Clima Organizacional y liderazgo transformacional

Based on promising performance of the p-Si MW in regenerative electrochemical cells dis- cussed in Chapters 3 and 4, and the dark catalytic performance discussed above, p-Si planar

and MW array electrodes with various HER catalysts were investigated as photoelectrodes for hydrogen evolution.

Due to the differences in light absorption and diffusion within an array of Si wires relative to the planar samples, new deposition conditions were needed to control the particle size and

uniformity for the high aspect-ratio wires. Uniform distributions of Pt, Ni, and Ni–Mo metal nanoparticle films were produced by electrodeposition and by using galvanic displacement

deposition, in which metal ions from solution are reduced as the Si is slowly etched by hydrofluoric acid (HF) without any external bias.[83]

5.4.1 Catalyst deposition

By modifying the deposition procedures from the literature, we were able to achieve uni- form distribution of nanoparticles of several metals that have high HER exchange current

densities on the high aspect ratio Si microwires.

Figure 5.4. SEM images of (L to R): Pt, Ni, and Ni–Mo nanoparticles deposited on p-Si MW arrays

Pt deposition. Pt nanoparticles were deposited from an aqueous solution of hydrofluoric acid and chloroplatinic acid (H2PtCl6) or K2PtCl6.[78] The HF concentration in the plating bath was varied between 0.25 M and 1.0 M, while the concentration of Pt salt was varied between 0.25 and 1 mM. Decreasing the concentration of HF increased the uniformity

of deposition along the length of the wires, while decreasing the Pt salt concentration resulted in more uniform particle sizes. Pt has been shown to follow a progressive nucleation

mechanism, so at higher concentrations, new particles are continuously forming on the

surface.[84] Further experiments used 0.5 M HF and 1.0 mM K2PtCl6. Deposition was carried out by immersing the electrode in the plating solution for 2 minute increments, and

then testing the electrochemical performance of each electrode. The optimal deposition time (maximized catalytic activity without adding too much catalyst to block the light)

was between 4 and 6 min. Pt was also evaporated onto p-Si samples using an e-beam evaporator, but no photovoltage was observed for any of the samples with evaporated Pt.

Ni deposition. Ni was deposited onto electrodes using either electrodeposition or elec- troless displacement. Nanoparticles were electrolessly deposited from an aqueous solution

of ammonium fluoride (NH4F) and nickel sulfate (NiSO4). Due to the instantaneous nu- cleation method for Ni, the particle sizes were much smaller than observed for Pt (center

image Fig. 5.4).[85] Deposition was carried out by immersing the Si sample in the electroless deposition bath for 4 min and then testing the electrochemical performance. Ni electrode-

position was carried out by depositing Ni from a bath that contained 1.3 M NiIIsulfamate and 0.5 M H3BO3. Depositions were carried out galvanostatically under illumination (20 mA cm−2 for 0.5 to 5 s).[79]

Ni–Mo electrodeposition. Ni–Mo was electrodeposited from a bath that contained

1.3 M NiII sulfamate, 0.5 M H3BO3, and 20 mM Na2MoO4, with the pH adjusted to be ∼ 4.5 using KOH.[79] Electrodeposition was performed using a PAR 273 potentiostat,

in a one-compartment cell, using a large area Ni foil counter-electrode and an Ag/AgCl reference electrode. Samples were immersed in the deposition solution and illuminated at

high intensity (∼ 400 mW cm−2) with an ELH-type tungsten-halogen lamp. Ni–Mo was deposited under galvanostatic conditions: 20 mA cm−2for 20–30 s (right image Fig 5.4).[79]

5.4.2 Planar Si with HER catalysts

Initial photoelectrochemical experiments were performed using planar p-Si electrodes coated with electrolessly deposited Pt nanoparticles in contact with acidic (pH = 2) electrolyte (Fig.

5.5). Performance was optimized at this pH between the catalytic activity of Pt (better in acid) and the measuredVoc, which decreased in more acidic electrolyte.[86] The electrodes showed an onset in photocurrent positive of RHE, which varied with illumination intensity,

-0.2 -0.1 0 0.1 0.2 0.3 Potential (V vs. RHE) -20 -15 -10 -5 0 11 23 48 100

Current Density (mA cm

-2)

Illumination (mW cm-2₎

Figure 5.5. Left: SEM image of electrolessly deposited Pt on planar p-Si. Right: J−Edata for planar p-Si electrodes with electroless Pt as the HER catalyst in a pH = 2 aqueous solution (0.5 M K2SO4 adjusted with H2SO4). Scan rate = 20 mV s−1. The cell was purged with H2 and the data are referenced to RHE = -0.362 V vs. SCE. Illumination = ELH-type solar simulation (intensity adjusted using neutral density filters).

but produced less than 300 mV Voc under 1 Sun illumination, similar to reports in the literature. All of the figures of merit for these devices are listed in Table 5.1.

To compare the performance of different catalysts on planar p-Si, electrodes were tested

in pH = 4.5 KHP with 0.5 M K2SO4supporting electrolyte to ensure stability of all catalysts. The left of Fig. 5.6 compares all three catalysts under 1 Sun ELH illumination.

5.4.3 p-Si MW with HER catalysts

Similarly, p-Si MW electrodes were compared with each of the catalysts under 1 Sun il- lumination in pH = 4.5 KHP with 0.5 M K2SO4 supporting electrolyte. Electrodes were fabricated from p-type Si MW arrays after catalyst removal, and then coated with catalyst immediately before electrochemical testing. The electrodes were tilted slightly to maximize

the measuredJsc. Figure 5.6 showsJ−E data for similar wire arrays coated with different catalysts. While all of the catalysts increased the onset of the HER by over 500 mV relative

to bare p-Si MW arrays, the photovoltage generated positive ofE(H+/H2), never exceeded 300 mV under 1 Sun illumination. The Voc for Ni–Mo electrodes was similar to that of Ni, but less than that of Pt, even though the catalytic onset andffs were better than pure Ni. Due to the small photovoltages generated, the overall energy conversion efficiencies for

-0.1 0 0.1 0.2 Potential (V vs. RHE) -25 -20 -15 -10 -5 0 Pt disk Electroless Pt 240s Electrodep NiMo 20s Electrodep Ni 0.5s

Current Density (mA cm

-2) -0.6 -0.4 -0.2 0 0.2 Potential (V vs. RHE) -14 -12 -10 -8 -6 -4 -2 0 no cat NiMo Pt Ni

Current Density (mA cm

-2 )

Figure 5.6. Left: J−E data for planar p-Si electrodes with Pt, Ni, or Ni–Mo as the HER catalyst. Right: J−E data for Ni, Ni–Mo, and Pt on p-Si MW electrodes. All data collected in a H2-purged pH = 4.5 KHP buffer and are referenced to the reversible hydrogen potential in the solution (E(H+/H2) = -0.51 V vs. SCE).[79]