In operando x-ray photoelectron spectroscopy studies of H₂ oxidation and H₂O electrolysis on gadolinia-doped ceria electrodes

Solid oxide electrochemical cells (SOCs) offer a means for clean and efficient conversion of chemical energy into electricity as fuel cells or a means of storing excess electrical energy into chemical fuels as electrolysis cells. Gadolinia-doped ceria (GDC) has been investigated widely for its potential as an electrolyte and electrode material in SOCs operating at intermediate temperatures between 500 °C and 700 °C [1–9]. Over this temperature range, GDC has an oxide-ion conductivity that is approximately 1–2 orders of magnitude higher than yttria-stabilized zirconia (YSZ) [2], which has been the most common SOC electrolyte material. GDC's higher ionic conductivity lowers ohmic resistances associated with bulk-ion transport in SOCs. Under reducing environments associated with the fuel electrode, reduction of some cerium cations (from Ce⁴⁺ to Ce³⁺) results in mixed ionic-electronic conductivity (MIEC) [10–12]. The MIEC behaviour lowers open circuit voltages (OCVs) due to electronic leakage, particularly at temperatures above 600 °C, which reduces efficiency in GDC-electrolyte fuel cells. On the other hand, the mixed conductivity enhances catalytic activity in composite metal/GDC electrodes for fuel oxidation (including carbonaceous fuels) and H₂O electrolysis, in part by extending the surface area available for the electrochemical reactions [13–16].

Extensive studies have characterized bulk transport and thermodynamic properties of GDC under conditions relevant for its application as either an electrolyte or a component in a composite SOC fuel electrode [17–20]. Some studies have used thin films to indirectly resolve near-surface oxide vacancies for GDC as a function of effective O₂ partial pressure (P_O2) and temperature characteristic of SOC electrodes [21, 22]. Although these studies correlated partial GDC reduction with MIEC behaviour and electrochemical activity due to high concentration of near-surface oxide vacancies, development of reaction mechanisms for GDC-based composite fuel electrodes requires more quantitative surface measurements of GDC in operando with relevant composite materials.

Ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) has received notable attention in recent years for studying high-temperature, electrochemical reactions in operando. Researchers have used this technique to study ceria and doped ceria under low effective P_O2 characteristic of fuel electrodes in SOCs [14, 15, 23–25]. In operando AP-XPS can measure surface oxidation states associated with electrochemical reactions, as shown in recent noteworthy studies on ceria-based electrodes to explore H₂/H₂O or CO/CO₂ electrochemistry [26–28]. These studies provided insight into the reducibility of ceria surface cations as a function of overpotential, temperature, and gaseous composition. On the other hand, they have not provided significant insight into the interactions between ceria and metal current collectors or within the metal/ceria-based composite electrodes (which can promote catalytic and charge transfer reactions). Understanding metal/GDC interactions is critical for developing quantitative electrochemical mechanisms for GDC-based electrodes such as Ni/GDC composites, which are now being used in intermediate temperature fuel cells. With appropriately designed metal/GDC electrodes, in operando AP-XPS can measure surface potentials and changes in some surface species fractions near the metal/GDC interfaces.

In this study, simultaneous electrochemical measurements and AP-XPS were performed at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory on thin-film Ni/GDC and Au/GDC electrodes operating for H₂ oxidation and for H₂O electrolysis [29, 30]. Spatially resolved measurements of the O 1s spectra and binding energy shifts for local surface potentials across the metal/GDC interfaces in combination with simultaneous electrochemical measurements provided critical information on the nature of the electrochemical reactions as a function of temperature and P_O2 characteristic of fuel electrodes in SOCs. With the increased catalytic activity for Ni compared to Au, differences in the electrochemical performance of the two different electrodes provided an indication on how catalytic activity on the Ni surface can significantly impact the GDC surface chemistry under conditions where reduced electron conductivity confines reactions closer to the metal/GDC interface. The AP-XPS measurements with extensive electrochemical characterization for H₂ oxidation and H₂O electrolysis on the thin-film electrodes at similar operating conditions provided mechanistic insight into the nature or electrochemical reactions across metal/GDC under reducing environments.

