Kenta
Kawashima
a,
Raúl A.
Márquez-Montes
a,
Hao
Li
ab,
Kihyun
Shin
ab,
Chi L.
Cao
c,
Kobe M.
Vo
c,
Yoon Jun
Son
c,
Bryan R.
Wygant
a,
Adithya
Chunangad
c,
Duck Hyun
Youn
d,
Graeme
Henkelman
ab,
Víctor H.
Ramos-Sánchez
e and
C. Buddie
Mullins
*acfg
aDepartment of Chemistry, The University of Texas at Austin, Austin, Texas 78712, USA. E-mail: mullins@che.utexas.edu
bOden Institute for Computational Engineering and Sciences, The University of Texas at Austin, Austin, Texas 78712, USA
cMcKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, USA
dDepartment of Chemical Engineering, Interdisciplinary Program in Advanced Functional Materials and Devices Development, Kangwon National University, Chuncheon, Gangwon-do 24341, South Korea
eFacultad de Ciencias Químicas, Universidad Autónoma de Chihuahua, Chihuahua, Chihuahua 7 31125, Mexico
fCenter for Electrochemistry, The University of Texas at Austin, Austin, Texas 78712, USA
gH2@UT, The University of Texas at Austin, Austin, Texas 78712, USA
First published on 1st March 2021
Nickel nitride (Ni3N) is known as one of the promising precatalysts for the electrochemical oxygen evolution reaction (OER) under alkaline conditions. Due to its relatively low oxidation resistance, Ni3N is electrochemically self-oxidized into nickel oxides/oxyhydroxides (electroactive sites) during the OER. However, we lack a full understanding of the effects of Ni3N self-oxidation and Fe impurity incorporation into Ni3N from electrolyte towards OER activity. Here, we report on our examination of the compositional and structural transformation of Ni3N precatalyst layers on Ni foams (Ni3N/Ni foam) during extended periods of OER testing in Fe-purified and unpurified KOH media using both a standard three-electrode cell and a flow cell, and discuss their electrocatalytic properties. After the OER tests in both KOH media, the Ni3N surfaces were converted into amorphous, nano-porous nickel oxide/(oxy)hydroxide surfaces. In the Fe-purified electrolyte, a decrease in OER activity was confirmed after the OER test because of the formation of pure NiOOH with low OER activity and electrical conductivity. Conversely, in the unpurified electrolyte, a continuous increase in OER activity was observed over the OER testing, which may have resulted from the Fe incorporation into the self-oxidation-formed NiOOH. Our experimental findings revealed that Fe impurities play an essential role in obtaining notable OER activity using the Ni3N precatalyst. Additionally, our Ni3N/Ni foam electrode exhibited a low OER overpotential of 262 mV to reach a geometric current density of 10 mA cmgeo−2 in a flow cell with unpurified electrolyte.
Metallic nickel nitride (Ni3N), on which we exclusively conducted experiments in this study, is a TM X-ide OER precatalyst. According to recent findings,22–27 it was found that the Ni3N electrode surface is transformed into layered nickel (oxy)hydroxide [Ni(OH)2 and NiOOH] due to its self-oxidation during OER testing. This newly formed nickel (oxy)hydroxide surface is primarily responsible for catalyzing the OER. Incidentally, the contribution of surface oxide/(oxy)hydroxide species to the OER was also discussed in the reports of other TM pnictides (e.g., CoN,28 FeP,29 CoP,30,31 Ni2P,32–34 Ni5P4,35 NiAs,36etc.). Recently, Trotochaud et al. reported that the incidental incorporation of trace Fe impurities from KOH electrolyte can increase the electrical conductivity and change the electronic structure of NiOOH, resulting in a dramatic increase in the OER activity of Ni(OH)2/NiOOH.37 According to several reports regarding (Ni,Fe)OOH,38–41 the Fe concentration in NiOOH for obtaining the maximum OER activity has been reported to range from 12–25%. Furthermore, pure NiOOH has also been revealed to be a poor electrocatalyst for the OER.37 Hence, in the Ni3N precatalyst, there is a possibility that trace Fe also plays a critical role in boosting the OER activity of the nickel (oxy)hydroxide formed during the OER.
