represents less 24 than 1% of the total amount of H 2 produced each year globally [5]. 25 The overall reaction for water electrolysis is shown in Eq. 1 [6].
The electrical work needed for the water electrolysis according to Eq. 1 at standard 27 temperature and pressure is given by the change in free energy, ∆ rxn G 0 -= 237.2 kJ/mol. 28 It marks the thermodynamic lower limit of energy required for producing hydrogen, 29 which in practical terms is equivalent to 33 kWh per 1 kg of hydrogen at an equilibrium The overpotential η is the difference between the equilibrium potential E eq and the applied potential E app (Eq. 20) at which the electrocatalyst provides a given current as a result of the excess potential introduced to overcome the energetic barrier of the reaction. [15,30].
The Butler-Volmer (Eq. 21) describes the rate of an electrochemical reaction through the current density (j) in terms of the overpotential [31]. As the scan rate controls the speed at which the potential is swept, j is proportional to the reaction rate [32]. The lower the η required to maintain a specific j, the more efficient the electrocatalytic material [33]. In this equation, j 0 is the exchange current density, α is the transfer coefficient, f represents the Faraday constant divided by the temperature and the real gas constant (i.e., f = F/RT), and n is the number of electrons transferred.
The Butler-Volmer equation represents the sum of reduction and oxidation reaction currents. These currents are proportional to the rate of reaction. In equilibrium, the total flux of current is zero because the system has reached a dynamic equilibrium in which the rate of oxidation-reduction (redox) reactions is the same [34]. Otherwise, only one process is dominant at high η values. This eliminates one term leading to Tafel's equation [35], which can be expressed as the linearized Eq. 22.
The Tafel slope, m T , describes how sensitive the current response is to the overpotential 145 and gives information about the reaction mechanism and the rate-limiting steps [35]. A 146 small Tafel slope is attributed to a rapid increase in the j at lower η values [15]. Equally, 147 the exchange current density (j 0 ) can be obtained by the Tafel equation (Eq. 22) that 148 corresponds to the intercept [14]. j 0 is proportional to the rate reaction at the equilibrium 149 when the total current is equal to zero, and the anodic and cathodic currents are equal 150 [15,36]. This kinetic parameter depends on the concentration, temperature, electrode 151 catalyst loading, and its specific surface area [36]. It reflects the intrinsic bonding/charge-152 transfer interactions between the electrocatalyst and the electroactive species [15]. A 153 high value of j 0 indicates that the material is a promising electrocatalyst for the target 154 reaction [15,36]. These three kinetic parameters can be determined experimentally by  The turnover frequency (TOF) is a performance parameter that evaluates the rate at which reactants are converted to products per catalyst site per second [42,43]. The TOF values depend on the temperature, and pressure [43]. The equation for the determination of TOF in gas evolving reactions is stated in Eq. 23, where j is the current density, N Av is the Avogadro number, n the number of electrons involved, F the Faraday's constant, and Γ is the surface or total concentration of active sites.
TOF values between 10 −3 and 10 −2 s −1 have been observed [  The electrochemical active surface area (ECSA) is a fundamental electrochemical property of an electrified interface, it is the electrode surface area that is accessible for charge transfer, or storage [45]. The values of ECSA depend on the electrochemical reaction that takes place in the interface and the materials involved [14,45]. Because every catalytic process takes place on the electroactive surface, extensive quantities obtained in electrochemical experiments must be normalized concerning ECSA. Since changes in the ECSA during an experiment may occur, it is necessary to monitor this property between experiments [46]. Despite its relevance in electrochemical processes, its determination is limited to inert and approximate planar electrode substrates, becoming complicated for porous materials [45]. Some methods that have been reported for the determination of ECSA are the integration of the redox peak areas, the hydrogen underpotential deposition, carbon monoxide stripping, and the double-layer capacitance (C DL ). Capacitive characteristics of an interface in an electrochemical system, like C DL , are associated with the charge storage phenomenon. This phenomenon occurs at potential values at which there is no solvent decomposition or electron transfer during the potential application in the interface [47]. C DL can be determined by two techniques: CV [23,48] and electrochemical impedance spectroscopy (EIS) [48]. For the CV technique, the first step is to identify the non-faradaic range of the system in a specific electrolyte for measurements at different scan rates. The current measured (i C ) in this potential window is attributed to the capacitive double-layer charge. When plotted against the scan rate (ν), the slope of the curve gives the value of C DL according to Eq. 25.
Finally, to obtain the ECSA, the value of C DL is divided by the specific capacitance of the sample at the same electrolyte (C S ) (Eq. 26) [48,49].

271
To produce economical and efficient electrocatalysts for water electrolysis is essential to develop new materials with tuned compositions, i.e. materials whose surfaces offer precise interactions with the reaction intermediates to promote the electron transfer at the lowest overpotential. For the case of the OER (in acid or alkaline media), the key intermediate adsorbed species are OH (Eqs. 12 or 17), O (Eqs. 14 or 18) and OOH (Eqs. 15 or 19). Nørksov showed that the adsorption energies of these intermediates are linearly related [69]. Ideally, the free energies for each of these processes should be 1.23 eV, but these energies correlate according to Eq. 27       activity towards OER and an enhancement in their activity due to the increased surface 344 and more exposed active sites [103].
The main deposition mechanism of bimetallic nickel films is the anomalous co-deposition,   Table 3 compares nickel-based electrocatalysts' performance parameters for OER and 583 HER in alkaline, acid, and neutral electrolytes. All the electrocatalysts listed in Table 3 584 and Table 4 were considered for the comprehensive study of nickel-based electrocatalysts 585 for water electrolysis in the present work, but not all of them are discussed in the 586 following sections.

618
After an increase in activity at 100 cycles, it remained stable until 500 cycles (Figure 8a).

619
In contrast, although β-Ni(OH) 2 also displayed an increase in activity at 100 cycles, it of pseudo-first-order OER rate constants, they observed that proceeded via two catalytic 635 sites, one with "fast" kinetics (k = 1.70 s −1 ) and other with "slow" kinetics (k = 0.04 s −1 ) 636 for Ni 0.82 Fe 0.18 OOH, the optimal composition. With a Fe content bigger than 25% un-637 controlled segregation was observed, and the rate of reaction decreased (k = 0.34 s −1 ).

638
The "fast" active site was attributed to Fe IV sites due to the match of the fraction of "fast" 639 sites (17.6%) and Fe content (18.19%). The "slow" active site was attributed to Ni due to    (Figure 11a). Ni 2 P had energy  for the industrial production of H 2 in acid, neutral and alkaline electrolytes is a challenge.

769
The precise control of composition, morphology, and structure of electrocatalysts can 770 lead to a material capable of efficiently catalyzing HER and OER in the same electrolyte.    tent ( Figure 19). So, to elucidate the correct mechanism, the development of purification 870 protocols for diver impurities and electrolytes is required.