2.1. Electrochemical cell fabrication and mounting

Single-chamber SOCs in this work were fabricated by radio frequency (RF) sputtering (Kurt Lesker PVD 75) of thin-film GDC on polycrystalline YSZ-support substrates. The substrates were made from YSZ powder (Tosoh, 8% by mole of Y₂O₃) pressed in a cylindrical die at 1400 bar. The resulting disk-shape pellet was machined into a square shape with interconnected grooves across the length of one side to provide a location for subsequent deposition of a platinum (Pt) counter electrode (CE) on the opposing side of the GDC thin-film working electrodes (WEs). Figure 1(a) shows how the grooves provided the CE with adequate exposure to gases in the single-chamber cell tests at the AP-XPS facility, where a ceramic contact heater on the CE side controlled the cell operating temperature. The machined-YSZ substrate was sintered at 1450 °C for 3 h to densify with dimensions of 1 mm thick and 10 mm × 10 mm in top-side surface area. The low current density demanded by the thin-film electrodes permitted the use of the thick YSZ electrolyte support. Two 300 nm thick films (2.5 mm × 4.0 mm in area) of GDC (20% by mole of Gd₂O₃, Gd_0.2Ce_0.8O_1.9) were deposited by RF-sputtering on the flat topside of the YSZ substrate. The two GDC thin films were separated by a 1.8 mm gap on the YSZ surface to avoid electrochemical interactions during independent electrochemical testing of each WE.

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Ni and Au (~300 nm thick) metal overlayers were sputter-deposited (AJA International, ATC 1800-V) separately on the two GDC films with a 300 μm border of exposed GDC around the edges of the overlayers. The deposited Ni and Au overlayers were annealed at 750 °C in 5% H₂ in volume (balanced with Ar) such that the overlayers formed interconnected open structure exposing pockets of the GDC surface over the entire area of each WE as illustrated in scanning electronic microscopy (SEM) images for a GDC thin-film WE with a Ni overlayer in figure 2(a) and with an Au overlayer in figure 2(b). The annealed metal overlayers consisted of 1–2 μm wide structures with adequate interconnections to provide both good in-plane conductivity across the thin-film WE and sufficient exposure of the GDC surface for significant length of the triple-phase boundary (TPB) between the metal, GDC, and gas phases on the WE surface. As such, the interconnected Ni and Au overlayers served both as current collectors and as a possible facilitator of charge transfer reactions between the metal and GDC as illustrated in figure 1(b).

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The substantial TPB can be characterized by the length per unit geometric area of the GDC surface (L_tpb). To characterize the L_tpb of both metal/GDC interfaces, scanning electron micrographs, such as those shown in figure 2(a) for the Ni overlayer and figure 2(b) for the Au overlayer, were processed to identify edges of the porous metal overlayer on the GDC surface. Image processing was done in MATLAB and the 'Edge' function was used with the Canny algorithm to detect edges with the porous metal on the GDC surface. In general, the annealing of the two different metal overlayers led to slightly smaller length scales and thus higher L_tpb for the Ni overlayer than for the Au. For this study, average L_tpb was 1.87 × 10⁶ m for Ni/GDC and 1.23 × 10⁶ m for Au/GDC, per unit surface area (in m²) of the GDC thin films covered by the Ni or Au overlayer. The 50% higher L_tpb for Ni/GDC would improve rates for any rate-limiting reactions involving charge and/or species transfer between the metal and the GDC surface. On the other hand, for reactions occurring completely on the GDC surface, L_tpb should play a very limited role in rates.

A 100 nm thick Pt CE was sputter-deposited (AJA International, ATC 1800-V) in the grooves on the backside of the YSZ as shown in figure 1(a). The Pt CE was fired at 800 °C to form a porous overlayer on the YSZ surface and thereby facilitate effective charge transfer for both H₂O electrolysis and H₂ oxidation on the CE. The grooves, in which the Pt was deposited, facilitated gas transport to and from Pt/YSZ/gas TPB on the CE, while allowing the cell to maintain good thermal contact with the surface heater during in operando XPS experiments. A Pt wire was attached to the Pt CE using a Pt paste and brought around to topside of the YSZ where a Pt mesh for electrical contact was fixed on an alumina blocking layer deposited on the topside of the YSZ substrate to facilitate an external electrical-lead connection. In the AP-XPS test station at ALS, the cell was mounted with three probes pressing separately onto the Ni overlayer, the Au overlayer and the Pt mesh to ensure good electrical connections as well as thermal contact between the backside of the cell and the ceramic heater as shown in figure 1(c).