However, until now, little attention has been paid to studying how trace Fe incorporation impacts the OER activity of the Ni3N precatalyst to understand the real active species for the OER. This is because almost all the reported Ni3N-based electrocatalysts have been tested in unpurified KOH electrolyte that contains trace quantities of Fe.23–27,42–56 As one example, Shalom et al. prepared the Ni3N/Ni(OH)2 electrocatalysts and tested them for the OER in both commercial and Fe-free 1 M KOH aqueous solutions.22 As the Ni3N/Ni(OH)2 electrocatalysts demonstrated almost the same OER activities, they concluded that the influence of Fe impurities on the OER activity of their samples was minor. However, prior to the OER activity tests, their Ni3N samples were oxidized to form Ni(OH)2via 500 cyclic voltammetry (CV) cycles in Fe-unpurified 0.1 M KOH. Accordingly, it is unknown if there was incorporation of trace quantities of Fe into the Ni3N/Ni(OH)2 samples during this CV oxidation process.
In this study, we prepared a Ni3N precatalyst layer on nickel foam (Ni3N/Ni foam) through room-temperature chemical oxidation and subsequent nitridation and investigated its transformation (self-oxidation) under the OER process in Fe-purified alkaline media (referred to here as “purified 1 M KOH”). Analytical techniques including X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy were employed to check the compositional and morphological changes of Ni3N before and after OER CV testing. The electrochemical changes to the Ni3N with OER CV testing were tracked using voltammetric techniques. Furthermore, to check the impact of Fe impurity incorporation on measured OER activities, the Ni3N/Ni foam electrodes were tested for the OER in Fe-unpurified alkaline media (“unpurified 1 M KOH”) as well. To more deliberately and evenly introduce trace Fe into the self-oxidized Ni3N/Ni foam57,58 and evaluate the OER activity in a condition close to practical conditions, a flow cell was used. Specifically, we conducted long-term OER chronopotentiometric (CP) tests to examine the relationship between the electrochemical changes and Fe incorporation over time. Importantly, the post-OER CV Ni3N sample in purified 1 M KOH exhibited a higher OER overpotential (η) of 482 mV at a geometric current density of 20 mA cmgeo−2 compared to the post-OER CP Ni3N sample (325 mV at 20 mA cmgeo−2) as well as the previously reported Ni3N/Ni foams (365 ± 24 mV at 20 mA cmgeo−2)22,26,42,55 in unpurified 1 M KOH. Additionally, the OER CV test in purified 1 M KOH showed a decline in measured OER activities [η: 442.5 → 463.2 mV at 15 mA cmgeo−2] whereas the OER CP test in unpurified 1 M KOH indicated an increase in measured OER activities [η: 315.7 → 293.3 mV at 15 mA cmgeo−2]. These experimental findings evidently imply that the superior OER performance of the Ni3N precatalyst in unpurified alkaline media mainly originates from the modified electronic properties of the self-oxidation-formed NiOOH due to the Fe incorporation.
For Ni3N/NF synthesis, the as-prepared NiO/Ni samples were subjected to further oxidation in a box-type furnace at 350 °C for 2 h, with a heating ramp of 10 °C min−1. Afterwards, the as-oxidized Ni foam pieces were placed onto a 101 × 67 × 3 mm quartz plate located in the center of a tube furnace and nitrided at 450 °C for 6 h at a heating ramp of 10 °C min−1 under an NH3 flow (120 mL min−1) and then cooled naturally to room temperature.
To investigate the electrode surface area change during the long-term OER CV test, electrochemically active surface areas (ECSAs) were estimated from the electrochemical double-layer capacitances (Cdls). Cyclic voltammograms (CVs) with different scan rates were obtained in the non-faradaic potential window (−0.2 to −0.1 VHg/HgO) to calculate Cdl from the scan-rate dependence of double-layer charging [Δj/2 = (ja − jc)/2].60 The potentials applied relative to Hg/HgO were converted into the reversible hydrogen electrode (RHE) scale via the Nernst equation . Here, was determined by using the saturated calomel electrode (SCE, saturated KCl aqueous solution, 0.241 V versus standard hydrogen electrode).
