Ebbert Mary Anna, Litster Shawn
Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA, USA.
Sci Rep. 2024 Dec 30;14(1):31637. doi: 10.1038/s41598-024-79935-6.
Polymer electrolyte membrane water electrolyzers (PEMWEs) are a critical technology for efficient hydrogen production to decarbonize fuels and industrial feedstocks. To make hydrogen cost-effective, the overpotentials across the cell need to be decreased and platinum-group metal loading reduced. One overpotential that needs to be better understood is due to mass transport limitations from bubble formation within the porous transport layer (PTL) and anode catalyst layer (ACL), which can lead to a reduction in performance at typical operating current densities. When operating at ultra-high current densities (UHCD), the rate of the OER may reach a critical point at which oxygen gas bubbles fill the pores of the ACL and PTL, completely blocking access of liquid water to the ACL. Because of this, there is a possibility that the cell will rely on water vapor diffusion through the evolving oxygen gas to deliver the water reactant to the OER catalyst. To assess the operational limitation of a PEMWE while relying on water vapor diffusion, a commercially manufactured membrane electrode assembly (MEA) was tested by flowing water vapor with an inert carrier gas into the anode as the reactant. To identify a limiting current density (i) of the electrolyzer under these conditions, potentiostatic polarization curves were obtained for a range of relative humidity (RH) and backpressures. The RH was varied to assess the impact of reactant concentration on the catalyst mass activity at low current and on the i, while the backpressure was varied to isolate the impact of the molecular gas diffusion coefficient on the i. Our findings highlight that water vapor diffusion through evolved oxygen is readily able to support the OER without notable mass transport overpotentials. However, our results show that water vapor feed inhibits high current density through reduced catalyst specific activity and polymer electrolyte membrane dry-out.
聚合物电解质膜水电解槽（PEMWEs）是高效制氢以实现燃料和工业原料脱碳的关键技术。为了使制氢具有成本效益，需要降低电池的过电位并减少铂族金属负载量。一个需要更好理解的过电位是由于多孔传输层（PTL）和阳极催化剂层（ACL）内气泡形成导致的传质限制，这可能导致在典型工作电流密度下性能下降。当在超高电流密度（UHCD）下运行时，析氧反应（OER）的速率可能达到一个临界点，此时氧气气泡会填满ACL和PTL的孔隙，完全阻止液态水进入ACL。因此，电池有可能依靠水蒸气通过不断产生的氧气进行扩散，将水反应物输送到OER催化剂。为了评估PEMWE在依靠水蒸气扩散时的运行限制，通过将水蒸气与惰性载气一起作为反应物通入阳极，对一个商业制造的膜电极组件（MEA）进行了测试。为了确定在这些条件下电解槽的极限电流密度（i），在一系列相对湿度（RH）和背压下获得了恒电位极化曲线。改变RH以评估反应物浓度对低电流下催化剂质量活性和i的影响，同时改变背压以分离分子气体扩散系数对i的影响。我们的研究结果表明，通过生成的氧气进行水蒸气扩散能够很容易地支持OER，而不会产生明显的传质过电位。然而，我们的结果表明，水蒸气进料会通过降低催化剂比活性和聚合物电解质膜干涸来抑制高电流密度。
