Differentiating polarization curve technique for determining the exchange current density of hydrogen electrode reaction

Abstract

The authors propose a new method to easily and reliably determine the exchange current density (i0) of hydrogen evolution reaction (her). The validity of a novel method named the differential polarization method (DPM) was ascertained by the estimation of i0 on platinum electrode in acid solutions. The hers in 0.005, 0.05 and 0.5 mol dm −3 H2SO4 solutions were visually categorized as reversible reactions. The obtained i0 had constant values of about 0.5 mA cm −2 , corresponding to the appearance of a vertical line on the anodic polarization resistance curve. This DPM shows the i0 is corresponds to the limiting diffusion current density of the hydrogen oxidation reaction. We believe that the DPM has the potential to be a beneficial and reliable method for estimating electrochemical parameters.

The Differentiating Polarization Curve Technique for the Tafel Parameter Estimation

“…Because the l/κ is independent of the i, it will be graphically emerged as the horizontal line when the |i| becomes large. We can estimate it using the following operation [8]:…”

“…We also know an important kinetic relation between net current density, i and overpotential, η ∅ (= E − E ∅ ) [8]:…”

“…Authors believe that there is a need for a more reliable technique for determining the i 0 in addition to the conventional TEM. To address this need, we have developed a technique termed the "differential polarization method (DPM)" [8]. Our previous work using the DPM reported that the i 0 for the her on the platinum electrode in the 0.5 mol dm −3 H 2 SO 4 solution is limited by the amount of dissolved hydrogen.…”

“…Such inconclusiveness data may be due to individual errors and difficulties in reading the proper region when applying the TEM. In addition, it is necessary to pay more attention to the physical factors such as solution resistance and oxide film on the electrode [8][9][10][11]. Authors believe that there is a need for a more reliable technique for determining the i 0 in addition to the conventional TEM.…”

Estimation of Exchange Current Density for the Hydrogen Evolution Reaction of Titanium in Sulfuric Acid Solution

“…電気化学の分野で最も基礎的かつ重要な反応の 1 つに水素 電極反応がある．水素電極反応に関する理解と応用は，燃料 電池や腐食防食工学ばかりでなく未来のエネルギー社会に とって極めて基礎的かつ重要な工学分野の 1 つとして認識さ れている [1][2][3] ．この反応は水素発生反応 (hydrogen evolution reaction；以下 her と略記する) と水素酸化反応 (hydrogen oxidation reaction; hor) の半反応から構成されており，特に her ～ hor 間の交換電流密度 (i 0 ) が学術上ばかりでなく産業的にも重 大な意味を持っている 1-7) ． i 0 に関する研究は今日も精力的に行われ，そのデータの蓄 積は着実に行われている．しかしながら既報データの中には 再現性や信頼性に乏しい例が散見される 2,4,7) ．例えば酸性溶 液中の Ti 電極の i 0 の値は 10 -2.7 ～10 -5.6 mA cm -2 とあり，およ そ 3 桁の違いがある． 3 桁の誤差があるデータは通常の工学 設計では使えない．この原因は個人的な測定誤差ばかりでは なく Tafel 外挿法 (Tafel extrapolation method; TEM) における 読み取りの困難さに起因しているものと言われている 2,4,7-9) ． 測定方法の 1 つとして微分分極法 (Differential polarization method; DPM) が提案されている [10][11][12] ．DPM は TEM を発展 させた手法で分極曲線，E (i) を微分した分極抵抗曲線，h (i) を 用いて解析する方法である．この DPMを用い 0.5 mol dm -3 H 2 SO 4 水溶液中の Pt 電極上での her は可逆反応 (i 0 ≈ 10 -0.3 mA cm -2 ) であり 10) ，Ag と Ni 電極上では非可逆反応 (それぞれ i 0 ≈ 10 -2.3 mA cm -2 ，移動係数，α c ≈ 0.39 と i 0 ≈ 10 -1.8 mA cm -2 , α c ≈ 0.7) で ある 12…”