Thus, to ascertain the presence of amorphous self-oxidized species on the post-OER Ni3N and Ni surfaces, EDX elemental mapping analyses were conducted on the pre- and post-OER samples. In the as-prepared Ni3N/Ni foam, Ni (green) and N (blue) elements were homogeneously distributed (Fig. 2a). Weak O (red) signals were also confirmed, which may correspond to the partial oxidation of the as-prepared sample surface due to exposure to ambient air. After the long-term OER CV test (Fig. 2b), Ni and N signals remained, while the O signal intensity increased. In the case of the pristine Ni foam, O signals were not observed (Fig. S4a, ESI†). However, relatively weak, evenly distributed O signals appeared after the OER test (Fig. S4b, ESI†). As just described, for both the Ni3N/Ni and Ni foams, O incorporation proceeded after the OER testing (Table S1, ESI†). In particular, the O/Ni atomic ratio of the Ni3N/Ni foam drastically increased from 0.016 to 0.320 following the OER test. Considering these facts, amorphous nickel (oxy)hydroxide layers might form on the post-OER Ni3N and Ni surfaces. More interestingly, the post-OER Ni3N/Ni showed a much higher O/Ni atomic ratio (0.320) than the post-OER Ni (0.013), which implies that Ni may be more resistant to oxidation than Ni3N. This point will be discussed in greater detail later in the paper.
To inspect the morphologies of the Ni3N/Ni and Ni foams before and after the long-term OER CV tests, SEM measurements were performed (Fig. 3 and Fig. S5, S6, ESI†). The low- and high-resolution SEM images of the as-prepared Ni3N/Ni foam are shown in Fig. S5a (ESI†) and Fig. 3a. Compared with the pristine Ni foam (Fig. S6a, ESI†), the as-prepared Ni3N/Ni foam surface is rougher. This roughness was likely due to (i) the amorphous NiO nanocluster growth on the Ni foam surface through low-temperature oxidation (Ni → NiO)59 and (ii) the subsequent lattice contraction that accompanies the replacement of O with N in the amorphous NiO precursor during the nitridation (NiO → Ni3N).69 After the OER test, the self-oxidized Ni3N/Ni foam macroscopically maintained the surface morphology of the as-prepared Ni3N/Ni foam (see Fig. S5, ESI†). However, from a microscopic view (Fig. 3), the relatively compact Ni3N surface was transformed into a nano-porous surface (and more available for electrolyte penetration) of its oxide counterparts [Ni(OH)2 and NiOOH]22,23,25,26 through the long-term OER process. By contrast, on the Ni foam, a compact layer (it doesn’t appear to be nano-porous like the Ni3N material) of its oxide counterparts formed after the OER test (Fig. S6b, ESI†). This morphological difference possibly resulted from the different extent of atomic packing of Ni in the hexagonal Ni3N and cubic Ni crystal structures (Fig. S2, ESI†). Specifically, Ni3N has a slightly longer Ni–Ni interatomic distance (dNi–Ni = 2.6 Å) than Ni (dNi–Ni = 2.5 Å). Accordingly, the highly porous structure of Ni(OH)2 and NiOOH derived from Ni3N can be realized due to the relatively rough surface and long Ni–Ni interatomic distance of Ni3N.
To investigate the surface chemical composition and states of the Ni3N/Ni and Ni foams before and after the long-term OER CV tests, XPS measurements were conducted. The Ni 2p3/2 and N 1s XPS spectra of the pre- and post-OER Ni3N/Ni foams are shown in Fig. 4. In the pre- and post-OER Ni 2p3/2 spectra (Fig. 4a and b), the deconvoluted peaks can be assigned to five components: (i) Ni3N (852.8 eV),23,26 (ii) NiO (854.5 eV),70 (iii) Ni(OH)2 (855.3 eV),70 (iv) NiOOH (855.8 eV),70 and (v) satellites (around 860.6 eV),26,71 respectively. The Ni 2p3/2 peaks of NiO and Ni(OH)2 in the as-prepared Ni3N/Ni foam imply that the product surface might be partially oxidized by ambient air exposure. After the OER test, the Ni 2p3/2 peak of Ni3N disappeared and NiOOH and Ni(OH)2 formation was confirmed. These phenomena indicate that the surface oxidation of Ni3N into NiOOH and Ni(OH)2, which agrees with previous reports.22,23,25,26 As for the Ni foam, similar surface oxidation was observed after the OER test. The XPS measurement results of the pre- and post-OER Ni foams are also available in Fig. S7 (ESI†). The N 1s spectrum of the as-prepared Ni3N/Ni foam was deconvoluted into two components (Fig. 4c). The high intensity peak at 398.3 eV is assignable to the lattice nitrogen species in Ni3N.22 The other peak is likely associated with surface-adsorbed NHx (400.0 eV).72 After the OER test, the N 1s signal almost disappeared, suggesting that the nitrogen species do not directly contribute to the OER process. From the above XRD, EDX, and XPS results (Fig. 1, 2, and 4), we conclude that a Ni3N@Ni(OH)2/NiOOH core@shell structure formed after the OER test. To further support this conclusion, Fig. S8 (ESI†) shows the electrochemical stability map (Pourbaix diagram and Gibbs free energy overlay) of Ni3N as theoretical evidence. Ni3N is unstable across a wide range of pH value and potentials. Thus, it is easily expected that the Ni3N surface, which is in contact with the electrolyte, can be oxidized under OER conditions and the Ni3N-derived nickel (oxy)hydroxide may act as the OER active sites rather than the initial Ni3N.
