Chapter 2. Nanostructured rhenium–carbon composites as hydrogen-evolving catalysts effective
2.3 Results and discussion
2.3.2 Electrocatalytic activity toward Hydrogen Evolution Reaction (HER)
(black), the ReO2−NPC/a-C (green), and the Re−NPC/a-C (blue). (e) HAADF-STEM image of a Re−NPC/a-C (scale bar = 20 nm) along with corresponding EDS mapping images for (f) Re and (g) C.
Figure 2.6Polarization curves of bulk Re powder (99.999%, Alfa Aesar), ReO2−NPC/a-C, Re−NPC/a- C, and Pt/C in (a) 0.5 M H2SO4aqueous solution and (b) the corresponding Tafel plot. (c) Polarization curves in 1 M KOH aqueous solution and (d) the corresponding Tafel plot. (e) Polarization curves in 1 M PBS aqueous solution and (f) the corresponding Tafel plot. (g) Electrochemical impedance spectra of different electrodes at overpotential 0.17 V versus RHE in 0.5 M H2SO4 and with an alternating current (AC) amplitude of 5 mV. (Inset) Equivalent circuit used for data analysis. Rs, solution resistance;
Rct, charge transfer resistance; CPE, constant-phase element; and Zw, Warburg impedance. Figure 2.13 shows the full impedance spectra for bulk Re powder and ReO2−NPC/a-C in 0.5 M H2SO4. (h) Capacitive current measured by CV at 0.15 V vs RHE (ΔJ0.15 V), plotted as a function of scan rate in 0.5 M H2SO4.
Next, we obtained the Tafel plots from the LSV polarization curves (Figure 2.6b,d,f). In control experiments, the Tafel slope recorded for Pt/C is 29.7 mV dec−1in 0.5 M H2SO4, 46.3 mV dec−1in 1 M KOH, and 47.9 mV dec−1in 1 M PBS; these are in agreement with the reported values and imply that H2is evolved at the Pt/C electrode according to the Volmer−Tafel reaction mechanism (Volmer step, H+(aq) + e−→ Hadin acid, H2O (l) + e− → Had+ OH−(aq) in base; Tafel step, 2 Had→ H2(g); and Heyrovsky step, H+(aq) + Had+ e−→ H2(g) in acid, H2O (l) + Had+ e−→ H2(g) + OH−(aq) in base).
The Tafel slopes for the Re−NPC/a-C are 56.3 mV dec−1in 0.5 M H2SO4, 53.8 mV dec−1in 1 M KOH, and 69.6 mV dec−1 in 1 M PBS, indicating fast kinetics of electrochemical hydrogen adsorption and desorption processes in acidic media, and water adsorption and dissociation processes in alkaline and neutral media. HER kinetic models suggest that the Tafel slope of about 30, 40, or 120 mV dec−1will be obtained when the Tafel, Heyrovsky, or Volmer reactions are the rate-determining steps (rds), respectively.55,74The fact that the Tafel slopes of the catalyst are close to the theoretical value of 40 mV dec−1and above 38 mV dec−1implies that the Heyrovsky-step-determined (electrochemical desorption of Hadbeing the rate limiting step) Volmer−Heyrovsky reaction mechanism may be dominant at the Re−NPC/a-C electrode.14,15In the Volmer (discharge) reaction, the fact that the Tafel slopes are ∼60 mV dec−1indicates that the adsorption of hydrogen at an electrode follows the Temkin-type adsorption isotherm.56−59Additionally, the values of 53.8 and 69.6 mV dec−1in basic and neutral media indicate facile dissociation of water in the Volmer step, the initial stage of HER.60 Overall, the observed overpotential and Tafel-slope parameters are, expectedly, worse than those for Pt (η = ∼0 mV and η10