2.2. Electrochemical characterizations

2.3. X-ray photoelectron spectroscopy measurements

Figures 3(a) and (b) compare the electrochemical performances for the GDC WEs with the Ni overlayer (i.e. Ni/GDC electrode) and with the Au overlayer (i.e. Au/GDC electrode) during in operando XPS measurements at ALS for different T.

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The IR_bulk-corrected potentials (V_{cell −} IR_bulk) subtract the ohmic overpotential from V_cell, where R_bulk is extracted from high-frequency impedance measurements. Because the tests were in a single chamber filled with mixtures of H₂ and H₂O, OCV was very close to 0 V for the H₂/H₂O electrochemistry. Negative V_{cell −} IR_bulk (positive/anodic current) indicated H₂ oxidation on the WE whereas positive V_{cell −} IR_bulk (negative/cathodic current) indicated H₂O electrolysis on the WE. Since both WEs shared the same YSZ electrolyte support and Pt CE, differences between V–I curves in figures 3(a) and (b) at the same operating conditions were attributed to differences in electrochemical behaviour of the Ni/GDC and Au/GDC electrodes.

Comparing figures 3(a) and (b) reveals that the Ni/GDC electrode achieved significantly higher current with increasing magnitude of V_cell, both for H₂O electrolysis (V_cell > 0) and particularly for H₂ oxidation (V_cell < 0). This ratio of currents for large positive and negative V_cell is much greater than the 50% increase in L_tpb of the Ni/GDC electrode in comparison to the Au/GDC electrode. For H₂O electrolysis, the Ni/GDC electrode shows smaller activation overpotentials at small V_cell than the Au/GDC electrode. The Ni/GDC interface enhances charge transfer reaction rates due to the improved ability of Ni to adsorb H and OH on its surface much more effectively than Au [37]. At P_H2 = P_H2O = 0.26 mbar and T = 620 °C, the Ni/GDC WE supported current densities of −8.7 mA cm⁻² at V_{cell −} IR_bulk = 0.5 V and 4.0 mA cm⁻² at −0.5 V (based on the area of the porous Ni overlayer). At the same conditions, the Au/GDC WE supported current densities of −1.9 mA cm⁻² at +0.5 V and 0.7 mA cm⁻² at −0.5 V. Under all conditions tested at the ALS, the Ni/GDC maintained significantly higher currents than the Au/GDC electrode with the exception of highly negative cell voltages at the lower T = 570 °C where the current densities of Ni/GDC and Au/GDC electrodes are within a factor of 2 which is close to the differences in L_tpb per area of the two WEs. The similar performance at highly oxidizing conditions (large negative V_cell) may be due to significant build-up of oxygenated species on the Ni surface reducing Ni/GDC charge transfer [38]. Non-oxidized H⁺ transfer across the Ni/GDC interface has been assessed to be the principal charge transfer reaction for H₂ oxidation and H₂O electrolysis [39, 40], and the increased affinity for H on Ni surfaces (vs. Au) provides higher exchange current densities and thus, lower activation overpotentials (η_act,WE) for both forward and reverse reactions.