Similarly to the CV-derived value for Cdl, the surface redox reaction of transition metal ions at transition metal (oxy)hydroxides (e.g., Ni0.8Fe0.2OxHy) can conventionally be a descriptor to estimate the number of active sites.76,77 Specifically, the redox active Ni species can be quantified as OER active sites using the integration of the main oxidation peaks of Ni2+/Ni3+ (A1 and A2 in Fig. S13, ESI†). However, in both the Ni3N/Ni and Ni foams, even though the CV-derived Cdl value changes (Fig. 5 and Fig. S10, S12, ESI†) it did not follow the changes in the Ni2+/Ni3+ oxidation peak intensities (Fig. S9a–h and S11a–h, ESI†). Importantly, despite the continuous increase in intensity of the Ni2+/Ni3+ oxidation peaks during CV cycling, the CV-derived Cdl values stabilized from 50 to 1000 cycles. Here, since the Ni2+/Ni3+ oxidation peak intensity may correlate with the amount of NiOOH species formed by the self-oxidation of Ni3N or Ni, we conclude that the amount of surface NiOOH species continuously increased over the CV cycling.36 Given that this does not correlate with an increase in OER performance, it instead implies that not all NiOOH species on the Ni3N and Ni surfaces can participate in the OER. It is well-known that both Ni(OH)2 and NiOOH possess layered structures and that the surface Ni2+/Ni3+ redox reactions involve changes in crystal structure [i.e., Ni(OH)2 ↔ NiOOH].75 According to recent studies about transition metal (oxy)hydroxide nanosheets,78,79 single-layered Ni(OH)2 nanosheets were transformed into nanoparticles during electrochemical Ni redox cycling. This shape transformation likely occurs because of (i) cracking from mechanical stress [e.g., Ni–Ni interatomic distance changes (dNi–Ni = 3.12 Å for Ni(OH)2 ↔ dNi–Ni = 2.86 Å for NiOOH)]80 and (ii) the ripening phenomena.79 Presumably, in our experiments, a similar cracking and ripening occurred as the Ni3N and Ni metal-derived nickel (oxy)hydroxide shells were amorphized during CV cycling. Furthermore, as shown in Fig. S14 (ESI†), the contraction of nickel (oxy)hydroxide shells could have proceeded, resulting in the formation of an underlying layer of compact nickel (oxy)hydroxide. Note that such a compact nickel (oxy)hydroxides may possess a “cornflake-in-milk”-like structure [cornflake: NiOxHy nanoflake; milk: electrolyte (see Fig. S14, ESI†)] and be able to participate in reversible Ni redox reactions but not the OER. Accordingly, only the outermost surface nickel (oxy)hydroxides may be able to contribute to the OER process. This may explain the observed mismatch between the trends of the CV-derived Cdl values and the Ni2+/Ni3+ oxidation peak intensities.