= 30 mV vs RHE) and recently reported Ru-based catalysts15but are significantly better than those for other known Re-based HER catalysts31−36. In addition, although catalytic HER activities in nonacidic media are usually about 2-3 orders of magnitude lower than in acidic ones with significantly increased overpotential,61−64the unique feature of the Re−NPC/a-C nanocomposite is that it performs comparably well over the entire pH range, from acidic, through neural, to basic. Of note, Re−NPC/a-C performs significantly better than bulk Re powder (orange curves in Figure 2.6a,c,e), evidence that this improved the performance for mass loading of 0.283 mg cm−2geometric; whereas comparisons against different mass loadings of Re powder are included in Figure 2.14. Such results indicate that combining nanosized Re particles with conductive a-C not only exposes abundant catalytic sites but also improves electrical conductivity.65Following this logic and to further enhance electrical conductivity of the catalysts, the clustered Re nanoparticles were interconnected with MWNTs (electrical resistivity, ∼3.00 × 10−5 Ω·cm66,67). The samples were prepared by the mixing of Re−NPC/a-C and MWNTs. The SEM images in Figure 2.15 show that in the materials thus made, the Re nanoclusters are enmeshed in a web of MWNTs. Importantly, the HER activity of these Re−NPC/a-C/MWNT constructs is further improved in both acidic (η = 85 mV, η10= 107 mV, and Tafel slope = 43.5 mV dec−1) and alkaline (η = 80 mV, η10= 107 mV, and Tafel slope = 50.7 mV dec−1) solutions and roughly unchanged in a neutral medium (η = 115 mV, η10= 163 mV, and Tafel slope = 69.2 mV dec−1) (Figure 2.16).
The faster kinetics of electrocatalysis is expected to correspond to lower charge-transfer resistance (Rct). To study charge-transfer effects, we conducted a series of electrochemical impedance spectroscopy (EIS) studies at frequencies varying from 0.1 to 100 000 Hz. The Re−NPC/ a-C catalyst shows an Rctvalue of 19.8 Ω in acidic solution (Figure 2.6g) and exhibits a much lower value of Rct
than ReO2− NPC/a-C (530.6 Ω) or bulk Re powder (1042.3 Ω). A similar trend is observed in basic and neutral electrolytes. Comparison between bulk Re powder and Re−NPC/a-C suggests that the electrically conductive a-C phase (resistivity 5.0-8.0 × 10−4 Ω·m68,69) not only supports the Re NPs within the high-surface-area berry-shaped clusters but also facilitates charge transfer at the nanocomposite’s surface.65,70We observe that under all conditions, the value of Rctfor Re−NPC/a- C/MWNT is lower than that for Re−NPC/a-C, which is in line with the expected role of MWNTs in enhancing the charge transfer at the material’s surface. Although geometric current density is a well- known and accepted measure of total catalytic activity from the catalyst electrode, we also analyzed current density normalized by ECSA to study the intrinsic activity of the samples. The ECSA values are determined by eq 7.71−76
ECSA = Cdl/Cs (7)
where Cdl and Cs are the double layer capacitance and the specific capacitance (for description of measurements and experimental data, see the Experimental section 2.5.4 and Figures 2.6h). In our case, it must be noted, that a-C present in the nanocomposites also contributes to the Cdl. Therefore, rather than directly comparing ECSA or Cdl values of Re−NPC/a-C vs bulk Re powder, we analyzed the intrinsic activity by dividing geometrical current densities by their respective ECSA. Figure 2.17shows the HER polarization curves normalized with respect to ECSA (for Re−NPC/a-C/ MWNT composite, see Figures 2.18and 2.19). In acidic media, the catalytic currents per ESCA for Re−NPC/a-C are much larger than those for the bulk Re powder (Figure 2.17a). The enhanced catalytic activity for Re−NPC/a- C is also verified by a lower Tafel slope exhibiting faster kinetics (Figure 2.17d). Enhanced intrinsic catalytic activity is also observed in basic and neutral media (Figure 2.17b,c,e,f), though it is not as pronounced as in the acidic media.