To explore the role of the metal and GDC surfaces on H₂ electrochemical oxidization and H₂O electrolysis, AP-XPS measurements were taken during electrochemical excitations to probe surface states on the metal and GDC as a function of V_cell. AP-XPS of electrochemically active surfaces measures shifts in core-level binding energies of specific spectral peaks, which correlates with changes of surface electric potentials [14, 15]. In this study, shifts of the O 1s binding energy spectra across the metal/GDC/YSZ interfaces around the uncoated GDC film border measured changes in surface potentials as a function of V_cell. Since the metal electrodes were grounded, the measured binding energy shifts provided measures of η_act,WE associated with charge transfer reactions across the metal/GDC interface phases. Figures 4(a)–(c) show linear mapping of the relative counts per second (CPS) of the O 1s spectra as a function of position along the Ni/GDC/YSZ border for WEs operating at P_H2 = P_H2O = 0.26 mbar and T = 620 °C. For highly positive V_cell (= 1.0 V) in figure 4(a), positive shifts of the O 1s peak on the GDC were small but non-negligible. At these conditions (as discussed further below), the GDC is significantly reduced and thus provides higher polaron concentrations, which facilitates electron transport from as well as H⁺ transport to the Ni electrode. On the other hand, much larger negative binding energy shifts with negative V_cell (−1.0 V) in figure 4(c) revealed a larger η_act,WE for a similar current across the Ni/GDC interface, indicating increased transfer resistance for H₂ oxidation (vs. H₂O electrolysis) on the Ni/GDC electrode. At negative V_cell, the GDC has lower electronic conductivity and thus likely relies on H⁺ surface transport to enhance charge transfer.

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The O 1s spectra for each location were fitted to Gaussian/Lorentzian line-shape peaks (GL(30) with a product formula) with Shirley-background corrections using standard XPS data analysis techniques in CasaXPS. The complex peak shapes were fitted with the three peaks related to three oxygen-containing species (O²⁻, OH⁻, and H₂O), exemplified at V_cell = 1.0 V in figure 4(d). The fits of the overlapping O²⁻ and OH⁻ peaks assumed identical full widths at half maximum with only a very small variation between the two different electrodes (FWHM(O²⁻) = FWHM(OH⁻) = 1.79 for Ni/GDC and FWHM(O²⁻) = FWHM(OH⁻) = 1.81 for Au/GDC). The integrated percentage area (percentage ratio relative to sum of O²⁻, OH⁻ and H₂O areas) for each component was then utilized for estimating surface species fractions. Figures 5(a) and (b) plot fitted O 1s spectra on the GDC surface at three representative V_cell and T = 620 °C near the Ni/GDC electrode and the Au/GDC electrode, respectively. In general, the local surface concentrations of OH⁻ and O²⁻ do not change significantly with V_cell for either WE, but Ni/GDC shows significantly higher concentration of OH⁻, which is likely tied to the higher activities (currents) for both H₂O electrolysis and H₂ oxidation. The higher OH⁻ occupancy on the GDC can serve as a marker for higher activity because key charge transfer steps in both H₂O electrolysis (reverse direction of reaction R1) and H₂ oxidation (forward direction of reaction R2) as shown here depend on OH⁻ activity on the GDC surface.

$$\begin{equation}{\text{H}}\left( {{\text{Ni}}} \right){{ + }}{{\text{O}}^{{{2 - }}}}\left( {{{\text{O}}_{{\text{GDC}}}}} \right)+{\text{ C}}{{\text{e}}^{{{4 + }}}}\left( {{\text{C}}{{\text{e}}_{{\text{GDC}}}}} \right) \leftrightarrow {\text{O}}{{\text{H}}^{{ - }}}\left( {{{\text{O}}_{{\text{GDC}}}}} \right)+{\text{ C}}{{\text{e}}^{{{3 + }}}}\left( {{\text{C}}{{\text{e}}_{{\text{GDC}}}}} \right)+{\text{ []}}\left( {{\text{Ni}}} \right){{ + }}{{\text{e}}^{{ - }}}\left( {{\text{Ni}}} \right)\end{equation} \tag{ R1 }$$

$$\begin{equation}{\text{H}}\left( {{\text{Ni}}} \right)+{\text{ O}}{{\text{H}}^{{ - }}}\left( {{{\text{O}}_{{\text{GDC}}}}} \right)+{\text{ C}}{{\text{e}}^{{{4 + }}}}\left( {{\text{C}}{{\text{e}}_{{\text{GDC}}}}} \right){\text{ }} \leftrightarrow {\text{ }}{{\text{H}}_{{2}}}{\text{O}}\left( {{{\text{O}}_{{\text{GDC}}}}} \right)+{\text{ C}}{{\text{e}}^{{{3 + }}}}\left( {{\text{C}}{{\text{e}}_{{\text{GDC}}}}} \right)+{\text{ []}}\left( {{\text{Ni}}} \right){{ + }}{{\text{e}}^{{ - }}}\left( {{\text{Ni}}} \right)\end{equation} \tag{ R2 }$$