After 1000 cycles, the Ni3N/Ni foam recorded a lower OER overpotential of 482 mV at 20 cmgeo−2 than the Ni foam (534 mV). This is mainly because Ni3N-derived nickel (oxy)hydroxide exhibited a unique nano-porous structure (see Fig. 3b) with an ECSA (∼246.8 cm2) approximately 10 times higher than that of Ni metal-derived nickel (oxy)hydroxide (∼24.02 cm2). Notably, in comparison to the previously reported Ni3N/Ni foams in unpurified electrolytes, our post-OER Ni3N/Ni foam in the purified electrolyte showed worse OER activity, despite having a higher CV-derived Cdl values (see Table S2, ESI†).22,26,42,55 The high OER overpotential can be attributed to the poor OER activity of pure NiOOH derived from Ni3N in purified electrolyte.37 The incidental incorporation of Fe impurities from unpurified electrolytes can increase the NiOOH conductivity and modify the NiOOH electronic structure, resulting in OER activity enhancement.37,39,81 As almost all the reported Ni3N/Ni foams were tested in unpurified electrolytes, Fe impurities were likely incorporated into the Ni3N-derived nickel (oxy)hydroxide electrocatalysts, resulting in the relatively low OER overpotentials (365 ± 24 mV at 20 mA cmgeo−2).22,26,42,55 Conversely, we used a purified electrolyte which prevented Fe incorporation and thus our Ni3N-derived nickel (oxy)hydroxides behaved electrochemically like a pure nickel (oxy)hydroxide with poor OER performance (see Fig. S13 and corresponding descriptions in the ESI†). Notably, contrary to previous findings in which samples are tested in purified KOH but pre-oxidized in unpurified KOH,22 our results and those of previous reports show that incorporation of trace Fe from the KOH electrolytes is essential if a Ni3N OER precatalyst is to achieve notable OER activity (more details are available in the descriptions below Fig. S13 in the ESI†).
To examine the intrinsic OER activity of the Ni3N/Ni and Ni foams after 1000 cycles, the current densities of each calculated using the ECSA at the overpotential of 450 mV are demonstrated in Fig. S15 (ESI†). From the XPS results (Fig. 4 and Fig. S7, ESI†), both the post-OER Ni3N/Ni and Ni foams possess similar Ni(OH)2/NiOOH surfaces, and thus one would expect that both the samples might show the same intrinsic OER activity. We find, however, that the Ni foam exhibited a higher intrinsic OER activity (−0.21 mA cmECSA−2) than the Ni3N/Ni foam (−0.05 mA cmECSA−2) (Fig. S15, ESI†). This intrinsic OER activity difference can be accounted for by the difference in the total electrode conductivity. As displayed in Fig. S13 (ESI†), the Ni oxidation peak intensities of the Ni3N sample were much larger than those of the Ni metal sample, indicating a larger yield of nickel oxide/(oxy)hydroxide for the Ni3N sample. In addition, the O/Ni atomic ratio of the post-OER Ni3N/Ni foam (0.320) was much higher than that of the post-OER Ni foam (0.013) (see Table S1, ESI†). Therefore, after the long-term OER CV tests, the as-formed nickel oxide/(oxy)hydroxide shell over Ni3N was probably thicker than the shell over the Ni metal, or at least the amount of the as-formed nickel oxide/(oxy)hydroxide over Ni3N may be greater than that of the as-formed nickel oxide/(oxy)hydroxide over Ni (see Fig. S14, ESI†). This thick oxide/(oxy)hydroxide shell over Ni3N may reduce hole access at surface-electroactive sites (reducing the electrical conductivity), thus lowering the intrinsic OER activity. Based on these results, we can also predict that compared to Ni metal, Ni3N may experience a higher extent of self-oxidation during the OER process.
Next, we performed density functional theory (DFT) calculations for the reaction on Ni3N and pure Ni metal surfaces. Specifically, Ni3N(100), Ni3N(110), and Ni(111) were analyzed with the computational hydrogen electrode (CHE) method with the OER mechanism proposed by Man et al. (Fig. 6).82 Herein, using the DFT results, we attempted to explain the oxidation resistances of Ni3N and Ni metal rather than their intrinsic OER activities. As demonstrated in Fig. 6a, since the first steps are exothermic, Ni3N(100) and (110) can easily be covered with HO* and O*. Compared to the first and second steps (i.e., formation of HO* and O*), HOO* formation on Ni3N(100) and (110) is much more endothermic, resulting in partial oxidation of Ni3N to Ni–O/Ni–OH during the OER. In the case of Ni(111), as HOO* formation is much more endothermic than the formation of HO* and O*, partial oxidation of Ni metal to Ni–O/Ni–OH may also occur. However, Ni(111) may experience slight difficulty in the formation of HO* and O* as the first step is more endothermic compared to that of Ni3N. This is due in part to the lower coordination number of Ni-sites on Ni3N structures compared to that on close-packed pure Ni metal surface, resulting in stronger binding energies to these oxygen-containing species. Consequently, Ni3N may have a lower oxidation resistance than Ni metal, in agreement with our experimental observations.
