Comparative Techno-Economic and Life Cycle Analysis of Water Oxidation and Hydrogen Oxidation at the Anode in a CO₂ Electrolysis to Ethylene System

Abstract

We compare the economic viability of employing hydrogen oxidation versus water oxidation at the anode of a commercial-scale electrolysis plant that converts CO 2 to ethylene. We vary the electrolyzer capital cost, membrane lifetime, and renewable electricity price to represent a current and future market scenario. We find that anodic hydrogen oxidation with membraneless reactor design can reduce the electrolyzer capital cost by up to 48% and reduce electricity demand by at least 50% with the current underdeveloped electrolyzer market. These capital and operating cost savings could further lead to a lower ethylene production cost from anodic hydrogen oxidation than the anodic water oxidation system with hydrogen supplied at less than $6/kg. In the future scenario with a fully developed electrolyzer market and cheap renewable electricity, we find that the anodic hydrogen oxidation system requires hydrogen cheaper than $0.7/kg to compete with the anodic water oxidation system. Moreover, hydrogen oxidation at the anode enables extremely low cradle-to-gate emission ethylene by utilizing negative emission hydrogen such as biomass gasification with carbon capture and sequestration, ∼240% lower than ethylene produced from the wind/solar electricity-driven water oxidation system. This low carbon footprint ethylene can further boost the economic competitiveness for anodic hydrogen oxidation with a future carbon credit market.

Analyzing Production Rate and Carbon Utilization Trade-offs in CO2RR Electrolyzers

“…The idea of using electrolyzers to transform CO 2 into value-added products has gained significant attention over the past decade. − The overall viability of this approach compared to alternative CO 2 mitigation strategies will depend on the confluence of reactor performance and numerous external economic factors. ,− When assessing reactor performance, a number of key performance indicators (KPIs) are crucial to consider when evaluating progress. For example, some of these KPIs include system-level characteristics such as specific energy consumption (e.g., kWh/kg product), productivity (e.g., kg product/time/system size or partial current density), and carbon utilization (e.g., single-pass CO 2 conversion efficiency).…”

“…Along these lines, here we employ a simple model to understand carbon utilization in CO 2 electrolyzers, enabling an exploration of the relationship between two KPIs: carbon utilization and reactor productivity. Productivity is important because it is directly tied to capital intensity for a given product output, while single-pass conversion efficiency is important because of the need to efficiently transform CO 2 into products while minimizing downstream separation costs. ,,, Previously, Weng et al , developed and examined models for both gas diffusion electrode (GDE) with liquid electrolyte and membrane electrode assembly (MEA) CO 2 electrolyzers and used them to examine the conversion and consumption of CO 2 as a function of applied current density. Subsequently, Kas et al developed a 2-D model for investigating how gradients along the gas channel impact CO 2 conversion and consumption.…”

“…Notably, here we ignore energy consumption, which would be another factor to consider when assessing the “highest performing” results. In the end, the relative value of conversion efficiency, productivity, and energy consumption must be weighed in economic terms within the context of the application. ,,,, In our view, though, trade-off curves like those illustrated in Figure can paint a more complete picture of electrolyzer performance and serve as essential inputs to techno-economic models. ,,, …”

Conversion of Reactive Carbon Solutions into CO at Low Voltage and High Carbon Efficiency

“…This reaction reduces electricity consumption relative to the OER, but the source and cost of H 2 must be considered. Ideally, H 2 would be produced from a low-carbon source such as biomass gasification or water electrolysis . The cost of generating H 2 by biomass gasification is reported to be as low as $0.9/kg , (with net negative emissions of 15–22 kg CO 2 /kg H 2 when it is coupled to carbon capture and sequestration , ), and the target price for clean H 2 determined by the DOE Energy Earthshots Initiative is $1 per kg of H 2 .…”

“…Ideally, H 2 would be produced from a low-carbon source such as biomass gasification or water electrolysis. 44 The cost of generating H 2 by biomass gasification is reported to be as low as $0.9/kg 45,46 (with net negative emissions of 15−22 kg CO 2 /kg H 2 when it is coupled to carbon capture and sequestration 47,48 ), and the target price for clean H 2 determined by the DOE Energy Earthshots Initiative is $1 per kg of H 2 . 49 We therefore used a forward-looking purchase price of $1/kg of H 2 as a basis for comparing the economics of HOR|CEM|HCO 3 − , which consumes H 2 , to the OER|BPM|HCO 3 − and conventional OER|AEM|CO 2 , which consume water at the anode (Table 1 and Figure S9).…”