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where (O_GDC) and (Ce_GDC) represent oxide and ceria sites respectively on the GDC surface. The ability for the Ni surface to supply and to receive adsorbed H atoms plays a significant role in enhancing the electrochemical activity of the WE. The weak interactions of adsorbed H₂O with lattice oxygen on the GDC surface [41] implies the importance of the metal surface in providing mobile H species to support more activity on the GDC surface. With the higher affinity of Ni for adsorbing H₂O than Au [41], it is not surprising that the Ni/GDC electrode shows higher intensity for near-surface/adsorbed H₂O in figure 5.

Primary peak (O²⁻) binding energies after the spectra fitting were plotted as a function of the cell position across the metal/GDC/YSZ border at different V_cell as shown in figures 6(a) and (b) for Ni/GDC/YSZ and Au/GDC/YSZ borders, respectively. Binding energy shifts on the GDC surface indicate changes in surface potential associated with the applied bias and resulting reactions. Due to surface charging, shifts in the YSZ surface do not necessarily correspond to surface potentials beneath the GDC film where the O²⁻ charge transfer occurred between those two phases. The relatively flat binding energy distribution across the GDC surface suggests sufficient electronic conductivity to sustain uniform potential distribution on the GDC surface at the measured current densities. Even though the Ni/GDC and Au/GDC WEs have similar magnitudes of η_act,WE at a given V_cell, much higher output currents associated with Ni/GDC electrode indicated superior electrochemical activity of the Ni/GDC electrode. Both WEs showed significantly higher currents for H₂O electrolysis than for H₂ oxidation at similar magnitude of η_act,WE. Plotting η_act,WE across the metal/GDC interface as a function of current per L_tpb (i') in figure 7 shows that for H₂O electrolysis, η_act,WE for the Au/GDC WE was only slightly higher than for Ni/GDC at the same i'. On the other hand, for H₂ oxidation, η_act,WE for the Au/GDC WE grew much faster with increasing magnitude of i' than for H₂O electrolysis. This is unlike Ni which had more similar slopes η_act,WE vs. i' for H₂O electrolysis and H₂ oxidation. Assuming that the two charge transfer reactions R1 and R2 determine the activation overpotential, Zhu et al [42] derived the following expression in equation (1) for a relationship between η_act,WE and i' across the Ni/oxide interface for H₂ oxidation based on the assumption that (R1) is in equilibrium and (R2) is rate limiting.

$$\begin{equation}i^{\prime} = {i^{\prime}_0}\left( {T,{P_{{\text{H2}}}},{P_{{\text{H2O}}}}} \right)\left[ {\exp \left( {\frac{{\left( {1 + {\beta _{{\text{R2}}}}} \right)F\left| {{\eta _{{\text{act,WE}}}}} \right|}}{{RT}}} \right) - \exp \left( { - \frac{{\left( {1 - {\beta _{{\text{R2}}}}} \right)F\left| {{\eta _{{\text{act,WE}}}}} \right|}}{{RT}}} \right)} \right]\end{equation} \tag{ 1 }$$

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where i'₀ is the exchange current density which is a function of T and the partial pressures P_H2 and P_H2O [42]. Fitting equation (1) to measured η_act,WE plotted in figure 7 provided a basis for estimating fundamental charge transfer parameters such as i'₀ and β_R2 as shown in table 1. Least squared fits of i'₀ and β_R2 for H₂O electrolysis currents and for H₂ oxidation currents were performed independently for each WE, and the results in table 1 (which are plotted as curves in figure 7) show that with the exception of H₂ oxidation all fits provide values of β_R2 close to 0.5. Exchange current densities i'₀ at 620 °C and P_H2 = P_H2O = 0.26 mbar are approximately 2.33 × 10⁻⁷ A m⁻¹ of L_tpb with the exception of H₂ oxidation on Au which has an i'₀ that is approximately one order of magnitude lower. While it cannot be presumed that these values represent specific rates of charge transfer reactions, they do provide a basis for estimating orders of magnitudes of specific reaction rate constants that might be used in a more detailed charge transfer reaction rate expression [39, 40].