First, we seek to explain the OER CP self-oxidation behaviors of Ni3N and Ni metal by using the Ni2+/Ni3+ oxidation peaks of the samples (Fig. 7a–d). After the 20 h CP tests, the Ni2+/Ni3+ oxidation peaks of both the Ni3N/Ni and Ni foams were more or less the same intensity (Fig. 7c and d), implying that the self-oxidation degree of Ni3N might be nearly equal to that of Ni metal (Fig. S21, ESI†). These similar degrees of self-oxidation are mainly due to the different applied potentials during the OER CP tests. Specifically, the Ni3N/Ni foam showed a lower applied potential (∼1.47 VRHE) than the Ni foam (∼1.67 VRHE) at a constant current density of 10 mA cmgeo−2, which may enable similar degrees of self-oxidation for Ni3N and Ni metal. More interestingly, both the Ni3N/Ni and Ni foams experienced slight increases in the intensity of the Ni2+/Ni3+ oxidation peaks during the long-term OER tests. However, the degree of increase observed in the Ni oxidation peaks during the OER CP tests is much smaller compared with those in the OER CV tests. These suppressions of the Ni oxidation peak intensity increases for the OER CP tests can be explained by the following two reasons: (i) reduction of structural damage in the OER process by the use of the CP technique and (ii) enhancement of electrical conductivity due to the Fe impurity incorporation. As described earlier, the CV-driven OER test involves the crystal structural changes [i.e., Ni(OH)2 ↔ NiOOH] of the self-oxidized Ni3N and Ni surfaces caused by the anodic and cathodic sweeps, which leads to crack formation.79 The as-formed cracks likely allow the electrolyte to penetrate the self-oxidation-derived passivation layer all the way to the un-oxidized initial material (i.e., Ni3N, Ni metal) and induce its further self-oxidation. In contrast, the CP-driven OER test does not require the surface Ni2+/Ni3+ redox reactions because of the roughly constant applied potentials. Thereby, the CP technique can restrain the crystal structural damage, electrolyte penetration, as well as self-oxidation. Fig. S22 shows the SEM images of the post-OER CP Ni3N/Ni and Ni foam surfaces. Especially for the Ni3N/Ni foam, the post-OER CP sample (Fig. S22a, ESI†) seemingly has a lower porosity than the post-OER CV sample (Fig. 3b), which represents evidence for the restrained structural damage. Furthermore, the self-oxidation of the precatalyst (i.e., Ni3N, Ni metal) during the OER may generally occur due to holes accumulated by the applied bias near the electrode surface. Thus, by increasing the electrical conductivity of the electrode surface, the holes can be utilized effectively for the OER and the self-oxidation may be suppressed. According to the study reported by Trotochaud et al.,37 Fe incorporation can increase NiOOH conductivity 30–60-fold (with 5–25% Fe), which may also lower the degree of self-oxidation. Incidentally, Trotochaud et al. also found that a NiOOH sample tested in unpurified KOH electrolyte possesses σ ≈ 2.5 mS cm−1, whereas a NiOOH sample under Fe-free conditions has values for σ ≈ 0.1 to 0.2 mS cm−1.37
To gain a better understanding of the OER activity transition during the OER CP testing, Fig. 7g and h show the OER overpotentials and CV-derived Cdl values as a function of time for the Ni3N/Ni and Ni foam electrodes. Unlike the findings in the purified electrolyte, both samples exhibited continuous reductions in the OER overpotential in the unpurified electrolyte. This OER activity enhancement is probably achieved by the continuous increases in intrinsic OER activity and ECSA owing to gradual Fe impurity incorporation and layered nickel (oxy)hydroxide formation, respectively.