The charge transfer relationship in equation (1) assumes that the GDC oxide surface is largely equilibrated with the gas phase and that the low effective P_O2 associated with the H₂/H₂O gas mixtures has not significantly impacted the GDC oxide vacancy site concentrations [V_O]_L and cation site concentrations of reduced ceria polarons [Ce³⁺]_L. Surface oxide vacancies are critical in providing exchange of H₂O with the gas phase through the following surface reactions.

$$\begin{equation}{{\text{H}}_{\text{2}}}{\text{O}}\left( {\text{g}} \right){{ + }}{{\text{V}}_{\text{O}}}\left( {{{\text{O}}_{{\text{GDC}}}}} \right)\, \leftrightarrow {\text{ }}{{\text{H}}_{\text{2}}}{\text{O}}\left( {{{\text{O}}_{{\text{GDC}}}}} \right)\end{equation} \tag{ R3 }$$

$$\begin{equation}{{\text{H}}_{\text{2}}}{\text{O}}\left( {{{\text{O}}_{{\text{GDC}}}}} \right) + {{\text{O}}^{2 - }}\left( {{{\text{O}}_{{\text{GDC}}}}} \right) \leftrightarrow {\text{ 2O}}{{\text{H}}^ - }\left( {{{\text{O}}_{{\text{GDC}}}}} \right)\end{equation} \tag{ R4 }$$

In addition, concentrations of reduced ceria polaron site [Ce³⁺]_L play a role in increasing electronic conductivity of the GDC and thereby extending the region of effective reaction area for charge transfer reactions [35]. Thus, effective P_O2 were calculated from chemical equilibrium with the H₂/H₂O mixtures and for the test conditions at the ALS that effective P_O2 ranged from 1.2 × 10⁻²⁴ bar at 570 °C to 6.3 × 10⁻²³ bar at 620 °C. A calibrated Gd_0.1Ce_0.9O_1.95 defect equilibrium model [36] was adopted for Gd_0.2Ce_0.8O_1.9 and used to calculate [V_O]_L and cation site concentrations of reduced ceria polarons [Ce³⁺]_L to assess whether changes in GDC defect concentrations significantly impact the electrochemical activity under the conditions tested. 20% Gd³⁺ doping on the Ce⁴⁺ sites results in lower boundary of [V_O]_L = 0.1 (out of a maximum of 2) at high P_O2. Figure 8 plots the change in [V_O]_L and [Ce³⁺]_L vs. effective P_O2 at OCV and at η_act,WE = +0.2 V. Circular markers show predicted [V_O]_L and [Ce³⁺]_L site fractions of tested conditions (both at ALS and UMD) at OCV. In general, for these relatively low T of solid oxide electrochemical cell operation, at open circuit, the calculated bulk [V_O]_L and [Ce³⁺]_L remain very close to their fully oxidized limits. While it is understood that surface site concentrations of [V_O]_L and [Ce³⁺]_L will be much higher [14], those concentrations will likely still close to equilibrated with the bulk and gas phase. On the other hand, when the GDC film experiences potentials η_act,WE, the voltage will shift surface and bulk concentrations as if it experienced a reduction or increase in effective P_O2 (P_O2,eff (η_act,WE)) according to the following equation.

$$\begin{equation}{P_{{\text{O2,eff}}}}\left( {{\eta _{{\text{act,WE}}}}} \right) = {P_{{\text{O2}}}}\exp \left( { - \frac{{4F{\eta _{{\text{act,WE}}}}}}{{RT}}} \right)\end{equation} \tag{ 2 }$$

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For η_act,WE < 0 during H₂ oxidation at the conditions in this study, the increase in P_O2,eff did not significantly impact [V_O]_L and [Ce³⁺]_L, but for η_act,WE > 0 during H₂O electrolysis, the drop in P_O2,eff was significant. For an η_act,WE = 0.2 V during H₂O electrolysis will shift the P_O2,eff down by a factor of ~10⁻⁵ and thereby increase [Ce³⁺]_L by a factor of 10 as indicated by the plots for η_act,WE = 0.2 V at the relevant T in figure 8. However, figure 8 also shows that [V_O]_L at these lower T remains close to the high P_O2 limit. As such, the effects of η_act,WE on surface chemistry are not likely due to an increase in H₂O exchange but may be due to an extension of the reaction region away from the Ni/GDC electrode TPB through improved electronic conductivity of the more reduced GDC surface. This explains the higher currents for positive η_act,WE in both the Ni/GDC and Au/GDC electrodes.

To more fully explore the surface chemistry as a function of η_act,WE, the fraction of O 1s peak area attributed to OH⁻ was evaluated on the GDC surface very near the Ni overlayer. Figure 9 plots the measured fraction of the O 1s peak attributed to OH⁻ which correlates with oxide surface site fractions. Because the AP-XPS detected oxygen a few nm beneath the surface, the surface site fractions of OH⁻ were likely higher than the area fractions plotted in figure 9. Figure 9 indicates a few interesting trends. Firstly, voltage does not seem to greatly impact the OH⁻ surface site fractions. Higher water partial pressure (P_H2O) and lower T both increased OH⁻ presence on the surface, but because these values do not represent actual site fractions, it remains unclear if surface OH⁻ creation or removal remains a rate limiting step. The relative site fractions from figure 9 provide valuable data for surface mechanism calibration relevant for Ni/GDC electrodes in solid oxide electrochemical cell modelling.

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Differences between binding energy shifts of OH⁻ peaks relative to shifts in the O²⁻ peak provided an indicator of electrochemically active region of the oxide surface [28]. Since the OH⁻ represents exclusively surface adsorbates and a large fraction of the O²⁻ peak comes from subsurface contributions, differences of binding energy shifts between OH⁻ and O²⁻ correlated with a local potential difference between the surface adsorbates and lattice oxygen in the GDC film during electrochemical reaction. Figures 10(a) and (b) plot the binding energy difference across the metal/GDC/YSZ interfaces at different V_cell for both Ni/GDC and Au/GDC, respectively. Under non-zero V_cell, no significant differences were seen for the region of exposed GDC surface and YSZ edge compared to OCV. Binding energies of O²⁻ and OH⁻ shifting by the same amount implies minimal surface charge build-up in that region associated with electrochemistry. At the metal/GDC interface, the results showed no divergence of binding energy difference except for H₂ oxidation reaction (V_cell = −1.0 V). This is consistent with the conclusion that surface polarization of the adsorbed OH⁻ on GDC was minimal when the GDC has higher conductivity (i.e. during electrolysis) or when charge transfer across the metal/GDC interface was low (such as the Au/GDC WE for H₂ oxidation). However, the increased charge transfer reactions at the Ni/GDC interface during H₂ oxidation indicates a significant change in the binding energy difference of OH⁻ and O²⁻. This indicates a significant surface charge build-up for ~40–50 μm from the Ni/GDC interface. This suggests the importance of charge transfer reactions R1 and R2 in facilitating electrochemistry and further supports the importance of activity of the metallic component in solid oxide fuel cell anodes [43].

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Due to the limited access at the ALS, further electrochemical characterization of the two WEs were performed over a range of T (up to 650 °C) and effective P_O2 using a range of H₂/H₂O mixtures. A higher range of P_H2/P_H2O ratios up to 10 were tested, and Ar dilution allowed low P_H2 and P_H2O values to be achieved comparable to those tested at ALS. Similar trends were observed between the Ni/GDC and Au/GDC electrode as shown in figures 11(a) and (b) compared to figures 3(a) and (b), respectively. The IR_bulk-corrected V–I curves in figure 11 showed higher currents for H₂O electrolysis for both WEs, but at 600 °C, the Ni/GDC electrode exhibited much lower polarization resistance than the Au/GDC electrode. At the higher T = 650 °C where electronic conductivity of the GDC significantly increased, the Au/GDC electrode showed significant increase in electrochemical activity. However, the much lower currents during H₂ oxidation suggests that the negative η_act,WE suppresses the increased conductivity and associated regions of electrochemical activity. In general, increased reduction of ceria on the GDC surface at higher T and under cathodic current extends the regions of electrochemical activity on the GDC surface, but such effects are suppressed by the negative η_act,WE during high anodic current.