To more deeply probe the intrinsic OER activity for each sample, we calculated the ECSA-based current densities at the overpotential of 450 mV. As seen in Fig. S15 and S23 (ESI†), the Ni3N/Ni and Ni foams in unpurified electrolyte reached much higher intrinsic OER activities (Ni3N/Ni foam: −0.76 mA cmECSA−2; Ni foam: −3.1 mA cmECSA−2) compared with the foams in the purified electrolyte (Ni3N/Ni foam: −0.05 mA cmECSA−2; Ni foam: −0.21 mA cmECSA−2). These OER activity improvements can be caused by the Fe impurities from unpurified electrolytes and enhanced mass transfer due to the use of the flow cell.37,39,68 Curiously, although both samples have the same surface chemical compositions (see Fig. S19, ESI†) and similar amounts of self-oxidized species [Ni(OH)2/NiOOH], the Ni foam demonstrated a higher intrinsic OER activity in comparison with the Ni3N/Ni foam after the OER CP tests in the unpurified electrolyte. This difference in intrinsic OER activity might be because of the different Fe incorporation level per ECSA for each sample. Klaus et al. reported that the Fe incorporation level increases logarithmically with respect to aging time for which a Ni(OH)2 electrode is immersed in unpurified 1 M KOH aqueous solution (in the absence of applied potential).39 In this work, since we immersed the Ni3N/Ni and Ni foams in unpurified electrolyte for almost same time (at least 20 h) during the OER CP tests, it is likely the total Fe uptake within both foams will be almost the same. However, because of the higher ECSA of the Ni3N/Ni foam, the Ni foam probably reaches a higher Fe incorporation level per ECSA than the Ni3N foam, resulting in the higher intrinsic OER activity of the Ni foam. To support this possibility, we show the magnified LSVs (Fig. 7c and d). In general, it is well-known that an anodic shift of the Ni2+/Ni3+ oxidation peaks and an increase in OER activity happen with increasing the Fe incorporation level within nickel oxyhydroxide (or increasing x in Ni1−xFexOOH).37,39,83–85 During the long-term OER CP tests in unpurified electrolyte, both the Ni3N/Ni and Ni foams showed the Ni2+/Ni3+ oxidation peak shifting and OER activity enhancement, which is evidence for the Fe uptake within our electrodes. However, compared with the Ni3N/Ni foam, the Ni foam exhibited larger anodic shifts of the Ni2+/Ni3+ oxidation peak during the OER test, indicating a higher x value in Ni1−xFexOOH (or a higher Fe incorporation level per ECSA). For this reason, the Ni foam obtained a higher intrinsic OER activity than the Ni3N/Ni foam after the OER CP testing. It is worth noting that Ni1−xFexOOH species act as OER electroactive sites for the Ni3N and Ni metal precatalysts in unpurified electrolyte.
Here, we also discuss a possible mechanism for Fe incorporation into the self-oxidized Ni3N/Ni and Ni foam electrodes. According to a previous report,38 initially, Fe impurity in unpurified electrolyte is incorporated at the edges/defects of the Ni3N- and Ni-derived NiOxHy nanoflakes. Subsequently, the Fe impurity is further incorporated into the bulk of the NiOxHy nanoflakes while Fe also remains at their edge/defect sites. Importantly, the Fe impurity is incorporated throughout the electrolyte-permeable regions of the Ni3N/Ni and Ni foam electrodes (self-oxidized part: NiOxHy layer).86
Finally, we further discuss the geometric OER activity of the foam-type electrodes in unpurified electrolyte. After the long-term OER CP tests, the Ni3N/Ni foam showed a higher geometric OER activity than the Ni foam despite its lower intrinsic OER activity (Table S2, ESI†). This higher geometric OER activity of the post-OER CP Ni3N/Ni foam may result from its higher ECSA. Furthermore, as listed in Table S2 (ESI†), the geometric OER activity of our Ni3N/Ni foam (in a flow cell) compares favorably with those of other reported foam-type Ni3N samples (in a standard three-electrode cell). This may be the result of the improved mass transfer due to the use of a flow cell.57,68 From the comparisons to literature values, the use of a flow cell is effective for further enhancing the geometric OER activity. This strategy can also be adapted to other foam-type electrocatalysts.
This study investigated Fe impurity incorporation effects on the OER activity of a Ni3N precatalyst. It is noteworthy that geometric OER activity degradation was observed after the OER CV test in the purified electrolyte whereas the geometric OER activity continuously increased during the OER CP test in the unpurified electrolyte. These different behaviors are likely caused by the presence or absence of Fe impurities in the electrolyte. From our findings, we would expect other Ni-based OER precatalyst performances to also be affected for the better by Fe impurities from the electrolyte, for which further investigation is necessary in the future.
Footnote |
† Electronic supplementary information (ESI) available: Experimental details, digital photographs, schematic illustrations, electrochemical stability map, additional XRD, SEM, EDX, and XPS results, and cyclic voltammograms. See DOI: 10.1039/d1ma00130b |
This journal is © The Royal Society of Chemistry 2021 |