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Dependency on the effective P_O2 of the electrochemical activity of the Ni/GDC and Au/GDC electrodes was further explored by increasing P_H2/P_H2O ratios at a constant T = 600 °C. Figures 12(a) and (b) plot IR_bulk-corrected V–I curves for both WEs ratios for different P_H2/P_H2O up to a ratio of 10 (corresponding to an effective P_O2 = 10⁻²⁵ bar at 600 °C). This lower P_O2 enables a substantial increase in [Ce³⁺]_L for positive η_act,WE as suggested in figure 8, but the improvement in H₂O electrolysis activity is not significant. Nonetheless, the increased reduction more than offsets the increased H₂ product concentration and the higher P_H2/P_H2O ratios improved H₂O electrolysis activity. Raising the P_H2/P_H2O ratio more significantly increased activity for H₂ oxidation, particularly for the Ni/GDC electrode. The higher P_H2/P_H2O ratios reduce the risk for buildup of oxide species on the Ni surface at high anodic current. The improvement of H₂ oxidation on Ni/GDC electrode at higher anodic current provides further proof of the role of the Ni activity on providing a key role in H₂ oxidation for fuel cell conditions.

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The presented study explored surface potentials, active surface species, and associated electrochemical performance of the Ni/GDC and Au/GDC electrodes in H₂ oxidation and H₂O electrolysis. Ambient pressure XPS measurements of electrochemically active GDC thin-film SOCs provided a means of measuring local surface potential through shift of characteristic binding energy and thereby overpotentials across the metal/GDC interface associated with charge transfer reactions. Specifically, investigation of O 1s core-level XPS of the GDC electrodes under H₂/H₂O atmosphere has revealed how near-surface oxidation states as well as active surface-absorbed species relate to electrochemical activity in this study. Fundamental charge transfer parameters such as i'₀ and β_R2 were derived to provide a basis for estimating specific reaction rate constants that can be used in a more detailed kinetics. Our study revealed that [V_O]_L and [Ce³⁺]_L in the GDC were affected by effective P_O2 and surface potential which significantly impacted the reactivity of the GDC. The relative site fractions of OH⁻ provided valuable data for surface mechanism calibration relevant for Ni/GDC electrodes. Differences between binding energy shifts of OH⁻ peaks relative to shifts in the O²⁻ peak revealed the region of electrochemically activity on the GDC away from the Ni/GDC interface.

Further electrochemical characterization of the Ni/GDC and Au/GDC showed the importance of the metal surface activity for increasing H₂ oxidation. The Ni electrode provides much higher H₂ oxidation currents than the less active Au electrode. On the other hand, the increased electronic conductivity of the GDC in ceria reduction-promoted voltages decreased the importance of the Ni activity, and both the Ni/GDC and Au/GDC electrodes showed similar and higher electrochemical activity for H₂O electrolysis. The simultaneous electrochemical measurements and surface characterization with XPS has provided key data to assist in developing a more mechanistic understanding of GDC electrodes for both solid oxide fuel cells and solid oxide H₂O-electrolysis cells.

This work was funded through the support of the Office of Naval Research through Contract # N000140510711 (Dr. Michele Anderson program manager). The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract # DE-AC02-05CH11231. We thank the University of Maryland Nanocenter and the University of Maryland Energy Research Center (UMERC) for support. We would also like to thank Dr. Chunjuan Zhang (now at BASF) who helped with the sputter deposition of the GDC films and the thin-film current collectors, and Drs. Sandrine Ricote and Huayang Zhu (Colorado School of Mines) who provided valuable feedback on the GDC kinetics.